Early Precambrian mafic rocks of the Fennoscandian shield as a reflection of plume magmatism: geochemical types and formation stages Текст научной статьи по специальности «Науки о Земле и смежные экологические науки»

RUSSIAN JOURNAL OF EARTH SCIENCES, VOL. 5, NO. 3, PAGES 145-163, JUNE 2003

Early Precambrian mafic rocks of the Fennoscandian shield as a reflection of plume magmatism: Geochemical types and formation stages

N. A. Arestova, S. B. Lobach-Zhuchenko, and V. P. Chekulaev

Institute of the Precambrian Geology and Geochronology of the Russian Academy of Sciences, St. Petersburg

E. G. Gus’kova

St. Petersburg Branch of the Institute of the Terrestrial Magnetism, Ionosphere, and Radio Wave Propagation (SPbF IZMIRAN), St. Petersburg

Abstract. The analysis of radiometric ages of Early Precambrian basites of the Fennoscandian shield, from the most ancient ones, >3.1 Ga, to 2.40 Ga, resolves five age groups of the basites. Each of these stages is shown to time span interval of 70-80 m.y. The early stages of the high-T mafic magmatism (>3.1 and 2.99-2.91 Ga) are confined to within the oldest core of continental crust in the Fennoscandian shield - the Vodlozero domain with crustal age of 3.2-3.4 Ga. The next stage of mafic magmatism (2.88-2.80 Ga) occurred within the Kola and western Karelian domains with crustal ages of 3.0 and 3.1 Ga and on the north of the younger, central Karelian domain. The last of the Archean stages of high-temperature mafic magmatism with ages of 2.72-2.66 Ga occurs in the north Karelian belts, in the Karelian part of the Belomorian area (the regions of Lake Notozero and the Tupaya Guba Bay of Lake Kovdozero) and possibly, in the western Karelian domain. This magmatism took place also immediately after the subduction processes at the boundary of the Karelian and Belomorian domains. The Early Proterozoic high-T mafic magmatism at 2.50-2.41 Ga was both the most areally extensive and continuos such episode in the Fennoscandian shield. Nearly all the researchers of the high-T basites of this stage attribute this magmatism to the ascent of a deep mantle super-plume. Paleomagnetic data provide further evidence that at 2.5-2.41 Ga a long-lived heat source occupied virtually the entire area of the present day Fennoscandian shield.

Introduction

The last decade witnessed how the previously governing plate tectonic paradigm of the Earth’s history gave way to

Copyright 2003 by the Russian Journal of Earth Sciences.

Paper number TJE03126.

ISSN: 1681-1208 (online)

The online version of this paper was published 18 July 2003. URL: http://rjes.wdcb.ru/v05/tje03126/tje03126.htm

the new theory of the global Earth Tectonics. From the standpoint of this theory, the Earth developed through the processes of core growth, plume tectonics, and plate tectonics, which first operated sequentially and then jointly [De-vias, 1997; Kumazava and Maruyama, 1994; Maruyama et al., 1994; etc.]. In this succession of mechanisms, plume tectonics, whose refinement was contributed to by many studies of the last decade [Campbell and Griffiths, 1990, 1992; Condie, 2001; Dobretsov et al., 2001; Grachev, 1998, 2000; Maruyama, 1994; etc.], is thought to have played the leading role at early phases of the Earth’s evolution. Studies by a number of workers in the Early Precambrian have shown

146

arestova et al.: early precambrian mafic rocks of the fennoscandian shield

Figure 1. Geologic map showing the Vodlozero domain [Lobach-Zhuchenko et al., 2002].

1 - areas where the oldest rocks of the domain have been dated: KV - mafic volcanics of the Vinela and Chereva rivers, TL - Lairuchei Creek tonalite, GAV - Vodla gneisses and amphibolites, TV - Vyg River tonalite, TPL - Palaya Lamba tonalite; 2 - gneissic tonalite, gneissic granite, and migmatite, undifferentiated; greenstone belts: the most ancient (3.0-2.92 Ga), 3 - with multimodal volcanism,

4 - with bimodal volcanism; 5 - younger (2.9-2.85 Ga), with bimodal volcanism, 6 - with bimodal volcanism, undated; (greenstone belts: 1 = Hautavaara, 2 = Koikary, 3 = Semch, 4 = Palaya Lamba,

5 = Oster, 6 = Shilos, 7 = Kamennoozero, 8 = Kenozero). Intrusions: 7 - gabbronorite, gabbro, gabbro-diorite, diorite; 8 - tonalite, trondhjemite; 9 - high-magnesian granite; 10 - subalkaline rocks: (a) granitoids, (b) mafic and intermediate dikes; 11 - granite; 12 - province of development of overprinted granulite facies metamorphism, including charnockite and enderbite massifs; 13 - boundary of development of the granulite facies assemblage; 14 - Matkalahti zone basites; 15 - central Karelian domain; 16 - Proterozoic rocks, 17 - rapakivi granite; 18 - Paleozoic rocks; 19 - basites dated at >3.1 Ga; 20 - basites dated at 2.99-2.91 Ga.

that the plume-tectonic mechanism was dominant during the Early Precambrian stages of geologic history [Abbott, 2001; Campbell and Griffiths, 1990, 1992; Vrevsky, 2000; etc.].

The most promising approach in unraveling mechanisms that were likely to operate in the Early Precambrian is the study of compositions of basites and ultrabasites, derivatives of mantle melts, to elucidate their source composition, melting conditions, and subsequent melt evolution.

The focus of our work is on Early Precambrian (3.4-

2.4 Ga) basites and ultrabasites of the eastern Fennoscan-dian shield. Our study draws on the recent results regarding the conditions and history of formation of Early Precambrian (Archean) crust in the eastern part of the shield [Lobach-Zhuchenko et al., 1998, 2000b, 2003]. Among these results is the conclusion that, alongside the previously established age heterogeneity of the Archean domains (Fenno-Karelian, Belomorian, and Kola) of the eastern Fennoscandian shield, there exists an age heterogeneity of the shield’s largest an-

cient entity, the Fenno-Karelian granite-greenstone province. The oldest portion of the Fenno-Karelian granite-greenstone province is the Vodlozero domain, whose crust started forming at 3.2-3.4 Ga. Later on, the crust of the western Karelian (3.1-3.0 Ga), Kola, and Belomorian (3.0-2.9 Ga) domains began to form. The crust of the youngest, central Karelian domain is less than 2.85 Ga old. Another approach in scrutinizing mafic-ultramafic magmatism is centered on establishing the principal stages of formation and evolution of Early Precambrian crust of the shield [Lobach-Zhuchenko et al., 2001].

Detailed petrologic and geochemical studies of Early Precambrian komatiites, basalts, and mafic intrusives of the eastern Fennoscandian shield, carried out in recent years [Arestova and Glebovitsky, 2003; Chekulaev et al., 2002, 2003; Lobach-Zhuchenko et al., 1998, 2002a, 2003; Puchtel et al., 1997, 1998, 1999; Vrevsky, 2000], enabled the researchers to identify (by analogy with modern basites generated in a variety of geodynamic settings) the rocks whose generation was likely related to mantle plumes [Campbell and Griffiths, 1992; Kerr et al., 2000; etc.]. Such basites, derived from high-temperature melts, are the focus of this study.

Characteristics of Early Precambrian Basites of the Fennoscandian Shield

Over the past 15 years, a large number of reliable isotope age determinations, most of which are listed in Table 1, have been carried out on Fennoscandian shield basites. These basites fall into five age groups: (1) >3.1 Ga, (2) 2.99-2.91 Ga, (3) 2.88-2.80 Ga, (4) 2.72-2.66 Ga, and (5) 2.50-2.41 Ga. Given below is the analysis of how basites of various age groups are distributed over the area of the Fennoscandian shield and of the geochemical types of the basites constituting these age groups.

Basites With Ages Older Than 3.1 Ga

Mafic rocks with the oldest age determinations (3.4 Ga) are found in the southeastern Vodlozero domain, the oldest in the Fennoscandian shield (Figure 1) [Kulikova, 1993; Puchtel et al., 1991]. The basites are represented by the Volotskaya Sequence komatiite-basalt assemblage (Table 2, nos. 1, 2). The peridotites of the Sequence are high-Mg rocks with ca. 27% MgO (no. 1, Table 2). Currently, there is no doubt that peridotitic komatiites with >24% MgO in spinifex-textured varieties are plume derived. Liquidus temperatures for melts initial to such komatiites, as calculated using the formula proposed by [Nisbet et al., 1993], range 1550-1600°C, which corresponds to a mantle temperature of ca. 1800° C and is far in excess of mantle temperatures in the Archean or Early Proterozoic, as calculated by [Richter, 1988]. Compositions of the Volotskaya Sequence peridotitic komatiites (27% MgO, 47% SiO2, CaO/A^Os = 0.7-1.29, Al2Os/TiO2 = 15-24, 1900-1300 ppm Ni, SNd(t) = +1.2)

♦ z'' y/Arc lavas / / _

S/ ■ A* —Js. / _ .. - " " 9 /\ s'

r ♦ ■ ■ ■

/ A // / V. a Plateau lavas sf MORB and

J Back-arc basin lavas

♦ komatiites

■ komatiite basalts a basalts

Figure 2. Zr/Y vs. Nb/Y plot for komatiites and basalts of the ancient Volotskaya Sequence of the Vodlozero domain showing fields for basalts from various geodynamic settings, after [Kerr et al., 2000].

suggest that they are derived from high-temperature mantle and are undepleted or slightly depleted in silica. The basalts associated with these komatiites are high in Ni (760150 ppm) and have Nb/Y and Zr/Y ratios (>0.1 and 2-3, respectively) that place them in the field of rocks generated in oceanic or continental-margin plateaus (volcanic rifted margins; Figure 2) [Kerr et al., 2000; Marsoli et al., 2000]. Although, according to our own geological data, such an old age requires additional validation, it can be safely assumed that high-temperature melts (plume derivatives) first appeared as early as >3 Ga ago.

Basites With 2.99—2.91 Ga Ages

The next (and longest) stage of mafic magmatism takes the time span between 2.99-2.91 Ga. The basites of this stage are widespread within the ancient Vodlozero domain and are represented by intrusions in the central part and by volcanics (komatiites and basalts) in the marginal parts of the domain (Figure 1). This stage, which lasted ca. 75-80 m.y., is divisible into three episodes.

The earliest episode is featured by intrusive magmatism, as exemplified by the Lairuchei layered intrusion (composed of gabbropyroxenite, gabbronorite, anorthosite, and dior-ite), situated in the central part of the domain and dated at 2.987±11 Ga. Geochemical features of the basites composing the intrusion (Table 2, nos. 3, 4) are: high mg# (0.790.68), 22-7% MgO, and high Cr (1000-350 ppm) and Ni (800-200 ppm). According to Campbell and Griffits’s data, NiO>600 ppm at 16% MgO may point to a plume prove-nace for the initial melt. Note, however, that characteristics of high-temperature melts, such as elevated (as compared to Archean komatiites) SiO2 contents, (Nb/La)N = 0.5, Ti/Zr = 40, an evolved distribution of the rare earth elements ((La/Yb)N = 5-10) (Figure 3), and eNd(t) = -0.8 to -2.5, suggest crustal contamination for the initial melt.

Table 1. Isotope age of Early Precambrian mafic rocks of the Baltic shield

age method used occurrence, massif rock SNd reference

Sm-Nd layered

2410±64 WR, Opx, PI Panikat assemblage —1.7±0.6 Huhma et al., 1990

2435±7 U-Pb, Zr Akanvaara layered assemblage Hanski et al., 2001

2436±5 U-Pb, Zr Koilismaa Alapieti, 1982

2439±3 U-Pb, Zr Koitilainen Hanski et al., 2001

2439±29 Sm-Nd, WR Kivaka gabbro Amelin and Semenov, 1996

2440±10 Sm-Nd, WR Kovdozero (Puakhta block) pegmatitic schliere —1.2±0.3 Efimov and Kaulina, 1997

2441±1.2 U-Pb, Zr Tsipringa gabbro -1; -0.5 Amelin et al., 1995

2442±1.4 U-Pb, Zr Lukkulaisvaara gabbro -1.5 Amelin et al., 1995

2443±10 U-Pb, Zr Tolstik gabbro 0, -1.5 Bogdanova and Bibikova, 1993

2445±2 U-Pb, Zr Kivaka gabbro -1; -0,5 Amelin et al., 1995

2446±5 U-Pb baddeleyite Lake Pyaozero (western Tikshozero dike assemblage) gabbronorite Vuollo et al., 1994

2448±42 Sm-Nd, WR Vetreny belt komatiites -1.2 Puchtel et al., 1997

2449±1.1 U-Pb, Zr Burakovsky gabbroids 2 - -2.8 Amelin et al., 1995

2449±35 Sm-Nd, WR, Opx, PI, Ol Mt. Golets, Vetreny belt komatiitic basalts Puchtel et al., 1997

2430±174 Sm-Nd isochr. WR+Ol+Px Vinela dike Peridotites —1.4±1.0 Puchtel et al., 1997

2450±10 U-Pb, Zr Kolvitsy anorthosite Mitrofanov et al., 1993

2453±

2406±3 U-Pb, Zr Main Range leucogabbro Mitrofanov et al., 1993

2452±3 U-Pb, Zr Pyrshin anorthosite Mitrofanov et al., 1993

2450±70 U-Pb, Zr Panaj arvi-T sipringa Ol-gabbronorites, dikes Buiko et al., 1995

2457±88 Sm-Nd, WR North Karelian zone gabbronorite dikes Amelin and Semenov, 1996

2460±9 U-Pb, Zr Zhemchuzhny gabbronorite —1.3±0.8 Kudryashov and Balagansky, 1999

2446±39 U-Pb, Zr Imandra gabbronorite Bayanova et al., 1999

2444±77 Sm-Nd, WR, Opx, PI, Cpx Imandra, Umba River massif gabbronorite Bayanova et al., 1999

2446±10 U-Pb, Zr General’skaya anorthosite —1.2±0.3 Bayanova et al., 1999

2496±10 U-Pb, Zr General’skaya gabbronorite Bayanova et al., 1999

2491±1.5 U-Pb, Zr Fedorovo-Panskaya gabbronorite Bayanova et al., 1994

2493±7 U-Pb, Zr Monchegorsk gabbronorite Bayanova and Mitrofanov, 1999

2668±2 U-Pb, Zr Tsaga intrusion Mitrofanov et al., 1993

2692±1.4 U-Pb, Zr Tupaya Guba Bay, Lake Kovdozero gabbro Lobach-Zhuchenko et al, 1993

2694±14 U-Pb, Zr Guba Mironova Bay, Lake Notozero gabbro Lobach-Zhuchenko et al, 1995

2705±7 U-Pb, Zr Hizovaara belt dacite cutting komatiites of the upper rock sequence, cutting komatiites Shchipansky et al., 1999

148 ARESTOVA ET AL.: EARLY PRECAMBRIAN MAFIC ROCKS OF THE FENNOSCANDIAN SHIELD

Table 1. Continued

age method used occurrence, massif rock £Nd reference

2760 U-Pb, Zr Kuhmo A mafic sill, cutting komatiites Patchet et al., 1981

2882±190 Sm-Nd, WR Uraguba Bay, Polmos-Poros komatiites 2.8 Vrevsky, 2000

2880±10 U-Pb, Zr Central Belomorian mafic zone felsic volcanics within basites Bibikova et al, 1999

2803±35 U-Pb, Zr Hizovaara belt dacites cutting komatiites of the lower sequence Kozhevnikov, 1982

2800 U-Pb, Zr Suomussalmi granodiorite, cutting rocks of the belt Gaal, et al., 1976

2843±39 Sm-Nd, WR Kostomuksha belt komatiites 2.8 Puchtel et al., 1997

2808±95 Sm-Nd, WR Kostomuksha belt komatiites and basalts 2.9 Lobach-Zhuchenko et al., 2000a, 2000b

2840±30 U-Pb, Zr Palaya Lamba intrusion Leucogabbro Lobach-Zhuchenko and Lavchankov, 1985

2849±3 U-Pb, Zr Semch Intrusion gabbro-diorite -0.8; -1.5 Sergeev, Arestova, et al., 1983

2916±117 Sm-Nd, WR, isochron Kamennye Ozera belt komatiites and basalts 2.7±0.3 Puchtel et al., 1999

2913±30 Sm-Nd, WR, isochron Shilos structure basalts 1.6±0.4 Lobach-Zhuchenko et al., 1999

2925±6 U-Pb, zircon suture between the Murmansk and Kola-Keivy domains evolved gabbro Kudryashov and Gavrilenko, 2000

2944±170 Sm-Nd, WR, isochron Koikary belt komatiites and basalts 1.7 Svetov and Huhma, 1999

2960±150 Sm-Nd, WR, isochron Kenozero belt komatiites and basalts 2.2 Sochavanov et al., 1991

2987±11 U-Pb, Zr Lairuchei gabbro-diorite -0.6; -2.5 Lobach-Zhuchenko et al., 1993

3128±86 U-Pb, Zr Vodla River amphibolite 1 Lobach-Zhuchenko et al., 1993

3320±100 U-Pb, Zr Vodlozero block amphibolite Sergeev et al., 1990

3391±76 Sm-Nd, WR, isochron Volotskaya Sequence komatiites and basalts 1.2 Puchtel et al., 1991

ARESTOVA ET AL.: EARLY PRECAMBRIAN MAFIC ROCKS OF THE FENNOSCANDIAN SHIELD 149

Table 2. Contents of major (%), trace, and rare-earth elements (ppm) in representative samples of high-temperature mafic rocks of the >3.1 Ga stage (nos. 1, 2) and the 2.99-2.91 Ga stage (nos. 3-17)

1 2 3 4 5 6 8 9 10 11 12 13 14 15 16 17

Samp. no. 352 Ar 319 Ar 393 Ar 367 Ar 427-7 Vr 7LZh 2103b Ar 517VB 565 Ar 348a Ar 5-103 11-116 41 Ar 25 Ar 400 Ar 605 Ar

Si02 45.86 45.98 52.77 58.75 46.98 50.78 46.23 48.48 45.65 50.43 51.44 51.06 46.38 50.36 45.15 49.79

Ti02 0.27 0.47 0.29 0.26 0.39 0.65 0.23 1.11 0.23 1.10 0.56 0.76 0.77 1.09 0.29 0.79

AI2O3 8.02 10.13 14.81 18.16 7.79 14.77 6.9 14.76 6.3 14.37 12.17 16.70 16.26 16.39 7.62 15.10

FeO 11.08 14.05 8.13 5.13 11.68 9.61 12.73 12.36 12.97 12.25 11.91 10.03 12.25 11.89 9.42 12.06

MnO 0.21 0.22 0.16 0.09 0.16 0.23 0.2 0.20 0.19 0.20 0.18 0.20 0.19 0.20 0.28 0.20

MgO 25.66 17.62 13.55 7.45 28.82 8.15 28.3 6.72 29.19 8.51 16.24 8.7 9.76 6.89 30.32 7.50

CaO 7.77 10.06 7.88 6.67 5.13 12.08 5.16 14.08 4.65 9.56 5.58 9.7 11.26 9.30 5.34 11.04

Na20 0.91 1.24 1.77 2.84 0.03 1.60 0.17 1.82 0.11 2.8 0.01 3.08 2.41 2.53 0.04 1.66

K20 0.19 0.20 0.31 0.27 0.01 0.17 0.1 0.02 0.02 0.64 0.01 0.05 0.08 0.01 0.07 0.07

P2O5 0.03 0.03 0.06 0.13 0.01 0.01 0.02 0.05 0.06 0.08 0.22 0.06

mg# 0.80 0.69 0.75 0.72 0.83 0.60 0.80 0.50 0.81 0.55 0.71 0.61 0.59 0.51 0.85 0.53

Rb 1 13 12 5 0 4 1 3 1 44 2 2 4 7 1 <5

Sr 21 37 210 295 7 102 3 112 30 125 21 99 75 159 3 109

Y 7 16 10 11 6 17 4 18 8 24 14 15 19 22 7 18

Zr 12 33 36 58 19 39 12 67 13 62 26 42 42 64 14 45

Nb 2 2 2 3 1 2.3 2 4 1 4.2 2 2 3.4 1 1.8

Ti 1162 3402 1996 1563 1653 4101 1380 6600 1283 7042 3238 4473 4030 6624 1740 5284

Ba 115 186 118 134 11 53 22 20 90 158 <100 163 <100 <30 <30

Cr 1035 2219 735 542 2170 414 3632 144 4501 366 1271 499 371 281 2527 400

Ni 1344 607 354 224 584 105 97 984 175 300 153 158 112 1437 145

Co 102 7 53 31 39 58 116 49 38 56

V 50 194 101 73 236 128 331 254 298 318 312

La 4.30 8.40 1.58 1.4 2.0 3.8 1.3 4 0.98 4.01 0.42 1.080

Ce 10.0 17.0 3.65 4.8 3.1 8.3 11 3.2 11.0 1.50 4.00

Nd 6.4 11 2.37 2 5.8 0.57 7.6 3.54 5.08 2.7 8.54

Sm 1.39 1.3 0.75 1.54 0.82 2.0 0.21 2.67 1.09 1.65 1.32 2.85 0.496 1.740

Eu 0.53 0.41 0.19 0.544 0.3 0.67 0.7 0.51 1.04 0.186 0.700

Gd 0.85 1.0 2.58

Tb 0.21 0.19 0.36 0.18 0.49 0.64 0.36 0.71 0.116 0.40

Yb 0.61 0.57 0.7 1.4 0.88 2.2 0.72 2.4 1.3 1.9 0.420 1.62

Ti/Zr 97 103 55 27 86 105 115 99 99 114 125 107 96 104 124 117

Nb/Y 0.29 0.13 0.20 0.27 0.17 0.14 0.50 0.22 0.13 0.18 0.11 0.15 0.14 0.10

Zr/Y 1.7 2.1 3.6 5.3 3.2 2.29 3 3.7 1.6 2.6 2.21 2.91 2.00 2.50

Nb/La 0.47 0.36 0.63 1.64 1.0 1.05 0.77 1.05 2.04 0.85 2.38 1.67

150 ARESTOVA ET AL.: EARLY PRECAMBRIAN MAFIC ROCKS OF THE FENNOSCANDIAN SHIELD

Figure 3. Spidergrams for gabbronorites and gabbro-diorites of the Lairuchei intrusion. Sample numbers on diagrams correspond to those in Table 2.

Mass balance calculations using major- and trace elements and AFC model calculations based on the (eNd-La/Sm) ratio suggest melt generation conditions that involved 15% assimilation of the Lairuchei tonalite by a plume melt with mantle REE abundances [Arestova, 1997].

Another episode is represented by mafic-ultramafic vol-canics of the western and eastern margins of the Vodlozero domain; it is dated at 2.96-2.94 Ga (Figure 1). Peridotitic komatiites with 24.8-29.6% MgO contents in spinifex tex-tured varieties and 44.4-47.2% SiO2 are found in all the greenstone belts (Table 2, nos. 5, 7, 9, 11). They belong to the silica-undepleted type (CaO/Al2O3 = 0.5-0.9, Al2O3/TiO2 = 15-25) and are high in Ni (950-1450 ppm) and Cr (2000-4000 ppm). The komatiites have unfractionated REE patterns ((La/Yb)N = 1±0.1, (Gd/Yb)N = 1±0.1) and REE abundances 1.5-4 times the chondritic at (Nb/La)N of ca. 1 (Figure 4a). Less frequently, the komatiites are depleted in the light REE and have (La/Yb)N =

0.6-0.7, (Gd/Yb)N = 1, and (Nb/La)N = 1-1.2. The £Nd(t) value in komatiites from the western surroundings of the domain is +1.7, and from the eastern, +2.2. In the Hautavaara belt komatiites (the domain’s western margin), Ni contents (650 ppm) are lower than in komatiites from the other belts. The Hautavaara komatiites are enriched in the light REE ((La/Yb)N = 1.3±0.1 and (Gd/Yb)N = 0.9±0.1) and have (Nb/La)N ratios of 0.5-0.7 and negative eNd(t) values; this set of evidence suggests crustal contamination for the ko-matiitic melt (Figure 4a).

Basalts found in the Oster, Palaya Lamba, and Hautavaara belts along the western margin the Vodlozero domain and in the Kenozero belt at its eastern margin, have different geochemical characteristics. Thus, basalts associated with komatiites have mantle Ti/Zr ratios (100-110), unfractionated REE patterns ((La/Sm)N = 1.0-0.9 and (La/Yb)N = 1.1-1.2), and REE abundances 7-14 times the chondritic (Table 2, nos. 6, 8, 10, 12; Figure 4b). The high Ni contents (>100-150 ppm), Nb/Y ratios in excess of 0.1, Zr/Y ratios of 2-3, and Nb/La = 0.9-1.11 of this basaltic group, are similar to those of oceanic plateau rocks. The £Nd(t) value in high-temperature uncontaminated basalts ranges from +0.5 to +3.2, suggesting source heterogene-

Figure 4. Spidergrams for komatiites (a) and high-temperature basalts (b) from greenstone belts of the western margin of the Vodlozero domain.

ity and/or mixing of melts from depleted and undepleted sources.

Geochemical features of the Hautavaara belt komatiites and basalts (reduced Ni contents, La/Yb>1, Nb/La<0.8 (Table 2, nos. 5, 6)) imply that these rocks make part of a plateau generated on continental crust. Komatiites occurring in association with basalts at Palaya Lamba are also likely to represent a fragment of a plateau generated on continental crust. This is evidenced by the surviving low-angle attitudes of volcanic flow units and by the superimposed deformations (high angle schistosity related to the subsequent accretionary phase and, at the same time, conformable to bedding planes, as should be expected in an oceanic plateau obducted onto a continental margin).

Detailed studies performed in the Oster and Semch greenstone belts have shown that alongside basalts similar to those just described, these belts contain basalts whose chemical features and spatial association with andesites suggest an analogy with modern volcanics generated in island-arc and backarc basin settings (Figure 5). Later on, basalts that were generated in various geodynamic settings underwent tectonic juxtaposition, to form a collage.

At the northern margin of the Vodlozero domain, mafic-ultramafic volcanics are dated at 2913-2916 Ma [Lobach-Zhuchenko et al., 1999; Puchtel et al., 1999; Sochevanov et al., 1991]. These volcanics occur in the Shilos and Kamen-nye Ozera bimodal greenstone belts. Komatiites are only encountered in the Kamennye Ozera belt, where they are

Figure 5. Zr/Y vs. Nb/Y plot for basalts from the western margin of the Vodlozero domain showing fields for basalts from various geodynamic settings, after [Kerr et al., 2000].

represented by peridotitic varieties that, where spinifex tex-tured, show high MgO, Cr, and Ni contents (Table 2, no. 16). The komatiites are depleted in the light REE ((La/Yb)N = 0.6-0.7), their medium- and heavy REE contents corresponding to those of primitive mantle (Figure 6). The ko-matiitic (Nb/La)N ratio is 0.9-1.0, suggesting lack of crustal contamination of the melts. Geochemical characteristics of the komatiites testify to their having originated from high-temperature plume melts in oceanic or rifted continental margin settings.

Figure б. Spidergrams for komatiites (a) and high-temperature basalts (b) from greenstone belts of the northern margin of the Vodlozero domain.

Figure Т. Zr/Y vs. Nb/Y plot for basalts from the northern margin of the Vodlozero domain showing fields for basalts from various geodynamic settings, after [Kerr et al., 2000].

Basalts of the northern margin have a broad range of compositions. In the Shilos belt, two basalt groups are discerned (Table 2, nos. 14, 15, Figure 6b). Both basaltic groups are high temperature rocks, considerable distinctions between them occur in their Ti and Zr abundances and REE enrichment degrees. Group 1 basalts are light REE depleted ((La/Yb)N = 0.5-0.7, (La/Sm)N = 0.6) and slightly HREE depleted ((Tb/Yb)N = 1.2). Their REE contents are 2.53.5 times the primitive mantle (Figure 4b). Group 2 basalts have (La/Yb)N = 1.9 and (La/Sm)N = 1 and REE contents 6-8 times the PM values. Both basaltic groups lack evidence of crustal contamination; their (Nb/La)N ratio ranges of 0.8-1.5. The early stage tholeiites of the Kamennye Oz-era greenstone belt (Table 2, nos. 17, 18) fall into groups with distinctive mg# (from 0.62 to 0.53) and high Cr and Ni abundances. These tholeiites are depleted in the light REE and show no crustal contamination, their (Nb/La)N ratio ranges of 0.8-1.7.

Geochemical characteristics of both basaltic sub-groups of the Shilos belt and early stage basalts of the Kamennye Oz-era belt are consistent with those of modern oceanic plateaus or continent margin plateau basalts (Figure 7). The discrepancies in the eNd(2916) values of the Shilos basite isochron (+1.6) [Sochevanov et al., 1991] and those of Kamennye Oz-era (+2.7) [Puchtel et al., 1999] are due to different isotope compositions of their initial melts and imply melt derivation from various parts of a heterogeneous source, which is feasible if melting occurs in the plume head.

Basites With 2.88—2.80 Ga Ages

The third episode of mafic magmatism, dated to the 2.88-

2.81 Ga time interval, is recorded in various domains of the Fennoscandian shield (Figure 8). During this stage, komati-itic and basaltic eruptions took place in the Kola Peninsula (Polmos-Poros belt, Uraguba, and Korvatundra, dated at 2.88 Ga [Vrevsky, 2000]), in northern Karelia (early vol-

Figure S. Sketch map of the Fennoscandian shield. 1 - ancient crustal domains with ages older than 2.9 Ga; 2 - greenstone belts with ages of 2.88-2.80 Ga; 3 - paragneissic belts; 4 - newly generated crust with an age of 2.85-2.74 Ga; 5 - basites dated at 2.88-2.80 Ga.

canism of the north Karelian system of greenstone belts, dated at >2.81 Ga [Kozhevnikov, 2000]), and in the western Karelian domain (Kostomuksha belt, dated at 2.84-

2.81 Ga [Lobach-Zhuchenko et al., 2000a; Puchtel et al., 1998]). High-temperature peridotitic komatiites with 2231% MgO and 44.4-47.2% SiO2 (Table 3, nos. 1, 4-6) are present in all the belts of this particular stage. They belong to the undepleted or slightly silica-depleted type (CaO/Al2O3 = 0.6-0.9, Al2O3/TiO2 = 15-25) and have high Ni (800-1600 ppm) and Cr (2000-4000 ppm) contents. Komatiites from most belts of this stage have unfractionated REE patterns ((La/Yb)N = 1±0.1, (Gd/Yb)N = 1±0.1), REE abundances 1.5-3 times the chondritic (Figure 9), and a (Nb/La)N ratio of ca. 1. Our own detailed study of komati-

ites from the Kostomuksha belt in the western Karelian domain shows them to be depleted in the light REE ((La/Yb)N = 0.4-0.6, (Gd/Yb)N = 1.04-1.16, and (Nb/La)N = 1-1.2). The £Nd(t) value in the komatiites ranges from +2.7 to +2.9, pointing to generation of their initial melts from a depleted source and to lack of crustal contamination. Basalts of greenstone belts of this stage (Table 3, nos. 3, 7-9) have mantle Ti/Zr ratios (100-116), unfractionated REE patterns ((La/Yb)N = 1.0-0.9), and REE abundances 7-14 times the chondritic (Figure 9). The high Ni contents (75-135 ppm), the ratios of Nb/Y = 0.1 and Zr/Y = 2-2.5 (Figure 10), and Nb/La >1.0 are earmarks of rocks generated in oceanic or continental plateaus in the absence of crustal contamination. However, the basaltic sequence of the Kostomuksha green-

Table 3. Contents of major (%), trace, and rare-earth elements (ppm) in representative samples of high-temperature mafic rocks of the 2.88-2.80 Ga stage

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Samp. no. 576-4 576-6 574-2 737-2 82 91155 44 123 25 90 43 1 868a 1002 849 112

SiO2 45.41 50.97 50.87 48.12 47.47 45.1 49.82 49.27 49.72 47.44 50.66 47.26 51.72 52.74 53.60 50.93

TiO2 0.37 0.53 1.36 0.35 0.32 0.41 0.88 0.81 1.00 1.48 1.45 1.36 1.94 0.38 0.40 0.32

Al2O3 7.5 13.06 15.28 6.04 6.70 7.01 15.14 13.72 15.89 14.66 14.76 18.11 19.55 18.26 19.09 21.85

FeO 13.66 10.43 12.28 10.14 11.19 12.69 12.86 13.01 12.03 12.14 13.59 12.16 9.87 6.86 5.68 6.07

MnO 0.32 0.24 0.22 0.14 0.14 0.16 0.20 0.19 0.14 0.23 0.21 0.13 0.14 0.22 0.10 0.07

MgO 31.74 10.15 5.86 29.40 27.92 27.20 6.75 7.06 7.31 8.00 6.19 4.82 3.01 6.62 5.80 7.22

CaO 0.52 13.69 10.15 5.62 5.46 5.93 11.95 11.47 9.97 11.69 9.45 9.78 7.05 9.38 9.40 9.38

Na2O 0.09 0.67 2.56 0.14 0.40 0.02 2.67 3.75 1.79 2.58 3.30 2.86 3.99 2.63 3.12 2.69

K2 O 0.02 0.26 1.23 0.04 0.16 0.02 0.23 0.35 0.20 0.18 0.17 0.72 0.74 0.48 0.51 0.75

P2O5 0.35 0.17 0.74 0.05 0.06 0.07 0.07 0.02 0.09 0.11 0.80 0.50 0.06 0.08 0.10

mg# 0.81 0.63 0.46 0.84 0.82 0.79 0.48 0.49 0.52 0.54 0.45 0.41 0.35 0.63 0.65 0.67

Rb 3 8 15 4 1.3 14 10 4 4 6 15 16 16 15 35

Sr 74 97 107 11 13.8 159 140 108 132 170 559 623 381 628 338

Y 5 12 30 8 9.6 20 25 22 29 28 16 22 12 15 13

Zr 19 24 69 20 20 24.5 52 50 55 75 83 69 176 57 60 36

Nb 1 3 0.768 2.5 2.5 3.8 4 3.5 4 10 5

Th 0.05 0.42 0.049 9 23 9

Ti 2209 1814 2460 6043 4832 6000 7755 8677 7653 11589 2344 2987 2012

Cr 4537 697 262 3184 3812 323 269 274 245 151 37 26 287 178 165

Ni 844 172 75 1627 1167 123 117 135 77 74 13 16 143 84 90

Co 92 62 59 103 113 58 47 51 15 22

V 233 363 339 107 202 405 330 432 265 143 114 86 96

La 0.68 2.80 0.25 0.505 2.5 2.6 2.6 4.7 4.7 7.8

Ce 2.02 8.2 1.32 1.66 7.8 6.5 12 10 52.2 61.8 19.4 14.5 13

Nd 1.78 6 1.043 1.32 1.87 5.9 6.2 4.9 9.2 7.5 25.8 28.7 10.1 7.6 6.6

Sm 0.72 2.2 0.430 0.52 0.81 1.96 2.2 2.13 3.27 2.13 5.18 6.12 2.35 1.72 1.54

Eu 0.123 0.9 0.020 0.27 0.9 0.79 1.12 0.76 2.4 1.66 0.82 0.35 0.69

Gd 1.00 3 0.960 1.21

Tb 0.130 0.56 0.53 0.79 0.51 0.57 0.73 0.36 0.3 0.32

Yb 0.74 2.3 0.58 0.834 1.69 2.3 3.2 2.2 0.95 1.24 0.84 0.72 0.82

Ti/Zr 98 116 110 91 100 116 97 109 103 105 111 66 41 50 56

Nb/Y 0.2 0.1 0 0.08 0.13 0.1 0.17 0.14 0.125 0.25 0.45 0.38

Zr/Y 3.8 2 2.3 2.5 2.55 2.6 2 2.5 2.59 2.964 4.31 8 4.75 4 2.77

Nb/La 1.47 1.07 1.52 1.0 0.96 1.46 0.85 0.745 0.64

stone belt, alongside uncontaminated basalts (Nb/La >1, SNd(t) = 2.9), hosts basalts that probably suffered contamination (Table 3, nos. 10, 11), their eNd(t) value ranging from -3.1 to +0.9 [Lobach-Zhuchenko et al., 2000a]. This implies that basalts of the Kostomuksha greenstone belt are plume-derived melts erupted in a volcanic rifted margin setting.

Intrusive mafic magmatism of the 2.85-2.84 Ga interval is recorded to have taken place on the western and northern margins of the Vodlozero domain, which had been formed through accretion. Mafic intrusives (the Semch, Palaya Lamba, and Shilos massifs; Table 3, nos. 12-16) have rather high SiO2 contents at elevated (0.5-0.8) mg#, Ti/Zr ratios of 40-80, an evolved REE pattern ((La/Yb)N = 5-12) (Figure 11), negative Nb anomaly ((Nb/La)N = 0.8-0.5), and negative eNd values (from -0.8 to -1.5), features that are suggestive of a variable degree of crustal contamination for the initial melts.

Basites With 2.72—2.66 Ga Ages

The Late Archean stage of mafic magmatism was less vigorous as compared to the preceding ones. It spans a ca. 2.72-2.66 Ga time interval. This stage is distinguished by moderate- and low-pressure granulite metamorphism first affecting a number of regions in southwestern and western Karelia (Lake Tulos, Lake Kuito), south-central Finland in the vicinity of Iisalmi (all the structures of the western Karelian domain), and between Lake Kovdozero and Lake No-tozero in the Belomorian domain. Basites of this stage are chiefly developed in northern Karelia and the Belomorian region. They are represented by: the late-stage komatiites of the north Karelian group of belts (>2705 Ma) [Shchipan-sky et al., 1999] and, apparently, of Finland [Gruau et al., 1990; Jahn et al., 1980]; the gabbronoritic and high-Ti, high-

Figure 9. Spidergrams for komatiites and basalts of the 2.88-2.80 Ga stage. Sample numbers on diagrams correspond to those in Table 3.

Fe gabbroic intrusions dated at 2.69 Ga in the regions of Lake Notozero and the Tupaya Guba Bay of Lake Kovdozero [Lobach-Zhuchenko et al., 1993, 1995]; and dikes developed throughout the shield (Figure 12; Table 4, nos. 1-5). Geochemical characteristics of both the volcanic and intrusive rocks alike (in particular, high Ni contents at high mg# of the rocks) testify to a plume related provenance for their initial melts [Campbell and Griffiths, 1992], whereas the elevated SiO2 contents, evolved REE patterns, Ti/Zr ratios of 60-70, and negative Nb anomaly (Nb/La <1) (Figure 13) point to a considerable crustal contamination of initial melts for both the intrusive and volcanic rocks. Evidence for a Late Archean plume within the Karelian province includes (besides the formation of high-temperature basites) the emplacement (in the 2.70-2.68 Ga time interval) of postoro-genic intracratonic granitoid intrusions featured by high contents of HFS elements such as Y, Zr, Ti, and Nb [Lobach-Zhuchenko, 2002].

Figure 10. Zr/Y vs. Nb/Y plot for basaltic komatiites and basalts of the 2.88-2.80 Ga stage (Hizovaara and Kostomuk-sha greenstone belts).

Figure 11. Spidergrams for intrusive basites of the 2.882.80 Ga stage.

It is open to discussion whether or not this particular age group includes the mafic-ultramafic volcanics from greenstone belts of eastern Finland. Not inconceivably, they belong to the preceding stage. Komatiites of these belts have the following compositional parameters: 22-27% MgO at 0.80-0.77 mg# and 9.5-19.8% MgO at 0.76-0.50 mg#; CaO/Al2Os ratios of 0.72-0.87, AhOs/T^ = 16-17, and Ti/Zr = 110; and Ni contents of 800-1500 ppm and 300650 ppm [Gruau et al., 1990; Jahn et al., 1980]. In this age group, komatiites make three sub-groups with dissimilar REE patterns: (1) light REE depleted ((La/Sm)N = 0.3-0.6 and (Gd/Yb)N of ca. 1.0); (2) with a flat REE pattern or a slight LREE enrichment ((La/Sm)N of ca. 1 and (Gd/Yb)N = 1.0-1.32); and (3) with REE abundances 1.5-2 times the mantle values and HREE depletion ((La/Sm)N = 0.7-0.95 and (Gd/Yb)N = 1.7-1.4). The basalts are also divisible into three groups: (1) uncontaminated, with Ti/Zr ratios of 100-110, (La/Sm) = 0.62, (Gd/Yb)N of ca. 1.0, (La/Sm)N of ca. 1.0-0.96, and (La/Yb)N of ca. 1.0, and with REE abundances 7-20 times the chondritic; (2) contaminated, with Ti/Zr = 70, (La/Sm)N of ca. 1.5-2, and (La/Yb)N of ca. 2-4; (3) subalkaline basalts with a high mg# (0.60), relatively high total alkali abundances (up to 6%) and Rb, and high Zr, P2O5, and REE. These rocks also have fractionated REE patterns ((La/Sm)N of ca. 4 and (Gd/Yb)N of ca. 4). The existence within the same greenstone structure of such a broad compositional variety of high-temperature volcanics is in good agreement with their plume origin and is not inconsistent with eruptions in a rifted continental margin setting.

Basites With 2.50—2.41 Ga Ages

The next stage of mafic-ultramafic magmatism belongs to the Early Proterozoic period and spans a 2.5-2.43 Ga time interval. Basites of this stage are widespread throughout the Fennoscandian shield and occur as both volcanic and intrusive varieties (Figure 14). These basites are represented by the layered intrusions of the Kola Peninsula, northern Karelia and Finland, Burakovskaya intrusion in southeastern Karelia, numerous drusite intrusions (coronitic gabbronorites) of the Belomorian region, and volcanics of the Vetreny belt and a number of smaller Sumian structures

Figure 12. Sketch map of the Fennoscandian shield. Symbols, as in Figure 8. Red circles denote radiometrically dated basites with ages of 2.72-2.66 Ga.

ations diagrams show silica enrichment (SiO2 2-4% higher than in Archean komatiites and basalts or in their later, Pro-terozoic counterparts; Figure 15), owing to which their initial melts are not infrequently referred to as being “boninite-like” [Sharkov et al., 1994]. On most variations diagrams, the drusites fall into two suites: magnesian (mg# = 0.81-0.49) and high-Fe (mg# = 0.53-0.29), featured by contrasting differentiation trends. According to their MgO/TiO2 ratios, the magnesian drusites plot in the komatiitic field, and the high-Fe ones, in the tholeiitic field (Figure 15). Drusites of the magnesian suite are dominant. They are typically high in Zr and have low Ti/Zr (50-80) and a high Zr/Y (3-6) ratios, as compared to the mantle. These rocks are high in Cr (130-3652 ppm) and Ni (up to 1300 ppm). In the high-Fe drusite suite, the Ti/Zr ratio ranges 60-140, and the Zr/Y ratio is similar to that in the magnesian drusites. Drusites of both suites are enriched in the light REE ((La/Yb)N = 5-10, (La/Sm)N = 2.5-3.5, and (Gd/Yb)N =1.2-2.0) (Figure 16), the total REE content in the high-Fe drusites being higher than in the magnesian ones. The drusites of both suites have negative Nb (Nb/La <1) and phosphorus anomalies and positive Pb anomaly, just like the crustal tonalites and gneisses. We interpret the contrasting chemistries of the drusites of the two suites within the same massif as evidence of cryptic layering, which is traceable even in the marginal portions of the massifs, where the drusites are turned to

in Karelia and the Kola Peninsula. In terms of geochemical and isotope characteristics, both the volcanic and intrusive basites of this age are closely similar (Table 4, nos. 6-16). All the basites of this group have elevated SiO2 contents at increased MgO and high Ni abundances. Among all the rocks of this group, our most detailed studies were centered on the Belomorian drusites. Drusite composition plotted on vari-

Rb Ba Th Nb К La Ce Nd Sm Eu Gd Tb Yb Sr Zr Ті Y

Figure 13. Spidergrams for komatiites and gabbronorites of northern Karelia of the 2.72-2.66 Ga stage. Sample numbers on diagrams correspond to those in Table 4.

garnet amphibolite. All the basites of this group (drusites, mafic rocks from layered intrusions, and volcanics) have eNd values between 0 and -2.5 (Table 1).

Melts initial to the high-Mg rocks with elevated silica contents and low Ti/Zr ratios may have been generated in three ways: (1) melting of water saturated mantle wedge in subduction zones (boninite model), (2) assimilation of felsic crustal material by mantle melt, and (3) mixing of high-temperature plume melts with partial melts of depleted harzburgitic lithospheric mantle. The boninite model for drusite melt generation is at odds with the high TiO2 contents and the low Al2O3/TiO2 ratio. Besides, oxygen isotope composition data from igneous minerals in the drusites (518O ranging 8-4) testify to their crystallization from dry melts [Salye et al., 1983]. The fact that the drusites are light REE enriched and have high Ni (600 ppm or more) with at least 15% MgO is rather suggestive of a plume nature for their parental melt [Campbell and Griffiths, 1992].

We have presented quantitative model calculations that involve contamination of picritic or komatiitic melts by crustal material (Belomorian granite or biotite-garnet gneisses) (Figure 17) and mixing of an EM1 type enriched melt with a high-magnesian, high-silica melt derived from residual harzburgite mantle [Lobach-Zhuchenko et al., 1998]. Mass balance calculations using both major and the rare

earth elements allow for both processes. The low Nb/La ratio, assimilation and mixing calculations in the La/Sm - £Nd reference frame, and the eNd variations and values in comparatively small massifs, all suggest that the most likely model is the one that invokes contamination of an undepleted plume melt by Late Archean crustal material.

Conclusions

Analyzing the timing, spatial position, and distribution of Early Precambrian basites throughout the Fennoscandian shield point to a number of regularities. Although it is impossible to establish the duration of the earliest radio-metrically dated stage of mafic magmatism, it can be ascertained that each successive stage spans a time interval of 70-80 m.y. The early stages of high-temperature mafic magmatism (>3.1 and 2.99-2.91 Ga) are confined to the most ancient core of continental crust on the Fennoscandian shield; namely, the Vodlozero domain with crustal age of 3.2-3.4 Ga. The first long-lasting stage of mafic magmatism (2.99-2.91 Ga) took place following a pattern that is classical to a number of modern plumes, and in which the first occurrences of high-temperature plume magmatism emerge in

Figure 15. SiO2 vs. MgO, TiO2 vs. MgO, Al2O3 vs. MgO, and CaO vs. MgO variation diagrams for Belomorian basites (drusites) with ages of 2.50-2.41 Ga. 1 - drusites of the magnesian suite; 2 - drusites of the high-Fe suite.

Figure 16. Spidergrams for 2.50-2.41 Ga komatiites and gabbronorites. Sample numbers on diagrams correspond to those in Table 4.

Table 4. Contents of major (%), trace, and rare-earth elements (ppm) in representative samples of high-temperature mafic rocks of the 2.72-2.66 Ga (1-5) and 2.50-2.41 Ga (6-16) stages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Samp. no. 683 VK 18 104a 3v 212 238 39 153 308d 979 158 157 1 5 291-5 91

SiO2 49.35 51.66 51.86 53.27 44.31 49.04 49.36 49.5 49.6 55.65 54.9 49.66 51.58 52.98 55.64

TiO2 0.57 0.37 1.07 0.6 0.2 0.3 0.46 0.49 1.4 0.83 1.35 0.55 0.64 0.88 1.32

Al2O3 9.95 13.67 15.08 13.85 3.97 6.92 12.03 10.00 15.70 13.50 12.10 11.46 12.71 10.24 12.90

FeO 10.39 8.50 10.70 9.21 11.32 10.23 7.26 10.32 8.73 11.19 14.22 10.36 9.74 9.75 12.12

MnO 0.24 0.14 0.09 0.14 0.20 0.14 0.16 0.16 0.10 0.15 0.16 0.19 0.19 0.17 0.19

MgO 20.36 11.40 8.00 9.57 31.45 23.72 15.87 17.10 5.50 6.47 3.45 14.24 12.05 8.90 6.09

CaO 7.82 9.08 8.80 8.35 2.65 6.20 8.53 8.20 10.10 7.30 7.07 9.00 8.89 11.25 5.48

Na2O 1.00 2.31 2.85 2.31 0.40 1.22 1.71 1.60 3.40 2.55 3.03 1.34 2.11 2.63 4.74

K2O 0.23 0.86 1.20 0.63 0.29 0.34 0.47 0.54 2.00 0.95 1.48 0.51 0.39 0.20 1.39

P2O5 0.09 0.08 0.17 0.12 0.11 0.06 0.08 0.10 0.27 0.01 0.01 0.06 0.07 0.09 0.14

mg# 0.78 0.71 0.57 0.65 0.83 0.81 0.80 0.75 0.53 0.51 0.30 0.71 0.69 0.62 0.47

Rb 18 22 35 14 21 6 9 10 16 44 16 41 5 4 30

Sr 44 234 274 388 232 17 141 155 145 339 213 249 150 427 109

Y 13 10 23 16 13 3 10 10 13 21 16 24 14 18 15

Zr 49 41 94 97 51 22 34 43 62 77 85 111 40 97 140

Nb 1 3 7 7 3 7 5 5 7 10 5 9 2 2 7 10

Th 0.8 12 13 <5 <5 16 10 9 15 15 9 12 1.5 3

Ti 3420 2860 6416 13856 3781 980 2004 2604 2883 7729 4400 8418 3300 3840 5280 7032

Ba 226 413 134 452 50 104 115 132 393 333 494 226 221

Cr 2252 344 156 49 195 3652 2751 1730 1659 97 197 54 1600 1700 550 107

Ni 509 292 107 43 47 1289 1005 489 659 92 59 15 386 403 298 57

Co 78 50 43 304 128 100 90 63 63 35 75 51 47 56 43 43

V 209 138 226 41 109 136 147 200 290 494 210 201

La 5.7 7.42 19.6 2.45 5.6 6.5 7.2 17 11.8 25.9 5.7 8.52 18

Ce 14.6 14.8 45.8 5.62 11.4 13.19 11.00 33.00 29.3 54.9 14 18 37

Nd 5.9 8.03 18.2 3.2 5.6 6.65 7.20 17.00 15.1 26.6 2.1 25

Sm 1.8 1.55 4.24 0.64 1.2 1.51 1.90 4.00 2.93 4.84 2.18 2.5 4.85

Eu 0.49 0.72 1.48 0.21 0.4 0.51 0.52 1.22 0.83 1.55 0.57 0.84 1.21

Gd 2.4 1.42 4.1 0.64 1.19 1.3 2.4 4.4 3.2 3.9

Tb 0.26 0.71 0.09 0.11 0.22 0.31 0.65 0.38 0.71 0.405 0.44 0.51

Yb 1.1 1.05 2.39 0.4 0.66 0.86 1.10 2.00 1.5 2.38 0.88 0.62 1.07

Ti/Zr 70 70 68 143 74 45 59 61 47 100 52 76 83 54 50

Nb/La 0.18 0.40 0.36 2.86 0.89 0.77 0.97 0.59 0.42 0.35 0.35 0.23 0.39

the central part of the continent, and subsequent ones, in its marginal parts. In the case in point, such first occurrence is the Lairuchei intrusion (2.99 Ga, in the central part of the Vodlozero domain) with the subsequent komatiite and high-temperature basalt volcanism along the western and eastern margins of the domain (2.94-2.96 Ga) and, possibly, at its northern margin (2.91 Ga). In all likelihood, beneath the Vodlozero domain there existed a deep seated plume (super-plume, or a first order plume, to use the terminology of Dobretsov and co-workers) that was rising from the interface between the core and the lower mantle. This particular inference is favored by the fact that the Nd isotope characteristics of a number of komatiites and mafic intrusions are explicable assuming mixing of melts derived from undepleted and depleted mantle sources or contamination of an undepleted mantle melt by crustal material. The rising mantle plume generated rift structures near the boundary and at

the margins of the Vodlozero domain. Brittle deformations that caused rifting related to the rising mantle plume did not result in breakup of the newly formed continental crust. The dominant values of model Nd ages (Tdm) in the 2.9-3.0 Ga time interval, obtained from rocks derivative from basalts that make up the younger domains [Lobach-Zhuchenko et al., 2000a, 2000b], suggest that basalts of this age were developed over a significant area outside the Vodlozero domain.

The next stage of high-temperature mafic magmatism (2.88-2.80 Ga) is expressed within the Kola and western Karelian domains with crustal ages of 3.0 and 3.1 Ga and on the north of the younger, central Karelian domain. Generation of komatiites and high-temperature tholeiite lavas in these domains provides evidence of a super-plume that ascended within these domains. The ascent of this plume initiated rifts beneath the continental crust of the Kola and western Karelian domains and beneath the oceanic plateau

La/Sm

Figure 17. AFC model [DePaolo, 1981] in the La/Sm-eNd. plane. The calculations assume (A) 10% melt assimilating depleted mantle with eNd(2.45) = +2.5 and (B) 10% CHUR melt assimilating crustal rock. Contaminants are Belomorian tonalite samples with a 2.75 Ga zircon age and Chupa Formation gneiss samples the with a 2.85 Ga zircon age. Geochemical characteristics of the samples are given in [Lobach-Zhuchenko et al., 1998].

in the northern part of what is now the central Karelian domain. According to [Vrevsky, 2000], the higher liquidus temperatures for the komatiites from greenstone belts of the Kola domain and the greater depth of generation of their initial melts, as compared to the liquidus temperatures for the older komatiites from the Vodlozero domain, should imply a higher ascent for the early plume.

As the deep mantle plume ascended beneath the northern to northwestern part of the shield, its south-southeastern part (namely, the northwest margin of the Vodlozero domain) provided the stage for extensive development of mafic magmatism and granitoids with ages of 2.85-2.80 Ga, whose generation is attributed to underplating [Lobach-Zhuchenko et al., 1999]. These facts suggest the existence, in parallel to the superplume, of another plume that may have been less deep seated, and that was initiated at the interface between the lower mantle and the upper mantle, inasmuch as this magmatism immediately postdated the termination of sub-duction processes at the western margin of the Vodlozero

domain. The ascent of this mantle plume may have been triggered by mantle slab sinking to the interface between the lower mantle and the upper mantle.

The last of the Archean stages of high-temperature mafic magmatism with ages of 2.72-2.66 Ga occurs in the north Karelian belts, in the Karelian part of the Belomorian area (the regions of Lake Notozero and the Tupaya Guba Bay of Lake Kovdozero) and, possibly, in the western Karelian domain. This magmatism took place also immediately after the subduction processes at the boundary of the Karelian and Belomorian domains. Accordingly, the mantle plume that ensured generation of high-temperature mafic melts, rose from the interface between the lower mantle and the upper mantle immediately following the end of the subduc-tion processes. The majority of high-temperature melts, of both volcanic and plutonic provenance alike, suffered crustal contamination, which points to the existence of thick continental crust.

Early Proterozoic high-temperature mafic magmatism at

2.50-2.41 Ga was the most extensive areally and the longest lasting on the Fennoscandian shield. Nearly all the researchers of high-temperature basites of this stage attribute this magmatism to the ascent of an extensive deep superplume [Amelin and Semenov, 1996; Arestova and Lobach-Zhuchenko, 1996; Hanski et al., 2001; Lobach-Zhuchenko et al., 1998; Puchtel et al., 1997; etc.]. Circumstantial evidence for the existence, in the 2.5-2.41 Ga time interval, of a long lived heat source that occupied virtually the entire area of what is now the Fennoscandian shield may be provided by paleomagnetic data. Nearly all the Archean basites measured yielded an additional magnetic component, whose age is estimated at 2.5-2.45 Ga [Arestova et al., 1999, 2000]. A hallmark of the mafic rocks of this stage is that all the volcanic and plutonic varieties without exception show varying degrees of crustal contamination, which is a further evidence that by the beginning of the Early Proterozoic there had been formed a thick continental crust, probably continuous beneath the entire eastern (Archean) part of the Fennoscan-dian shield.

Our analysis of the spatial position, timing, distribution, and geochemical characteristics of the high-temperature Early Precambrian basites of the Baltic shield suggests the following conclusions:

1. In the Early Precambrian of the Baltic shield (3.4-

2.4 Ga), established are five stages of high-temperature mafic-ultramafic magmatism, most of which is attributable to the action of the plume-tectonic mechanism that ensured the inflow of lower mantle material and heat to cause melting in the upper mantle and crust.

2. The plume-derived high-temperature komatiitic melts, showing no crustal contamination and originating from depleted sources, are heterogeneous in terms of their Nd isotope compositions; accordingly, they either are derivatives from second-order plumes or result from mixing of plume material and plume-entrained portions of depleted upper mantle.

3. The plume-derived melts intruded both the newly formed continental crust and surviving oceanic crust, giving rise to deep-seated intrusions in the rather thick continental crust and to volcanics in continental and oceanic plateau settings. As a rule, intraplate rifting occurred in marginal parts of the sialic domains.

4. The process of interaction between initial mafic melts and crustal material (plume-crustal interaction) is established starting from the second recorded stage of mafic-ultramafic magmatism.

5. With increasing thickness of the continental crust of the Baltic shield, the degree of crustal contamination of mafic melts became progressively higher, to reach a maximum at the end of the Archean and beginning of the Proterozoic, during the fourth and, especially, the fifth stage of magma-tism.

Acknowledgments. This work was supported by the Russian Foundation for Basic Research (project nos. 01-05-64930 and 0205-65052).

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(Received 1 July 2003)