Continuous record of geomagnetic field variations during cooling of the Monchegorsk, Kivakka and Bushveld Early Proterozoic layered intrusions Текст научной статьи по специальности «Науки о Земле и смежные экологические науки»

RUSSIAN JOURNAL OF EARTH SCIENCES, VOL. 6, NO. 6, PAGES 391-456, DECEMBER 2004

Continuous record of geomagnetic field variations during cooling of the Monchegorsk, Kivakka and Bushveld Early Proterozoic layered intrusions

D. M. Pechersky1, V. S. Zakharov2, A. A. Lyubushin1

Abstract. A continuous record of the geomagnetic field direction during of cooling of the Monchegorsk (2.5 Ga), Kivakka (2.45 Ga) and Bushveld (2.06 Ga) layered intrusions was conducted for the first time. The low-Ti titanomagnetites (Tc = 530-580oC) are the main natural remanent magnetization (NRM) carriers. The high temperature NRM component according to petromagnetic characteristics is thermoremanent and was acquired at the cooling phase of the intrusion. The magnetic grains above 540oC are close to a single-domain state. The temperature versus time and cooling velocity versus temperature and time dependences during cooling of intrusions were calculated on the basis of solution of the Stefan problem and the nonstationary heat conduction problem. The geothermal gradient value was taken as 20o km-1. The unblocking temperatures in the course of thermal demagnetization of samples (Td) and the blocking temperatures during cooling from high temperatures (Tb) rate dependences were used for conversion of the Td to Tb [Dodson and McClelland, 1980]. A detailed thermal demagnetization at 2o-3o interval from 530oC up to 580oC was made and a pattern of behavior of the geomagnetic field direction was obtained (a) for the time of “running” the Curie points over the section and (b) for the cooling time of the intrusion at each sampling point from Tb = 580oC. A wavelet analysis has been used to study the field variation etc. The main rhythms of the direction variation spectrum are 3-4.5, 5-7, 8-10, 12, 15-17, 19-20, 30-40, 50-60, and 90-100 kyr and they change in time. The length of rhythms is different and ranges from 1-2 oscillations (“splashes”) up to 10-12. The Monchegorsk intrusion. The pole 265.3oE, 1.3oN. The length of record is ^70 kyr. The geomagnetic excursion of the reversal polarity lasting for less than 2000 years and the R-subchron of ^60 kyr were recorded. Kivakka intrusion. The A1 prefolded and A2 synfolded components which occurred at the stage of cooling of the intrusion are isolated. The body started to tilt approximately 85 kyr after intrusion emplacement and it lasted for 20-25 kyr. The pole of the A1 is 17.8oS and 247oE. Duration of the record is 35 kyr and only one R-polarity existed during this period. Bushveld intrusion. Orientation of the horizontal plane of the samples has been restored from viscous magnetization and a paleomagnetic pole (12oN, 35.4oE) has been determined from a high-temperature N-component of the NRM. A paleomagnetic record for ^500 kyr was obtained; polarity of the field has changed only once during this period.

1 Institute of Physics of the Earth, Russian Academy of Sciences, Moscow, Russia

2 Moscow State University, Moscow, Russia

Copyright 2004 by the Russian Journal of Earth Sciences.

Paper number TJE04158.

ISSN: 1681-1208 (online)

The online version of this paper was published 12 January 2005. URL: http://rjes.wdcb.ru/v06/tje04158/tje04158.htm

Introduction

A fine structure of the geomagnetic field is commonly studied from paleomagnetic records in sections of the sedimentary and volcanogenous deposits. The overwhelming majority of such sections, even the most “continuous” have gaps and, consequently, the paleomagnetic records of the geomagnetic field are discontinuous. The gaps are not always detected and they cover time intervals of several years to

a few million years and their distribution along sections is usually irregular.

Only the cooling magmatic bodies ranging from lava flows (cooling time of days, months and years) to large intrusive bodies, whose cooling time covers dozens of thousands and millions of years, represent an actual continuous record of the geomagnetic field behavior. Samples from such bodies yield two versions of continuous records: 1) detailed sampling from the contact to the inner zone of the body; the isotherm the Curie point of magnetic minerals which are present in the body shifts from the contact, where the cooling is relatively fast, to the maximum time approximately in the center of the body (one should have in mind that by the time of record the contact zone and the enclosing rocks are heated noticeably and the cooling pattern may be either uniform or reverse); 2) detailed thermal demagnetization applied to each sample will make it possible to read the record of the geomagnetic field behavior at the sampling point during cooling of the magmatic body at the sampling point from the Curie point of a magnetic mineral present in the sample to the blocking temperature, at which a significant portion of the thermoremanent magnetization is still recognizable within the accuracy of measurements. The theoretical and experimental paleomagnetic investigation of the artificial and natural samples containing the magnetite grains of different size, from single-domain to multidomain [Pechersky et al., 2002, 2004; Shcherbakov and Shcherbakova, 2002] have shown that “time” (temperature) of the field direction variations in the pTRM directions is retained during the thermal demagnetization, whereas amplitude of the direction variations reduces notably with respect to actual value (90o) from the single-domain (error is close to zero) to 40o in the case of the multidomain grains. Consequently, presence of the single-domain grains is required in the second version for a reliable record.

An important point is the choice of a body for a thermophysical calculation of its cooling process without a notable participation of fluids (“dry” melt) at all the stages. The latter accelerate considerably the cooling process and add complexity to the pattern. The heterophase and singlephase change of the primary magnetic minerals at different stages of existence of the body make the record still more complicated.

All the above considered, the gabbro-pyroxenite layered intrusions reasonably uniform by the magmatic conditions (nearly dry magma) and cooling, which is close to conductive, are the most favorable object. The cumulative parts of the complex are most favorable from the paleomagnetic point of view. They do not contain the primary igneous magnetic minerals, but magnetic minerals appear as inclusions in pyroxenes, olivines and plagioclases as a result of exsolution during the cooling phase, approximately from 550-600oC, i.e., during the cooling of a nearly homogeneous solid body. These inclusions are dominated by nearly singledomain low-Ti titanomagnetite and magnetite grains. It is important to select bodies that show no evidence of hydrothermal and similar alterations during both the cooling and subsequent existence of the body.

This is the first study of a continuous record of paleovari-ations of the geomagnetic field on the whole and, in par-

ticular, conducted on such objects, which are described in this article. There are some publications devoted to records of geomagnetic reversals in cooling magmatic bodies. This is mostly the first version of a record of an NRM component isolated at a temperature close to the Curie point of magnetite. For example, researchers have obtained fairly detailed records of the geomagnetic field before, during and after reversals during the cooling of the Miocene granitoid intrusions of Tatoosh (~17-18 Ma) and Loreal Hill (~8 Ma) in the northern USA [Dodson et al., 1978] and the Agno diorite intrusion (~15 Ma) on Luzon Island, Philippines [Williams and Fuller, 1982]. It was shown that the primary thermoremanent magnetization is associated with low-Ti titanomag-netite and magnetite. Being a carrier of a stable NRM component, the magnetite occurs as fine inclusions in silicates, products of their exsolution. As a rule, more than 90% of the NRM is unblocked in a range 540-580° C. Based on several thermophysical models, the cooling times of the intrusions in this temperature range and, accordingly, the duration of transitional zones were roughly estimated at less than 6000 to 14000 years, depending on whether water was involved or the cooling was purely conductive. Detailed sampling of a drill core from the Tatoosh intrusion revealed (against the background of appreciable noise) variations in the direction and value of magnetization; the vector rotates counterclockwise, producing variously shaped loops similar to secular variations.

Coe and Prevot [1989] and Coe et al. [1995] described a case of a very rapid geomagnetic reversal that was also discovered by the first method in the cooling record of a lava flow on Mount Steens. This interpretation, however, is not indisputable, because it was shown that this feature may have resulted from remagnetization of the upper part of the flow due to its heating by the overlying flow that erupted when the geomagnetic field had an opposite polarity. This is supported by the existence of a similar situation (high-temperature NRM components of opposite polarities present in one flow) observed, first, in lava flows on La Palma Island, where the Matuyama-Brunhes reversal was recorded, and second, in the Gamarri lavas in Ethiopia [Valet et al., 1998]. Paleomagnetic, petromagnetic and other detailed studies proved that this is a result of the remagnetization of basalts by an overlying flow.

There are examples of the second version of record of the geomagnetic reversals found in samples containing singledomain pyrrhotite from Jurassic sedimentary sequence in the western Alps that slowly cooled in the Miocene and underwent deformations and metamorphism at temperatures of 300°C to 350°C in the Oligocene [Crouzet et al., 2001a; Rochette et al., 1992] and in the Himalayas where carbonates underwent metamorphism in the Tertiary period [Crouzet et al., 2001b]. It is revealed a record of up to ten reversals in the course of detailed thermal demagnetization of NRM of each sample [Crouzet et al., 2001a]. A correlation of magnetic chrons between samples and sampling points was then performed. As a result, a magnetostratigraphic section comprising 21 polarity reversals was constructed. The thermo-magnetic study showed that the pyrrhotite grains have a single-domain structure and obey the law of additivity and independence of partial thermal magnetizations. The tem-

perature range within which the magnetic chrons was identified from unblocking temperatures was 302-182.5°C, and the record was ~4 Myr long.

Finally, first paleomagnetic and thermophysical investigation of a typical layered gabbro-pyroxenite Monchegorsk intrusion were conducted [Pechersky et al., 2002, 2004]. A magnetochronological pattern (Figure 1) was obtained for each sample taken from selected section 375 m thick for the second method used. The blocking temperature intervals were translated into the relative cooling time so to construct magnetochronological columns of magnetic polarity during cooling of the intrusion [Pechersky et al., 2002, 2004]. Figure 1 shows that all the columns begin from N-polarity (chron N1) and at Td = 550-560°C the boundary is between the N and R polarity, the N-polarity in columns (samples) covers an interval of 30-40 kyr. This pattern is seen from the base of the section up to ~250 m, and behind the fault, which is between samples 43 and 45, stops sharply and only N-polarity (N2) is reliably noted. Evidently, a certain shift occurred along the fault, which interrupted a continuous record and a more high horizons of the section are fixed behind the fault. At later portions of the columns (Td < 540° C) either partial remagnetization or unblocking temperatures are above 500°C (multidomain grains). In the latter case conversion of the unblocking temperatures (Td) into blocking temperatures (Tb), and, consequently, estimation of the cooling time is not correct. Therefore, only unblocking temperatures above 530°C are used for magne-tochronological assessment. A summary magnetochronolog-ical column from the moment of cooling of intrusion from 580°C looks as follows: N 1-chron - from 0 to 30-40 kyr, R-chron - from ~50 to ~130 kyr, N2-chron - from 130 kyr and further. The reversed polarity duration is similar to the late Cenozoic subchrons.

This paper is dedicated, first of all, to continuation of the paleomagnetic and thermophysical study of the Monchegorsk intrusion and of two more intrusions of Kivakka and Bushveld. The main purpose of this is to obtain a paleomagnetic record of the geomagnetic field direction variations. We preferred to conduct this type of investigation with a rather ancient objects not only because they meet the aforementioned conditions, but also due to the fact that no sections of sedimentary and volcanogenous deposits are available in the Early Proterozoic for the study of the fine structure of the geomagnetic field.

Brief Geological Description

All three foregoing intrusions are typical layered gabbro-pyroxenite magmatic bodies, of a different age, located in different geological structures and even on different continents. All of them, despite their ancient age, are well preserved.

A. The Monchegorsk pluton is a part of the large layered Early Paleoproterozoic Monchegorsk intrusive complex, located in the central part of the Kola Peninsula. The age of the Monchegorsk intrusion is ~2.5 Ga [Amelin et al., 1995; Balashov et al., 1993]. As a result of intensive collision processes, which accompanied the Svecofennian Ocean closure

in the central part of the Baltic Shield about 2.0-1.9 Ga, this craton was separated into several tectonic blocks and was subjected to metamorphism: metamorphism weakens from the western boundary to the east from granulite to the greenstone facies. The Monchegorsk pluton was slightly affected by tectonic and metamorphic processes; metamorphic alterations localize here mainly in fault zones. The primary bedding of the intrusive rocks has been left intact, and minor variations in the layering attitude are primary.

The Monchegorsk pluton has an arcuate (in plan) shape; one of its two branches is elongated in a NE direction and the other trends E-W (Figure 2). The total area of the intrusion is about 55 km2. Its W-SW continuation is break off by the Monche-Tundra fault; its southern continuation is buried under Early Paleoproterozoic volcanic-sedimentary rocks of the Imandra-Varzuga group, partially thrusting on the intrusion. The primary intrusive relationships have been preserved only in the northern part of the intrusion, where a rapidly cooled marginal zone consisting mainly of fine-grained norites and gabbro-norites exists all along the contact. Thickness of the zone ranges from 10 m to 100 m. A layer of peridotites (dunite cumulates) 100 m to 200 m thick is observed over gabbroids of the lower marginal zone. This layer is overlain by a 250 m to 400 m thick zone of rhythmically alternating peridotites, olivine orthopyroxenites, and orthopyroxen-ites. This zone underlies a layer of bronzitites 300 m to 700 m thick, which consists further of norites (plagioclase-orthopyroxene cumulates) and gabbro-norites (plagioclase-orthopyroxene-clinopyroxene cumulates). The only ore mineral observed in thin sections is chromite (in peridotites). The reconstructed total thickness of the intrusion is about

3 km; its upper part (~1 km) was eliminated by erosion. The formation depth of the intrusion is 10-15 km (4-4.5 kbar), as indicated by very scarce evidence of the reaction between basic plagioclases and olivines, which proceeds at a pressure of at least 5 kbar.

Oriented samples were selected from outcrops along the eastern crest of Mountain Nittis. Rocks exposed here belong to the endocontact zone of rhythmically alternating peridotites (bronzitites) replaced in the upper part of the section by monotonous bronzitites of the pyroxenite zone.

Samples were collected uniformly from the base of the mounting to the top at 10 m intervals (vertically). The total thickness from which the samples were collected was 375 m. A total of 39 samples were collected from the intrusion, and 14, from its surrounding Archean gneisses. Aside of this, 46 samples were collected every 1-2 m in the interval from 100 m to 161 m from the lower contact.

B. The Kivakka intrusion (Figure 3) belongs to the Olang group of coeval layered plutons located within the Pyaozersk-Tiksheozersk uplift (Northern Karelia). The initial phase of the Proterozoic activity has shown in formation of the granite intrusions of the Nuorunen-Karmanka type, whose age is 2450±72 Ma [Buiko et al., 1995] and 2449.3±3.7 Ma [Levchenkov et al., 1994]. Intrusion of the layered plutons and gabbro-norites dikes was controlled by a series of the N-W deep faults [Saltikova, 1991; Silvennoinen, 1991]. The Kivakka intrusion isotope age is 2445±3.4 Ma [Balashov et al., 1993]. A younger geological age of the layered plutons as compared with granites is confirmed by the

Figure 1. Magnetochronologic columns, arranged according to data of the component analysis of the difference vectors of the NRM in the process of the detailed thermal demagnetization [Pechersky et al, 2004]. Blue color - normal (N) magnetic polarity, primary cooling intervals - bright color, weak color - partial or complete more young remagnetization; red color - reverse (R) polarity, bright color -primary cooling intervals, weak color - partial or complete more young remagnetization; white color -unreliable intervals. Fully unreliable samples are not included into the figure. Distance between columns is proportional to the vertical distance between the samples. The time scale is made on the basis of the thermophysical calculations. The “zero” point (magnetite Curie temperature) shifts from the base of the section to the top (375 m) for approximately 10 thousand years. Td - NRM unblocking temperature during the laboratory thermal demagnetization, Tb - NRM blocking temperature in the process of cooling of the Monchegorsk intrusion, C. Time is in kyr. Figures on the top of the columns are sample numbers.

394 PECHERSKY ET AL.: CONTINUOUS RECORD OF GEOMAGNETIC FIELD VARIATIONS

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Figure 2. Diagram of the geological structure of the Monchegorsk pluton [Sharkov, 1980] a) Position of the Monchegorsk complex in the main tectonic structures of the N-E part of the Baltic shield. MB -1 - Murmansk block; TsKB - Central Kola block; T and L - Tersk and Lottinsk fragments of the Tersk-Lottinsk block; P and I-V - Pechenega and Imandra-Varzuga structures of the Pechenega-Varzuga volcanogenic-sedimentary belt; LGP and UGP - Laplandia and Umbina fragments of the Laplandia-Umbina granulite belt; GLR - Laplandia main deep fault; MK - Monchegorsk complex. b) Diagram of the geological structure of the Monchegorsk pluton 1 - host diorite-gneiss, crystalline schist of the Kola series and other rocks of the recent Archean; 2 - peripheral endocontact zones of the intrusion;

3 - dunite, peridotite; 4 - rhythmic interbedding of dunite, peridotite, olivine bronzitite and bronzitite; 5 - predominantly bronzitite; 6 - zones of the sulphide mineralization; 7 - position of the core holes; 8 - area for taking oriented samples for the paleomagnetic study; 9 - norite; 10 - altered norite, 11 - faults.

Figure 3. a) Schematic geological map of development of the North Karelia layered intrusions (lattice) and Kivakka intrusion position. b) Scheme of the geological structure of the Kivakka intrusion [Koptev-Dvornikov et al., 2001]. 1 - contact zone rocks; 2 - peridotite of the olivinite zone; 3 - norite with pyroxenite horizons; 4 - norite; 5 and 6 - gabbro-norite; 7 - geological boundaries; 8 - faults; 9 - sampling profile of oriented samples, sampling points and sample number.

intersecting contacts of the gabbro-norites dikes.

The Svecofennian tectonomagmatic activity within the Pyaozersk-Tiksheozersk uplift is noted by intrusion of multiphase alkaline-ultrabasic bodies and metamorphic events in the interval of ~2-1.5 Ga [Amelin and Semenov, 1996; Amelin et al., 1995; Kogarko, 1995].

The Kivakka massif has a truncated ellipse shape (in plan) with 4.9 km and 3.15 km semi-axes. An overturned circu-

lar cone with initially vertical axis is proposed as a model. Height of the cone is 3.9 km, the vertex angle is 40°, diameter is 6.5 km, volume of the cone is 44 km3 [Khvorov et al., 2000; Koptev-Dvornikov et al., 2001]. It is presumed that the intrusion has developed as a result of a single injection of magma and its further differentiation in a chamber at the formation place [Koptev-Dvornikov et al., 2001]. According to the modeling the geostatic pressure at the place of for-

Figure 4. Geological scheme of the Bushveld intrusion [Buchanan and Reimold, 1998]. Red circle shows position of well WP-16.

mation of the intrusion was about 3 kbar that corresponds to a depth of ~10 km, the magma initial temperature was 1380-1400° C, crystallization of magma was under dry conditions [Khvorov and Azzuz, 2003]. Features of insignificant local hydrothermal manifestation as fine quartz-chlorite veins whose formation temperature does not exceed 350° C show up much later (M. V. Borisov, verbal information, 2003). No other secondary alterations in the intrusion rocks were found.

The massif is divided into several blocks by a series of faults (the dip azimuth is 210°-220°, angle of dip is 70°-80°) and is tilted to the NW. The bedding elements of the primary magmatic lamination vary insignificantly and the average dip azimuth is 306° and the angle of dip is 35°. The dolerite dikes, which intruded to the rocks of the layered complex, have retained the primary bedding and are not disturbed by deformations. The contact zones and the layered series reflecting the order of alternation of cumulative paragenesises are distinguished in the vertical section of the Kivakka intrusion [Lavrov, 1979]. Thickness of the contact zone of the intrusion does not exceed 100 m and it is composed mainly of norites and pyroxenites. Olivinites and olivine-bearing pyroxenites 400 m thick occur in the base of the layered series. A 700 m thick packet of norites thick builds up the section; the lower part of this packet contains a group of coarse-grained pyroxenites. Sometimes small horisonts of olivine norites and gabbro-norites occur among norites. Banded rocks is a characteristic feature of the norite packet. A gabbro-norite layer 420 m thick is overlying the latter. The gabbro-norite with pigeonite stratum 320 m thick ends the section. Rocks of the layered intrusion

complex do not contain primary magmatic magnetic minerals, i.e. minerals which crystallized from the melt. This is typical of the cumulative part of layered intrusions. The top 50 m of the upper zone are composed of minerals which characterize the residual melt of the basalt magma differentiation. The melt accumulates the iron group elements and, accordingly, an increased titanomagnetite content is observed.

The most complete section of the layered series is represented in the central block of the Kivakka intrusion (Figure 3). Thickness of the section reach to 1800 m. Oriented 70 samples were collected in a well exposed interval from 330 m to 1650 m of the section transversely to the strike of the magmatic lamination for petromagnetic and pa-leomagnetic study (Figure 3). Sampling began in the olivine zone where 7 samples of peridotites were collected. Two portions of the section were characterized in more detail. The first portion up to 300 m thick covers several bands of norites, plagioclase-bearing pyroxenites and gabbro-norites in the zone of a contrast lamination of the central part of the section. The second portion covers several bands at the top of the norite zone. Several samples were collected from massive gabbro-norites close to the intrusion roof.

C. The Bushveld intrusion (Figure 4) [Eales and Cawthorn, 1996; Sharkov, 1980; Wager and Brown, 1970] is located in the center of the South African Shield, on the southern periphery of an arcuate East-African rift. The intrusion is cup-shaped with the concave bottom (lopolith). The series of layered rocks falls inside the massif, the lamination dip angles 10°-15°prevail; they become greater towards the lateral contacts. The massif (in plan) is elongated in

the latitudinal direction. Its longer axis is 480 km and the cross-section dimension is about 80 km. A total thickness of the intrusion is 7-9 km. The intrusion penetrated into the upper part of the Lower Proterozoic Pretoria suite which was underlying at that time sediments and vulcanite of the Megalisburg and Rooiberg series. Thickness of the Rooiberg series lying on the roof of intrusion is about 3 km maximum, i.e. thickness of the Bushveld intrusion roof during its penetration did not exceed 3 km and, consequently, depth of the intrusion during its cooling was in the interval from 3 km to 10-12 km. The roof during formation of the intrusion raised and melted partially. This palingenous acid magma could have remained liquid throughout the period of crystallization of basic rocks of the massif. Judging from associations of the contact-metamorposed rocks, pressure in the intrusion walls varied from 2 kbar to 3 kbar [Engelbrecht, 1990], i.e., in case of a geostatic pressure the depth was 6-10 km ignoring pressure of magma onto surrounding rocks and this does not contradict the geological estimation of depth of formation of the intrusion.

Age of the Rooiberg vulcanites is 2.06 Ga, age of the Bushveld complex is 2.06 Ga [Buick et al., 2001], age of

4 pyroxenites from different levels of the Merensky reef, which was determined by the Re-Os isochronous method is 2043±11 Ma, age by the Rb-Sr method is 2061±27 Ma [Schoenberg et al., 1998]. The Makhuzo granites (a final granite magmatism) form small stocks, sills and dikes; the Pb-U and Rb-Sr-isochronous age is about 2050 Ma. Age of the red granites intruded the Busveld massif according to the Rb-Sr isochroous method is 1950±50 Ma. The latitude strike dikes of the Karoo age intruded the Busveld massif.

Majority of researchers give preference to a hypothesis of the layered magmatic rock formation as a result of crystallization differentiation of the basalt magma during one magmatic phase within a vast magmatic chamber. Another hypothesis suggests multiple injections of magma so close to each other in time, that the previous portion of magma had no time to differentiate. It is an important statement for our problem that in all variants of the Bushveld intrusion formation model all actions of intrusion or mixing of melts occurred at the magmatic phase; at the cooling phase below 600°C the Bushveld massif was a single, sufficiently solid body.

All rocks of the Bushveld complex are exceptionally fresh and not deformed. No signs of hydrothermal phenomenon were discovered; only two cases of a local minor quantity of the secondary amphibole have been noted. Alkaline intrusions and the red granite intrusions breaking through rocks of the Bushveld complex are at least 50 km away from the point of drilling the well WP-16. Their influence (heating) must not have a serious effect on the paleomagnetic results.

The well WP-16 (25°42' S and 27°31' E) was drilled in the south of the western part of the Bushveld intrusion (Figure 4). Section of the intrusion in the region of the well is represented by five zones: 1) marginal zone (norites);

2) lower zone consists of rhythmically alternating pyrox-enites and harzburgites (820 m thick); 3) critical zone is characterized by a distinct fine lamination and it consists of pyroxenites and norites (1300 m); 4) main zone consists of norites, gabbro-norites and anorthosites (3100 m), a fine

rhythm is weakly expressed; rocks of zones 2-4 are cumulative crystallization products; 5) upper zone is a final product of crystallization differentiation; total thickness of this zone is 1700 m.

The well and the oriented core samples for the paleomag-netic study cover upper 240 m of the critical zone and lower 540 m of the main zone, 173 specimens altogether. The well section is composed of norites (72% of the thickness), anorthosites (22% of the thickness) and pyroxenites (6% of the thickness). The main rock-forming minerals in the rocks of the well section are bronzite and basic plagioclase, clinopyroxene is rare in occurrence, olivine occurs sporadically, occurrence of biotite and amphibole is even more rare, the ore minerals are represented by chromite.

The main and the critical zones are separated by the Merensky reef. It is 2-6 m thick. This is a layer of a porphyrite pyroxenite, which includes two interlayers of chromite 1-5 cm thick (near the lower contact) and 2050 cm (below the upper contact). A sulfide mineralization is concentrated near the upper contact. The pegmatite veins sometimes with a massive sulphide mineralization occur in rocks of the reef. These veins outcrop rapidly at transition from the reef into overlying and underlying anorthosites.

According to this publication [Hattingh, 1986a, 1986b], the electron-microscopic study of rocks of the main and critical zones, products of high-temperature exsolution were found in the pyroxene and plagioclase grains; these were thin acicular inclusions of magnetite of 1-1.5 ^m average width. The single-domain and pseudosingle-domain magnetic state of such grains is most likely and a natural remanent magnetization associated with this magnetite is considered, in paleomagnetic terms, as primary. Isolated fine grains of magnetite are few and far between. Aside of this, there are grains of ilmenite, in a more rare case these are thin acicular inclusions, more frequently there are isolated grains up to 100 ^m, gematite lamellae occur inside these. Composition of isolated grains of ilmenite x ~ 0.85. Iron sulphide grains, mainly in narrow layers of the ore zones occur less frequently than ilmenite. A part of these grains are identified as pyrrhotite.

Rocks of the intrusion in the region of the well (but not in the well proper) here and there are replaced by holocrys-talline magnetite-pyroxene pegmatoids. No alterations in the rocks enclosing pegmatites and baked contacts are seen, i.e. this is an in-situ replacement process without addition and evacuation of material, without metamorphism.

Techique of Paleomagnetic and Petromagnetic Measurements

The natural remanent magnetization (NRM or Jn) value and direction were measured in all samples with a JR-4 magnetometer, and the magnetic susceptibility k and its anisotropy, with KLY-2 measuring instrument. The magnetic susceptibility was measured after regular heating in the laboratory to control mineralogical alterations. For a series of representative samples, we performed a ther-

momagnetic analysis in the laboratory of the Institute of Physics of the Earth and in the Borok observatory (with the vibromagnetometers and magnetometers designed by N. M. Anosov, K. S. Burakov and Yu. K. Vinogradov) of saturation magnetization Js, saturation remanent magnetization Jrs and thermoremanent magnetization Jrt (TRM), coercive force Hc and remanent coercive force Hcr were measured. Paramagnetic minerals were excluded from values Js and Hc of weakly magnetic samples according to magnetization curves or according to correlation of Jn and k. Thus, a set of pyroxenite samples with Jn < 0.1 mA m-1 has k ~ 4 x 10-4 SI units. This value was adopted as a paramagnetic susceptibility of pyroxenites and peridotites kp, and it was subtracted from the total value of magnetic susceptibility to determine ferrimagnetic susceptibility and, accordingly, the Koenigsberger ratio for the ferrimagnetic fraction of material: Qnf = Jn/50Kf, where Kf = k — kp = k — 400.

Preliminary thermal demagnetization from 120°C to 580° C (12-15 steps) was performed on duplicates of all samples. A detailed thermal demagnetization of the Monchegorsk intrusion samples from 440°C to 600°C (thermal demagnetization step of 5°) and for samples collected every 1-2 m - from 540° C to 580° C (2-3° step) was performed to study the geomagnetic field record in the course of cooling of intrusive body. For samples from the Kivakka intrusion thermal demagnetization was performed from 420°C to 530°C (5° step) and from 530°C to 580°C (2°-3° step). The thermal demagnetization interval was selected on the basis of preliminary experimental investigations on artificial and natural samples [Pechersky et al., 2002; Shcherbakov and Shcherbakova, 2002] and on data of the preliminary thermal demagnetization. Results of the latter have shown that below 400° C contribution of unblocking temperatures even in the case of single-domain grains is nearly the same as the accuracy of measurements with the JR-4 magnetometer. Specific character of collection of samples from the Bushveld intrusion is first, in their size and second, in their orientation. These are cubes 1x1 cm at the bottom and 1-0.4 cm high. Two cubes represent each level usually. The top and bottom of these cubes are known (the plane in some samples is not strictly horizontal) and their orientation in the horizontal plane is unknown. We were unable to conduct various paleomagnetic and petromagnetic experiments with the samples in their natural state because of their small size and quantity. By heating up to 200-300°C we tried to extract the NRM viscous component so as to restore through it orientation of samples in the horizontal plane. The core was lifted in 1995 and we received it in 2002. It is reasonable that samples changed their position more than once and every time they acquired a new viscous magnetization. To remove at least partially the “parasitic” viscous components, samples were kept for about a year in a permalloy screen and only after that they were subjected to thermal demagnetization. Declination was restored in the following way: declination of the difference vector Jn20 — Jn200,300 was subtracted from declination of the ancient NRM component. Inclination of the difference vector, which had to be negative and close to inclination of the recent magnetic field of the Earth at the point of drilling of well WP-16 (-44°), served as a control of the

fact that we deal with a viscous component generated by a recent geomagnetic field at the time of lifting the core from the well. Restoration of declination of the NRM primary component is important for the analysis of behavior of a complete vector of the geomagnetic field. Due to a reason mentioned above, however, the “restored” inclination has a wide scatter and is no good for the analysis of a fine structure of the geomagnetic field in the first version. Therefore, analysis of behavior of the geomagnetic field in the first version was according to inclination only. In the second version it is possible to analyze a completer magnetization vector, if to consider only relative variations of declination in the course of a detailed thermal demagnetization. The restored declination was of use to determine the mean paleomagnetic direction of the whole collection. It coincides practically with the paleomagnetic direction in samples from natural outcrops, collected in the same locality [Hattingh, 1986a]. Based on results of preliminary thermal demagnetization, samples were chosen for a detailed thermal demagnetization wit a three degrees step and interval for their thermal demagnetization was selected from 520°C to 580°C to analyze components associated with magnetic minerals, which are close to magnetite.

Titanohematite with Tc = 600-660° C is present in some samples of all collections. Time and temperature of its origination are not known (it is present more frequently in il-menite as lamellae [Hattingh, 1986a, 1986b]), therefore, the NRM component associated with titanohematite is not used in the analysis of the geomagnetic field behavior.

To reduce contribution of magnetic soft multidomain grains, all samples from 5 mT to 30 mT underwent an alternating magnetic field demagnetization (AF demagnetization) on the unit designed by D. Kevlashvili (IFZ RAS), before the detailed thermal demagnetization. The amount of 10-15 mT will suffice for majority of samples to get a stable NRM component.

Samples were heated in a furnace placed into a four-layer screen of annealed and AF cleaned permalloy; the magnetic field inside the furnace did not exceed 10 nT.

The blocking temperature of the remanent magnetization depends strongly on the cooling rate [Dodson and McClelland-Brown, 1980; Pullaiah et al., 1975]. In case the intrusive body cools slowly the blocking temperature (Tb) of the natural thermoremanent magnetization will differ considerably from the unblocking temperatures (Td) of the natural remanent magnetization in the course of thermal demagnetization of samples in a laboratory. We used the plots of Tb and Td versus the cooling rate for single-domain magnetite particles, presented in [Dodson and McClelland-Brown, 1980] (Figure 5). In order to use these plots, it is necessary to choose the samples whose carrier of the given NRM component is the single-domain magnetic mineral (in our case it is magnetite and a low-Ti titanomagnetite). In case of a noticeable role of the multidomain grains all magnetization will take place near the Curie point. If there are several magnetic minerals with different Curie points in the sample it becomes difficult to translate temperatures into time.

To estimate the NRM nature a set of indicators is used. These are the magnetic hardness expressed in terms of Hcr,

0.65 0.70 0.75 0.80 0.85 0.90 0.95 1.00 1.05

Td/Tc

Figure 5. Diagram of dependence for unblocking temperature Td and blocking temperature Tb curve of the thermoremanent magnetization of the single-domain grains a) magnetite and b) titanomgnetite at different velocities of cooling of the body [Dodson and McClelland-Brown, 1980]. Numbers at the lines are cooling velocity (°/Myr).

Hcr/Hc, Qn, Qnf, the shape of the curve Jn(T), the linearity of the Arai-Nagata diagrams (dJn — dJrt), the similarity of variations in dD and dI between several neighboring samples, and the ratio Jrt/Jri. It is known [Sholpo, 1977] that, in an ensemble of multidomain grains, the ratio Jrt/Jri ~ 3 and decreases with an increase of single-domain particles in the ensemble of magnetic grains. However, in the case of crystallization magnetization, we have Jrk/Jri ~ 1 [Nguen and Pechersky, 1985]. Therefore, if Jn and Jrt are acquired in similar external magnetic fields, Jn/Jri and Jrt/Jri ~ 3, this will indicate that Jn is mostly associated with mul-

tidomain grains and is a thermoremanent magnetization; if Jrt/Jri ~ 3, and Jn/Jri ~ 1, this means that Jn is likely to be of the crystallization or chemical origin (in the latter case, usually Jrc/Jri < 1 [Nguen and Pechersky, 19S7]).

The component analysis based on the thermal demagnetization data was performed with the use of the program developed by Enkin [1994] was applied to single out the NRM components obtained in the course of thermal demagnetization and to determine mean paleomagnetic directions.

Petromagnetic Results

A. The Monchegorsk intrusion. According to the thermomagnetic analysis of Js, Jrs, Jrt, rocks of the intrusion are clearly dominated by magnetite with Tc = 550-5S5oC and occasionally contain pyrrhotite with Tc = 325oC. In contrast, pyrrhotite prevails in the host rocks and is the only magnetic mineral in some gneiss samples.

According to a hyperbolic form of Js(T) the paramagnetic contribution is substantial in rocks of the intrusion. The great majority of samples are dominated by paramagnetic к. Judging from the susceptibility and saturation magnetization, rocks of the intrusion contain commonly <0.1% of magnetite. In general, the magnetization of periodites is higher than that of pyroxenites (Table 1). The distribution of к and NRM along the section is mainly controlled by distribution of periodites, which are predominant in the lower part of the intrusion. The rocks are little affected by heating: the susceptibility кт measured after heating of samples to 540-5S0oC changes insignificantly, and the кт/к0 ratio varies from 0.S to 1.0S, averaging 0.9S. Only in samples containing appreciable amounts of iron sulfides, in particular, pyrrhotite, does кт/к0 reach 1.S (sample 44). This ratio is even higher in the surrounding gneisses, where the main magnetic mineral is pyrrhotite.

The magnetic hardness of rocks was estimated from Hcr, it varies from 12 mT to 37 mT, irrespective of the rock composition, concentration of magnetic minerals, and, к, variation, and likely reflects the predominance of nearly singledomain and multidomain grains.

The rocks above the edge zone of the intrusion are isotropic, the maximum-to-minimum susceptibility ratio of the sample is AK = 1.0-1.0S (averaging 1.02); in the edge zone AK reaches 1.19 and averages 1.1 in the lower 100 m of the section. Such behavior of the magnetic anisotropy is common for magmatic bodies whose magnetic fabric forms in their peripheral zones at the stage of crystallization. Higher anisotropy is inherent to a more magnetic samples, i.e., is associated with magnetic minerals, whereas weakly magnetic pyroxenites, whose susceptibility is predominantly paramagnetic, are as a rule, nearly isotropic. The magnetic anisotropy of the host gneisses is AK = 1.02-1.19, averaging 1.12.

As seen from the Jn /Jri and Jrt /Jri values and the Qnf values correlating with these ratios (Table 2), the main contribution to Jn and Jrt at room temperature is made by multidomain grains, and Jn is a thermoremanent magnetization in ~70% of samples. Evidently, magnetic grains in

Table 1. Magnetization of Monchegorsk intrusion rocks

Rock no. k, 10 4 SI units Jn, mA m 1 Qnf

average range average range average range

Peridotite 8 16.8 9.2-153 90.4 50-1850 11.7 1.5-60

Pyroxenite 29 8.9 4.0-59.3 26.6 <0.1-285 6.5 1.0-278

the interval 540-580° C are close to the single-domain state. Below this interval they are close to the mulltidomain state, as is seen from the NRM direction, which practically does not change within the measurement error, in the course of the thermal demagnetization up to 500-530°C (see below).

B. The Kivakka intrusion. Rocks of the intrusion are weakly magnetic, as a rule, (Table 3) the feature which is usual for cumulative rocks. Figure 6 shows typical curves of Jrs(T). One can see the heating stability of the magnetic

material (Figure 6, Table 3). According to data of the ther-momagnetic analysis (Table 3) the Curie point vary from 525°C to 560°C, and this corresponds to concentration of titanomagnetite with x = 0.11-0.05. The saturation magnetization of such titanomagnetites averages approximately 415 x103 A m-1. The saturation magnetization of the investigated samples does not exceed 103 A m-1, accordingly, their titanomagnetite content is no more than 0.3%; in most cases it is less than 0.1% and frequently it is less than 0.01%. Some samples contain the magnetic phase, which disappears

Table 2. Magnetic properties of the Monchegorsk intrusion rocks

no. Jrt/Jri Jn /Jri Qnf Domain state Jn origin Main Td

2 2.45 2.1 17.9 MD+SD TRM 575 (585)

3 3.65 9 MD TRM 575 (585)

4 3.89 11.4? 25.8 MD TRM? 575 (585)

5 2.39 5.1 26.4 MD+SD TRM 575

6 3.47 3.6 17.5 MD TRM 575 (585)

7 1.9 6.9? 19.3 SD+MD TRM 575

9 2.4 0.52 1.4 MD+SD CRM? 555, 575

10 4.6 3.8 6.3 MD TRM 545

12 3.13 4.6 13.2 MD TRM 560

30 2.17 7.7? 42.7 MD+SD TRM 565, 580

33 2.4 1.4 23.8 MD+SD KRM+TRM 565

35 5.4 2.7 12 MD TRM 565

37 4.3 0.84 4 MD KRM 565

38 2.24 0.65 3.2 MD+SD CRM? 545(560, 80)

39 3.14 1.07 2.1 MD KRM 565

40 4.7 0.6 1 MD CRM? 545 (560)

42 3.5 3.26 15 MD TRM 575 (585)

43 2.3 4.2 <0.5 MD+SD TRM? 575

45 2.1 0.33 0.6 MD+SD CRM? 585

46 2.65 0.66 3.2 MD CRM 545, 565 (580)

47 6.2? 9.2? 116 MD TRM 545(560, 585)

48 2.64 6.5? 19 MD TRM 545, 565

49 2.96 0.72 1.5 MD CRM 545? 585?

51 1.92 2.16 16 MD+SD TRM 565 (580)

53 1.9-2.4 3.4-4.3 29 MD+SD TRM 560

Note: TRM, KRM, and CRM are the thermal, crystallization, and chemical remanent magnetizations, respectively. Higher values of the ratios are possibly due to an incomplete creation of the anhysteretic remanence in an alternating field of 90 mT and other factors; in the case of high ratios Jn/Jri, this can be related to anomalously high NRM values in the region 540—550° C and an inaccurate NRM interpolation. Qnf is the Koenigsberger factor (Jn/50K) for the ferrimagnetic part of the susceptibility Kf that is the difference between the measured value of susceptibility and its paramagnetic part (about 4x104 SI units); the Qnf and NRM values are interpolated in samples with NRM splash. MD and SD are multi- and single-domain grains. respectively. The main values of Td are unblocking temperatures of NRM close to Curie points, and the Td values given in parentheses account for an insignificant portion of the NRM value. Large determination uncertainties in Jn/Jri and Qnf in sample 43 are possibly due to a very small value of Jn (<0.1 mA m-1).

Table 3. Magnetic properties of the Kivakka intrusion rocks

no. rock Jrs Her Js Hc Hcr/Hc Jrs j Js K Qn AK Tc i Tc2 Jrst j Jrs Td

4a41 norite 0.0056 581 0.022 144 4 0.26 219 3.45 1.01 280(30)540 540 1.46 560

lla36 harzb 0.086 22 0.518 15 1.53 0.17 4530 2.27 1.33 560 540 1.2 555

16b32 harzb 0.0027 141 0.042 43 3.3 0.064 245 5.96 1.004 525 520 1.19 555

20sll norite 0.0039 72 0.0173 32 2.28 0.225 305 31.2 1.017 530 530 1.09 555, 575

25s3u bronz 0.075 32 0.858 12 2.73 0.09 3847 3.8 1.375 240(14)540 530 1.02 575

26s41 norite 0.0115 45 0.047 21 2 0.25 250(10)535 540 0.87 535, 565

30s61 norite 0.0052 67 0.0205 33 2.02 0.254 471 5.34 1.006 170(5)540 540 0.95 5507580

32v53 bronz 0.0285 31 0.164 15 2.01 0.17 4623 3.6 1.43 330(8)530 290(10)530 1.28 545, 570

33s6u norite 0.0276 32 0.158 14 2.2 0.18 1456 3.9 1.09 160(13)540 540 0.9 545, 570, >600

35v46 norite 0.051 22 0.41 9 2.34 0.13 3083 4.98 1.25 525 525 1.06 545

39pcl4 norite 0.0094 32 0.032 15 2.09 0.3 392 6 1.026 530 530 1.2 530, 555

40v34 bronz 0.0074 46 0.049 24 1.93 0.15 412 5.39 1.01 545 540 1.07 530, 550

42pc30 norite 0.022 28 0.205 11 2.56 0.11 2692 3.54 1.12 540 525 1.04 530, 550

43spl3 norite 0.038 31 0.248 14 2.19 0.15 2040 4.64 1.08 535 530 0.99 520, 535

44spll norite 0.018 33 0.089 16 1.99 0.2 1008 5.55 1.048 535 540 0.97 520, 535

46pvl norite 0.0136 47 0.122 22 2.15 0.11 250(97)540 540 1.14 520, 535

47spm0 bronz 0.0064 108 0.016 74 1.46 0.4 251 25.9 1.008 525 530 1.08 525, 560

50v35 norite 0.004 203 0.017 85 2.39 0.235 273 10.7 1.004 540 540 1.06 550

51v38 norite 0.037 46 0.165 23 1.97 0.22 1129 4.05 1.007 170(10)540 540 0.8 575, 610

53v41 norite 0.0013 50 0.008 24 2.03 0.163 222 2.73 1.009 170(13)540 540. 580 0.94 550, 580, 605

56ptl5 norite 0.0069 38 0.0314 19 1.8 0.22 765 3.8 1.03 540 540 1.06 555

61t50 norite 0.0128 33 0.095 12 2.83 0.14 1228 2.78 1.026 535 540 1.18 555

63pt32 gabn 0.0231 51 0.125 22 2.37 0.18 1413 5.3 1.15 150(10)540 540 0.9 555

64t44? gabn 0.034 28 0.206 13 2.14 0.16 255 3.1 1.012 200(12)550 550 0.87 570

67t53 gabn 0.0089 95 0.0285 48 1.99 0.31 315 4.16 1.032 540 550 1.34 555

68t58 gabn 0.029 68 0.091 33 2.05 0.32 857 9.25 1.065 425(23)540 440(15)540 0.93 490, 555

70t61 gabn 0.066 45 0.499 14 3.18 0.13 3730 15.7 1.041 160(9)550 550 0.95 560, 660

Note: no. — sample number, paleomagnetic unreliable samples are excluded from the table; rock-bronz — bronsitite, gabn — gabbro-norite, harzb — harzburgite; Jrs — saturated remanent magnetization, A m— Hcr — remanent coercive force, mT; Js — saturation magnetization, A m— Hc — coercive force, mT; paramagnetic part along magnetic curve is excluded from values of Js and Hc\ k — magnetic susceptibility, 10—6 SI units; Qn = Jn/40«; An = Kmax/%in; Tc\ and Tc2 — Curie points after the first and second heating of the sample, magnetic phase portion is given in parenthesis; Jrst — saturation remanent magnetization after the samples is heated up to 600° C; Tj — unblocking temperature of the high-temperature NRM component.

402 PECHERSKY ET AL.: CONTINUOUS RECORD OF GEOMAGNETIC FIELD VARIATIONS

Figure 6. Typical examples of curves of thermal destruction of the saturation remanent magnetization. Kivakka. 1 - first heating, 2 - second heating. pt36, pc14, and pc30 - sample numbers.

in the interval of 150-425°C. Around 5-30% of Jrs fell on this phase. This phase is usually absent on curves Jrs(T ) of the second heating, i.e. more likely it is associated with maghemite or/and with internal stresses. Disappearance of both during heating causes reduction of the remanent magnetization, which is observed in many cases (Table 3). The magnetic phase does not disappear only in two samples during second heating (samples 32v53 and 68t58, Table 3). Probably this is hemoilmenite. It is confirmed by the following fact: sample 32v53 Tc = 330°C, and it falls down to 290°C during second heating and magnetization in this case rises, which is an inherent feature for hemoilmenite. The magnetic phase examined between 150° C and 300° C is of no importance for us as we study the paleomagnetic record above 500°C, where these phases are not present. It is a very rare case when the samples contain a minute amount of magnetite; on curve Jrs(T) it is found only in one sample, judging by Jn(T), approximately 50% of the investigated samples contain “tails” of magnetite with the minimal percentage of magnetization. Titanohematite with Tc = 605-660°C has been observed in 6 samples during the thermal demagnetization and its concentration is minute. It is not recorded on the Jrs(T) curves because of its low concentration.

Thus, titanomagnetite with Tc = 525-560°C, which is

close to magnetite is the main NRM carrier, that defines the paleomagnetic record above 400° C.

Let us consider now the magnetic state of this mineral group. As is seen from the value of Hcr = 22-200 mT (Table 3) a magnetic hard material prevails in the samples. Values of Qn = 3-31 and Jrs/Js = 0.1-0.4 (<0.3 prevail) are relatively not high (Table 3). This obviously speaks about an appreciable portion of the multidomain grains with internal stresses and pseudosingle-domain grains and their deciding contribution into the NRM. The majority of points on the Hcr/Hc — Jrs/Js diagram (Figure 7) are near the line separating multidomain and pseudosingle-domain grains [Day et al., 1977] and they are along the theoretical curves for ~50% mix of the multidomain and single-domain grains [Dunlop, 2002]. Consequently, an appreciable portion in the NRM falls on the single-domain and pseudosingle-domain magnetic grains, in the investigated rocks of the Kivakka intrusion. This is at a room temperature, but at temperatures nearing their Curie points, the majority of these grains will behave as single-domain grains.

The majority of the investigated samples of the Kivakka intrusion are magnetically isotropic (Table 3). Anisotropy of magnetic susceptibility exceeds 10% only in 12 samples. Usually these are relatively magnetic samples,

04

0.3

0.2

0.1

JrsUs

♦ ♦,

♦ ♦ ♦ ♦

♦ # ♦♦ ♦ ♦ ♦

♦ ♦ ♦ ♦ ♦

♦

1.5 1.7 1.9 2.1 2.3 2.5 2.7 2.9

3.1

H,

3.3

r/Hc

3.5

Figure 7. Experimantal values of Hcr/Hc and Jrs/Js. Kivakka.

i.e. their anisotropy is determined first of all by distribution and fabric of magnetic minerals. Rocks, whose anisotropy is determined by paramagnetic minerals (weakly magnetic rocks containing less than 0.1% of magnetite) have usually anisotropy of magnetic susceptibility less than 5%. Anisotropic samples have a foliation fabric

(E — KinKin/Kmin Kmax —

1.08 1.3, Emean — 1.17), which confirms the solid phase conditions of crystallization of the magnetic minerals, probably, under directional pressures (deformations) and/or as a result of crystallization along slice planes of silicates during their exsolution.

Composition, concentration of magnetic minerals, their magnetic fabric (anisotropy of magnetic susceptibility) and magnetic state do not depend on the petrological and min-eralogical characteristics of rocks.

C. The Bushveld intrusion. The magnetic susceptibility in the investigated part of the intrusion section varies from units up to 1825 x10-5 SI units and in the main group of samples it is in the range of 20-60x10-5 SI units and correlates clearly with the petrological composition of rocks

10000

J„

1000-

100-

10

♦ V * ♦ ♦ ♦ ♦

♦ ♦ ♦ ♦ ♦♦♦♦.♦ ♦. * ♦ ♦ J.. ♦ ♦

♦ ♦ ♦ • ■^0 ► ♦ A ♦

V* ♦ ♦ ♦ ♦ ♦

10

100

1000

10000

Figure 8. Values of magnetic susceptibility

(k, 10-5 SI units) and of natural remanent magnetization (Jn, mA m-1). Bushveld. Logarithmic scale.

Figure 9. Bushveld intrusion natural remanent magnetization of samples versus their density.

(Table 4). The latter is connected with a paramagnetic component prevailing in samples with k < 10-4 SI units. If to draw lines, which restrict the swarm of points at the top and bottom on Figure 8, they will cross the abscissa axis at points k — 0.5 x10-5 and 3x10-5 SI units. This, evidently, is the portion of paramagnetic susceptibility in samples under investigation, from anorthosites, where and appreciable portion of the negative diamagnetic susceptibility is likely to be present, to pyroxenites, when concentration of iron in the rock is maximum and this concentration determines the paramagnetic susceptibility. If we consider the NRM versus rock density dependence, which is unambiguously governed by the rock composition, a tendency of growth of the NRM value with reduction of density, i.e. from pyroxenites (density exceeds 3 g cm-3) to anorthosites (density is less than 2.8 g cm-3) (Figure 9). A group of narrow horizons with a relatively high NRM stands out. These belong, as a rule, to anorthosites (Table 4). But this does not mean that all anorthosite samples have a high NRM: only 20% have NRM>1000 mA m-1, and 40% of samples have NRM<100 mA m-1. This ratio is much less in the case of norites and pyroxenites: less than 4% of samples have NRM>1000 mA m-1 and more than 80% have NRM<100 mA m-1. Consequently, the bulk of magnetic minerals, close to magnetite, is concentrated in plagioclases of anorthosites.

As it is impossible to perform the thermomagnetic analysis on the Bushveld intrusion samples, we estimate composition of the magnetic minerals according to data of the thermal demagnetization. It is possible to identify four major unblocking temperatures, which are, evidently, the Curie points: 1) an abrupt drop of magnetization near 300-350°C (6 samples), which is, probably, associated with pyrrhotite [Hattingh, 1986a], does not participate in the paleomagnetic analysis as the main attention is paid to magnetite and ti-tanomagnetite; 2) an abrupt drop of magnetization between 553°C and 556°C, from 20-30% up to more than 90% of NRM, the Curie point of 556°C has been adopted; judging by data from [Hattingh, 1986a], this is a low-titanium titanomagnetite - the exsolution product, mainly, of plagio-

Table 4. Magnetic properties of samples from the Bushveld intrusion

no. m rock Jno /Jn max K dTin Iin pol dThi Ihi pol dNR Td I556 5 GO O

1 30.81 norite 109 53 450-520 -29.4 555-600 78.3 N 556, 580,TH 77.8 78.1

2v 32.73 anorth 120 76 555-580 72.3 N 556, 580 55 74.2

3v 34.36 norite 180 17 20-580 68.9 N 556, 580 77 78.9

4 35.96 norite 16.2/20.2 38 200-520 -48.9 R 520-580 74.2 N 27 556, 580 75.5 75.9

5 40.34 norite 494/562 35 520-580 72.7 N 556, 580 73.2 71.8

6 44.8 anorth 634/690 51 400-520 -49.8 R 520-620 78.1 N 31 556, 580,TH 78.5 79.6

7v 46.28 anorth 5906 115 540-620 80.6 N 556, 570,TH 80.5 80.7

8v 53.6 anorth 2957 44 20-620 77 N 556, 580,TH 85 83.4

9v 59.58 anorth 3257/3290 187 450-620 86 N 556, 580,TH 87.6

10 62.22 norite 55.8/57.4 34 20-520 46.7 N 580?

11v 65.4 anorth 185 8 540-570 67.4 N 556, 580 70.6 74.4

12 68.64 norite 29.6/36.6 43 450-580 59.2 N 556, 580 64 70.2

13 74.63 norite 37.8/49.6 58 350-580 50.4 N 556, 580 62.3 53.5

14v 80.51 norite 66.6 53 400-580 71.4 N 556?575? 64.8 57.6

15v 87.94 norite 7.5/24.1 57 400-520 -51.7 R 520-580 64.8 N 13 556, 572 76.7 84.6

16 93.93 norite 35.6/32.6 54 500-520? -39.3 R 540-580 72.7 N 37 556, 580 75.3 69.6

17 101.38 norite 34/49.2 51 500-540 -45.1 CN? 555-580 71.3 N 64 556, 580 73.6 59.9

18 107.22 norite 14.4/41.4 52 500-540 -53.8 R 540-580 63.5 N 20 556, 580 71.5 68.8

19 117.39 norite 23/24.8 46 555-580 70.4 N 556, 580 68.1 78.6

21 132.35 norite 19 47 300-580 54 N 556, 570 53.8 64

22 136.62 norite 31/40.4 46 450-520 -76.6 R? 520-580 55.5 N 43 556, 580 54.7 70.1

23 147.08 pyrox 44.2/41.2 36 350-450 -44.5 R 450-580 54.8 N 11 556, 570 57.3 62.5

24 151.55 norite 31.4/31.6 46 500-540 -65.1 R 540-580 60.3 N 9 556, 580 51.1 73.4

25 160.39 norite 26.5/30.8 42 350-450 -46.1 R 540-580 62.1 N 19 556, 580 60.9 72.6

26 167.48 norite 21.3/21.1 40 450-520 -74.7 R 540-580 56.4 N 23 556, 580 62.6 57.7

27 169.43 anorth 82.2 4 520-580 73.5 N 556, 580 72 71.5

28 171.14 anorth 4.2/3.2 59 450-540 -52.6 R 540-565 60.8 N 49 556?, 570 57.4 66.5

29v 171.72 anorth 20.8/20.1 24 500-540 -43.5 R 555-580 73.2 N 31 556, 575 74.7 76.1

30 174.7 anorth 30.8/26.4 41 450-520 -47.3 R 520-580 63.6 N 21 556, 570 68.5 66.2

31 176.57 norite 31/38 44 200-540 -57.8 CN? 540-580 72.7 N 20 556, 580 75.2 70.5

33 187.91 pyrox 20.6/44.2 48 500-540 -63.8 R 540-580 66.8 N 4 556, 580 65.2 74.5

34 190.85 norite 16/27.2 48 500-520 -40.9 R 540-580 53.9 N 36 556, 570? 69.8

35 195.3 pyrox 21.2/29.2 43 450-540 -54.7 R 540-580 58.2 N 5 556, 580 57.5 63.4

36v 198.15 norite 21.6/21.8 43 450-540 -36.9 R 540-570 60.4 N 25 556, 573 68.2 75.7

37v 206.82 norite 15.5/12 40 450-540 -37.4 R 540-580 59.9 N 24 556?, 580 62.3 77.9

38 213.28 norite 11.3/27.1 51 350-540 -71.8 R 540-580 54.1 N 19 556, 580 56.1 55.4

39 221.87 norite 22.4/41.6 42 500-520 -37.2 R? 520-570 68 N 32 556, 575 67.8

40 229.34 norite 16.8/44.4 45 500-540 -40.3 R? 540-570 70.4 N 34 556, 570 60 84.6

41 236.86 norite 52.8/49.6 42 450-520 -37.8 R? 520-580 67.9 N 64 556, 580 61.8 70.7

42 241.32 norite 56/74.2 43 500-520 -35.8 R? 540-580 70.5 N 35 556, 580 60.7 73.4

43 251.77 norite 30.8/48.8 46 500-540 -76.8 R? 540-580 54.8 N 22 556, 570 55.1 64.5

44 255.64 norite 18.8/30.5 42 520-540 -38.3 R? 555-580 56.1 N 19 556, 570 53.4 62.7

45 265.32 norite 36.6/48.6 43 450-520 -59.2 R 540-580 63.8 N 8 556?, 580 45.3 85.2

46 272.77 norite 17.7/23.5 43 450-540 -68 R? 540-580 54.5 N 46 556, 580 50.3 68.2

47 280.24 norite 26.6/24.3 49 450-520 -47.9 R? 555-580 64.2 N? 35 556, 580 61.1 81

48 286.24 norite 15/16 42 450-520 -45.5 R? 540-580 60.9 N 16 556. 580 54 66.7

49v 295.2 norite 16.7/23.5 45 500-540 -50.4 R 540-580 58.1 N 9 556, 578 60.6 72.7

50 302.31 norite 9.6/14.9 40 500-540 -53.3 R 540-580 75.3 N 24 556, 580 74.9 75.8

51 309.76 norite 15.7/19 41 500-540 -50.3 R 540-580 69.3 N 20 556, 580 65.6 76.8

52 317.23 norite 53 42 450-520 -75 R 555-580 73.6 N 7 556, 580 72 76.6

53 324.49 norite 19.1/23.6 40 450-520 -49.4 R? 540-580 66.7 N? 19 556, 580 65.2 81.5

54 328.56 norite 34.8/32.2 37 450-520 -47.8 R? 540-580 74.5 N 30 556, 580 72.5 77.2

56 335.15 norite 102 41 400-540 -32.2 R? 565-580 70.3 N 38 556, 580 70.3

57v 340.32 norite 44.6/68.8 41 450-540 -30 R? 540-580 73.2 N 43 556, 580 72 81

58 344.86 norite 61.8/65.2 36 500-520 -28.6 CN? 540-580 76.6 N 50 556, 580 76.6 87.6

59 356.13 norite 158 38 400-540 -36.5 R 540-580 62.7 N 26 556, 580,TH 58.1 74.5

Table 4. Continued

no. m rock Jno /Jn max K dTin Iin pol dThi Ihi pol dNR Td CO iO iO I580

60v 359.82 norite 35.6/36 42 450-540 -38.2 CN? 540-580 65 N 29 556, 580 76.6 72.4

61 368.75 norite 16.9/42.9 36 450-540 -53.2 R 540-580 66.9 N 15 556, 580 76.1 72.7

62 372.65 norite 36.2/43.9 34 300-520 -64.8 CN? 520-620 67.4 N 38 556, 580,TH 51.6 74.6

63v 376.23 norite 1858 36 300-500 -38.4 R 540-580 67 N 30 556, 580 50.4 79

64v 376.79 anorth 1911 47 555-600 59.1 N 556?, 570,TH 48.2 62

65 385.17 anorth 82.4/90.8 35 520-540 -49.1 R 540-580 54 N 5 556, 580 54.5 72.5

66 393.66 norite 56.2/51.4 37 350-520 -76 R?? 555-580 66.7 N 28 556, 580 66 61.2

67 395.14 pyrox 13.1/12.2 62 450-540 -39.5 R 540-570 43.4 N 10 556, 565 53.9

68v 398.75 norite 75/116 35 500-520 -38.9 R? 555-580 73.7 N 35 556, 580 82.3 75.4

69 404.56 norite 125/150 38 500-520 -46.9 CN? 520-580 69.7 N 25 556, 580 74.3 77.8

70v 409 norite 88.6/91.9 28 450-520 -36.2 R 540-580 74.9 N 40 556, 580 80.7 84.4

71 414.58 pyrox 8.6/14.1 59 200-500 -65.1 CN? 520-570 54.6 N 26 556, 570 54.2 57.4

72v 418.76 norite 236/214 51 500-540 -38.6 R 540-580 72.2 N 34 556, 582 66.2 78.1

73v 429.61 norite 61/90 48 500-520 -38.1 CN? 555-580 73.2 N 35 556, 577, 585 72.8 77.5

74 435.73 norite 12.3/30 47 500-540 -54.6 R 540-580 79.3 N 28 556, 580,TH? 75.1 77.2

76 444.51 norite 4.1/12.2 42 200-450 -51.6 CN? 520-570 56.4 N? 19 550, 570 63.4 60.3

77 448.85 norite 33.6 37 350-540 -36.4 CN? 540-580 51 N 19 556, 580 40.1 70.7

78 449.92 norite 13.4/12.8 35 400-520 -35.7 R? 520-580 53 N 26 556, 580 53.6 55.4

79 453.91 norite 10.6/32 20 400-520 -46.3 R 540-580 53.3 N 15 556, 580 54.5 58

80 455.3 anorth 11.3/23.2 51 350-540 -42.6 R 555-580 69.7 N 27 556, 580 67.1 74.9

81 455.78 norite 25.8/34 23 500-540 -21.3 CN? 565-580 69.9 N 56 556, 580 58.9 69.9

82 456.17 norite 8/12.9 41 500-540 -46.8 R 565-580 52.6 N 10 556, 580 56.4 52.6

83 457.74 anorth 27.1/42.3 39 300-540 -45.2 R 555-580 70.1 N 30 556, 580 58.3 70.1

84-1v 463.06 anorth 237 25 500-520 -35 CN? 520-580 71.3 N 46 556, 582 57.9 87.4

85 465.32 norite 20.2/22.8 47 500-540 -32.5 R? 540-600 49.5 N 19 556, 580,TH 52.6 45.4

86 474.22 anorth 13.2/24.9 27 350-540 -43.4 R 540-570 63.9 N 25 556, 570 59.9 64.9

87 475.47 anorth 6.8 41 555-570 39 N? 556?, 570 32 39

89 483.33 norite 10.1/21.9 37 520-540 -68.9 CN? 540-580 67 N 32 556, 580 64 65.9

90 489.29 norite 28.7/42.7 36 400-540 -21.3 R 540-580 65.7 N 45 556, 580 61.2 72.6

91 491.56 norite 5.6/11.9 41 500-540 -38 R? 540-580 58.1 N 23 556, 580 61.8 65.3

92 493.18 anorth 24.3/25 38 555-580 52.2 N 556, 580 50.3 53.4

95 507.84 norite 55.7/77.2 52 200-350 -68 R?? 555-580 74.9 N? 26 556, 580 78 63.6

96 511.65 nprite 4.6/8 44 450-540 -58.3 R 555-570 54.7 N 7 556, 570 52.8 61.7

97 520.41 anorth 424 33 520-580 79 N 556, 580,TH? 77.9 79

98 523.1 anorth 13.6 29 500-520 -23.1 CN? 540-570 52.7 N 51 556, 570 53.2 48

100v 529.29 pyrox 110 34 20-580 74 N! 556?, 580 67.8 75.7

101 533.96 pyrox 17.6 47 20-540 -81 580? 515

102v 535.14 anorth 16.8 3 555-580 75.5 N 556, 580 69.4 78.9

103v 537.16 anorth 934 39 555-580 78.4 N 556, 570,TH 71.2 75.6

104 539.07 norite 13.5/11.6 61 500-540 -31.3 CN? 555-580 44.3 N? 22 556, 580,TH? 43.6 45

105 542.65 anorth 23.9 33

106v 548.7 anorth 84.4/101 28 200-520 -71.4 R? 555-580 71.2 N 5 556, 580 69.8 75.1

107v 550.14 norite 26.7/32.6 53 500-540 -47.4 CN? 565-580 74.8 N 43 556, 580 80.8 79.4

108v 554.56 anorth 586 26 520-580 75.4 N 556, 580 66.1 81.3

109 556.34 anorth 70.6/85.6 37 500-540 -47.5 R? 555-580 72 N 27 556, 580 73.8

110 563.5 norite 20.9 48 555-580 63.6 N 556, 570 64 63.6

111 567.4 anorth 44.2/40 35 500-540 -57.5 R 540-580 55.1 N 6 556?, 580 58.1 69

112v 572.19 anorth 228/246 29 200-520 -35.5 R 520-580 70.8 N 36 556, 570,TH 71.4 76.1

113v 576.78 anorth 831 26 500-520 -20.2 R? 540-620 76.4 N 56 556,TH 77.7 78.1

114 582.65 anorth 982/1071 45 555-620 78.6 N 556, 580,TH 78.8 63.1

115 587.84 anorth 1398/1564 42 200-520 -34.2 R 540-630 66.4 N 35 556, 580,TH 68.1 64.4

116v 592.2 anorth 741/762 65 555-580 69.5 N 580,TH

117v 596.58 anorth 1788 47 555-630 75.8 N 556, 580,TH? 68.8 65.6

118v 601.1 anorth 246 15 540-600 70.5 N 556, 575,TH? 80.4 75

119v 606.33 anorth 279 17 540-600 76.4 N 556, 570,TH 69.5 75.1

Table 4. Continued

no. m rock Jno /Jn max K dTin Iin pol d Ihi pol dNR Td I556 I580

120v 611.76 anorth 427 21 540-600 66.7 N 580?,TH? 49.4

121v 616.6 norite 40/49 19 300-520 -49.6 R? 555-580 55.2 N 6 556?, 585 42.9 67.6

122 621.38 norite 4.3 34 520-540 -63.4 R? 555-580 60 N 6 556, 570 44.6 59

123v 625.69 norite 96.8 33 520-580 61 N! 556, 580 62.8 67.4

124 627.75 norite 27.3/28.3 85 520-540 -34.9 CN? 555-580 51.1 N? 85 556?, 570? 51.1

125 632.68 norite 37.8 38 500-540 -46 R? 555-580 49.8 N? 9 556?, 580? 54.4

127v 643.08 anorth 132/145 7 540-580 70.7 N 556, 580 71.9 77.1

128 645.52 anorth 154/278 20 200-300 -56 CN? 540-580 70.3 N 19 556, 580,TH 70.5 76.4

129v 647.75 anorth 203/118 42 200-400 -54.8 R?? 500-580 68.4 N? 48 556, 568, 578,TH 64 65.5

130 648.6 norite 1890 206 R??

131 649.1 norite 289 80 500-? -35.5 R? 300, 570 -24.2

132 650.95 anorth 31 61 520-555 -14 CN? 555-580 69 N? 57 556, 580 67.8 68.3

133v 653.05 anorth 158/260 12 200-300 -62.5 R? 540-600 75.4 N 24 556, 580,TH 78.3 78.6

134 657.46 anorth 6.6/13.4 9 450-540 -32 CN? 540-580 54.7 N? 24 556?, 570 40.5 66.2

135v 659.3 anorth 125 32 200-580 70.3 N! 556, 580 67.9 72.2

136 661.39 norite 10.1/14.8 17 500-540 -67.2 R 540-580 60.9 N 16 556, 580 59.7 62.3

137 664.6 norite 2.3 24 350-520 -50 CN? ?

138 670.23 norite 7.2 33 400-540 -66.5 CN? 560

139 675.72 norite 5.5 38 500-540 -35 CN? 545

140 680.92 norite 42.3 98 450-550 -47.8 R 556 -51.8

141 683.28 norite 6.7 35 500-540 -55 R 540-565 45.5 N? 12 556? 51.3

142 690.22 norite 1.7 40 300-500 -61.1 CN? ?

143 692.78 norite 2.4 43 500-540 -45 CN? 540?

145 702.39 norite 13.3 44 200-350 -58.8 R? 745

147 711.7 norite 11 36 500-540 -47.2 CN? 545?

148 718.17 norite 13 36 520-540 -47.4 R? 540-570 78.7 N? 37 570 78.7

149 721.16 norite 23.3 47 450-540 -55.7 540 765

150 726.56 norite 76.2 49 520-565 -60.9 R? 560?

151 730.34 norite 57.7 36 520-540 -76.1 R 540-570 66.3 N 12 556, 570 78.6 62

152 734.96 norite 40.7 22 200-350 -61.8 R? 350 785

153 739.51 norite 57.4 30 200-300 -85.5 CN? 300 795

154v 745.15 norite 17.7/55.8 14 400-540 -39 CN? 540-580 56.8 N 20 556, 572,TH 55.6 69.8

155v 749.43 anorth 2178 87 200-400 -67.2 R 540-570 75.9 N 24 556, 580,TH 59.8 70.5

156 755.21 norite 241 45 500-555 -35.3 CN? 570-580 57.7 N? 79 350, 580 57.7

157 759.79 norite 30 40 500-540 -51.6 R 555-580 47 N 7 556?, 580 26.4 69.7

158 764.66 norite 77 50 520-555 -67.9 R 555-580 50.7 N? 34 575? 59.7

160 773.31 anorth 72 28 200-540 -65.1 R 555-580 61.3 N 7 556, 570 52.4 63.3

161 779.6 anorth 100 33 520-555 -37.7 R? 555-570 51.9 N? 24 350, 556?, 570? 51.9

162 783.8 anorth 15/48 12 500-540 -45.2 R 555-580 57.8 N 14 556, 580 53.1 61

163v 788.09 anorth 7/84.8 18 350-540 -29.1 CN? 540-580 29.9 N 8 556?, 580 53.5 72.5

164v 794.54 anorth 35.8/47.6 17 555-580 69 N? 556, 580 54.8 58.8

167 802.42 pyrox 1547 182 5 500-520 -32.6 R 580?,TH -38.1

168 802.42 chrom 106

169 803.45 pyrox 9.7/7.6 88 450-520 82 N? 520-540 -66.3 R? 19 540 -66.3

172 809.39 norite 25 245 400-500 -55 520-550 -72 R 560 -72

173 814.15 norite 554 387 450-520 -85 CR 520-540 -41.4 R 580? -41.4

Note: no. — sample number, letter “v” is used to mark samples, which were used for analysis of the field variations in the course of cooling of the intrusion at the sampling point; m — sampling level in meters; rock — anorth — anorthosite, pyrox — pyroxenite; k — magnetic susceptibility, 10-5 SI units (data from B. N. Pisakin, 2003); Jno/Jnmax — initial and maximum natural remanent magnetization NRM of the sample in the course of thermal demagnetization, respectively, mA m—1; dT-n — temperature of isolation of intermediate-temperature NRM component, I-n — inclination of intermediate-temperature NRM component; pol — polarity of this component; dThi — temperature of isolation of high-temperature component; /hi — inclination of high-temperature NRM component; N — normal polarity, R — reversal polarity, C — direction close to the recent field; dNR — difference of directions of the intermediate-temperature and high-temperature components of each sample; Td — main unblocking temperatures of high-temperature components (Curie points); /556 and /580 — inclinations of high-temperature NRM components near their Curie points.

Figure 10. Examples of Jn(T) I, II and III type dependences (see text). A smooth transition from type II to type I. Temperature, ° C - everywhere along the horizontal axis, magnetization mA m-1 - along the vertical axis. moXX - sample numbers.

clase, more rare of pyroxene (see above). Since this Curie point remains constant throughout the section this speaks in favor of a “one action” formation of this magnetic mineral, which occurred at the phase of a high-temperature exsolution of silicates, the secondary alterations would have definitely caused dispersion of composition of such minerals; 3) after being heated to 580° C and most rare to 570° C the NRM in majority of samples becomes less than 0.1 mA m-1 and this is the Curie point of magnetite (Table 4); 4) an appreciable magnetization is recorded in 27 samples (20 of them are anorthosites, 6 are norites and 1 is pyroxenite) at a temperature above 5800C which last to 600-6600C according to data from [Hattingh, 1986a]. These are lamellas of titanohematite in the ilmenite grains, the paleomagnetic di-

rections associated with this mineral are very close to “magnetite” directions. The titanohematites fall approximately on the horizons enriched by magnetite and titanomagnetite, i.e. having a high magnetization (Table 4).

Results of Thermal Demagnetization and of Component Analysis

A. The Monchegorsk intrusion. According to the magnetization behavior in the process of thermal demagnetization, the curves Jn (T) can be divided into three types (Figure 10):

I) a parabolic curve of the Q type or has a tablelike shape; its Td value varies from 550° C to 585° C, and one to three phases with different Td values are present in a number of samples;

II) Jn sharply increases beginning from 545-555° C, and its peak varying in height is observed at 550-560°C.

III) The peak degenerates into a step in the curve Jn(T). A smooth continuous transition exists between types II and III on one side and between III and I (Figure 10); therefore, they are of the same or a similar origin. Type III is intermediate between types I and II. The division of samples into three types in accordance with the shape of the curve Jn(T) does not depend on the susceptibility, NRM and its unblocking temperature (Table 2), Qn, magnetic anisotropy, and the over-printing amount (the last factor has an effect mainly at temperatures below 520-540°C). In all samples of types II and III, the curves Jrs(T) and Jrt(T) have the shape typical of magnetite and similar to the Q type [Pechersky et al., 2002]; i.e., peaks in the curves Jn(T) of type II, a tablelike shape of the curves of type I, and steps in the curves of type III are unrelated to the com-position and structural properties of magnetite grains. However, their dependence on the magnetic polarity is evident. Thus, the thermal demagnetization reveals a single magnetic polarity in samples of type I, mostly normal (N) samples 32, 38, 40, 41 and 45-53 and more rarely reversed (R) samples 9-11 and 13. Both polarities are fixed in samples of type III, and this is their main distinction from samples of type I. Finally, 10 samples of type II have the N and R polarities, one sample has the R polarity, and none of the samples has the N polarity. Along the section, samples are located quite regularly: samples of type II obviously prevail in its lower part (the lower ~250 m), where two magnetic zones of the normal (580-550° C) and reversed (below 550° C) polarities are clearly identified, whereas samples of type I prevail in the upper part of the section (the upper ~150 m), where the normal polarity alone is observed.

Such behavior of Jn (T) can be interpreted as follows. Two Jn components of opposite polarities exist in samples of type

II. One of these polarities (usually reversed) has lower unblocking temperatures (below 550° C), and/or its carriers were formed below 550°C, whereas normal polarity grains formed at a higher temperature and have higher values of Td and Tc. The relative contributions of these two components are different. The aforesaid explains the appearance of the curves Jn(T) of type II and the smooth transition between type II and type I through type III. This interpretation is valid if the overwhelming majority of magnetite grains are in the single-domain state in the interval 500580° C. Considering that Jn/Jrt ~ 1 (Table 2), the crystallization remanent magnetization (CRM) prevails in one-third of the samples studied, and some of them have a primary paleomagnetic direction similar to that of the samples having thermoremanent magnetization coeval with the intrusion cooling (see below). This does not contradict the fact that such samples were magnetized at the stage of the intrusion cooling. If magnetite grains formed at 450-500° C (i.e., below the Curie point of magnetite), they acquired the crystallization magnetization. In this case, their unblocking temperature during the thermal demagnetization,

considering the cooling rate of the intrusion (see below), is 500-540° C (Figure 5). Evidently, such a process also took place later, at 2-1.9 Ga, during intense tectonic movements accompanied by heating to ~400°C (the greenstone metamorphism observed near faults), which corresponds to an ~500°C unblocking temperature of the thermal demagnetization (Figure 5). This interpretation is supported by the prevalence of the magnetization of this type near the fault crossing the sampling profile (samples 37-46, Table 2). Magnetic grains with the KRM or chemical remanent magnetization (CRM) have lower values of Qnf (Table 2).

The thermal demagnetization results distinguish three ancient NRM components: 1) a conventionally low-temperature component is distinguished in a number of samples below 500° C (frequently below 400° C), 2) a middle-temperature component is detected usually between 500° C and 540° C,

3) a high-temperature component is detected above 540° C (Figure 11a). The middle-temperature and high-temperature NRM components have opposite polarity (Table 5) and they are frequently fixed in one sample. The single-component, two-component and three-component samples according to NRM do not differ by Jrt(T), Jrs(T), Js(T) - all of them are single-component [Pechersky et al., 2002], i.e. presence of components of opposite polarity is not a result of selfreversal. Absence of any secondary changes in the intrusion rocks, independence of magnetic polarity from composition of the rocks (peridotites, pyroxenites), content and composition of magnetic minerals (magnetite, phyrrotite), from the domain state of magnetic grains (Hcr), it is possible to suppose that opposite polarity of the NRM components is the result of variation of the geomagnetic field polarity in the process of cooling of the intrusion.

As seen from Table 5 the reversal test is performed within a95 for the intrusion rocks. The pole coordinates which belong to the Early Proterozoic layered intrusions and mafic dikes of the eastern part of the Baltic shield (Karelia, Finland and the Kola Peninsula), whose age is 2.4-2.45 Ga, have a great scattering: latitude is 11-41°S, longitude is 245-305°E, average coordinates 23°S and 273°E differ from our results. Contradiction can be removed, if the block with the Monchegorsk intrusion to rotate counterclockwise and to tilt in the NW direction at 15°-20°, the Monchegorsk intrusion pole (for example, lines 4 and 5, Table 5) will occupy a place, within a95, in the group of the paleomagnetic poles of Karelia and the Kola Peninsula, whose average age is 2.45 Ga and the average coordinates of this pole will be 23°S, 273°E (Table 5) [Pechersky et al., 2002]. It is very likely that a great scattering of the Early Paleo Proterozoic poles is caused by a neglect of local tectonic movements.

A secondary NRM component associated with both magnetite and pyrrhotite (Table 5) is identified in a wide range of demagnetization temperatures up to the Curie point of magnetite (typically, below 520° C) in many samples of the intrusion and host rocks. Comparison of the pole position calculated from this component with the APWP of the Baltic Shield shows that the age of this component is ~1.9-2 Ga [Pechersky et al., 2002, 2004]. An intense tectonic reorganization of the region is dated at this time (see Brief geological description). A certain scatter in the pole coordinates and the presence of normal (predominantly) and reversed (occa-

Figure 11. Examples of the orthogonal projections in the process of thermal demagnetization of the natural remanent magnetization (Zijderveld diagrams). Filled signs - horizontal plane, open signs -vertical plane. A (this page). Monchegorsk intrusion: sample 9, the primary reverse polarity KRM is detected in the 480-515°C range, the secondary normal polarity KRM - between 540°C and 555°C, the primary reverse polarity KRM - between 555°C and 580°C; sample 34, the secondary normal polarity NRM is detected below 540° C, the primary reverse polarity component - between 540° C and 550° C and

the primary normal polarity component - between 550°C and 570°C; sample 35, the secondary reverse polarity component is distinguished below 515°C, the primary reverse polarity component is detected from 540°C to 550°C, the primary normal polarity component is detected from 555°C to 580°C; sample 37, the secondary reverse polarity component is detected from 20°C to 500°C, the primary reverse polarity KRM is fixed from 500° C to 540° C, the primary normal polarity component is fixed from 540°C to 565°C; sample 40, the secondary normal polarity component is fixed up to the magnetite Curie point, this is most likely a chemical magnetization; sample 42 - example of a complicated behavior of NRM near the fault, this is predominantly the KRM, up to 545°C this is the secondary normal polarity component, between 550°C and 560°C - primary reverse polarity component, above 560°C - primary normal polarity component; sample 47, from 20°C to 580°C the primary normal polarity NRM (TRM) component is fixed; sample 53, up to 550°C the secondary normal polarity component is detected, above up to 575° C - the primary normal polarity component is detected.

550°C component A2 is detected; samples 15, 17 and 47 - component A1 is detected above 535-550°C, component B - between 420-520°C and 550° C, between 420°C and 520°C - a component close to the recent; sample 25 - component A2 is detected from 550°C, component B - between 450°C and 550°C, below a component close to the recent is fixed; sample 33 - up to 545°C component B prevails, above -component A1; sample 34 - component B; sample 41 - component A1 (A2) is detected from 535°C, the sum of components A1+B with similar spectra of unblocking temperatures is fixed between 420° C and 535°C, a component, evidently, close to the recent is present up to 500°C; sample 48 - component A2 is detected above 540° C, below the sum of components A2+B is fixed; sample 50 - the sum of components A+B with similar spectra of unblocking temperatures; samples 51, 55, 57, 59 and 66 - component A1 is detected above 520-550°C, below the component B is fixed or B+A and a component close to the recent (below 500°C); sample 70 - components A1 and A2 are detected from 520°C to 660°C (smooth transition), below is the sum of components A2+B.

Table 5. Average paleomagnetic directions and pole coordinates of the Monchegorsk intrusion and of the neighboring regions

Number of samples and magnetization age Polarity D I K «95 Pole longitude latitude

1) 27, primary N 305 -15 19.7 6.1 267.6 E 5.2 N

2) 23, primary R 127 30 10.4 9 263.2 E 4.7 S

3) Difference between 1 and 2 N+R 306 -23 57.6 13 265.3 E 1.3 N

4) D-50 N+R 256 -23 310 E 16 S

5) D-30, I+17 N+R 276 -40 289 E 19.7 S

6) 19, secondary N+R 334 33 12.5 9.1 244.6 E 37.3 N

7) 13, pyrrhotite, primary N+R 328 -33 5.8 16.1 255.3 E 2S

8) 14, pyrrhotite, secondary N+R 316 46 12.3 10.7 282.7 E 37.4 N

9) Dikes of the Main Range N 251.1 E 7.9 N

10) Diabases, Voche-Lambina N+R 247.6 E 5.7 N

11) Average on 9 poles N+R 12.2 273 E 23 S

Note: The polarity column presents polarities of the high-temperature NRM component (N — normal, R — reversed) that were determined after [Elming et al., 1993; Khramov et al., 1997]; D and I are, respectively, the declination and inclination of this component; K is the precision parameter; «95 — is the confidence angle; pole — coordinates are determined from the directions of the high-temperature primary and secondary NRM components. Poles 9 and 10 [Khramov et al., 1997] were derived from structures similar in location and age to the Monchegorsk intrusion. Pole 11 — average pole of 9 determinations for Kola Peninsula and Karelia, its average age 2448 Ma [Ein and Sokolov, 2000; Elming et al., 1993; Khramov et al., 1997; Krasnova and Gooskova, 1995; Mertanen, 1995; Mertanen et. al., 1989, 1999].

sionally) polarities in the secondary component indicate that the process of its acquisition was long. The preservation of the primary NRM component in several pyrrhotite-bearing samples (Table 5) indicates an irregularity of this process. The secondary component exhibits properties of the TRM, KRM and CRM (Table 2), indicating its association with a later (1.9-2 Ga) heating to very high temperatures and a possible formation of new magnetic minerals under these conditions; this resulted in the acquisition of the crystallization magnetization (see above). As seen from position of the pole associated with the secondary NRM component with respect to the APWP of the Baltic Shield, the tectonic

movements are likely to have preceded the acquisition of the ancient secondary NRM component. It is conceivable that the northward displacement of the Monchegorsk intrusion pole toward the poles of remagnetization (1.9-2 Ga) is related to an incomplete destruction of the high-temperature remagnetization at 1.9-2 Ga, because the range of Td values in newly formed magnetite grains is close to that of grains formed at the stage of the intrusion cooling. This must adversely affect quality of the paleovariation record.

B. The Kivakka intrusion. Two NRM components are identified in the process of thermal demagnetization (Figure 11b): a high-temperature component A has a re-

Table 6. Paleomagnetic directions and coordinates of poles of the NRM components of the Kivakka intrusion

NRM Component no. co Ig/s K «95 Polar Latitude Pole Longitude

Aig 17 134° 2 O 0 11.6 10° R 5.9°N 258.4°E

Ais 17 138.7° 55.6° 11.6 10° R 17.8°S 247°

Ais(D-30) 17 108.7° 55.6° R 26.2°S 271.5°

A2g 10 130.3° 60.9 26.3 7. 3 0 R 25.3°S 249.9°E

Bg 47 340° 0 5 4 20.6 4.5° N 47.7°N 240.1°E

Sum of mean (Ai+B)g 2 88.5° 5 0

Sum of mean (A2+B)g 2 22.2 7 00 0

Mean pole 9 12.2 N 23° S 273°E

Note: NRM component — high-temperature component, Ai — high-temperature, primary, prefolded; A2 — high-temperature, afterfolded or synfolded, its indication temperature is usually slightly below Ai component; B — secondary component, result of the second heating of the intrusion approximately 1.9 Ga ago, maximum temperature at which it is identified varies from 500°C to 580°C; no. — number of samples; D,I — paleomagnetic declination and inclination, respectively; g, s — geographic and stratigraphic coordinates; K — precision parameter; «95 — confidence angle; Polar — magnetic polarity; N — normal; R — reversed; Pole — coordinates of the north paleomagnetic pole; Mean pole — mean coordinates of the north paleomagnetic pole of the Kola Peninsula, Karelia and Finland (mean for 9 definitions of different authors, average age 2448 Ma), [Pechersky et al., 2002].

Figure 12. Kivakka. Stereogram of paleomagnetic directions of NRM components detected during the thermal demagnetization and a component analysis in the geographic coordinates. Circles show components A (the second quadrant and low inclinations - component Ai, high inclinations - component A2, acquired after tilting of the Kivakka block), crosses show component B. Circles in the 4 quadrant are high-temperature components, possibly normal polarity A, a typical component B is distinguished in these samples, two of these points are included in calculation of the mean direction of Ai , for which purpose their polarity was changed for a reversal. “Jumped off” crosses refer to component B as an ordinary high-temperature reverse polarity component A is detected in the same samples. The dotted line show areas of points where components A and B are not separated.

versed polarity (declination in the second quadrant, a positive inclination, polarity is adopted according to [Elming et al., 1993; Khramov et al., 1997]); the component B has a normal polarity (declination in the fourth quadrant, a positive inclination). Components A and B on the stereogram (Figure 12) in the geographic coordinates form a swarm of points where three groups can be distinguished: 1) the second quadrant, positive and occasionally negative low and intermediate inclinations (circles on Figure 12), here are only high-temperature NRM components of the reverse polarity, a component Ai; 2) the second quadrant, high positive inclinations, a high-temperature component A2; 3) the fourth quadrant, high and intermediate positive inclinations (crosses on Figure 12), a component B, in samples containing both components A and B, the latter is isolated at temperatures below temperature that indicates component A, but only one component B is quite often is recorded at a temperature up to 580°C (Figure 11b).

Average paleomagnetic directions and coordinates of poles

are defined for three mentioned above groups (Table 6). If elements of the layered intrusion bedding are taken into consideration, i.e. to adopt its original bedding as horizontal, coordinates of the Kivakka paleomagnetic pole (component Ai) in the stratigraphic coordinates are close to mean coordinates of the pole of a similar average age (Table 6, Figure 13). If the Kivakka block is turned around the vertical axis through 30°, they will coincide (Table 6, Figure 13). This is the evidence, first of all, in favor of a close to the time of cooling of the Kivakka intrusion component Ai , and second, of the tectonic origin of divergence of the Kivakka pole and the mean pole of the region of the same age. The next important fact is the practical coincidence of the pa-leomagnetic directions and, consequently, of the paleomag-netic poles of the Ai component in the stratigraphic coordinates and of A2 component in the geographic coordinates. According to the geological data the Kivakka block was tilted prior to intrusion of the dolerite dikes, which are dated to the same magmatic phase to which the Kivakka intrusion

belongs. Thus, tilting of the Kivakka intrusion occurred at the stage of its cooling at temperatures close and/or exceeding 500° C. The paleomagnetic fixing of this event is also evidence about acquisition of Ai component and about variation of its direction at the stage of cooling of the Kivakka intrusion, i.e. the Ai component from the paleomagnetic point of view is primary. The block turned around the vertical axis after the A2 component has been acquired (as it coincides with the A1 component).

The paleomagnetic pole of the B component almost falls on APWP of Baltic shield close to 1.9 Ga (Figure 13). Most probably, occurrence of the B component is associated with the secondary heating of the block during the Svecofennian tectonomagmatic activation. No appreciable secondary alterations in the Kivakka intrusion rocks in the area in question are found; samples where component A or B prevails do now show any peculiarity. Therefore, it would be more logical to assume a pure effect of heating approximately to 400500° C, when a part of the single-domain and pseudosingledomain magnetic grains with the blocking temperatures below 500°C, which were formed, mainly, at the stage of the primary cooling of the intrusion, would acquire a new magnetization. The unblocking temperatures of such magnetization will be 500-550°C respectively (Figure 5). As this is practically the same ensemble of magnetic grains, it is impossible to fully divide components A and B by the thermal demagnetization and, depending on such division the component B or A and their indivisible sum with similar unblocking temperatures will prevail in the sample (Figure 11b). Thus, in case of the sum of components A1+B declination is towards E-N-E and inclination is higher than for A1 or B, in case of the sum of components A2+B declination becomes more northern and inclination is close to a vertical (Figure 11b and 12, Table 6).

The components A1 and A2 are detected, as a rule, above 500° C, and frequently above 540-550° C (Figure 11b). Some samples are completely remagnetized and this is in agreement with heating of a portion of minerals up to the Curie points (quite often Tc = 525-540°C, see Table 3). Thus, for example, intervals between 358 m and 370 m, between 421 m and 500 m, between 665 m and 746 m, between 1030 m and 1132 m were remagnetized completely (Figure 14a,b).

C. The Bushveld intrusion. According to behavior of the NRM during of thermal demagnetization all samples can be divided into 4 types: 1) tablelike shape, up to T >550°C the NRM intensity does not change practically and afterwards it drops abruptly (21 samples), 2) parabolic Q-type (11 samples), 3) linear decrease, L-type (8 samples), 4) majority of samples show increase of magnetization during of thermal demagnetization; this increase is smooth (P-type of thermal demagnetization curve show (30 samples) and/or sharp (45 samples), which is connected with presence of two NRM components of a normal and reversed polarity. The NRM component with a negative inclination is identified in 119 samples; temperature of its detection is always below the NRM component with a positive inclination, the latter is found in majority of samples (142), the samples with pyrrhotite (6) and low-magnetic samples (16). Declination was restored for both components according to a viscous magnetization. Due to reasons indicated in the “Methods”

V *)B *-—^— hi ' ' ^ *2120

1760 1830N| 1880IT

• /

A„( • 1 # f * 450 ]:

2675 ^2g / V • © < / P

..jjk

30

■30°

-60

210°

240°

270°

300°

330°

Figure 13. Position of the paleomagnetic poles of components A1, A2 (before and after tilting of the Kivakka block) and B relative to APWP of Fennoscandia \Elminq et al., 1993].

section a great scatter of the unit paleomagnetic directions occur (Figure 15). The average direction, however, does not differ practically from that obtained for samples selected from the natural outcrops in the western and northern parts of the Bushveld intrusion \Hattingh, 1986a; Hattingh and Pauls, 1994], both in the geographic and in the stratigraphic coordinates (Table 7).

The average bedding of the intrusion lamination in the region of well WP-16 with the dip azimuth 20°, dip angle 13.5°, was calculated according to data taken from \Hattingh, 1986a].

An ever greater scatter of directions is observed in the case of a intermediate-temperature component of the reverse polarity; its mean direction differs appreciably by inclination from the high-temperature component with the normal polarity in one system of coordinates (Table 7). It is possible to state that this is not a remagnetization affected by the field, which is close to the the present day field direction first, the inclination is notably more steep, second, and, more important - the mean declination is opposite to declination of the recent field. It should be noted that the paleomag-netic direction of the high-temperature component in the stratigraphic coordinates, and, consequently, coordinates of

Figure 14. Kivakka. Distribution of declination and inclination of components A1 and A2 over the section (a, b) and in time (c, d ). a, b - data of the component analysis, the range close to the Curie point of the main magnetic mineral in the sample. c, d - direction of NRM close to Curie point. e, f - averaged data, averaging interval is 3 thousand years, averaging step is 1 thousand years. Time is given from the moment of emplacement of intrusion. Direction of component A1 is in the stratigraphic coordinates; A2 is in the geographic coordinates.

the paleomagnetic pole do not differ practically from the paleomagnetic direction and coordinates of the pole of the intermediate-temperature component in the geographic coordinates (Table 7). It is likely that the tectonic tilting has occurred at the stage of cooling of the intrusion and, consequently, the normal polarity component was acquired before tilting, and the reverse polarity component was acquired after the intrusion was tilted. The situation is similar to that

of the Kivakka intrusion (see above).

Following \Hattingh, 1986a, 1986b, 1989] we adopt a normal polarity for the NRM high-temperature component and the fact that this component is associated with thin inclusions of titanomagnetite-magnetite in plagioclases and pyroxenes, formed during their exsolution, and was acquired at the time of cooling of the Bushveld intrusion from the temperature exceeding the magnetite Curie point.

Table 7. Paleomagnetic directions and the pole of the Bushveld intrusion

NRM no. D I K «95 paleopole

Latitude Longitude

ANg 101 2° 81° 10.1 4.4° 8.1°S 21.1°E

Ans 101 12.5° 68.3° 10.1 4.4° 12°N 35.4°E

ARg 58 182° - Oi o 5 8. 2 O 16°N 28.9°E

Ars 58 188° -52.9° 5 8.2° 30.3°N 35.2°E

A 32 336.1° 82° 58 3. 4 o 11.2°S 28.1°E

B 32 357.8° 70.9° 80 2. 8 o 9.2°N 27.3°E

North 12 15 1.6°N 22.3°E

Note: NRM — natural remanent magnetization component, An — normal polarity high-temperature component, Ar — reverse polarity intermediate-temperature component, our results; g — geographic coordinates, s — stratigraphic coordinates; A, B, and North — a stable NRM component, identified in samples from outcrops of the Main Zone in the western (A, B) and in the northern part of the Bushveld intrusion as a result of an an AF demagnetization, in the geographic (A, North) and stratigraphic (B) coordinates [Hattingh, 1986a; Hattingh and Pauls, 1994].

Thermal Physical Estimation of the Cooling Process of Intrusions

In order to analyze variations of the geomagnetic field, recorded in the cooling process of the intrusion, it is necessary to determine the cooling time within the range of blocking temperatures and the total duration of cooling. To model the dynamics of temperature variation after the emplacement of the intrusive body and estimate these time intervals, we examined the nonstationary problem of heat

N

Figure 15. Stereogram of directions of the high-temperature NRM component of the specimens from the core of well WP-16. Bushweld. Declinations were determined with aid of the viscous remanent magnetization (difference vector Jn20 - Jn200,300).

conduction (with regard for crystallization of the intruded melt).

We assume that the emplacement of the intrusive body was relatively rapid (as compared with the characteristic time of cooling). Then, the heat exchange during the emplacement can be neglected, and the temperature of the surrounding rocks at the initial time can be assumed to be the same at the upper and lower boundaries of the intrusion (this temperature is defined by the undisturbed geotherm). The temperature of surrounding rocks at the time of emplacement of the intrusion has to be estimated. The resent geothermal gradient in the region of the Kola ultradeep borehole which is about 20° km-1 is known. The steady (balanced) geothermal gradient is determined by thermophysical properties of the environment and, accordingly, must not differ notably from the recent gradient. According to [Berk et al., 1980], it is unlikely that the ancient continental crust was affected exclusively by high geothermal gradients (except for a rather narrow accretion belts). Based on similarity of behavior of the ancient continental fragments with the regime of more recent epochs, these authors come to a conclusion that the equilibrium geothermal gradients in the Archean time did not exceed, usually, 25° km-1. According to E. V. Koptev-Dvornikov (verbal information, 2003), the best agreement (on energy- and mass exchange) of the model results with those observed, during modeling the Kivakka intrusion crystallization, can be obtained if temperature of the surrounding rocks at the depth of emplacement of the intrusion is adopted to be about 200° C. In accordance with the above and considering the depth of emplacement of this intrusion about 10 km, the geothermal gradient becomes equal to 20° km-1. This gradient was adopted for all intrusions under study.

The problem of cooling of the intrusion can be divided into two stages. First, we estimated the cooling time of the intruded melt at the time of emplacement of the intrusion (about 1400°C) to the solidus temperature (about 1200°C) and obtained the temperature distribution in the intrusion and surrounding rocks at the time of complete solidification. To do this, we solved the Stefan problem. Two phases were discussed and in each phase the temperature satisfies the

heat conduction equation:

<9t

p=1

dT

(1)

where T - is the temperature; t - is the time; xp - is the coordinate, p takes the values from 1 to N (dimension of the problem); p - is the density, C - is the thermal capacity at a constant pressure, k - is the coefficient of thermal conductivity, s = 1 (solid phase), s = 2 (melt). The temperature is constant T = T* (crystallization temperature) at the phase interface, the heat flows are discontinuous and their difference is equal to Av, where A - is the melting heat, v - is the velocity vector of the phase interface.

It is rather difficult to follow the movement of the phase interface for a three-dimensional case. Aside of this, the task of this work is to model the cooling within the range of temperatures which is much lower than the solidus temperature (below 600°C). Therefore, to solve the Stefan problem we use the approach of [Dudarev et al., 1972; Tikhonov and Samarsky, 1966], which being relatively simple, allows with an adequate accuracy for our purpose to describe the thermal exchange during crystallization. We write equation (1) as follows:

A d

p(C (T)+ A*(T - T *) - = ^ W

p=1

dT

(2)

where 5(T — T*) denotes 5-function, the thermal capacity function C(T) is given by:

C(T)

C1,T < T * C2,T > T*

in a similar way it is possible to write down also for the coefficient of thermal conductivity. Further, a function of effective thermal capacity is introduced

Ceff (T) = C(T) + A5(T — T*) ,

Then we have the following equation for T

(2*)

When all the points of the system have cooled below the crystallization temperature T*, equation (2) becomes an ordinary equation of the thermal conductivity for an inhomo-geneous medium.

The finite difference method was used to solve the equation. With the computational solution the 5-function is substituted in the rough for the function 5(T — T*, A), which differs from the 0 only in the interval (T* — A, T* + A) and satisfies the normality condition of 1.

Calculations were made for different values of the thermal physical parameters, geometric characteristics and dimensions of bodies, depth of their formation, which are given in the geological section.

Below we give the calculation results for every intrusive body.

A. The Monchegorsk intrusion. The geometry of the intrusive body (length is ~10 km, thickness is ~3 km) allows examining a one-dimensional problem (depending on the depth only) for the sampling area 375 m thick. Values of the thermal and physical parameters were taken from [Dudarev et al., 1972; Thermal field of Europe, 1982; Turcotte and, Schubert, 1985]. The thermal conductivity for surrounding rocks is k = 3.05 W m-1 K-1, thermal capacity is C = 1150 J kg-1 K-1, density is p = 2.75 kg m-3. The melting heat for the intrusion rocks is A = 350 kJ kg-1, the thermal conductivity is k = 4.35 W m-1 K-1, the thermal capacity is C = 1005 J kg-1 K-1, density is p = 3.31 kg m-3. With the computational solution the step in spatial coordinate was 100 m, and the step in time was 100 years.

According to these calculations it takes approximately 25000 years for the melt to solidify completely. As is evident from the temperature versus time curve (Figure 16) the initial difference in temperature between parts of the section under study (about 150°) rapidly disappears. Thus, the difference between the times at which the upper and lower parts of the section attain the Curie point of magnetite (580°C) is ~12,000 years. The entire section attains a blocking temperature of 480° C almost simultaneously as is indicated by the coinciding curves T(t) (Figure 16). The period between the times when the section assumes temperatures of 580°C and 480°C, is about 160,000 years. During further cooling, the curves once again diverge; the direction of the temperature gradient becomes normal (to the surface) and tends with time to the undisturbed direction. The time of cooling of the lower part of the intrusion to the undisturbed temperature (250° C) is estimated as 15-20 Myr.

It is seen from calculations that the surrounding rocks are heated; those adjacent to the intrusion are heated up to 550-650° C, and those which are 2 km below the intrusion foot are heated up to 450°C.

The cooling velocity changes appreciably: at 580° C it varies from 1100°/Myr at the contact to 1170°/Myr at a distance of 400 m from the contact, at 480° C it varies from 400° /Myr at the contact to 450°/Myr at a distance of 400 m from the contact (Figure 17).

B. The Kivakka intrusion. A tapered shape of this intrusion has required a solution of a three-dimensional problem. On the basis of data presented by E. V. Koptev-Dvornikov and colleagues the following thermal physical characteristics were adopted for modeling. The surrounding rocks: the thermal conductivity k = 2.51 W m-1 K-1,

the thermal capacity C

1050 J kg-1 K-1,

the density

p = 2.65 kg m- . The thermal and physical properties of the intrusion proper were determined on the basis of data about its composition, assuming the additive model. The average weighted composition of the Kivakka intrusion is as follows (mole fractions): plagioclase - 31.9%, clinopy-roxene - 11.7%, orthopyroxene 23.2%, olivine - 32.6%, il-menite 0.6%. Data on the thermal conductivity, thermal capacity and melting heat of minerals of the were presented by S. V. Bolikhovskaya (verbal presentation, 2003). Finally the following values of parameters were adopted for our model: k = 3.52 W m-1 K-1, the thermal capacity C = 1110 J kg-1 K-1, the density p = 3.22 kg m-3, the melting heat A = 550 kJ kg-1.

,° Myr"1 T,° C

Time, Myr

Figure 16. Temperature (T) versus Monchegorsk intrusion cooling time at different parts of the section (at the contact, 160, 200 and 400 m from the contact). a, b, c - different time ranges.

T, °c

Figure 17. Calculated velocity of cooling of Monchegorsk intrusion versus temperature. Symbols are the same as in Figure 16.

As is evident from Figure 18, cooling of the intrusion lasted for about 25,000 years within the temperature range from 580° C to 500° C. As the intrusion approached the upper contact it cooled rapidly as seen by the shift of the T(t) isolines to the left, beginning approximately from 1200 m (Figure 18). The cooling velocity of intrusion at different points of the section and at different temperatures changes within a relatively narrow range: from 4.5° to 5° in one thousand years at 575°C and from 3° to 3.5° in one thousand years at 515°C (Figure 19).

C. The Bushveld intrusion. Since dimensions of the intrusion are quite large (approximately 480x80 km), and WP-16 well is located near the side boundary, it is reasonable to examine a two-dimensional model of cooling, i.e. a vertical section crossing the well region. The shape of the intrusion is a flat-laying truncated ellipsis with the overlying rocks and whose maximum dip angle on the lateral contact is 20°.

When selecting values of the thermal physical parameters for the model of cooling we were based on information about structure and rocks of the intrusion and the host rocks offered by [Eales and Cawthorn, 1996; Sharkov, 1980; Wager and Brown, 1970] and the reference data. We adopted the following thermal physical characteristics for modeling. For the host rocks: the thermal

conductivity k = 2.1 W m-1 K-1, the thermal capacity C = 880 J kg-1 K-1, the melting heat A = 285 kJ kg-1 -for the overlying rocks which were submelting, the density p = 2.6 kg m-3. The thermal physical properties for the intrusion proper were determined on the basis of data about its composition and the additive model. The average weighted composition of the Bushveld intrusion was adopted as follows on the basis of data taken from the literature mentioned above: plagioclase - 47%, clinopyroxene - 53% (mole fractions). As a result the following values of parameters were adopted: the thermal conductivity k = 3.19 W m-1 K-1, the thermal capacity C = 1117 J kg-1K-1, the melting heat A = 472 kJ kg-1. The value for density p = 2.9 kg m-3 was adopted on the basis of averaging data on the core from WP-16 well (data of B. N. Pisakin, 2003).

According to the geological and experimental data (see Brief geological description), the intrusion roof at the time of emplacement was at a depth of about 3 km. The temperature of the intruded melt has bee adopted about 1400° C, the crystallization temperature has been taken as 1200°C. We have taken into consideration the fact when modeling that the roof over the intrusion was rising at the time of emplacement (the value of 800 m was adopted as no precise data is available) and afterwards this uplift eroded. Though there are evidences that the intrusion could have taken place in several stages (see Brief geological description) we considered a single-act intrusion in a simplified way as no specific information about the number of these stages, their sequence and volume is available, and we are mostly interested in the stage of cooling of the intrusion at a temperature 600°C.

It follows from modeling of cooling of the intrusion in the zone corresponding to the well (Figures 20, 21) that the higher the sampling points are in the bore hole, the lower their temperature at the similar times is (Figure 20). This result needs a certain explanation as at a glance an inverse dependence must be observed; points from lower levels are

most close to the inner layers of the intrusion and, consequently, they must cool slowly. In fact, first lower points of the well (which are closer to the lower boundary of the intrusion) cool rapidly, whereas the temperature gradient here has a reverse direction (with regards to the normal). Then, after some time (which depends on properties of the system) of the beginning of cooling, the temperature alonge the well-section equalizes practically and at a further cooling the curves again diverge and direction of the gradient becomes normal, and with time it tends to the undisturbed direction. In this case the reason of a rather rapid (approximately 0.5 Myr) overturn of the gradient is the influence of a relatively close located cold upper boundary - the day surface where temperature is adopted to be constant and equal to 0° C. As a result by the time the system has cooled to 580°C, i.e. to the beginning of the paleomagnetic record, a temperature rise with depth is observed everywhere in the area under study (in intrusion and in the host rocks).

The cooling velocity (Figure 21) varies from 580° C to 480°C, accordingly, from 290-330°/Myr to 200-230°/Myr, i.e. the velocity changes insignificantly along the section.

Paleomagnetic Record of Variations of the Geomagnetic Field During Cooling of Intrusions, Wavelet Analysis Results

Let us examine two variants of the record of variations described in the introduction: 1) record for the time the Curie point is “running” along the section and 2) record in every sample from the result of the detailed thermal demagnetization.

Because of finite intervals of the thermal demagnetization step (3°-5°) in the first and second variants we do not obtain a continuous record of the field, but we obtain difference magnetization vectors between adjacent temperatures. We obtain a certain averaged pattern of behavior of the remanent magnetization vector. In the first variant of the paleo-magnetic record we adopted the magnetization density to be constant in the three-degree intervals, though the spectrum of blocking temperatures of the remanent magnetization may be non-homogeneous and quite narrow.

In order to tie the obtained paleomagnetic results to the time in the second variant of the record it is necessary first to go from unblocking temperatures to the NRM blocking temperatures in the process of cooling of intrusions. To convert unblocking temperatures during of thermal demagnetization into the intrusion cooling temperature, we assume, that the magnetic grain state is sufficiently close to the single-domain state, in the temperature range under study, then blocking temperatures of the magnetic minerals will correspond to the intrusion cooling temperature at the sampling point. Should the multidomain grains play an appreciable role, their magnetization will take place near the Curie point. The NRM carriers behave as the single-domain and pseudosingledomain particles at Td > 530° C, as it was shown above.

The cooling velocity VCooi, was determined for every unblocking temperature, obtained in the course of thermal de-

72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97

Time, kyr

Figure 18. Temperature at different points of the section versus time of cooling of the Kivakka intrusion. Time is given from the moment of magma intrusion. Numbers in the figure show thickness in meters from the lower contact to the given point of the section.

3.0 I......................................................................................................................................................................................................................

580 575 570 565 560 555 550 545 540 535 530 525 520 515 510 505 500

T, °c

Figure 19. Velocity of cooling of the Kivakka intrusion versus temperature. Symbols are the same as in Figure 18.

Figure 20. Calculated temperature versus time of cooling of the Bushveld intrusion. Numbers near the lines indicate depth in meters from the well mouth (0) top-down.

330

230

220 I...................... .................... .................... .................... .................... ...................

580 570 560 550 540 530 520

Figure 21. Velocity of cooling of the Bushveld intrusion versus temperature. Symbols are the same as in Figure 20.

magnetization, according to data of Figure 17, 19, 21, and, afterwards the NRM blocking temperature Tb in the course of cooling of the intrusion was found with the help of a transparency Tb(Td, VCooi) (Figure 5). The Curie points of the samples under study vary appreciably from 525° C to 580° C (see petromagnetic section). For such cases Figure 5b is used, where relative values of Td/Tc and Tb/Tc, are given which can help to define Tb easily.

The next step is the transition from temperatures to the cooling time for which purpose the T (t) dependences are used for various points of the section (Figure 16, 18, 20).

Thus, we obtain a pattern of behavior of the NRM direction (D,I) for every sample, for each five-degree and/or three-degree range depending on the unblocking temperature (heating in the laboratory) calculated blocking temperature and the estimated relative cooling time. To conduct a further analysis, the samples were selected where we can judge with a greater confidence about the record of behavior of the geomagnetic field direction. The main criterion is proximity of the NRM components detected in a certain temperature range to the average paleomagnetic direction throughout the section, which is isolated above 540° C.

In the first variant of the paleomagnetic record conversion of Td into Tb becomes simple as Tb = Td near the Curie point. However, errors in estimation of the Curie point, cooling time have a more appreciable effect, and, consequently, the scatter of determinations increases (see below); besides, the cooling time is by three degrees comparable with the Curie temperature “running” along the section, for example, in the case of a detailed sampling in the Monchegorsk intrusion (see below).

Time series wavelet analysis. The initial or basic data are a sequence of couples of numbers {tj , Zj, }, J = 1,..., N where the first are the time marks, the second is the angle of orientation of the paleomagnetic field component. The times marks tj form a nonuniform mesh of values, characterized by presence of the closeness intervals (a very frequent measurement), and by sufficiently long intervals of absence of data. The measured values of zj are characterized by a high noise of measurements; this can be judged by large amplitude of their variations in the intervals of concentration the time marks. To proceed with the analysis of data it is necessary to pass to a uniform time step, for which purpose the initial (nonuniform) data about each time point is to be smoothed. First of all a quantization time interval must be specified. A nuclear averaging method of Nadaray-Watson [Hardle, 1989] was employed to obtain values of the uniformly numbered signal x(t) :

where, in its turn:

<^(s) = exp(—s2)

(5)

- is the Gaussian averaging kernel, and, H > 0 - is the parameter of the averaging kernel width. By implication of formulas (3) and (4), the initial measurements (tj, Zj) for which the time marks satisfy the condition | tj — t | < H, make the main contribution into the average value x(t). The choice of the parameter H follows herefrom; it must be equal to a half of the selected time step. While implementing this method of transition to the uniformly numbered signal a situation may occur when a direct use of formula (3) will lead to an uncertain type of zero division (in the case when the point t is inside a sufficiently long period when measurements are not available). In this case it is necessary to increase the averaging parameter H so that the averaging kernel would be able to reach from the point t the nearest time marks tj. The averaging program envisages this possibility and begins to increase H gradually in the case if |Wo(t|H)| < 10—6 - till this inequality does not change for a reverse and, thereby, the zero division situation is eliminated.

For a further application of the wavelet analysis, removal of the low-frequency trends is a useful operation to avoid the fringe effects, which occur in case of sufficiently strong low-frequency variations of the signal. After a number of trials, we have selected a polynomial of the 3-rd order. Further we have taken a difference between the base curve and the smoothing result and a wavelet diagram was drawn for this difference. The smaller radius of the sliding time slot of the polynomial smoothing is the boldly small-scale (high-frequency) peculiarities of the signal behavior seen.

Let x(t) - is the times series being analyzed. We are interested in its time-and-frequency structure. A continuous wavelet analysis is the most sensitive tool for the purpose [Chui, 1992; Daubechies, 1992; Mallat, 1998]. Let ^(t) -is a certain function satisfying the admissibility condition:

j ^(t)dt = 0 and the normality condition: j |^(t)|2dt = 1.

A value which depends on two parameters (t,a), a > 0 is called a continuous wavelet transform:

Wx(t,a) = —^ x(s) • ds

aa

/ x(t + av) • ^(v)dv .

(6)

where:

x(t) = Wi(t |H)/Wo(t |H)

(3)

wi(t|h) = X/j • ^((tj — t)/H)

j=1

N

wo(t|h) = X^((tj — t)/H) ,

(4)

Here t - is the time point, a > 0 - is the scale parameter, which we will further give a more usual term as “period” or “rhythm”. The value (6) reflects behavior of the signal under study about a point t with a typical period of variations a. It is reasonable that the value (6) depends greatly on the choice of the function ^(t). Further we will use the so-called Morlet wavelet or a complex-valued modulated Gaussian:

j=i

1

^(t) = nT/4 exp(—t /2 — int) . (7)

This wavelet is adapted best of all for distinguishing short-lived harmonic peaks (trains) and has certain optimality properties in the search of a compromise between the frequency and time resolution (yielding the so-called Heisenberg bound). Our primary aim is to construct a twodimensional map of the module of value (6) (|Wx(t,a)|), which gives a pictorial presentation of the dynamics of the onset, evolution and disappearance of typical periods of the harmonic peaks of the signal under study.

A. Monchegorsk intrusion. Taking into consideration the above statements (see sections Petromagnetic measurements and Thermal demagnetization results), it is reasonable to examine variations of the field direction at temperatures above 530-540°C. We tried to reveal the long-period variations by the thermal demagnetization from 400° C to 580°C with the five-degree steps [Pechersky et al., 2002, 2004]. Here we will discuss the results of the wavelet analysis data of demagnetization with the five-degree steps above 530° C along the 375 m section (samples were taken every 10 m at the average), and of a more detailed thermal demagnetization with the three degrees steps from 540° C to 580°C in the 100-161 m range of the section (samples were taken every 1-2 m). Both in the first and in the second variant of the record the amplitude of variations of declination reaches ±20°, and that of the inclination usually does not exceed ±10°.

The first variant of the record of paleovariations. Let us examine data of the component analysis and behavior of the direction at unblocking temperatures near the Curie points (Tables 8, 9). As has been stated above, two NRM composites of the opposite polarity are distinguished according to data of the component analysis (Table 5). Directions of the medium-temperature (R), and high-temperature (N) NRM components vary within the range usual for variations as is evident from the standard deviations, D=134.7°±14.2° and 297°±11.9°, I=23.2°±11.6° and —8.8°±6° respectively. Appreciable deviations from the average refer, first of all, to several samples with a relatively high magnetic susceptibility (evidently, due to an incomplete demagnetization of partially remagnetized multi-domain magnetic grains) and second, a paleomagnetic anomaly (excursion) with a complete change o the polarity (Table 9 and Figure 22) is fixed in the interval from 150 m and 157 m of the section in the direction of the high-temperature component; the NRM vector circumscribes a complicated, close to a circle movement during the excursions (Figure 23).

Data of the component analysis are no good for conversion of temperature into time as direction of the NRM components distinguished is averaged from a large temperature range (Table 8). Therefore, to convert temperatures into time the unblocking temperatures close to the Curie points were used (Table 9). These are temperatures of an appreciable sharp drop (more than 50%) of the NRM value in a three-to-five degrees step in the process of the thermal demagnetization. However, even in this case difference between the adjacent three-degrees points amounts to 3-4 thousand years, i.e. a certain smoothing takes place all the same. We

refer the value of D, I to the point one degree above the Curie point and the smoothing interval will be less than two degrees.

As seen from Table 9, the NRM high-temperature component has only a normal polarity during its movement along the 375 m section, whereas in a more detailed case during the movement along the 61 m section the excursion is fixed between ~128 and 130 thousand years. Its absence in the record of a long section is explained by the fact that the time intervals between the observation points are approximately 5 thousand years and our excursion of about 2 thousand years falls in one of these gaps.

The number of field direction points over a “long” section are insufficient for the wavelet analysis and therefore, only a “short” section, where an excursion is cut off, is analyzed. The record analyzed covers the time period from 116 to 155 thousand years from emplacement of the intrusion.

The time marks obtained for a nonuniform mesh of values and the values D, I proper are characterized by a high noise (Table 9, Figure 24). This is mainly due to inaccurate determination of the Curie points and the relevant temperatures and time. Therefore, a transition is made to a uniform time step, smoothing has been made and the trend has been extracted (Figure 24a,b). After these operations are made the wavelet diagram shows distinctly a 7-9 thousand years rhythm in declination and in inclination (Table 10); it can be traced over the entire record and reduces slightly with time. Aside of this, a series of splashes ranging from 4.5 to 1.7 thousand years, forming the chains, is fixed in the entire record (Figure 24b,f).

In the second variant let us examine only those samples where the record of behavior of the field direction is clearly seen (Table 11). A short section of record of the reverse polarity (Table 11) is insufficient for an independent analysis, therefore, its polarity was changed for a normal polarity and it was combined with the remaining record (Figure 25). It should be noted that the reverse polarity interval takes time between 160 kyr and 220 kyr from emplacement of the intrusion (Table 11, see also Introduction and Figure 1). Since there is no reverse polarity behind the fault (Figure 1, Table 11), the shift in time between the sections before and after the fault is approximately 100 kyr and these two sections differ in the paleomagnetic declination, their average values are: mo-1-42 - D=302.4° and mo-45-53 - D=290.3°. It is quite likely, that aside of the interval in time the upper block has turned counterclockwise relative to the lower block. The time for the upper part of the section is estimated from a single curve of the intrusion cooling (Figure 16) and then was increased by 100 kyr.

The excursion of the reverse magnetic polarity registered in the first variant of the record has not been fixed in the given variant of the record in none of the samples in the process of their detailed thermal demagnetization. It will be recalled that the thermal demagnetization step is three degrees that equals to 3-4 thousand years when converted into time. The excursion at such an interval of the thermal demagnetization (three degrees) is simply omitted, whereas less than 100 years pass between adjacent samples of the “short” section (the first variant of the record) near the Curie point.

Table 8. Some magnetic properties and paleomagnetic data. Monchegorsk intrusion

no. M Jnt K Q nt Td D I Td D I

ch46 161 3.61 522 0.74 530-540 126.2 21.9 540-543 280.4 —14.6

ch45 160 7.06 485 2.09 530-543 134.3 19.1 543-552 303.8 —9

ch44 159 12.9 500 3.24 540-546 119.2 24.1 546-555 285.2 —12.7

ch43 158 11.9 415 19.9 500-540 129.5 15.7 540-558 286.8 1.8

ch42 157 54.8 1261 1.6 500-530 159.5 45.1 543-558 299.4 —17.6

ch41 156 86.1 1046 3.35 500-552 132.3 —16.2 561-579 102.6 —22.1

ch40 155 6.06 460 2.54 530-543 129.5 20.1 555-579 87.2 40.5

ch38 153 561 715 44.7 520-561 156.3 29 567-579 165.6 35.4

ch37 152 593 707 48.5 500-549 143 31.9 552-579 150.1 39.6

ch36 151 128 490 35.7 564-579 321.1 —52

ch35 150 166 820 9.93 540-579 268.6 —7.4

ch34 149 22.4 407 80.4? 540-543 140 14.6 549-558 297.1 —13.9

ch32 147 14.07 451 6.94 540-549 117.2 24.4 552-558 310.6 —20.5

ch31 146 6.08 426 5.87 543-549 293.7 —16.4

ch30 145 121 1335 3.25 543-546 145.3 34.2 555-576 306.6 —10.5

ch29 144 20.3 771 1.37 500-530 142.1 26.4 543-564 320.4 —6.3

ch28 142 14 400 100? 500-530 75.8 18.3 530-576 244 —9

ch27 141 60.6 615 7.08 540-543 120.7 19.7 543-576 296.6 —7.2

ch26 140 36.9 540 6.62 520-543 133.3 19.2 543-570 314.6 —9.1

ch25 139 14.7 490 4.1 530-540 124.4 34.1 549-561 297.9 —12.4

ch24 138 0.73 353 100? 530-543 123.7 18.7 543-546 303.7 —10.4

ch23 137 105 790 6.76 530-546 127.4 21.9 549-567 302.6 —8.1

ch22 136 93.3 664 8.9 520-530 104 40.8 540-573 323.7 —3.8

ch21 135 117 723 9.1 530-543 133.9 15.1 549-570 307.3 —6.5

ch20 134 22.8 422 26 520-540 142.4 43.4 540-570 286.4 —6.7

ch19 133 3.04 432 2.39 530-546 161.4 26.5 543-555 284 —13.2

ch18 132 41.4 1387 1.05 540-549 153.9 62.6 552-573 304.6 0.3

ch17b 130.5 316 5030 1.71 540-564 157.3 6.9

ch16 128.5 82.5 640 8.63 520-546 126.7 21.3 552-567 300.6 —19.3

ch15 126.5 186.6 934 8.8 530-546 125.3 23.5 555-567 294.1 —12.5

ch14 124.5 138.1 459 58.8 540-579 153.7 6.5

ch12 120.5 43.8 500-543 112.4 4.8 552-570 292.2 —2.7

ch11 118.5 65.5 690 5.67 520-540 129.4 8 543-570 303.8 0.6

ch10 116.5 120 673 11 520-540 123.9 4.1 546-567 298.7 —2.7

ch9 114.5 87.2 860 4.76 520-546 144.3 10 546-576 275.5 —2.6

ch8 112.5 177.6 4380 1.11 530-540 208.3 46.3 540-567 292.3 —3.4

ch7 111 101 2760 1.08 540-546 130.6 24.8 546-567 293.2 —12.5

ch6a 109 75.6 861 4.12 530-543 130.4 25.6 546-579 292.7 —12

ch5 107.5 61.5 720 4.83 530-543 132.7 30.8 555-573 293.8 —13.6

ch4 105.5 42.2 873 2.24 540-543 121.3 19 543-576 302.5 —7.3

ch3 103.5 21.7 540 3.9 530-552 110.8 4.9 552-567 309 —20.9

ch2a 101.5 25.5 517 5.37 540-549 142.4 18.5 549-567 299.4 —5

ch1 100 98.5 1047 3.82 540-543 151.1 48.8 558-570 279.7 —7.9

Note: no. — sample number; m — thickness in meters; Jnt — NRM value in the process of thermal demagnetization, mA m—1; k — magnetic susceptibility, 10-5 SI units; Qntf = Jnt/k-400; Td — NRM component identification interval; D,I — declination and inclination of the identified component, respectively.

An irregular distribution of the points in time and an appreciable noise is noted in the case of the first variant of the record. Because of it a transition was required to a uniform time step and smoothing of the observation data (Figures 25, 26).

A wavelet analysis has been performed to make a quantitative assessment of the spectral components of the field oscillations. The pattern of variations is complicated

(Figures 25, 26). Rhythms that are stably traced during the record obtained are distinguished, as well as rhythms that change with time and individual splashes (Table 10). In general, the spectrum of rhythms of variations of the geomagnetic field direction covers the periods from 1 thousand years to 30-40 thousand years (Table 10 and Figures 25, 26). In case of a “short” section (Table 10, Figure 25) the most distinct and clear, with the maximum amplitudes

Table 9. Paleomagnetris directions for the high temperature (near Tc NRM component. Monchegorsk intrusion

no. meters Td time D I no. meters Td time D I

mo2 34 570 120 311.6 -17.4 ch1 100 558 134.5 301.1 7.1

mo3 40 570 120.3 307.4 -8.3 ch2A 101.5 551 142.4 300.5 -7.9

mo4 49 570 120.5 336.6 -17.1 101.5 569 123.3 284.5 10.3

mo5 64 570 121 302.6 -6.4 ch3 103.5 556 137 302.3 -14.3

mo6 74 570 121.3 310.4 -10.2 103.5 566 126.2 312.8 -24.8

mo7 83 570 121.6 315.7 -10.1 ch4 105.5 567 125.2 299 -8.8

mo8 92 570 121.8 311.6 8.7 105.5 577 116 278 2.3

mo10 108 570 122.5 329.9 -12.1 105.5 545 149.8 302.6 -6.9

mo11 109 575 117.8 288 -7.7 ch5 107.5 560 132.5 296.1 -16

mo12 109 545 149.8 317 -5.7 107.5 572 120.2 296.8 -11.8

110 555 138 320 1 ch6b 109 562 130.4 298.7 -12.8

mo30 138 560 133.4 318 -2.7 ch6a 109 554 139.2 287.5 -12.8

mo32 148 555 138.8 282.2 -16.5 109 569 123.4 304.2 -1.4

148 575 118.8 270.2 -12.8 ch7 111 566 126.2 299.3 -0.7

mo33 158 560 134 313 -9.6 ch8 112.5 566 126.5 296.1 -4.2

mo34 168 550 145 304.6 -3.1 ch9 114.5 554 139.3 284.2 -2.7

mo35 170 560 134.2 298.9 0.2 ch10 116.5 566 126.4 300.4 1.9

mo37 193 555 140 304.5 -25.8 ch11 118.5 548 146.5 302.5 0.7

mo41 232 570 125.7 292.3 -28.3 ch12 120.5 566 126.5 291.1 -7.8

mo42 246 575 121.2 312.4 -5.2 ch15 126.5 566 126.8 294 -9.3

mo43 257 570 126.2 297.9 -23.3 ch16 128.5 554 139.5 300.1 -19

mo47 302 540 259.7 275.9 -17.3 128.5 566 126.8 287.5 2.5

mo51 356 575 223.6 280.5 -5.1 ch19 133 549 145.5 276.5 -21.7

mo52 367 545 254.6 277.8 -13.4 ch20 134 570 123 289.5 -2.1

367 560 238.3 282.4 -12.6 134 551 143 288.4 -6.1

367 575 224 279.5 -5.3 ch21 135 557 136.5 306.3 -5.6

mo53 375 555 243.7 288.4 -8.8 135 566 127 308.5 -9.1

ch22 136 547 147.8 323.6 -6.6

136 560 133 317.1 0.5

136 569 124.2 336.6 5.4

ch23 137 553 141 303.8 2.4

137 567 125.8 300.5 -4

ch24 138 545 150.2 303.7 -10.4

ch25 139 559 134.5 302.5 -17

ch26 140 560 133.3 315.6 -6.4

140 570 123.3 309.2 -4.2

ch27 141 560 133.2 314.8 -14

141 569 124 301.6 -0.7

ch29 144 570 123 291.1 -14.6

ch30 145 569 124.2 317.1 -8.3

145 566 127.1 305.3 -11.7

ch31 146 548 146.7 293.7 -16.4

ch32 147 557 136.8 310.7 -20.4

ch34 149 554 140 294.2 -11.2

ch35 150 563 130.2 275 -5.6

ch36 151 566 127.2 320.8 -50.5

ch37 152 566 127.5 150.2 34.7

ch38 153 564 129.2 173.6 27.7

ch40 155 551 143.5 312.9 -8.4

ch41 156 564 129.6 102.2 -15.8

ch42 157 557 137 298.6 -18.4

ch43 158 557 137 256.8 6.5

ch44 159 554 140.3 276.9 -0.6

ch45 160 551 143.8 303.8 -3.2

ch46 161 542 154.8 280.4 -14.6

Note: no. — sample number, mo — 375-meter section, ch — 61-meter section; Td — unblocking temperature close to the Curie point, °C; time in kyr converted, from Figures 16 and 17; D,I — NRM declination and inclination at Td.

Figure 22. Variations of the geomagnetic field direction obtained as a result of the component analysis of the NRM thermal demagnetization of samples along the section: a) declination D, b) inclination I, obtained in the 500-540°C range; c) declination D, d) inclination I, obtained in the 540-576° C range. Monchegorsk intrusion.

Figure 23. Behavior of direction of the high-temperature NRM component during the excursion. a - results of the component analysis, record along the section, between 35 m and 52 m; b - record during time interval between 127 kyr and 130 kyr Monchegorsk intrusion.

long-lived declination rhythms are: 30-40, 16, 11.5 and 9 thousand years, the inclination rhythms are: 40-36, 27, 16 and 8-10 thousand years; in case of a “short” section (Table 10, Figure 26) - the declination rhythms are 11-9, 7.6-5.5, and 4-6 thousand years, the inclination rhythms are 14-10, 9-6.5 and 3-4 thousand years.

Attention is attracted by a similarity of the wavelet diagrams in the time interval of 110-150 thousand years in the first and second variant of the record (see Figures 24, 25, 26), that counts in favor of objectivity of the results obtained.

B. Kivakka intrusion. The time distribution of the observation points is nonuniform therefore, the declination and inclination versus time curves were plotted from which values of D, I were obtained with the equal intervals (every 0.5 thousand years) by means of a linear interpolation.

For the first variant of the analysis of behavior of the geomagnetic field direction, directions of the A1 component (inclinations no more than 40° ) are converted into the strati-

graphic system of coordinates and directions of the A2 component (inclinations more than 40°) are retained in the geographic system of coordinates (Table 12). Such simplification of the situation (an momentary tilting of a body) caused scattering of data. As has been stated above, the wide intervals in the section of the intrusion were remagnetized in the direction of component B, and thus, the pattern has large interruptions (Figure 14a,b). Despite the interruptions it is evident that during cooling of the intrusion the geomagnetic field had predominantly one reverse polarity. The declination and inclination at a temperature which is standing no more than 3° from Tc are written out in the samples where it becomes possible to detect the A component (Table 13). A relative time of acquisition of component A has been determined from temperature and position in the section according to Figure 18. Thus, behavior of the paleomagnetic direction over the section in time was obtained (Figure 14c,d). A great scatter of data should be noted that, first of all, is as-

Table 10. Wavelet analysis results. Monhegorsk intrustion

A. First variant of record. The 60 m section, sampling every 1-2 m (ch). Figure 24

D, I Rhythm “length” Standard Note

D and I ~15? 115-145 Unclear and weak

I 9.1-7.6 115-155 ±1.3 Intensive, clear

I 5.6? 115-125? Weak

I 3-3.4-3 115-147 ±1.3 Chain of splashes or a single rhythm with varying amplitude

I 1.9 144-147 ±1.1 Splash

D 8-8.3-7.2 115-155 ±1.2 Intensive, clear

D 4.5 115-125 ±1.1 Splash

D 3.3-2.5 124-145 ±1.1 Chain of splashes or a single rhythm, decreasing with time

D 2 146 ±1.1 Splash

D 1.7 125 ±1 Splash

D 1 143 and 147 Splashes

B. Second variant of record. The 375 m section, sampling every 10 m (mo). Figure 25

D 30-40 135-260 ±1.2 Intensive

D 20 140-180? ±1.2 ?

D 15.8 195-230 ±1.2 Intensive

D 11.5 160-190? ±1.2 Intensive

D 9.1 180?-230 ±1.1 Nonuniform amplitude

D 8.1 140 ±1.1 Weak splash

D 5.2 180-190 ±1.1 Splash

D 4.5 160 ±1 Weak splash

D 2.7 116 ±1 Weak splash

I 40-36 155-200 ? Weak

I 26.9 190-260 ±1.5 Intensive

I 15.8 140-245 ±1.2 Intensive

I 10-9-8-10 125-270 ±1.2 Intensive, amplitude varies

I 7.2 135 ±1.1 Splash

I 5.4 155-170 ±1.1 Intensive splash

I 5.4 220-240? ±1.3 Most likely two splashes

I 4.7 255 ±1.1 Splash

I 4.5 196 ±1 Weak splash

I 4.4 117 ±1.1 Splash

I 3.7 139 and 239 ±1.1 Splashes

C. Second variant of record . The 61 m section, sampling every 1-2 m (ch). Figure 26

D 18 115-150? Runs into intensive rhythm 11-9

D 11.2-9 115?-185 ±1.3 Intensive, fades away in the middle of the interval

D 7.6 115?-165? Weak

D 7.6-5.5 115-145 ±1.2 Intensive

D 4-5.8 160-185 ±1.2 Intensive

D 5-2.4 115-130 ±1.1 Chain

D 2.8 130-145 ±1.1

D 3.7 182 ±1.2 Intensive splash

D 2.4 170 ±1.1 Intensive splash

D 2.1 181 ±1.1 Intensive splash

D 1.7 141 Weak splash

D 1.5 161 ±1 Splash

D 1.3 135 ±1 Splash

D 1 141 and 155 Splashes

I 14-10 130-180 ±1.2 Amplitude increases sharply with time

I 9.1-7.2-6.5 125-185 ±1.2 Rhythm fades away in the middle of the interval, then, amplitude increases sharply

Iga (c)

120 125

130 135 140

Time, kyr

145 150

n i34 1.0-

-30

-26 0.8-

-22

-18 0.6-

-14

-10 0.4-

-6

. 0.2-

-2

o.o-

120 125

130 135 140

Time, kyr

145 150

Figure 24. The first variant of recording the behavior of the geomagnetic field declination and inclination. Monchegorsk intrusion; 61m section, sampling every 1-2 m (ch). Points - initial data; line - averaging result (H=0.25) and transition to a uniform step in time (At = 0.5); thick blue line - assessment of the adaptive trend with the help of a local-polynomial smoothing by the 3rd order polynomial in a running window of a radius of 20 counts (10 kyr). Time row D, b) result of subtraction of the trend (D), c) wavelet diagram of the remainder after subtraction of the trend (D); d) time row I; e) result of subtraction of the trend (I); f) wavelet diagram of the remainder after subtraction of the trend (I). Lga - logarithm of the time scale (period).

Table 10. Continued

C. Second variant of record. The 61 m section, sampling every 1-2 m (ch). Figure 26

D, I Rhythm “length” Standard Note

I 6.5 180 ±1.2 Intensive splash

I 3.8-4.7-4.3-3.2 126-185 ±1.2 Rhythm fades away in the middle of the interval, then, amplitude increases sharply

I 2.8-3.3 150-160 ±1

I 2.8 and 2.7 126 and 166 ±1 Splashes

I 2 157 and 174 ±1 Splashes

I 1.7 127 ±1 Splash

Note: D, I — paleomagnetic declination and inclination accordingly; rhythm — the limiting values are given, if the rhythm changes with time; standard — standard deviation of rhythm (±); “length” — duration of oscillation of the given rhythm. All data are given in kyr.

Figure 25. The second variant of recording the behavior of the geomagnetic field declination (a-c) and inclination (d-f). Monchegorsk intrusion; 375 m section, sampling every 10 m (mo). Symbols are the same as in Figure 24, radius of smoothing - 40 counts (20 kyr).

sociated with impossibility to divide completely components Ai and A2, in the process of the thermal demagnetization, and inaccuracy in assessment of the Curie points and, consequently, of the time of acquisition of components A1 and A2 had a certain effect. As a result of averaging data in the interval of 74-82 thousand years (Figure 14e,f) rhythmic oscillations of D and I are seen in the antiphase, and the rhythm is equal to 2 thousand years.

The second variant of the record we shall examine only for those samples, where components A1 and A2 are separated from B most reliably as a result of the detailed thermal demagnetization (Figure 27, Table 12). The pattern of variation of the field direction is “smearing” especially in the case of inclination because of a tilt of the intrusion. Therefore, and also because of the short time series of every sample we shall examine behavior of declination and inclination averaged from records for 9 samples (Figures 28a, 29a). After extraction of the trend it is evident that variations exist on its background, where rhythm is observed even visually (Figures 28b, 29b). Oscillations of D and I

in the left part of the record are synphasal, amplitudes are different, the rhythm is equal to approximately 10 thousand years; oscillations of D and I in the right part of the record are antiphase and their rhythm “extends” up to 12-20 kyr. The complexity of the situation is reflected in a complicated process of oscillations of the field vector: the DI diagram (Figure 30) shows that the vector performs complicated oscillations, forming frequently incomplete loops of 2.5-9 thousand years, the vector in the loops rotates anticlockwise and counterclockwise (Figure 30). The loops with the rhythm of ~10 kyr are seen best of all.

An appreciable change of the rhythm pattern and of D, and I falls on the period of 85 thousand years from the moment of emplacement of intrusion, a sharp deflection of the vector occurs during this interval (Figure 30). This is, evidently, associated with the beginning of tilting of the intrusion which disturbed cooling condition of the body. A pattern of the trend for the tilting counts in favor of this association: just from 85 thousand years there is a linear increase of I from values typical for the A1 component (in the

Figure 26. The second variant of recording the behavior of the geomagnetic field declination (a-c) and inclination (d-f). Monchegorsk intrusion; 61 m section, sampling every 1-2 m (ch). Symbols are the same as in Figure 24, radius of smoothing - 40 counts (20 kyr).

geographic system of coordinates (Table 6)) to values typical for A2 to 105-100 thousand years (Figure 29a, Table 6). Thus, it is possible to assume that this linear trend had fixed the process of tilting of the intrusion that lasted for 20-25 thousand years.

A wavelet analysis has been performed to make a quantitative assessment of the spectrum of the field oscillations. The pattern of variations is complicated (Figure 31). In general, the spectrum of rhythms of variations of the geomagnetic field direction covers the period from 1 thousand years to 17 thousand years (Table 14 and Figure 31). Out of these the most distinct and bright, with the maximum amplitudes declination rhythms are 15-17, 10.5-9.5 and 5-6 thousand years (Figure 31a,b), the inclination rhythms are 12, 8-9 and 5 thousand years (Figure 31c,d). It is quite natural that with a greater smoothing window the short rhythms disappear partially or completely.

C. Bushveld intrusion. To analyze behavior of the geomagnetic field in the process of cooling of the Bushveld in-

trusion, meters of the well WP-16 section are converted into time (Table 15). In the first variant we have examined three sections of the T-Time diagram (Figure 20), corresponding to three NRM components of normal and reverse polarity: 1) a sharp drop of magnetization between 553°C and 556°C; the paleomagnetic direction has been determined from the difference vector between 553°C and 556°C; the inclinations obtained in this way are given in Tables 4 and 15; 2) a sharp drop of magnetization near 580°C, this is the Curie point of magnetite, the paleomagnetic direction has been determined from the difference vector between 577° and 580° (Table 15); 3) temperature of detecting the reverse polarity component in a vector analysis of the detailed thermal demagnetization covers more frequently a 520-540° C interval (Tables 4, 15). The temperature of 530°C has been adopted conventionally, when the end of the record of “running” of temperature 556°C is slightly overlapped and when negative inclinations appear with the beginning of the record of the negative polarity component (Tables 4, 15). As a result a

Table 11. Paleomagnetic data of the Monchegorsk intrusion. The second variant of record

Table 11. Continued

no. time D I no. time D I

mo1 160 292.7 -3.2 ch3 184 313.7 -27.1

150 292.8 -4 176 312.5 -25.3

141 292.6 -4.2 168 312.4 -25

132 292.6 -4.1 162 312.5 -24.1

123 292.8 -4.2 156 311.7 -24.4

114 308.8 -6.6 151 315 -27.6

mo2 160 309.6 -13.6 143 314.6 -28.4

150 311.6 -15.1 137 312.2 -29

141 310.9 -15.3 132 317.8 -30.5

132 310.9 -15.6 126 342 -48.8

123 311.6 -17.4 ch4 180 302.6 -6.9

114 321 -19 172 302.4 -7.9

110 319.9 -19.5 166 301.9 -8.1

mo3 161 304.2 -4.1 160 303.2 -7.9

151 304.6 -5.1 153 302.2 -8

142 307.7 -6.7 147 302.4 -7.5

133 307.2 -6.9 140 301.4 -7.7

124 307.4 -8.3 134 303.2 -9

122 304.4 -3.8 129 309.1 -9.1

115 304 -6.4 125 301.1 -0.5

mo4 162 337.3 -15 120 301.7 -0.7

154 334 -15.7 116 294.7 -11.8

144 330.1 -16.6 ch5 164.5 286.8 -5.2

134 328.8 -17.6 159.5 293.2 -13.5

126 336.6 -17.1 154 294.5 -14.2

mo5R 183 137 14.5 148.5 295.1 -14.4

171 136.4 14.1 142.5 293.1 -13

159 138 12.9 137.5 294.5 -13.2

mo5N 150 297 -6.7 132.5 290.6 -5.6

142 300.6 -7.5 134.5 292.6 -15.4

134 302.6 -7.5 130.5 293.6 -9.9

127 303.1 -7.1 7 126 293.5 -8.1

120 302.6 -6.4 ch6a 164 291.9 -11.7

mo6 160 305.6 -8.6 158.5 292.9 -11.5

150 307.9 -9.1 151 292.7 -11.9

141 310.2 -9.9 146 291.5 -11.1

132 309.8 -10.1 140 290.3 -9.2

123 310.4 -10.2 136 291.2 -7.9

114 319 -15.1 140 293.6 -9.6

110 316.7 -13.8 134.5 288.8 -8.6

mo7 162 312.3 9.9 128 290.9 -4.3

152 313.7 -2.4 ch7 166.5 299.1 -0.3

142 315.3 -10.1 158.5 297.4 1.3

133 315.2 -10.8 151 297.4 -0.1

125 315.7 -10.1 144 299.6 0.3

117 322.4 -2.7 139 299.7 -1.9

112 325.1 -5.8 125 300.1 0.4

mo8? 162 308.2 -3.3 130 300 0.6

152 309.2 -6.8 ch8 174 289.7 -2.5

142 310.4 -2.5 168 295.6 -2.7

134 309.8 -2.4 161 292.3 -3.9

125 311.6 8.7 157 293.3 -3.2

mo10R 197 107 15.4 152 296.4 -3.6

184 107.6 15 146 294.4 -3

172 108.1 15 140 291.6 -2.3

161 109.7 134.5 292.6 -3.5

no. time D I no. time D I

mo10N 153 315.4 -6.5 130.5 296.1 -4.2

142 319 -9.2 ch9 169.5 263.4 -1.6

133 308.9 -21 161.5 264.6 -1.5

125 329.9 -12.1 155 270.3 -2.3

mo11 184 98.3 11.5 148.5 274 -2.6

172 98.7 12.1 143.5 274.4 -2.2

161 101.1 12.4 ch10 174 298.9 -3.2

149 322.9 -1.1 167 300.3 -1.6

153 323.1 -3.5 160 298.7 -3.6

142 317 -9.5 155 299.7 -2

132 311.2 3.2 148.5 299.5 -1.4

125 279.4 -16.7 144 297.7 -2.1

118 287.7 -7.7 139.5 299.8 -1.5

113 275.4 -6.6 134 301.1 -3.6

mo12 161 313.9 -8.2 129.5 300.6 2.4

149 317.1 -5.7 ch12 174 290.7 -3.5

149 315.6 -6 167 290.9 -3.9

141 319.7 1 160 291.5 -4.8

132 361.6 5.7 155 292.6 -3.9

mo30 167 319.9 1.1 149 291.8 -3.2

158 319.1 0.3 144 295.2 -5.2

148 318.4 -3 139.5 294 -4.9

138 318 -2.7 134 295.1 -11.5

128 313.7 1.4 129.5 292.4 -15.2

126 317 0.2 ch15 167 291 -9.2

120 318.1 1.7 160 293.8 -11.4

mo32 149 280.1 -16 155 293.6 -11

153 280.5 -16.4 149 293.6 -10.9

143 282.2 -16.5 144 294.6 -12.6

134 275.6 -14.8 139.5 293.6 -12.1

mo33R 220.5 146.8 27 134 294.7 -10.9

204 146.3 26.6 130 293.8 -8.9

191 146.9 25.4 126.5 292.5 3.4

mo33N 177.5 261.7 34.1 ch16 158.5 298.1 -12.2

165.5 312.6 -12.5 152.5 298.7 -12.4

153.5 312.5 -11.7 147.5 300 -13.2

144 313.1 -12.7 144 299.9 -18.2

134 313 -9.6 138.5 297.3 -6.9

mo37R 220 151.3 26.4 ch19 162 280.9 -17.2

205 151.5 27.8 156 279.3 -18

190 163.5 19.7 150 276.5 -21.7

mo41 165 296.1 -19.7 ch20 174 286.6 -3.4

155 303.3 -15 167 287 -5.6

145 292.1 -18.2 160 274.8 -5.1

137 285.5 -24.5 155 287.5 -6.1

128 292.3 -28.3 148.5 289.3 -5.4

mo42 155 323.5 -2.7 143 289 -5.2

146 326.6 -10.5 137.5 290.5 -7

137 325.9 -13.5 132 289.7 -2.8

129 320.8 -14.6 134 285.9 -5.3

123 312.4 -5.2 128.5 288.2 -3

117 314.3 -4.2 124.5 120 289.5 293 -2.1 4.3

fault, gap 117 284.5 -0.3

~100 kyr ch21 171 304.3 -0.2

162 304.5 -2.4

Table 11. Continued

no. time D I no. time D I

mo45 260 275.9 -17.3 157.5 308 -6.7

264 273.8 -25.4 152 306.9 -6.9

255 278.3 -16.5 146.5 307 -6.8

247 275.4 -15.9 141 307.2 -6.7

237 273.3 -22.3 137.5 308.5 -8.3

mo47 279 266 2.4 133 306.8 -8.1

268 263.9 1.8 129 307.4 -8.9

257 275.6 -4.9 128.5 296.6 -6.7

248 283.2 -7.6 124.5 293.3 -8

240 276.8 -1.4 120 266.2 -4.9

232 278.3 -2.1 ch22 182 327.1 -1.7

224 280.5 -5.1 174 326.5 -2.6

mo51 270 277.8 -13.4 166.5 329 -0.4

261 279.7 -14.3 159.5 326.6 -1.6

251 282.1 -16.9 152.5 326.1 -1.9

243 282.4 -12.6 147.5 325.8 -2

233 275.3 -1.9 143 330.7 2.1

232 269.3 -3.4 137 339 4.1

226 279.5 -5.3 131 338.3 4.8

mo52 267 290.7 -13.6 126.5 338.5 6.4

257 289.6 -7.5 ch23 167 293.9 4.5

247 288.4 -8.8 160.5 299.9 -3.5

238 281.6 -1 154 301.5 -5.8

mo53 267 290.7 -13.6 148 302.7 -7.3

257 289.6 -7.5 142 302.3 -6.7

247 288.4 -8.8 146.5 303.7 -7.2

238 281.6 -1 141 303.7 -7.4

135.5 302.4 -2.2

130.5 302.5 -2.7

ch25 174 298.9 -11.5

167 298.7 -11.4

160 298.4 -11.6

154.5 298 -12.2

149 298.5 -12.6

143 299.5 -11.7

137.5 302.5 -17

ch26 167 314.7 -7.9

160 309.2 -2.1

154.5 310.7 -3.5

149 313.1 -3.8

143 312.1 -3.5

137.5 312.1 -4.1

132 309 -2.1

ch27 167 296.4 -6.1

160 297.9 -6.1

154.5 297.5 -7.2

149 296.3 -5.7

143 298.9 -7.4

137.5 303.2 -4.3

132 298.3 0

139.5 294.4 -4.2

131 301.6 0.5

ch29 181.5 312.3 -4.9

175 311.2 -5.7

168.5 311 -5.8

162 311.2 -3.3

Table 11. Continued

no. time D I no. time D I

155.5 313.4 -6

150 312.1 -5.4

145 308.6 -4.1

139.5 306.9 -6.8

134 310.9 -4

128.5 296.7 -8.7

ch30 174.5 298.5 4

167.5 306.6 -9.3

160.5 305 -8.8

154 306.6 -7.5

147.5 306 -9.5

142 307.1 -9.8

137 304.6 -10

131 305.4 -10

126.5 306 1.9

165 296.4 -13.6

ch34 158.5 296.5 -12.4

152 296.6 -13.4

146.5 296.4 -13.5

141.5 294.2 -11.2

ch35 185 277.1 -8

181 275.4 -8

167.5 275.9 -6.4

160 276.8 -8.4

152.5 277.3 -8.6

146.5 277.8 -9.2

140.5 272.6 -6.3

135 272.4 -4.3

130 264 0

124.5 262.8 -4.3

120.5 258 -0.2

ch42 172 295 -13.2

165 295.7 -13.9

158.5 295.8 -14.5

153 296.2 -14

147.5 294.1 -14.2

141.5 292 -11.6

ch45 166.5 300.5 -5.5

160.5 301.7 -6

154.5 302.6 -5.4

149 301.3 -5.1

Note: Symbols are the same as in Table 9.

record of variations for ~270 thousand years was obtained (Figure 32). If to convert the reverse polarity distinguishing temperature component Td = 520-540° C, into blocking temperatures Tb = 480-515° C then the gap between the end of record of the 556-degree component and beginning of the 530-degree component will be 100 thousand years according to Figure 20. Absence of the reverse polarity at the thermal demagnetization above 540°C, used in the second variant of the record indicates that that this gap exceeds 300 thousand years. An artificial removal of this gap does not change the pattern of the spectrum of variations.

Sign of the reverse geomagnetic polarity interval has been

Table 12. Selected samples of paleomagnetic record of the Kivakka intrusion

no. m Tc Comp. Interval Td Interval Tb Time, kyr

2pk19 344.6 560 Ai 560-532 560-521 76.9-87.9

17b27 563 580 Ai 580-537 580-516 73.9-89.6

41b31 750 572 Ai 572-530 572-509 76.8-92.7

48pc25 769 580, 562 Ai 577-520 577-500 75.7-95

51v38 806 585, 570 A2+A1 585-530 585-510 74.2-92.4

55t52 912 575 A2 575-537 575-519 76.9-90

66t48 1158 562 A2 562-530 562-516 79.6-91.4

68t58 1413 565 A2 565-505 565-470 78.9-111

70t61 1650 562 A2 562-500 562-464 78.5-109.4

Note: no. — sample number; m — sampling level in the section in meters from the lower contact; Tc — Curie point of the essential mineral, carrier of the magnetic record; Comp. — NRM component (see text) from which the paleomagnetic record is fixed; interval Td of paleomagnetic record; interval Tb of paleomagnetic record; time — time of cooling of the intrusion in the mentioned interval Tb (according to Figure 18).

changed for a more convenient statistic analysis of the spectrum of variations of the geomagnetic field (Figure 33a). The time marks form a very irregular net of values (Tables 4, 15). The values D, I are characterized by a high noise, which

Table 13. Paleomagnetic data of the Kivakka intrusion

no. m Tc t D I Comp.

2 344.6 560 76.9 140.1 40.3 Ai

3 358.9 560 76 143.4 63.6 A2

11 423.6 555 78.4 156.7 42.3 A2

13 523.1 560 78 139.9 56.7 A2

14 563.3 560 78.2 127 45.9 Ai

15 563.3 555 79.5 120.3 57.9 A2

17 563.4 580 73.9 137.9 63.4 Ai

25 636.8 560 79 122.7 75.1 A2

26 637.9 560 78.5 127.5 68.4 A2

28 640.6 570 76.6 116.7 77.7 A2

29 641.4 570 76.6 137.1 72.6 A2

41 750.4 572 76.8 144 62.7 Ai

44 760.3 550 81.7 123.5 50.6 A2

46 760 515 91 164.8 56.2 A2

46 760 530 87 127.5 50.4 A2

47 767.5 555 80.6 140.6 49.4 Ai

48 769.2 560 79.3 148.3 43.4 Ai

48 769.2 585 74.1 168.7 45.4 Ai

49 771 555 80.8 117.4 54.9 A2

51 806 585 74.2 140.3 61.5 Ai

51 806 570 78.3 164.9 60.7 Ai

55 912 570 76.9 125.8 74.7 Ai

57 971.3 555 81.3 173.1 75.7 A2

59 978.5 550 82.4 141.8 56.2 A2

66 1157.6 555 79.6 153.9 73.9 Ai

67 1279 580 76 110 52.2 Ai

68 1413 560 79.5 127.6 46.1 A2

70 1650.2 560 78.6 136.9 41.7 A2

70 1650.2 560 78.6 160.9 75.7 Ai

Note: no. — sample serial number; m — distance (thickness) from the lower contact in meters; Tc — Curie point, °C; t — time from the moment of emplacement of intrusion in kyr; D and I — declination and inclination of the NRM component; Comp. — NRM component.

can be judged about by a large amplitude of their variation in the interval of the time mark closeness (Figures 32, 33). Therefore, we have made smoothing and a transit to a uniform time step. The value of 0.5 thousand years has been selected as a step as this value is most typical for an increment. The basic (nonuniform) data about each time point were averaged. A nuclear averaging method was employed to obtain values of the uniformly numbered signal (see above).

The Figures 32 and 33b show a smooth increase of the amplitude of the inclination variations during the recording. A sharp increase of the amplitude, distinguished on the wavelet diagram (Figure 34) is fixed at the end of the normal polarity interval; the reverse polarity interval begins with an appreciable decrease of the amplitude, and further a smooth increase of the amplitude of the inclination variations con-

Table 14. Wavelet analysis results. Kivakka

1—1 D, Rhythm Standard “length” Note

D 15-17 1 35 uniform

D 10.5-9.5 1 >40

D 5-6 1 >40

D 4 1 10

D 3 splash

D 2.5 splash

D 2.2 splash

D 1 splash

I 12 1 >40

I 8-9 1 >40 Overlapped by splash

I 5 1.5 20

I 5.5 1 >10

I 2.5 splash

I 1.3 splash

I 1? splash

Note: D, I — paleomagnetic declination and inclination ac-

cordingly; Rhythm — limiting values are given, if the rhythm varies with time; Standard — standard deviation of rhythm (±); “length” — duration of oscillation of the given rhythm. All data are in kyr.

Figure 27. Kivakka. Variations of declination and inclination of components A1 and A2 in each sample versus cooling time. Left column - declination, right column - inclination. Numbers in circles - sample numbers, numbers in rectangles - distances from the lower contact vertically in meters.

tinues. The rhythm of oscillations inside the observed strong splash is 7.6±1 thousand years and similar splashes (6) with a similar rhythm but less intensive amplitude are seen almost on the entire interval of the record (Table 16, Figure 34).

In general, a series of oscillations of inclination with the rhythms from 68 to 6.5 thousand years is seen from the data of the wavelet analysis (Table 16, Figure 34). All of them are either not stable or their duration is short (different rhythms have the number of oscillations ranging from 1-2 to 7-9), or the rhythm value changes in time, more frequently it increases with time (Table 16, Figure 34) or oscillations form the chains of short and rapidly fading and appearing again

splashes (for example oscillations with the rhythms of б.5-7.5 and 2.5-4.5 thousand years. Distances between splashes with a close rhythm are uneven (Figure 34).

It will be recalled that the paleomagnetic direction in each case was determined with the use of a three-degrees interval to the Curie point; such temperature interval covers a period of about 10 thousand years. Consequently, with the linear magnetization - temperature and time relationship we obtain a smoothed direction of the field for 10 thousand years. This can be slightly less if the multidomain grains are present in this interval (then the drop of magnetization comes closer to the Curie point). The most reliable are rhythms of no

Figure 28. Kivakka. Charts of time series of the mean values of declination D (on 9 samples). The second variant of recording. a - initial data (black line) and averaging by the 3rd order local polynomial in a running window of a radius of 20 counts (10 kyr); b - initial data (black line) and averaging by the 3rd order local polynomial in a running window of a radius of 10 counts (5 kyr); c, and d - difference between initial data and a smoothed curve.

Figure 29. Kivakka. Charts of time rows of the mean values of inclination I (on 9 samples). The second variant of recording. a - initial data (black line)and averaging by the 3rd order local polynomial in a running window of a radius of 20 counts (10 kyr); c - initial data (black line) and averaging by the 3rd order local polynomial in a running window of a radius of 10 counts (5 kyr); c, and d - difference between initial data and a smoothed curve.

Table 15. Paleomagnetic data of the Bushveld intrusion

m Td(R) I530 Time, kyr I556 Time, kyr I580 Time, kyr

30.81 450-520 -29.4 167.85 77.8 83.4 78.1 3.08

32.73 168 55 83.56 74.2 3.27

34.36 77 83.74 78.9 3.44

35.96 200-520 -48.9 168.5 75.5 83.91 75.9 3.6

40.34 73.2 84.39 71.8 4.03

44.8 400-520 -49.8 169.6 78.5 84.87 79.6 4.48

46.28 80.5 85.03 80.7 4.63

53.6 85 85.83 83.4 5.36

59.58 87.6 86.48 5.96

62.22

65.4 70.6 87.11 74.4 6.54

68.64 64 87.46 70.2 6.86

74.63 62.3 88.12 53.5 7.46

80.51 64.8 88.76 57.6 8.05

87.94 400-520 -51.7 175 76.7 89.56 84.6 8.79

93.93 500-520 -39.3 165.74 75.3 90.16 69.6 9.39

101.38 500-540 -45.1 176.67 73.6 91.03 59.9 10.14

107.22 500-540 -53.8 177.4 71.5 91.66 68.8 10.72

117.39 178.67 68.1 92.77 78.6 11.74

132.35 53.8 94.39 64 13.24

136.62 450-520 -76.6 181.08 54.7 94.86 70.1 13.66

147.08 350-450 -44.5 182.38 57.3 96 62.5 14.71

151.55 500-540 -65.1 182.94 51.1 96.48 73.4 15.16

160.39 350-450 -46.1 184.05 60.9 97.44 72.6 16.04

167.48 450-520 -74.7 184.94 62.6 98.21 57.7 16.75

169.43 72 98.42 71.5 16.94

171.14 450-540 -52.6 185.39 57.4 98.61 66.5 17.11

171.72 500-540 -43.5 185.47 74.7 98.67 76.1 17.17

174.7 450-520 -47.3 185.84 68.5 99 66.2 17.47

176.57 200-540 -57.8 186.07 75.2 99.2 70.5 17.66

187.91 500-540 -63.8 187.5 65.2 100.44 74.5 18.79

190.85 500-520 -40.9 187.86 69.8 100.75

195.3 450-540 -54.7 188.41 57.5 101.24 63.4 19.53

198.15 450-540 -36.9 188.78 68.2 101.55 75.7 19.82

206.82 450-540 -37.4 189.85 62.3 102.49 77.9 20.68

213.28 350-540 -71.8 190.66 56.1 103.19 55.4 21.33

221.87 500-520 -37.2 191.73 67.8 104.13 22.19

229.34 500-540 -40.3 192.67 60 104.94 84.6 22.93

236.86 450-520 -37.8 193.6 61.8 105.76 70.7 23.69

241.32 500-520 -35.8 194.17 60.7 106.24 73.4 24.13

251.77 500-540 -76.8 195.47 55.1 107.38 64.5 25.18

255.64 520-540 -38.3 195.96 53.4 107.8 62.7 25.56

265.32 450-520 -59.2 197.17 45.3 108.85 85.2 26.53

272.77 450-540 -68 198.1 50.3 109.66 68.2 27.28

280.24 450-520 -47.9 199.03 61.1 110.48 81 28

286.24 450-520 -45.5 199.78 54 111.13 66.7 28.63

295.2 500-540 -50.4 200.9 60.6 112.1 72.7 29.52

302.31 500-540 -53.3 201.79 74.9 112.88 75.8 30.23

309.76 500-540 -50.3 202.72 65.6 113.69 76.8 30.98

317.23 450-520 -75 203.65 72 114.5 76.6 31.72

324.49 450-520 -49.4 204.56 65.2 115.29 81.5 32.45

328.56 450-520 -47.8 205.07 72.5 115.73 77.2 32.86

335.15 400-540 -32.2 205.89 116.45 70.3 33.52

340.32 450-540 -30 206.54 72 117.01 81 34.03

m Td(R) I530 Time, kyr I556 Time, kyr I580 Time, kyr

344.86 500-520 -28.6 207.11 76.6 117.5 87.6 34.49

356.13 400-540 -36.5 208.52 58.1 118.73 74.5 35.61

359.82 450-540 -38.2 208.98 76.6 119.13 72.4 36

368.75 450-540 -53.2 210.09 76.1 120.1 72.7 36.9

372.65 300-520 -64.8 210.58 51.6 120.53 74.6 37.27

376.23 300-500 -38.4 211.29 50.4 120.92 79 37.62

376.79 48.2 120.98 62 37.68

385.17 520-540 -49.1 212.15 54.5 121.89 72.5 38.52

393.66 350-520 -76 213.21 66 122.81 61.2 39.37

395.14 450-540 -39.5 213.39 53.9 122.97 39.51

398.75 500-520 -38.9 213.84 82.3 123.36 75.4 39.88

404.56 500-520 -46.9 214.57 74.3 124 77.8 40.45

409 450-520 -36.2 215.13 80.7 124.48 84.4 40.9

414.58 200-500 -65.1 215.82 54.2 125.09 57.4 41.46

418.76 500-540 -38.6 216.35 66.2 125.54 78.1 41.88

429.61 500-520 -38.1 217.7 72.8 126.72 77.5 42.95

435.73 500-540 -54.6 218.47 75.1 127.39 77.2 43.58

444.51 200-450 -51.6 219.56 63.4 128.34 60.3 44.47

448.85 350-540 -36.4 220.11 40.1 128.8 70.7 44.88

449.92 400-520 -35.7 220.24 53.6 128.93 55.4 45

453.91 400-520 -46.3 220.74 54.5 129.36 58 45.39

455.3 350-540 -42.6 220.91 67.1 129.51 74.9 45.53

455.78 500-540 -21.3 220.97 58.9 129.57 69.9 45.57

456.17 500-540 -46.8 221.02 56.4 129.61 52.6 45.62

457.74 300-540 -45.2 221.22 58.3 129.78 70.1 45.77

463.06 500-520 -35 221.88 57.9 130.36 87.4 46.31

465.32 500-540 -32.5 222.16 52.6 130.6 45.4 46.53

474.22 350-540 -43.4 223.28 59.9 131.57 64.9 47.42

475.47 32 131.71 39 47.55

483.33 520-540 -68.9 224.4 64 132.56 65.9 48.33

489.29 400-540 -21.3 225.16 61.2 133.21 72.6 48.93

491.56 500-540 -038 225.45 61.8 133.46 65.3 49.16

493.18 50.3 133.63 53.4 49.32

507.84 200-350 -68 227.48 78 135.23 63.6 50.78

511.65 450-540 -58.3 227.96 52.8 135.64 61.7 51.19

520.41 77.9 136.57 79 52.04

523.1 500-520 -23.1 229.39 53.2 136.89 48 52.31

529.29 67.8 137.56 75.7 52.93

533.96 20-540 -81 230.75

535.14 69.4 138.2 78.9 53.51

537.16 71.2 138.42 75.6 53.71

539.07 500-540 -31.3 231.38 43.6 138.62 45 53.91

542.65

548.7 200-520 -71.4 231.59 69.8 139.67 75.1 54.87

550.14 500-540 -47.4 232.77 80.8 139.83 79.4 55.01

554.56 66.1 140.3 81.3 55.46

556.34 500-540 -47.5 233.54 73.8 140.5 55.64

563.5 234.44 64 141.28 63.6 56.35

567.4 500-540 -57.5 234.93 58.1 141.7 69 56.74

572.19 200-520 -35.5 235.52 71.4 142.23 76.1 57.22

576.78 500-520 -20.2 236.1 77.7 142.72 78.1 57.66

582.65 78.8 143.36 63.1 58.26

587.84 200-520 -34.2 237.48 68.1 143.93 64.4 58.78

592.2

m Td (R) I530 Time, kyr I556 Time, kyr I580 Time, kyr

596.58 68.8 144.88 65.6 59.67

601.1 80.4 145.37 75 60.13

606.33 69.5 145.94 75.1 60.63

611.76 49.4 61.17

616.6 300-520 -49.6 241.08 42.9 147.06 67.6 61.66

621.38 520-540 -63.4 241.67 44.6 147.58 59 62.14

625.69 62.8 148.04 67.4 62.57

627.75 520-540 -34.9 242.47 51.1 148.27

632.68 500-540 -46 243.09 54.4 148.8 63.27

643.08 71.9 149.93 77.1 64.31

645.52 200-300 -56 244.69 70.5 150.2 76.4 64.55

647.75 200-400 -54.8 244.97 64 150.44 65.5 64.78

648.6

649.1 500-? -35.5 245.14

650.95 520-555 -14 245.37 67.8 150.79 68.3 65.1

653.05 200-300 -62.5 245.63 78.3 151.02 78.6 65.3

657.46 450-540 -32 246.18 40.5 151.5 66.2 65.75

659.3 67.9 151.7 72.2 65.93

661.39 500-540 -67.2 246.67 59.7 151.93 62.3 66.13

664.6 350-520 -50 247.08

670.23 400-540 -66.5 247.78

675.72 500-540 -35 248.47

680.92 450-550 -47.8 249.12 -51.8 154.05

683.28 500-540 -55 249.41 51.3 154.31

690.22 300-500 -61.1 250.28

692.78 500-540 -45 250.6

702.39 200-350 -58.8 251.8

711.7 500-540 -47.2 252.96

718.17 520-540 -47.4 253.77 78.7 71.82

721.16 450-540 -55.7 254.15

726.56 520-565 -60.9 254.82 62.9 72.66

730.34 520-540 -76.1 255.29 78.6 159.42 62 73.03

734.96 200-350 -61.8 255.87

739.51 200-300 -85.5 256.48

745.15 400-540 -39 257.14 55.6 161.04 69.8 74.52

749.43 200-400 -67.2 257.68 59.8 161.5 70.5 74.94

755.21 500-555 -35.3 258.4 57.7 75.52

759.79 500-540 -51.6 258.97 26.4 162.63 69.7 75.98

764.66 520-555 -67.9 259.58 59.7 76.47

773.31 200-540 -65.1 260.66 52.4 164.1 63.3 77.33

779.6 520-555 -37.7 261.45 51.9 164.78 77.96

783.8 500-540 -45.2 261.98 53.1 165.24 61 78.38

788.09 350-540 -29.1 262.51 53.5 165.7 72.5 78.81

794.54 263.32 54.8 166.41 58.8 79.45

802.42 -38.1 167.27

803.45 520-540 82 264.43 -66.3 167.38

809.39 400-500 -72 168.02

814.15 450-520 -85 265.77 -41.4 168.54

Note: m — sampling level in meters; Td(R) — temperature of detecting the intermediate-temperature reverse polarity NRM component, the component analysis result; /530 — inclination of the intermediate-temperature NRM component; /556 and /580 — inclinations of the high-temperature NRM components close to their Curie points.

Table 16. Bushveld intrusion. First variant of record

Rhythm, kyr Standard Length, kyr Number of rhythms Note

56-67 ±7.8-5.8 10-270 4-5

29.5-40 ±3-4.6 10-270 7-9

20-25 ±2 120-220 5

16 ±2.8 60-110 3.2 Chain of trains

15 ±2 140-160 1.5

15.8 ±2.8 185-245 4

11 ±1.6 160-170 1 Splash near N-R

6.3 ±1 65-70 1 Series of splashes

7.1 ±0.6 80-100 3 of close rhythm

7.6 ±1.2 140-170 4 Most intensive

7.1 ±0.6 195-210 2

7.4 ±1 225-245 3

8.5 1 250-270? 2.5

less than 10 thousand years, considering the above statement, and smaller rhythms should be viewed with caution.

We selected 44 samples with the most reliable results of the thermal demagnetization (Tables 4, 15) for the second variant of the field variation record. We can watch the behavior of not only inclination but also declination in each sample (Figure 35). A direct comparison of declinations is impossible, however, due to an arbitrary orientation of the horizontal plane. We are not so interested in the values of

/ i ► 80

79 /

\\ 11 ►94 a 75

100» 02 D

) . lp83 T 105

87#*® 84 ^ ♦— -#107

Figure 30. DI-diagram of behavior of the geomagnetic field vector in the process of cooling of the Kivakka intrusion. The second variant of recording.

declinations as their behavior in time. Therefore, the following operation was made: a mean value of D was calculated for each sample in the process of the thermal demagnetization and a difference value of declination was plotted relative to the mean value. We can compare the difference values of samples and study the record of behavior of direction of a complete vector of the magnetic field. However, due to a sharp tilt in values of declination great mistakes are likely to happen (deviation of the vector by 2°-3° through errors of measurements causes alteration of declination by 10° and more!).

As has been stated above, two magnetic minerals with the Curie points 556°C and 580°C are regularly present in samples of the intrusion. Two types of the record were composed: 1) it is assumed that a sample has one magnetic phase with one of the Curie points prevailing (if the NRM intensity drop at 556°C certainly exceeds 70%, the record is analyzed only up to 556°C, if the drop is less than 50% the first Curie point is ignored); 2) both Curie points are taken into consideration. In the first case the record covers ~0.4 million years. In the second case the magnetic record is shifted relative the titanomagnetite record and the stacked record looks as if it is shorter as compared with the first case by ~60 thousand years; this portion of ~60 thousand years is “cut short”. We analyze both types of the record.

After the results on all the samples selected for the analysis are averaged and after a transition to a uniform time step, cyclicity becomes apparent in declination and especially in inclination (Figure 36).

A series of the inclination oscillations with the rhythms from 100 to 5 thousand years is seen from the wavelet analysis data as in the first variant of the record (Tables 16, 17, Figure 37). They are usually unstable or their duration is not long (no one of the rhythms does not cover the entire interval of the record, the number of oscillations in different rhythms is from 1-2 to 10-12), or the rhythm value varies in time (Tables 16, 17, Figure 37), or oscillations form chains of rapidly fading and appearing again splashes that are especially appreciable in the zone of no less than 10 thousand years. The pattern of behavior of declination and inclination

Figure 31. Kivakka. Wavelet diagram of difference between initial data and the mean curve D, a) window of 20 counts (10 kyr, see Figure 28c); b) window of 10 counts (5 kyr see Figure 28d). Wavelet diagram of difference between initial data and the mean curve I, a) window of 20 counts (10 kyr, see Figure 29c); b) window of 10 counts (5 kyr see Figure 29d). Lga - logarithm of the time scale (period).

Figure 32. Bushveld. Behavior of inclination of different NRM components during a relative time of “running” of 580°C temperature (0-78 kyr), 556°C (80-168 kyr) and 530°C (more than 168 kyr) along the section uncovered by well WP-16 (Table 15). Meters are converted into time according to Figures 20, 21. The start of the count is the moment when the temperature of 580°C was at the beginning of the well section, cooling was from the top down (see text).

Table 17. Bushveld intrusion. Second variant of record

A. One Curie point. Declination

Rhythm Standard “length” Number of rhythms Note

89-100 >400 >4 Very weak

55-63 ±2.5 1200-1400 4 bright

55-53 ±5 1580-1700 2-3 bright

35-37 ±3 1200-1360 4-5 weak

31.5-34 ±4 1620-1700 2-3 bright

20-24 ±2 1400-1500 4-5 very weak

18 ±3 1300-1350 3 Chain of splashes, the first is bright

18 ±2 1400-1430 1-2 -*-

19 ±2 1500-1540 2 -*-

15 ±5 1200-1320 6-10 bright

12 ±2 1650 1 Splashes, weak

10 ±3 1240 1-2 bright

10 ±2 1270 1

10 ±2 1340 1

7.5 1490 1

6.6 1290 1-2

6 1370 1

6 1410 1

5.5 and 4.5 1310 2 splashes

B. One Curie point. Inclination

83-60 ±3 1240-1320 1? weak

63-74 ±3 1550-1700 3-4

55-58 ±3 1240-1360 2-3

24 ±5 1640-1720 3 Bright splash

24-19 ±3 1530-1640 4-5

20-17 ±5-2 1240-1500 10-12 Series of trains (4), first is bright, last is weak

14 ±3 1620 Series of splashes

12 ±3 1570

11 ±2 1660

10 ±3 1260

7.5 ±2 1550 bright

7 ±2 1630 bright

7 ±2 1650 bright

6.5 and 6.3 1450 2 splashes

4.7-5.3 1300-1510 8 splashes

C. Two Curie points, 556°C and 580°C. Declination

100-87 1360-1660 > 3 ? very weak

63-50 ±4 1560-1660 2 bright

45 ±2 1300-1350 1 splash, weak

36-42 ±4 1360-1460 3 weak

34-36 ±4 1560-1660 > 3 bright

21 ±2 1240-1300 >3

18-16 ±2 1240-1370 7 bright

17 ±2 weak, splash

14-18 ±4 1570-1650 5 bright

13 ±3 1260-1320 4 bright

9.5 ±3 1290 bright, splash

9 ±2 1440 splash

10.5 1550 splash, weak

10 ±3 1610 splash

9.3 ±2 1580 bright, splash

8.5 and 8 ±4 1500, 1590 splash

7 1410, 1440 2 splashes

6.6 ±2 1340 splash

5.8, 5.4 1310, 1390 splashes

Figure 33. Bushveld. The first variant of the paleomagnetic record. Paleomagnetic inclination after the inclination sign has been changed in the R-polarity interval. a) points “x” - initial data; solid thin line - after transition to a uniform step of 0.5 kyr with the help of a nuclear assessment with the radius of H=0.25; solid thick line - result of a subsequent smoothing of the curve with a uniform step by the 3rd order polynomial. b) difference between the smoothed curve and a signal after transition to a uniform time step.

has no difference in principle in both cases of the recording, with the second Curie point taken into account and ignored.

We connect the difference, first of all, with inaccurate fixing of unblocking and blocking temperatures, with determination of the Curie points and time.

Despite errors of measurements and in processing of the results, the following intervals of the main oscillations can be distinguished with confidence practically in all variants of the paleomagnetic record (three records of inclination and two records of declination) (Tables 16, 17): 90-100, 50-65,

30-40, 19-20, 15-17, 10-8 and 5-7 thousand years. Intervals, aside of errors, reflect first of all, instability of rhythms in time (Figures 34, 37). Duration of the main rhythms (no less than 10 thousand years) spans frequently 5-10 oscillations. The intervals of oscillations of D and I that are fixed

Table 17. Continued

D. Two Curie points, 556° C and 580°C. Inclination

Rhythm Standard “length” Number of rhythms Note

100-93 ±4 1500-1700 2?

89 ±6 1240-1300 1?

50 ±3.5 1240-1340 2

24-34 ±3.5-12 1470-1700 8-9 bright

21-25 ±4.5 1240-1310 3

16-18 ±2.5 1300-1430 7-8 weak (train?)

12-15 ±4.5 1440-1590 10-11 bright

9-10 ±3 1270-1640 5 splashes

6.3-7.2 ±2 1300-1610 5 splashes

4.8-5.8 1300-1580 5 splashes

0.0-1---,----,-----------------------------------------------------,-,-,-,-,-,-,-,-,-,—M

20 40 60 80 100 120140 160 180 200 220 240 260 Time, kyr

Figure 34. Wavelet diagram for the curve plotted in Figure 33b.

Figure 35. Behavior of declination and inclination of the field in the process of cooling of the Bushveld intrusion in each sample. Sample numbers are in rectangles.

Figure 35. Continued.

Figure 35. Continued.

on the wavelet diagrams (Figures 34 and 37) are confirmed by the maxima obtained by the maximum entropy method (Figure 38). However, as can be seen from comparison of Figures 37 and 38, the maximum entropy method fixes only the most intensive maxima of the oscillation amplitudes irrespective of whether it is a splash or a long oscillation, which is traced almost along the entire recording interval. In the last case the rhythm value is changing frequently with time, but the maximum entropy method is fixing the rhythm value with the maximum amplitude (for example, rhythm in Figure 37d is traced from 1440 kyr to 1590 kyr and changes during this period 12 kyr to 15 kyr years, whereas the maximum entropy method is fixing a rhythm of 12.5 kyr, relating to the maximum amplitude falling at 1480-1490 kyr; a similar situation is also for the rhythm of 24-34 kyr which is traced from 1480 kyr to 1660 kyr, a rhythm of 28 kyr with the maximum amplitude at 1640 kyr was fixed by the maximum entropy method etc.).

Conclusion

1. As a result of the paleomagnetic and petromagnetic study of the Monchegorsk, Kivakka and Bushveld intrusions we have shown for the first time that it is possible in principle

to obtain a continuous record of the geomagnetic field behavior, fixed in the process of cooling of the gabbro-pyroxenite layered intrusions. Such intrusions are the most suited for this type of investigation: a) these are dry magmas, their cooling is in a conductive way, without participation of fluids which is simple to model knowing the main thermo-physical parameters, geometry and depth of forming the intrusion; it is possible to calculate temperature, time and velocity of cooling and this process is simple and homogenous at the cooling temperature below 600°C when the paleomagnetic recording begins; b) the mechanism of formation of magnetic minerals in the layered intrusions is known and is quite uniform - these are thin inclusions of the low-Ti titanomag-netites in plagioclases and pyroxenes as a result of a high-temperature exsolution of the latter; magnetites sealed in the silicates well preserved for billions of years and, accordingly, the paleomagnetic record is also retained. True, there is a certain difficulty here - the existing estimations of the plagioclase and pyroxene exsolution temperature with formation of the magnetite inclusions (550-600°C) are approximate. According to data of the thermo magnetic analysis and of the thermal demagnetization the low-Ti titanomag-netites are the main carriers of NRM in all the intrusions (Tc = 530-580°C). Usually their concentration is less than 0.1%. The NRM component acquired at the stage of cooling of the intrusion is thermoremanent and is distinguished

Figure 36. Behavior of declination (a, c) and inclination (b, d) of the field in the process of cooling of the Bushveld intrusion. The second variant of the record. a) Result of combining data of D for the first set of data (one Curie point is 580° C), after subtraction of the mean value from each sample. b) Result of combining data of I for the first set. c) Result of combining data of D for the second set (two Curie points, 556°C and 580°C), after subtraction of the mean value from each sample. d) Result of combining data of I for the second set. Points - initial data, line - result of averaging (H = 0.25) and transition to a uniform time step (At = 0.5).

“I 1 I 1 I 1 I 1 \~

1300 1400 1500 1600 Tlme ^ 1700

Figure 36. Continued.

by such characteristics as Hcr/Hc, Jrs/Js, Qn, shape of the curve Jn(T), Jrt/Jri and Jn/Jri relationship the majority of samples above 500°C. Formation of a portion of titanomag-netites below their Curie points, i.e. below 520°C, which will acquire the crystallization remanent magnetization and partial thermoremanent magnetization during a further cooling is not ruled out. Calculations that we used refer only to a complete thermoremanent magnetization. It is not incon-

ceivable that a similar effect is present in the objects studied by us and creates additional noise and biased errors.

2. We were reading the paleomagnetic record in two variants: a) first variant - a detailed sampling from the contact deep into the body, the isotherm of the Curie point of magnetic minerals present in the body shifts at different stages of cooling from the contact deep into the body or vice versa; b) second variant - a detailed thermal demagnetization of

Figure 37. Wavelet diagrams of behavior of declination and inclination of the geomagnetic field in the process of cooling of the Bushveld intrusion. The second variant of the record. a) Wavelet diagram for D of the first set of data (one Curie point is 580° C), after averaging and transition to a uniform time step, a final part of data (after 1692.5 kyr) is excluded from processing due to strong peaks. b) Wavelet diagram for I from the first set of data after averaging and transition to a uniform time step and removal of a general linear trend. c) Wavelet diagram for D of the second set of data (two Curie points, 556°C and 580°C), after averaging and transition to a uniform time step, initial and final parts of data are excluded from processing due to strong peaks. d) Wavelet diagram for I of the second set of data after averaging and transition to a uniform time step and removal of a general trend by the 3rd order polynomial. Lga - logarithm of the time scale (period).

every sample permits to read the record of the geomagnetic field behavior in the process of cooling of the magmatic body at the sampling point from the Curie point of the magnetic mineral which is present in it to the blocking temperature at which a significant share of the thermoremanent magnetization can be still fixed within the range of accuracy of measurement. As a result of the petromagnetic study we have found that an appreciable and sometimes a considerable share of the ensemble of the magnetic grains are multidomain grains. Thermal demagnetization of such grains is close to their Curie points. This is of no significance for obtaining the first variant of recording. As to the second variant, due to the above reason it becomes possible to fix, as a rule, the record of the magnetic field behavior only above 500°C, and more frequently, above 540°C, when contribution of the single-domain and pseudosingle-domain magnetic grain contribution in the NRM begins to prevail. Hence, in

case of magnetite (Tc = 580°C) with the thermal demagnetization step of 5° less than 10 points are obtained and this, of course, is insufficient for analysis of the record. Even the step of 3° will not save the situation. We had to average results on many samples to improve reliability. It is necessary to have a step of maximum 1° for the thermal demagnetization for which purpose a high-quality temperature control, a very high temperature uniformity and a high-quality shielding of the external magnetic field will be required. Even this 1° step does not ensure a continuous recording, both of our recording variants are discrete (in the first variant samples were taken from the Monchegorsk intrusion every 1-2 m that makes 100 years; in case of the Bushveld intrusion, 1 m of the section represents more than 500 years), though the process of cooling of the intrusion is undoubtedly continuous and, consequently, ensures a continuous paleomagnetic record. It is necessary to conduct a continuous thermal de-

Figure 38. Spectra of power (maximum entropy method, ARorder = 80) of signals after averaging and transition to a uniform time step. (a) and (b) - D and I of the first set of data (one Curie point); (c) and (d) - D and I of the second set of data (two Curie points), after exclusions or peripheral intervals with peaks or of general trends (see Figure 37).

magnetization of the samples and this is now possible due to the fact that Yu. K. Vinogradov (Borok Observatory) developed a three-component thermomagnetometer performing a continuous recording automatically.

A more thorough sampling must be done for the first variant of the paleomagnetic recording, but even in this case the method will remain discrete.

Temperature accuracy determination is of great importance. In our experiments a 1° error in setting temperature in the furnace, in determination of the Curie point, in con-

version of unblocking temperature during thermal demagnetization into blocking temperature during cooling of the intrusion etc. amounts to about 400 years in the most rapidly cooled Kivakka intrusion, 1000 years in the Monchegorsk intrusion and 3000 years in the larges Bushveld intrusion! Hence we have appreciable scatter in the time series of D,I, both in the first and in the second variants of the record.

In general, the second variant of obtaining the paleomag-netic record is more informative and efficient: a) it does not require a strict sampling across thickness of the body,

it is sufficient to select the unaltered localities (for example, great gaps appeared in the first variant of the pale-omagnetic record due to the secondary alterations in the Kivakka intrusion), however, the intrusion must be a single uniformly cooled body (for modeling the cooling process); b) free choice of the sampling area and pieces of the unaltered rocks containing homogeneous magnetic minerals by composition, that is expressed in one or definitely prevailing Curie temperature, and in prevalence of the single-domain grains. All this almost impossible to observe in the first variant of obtaining the record; c) non-oriented samples can be used as it is possible to obtain relative alterations of the field direction as, for example, in the case of the Bushveld intrusion, where we were dealing with the specimens from the core which were not oriented in the horizontal plane, and obtained information about relative alterations of the paleomagnetic declination.

3. The secondary alterations appreciably distort the primary recording down to its complete destruction. We do not dwell on such phenomena as viscous remagnetization that is removed easily by thermal demagnetization. Naturally, so ancient formations as Early Proterozoic layered intrusions could not avoid various secondary phenomena during their long life such as heating, metamorphism, tectonic deformations etc., which had an effect on preservation of the primary record. Partly this is eliminated by the choice of the objects of the study. Thus, the whole blocks are preserved perfectly in the intrusions under study as was mentioned above (item 1). Samples for our studies were taken from such blocks. However, despite the choice of the objects, two out of three intrusions (Monchegorsk and Kivakka) were subjected to a considerable secondary heating (evidently, up to 400500° C), which has sharply reduced the interval of the primary paleomagnetic record and even completely destroyed it here and there. This heating is dated to the Svecofennian tectonomagmatic activity (judging from the paleomagnetic data, age of this activity is 2-1.9 Ga).

4. Despite the difficulties listed above (it. 1-3), we succeeded in obtaining for the first time certain information about the state of the geomagnetic field in the process of cooling of three Early Proterozoic intrusions. Duration of the record obtained is ~30-40 kyr (Kivakka intrusion), ~70 kyr (Monchegorsk intrusion) and ~400 kyr (Bushveld intrusion). From the viewpoint of magnetostratigraphy the record is characterized presence of chrons and subchrons of opposite polarity with duration of hundreds of thousand years to several dozens of thousand years; an excursion which lasted less than 2 thousand years has been fixed. A wavelet analysis was used to study the spectrum of the geomagnetic field variations peculiarities of their behavior in time (alteration of amplitude and rhythm in time, stability and duration of oscillations etc.). The spectra of variation recorded in the process of cooling of three intrusions are given in Figure 39. The spectra of record of three intrusions have both similarity and an appreciable difference comparable with one another, though the Monchegorsk and the Kivakka intrusions are older than the Bushveld intrusion by about 400 Ma and are located at a huge distance from it. The latter is not surprising if we recollect that even during the recording (dozens and hundreds of thousands years)

some rhythms disappear and some appear, and the rhythm values vary in time. Duration of the rhythms varies from 1-2 oscillations (splashes) up to more than 10. For 50 million years and all the more for 400 million years (the age difference of the Monchegorsk, Kivakka and Bushveld intrusions) the spectrum of variations could have changed considerably. Besides, the distinguished rhythms have standard deviations, which exceed, as a rule, 1 thousand years; the technical errors (it. 2 of the Conclusion) are increasing the possible scatter of the results. At the same time, the spectra are comparable with one another.

Let us sum up all histograms of all three intrusions, declination and inclination, all variants of obtaining the records; to be more correct, let us calculate the average rhythms by intervals (Figure 40). As a result, the modes of stable oscillations: 3.2; 4; 4.4-4.7; 5; 5.5; 7; 7.5; 8-10; 14-15(?); 17.5-18.5; 20(?), 30-40, 52-64 and 90-100 are distinguished against a background of a uniform noise caused by a combination of actual short “splashes” (1-3 oscillations) and instability of rhythms in time plus errors in assessment of temperature and time (Figure 40). The last two rhythms 52-64 and 90-100 kyr are fixed only in a long record of the Bushveld intrusion. On the other hand, there are practically no rhythms below 5 thousand years in the record of the Bushveld intrusion, whereas rhythms of 2-3-4 thousand years a definitely observed in records of the Monchegorsk and Kivakka intrusions. However, accuracy in determination of the value of these rhythms is not reliable.

The pointed out features of the geomagnetic field behavior do not differ in principle from characteristics of the geomagnetic field in the Recent Cenozoic era. Hence, by the beginning of Proterozoic the liquid core of the Earth, which generates the geomagnetic field, was formed and was further changing insignificantly.

5. Let us point to some standard paleomagnetic results. The Monchegorsk intrusion. A paleomagnetic pole 265.3°E, 1.3°N, has been determined, which differs appreciably from APWP of the Baltic shield that is associated with the tectonic clockwise rotation through ~ 30° and a tilt of the intrusion by 15°-20° to NW. The Kivakka intrusion. The prefolded component Ai and synfolded/afterfolded component A2 that were acquired at the stage of cooling of the intrusion are distinguished in the process of the thermal demagnetization of the samples. Judging by the trend of inclination, tilting of the body by 36° began approximately 85 thousand years after emplacement of the intrusion and it lasted for 20-25 thousand years. Position of the pole of the Ai component in the stratigraphic system of coordinates (17.8°S and 247° E) is close to the mean coordinates of Fennoscandia pole of a similar age (they coincide after rotation of the intrusion through 30° counterclockwise). A great scatter of the paleomagnetic directions, first of all, is associated with an incomplete separation of the A1 and A2 components during the thermal demagnetization. The Bushveld intrusion. Orientation of the horizontal plane of the samples was restored on the basis of the viscous magnetization and from the high-temperature N-component of NRM we determined a mean paleomagnetic direction and a pole (12°N, 35.4°E), which coincides practically with the pole that was defined from the oriented samples of similar rocks taken from the

Figure 39. Summary of spectra of variations of the geomagnetic field direction in the process of cooling of the Bushveld, Kivakka and Monchegorsk intrusions. Results of the wavelet analysis (see Tables 10, 14, 16 and 17). Black bars are rhythms of oscillations of D and I. Crosshatched rectangles are intervals of rhythms changed in time. Height of the bars and rectangles reflects the number of oscillations (or the number of splashes) of one rhythm. Different methods of obtaining the paleomagnetic record are marked in different hatching. The most striking rhythms with the highest amplitude are marked with red.

natural outcrops [Hattingh, 1986a]. A tilt similar to that of the Kivakka intrusion has been noted at the stage of cooling of the Bushveld intrusion. Its angle is about 15° and it occurred after the normal polarity of the geomagnetic field has

Чпеап

8 1--------

7----------

6----------

5----------

4----------

3----------

2----------

1----------

0 -I-------'-----1

1 10 Time, kyr 100

Figure 40. Results of the interval averaging of histograms Figure 39. Up to 20 kyr - average values on 6 histograms, from 20 kyr to 40 kyr - average values on 4 histograms (the Kivakka record is short and it does not cover the rhythms of 20 kyr and more); more than 40 kyr these are average values on 2 histograms (only Bushveld).

changed. Rotation of blocks, including the Monchegorsk and Kivakka intrusions (or their part) about the vertical axis occurred after tilting of the bodies but before their secondary heating.

Acknowledgments. The authors would like to express their gratitude to V. N. Dech and S. R. Kotov for presenting a collection of specimens from the core of well WP-16; A. V. Shatsillo and

D. M. Khvorov for their help in selection of the oriented samples from the Monchegorsk and Kivakka intrusions; M. V. Borisov, S. V. Bolikhovskaya, E. V. Koptev-Dvornikov, D. M. Khvorov, A. V. Chistyakov and E. V. Sharkov for their consultation and presentation of the geological and other materials; S. Kornev, Z. V. Sharonova and G. S. Yanova for their help in the paleo-magnetic and petromagnetic measurements. This work was done due to a financial support of the RFFl, Grant no. 01-05-64240.

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D. M. Pechersky, A. A. Lyubushin, Institute of Physics of the Earth, Russian Academy of Sciences, 10 Bol’shaya Gruzinskaya ul., Moscow, 123995 Russia

V. S. Zakharov, Moscow State University, Vorobjovi Gori, Moscow, 119899 Russia

(Received 24 October 2004)