Silicic Fe-Ti-oxide series of slow-spreading ridges: Petrology, geochemistry, and genesis with reference to the Sierra Leone segment of the Mid-Atlantic Ridge axial zone at 6\deg N

Sharkov E. V.; Bortnikov N. S.; Zinger T. F.; Chistyakov A. V.

RUSSIAN JOURNAL OF EARTH SCIENCES, VOL. 7, ES4001, doi:10.2205/2005ES000181, 2005

Silicic Fe-Ti-oxide series of slow-spreading ridges: petrology, geochemistry, and genesis with reference to the Sierra Leone segment of the Mid-Atlantic Ridge axial zone at 6°N

E. V. Sharkov1, N. S. Bortnikov1, T. F. Zinger2, and A. V. Chistyakov1

Received 15 December 2004; revised 10 February 2005; accepted 1 March 2005; published 14 August 2005.

[1] Silicic Fe-Ti-oxide magmatic series was first recognized in the Sierra Leone axial segment of the MAR near 6°N. The series consists of intrusive rocks (harzburgites, lherzolites, bronzitites, norites, gabbronorites, hornblende Fe-Ti-oxide gabbronorites and gabbronorite-diorites, quartz diorites, and trondhjemites) and their subvolcanic (ilmenite-hornblende dolerites) and, possibly, volcanic analogues (ilmenite-bearing basalts). The deficit of most incompatible elements in the rocks of the series suggests that the parental melts were derived from a source that had already been melted. Correspondingly, these melts could not be MORB derivatives. The origin of the series is thought to be related to the melting of the hydrated oceanic lithosphere during the emplacement of an asthenospheric plume (protuberance on the surface of a large asthenospheric lens beneath MAR). The genesis of different melts was supposedly controlled by the ascent of a chamber of hot mantle magmas thought this lithosphere in compliance with the zone melting mechanism. The melt acquired fluid components from the heated rocks at the peripheries of the plume and became enriched in Fe, Ti, Pb, Cu, Zn, and other components mobile in fluids. Index

TERMS: 1032 Geochemistry: Mid-oceanic ridge processes; 1037 Geochemistry: Magma genesis and partial melting;

1065 Geochemistry: Major and trace element geochemistry; 3619 Mineralogy and Petrology: Magma genesis and partial melting; KEYWORDS: Slow-spreading ridges, Ultramafic cumulates, Fe-Ti-oxides.

Citation: Sharkov, E. V., N. S. Bortnikov, T. F. Zinger, and A. V. Chistyakov (2005), Silicic Fe-Ti-oxide series of slow-spreading ridges: petrology, geochemistry, and genesis with reference to the Sierra Leone segment of the Mid-Atlantic Ridge axial zone at 6°N,

[3] These rocks are still studied relatively poorly because they are commonly regarded as late derivatives of tholeiitic basalts in mid-oceanic ridges (MORB) [Dick et al., 1992; Silantyev et al., 1998]. However, Simonov et al. [1999] were the first to note that the parental melts of the Fe-Ti-oxide gabbroids could not be derivatives of either N-MORB or E-MORB, because they are oversaturated with TiO2. These rocks are sometimes regarded even as evidence of the existence of anomalous mantle sources [Cannat et al., 1992]. The nature of the trondhjemites is also uncertain: according to Dick et al. [1991], these rocks were generated by the fractional crystallization or anatexis of the host am-phibolized gabbroids, whereas Silantyev [1998] believes that they had had a separate deep source with anomalous geochemical characteristics typical of source like E-MORB.

[4] Our petrological and geochemical data obtained on these rocks from the Sierra Leone MAR segment indicate that these rocks display some unusual features which have not been adequately fully highlighted before. First of all, these rocks are characterized by low contents of incompatible elements, including LREE, which suggests that the parental

Russ. J. Earth. Sci., 7, ES4001, doi:10.2205/2005ES000181. Introduction

[2] The third crustal layer in slow-spreading ridges, such as the Mid-Atlantic Ridge (MAR) and the Southwest Indian Ridge, is known to consist of a broad spectrum of plutonic rocks, which include, along with primitive magnesian trocto-lites and gabbro, also ferrogabbro (Fe-Ti-oxide gabbro) and closely associated with it norites, gabbronorites, hornblende gabbronorites, and gabbronorite-diorites, quartz diorite, as well as veins and small bodies of plagiogranites (trond-hjemites) [Dick et al., 2000; Ozawa et al., 1991; Pearce, 2002; Silantyev, 1998].

1 Institute of the Geology of Ore Deposits, Petrography, Mineralogy, and Geochemistry (IGEM), Russian Academy of Sciences, Moscow, Russia

2Institute of Precambrian Geology and Geochronology, Russian Academy of Sciences, St. Petersburg, Russia

ISSN: 1681-1208 (online)

60° 50° 40° 30° 20° 10° 0° 10°

Figure 1. Location map of the study area (black square) among seafloor structures in the Central Atlantic (based on the data of satellite altimetry from [Sandwell and Smith, 1997].

melts of these rocks were derived from sources that had already been remelted, and, thus, these rocks could hardly be MORB derivatives. Moreover, these melts exhibit some extraordinary compositional features: on the one hand, they were saturated or even oversaturated with silica and had high water contents, as is typical of magmas of the supra-subduction calc-alkaline series, and, on the other hand, were rich in Fe, Ti, and Nb, as is characteristic of within-plate (plume-related) magmas.

[5] All of these features seem to suggest that slow-spreading ridges contain, along with MORB (which are typical of such environments) and their derivatives, also an unusual type of magmatic melts, which are enriched in silica and do not fall into any of the currently adopted systematics for magmatic. This publication is devoted to the petrological-geochemical characterization of these rocks and their genesis.

Geological Overview

[6] The axial MAR segment described in this paper lies between the Bogdanov Fracture Zone (7° 10' N) and 5° 00' N [Pushcharovskii et al., 2002; Skolotnev et al., 2003a], near the Sierra Leone Fracture Zone (Figure 1). The local seafloor

has a highly dissected topography, which is characterized by the absence of transform faults and the variable strike of the rift valley, which consists here of graben-shaped depressions up to 4-5 km deep, such as the Markov depression (Figure 2). This MAR segment also differs from adjacent areas in having lower seismicity.

[7] The results of dredging during Cruise 10 of the R/V Akademik Ioffe in 2001-2002 and Cruise 22 of the R/V Professor Logachev in 2003 suggest that long (>300 km) segments of both walls of the rift valley consist of plutonic rocks, including serpentinized peridotites, various gabbroids, cata-clasites and mylonites developing after mafic and ultramafic rocks, and various metasomatic rocks. Strongly altered and tectonized basalts and dolerites crop out in the floor of the valley, its northern and southern flanks, and in the upper parts of its walls. Fresh basalts with crusts of volcanic glass were found on valley floor and on the slopes of neovolcanic rises.

[8] In spite of the uneven character of the sampling, it can be definitely concluded that both walls of the valley contain similar rock complexes that characterize the whole sequence of the oceanic crust. Judging from the results of remote sensing (conducted with the use of a MAK marine acoustic complex during Cruise 22 of the R/V Professor Logachev in 2003), the local crust is clearly layered. This suggests that most of the rocks discussed in this paper compose a

Figure 2. Schematic map of the Sierra Leone MAR segment [after Skolotnev et al., 2003b]. 1 - site of dredging; 2 - site of CTD-sounding; 3-5 - profiles: 3 - multibeam, 4 - parasound, 5 - transition between sites.

layered complex, which is analogous to those in ophiolitic wide occurrence of ultramafics and gabbro indicate that the

associations like Monviso in the Western Alps [Lombardo et modern extension of the oceanic crust in this MAR segment

al., 2002]. proceeded at the predominance of tectonic over magmatic

[9] The abundance of sediments in the axial valley and the processes. All of the examined and sampled neovolcanic

Figure 3. Primitive leucocratic troctolite, sample Figure 5. Plagiogranite (trondhjemite) vein in ilmenite-I1069/16. Photo: A. A. Peyve. hornblende dolerite, sample I1060-57. Photo: A. A. Peive.

ridges are variably shifted to the east relative to the rift axis.

General Charactrization of the Rocks

[10] The rock samples discussed in this paper were dredged mostly from the slopes of the Markov depression (sites I1032, I1060, I1063, and I1069), and basalts were uplifted from a volcanic plateaus north of it (sites I1026, I1027, and I1072). The dredged material included both fresh and variably altered rock material. The completely metamorphosed rocks were classified into (a) those developing after ultramafic rocks, gabbro, and dolerites and (b) metamorphic rocks.

Figure 4. Texture of olivine hornblende gabbronorite with Fe-Ti oxides. Micrograph, thin section I1040-14, magnification 40x , one polarizer.

Petrography

[11] According to their petrographic characteristics, the plutonic rocks can be subdivided into two groups: (i) primitive magnesian troctolites (Figure 3) and gabbro and

(ii) gabbronorites and gabbronorite-diorites containing, Opx, Fe-Ti oxides, and kaersutite (Figure 4) and associated with norites, pyroxenites, granodiorites and plagiogranites (Figures 5 and Figure 6). As was mentioned above, the latter rocks occur mostly as veins and dikes up to 10-20 cm thick, most often 1-5 cm, and usually contain kaersutite and il-menite. These rocks seem to typical occur in slow-spreading ridges as such bodies, because they were also found as analogous bodies in other MAR segments [Silantyev, 1998] and in the Southwest Indian Ridge [Dick et al., 1991; Holm, 2002; Ozawa et al., 1991]. These peridotites are sometimes notably different from mantle residues in bearing relics of

Figure 6. Texture of plagiogranite (trondhjemite). Thin section I1060-57, crossed polarizers.

cumulus textures (Figure 7) and in having different compositions of minerals, particularly chromite (see below), which suggests that the rocks are intrusive.

[12] The dredged dolerite samples are fairly diverse: they include common olivine-clinopyroxene and olivine-free varieties, sometimes with elevated contents of ilmenite (up to 3 vol %), and plagioclase-phyric Ilm-Hbl dolerites. Along with fresh rocks, the dredged material contained altered varieties, whose pyroxenes and hornblende are partly or completely replaced by fibrous actinolite.

[13] Both the fresh and the altered basalts can be aphyric or porphyritic, with phenocrysts of Ol, Ol+Pl, or Pl alone. The groundmass of these basalts is usually ophitic or has an unusual texture with sheaves of recrystallized spherulites, which are clearly seen in glassy varieties. The latter type of the basalts usually bears abundant small ilmenite grains, a feature suggesting that these rocks are volcanic analogues of the second group of the plutonic rocks. The rocks often exhibit traces of high-temperature cataclasis with the development of deformation structures in magmatic minerals (Ol 1, Opx, Pl, and Hbl) and the appearance of small subequant neoblasts. The genesis of such high-temperature cataclasites was likely related to deformations in already solid but still hot rocks, because the minerals show no traces of low-temperature alterations, and the composition of the neoblasts is close to that of the cataclasts.

[14] Rocks of particular interest are the volcanic breccias that were cataclased under such conditions and were found among rocks from sites I1060 and I1063. Boudin-, oval-, and lens-shaped fragments (15-20 cm long) in these rocks consist of cataclased peridotites with relict cumulus textures (harzburgites and lherzolites) that were not affected by low-temperature alterations and are cemented by weakly cataclased fine-grained porphyritic rocks, whose composition varies from gabbronorite-diorite to granodiorite with ilmenite and magmatic kaersutite. It is worth noting that fragments (boudins) of these ultramafics bear practically no traces of low-temperature alterations at contacts with the diorite-granodiorite cement, as is also typical of relations between syngenetic vein derivatives and their host cumulates.

[15] The low-temperature alterations are spread much more broadly: the peridotites are usually extensively ser-pentinized, and the gabbroids are amphibolized with the development of fibrous actinolite after pyroxenes and parg-asite. The rocks are commonly cut by veinlets of carbonate, prehnite and chlorite. In places, the rocks were also affected by low-temperature shearing and brecciation. The thickest of these zones are associated with the development of diverse metamorphic rocks, which often have disseminated-stringer or massive sulfide mineralization [Pushcharovskii et al., 2002].

[16] In order to characterize the typical varieties of the

1 Mineral symbols: Ol — olivine, Opx — orthopyroxene, Cpx — clinopyroxene (Aug — augite), Pig — inverted pigeonite, Pig-Aug — inverted pigeonite-augite, Pl — plagioclase, Hbl — brown kaersu-titic hornblende, Qtz — quartz, Cht — chromite, Ilm — ilmenite, Ti-Mag — titanomagnetite, Zr — zircon, Ap — apatite, Am, — am-phibole, Act — fibrous actinolite; end members: Fo — forsterite, Fa — fayalite, En — enstatite, Fs — ferrosilite, Wo — wollastonite, An — anorthite, Ab — albite.

-

Opx

Figure 7. Cumulus orthopyroxene porphyroclast among olivine and pyroxene neoblasts in cataclased lherzolite. Thin section I1063/17, micrograph, magnification 40x, crossed polarizers.

rocks, we conducted their detailed petrological and geochemical examination, whose results are summarized below. The petrography of the rocks is briefly characterized in Table 1, and microprobe analyses of their minerals are presented in Tables 2-7.

Chemistry of Major Rock-Forming Minerals

[17] The mineralogy of many of our samples is quite unusual and is generally atypical of both mantle rocks and classic cumulates in continental layered intrusions. First of all, the chemistry of the same minerals varies even within a single thin section, and mineral crystals bear “seed” inclusions that are uncommon in these mineral assemblages.

[18] The composition of olivine in the rocks broadly varies from Fo91-85 in the peridotites and troctolites to Fo26 in the olivine gabbro-diorites (Table 2). A sample of the Fe-Ti-oxide olivine dolerite contains zonal olivine crystals with Fo82 in the cores and Fo62 in their peripheral portions. The composition of the olivine microphenocrysts in olivine plagioclase-phyric basalt I1072/1 corresponds to Fo87, i.e., is close to the composition of olivine in the primitive troctolite. An unusual feature of olivine in the peridotites with relics of cumulate textures is its variable composition: the predominant grains have the composition Fo86, and occasional grains correspond to Fo70 (sample I1063/17, Table 2).

[19] The composition of pyroxenes from the rocks is demonstrated in Table 3 and Figure 5. A noteworthy feature of the rocks is the presence of unexsolved subcalcic augite (pigeonite-augite). Such pyroxene was found even in the ul-tramafic rocks (sample I1063/2), a feature absolutely atypical of mantle rocks. It is also pertinent to mention that the composition of the pyroxenes varies even within a single thin section (Figure 8, Table 3).

Table 1. Petrography of the studied samples (minerals contents in vol. %)

no.no./ord. no. sample Name of the rock Texture and mineral composition

1 I1026/21 Pl-phyritic basalt, fresh Pl phenocrysts, microphenocrysts of Cpx and Ol; groundmass: mainly Pl with small amount of volcanic glass.

2 I1027/5 Aphiritic plagiobasalt, fresh Phenocrysts Ol; groundmass - mainly Pl with small amount of volcanic glass

3 I1032/7 Cataclased Ol gabbro (Ol+Cpx+Pl cumulate) Ol - 35%, Cpx (Aug - 30%, Pig - 10%), Pl - 30%; small amout og Chr. Ol partly serpentinous, and Pl - partly saussurited.

4 I1060/1 Small-grained dolerite Microdoleritic texture, formed by Cpx and Pl with small amount (~5-6%) of Ol and 3-4% Ilm. Cpx has specific paniculate texture.

5 I1060/2 Hornblende basalt with Ilm Fine-porphyritic texture with Pl phenocrysts; groundmass formed mainly by Hbl and Pl with minor Cpx (~10%), Ilm (~1%) and rare Ap.

6 I1060/10 Cataclased coarse-grained gabbro (Pl cumulate) Pl - 75% (An65); Cpx: Pig-Aug (W27En3gFs34) - 20%, Aug (W46En45Fsg) - 5% (relics within Pig-Aug); Pl often chloritized, pyroxenes partly replaced by fibrous Act.

7 I1060/12 Leucocratic gabbronorite (Opx+ Cpx+Pl cumulate) Pl - 70%, Opx - 20%, Cpx - 8%; interstitual Hbl - 2%; Opx partly replaced by talc and fibrous Act

8 I1060/18 Small-grained Fe-Ti-oxide granodiorite Mineral composition: Pl (An10.5) - 60%, Hbl - 25%, Qtz - 8%, Ilm - 6%, Ap - 1%.

9 I1060/57 Small-grained plagiogranite (trodhjeimite) Acid Pl (65%) and Qtz (30%) with small amount (5%) of Hbl and Cpx; small grains of Ilm, Ap and Zr are presented.

10 I1063/1 Small-grained cataclased Fe-Ti-oxide gabbronorite Porphyritic texture with lens-like grains of inverted Pig; Pl (An4o) - 55%, Cpx (W41 En32Fs27) - 30%, Opx (W3En46Fs51) - 10%, Ilm - 1%, Mag - 2%, Hbl - 2%; very rare - hyalosiderite Fo26.

11 I1063/2 Cataclased harzburgite (Ol+Opx+Crt cumulate), non serpentinized Porphyroclastes of Ol and Opx within fine-grained aggregate neoblastes the same composition: Ol (Fog1) - 75%, Opx (W1EnsrFs12) - 25%, rare Cpx (W45En47Fs8) and Chr.

12 I1063/5 Cataclased Fe-Ti-oxide Hbl gabbronorite-diorite Mineral composition: Pl (An40) - 75%, Hbl - 10%, Cpx (W43En3oFs27) - 5%, Opx (W2En48Fs5o) - 3%, Ilm - 3%, Ti-Mag - 2%, Ap - 2%. Pyroxenes and Hbl slightly amphibolized with evolving of fibrous Act.

13 I1063/6 Cataclased Fe-Ti-oxide Hbl gabbronorite Pl - 60%, Opx - 10%, Cpx - 15%, Hbl - 10%, Ilm - 5%; pyroxenes and Hbl partly replaced by fibrous Act.

14 I1069/16 Leucocratic troctolite (Ol+Pl cumulate) Ol (Fo85) - 15%, Pl (An76-80) - 75%, interstitial Cpx (W45En47Fs38) - 10%; rare Chr. Ol partly replaced by talc.

15 I1072/1 Ol-Pl basalt with crust of volcanic glass Small-porphyritic texture. Microphenocrysts Ol (Fo87) and Pl (An71_73) into the glassy basis; it is find oval xenocrysts of Pl (An76-78)

16 L1140/2 Ol-Pl basalt with crust of volcanic glass Small-porphyritic texture. Needle zonal Pl and microphecrysts of Ol; groundmass formed by paniculate grains of Cpx and needles of Pl; small grains of Ilm present.

Note. Ol — olivine, Opx — orthopyroxene, Cpx — clinopyroxene (augite), Pig — pigeonite, Hbl — brown magmatic hornblede (pargasite), Pl — plagioclase, Chr — chromite, Ilm — ilmenite, Mag — magnetite, Ap — apatite, Act — actinolite. Composition of minerals studied by microprobe.

Table 2. Representative analyses of olivine in rocks from the Sierra Leone MAR segment

Analyt. spots SiO2 TiO2 Al2O3 Cr2O3 FeO MnO MgO CaO Na2O NiO V2O5 P2 O5 Cl Fo

I1063/1-6 43.08 0.10 1.06 0.01 45.51 0.50 9.00 0.14 0.43 0.00 0.10 0.01 0.15 0.26

I1063/2-5 42.01 - - - 8.60 0.12 48.90 0.04 - 0.33 - - - 0.91

I1063/17-2 37.07 0.01 0.05 0.02 26.55 0.43 35.35 0.03 0.14 0.34 0.00 - 0.01 0.70

I1063/17-9 40,66 0.12 0.02 0.04 12.77 0.27 45.95 0.08 0.02 0.07 0.00 0.00 0.00 0.86

I1069/16-4 39.90 0.00 0.13 0.00 14.31 0.11 45.23 0.06 0.11 0.15 0.00 - 0.00 0.85

I1072/1 40.00 - - - 12.46 0.21 46.87 0.30 - 0.15 - - - 0.87

L1141/15(c) 38.88 0.00 0.00 - 17.41 0.37 42.79 0.38 0.17 - - - - 0.82

L1141/15(r) 36.33 0.16 0.00 - 33.10 0.49 29.53 0.24 0.15 - - - - 0.62

Note. Analyses presented here and below were conducted on a CamScan and Cameca microprobes (at the Moscow State University and the Institute of the Geology of Ore Deposits, Petrography, Mineralogy, and Geochemistry, Russian Academy of Sciences, respectively). All analytical totals are normalized to 100 wt %; dashes mean that the components was not analyzed. Samples: I1063/1 — Fe—Ti-oxide hornblende gabbronorite; I1063/2 — fresh cataclased harzburgite with relict cumulate textures; I1063/17 — fresh cataclased lherzolite with relict cumulate textures; I1069/16 — fresh leucocratic troctolite (forellenstein); I1072/1 — microphenocrysts from basalt; L1141/15 — Fe—Ti-oxide olivine dolerite, the olivine is zonal: c — core, r — rim of the grain. No zoning was detected in the thin sections.

Table 3. Representative analyses of pyroxenes in rocks from the Sierra Leone MAR segment

Analyt. spots SiO2 2 iO2 Ti 3 O3 2 Al FeO 3 O3 2 r C MnO MgO CaO O 2 a N O 2 V to O cn NiO P2O5 Cl Wo En Fs

I1028/3-1 51.64 0.46 1.77 10.03 - 0.28 13.12 21.84 0.56 0.10 0.07 0.13 - - 45.4 37.9 16.7

I1028/3-4 51.28 0.92 2.19 12.04 - 0.25 13.23 19.42 0.62 0.00 0.00 0.05 - - 41.0 38.8 20.2

I1028/3-8 51.70 0.24 0.80 25.57 - 0.47 19.94 0.82 0.33 0.04 0.06 0.00 - 0.02 1.7 57.1 41.2

I1060/3-3 53.30 0.09 0.90 8.68 0.11 0.22 14.41 21.80 0.32 0.06 0.00 0.00 0.03 0.07 44.9 41.2 13.9

I1060/10-1 51.94 0.43 2.58 5.81 0.25 0.26 15.83 22.17 0.36 0.02 0.02 0.20 0.10 0.02 45.5 45.2 9.3

I1060/10-3 52.17 0.15 3.90 18.44 0.01 0.32 12.97 11.16 0.72 0.07 0.00 0.00 0.03 0.07 25.6 41.4 33.0

I1060/10-4 51.54 0.05 3.90 20.42 0.06 0.32 11.26 11.62 0.48 0.06 0.06 0.02 0.17 0.02 28.9 36.2 36.9

I1063/1-1(m) 50.83 0.25 1.25 16.36 0.00 0.58 10.90 19.43 0.34 0.04 0.00 0.00 0.03 0.03 41.0 32.0 27.0

I1063/1-2l 50.07 0.07 0.25 31.68 0.04 0.91 14.22 2.52 0.13 0.02 0.00 0.00 0.08 0.01 5.4 42.0 52.6

I1063/1-7 50.15 0.24 0.50 31.04 0.17 0.71 15.72 1.34 0.00 0.07 0.00 0.00 0.05 0.00 2.8 46.1 51.1

I1063/2-1 56.37 0.03 3.05 5.56 0.50 0.13 33.33 0.97 0.00 0.00 - 0.06 - - 2.0 89.5 8.5

I1063/2-2 55.71 0.01 2.91 5.75 0.69 0.14 33.86 0.87 0.00 0.00 - 0.06 - - 1.6 89.8 8.6

I1063/2-4 54.81 0.03 2.48 2.66 0.75 0.06 22.30 16.70 0.18 - - 0.03 - - 33.5 62.2 4.3

I1063/2-7 54.21 0.08 1.55 2.16 0.61 0.06 17.72 23.24 0.32 0.00 - 0.05 - - 46.8 49.7 3.5

I1063/5-2 50.79 0.20 0.90 16.14 0.04 0.55 10.26 20.49 0.30 0.04 0.00 0.18 0.12 0.00 43.3 30.1 26.6

iНе можете найти то, что вам нужно? Попробуйте сервис подбора литературы.

I1063/5-3 54.20 0.06 0.23 27.59 0.01 1.14 15.24 0.99 0.45 0.03 0.00 0.00 0.07 0.00 2.2 48.3 49.3

I1063/5-7 52.73 0.04 0.13 34.65 0.04 0.98 10.00 1.27 0.08 0.05 0.00 0.00 0.00 0.02 3.0 33.0 64.0

I1063/17-2 52.15 0.32 0.87 10.95 0.14 0.26 14.48 20.32 0.33 0.04 0.00 0.00 0.12 0.00 41.5 41.1 17.4

I1063/17-3 52.39 0.12 0.27 23.49 0.06 0.55 21.36 1.47 0.00 0.05 0.04 0.00 0.20 0.00 3.0 60.0 37.0

I1063/17-4 53.78 0.00 1.42 4.17 0.61 0.19 16.32 23.00 0.36 0.00 0.00 0.15 - 0.00 46.8 46.3 6.9

I1063/17-5 51.92 0.29 0.75 13.50 0.01 0.39 12.52 20.06 0.39 0.02 0.00 0.00 0.13 0.01 41.8 36.2 22.0

I1063/17-6 56.38 0.05 1.72 7.50 0.60 0.30 32.82 0.52 0.00 0.00 0.00 0.08 - 0.04 1.0 87.3 11.7

1 0 01 / 1 - 1 ( m) 51.69 0.01 4.43 2.37 1.41 0.05 16.06 23.17 0.40 0.04 0.00 0.21 0.15 0.02 48.9 47.2 3.9

I1063/17-11(l) 54.64 0.13 4.16 6.27 1.08 0.11 32.34 0.61 0.35 0.07 0.00 0.14 0.08 0.01 1.1 89.2 9.7

I1063/17-12 56.40 0.00 2.56 5.63 0.76 0.25 32.55 1.72 0.00 0.00 0.00 0.02 0.04 0.07 3.3 88.2 8.5

I1069/16-2 51.82 0.45 3.14 4.19 1.13 0.05 16.16 22.58 0.24 0.05 0.00 0.10 0.10 0.00 46.7 46.6 6.7

L1141/15 53.03 0.05 2.55 4.21 0.00 0.13 16.44 23.38 0.21 0.00 - - - - 44.8 49.7 5.5

L1163/34-1 51.34 0.57 1.83 13.95 - 0.40 10.86 20.71 0.31 - - 0.03 - - 44.0 32.1 23.8

L1163/34-9 50,.53 0.52 1.49 13.71 - 0.44 11.44 21.55 0.29 - - 0.03 - - 44.4 32.8 22.8

L1163/34-3 52.32 0.27 0.53 27.51 0.06 0.74 16.40 2.14 - - - 0.03 - - 4.5 48.6 46.9

L1163/34-7 51.16 0.30 0.62 29.83 - 0.78 15,60 1.63 - - - 0.05 - - 3.5 45.9 50.6

Note. All analytical totals are normalized to 100 wt %. Samples: I1028/3 — Fe—Ti-oxide gabbronorite; I1060/3 — hornblende dolerite; I1060/10 — coarse-grained gabbro, I1063/1 — Fe—Ti-oxide hornblende gabbronorite; I1063/2 — fresh cataclased harzburgite; I1063/5 — Fe—Ti-oxide hornblende gabbronorite-diorite; I1063/17 — fresh cataclased lherzolite (m — rock matrix, l — lamella); I1069/16 — fresh troctolite, L1141/15 — Fe—Ti-oxide olivine dolerite; L1163/34 — Fe—Ti-oxide gabbronorite.

Table 4. Representative analyses of plagioclase in rocks from the Sierra Leone MAR segment

Analyt. spots SiO2 2 iO2 Al 2 O co FeO MnO MgO CaO O 2 a £ K2O Cl co O 2 r C V to O cn NiO P2O5 Or Ab An

I1028/3-5 56.47 0.00 27.38 0.18 0.00 0.10 9.55 6.20 0.11 - - - - - 0.6 53.7 45.7

I1028/3-2 56.14 0.14 27.31 0.30 0.01 0.03 9.48 6.24 0.22 - - - 0.13 - 1.3 53.7 45.0

I1060/3-1 47.65 0.00 32.59 0.33 0.00 0.08 16.62 2.42 0.04 0.02 0.00 0.01 0.12 0.12 0.2 20.9 78.9

I1060/3-2 51.59 0.00 30.28 0.16 0.00 0.00 13.62 3.97 0.96 0.02 0.19 0.00 0.01 0.07 0.5 34.4 65.1

I1060/18-3 67.27 0.00 20.75 0.00 0.05 0.00 1.98 9.72 0.03 0.00 0.10 0.03 0.02 0.06 0.05 89.5 10.5

I1060/63-1 60.70 0.09 24.70 0.23 0.00 0.06 6.13 7.71 0.23 0.04 - 0.00 0.12 - 1.3 68.5 30.1

I1063/1-8 58.71 0.01 25.42 0.30 0.00 0.07 8.19 6.60 0.14 0.00 0.00 0.00 0.12 0.43 0.8 58.9 40.3

I1063/1-10 57.74 0.07 26.10 0.08 0.00 0.00 8.66 6.69 0.20 0.03 0.16 0.00 0.15 0.12 1.1 57.7 41.2

I1063/5-1 58.32 0.05 25.78 0.25 0.11 0.00 8.22 6.91 0.085 0.03 0.10 0.00 0.00 0.13 0.5 60.0 39.5

I1063/17-1 60.39 0.02 25.16 0.01 0.00 0.00 6.88 7.33 0.21 0.01 0.00 0.00 0.00 - 1.3 65.0 33.7

I1063/17-4 60.16 0.00 24.67 0.11 0.00 0.00 7.04 7.48 0.15 0.01 0.05 0.00 0.07 0.26 0.1 65.0 33.9

I1063/17-14 54.03 0.00 28.57 0.21 0.00 0.08 11.58 5.17 0.00 0.00 0.28 0.00 0.00 0.08 - 44.7 55.3

I1069/16-2(l) 49.68 0.04 31.57 0.18 0.00 0.05 15.58 2.69 0.02 0.03 0.00 0.00 0.00 0.07 0.1 23.8 76.1

I1069/16-3(s) 47.73 0.08 32.73 0.29 0.00 0.03 16.45 2.21 0.08 0.05 0.05 0.07 0.00 0.03 0.5 19.3 80.2

I1072/1(x) 49.08 - 32.26 0.25 - - 15.90 2.51 - - - - - - - 22.2 77.8

I1072/1(l) 50.65 - 31.20 0.42 - - 14.68 3.05 - - - - - - - 27.3 72.7

I1072/1(l) 50.54 - 31.31 0.38 - - 14.50 3.27 - - - - - - - 29 71

L1141/15(c) 51.30 0.00 30.37 0.56 0.00 0.00 14.20 3.49 0.08 - - - - - 0.4 31.0 68.6

L1141/15 (r) 55.66 0.14 27.42 0.79 0.00 0.00 10.50 5.36 0.13 - - - - - 0.7 47.5 51.6

L1163/34-14 57.12 - 26.95 - 0.25 - 9.30 6.32 0.06 - - - - - 0.3 55.0 44.7

L1163/34-18 57.50 - 26.84 - 0.24 - 8.94 6.41 0.07 - - - - - 0.4 56.2 43.4

Note. All analytical totals are normalized to 100 wt %. Samples: I1028/3 — Fe—Ti-oxide hornblende gabbronorite; I1060/3 — plagioclase-phyric hornblende dolerite (I1060/3-1 — aggregate of phenocrysts, I1060/3-2 — thin lath); I1060/18 — granodiorite; I1060/63 — hornblende quartz diorite; I1063/1 — hornblende gabbronorite (I1063/1-8 — subhedral crystal, I1063/1-10 — inclusion in clinopyroxene); I1063/5-1 — Fe—Ti-oxide hornblende gabbronorite-diorite; I1063/17-1 and I1063/17-4 — thin granodiorite vein; I1063/17-14 — lherzolite (rim around chromite); I1069/16 — troctolite, cumulate plagioclase: I1069/16-2(s) — large individual grain, I1069/16-3(I) small chadacryst in clinopyroxene oikocryst; I1072/1 — basalt: x — xenocryst, l — lath; L1141/15 — dolerite (c — core, r — rim); L1163/34 — Fe—Ti-oxide gabbronorite.

Table 5. Representative analyses of kaersutite in rocks from the Sierra Leone MAR segment

Analyt. spots SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O V2O5 NiO Cl Cr2O3 P2 O5

I1028/3-5 46.56 1.63 8.85 13.74 0.14 13.99 12.01 2.36 0.08 0.42 0.23 - - -

I1028/3-6 46.47 1.63 8.83 13.72 0.14 13.96 12.00 2.36 0.08 0.42 0.23 0.17 - -

I1028/3-9 46.44 1.77 8.94 12.85 0.03 14.33 12.40 2.18 0.15 0.45 0.21 0.26 - -

I1060/3-5 48.25 1.76 8.23 12.88 0.21 14.80 11.35 1.98 0.22 0.11 0.10 0.00 0.00 0.10

I1060/18 46.55 1.35 5.36 29.68 0.50 4.82 9.69 1.46 0.36 0.08 0.00 0.10 0.01 -

I1063/1-11 44.95 2.08 9.38 18.78 0.25 10.65 11.00 2.50 0.17 0.09 0.04 0.00 0.01 0.10

I1063/2 51.92 1.06 4.41 14.75 0.20 15.36 11.16 0.81 0.12 0.16 0.17 0.06 0.09 -

I1063/17-5 46.65 0.32 11.89 5.95 0.09 18.20 12.08 2.86 0.21 0.00 0.04 0.00 1.71 0.03

I1063/17-6 45.08 3.53 10.35 12.26 0.02 14.08 11.12 2.78 0.24 0.03 0.21 0.02 0.13 0.14

I1068/33-3 49.50 1.23 9.09 7.44 0.02 18.26 11.59 2.20 0.30 0.05 0.00 0.05 0.27 -

L1124/13-1 49.16 2.39 8.39 18.03 0.30 11.52 10.16 2.49 0.24 - - 0.05 - -

L1163/34-11 49.98 0.58 6.78 20.37 0.34 9.64 11.09 1.08 0.11 - 0.03 - - -

L1163/34-12 48.20 1.20 8.71 19.96 0.26 8.96 10.91 1.59 0.18 - 0.03 - - -

L1163/34-13 51.77 0.65 5.31 19.36 0.32 10.39 11.17 0.92 0.08 - 0.03 - - -

Note. All analytical totals are normalized to 100 wt %. Samples: I1028/3 — hornblende gabbronorite; I1060/3 — plagioclase-phyric hornblende dolerite; I1060/18 — ilmenite—hornblende granodiorite; I1063/1 — Fe—Ti-oxide hornblende gabbronorite; I1063/17 — thin vein of two-pyroxene hornblende granodiorite; I1068/33 — coarse-grained gabbro: L1124/13 — hornblende gabbro; L1163/34 — hornblende gabbronorite.

Cr

A1 Fe3

Figure 8. Cr-Al-Fe3+ diagram for the composition of chromite from the rocks.

[20] The composition of the plagioclase varies within broad ranges, from bytownite An80 to oligoclase-albite An10. Basalt I1072/1 contains plagioclase xenocrysts of composition An76-78 and microcrysts of composition An71-73. A notable feature of plagioclase in these rocks is its very low K contents (Table 4, Figure 9).

Figure 10. Corroded Cr-spinel grain (Table 6, analysis I1063/17-15) in lherzolite I1063/17. The grain is surrounded by small grains of plagioclase (An55.3; Table 4, analysis I1063/17-14). Darker gray corresponds to clinopyroxene, paler gray is olivine, and black patches are remnants of the carbon sputter coating. BSE image, CamScan electron microscope, Moscow State University.

Figure 9. Zonal chromite grain in lherzolite I1063/17. The brighter central part of the grain corresponds to Fe-richer chromite (Table 6, analysis I1063/17-7(c)), and its grayish peripheral part corresponds to Mg-richer chromite (Table 6, analysis I1063/17-8(r)); dark gray grains are olivine. The sample is cut by serpentine veinlets with small magnetite grains (white) and a younger serpentine-chlorite veinlet. BSE image, CamScan electron microscope, Moscow State University.

[21] Magmatic kaersutitic brown hornblende occurs in the gabbroids both as a cumulus mineral and as intercumulus grains, and the dolerites contain small kaersutite grains in the groundmass. Similarly to the pyroxenes, the hornblende is often replaced by fibrous actinolite. The composition of the hornblende varies from high-Ti varieties in the hornblende gabbroids to relatively low-Ti varieties in the dior-ites and quartz diorites (Table 5). According to the results of Pl-Am thermometry [Blundy and Holland, 1990], the rock crystallized at temperatures from S00oC (Fe-Ti-oxide hornblende gabbronorite L1124/13-11) to 650-5S0oC Fe-Ti-oxide gabbro-diorite and quartz diorite) (unpublished data of S. S. Abramov).

[22] The apatite contains small amounts of Cl (0.09-

0.24 wt%). Fluorine was not determined in this mineral because of methodical difficulties.

[23] The oxide minerals (Cr-spinel, titanomagnetite, and ilmenite) are particularly interesting as exhibiting some noteworthy structural features.

[24] A remarkable feature of Cr-spinel in our samples of the cataclased peridotites with relics of cumulate textures is its broad chemical variations (Table 6, Figure 10), which are atypical of this mineral in mantle rocks but is typical of Cr-spinel in the cumulates of layered intrusions. Moreover, some chromite grains have corroded cores (Figure 11) consisting of Fe-rich chromite and surrounded by more magne-sian peripheral zones, which contain P and Cl (Table 6).

[25] It is also worth mentioning the development of rims of fine-grained plagioclase Ang6.3 around a resorbed chromite

Figure 11. Character of ilmenite and magnetite crystallization in Fe-Ti-oxide hornblende gabbronorite-diorite I1063/1. Brightest white-magnetite (Table 7, analysis I1063/1-3), darker grayish white-ilmenite (Table 7, analysis I1063/1-4), gray-brown hornblende (partly chloritized), dark gray-augite with thin orthopyroxene lamellae, almost black-quartz. BSE image, CamScan electron microscope, Moscow State University.

grain (Figure 12), whose composition is close to that of the zonal chromite grain described above in the same thin section (Table 4, analysis I1063/17-14). The outer part of this rim consists of fine clinopyroxene grains, and the grain itself is located among olivine neoblasts. Similar rims often develop around chromite grains during high-temperature

Figure 12. Fragment of a triangular Wo-En-Fs plot showing the composition of pyroxenes from the studied rocks.

and low-pressure transformations in ultramafics occurring in ophiolitic associations [Laz’ko and Sharkov, 1988].

[26] The composition of most chromite grains in this thin section and the chemistry of the vermicular aggregates among olivine and pyroxene neoblasts, as well as the composition of chromite in the rim of the zonal crystal, are characterized by very low Fe3+ contents and variable Cr/Al ratios. As in this rock, Cr-spinels in harzburgite I1063/2 can be subdivided into two groups: one close to the predominant type of grains in sample I1063/17, and the other characterized by low Al2O3 contents and high concentrations of Cr2O3.

Table 6. Representative analyses of Cr-spinel in rocks from the Sierra Leone MAR segment

Analyt. spots SiO2 TiO2 Al2O3 Cr2O3 FeO MnO MgO CaO Na2O K2O V2 O5 NiO ZnO P2O5 Cl

I1030/1-1 — 0.85 24.33 40.00 19.49 0.13 15.20 - - - - - - - -

I1030/1-2 - 0.88 24.55 40.08 19.44 0.13 14.92 - - - - - - - -

I1063/2-1 - 0.17 26.45 41.88 19.45 n.d. 11.29 - - - 0.38 0.09 0.29 - -

I1063/2-2 - 0.08 29.73 38.81 18.09 n.d. 12.67 - - - 0.31 0.09 0.22 - -

I1063/2-3 - 0.03 33.23 35.40 16.94 n.d. 13.68 - - - 0.38 0.13 0.21 - -

I1063/2-4 - 0.33 6.96 46.06 40.85 0.82 3.08 - - - 0.48 0.11 1.32 - -

I1063/2-5 - 0.94 4.61 37.12 52.32 0.64 2.70 - - - 0.55 0.20 0.92 - -

I1063/17-3 0.29 0.05 38.64 28.97 16.35 0.06 15.00 0.00 0.42 0.00 0.06 0.16 - - -

I1063/17-7(c) 0.35 0.11 12.89 43.33 38.72 0.31 3.36 0.09 0.77 0.13 0.37 0.00 - 0.00 0.00

I1063/17-8(r) 0.00 0.00 27.32 37.38 24.13 0. 35 9.43 0.07 0.62 0.00 0.37 0.00 - 0.18 0.04

1063/17-13 0.14 0.06 23.60 42.89 22.09 0.46 10.06 0.07 0.35 0.01 0.24 0.00 - 0.00 0.02

I1063/17-15 0.03 1.17 19.08 37.56 36.31 0.19 4.67 0.08 0.61 0.02 0.22 0.00 - 0.00 0.07

I1069/16-1 0.00 1.83 10.13 41.37 42.66 0.53 2.48 0.04 0.29 0.00 0.61 0.00 - 0.00 0.06

Note. All analytical totals are normalized to 100 wt %. Samples: I1030/1 — basaltic glass; I1063/2 — cataclased harzburgite with relict features of a cumulate texture; I1063/17 — cataclased lherzolite with relict features of a cumulate texture (c — inner part of the grain, r — its peripheral portion (see Figure 4); I1063/17-13 — vermicular grain: I1063/17-15 — chromite with a reaction rim of columnar plagioclase grains); I1069/16 — fresh leucocratic troctolite (forellenstein); n.d. — not determined.

Table 7. Representative analyses of Fe-Ti oxides and apatite in rocks from the Sierra Leone MAR segment

Analyt. spots SiO2 TiO2 Al2 O3 Cr2O3 Fe2O3 FeO MnO MgO CaO Na2O K2O V2 O5 NiO Cl P2O5

I1028/3-3 0.06 49.85 0.08 0.00 — 48.40 0.71 0.58 0.03 0.00 0.02 0.00 0.26 0.00 0.00

I1060/3-1 0.15 51.28 0.05 0.12 — 45.57 2.36 0.15 0.10 0.00 0.00 0.14 0.00 0.04 0.04

I1060/18-2 0.04 51.40 0.11 0.10 — 46.78 1.30 0.12 0.06 0.00 0.00 0.04 0.00 0.03 0.02

I1060/18-4(ap) 0.37 0.12 0.02 0.01 — 0.30 0.06 0.00 56.36 0.44 0.05 0.00 0.03 0.26 41.98

I1060/63-3 0.21 50.04 0.22 0.03 — 46.84 0.50 1.51 0.35 0.02 0.06 0.19 0.00 0.04 0.00

I1063/1-3 0.16 9.76 2.75 0.15 39.53 45.55 0.19 0.33 0.11 0.52 0.00 0.90 0.00 0.05 0.00

I1063/1-4 0.07 51.62 0.03 0.07 — 47.16 0.93 0.00 0.05 0.00 0.02 0.00 0.00 0.05 0.00

iНе можете найти то, что вам нужно? Попробуйте сервис подбора литературы.

I1063/1-9(inc) 0.09 12.37 1.50 0.11 41.68 42.64 0.37 0.34 0.09 0.43 0.01 0.33 0.00 0.04 0.00

I1063/5-4 0.08 50.44 0.10 0.03 — 47.40 1.40 0.14 0.00 0.27 0.00 0.00 0.00 0.00 0.14

I1063/5-5 0.02 3.29 0.92 0.00 33.98 61.51 0.00 0.17 0.01 0.06 0.00 0.00 0.00 0.01 0.03

I1063/17-1 0.11 51.66 0.17 0.16 — 45.67 0.65 0.97 0.07 0.15 0.00 0.00 0.01 0.01 0.38

I1063/17-7 0.27 52.08 0.09 0.34 — 45.80 0.95 0.00 0.26 0.00 0.03 0.00 0.15 0.03 0.00

I1063/17-8(ap) 0.44 0.00 0.00 0.00 — 0.14 0.05 0.00 56.29 0.00 0.12 0.00 0.00 0.09 42.87

L1163/34-26 — 8.85 3.35 0.07 38.46 48.04 0.95 0.28 — — — — — — —

L1163/34-29 — 52.92 0.04 — — 45.83 0.96 0.25 — — — — — — —

Note. All analytical totals are normalized to 100 wt %. Samples: I1028/3 — hornblende gabbronorite with ilmenite; I1060/3 — hornblende dolerite; I1060/18 — granodiorite; I1060/63 — fine-grained hornblende dolerite with ilmenite; I1063/1 — Fe—Ti-oxide hornblende gabbronorite (3 — zonal grain, analyzed by scanning over an area, 4 — Ilm grain, 9 — inclusion in clinopyroxene with exsolution lamellae); I1063/5 — Fe—Ti-oxide hornblende gabbro-diorite; I1063/17 — gabbronorite-diorite veinlet in lherzolite; L1163/34 — Fe—Ti-oxide gabbronorite; ap — apatite; inc — inclusion.

[27] All of these data evidently indicate that the early Fe-rich chromite with elevated TiO2 and relatively low Al2O3 and MgO contents are xenogenic for these peridotites. They could be preserved as inclusions in other minerals (when captured by growing crystals), but more commonly they reacted with the magmatic melt and were dissolved in it. Likely evidence of this processes is displayed in Figure 12.

[28] The Fe-Ti oxides of the rocks are unusual. For example, titanomagnetite with exsolution ilmenite lamellae, which are typical of continental mafic intrusions [Bogatikov, 1996; Wager and Brown, 1968], is contained in the gab-bronorites only in the form of small inclusions in clinopy-

Or

A------A---A-----A-----A----A------A---

/ —v-------Jg[l| ^

Ab An

O 11028/3 ♦ 11063/2

■ 11060/3 H 11063/3

3 11060/18 V 11063/5

« 11060/63 011063/17

▲ 11063/1 #11069/16

▼ 11163/34

Figure 13. Composition of plagioclase in the rocks.

roxene. These oxides (Mag containing <10 wt % TiO2 and Ilm) usually crystallize together or compose individual grains (Table 7, Figure 13). According to Fershtater et al. [2001], this mode of occurrence of Fe-Ti oxides is typical of rocks that crystallized from hydrous melts at relatively low temperatures (600-800°C, —logfO2 = 8-11).

[29] As obviously follows from all of these data, the rocks contain minerals belonging to at least two distinct assemblages. Relics of the earlier of them are contained as inclusions (“seeds”) in the typical minerals of the magmatic assemblage itself. These are Fe-rich Cht overgrown by rims of a more magnesian and less titanian variety in the peri-dotite (sample I1063/17) and Ti-Mag with exsolution lamellae in clinopyroxene from the gabbronorite (sample I1063/1). Moreover, the composition of the olivine significantly varies, and volumetrically predominant magnesian varieties are in places associated with moderately magnesian grains. All of these facts testify to very unusual conditions under which the rocks were produced.

[30] A noteworthy feature of the rocks is the variable composition of the same minerals in them. This can be illustrated most glaringly by the example of the peridotites wit relict cumulus textures: a single sample of these rocks can contain olivine of the composition Fo86 and Fo70 (Table 2, sample I1063/17). The broadest compositional variations in this sample and in I1063/2 are exhibited by their Cr-spinel (Table 6, Figure 10). Some of their grains are Fe-rich, which is atypical of mantle rocks but is characteristic of ultra-mafic layered complexes [Sharkov et al., 2001]. Moreover, some of the chromite grains contain corroded cores of Fe-rich chromite overgrown by more magnesian outer zones, which contain P and Cl (Figure 11, Table 6).

[31] As can be seen from Table 8, the Fe-Ti-oxide gabbro-norites crystallized, according to Ilm-Mag geothermometry,

Figure 14. Morphology and inner structure of zircon grains from sample I1028/1 (cathode luminescence data). Determinations were conducted at the University of Granada, Spain. A, B - Fragments of deformed zircon grains with sectorial zoning; C - dipyramidal zircon grain with cross zoning; D - fragment of a zircon grain whose inner part has a sectorial zoning and the outer rim exhibits oscillatory zoning.

at average temperatures of 621±41°C, which is close to the analogous values obtained by the same method for Fe-Ti-oxide gabbroids in the Southwest Indian Ridge [Natland et al., 1991].

U-Pb Age of the Rocks

[32] As was mentioned above, the Fe-Ti-oxide rocks contain zircon. Zircon for radiological dating was separated from one of our rock samples (cataclased hornblende gab-bronorite, sample I1028/1, dredged from the rift valley wall at a depth of close to 4 km). The rock was dominated by clinopyroxene and plagioclase and contained subordinate amounts of orthopyroxene, magmatic brown hastingsitic hornblende, ilmenite, and accessory apatite and zircon. Cataclasis resulted in this rock in the deformations of large crystals (cloudy extinction and deformed twinning boundaries in pla-gioclase and bent exsolution lamellae in pyroxenes) and the development of small subequant neoblasts. The zircon was also cataclased and disintegrated into small fragments. The

secondary alterations were generally insignificant and involved the replacement of mafic minerals by fibrous acti-nolite.

[33] Analogous rocks were dredged from the walls of other parts of the Markov depression and neighboring depressions in this MAR segment, which testifies to their broad in-situ occurrence. Zircon grains were obtained from sample I1028/1 using magnetic separation and heavy liquids, and by hand-picking under a binocular magnifier. The grains were classified according to their size, color, and morphology. Approximately 70-75% of each population were utilized to prepare epoxy pellets, which were polished and examined under an electron microscope with a cathode-luminescence detector. The rest of the populations were dated (TIMS) by the U-Pb method [Sharkov et al., 2004a].

[34] The largest fraction (>450x250 ^m) consisted of colorless transparent and semitransparent zircon grains disintegrated into small fragments during cataclasis (Figure 14). The weak erosion of their face surfaces suggests that the zircon was partly dissolved. The cathode luminescence examination of more than 30 individual zircon grains revealed their

206tyi /23 81

Figure 15. Isochron diagram.

zoning, which is likely of magmatic genesis. We detected weak oscillatory and peculiar sectorial zoning (Figures 14a, 14b), as is typical of zircon from mafic crystalline rocks, particularly those in ophiolitic complexes [Rubatto and Gebaver,

2000]. The primary nature of the sectorial zoning is clearly seen in Figure 2c, in which a fragment of this zoning cuts across the morphological features of a small fragment crystal of a zircon crystal.

[35] The smaller fraction includes euhedral zircon grains with clearly pronounced dipyramid faces (Figure 14c). The predominance of euhedral grains was caused both by the crystallization of this zircon from a melt and by the fragmentation of larger grains during cataclasis. This follows form the fact that the zoning of some grains is not conformable with their morphology (Figure 2c), whose euhedral faceting

seems to be secondary and imitates the habit of the larger primary grain.

[36] Zircon dating was conducted on a Finnigan MAT-261 solid-source eight-collector mass spectrometer using samples of three size fractions of the most transparent and euhedral grains and grain fragments. Fraction I consisted of 18 grains, which ranged form 500 ^m to 300 ^m along the long axis and from 250 ^m to 150 ^m along the short one. Fraction III comprised 35 grains with average sizes of 250x175 ^m, and fraction IV included 30 grains and grain fragments with average sizes of 250x140 ^m. A subcordant age value was obtained for small and the most euhedral and transparent crystals (Figure 15, Table 9). The age of the rock is 97.42±0.15 Ma.

[37] Our experience in studying zircon from blastomyloni-

Table 8. Ilmenite-magnetite thermometry of Fe—Ti-oxide gabbronorites and gabbronorite-diorites

Analyses in Table 5 [Powell and Powell, 1977] [Spenser and Lindsley, 1981] [Andersen and Lindsley, 1985]

I1063/1-3 and I1063/1-4 588-605° C; 510-545°C; 539-572°C;

— lg f O2 = 24.34 - 26.40 — lg fO2 = 25.32 - 25.40

I1063/5-4 and I1063/5-5 542-543°C; 586-590°C; 607-608°C;

— lg fO2 = 20.76 - 20.90 — lg fO2 = 20.28 - 20.39

L1163/34-27 and L1163/34-29 571-580°C 601-613°C

14 of 30

Table 9. U-Pb isotopic dating of zircon from cataclased hornblende gannronorite, sample 11028/1, Markov depression, Central Atlantic

no. sample (mg) [Pb]rad ppm [U] ppm 2°6pb/2°4pb 2°7pb/2°6pb 2°8Pb/2°6Pb 207Pb/23BU (2(7%) 206Pb/238U (2(7%) Rho Age 207Pb/23BU (Ma) Age 206Pb/238U (Ma) Age 2°7pb/206pb (Ma)

1 0.068 106.35 1263.9 98.68 0.04787±71 0.38726 0.10059 0.01524 0.58 97.3±1.7 97.5±0.6 92.9±35

3 0.14 108.71 1336.2 124.6 0.04813±62 0.35653 (1.76) 0.10085 (0.56) 0.01519 0.44 97.6±1.4 97.2±0.3 106.0±30

4 0.077 75.69 1281.8 79.08 0.04793±91 0.66831 (1.42) 0.08038 (2.45) (0.34) 0.012163 (0.77) 0.57 78.5±1.9 77.9±0.6 96.1±49

Note. Zircon fractions: 1 — 18 grains, 3 — 35 euhedral grains, 4 — 30 grains.

ES4001 SHARKOV ET AL.: SILICIC FE-TI-OXIDE SERIES OF SLOW-SPREADING RIDGES ES4001

tization zones in the Early Proterozoic Pezhostrov Island gabbro-anorthosite massif in the Belomorian Mobile Belt in the White Sea area [Sharkov et al., 1994] indicates that zircon recrystallization coupled with a diminish of its grain sizes does not affect the isotopic characteristics of this mineral, and its age evaluated using these grains is close to the age of zircon crystallization form the melt [Zinger et al., 2001]. This led us to believe that the Mesozoic age obtained for zircon from gabbronorite from the Markov depression corresponds to the crystallization age of this mineral.

[38] Finds of ancient zircon in gabbroids form the axial MAR zone put forth the problem of interpreting the nature of this mineral itself, as to whether it crystallized from a magmatic melt and, hence, its age corresponds to the crystallization age of the rock, or the zircon is a relict and xenogenic mineral that was not decomposed during the melting of the rock. According to currently adopted concepts, the opening of the Central Atlantic started at approximately 170 Ma and continues until now [Pushcharovskii, 2001]. This led Pilot et al. [1998] to hypothesize that zircon could be preserved in the mantle at temperatures of about 1000° C for 150 m.y. without losses of its radiogenic lead. These researchers proposed two possible explanations for the nature of ancient zircon in the gabbroids:

[39] (1) The rifting coupled with the opening of the Atlantic resulted in the tectonic disintegration and imbrication of the continental crust, which was broken into a series of tectonic slabs as a consequence of subhorizontal thrusting in the continental lithosphere. Involved in small-cell convection in the upper mantle, these continental lithospheric fragments were remelted. The convection cells that circulated on both sides of the ridge facilitated the displacement of these semi-molten zircon-bearing rocks toward the spreading axis, where they took part in the formation of the gabbroids.

[40] (2) The continental crustal material has been preserved in the Kane zone since the opening of the Atlantic because of the migration of the transform fault and jumps of the ridge axis. Part of this material later subsided in the axial zone of the ridge and was involved in the origin of the gabbroids.

[41] Thus, these researchers believe that the ancient zircon is of continental genesis, is not related to oceanic magma generation, and, hence, cannot provide information on the age of its host rocks. At the same time, geological, petrological, and isotopic-geochemical data provide no evidence that continental material can occur either in the Kane Fracture Zone or in the Markov depression. Moreover, the Fe-Ti-oxide silicic rocks found in the walls of the depression are typical of the third layer of slow-spreading ridges, such as MAR or the Southwest Indian Ridge, and of some ophiolitic associations, such as Monviso in the Western Alps [Lombardo et al., 2002; Rubatto and Gebaver, 2000]. These rocks typically bear zircon with analogous unusual sectorial zoning, which is uncommon in this mineral from continental rocks.

[42] These facts demonstrate that there is no need to propose any specific mechanism for the genesis of the zircon. Furthermore, according to experimental data, zircon can be easily dissolved in basaltic magmas and can crystallize only from melts oversaturated with silica [ Watson and Harrison, 1983], and, thus, the occurrence of this mineral in rocks of

the silicic Fe-Ti-oxide association can hardly be accidental. This led us to believe that the zircon crystallized from melts that were emplaced into the oceanic lithosphere, and the zircon age corresponds to the age of its host rocks.

Rock Chemistries

[43] The concentrations of major components in the rocks (Table 10) were determined at the Geological Institute of the Russian Academy of Sciences by conventional techniques. The rocks were analyzed for trace elements at the Department of Mineralogy and Petrology of the University of Granada, Spain. Zr was determined by XRF, using the technique that involves fusing with Li tetraborates (the accuracy of the method is ±5% per 100 ppm Zr). Other trace elements were analyzed by ICP-MS after decomposing 0.1000 g of the powdered material in an HNO3 + HF mixture in a teflon vessel at 180° C and a pressure of 200 pounds per square inch for 30 min and subsequent dissolving in 100 ml of 4% HNO3. The accuracy of the analysis was close to ±5% at an analytical concentration of 10 ppm. The results are summarized in Table 11.

[44] Our analyses of the basalts and plutonic rocks from the study area for major and trace elements and for REE indicates that, having many common features, these rocks nevertheless show broadly varying geochemistry. The example of sites I1060 and I1063 demonstrates that samples from a single dredge are characterized by similar fractionation patterns of elements, but these patterns for analogous rocks vary from site to site. This suggests that the dredge captured rock fragments mostly from a single locality but not from the whole dredging course.

Major Elements

[45] As follows from Table 10 and Figure 11, all of the rocks affiliate with the tholeiitic series. The composition points of the basalts and hornblende dolerite lie practically exactly on the boundary line separating the tholeiitic and calc-alkaline series, and sample I1072/1 is closer to MORB. All other rocks can be subdivided into two groups according to their major-element compositions: magnesian and ferrous. The rocks of the first group are primitive gabbroids and troctolites from sites I1032, I1060 (samples 10 and 12), and I1069. They are compositionally close to N-MORB and were, perhaps, produced by such melts. The rocks of the second group are Fe-Ti-oxide gabbronorites, gabbronorite-diorites, and diorites from sites I1060 and I1063. They are characterized by high contents of TiO2 (up to 4.47 wt %) and elevated concentrations of P2O5 (up to 1.44 wt % in gabbronorite I1063/5). It is interesting that the K2O concentrations of the rocks remain low and reach a maximum of 0.40 wt % in the same gabbrodiorite, whereas this concentration in the trondhjemites is as low as 0.16 wt %. The trondhjemites and harzburgites are quite different and have the lowest contents of these elements.

Table 10. Major-element composition (wt %) of the rocks

Analyt. spots SiO2 TiO2 Al2O3 Fe2O3 FeO MnO MgO CaO Na2O K2O P2O5 LOI Total

I1026/21 49.38 1.69 14.99 4.22 5.29 0.08 8.86 10.28 2.91 0.33 0.19 1.39 99.61

I1027/5 50.67 1.75 14.65 3.30 7.08 0.10 6.92 9.59 2.92 0.46 0.17 1.77 99.38

I1032/7 46.77 0.15 11.78 3.68 5.39 0.09 18.20 9.76 1.38 0.10 0.01 2.53 99.83

I1060/1 47.80 1.23 15.70 1.33 7.08 0.17 8.77 11.30 3.25 0.12 0.15 3.26 100.16

I1060/2 50.20 0.88 15.20 1.88 6.85 0.17 7.67 11.61 3.31 0.12 0.14 1.75 99.78

I1060/10 47.56 0.42 15.23 3.72 5.98 0.19 10.91 12.72 1.77 0.06 0.01 0.89 99.46

I1060/12 52.12 0.31 16.54 2.01 4.79 0.19 10.05 10.06 2.63 0.07 <0.01 1.00 99.77

I1060/18 59.72 1.65 13.04 2.23 8.50 0.15 2.43 5.57 4.92 0.29 0.44 0.80 99.74

I1060/57 77.30 0.23 12.48 0.41 0.63 0.02 0.16 1.68 5.82 0.16 0.02 0.40 99.31

I1063/1 47.38 4.47 12.90 3.82 11.38 0.18 5.78 9.08 3.18 0.10 0.09 1.20 99.56

I1063/2 45.44 0.21 3.28 2.73 7.92 0.16 36.75 1.19 0.42 0.10 0.05 1.30 99.55

I1063/5 47.20 2.98 13.40 5.33 11.65 0.27 4.97 7.62 3.86 0.40 1.44 0.55 99.67

I1063/6 43.99 4.39 14.13 5.24 12.37 0.19 5.02 8.14 3.34 0.27 0.65 1.74 99.47

I1069/16 45.61 0.11 18.65 2.19 2.86 0.11 12.11 14.02 1.35 0.04 <0.01 2.43 99.48

I1071/1 65.50 0.93 14.43 2.90 3.12 0.07 2.33 2.68 5.64 0.14 0.27 2.00 100.01

I1072/1 48.49 1.06 15.86 3.69 7.28 0.17 8.93 11.50 2.55 0.09 0.09 0.37 100.08

L1140/2 48.00 1.28 15.80 1.95 7.16 0.15 9.24 12.13 2.42 0.28 0.11 1.37 99.89

Note. See text for petrographic characteristics of the rocks.

[46] As is seen in Figure 16, all analyses of the rocks can be approximated by a single line, i.e., judging from their major-element compositions, all of these rocks can be regarded as derivatives of a single parental melt like MORB. However, the analysis of the concentrations of trace elements points to a amore complicated situation (see below).

Trace and Rare-Earth Elements

[47] The chondrite-normalized REE patterns of the rocks are presented in Figure 17. The basalts and dolerites are characterized by practically unfractionated REE patterns with unclearly pronounced negative Eu anomalies, as is also typical of the cumulate harzburgites (sample I1063/2). In contrast to them, all gabbroids are depleted in LREE and have positive Eu anomalies. The primitive troctolites and gabbro bear the lowest concentrations of REE, particularly LREE, which are close to chondritic ones. The ilmenite-hornblende (Fe-Ti-oxide) varieties of the rocks are notably different from other varieties and have higher REE concentrations, which are close to those in the basalts; but these rocks differ from the basalts in having lower LREE contents and well pronounced positive Eu anomalies. The dior-ites and granites have REE concentrations by one order of magnitude higher than in the other rocks and generally flat REE patterns with clearly pronounced negative Eu anomalies. Slight LREE enrichment was detected only in the pla-giogranites.

[4S] The spidergrams of the rocks normalized to the primitive mantle (PM), N-MORB, and E-MORB are only insignificantly different from one another (Figures 18-20) and, thus, are considered collectively. As can be seen from these patterns, the contents of incompatible elements in basalts and

plutonic rocks from site I1063 are the closest to the analogous values of E-MORB, whereas gabbroids from other sites are close to N-MORB. The simplest fractionation patterns of these elements are characteristic of the basalts, and the most complicated ones are typical of the plutonic rocks. As could be expected, the highest concentrations of these elements typically occur in the diorites and granites, interme-

FeOt

Na20+K20 MgO

Figure 16. AFM diagram for the compositions of the rocks. 1 - plutonic rocks of silicic Fe-Ti-oxide series; 2 - basalts and dolerites; 3 - primitive troctolites and gabbro.

Figure 17. REE patterns of the rocks normalized to C1 chondrite [McDonough and Sun, 1995].

diate values are most common in the Fe-Ti-oxide gabbroids, and the minimum ones were detected in the troctolites and gabbro.

[49] Most of the rocks are characterized by positive Pb anomalies and, often, also U anomalies; the opposite situation is typical only of the diorites, granodiorites, and pla-giogranites. Most of the gabbroids have positive Sr and Eu anomalies, whereas the intermediate and acid rocks have negative anomalies at these elements. The only exceptions are the harzburgites, which have negative Sr and Eu anomalies. The overwhelming majority of the rocks is characterized by negative anomalies at Zr, Hf (the only exceptions are basalts I1027/5 and I1026/21), and Th (except only for the granites, which have small positive anomalies). Rocks from site I1063, particularly the Fe-Ti-oxide gabbroids, have strong positive anomalies at Ta and Nb and negative anomalies at Hf. Along with basalts from sites I1026 and I1027, these rocks are characterized by clearly pronounced positive anomalies at Cs and Rb, whereas the diorites and pla-giogranites from site I1060 have negative anomalies at Rb.

[50] The concentrations of incompatible elements in our sample of the cumulus harzburgite (sample I1063/2) are generally notably higher than in the primitive mantle (Figure 18) at a generally similar character of fractionation, which is

iНе можете найти то, что вам нужно? Попробуйте сервис подбора литературы.

different from those in other rocks from the same site. These concentrations are only insignificantly lower than in E-MORB (Figure 20). According to the geochemical criteria in [Reverdatto et al., 2005], this harzburgite, which contains 1.35 ppm Sm, 1.64 ppm La, and 1.46 ppm Yb at a sum of REE of more than 20 ppm (Table 11), belongs to peridotites of crustal provenance. It should be stressed that some cumulate ultramafics from this site contain rare grains of kaersutite and ilmenite. This suggests that the ul-tramafic cumulates are the highest temperature members of the Fe-Ti-oxide association. The assemblages of trace elements contained in these rocks are generally the same as in other derivatives, but their concentrations are much lower.

[51] The ore-element compositions of the samples were normalized to the primitive mantle (Figure 21). Note that none of the rocks, including fresh basalts containing volcanic glass, shows elemental patterns notably different from that of PM. As was mentioned above, all of the rocks are enriched in Pb, with its highest contents detected in basalts I1072/1, Fe-Ti-oxide gabbroids I1063/1 and I1063/6, diorites I1060/18, and trondhjemites. Practically all of the rocks display positive Ga anomalies. The primitive gabbroids are characterized by negative Sn and Mo anomalies, whereas most of the Fe-Ti-oxide gabbroids and, particularly, the late derivatives

Table 11. Trace-element and REE composition (ppm) of rocks from the Markov depression

Sample Ba Rb Sr Y Zr Nb Th Pb Ga Zn Cu Ni V

I1026/21 35.648 7.95 177.122 35.196 122.744 7.446 0.497 1.417 17.365 75.177 56.48 134.099 257.98

I1027/5 21.888 7.321 135.835 37.37 111.989 4.71 0.277 1.061 17.754 86.588 69.446 118.475 289.412

I1032/7 46.156 1.306 84.763 4.826 8.111 0.773 0.027 0.555 7.55 37.629 153.402 435.832 111.914

I1060/10 8.125 0.411 98.198 8.552 11.526 0.979 0.043 0.525 13.526 49.439 9.125 128.728 176.38

I1060/12 12.822 0.896 124.083 5.636 6.756 0.747 0.019 0.54 12.801 35.746 54.355 86.251 149.879

I1060/18 76.999 5.259 116.074 155.818 29.531 20.939 1.192 2.208 32.472 117.254 13.484 9.959 38.326

I1060/57 110.01 1.336 69.532 107.77 10.435 23.313 5.231 2.526 26.951 9.345 4.212 6.341 6.41

I1063/1 30.388 2.191 142.374 30.012 49.038 14.38 0.042 0.978 21.501 150.772 69.233 49.506 318.748

I1063/2 12.934 2.897 18.636 13.067 11.177 1.704 0.068 0.577 3.604 78.743 4.347 1786.31 49.543

I1063/5 77.58 11.806 150.814 112.006 30.375 25.437 0.486 1.116 25.11 152.912 39.264 105.781 83.563

I1063/6 39.153 12.96 167.428 32.855 35.647 10.257 0.123 1.282 21.734 147.207 43.94 29.566 218.413

I1069/16 4.857 0.474 99.917 3.811 6.681 0.727 0.027 0.631 8.994 18.473 106.216 361.131 93.917

I1072/1 25.551 1.509 98.771 25.218 57.158 3.488 0.273 1.686 14.25 66.177 96,864 105,781 244,544

I1060/1 13.576 0.916 118.156 22.276 49.61 3.454 0.273 0.740 14.236 69.304 74.017 136.55 227.011

I1060/2 34.573 0.797 91.786 48.466 43.93 6.629 0.258 1.152 15.573 67.611 83.415 77.36 210.981

L1140/2 67.925 3.925 153.492 21.049 63.40 9.820 0.661 0.684 14.089 65.355 74.688 176.26 229.835

Sample Cr Hf Cs Sc Ta Co Li Be U Sn Mo La Ce

I1026/21 237.341 3.10 0.297 34.809 0.51 39.613 7.079 0.776 0.233 1.209 0.529 6.58 17.35

I1027/5 208.26 3.041 0.246 39.58 0.354 41.12 5.227 0.764 0.145 1.025 0.246 4.93 14.41

I1032/7 473.634 0.25 0.01 28.658 0.031 66.533 7.01 0.149 0.044 0.01 0.01 0.30 0.88

I1060/10 713.353 0.383 0.01 36.943 0.051 40.797 5.837 0.318 0.037 0.05 0.079 0.58 1.74

I1060/12 44.349 0.218 0.01 34.307 0.03 37.352 2.179 0.27 0.05 0.06 0.095 0.33 0.96

I1060/18 21.893 2.416 0.213 25.269 1.649 18.831 1.552 2.895 0.253 7.487 0.162 25.83 72.16

I1060/57 6.041 0.668 0.029 2.787 2.866 2.23 1.123 5.175 0.86 5.462 0.001 41.73 97.36

I1063/1 44.306 1.501 0.117 53.137 1.015 47.796 4.334 0.561 0.029 0.209 0.558 1.84 5.68

I1063/2 3134.9 0.381 0.061 15.352 0.144 93.668 4.136 0.278 0.045 0.01 0.141 1.64 4.53

I1063/5 81.606 1.241 0.049 29.537 1.461 32.291 3.63 1.288 0.131 2.374 1.187 19.02 56.09

I1063/6 24.283 1.11 0.168 37.446 0.733 43.382 4.064 0.581 0.052 0.479 0.448 4.27 12.45

I1069/16 2574.48 0.201 0.01 24.892 0.035 34.229 1.592 0.187 0.028 0.01 0.041 0. 26 0.73

I1072/1 273.673 1.739 0.01 38.073 0.324 41.12 0.886 0.538 0.111 0.69 0.338 3.67 10.32

I1060/1 351.699 1.465 0.157 34.577 0.295 38.577 3.852 0.574 0.041 1.019 0.368 3.294 8.852

I1060/2 224.269 1.461 0.103 34.878 0.432 37.516 3.442 1.022 0.007 1.461 0.501 6.636 18.999

L1140/2 357.079 1.724 0.186 36.083 0.659 41.936 4.482 0.451 0.196 0.965 0.778 6.732 14.65

Sample Pr Nd Sm Eu Gd Tb Dy Ho Er Yb Tm Lu

I1026/21 2.65 13.40 4.19 1.35 5.06 0.90 5.84 1.30 3.61 3.43 0.55 0.52

I1027/5 2.39 12.31 4.36 1.50 5.53 1.00 6.37 1.38 3.99 3.79 0.59 0.57

I1032/7 0.17 1.00 0.42 0.30 0.72 0.13 0.82 0.18 0.49 0.47 0.07 0.07

I1060/10 0.32 1.72 0.76 0.40 1.17 0.21 1.42 0.31 0.88 0.83 0.13 0.13

I1060/12 0.18 0.99 0.46 0.35 0.65 0.13 0.93 0.21 0.64 0.66 0.10 0.09

I1060/18 11.00 53.12 16.13 3.52 20.63 3.62 24.52 5.33 15.59 14.99 2.38 2.24

I1060/57 12.38 50.60 12.82 1.61 13.19 2.38 16.00 3.55 10.37 11.16 1.68 1.75

I1063/1 1.10 6.55 2.79 1.92 4.07 0.76 5.08 1.14 3.20 3.17 0.48 0.51

I1063/2 0.75 4.16 1.35 0.44 1.79 0.31 2.13 0.48 1.39 1.46 0.23 0.23

I1063/5 9.30 50.29 15.95 4.11 19.04 3.16 19.70 4.11 10.67 9.39 1.54 1.33

I1063/6 2.22 11.83 4.37 2.06 5.64 0.91 5.82 1.21 3.16 2.87 0.46 0.42

I1069/16 0.12 0.74 0.35 0.24 0.54 0.10 0.65 0.14 0.40 0.39 0.06 0.06

I1072/1 1.62 8.00 2.74 0.96 3.64 0.66 4.35 0.96 2.78 2.69 0.43 0.42

I1060/1 1.45 7.64 2.54 0.90 3.07 0.55 3.81 0.82 2.21 1.99 0.33 0.29

I1060/2 3.04 15.54 5.03 1.50 6.38 1.16 8.07 1.76 5.13 4.63 0.78 0.69

L1140/2 2.03 9.37 2.57 1.00 3.04 0.54 3.65 0.82 2.22 2.07 0.34 0.31

Table 11. Continued)