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RUSSIAN JOURNAL OF EARTH SCIENCES, VOL. 3, NO. 3, PAGES 197-217, AUGUST 2001
Evolution of magmatism in the zone of junction between granite-greenstone and granulite-gneiss regions, Sayan mountains, Siberia
V. I. Levitskii and G. P. Sandimirova
Vinogradov Institute of Geochemistry, Siberian Division, Russian Academy of Sciences
A. I. Melnikov
Institute of the Earth’s Crust, Siberian Division, Russian Academy of Sciences
Abstract. The crust in the south of the Siberian Craton includes the tonalite-trondhjemite complex of the basement, the highly metamorphozed rocks of the Kitoi Suite, the rocks of the Onot greenstone belt, ultrametamorphic rocks, the gabbroids of the Arban Complex, the metaultramafics of the Ilchira Complex, the rocks of the postultrametamorphic phase, and the metasomatic rocks of deep fault zones. The continental sialic tonalite-trondhjemite crust and the oceanic basaltic crust coexisted during the Early Archean. The rocks of the Onot greenstone belt are restricted to the linear troughs (paleorifts) of the early sialic tonalite-trondhjemite crust. The bottom of the belt is dominated by calc-alkalic bimodal rocks, the middle, by carbonate rocks, and the top, by clastic rocks and flysch. The processes of ultrametamorphic and postultrametamorphic allochemical transformations altered significantly the preexisting rocks. The junction zones of the Baikal granulite-gneiss and East Sayan granite-greenstone regions are marked by the wide development of the rapakivi-like granites of the Shumikha Complex, similar petrogeochemically to the maritime complex of the West Baikal region. The subconcordant linear distribution of the rocks of the Onot Belt, the ultrametamorphic and postultrametamorphic rocks, the granites of the Shumikha Complex, and the metasomatic rocks of deep fault zones testifies to their long-lasting connections with mantle sources.
Introduction
Granite-greenstone and granulite-gneiss regions are classified among the main geostructural elements of the Precam-brian continental crust, and the establishment of relations between these low- and high-grade metamorphic formations is a fundamental problem of modern geology. The main aim of this paper is to discuss regular mechanisms in the oper-
Copyright 2001 by the Russian Journal of Earth Sciences.
Paper number TJE01063.
ISSN: 1681-1208 (online)
The online version of this paper was published August 21, 2001. URL: http://rjes.agu.org/v03/TJE01063/TJE01063.htm
ation and evolution of petrogenic processes in the zone of junction between the East Sayan granite-greenstone region (using the largest Onot greenstone belt as an example) and the high-grade metamorphic rocks of the Sharyzhalgai Complex of the Baikal granulite-gneiss province in the southern marginal salient of the Siberian Craton basement (the interfluvial area of the Kitoi, Bolshaya and Malaya Belaya, Onot, and Tagna rivers in the Southeast Sayan region).
Earlier a wide development of tonalite-trondjemite rock associations was discovered there [Sandimirova et al., 1992]. Geochronological and isotopic studies were carried out [Levitskii et al, 1995; Sandimirova et al, 1992, 1993]. The compositions of the basement rocks and some of the Onot Greenstone Belt rocks were determined [Mekhonoshin, 1999; Nozhkin et al, 1995; and others]. The origin and petrochemistry of ores in the Onot mineral deposit were estab-
lished [Levitskii, 1994], and the granites of the Shumikha Complex were classified as rapakivi-like granites [Levitskii et al., 1997a, 1997b]. The aim of this paper is to generalize and summarize the earlier and new evidence on the geology, geochronology, petrology, and geochemistry of the region.
Methods of Study
The main aim of the investigations discussed was to study the rocks of different origins, reveal relationships between, and investigate their geological, petrological, mineralogical, and geochemical evolution. As a result of this work, the following associations of rocks were distinguished: (1) igneous rocks; (2) metamorphic rocks which had experienced isochemical transformations; (3) ultrametamorphic rocks, represented by plagioclase and K-feldspar migmatites and granitoids in the aluminosilicate substrate and by skarn in the marble: (4) metasomatic rocks of the postultrametamorphic stage (post-migmatitic metasomatic rocks after Glebovitskii and Bushmin, [1983]) and of the deep fault zones. The postultrametamorphic rocks developed under the conditions of declining temperature and are usually subdivided into subclasses of different temperatures, which are not discussed here.
The geochronological investigations and analytical work done in the Vinogradov Institute of Geochemistry can be summarized as follows: Rb-Sr isochron analysis (analysts G. P. Sandimirova and Yu. A. Pakholchenko); X-ray fluorescence analysis of petrogenic elements and Ba, Sr, and Zr (analysts T. N. Gunicheva and A. L. Finkelshtein); atomic absorption analysis (Li, Rb, Cs, analyst D. Ya. Orlova); quantitative spectral analysis (La, Ce, Nb, Yb, Y, Co, Ni, Cr, V, Sc, Zr, Sn, Mo, Zn, Pb, B, Ge, Ag, Ba, Sr, F, B, and Be, analysts E. B. Smirnova, L. N. Odareeva, A. I. Kuznetsova, S. K. Yaroshenko, and L. L. Petrov); scintillation analysis (Au, Pd, analyst S. I. Prokopchuk). In this work we also used the REE analyses performed using the methods of preliminary sample enrichment and quantitative spectral analysis in the Vinogradov Institute of Geochemistry (analysts L. I. Chuvashova and E. V. Smirnova) and the method of instrumental neutron activation analysis in the Institute of Geology and Geophysics, Siberian Division, Russian Academy of Sciences (analyst V. A. Bobrov) [Nozhkin et al, 1995].
Geochronological methods. The samples were prepared chemically for the isotopic analyses using one specimen, the decomposition by a (HF+HNO3+HCIO4) mixture, and two stages of Rb and Sr partitioning by the method of ion-exchange chromatography using a BiORad AG 50 Wx8 (200-400-mesh) cation exchanger in an H+ form. Isotope compositions were measured on a MI 1201T mass spectrometer completed with a PRM-2 unit and an Iskra-1256 microcomputer using a mode of one-ribbon source. To enhance the ionization effect and stabilize the ion beam, the specimen was applied to the ribbon using a Ta2 0sXn H2 0-based activator in the form of a suspension in (HF+HNO3+H3PO4) acids in a ratio of 1:1:1 [Tauson et al., 1983]. The concentrations of rubidium were determined by the method of
isotope dilution, and those of strontium, by the method of double isotope dilution. The validity of the isotopic analyses was estimated using SRM-987, VNIIM-Sr, and ISG-1 (granite) standards. The isochron parameters, such as the Rb/Sr ages and primary (87Sr/86Sr)o ratios were calculated using the Isoplot computer program [ York, 1966] and a polynomial method using models [McIntyre et al., 1966] taking into account ±2a errors for both axes of the coordinates (0.5% for 87Rb/86Sr and 0.05% for 87Sr/86Sr).
Analytical methods. The lower limits of detecting petrogenic elements (Si, Ti, Al, Fe, Mn, Mg, Ca, P, Na, and K) were 0.01%. Those for trace elements (ppm) were 5-10 for Zr, Ba, Sr, and Zn; 0.5-1 for Li, Rb, Cs, and Pb;
0.1-15 for La, Ce, Nb, Yb, and Y (direct quantitative spectral analysis), and 0.01-1 (instrumental neutron activation analysis); 1 for Co, Ni, V, and Sc; 3 for Cr; 5 for Cu; 0.8 for Sn and Ge; 1-5 for B; 100 for F; 0.05 for Be; 0.3 for Mo; 0.01-1 for Ta, Nb, and Hf (spectrochemical analysis with preliminary enrichment); 0.01 for Ag; and 0.0001 for Au and Pd. The analytical procedures were described in the literature [Emission..., 1976; Finkelshtein and Afonin, 1996; Smirnova and Konusova, 1982; and others]. We verified our analytical results using international and Russian standards: BCR, ST-1A, SGD-1A, AGV-1, G-2, SM, SG-1A, SG-2, SI-
1, BM, TB, KH, GXR 1-5, and others, and also using repeated analyses of the same elements in selected samples by different methods, in different laboratories, and in different institutes. The correlation of the results of analyzing petrogenic, trace, and rare-earth elements was done repeatedly and showed good agreement [Levitskii, 2000; Petrova, 1990]. The representativeness of the samples and the reliability of the analytical data were high enough to derive the trustworthy geochemical characteristics of the rocks.
Data
The main geostructural elements of the crust in the Sha-ryzhalgai salient of the Siberian Craton basement are the Baikal granulite-gneiss region and the East Sayan granite-greenstone region. Generally, the junction between these high-and low-grade metamorphic rock regions is marked by faults and, in some areas, by imbricate structures, as reported by previous investigators [Shafeev, et al., 1981]. Usually, the stratigraphic units and complexes have tectonic contacts with the rocks of higher TP values resting on the lower-grade metamorphic rock strata.
The Baikal granulite-gneiss region was not classified earlier as an individual geostructural element of the Pre-cambrian crust. In our understanding, this region includes the outcrops of the granulite facies rocks in the Irkut, Zhi-doi, Kitoi, Bulun [Grabkin and Melnikov, 1980; Levitskii et al., 2000], and other blocks of the Sayan marginal salient of the Siberian Craton basement (Figure 1).
The rocks of the Sharyzhalgai Series dominate in the Irkut and Zhidoi blocks in the belt stretching from the Katoi River to the Baikal Lake shore between Kultuk town and Port
Figure 1. Schematic map 011 the basement of the study area prepared from geological and geophysical data [Lobachevskii and Melnikov, 1985].
(1) Main Sayan Fault zone; (2) large interblock faults (figures in circles): (1) Tocher. (2) Alar. (3) Onot. Khort.agua, (4) Savina. (5) Kit.oi Zalari. and (6) Angara; (4) The rocks of the Baikal granulite-gneiss region: (a exposed on the Sliaryzliarlgai salient and on the Bulun (I), Kit.oi (II), and Zliidoi (III)
blocks; b covered by sediments); (5) Late Arcliean rocks of the Onot Belt: (a) exposed, (b) covered by sediments; (6) Prot.erozoic rocks of the Urik Iya trough; (7) Present-day boundary of the crat.on’s sedimentary cover; (8) granitoids of the Sliumikha (Sayan) Complex; (9) granitoid bodies mapped from gravity data under the crat.on’s sediments; (10) inferred basement rises: (A) At.ovka, (T) Tyret; (11) deep drill holes; (12) isobases (m) of the top of the Mot Formation.
Baikal. This area was described by many geologists [Evolution..., 1988; Grabkin and Melnikov, 1980; Petrova, 1990; Petrova and Levitskii, 1984; and others]. The age of the early met.amorphism, derived in the laboratories of the Institute of Geochemistry and the Institute of the Earth’s Crust in different periods of time by the Rb-Sr isocliron method
for basic two-pyroxene schists ranges between 3.72±0.3 and
3.1 Ga [Gornova and Petrova, 1999; Mekhonoshin et al., 1987; Sandimirova et al., 1979; and others]. The high-precision zircon dating and the Rb/Sr and Nd/Sm data [Af-talion et al., 1991; Bibikova et al., 1990] yielded a broad range of values: from 2.84±0.72 to 1.8±0.30 Ga. However,
102°
102° o 5 10 km
I_____________I___________I
Figure 2. Schematic geological map of the Onot greenstone belt reproduced from \Nonmetallic..., 1984]. (1) alluvium; (2) red beds of the Ushakovskaya Formation; (3-6) Onot greenstone belt: Sosnovyi Baits Suite (3), the upper subsuite of the Kamchadal Suite (4), the lower subsuite of the Kamchadal Suite (5): mainly carbonate (a) and mainly noncarbonate (b) rocks, Maloiret and Burukhtui suites (6); (7) Kitoi Series; (8) peridotites and pyroxenites of the Il’chir Complex; (9) dolerites of the Nerchinsk Complex; (10) granitoids of the Shumikha Complex; (11) gabbro, gabbro-diabase, and apogabbro amphibole rocks of the Arban Complex; (12) tonalites and trondjemites of the basement of the Onot Belt and ultrameta-morphic rocks (migmatites and granites) developed after them; (13) faults: (a - proved, b - inferred); (14) deep fault zones.
all of these dates were obtained for the rocks of the ultra-metamorphic stage. Moreover, M. Aftalion et al. [1991] analyzed altered rocks and do not mention any analyses for the primary rocks.
The metamorphic rocks of the Kitoi Series, developed in the Bulun and Kitoi blocks (Figure 1 and 2), are medium-Al, with biotite, amphibole, pyroxene, and garnet, and high-Al, with sillimanite, cordierite, biotite, and garnet, plagiogneisses and two-pyroxene plagioschists and plagiogneisses (occasionally with garnet), metagabbro-anorthosites, dolomite and calcite marbles, and less common sillimanite and biotite quartzitic gneisses and monomineral quartzite. The compositions of these rocks are given in Ta-
ble 1 and their REE distribution patterns, in Figure 3c. The age of the Kitoi plagiogneisses was found to be 2.827±180 Ga with (87Sr/86Sr)o = 0.7055±20.
The rocks of the ultrametamorphic stage cut the
metamorphic rocks, contain their relicts, and show often observed transitions from the unaltered early rocks via pla-giomigmatite, K-feldspar, and schlieric K-feldspar migmatites to autochthonous and allochthonous granites. The compositions of these rocks are presented in Table 2. Compared to their parental rocks, they are lower in iron, CaO, MgO, Li, F, iron-group elements, and Tb and higher in Si02, K2O, Rb, Ba, light REE, Zr, and Pb (Table 2, Figure 3c). A se-
Table 1. Chemical compositions (wt.%) and the contents of minor and rare-earth elements (ppm) in the metamorphic rocks of the Kitoi Series
no. 1(1) 2(1) 3(1) 4(1) 5(3) 6(2) 7(2) 8(1) 9(6) 10(1) 11(1)
Si02 48.35 78.21 89.84 0.92 55.74 66.7 57.72 68.71 64.4 54.96 51.07
Ti02 1.18 0.21 0.1 0.4 1.77 0.69 2.41 0.6 0.77 0.29 1.24
A1203 19.08 8.91 3.78 0.26 13.22 15.05 14.67 12.93 14.65 5.47 13.01
FeO 12.88 2.18 1.01 12.31 5.70 10.10 6.67 8.74 10.64 15.12
MnO 0.16 0.77 0.14 0.07 0.11 0.08 0.05 0.19 0.21
MgO 6.16 5.06 2.75 0.9 3.96 2.08 2.98 2.83 3.33 10.41 6.09
CaO 1.88 0.26 0.09 53.83 7.73 3.01 5.07 1.43 1.72 5.14 8.64
P2O5 0.02 0.02 0.05 0.39 0.16 0.42 0.07 0.09 0.03 0.11
K20 1.24 2.81 1.46 0.04 2.19 2.14 2.86 2.52 2.07 0.57 0.84
Na20 5.32 1.16 0.37 0.12 1.98 3.15 2.76 2.47 2.24 0.84 2.16
LOI 4.06 1.09 0.57 41.78 0.53 1.085 0.405 1.5 1.835 1.55 1.56
Li 18 4 3 0.1 22 36 36 22 49 64 14
Rb 22 140 65 0.1 47 89 92 13 114 16 42
Cs 1 4 0.1 2 2 2 2 4 0,1
Ba 10 330 170 14 410 597 710 330 395 69 80
Sr 145 70 20 100 243 412 440 195 110 47 120
B 1 7 20 10 11 22 2
Be 0.15 1.35 2 3 2.35 0.45 0.8
F 230 875 1000 370
Mo 1 0.5 3 1.5 1.9 0.5 0.2
Sn 2.9 4 1.5 0.5 2.1 1.7 0.5
La 5 25 30 22 38 2 3
Ce 20 69 60 52 74 3 30
Nd 5 48 29 34 2 21
Yb 0.1 4.15 3 2.2 2.5 1.5 3.8
Y 9 37 30 17 3 11 25
Zr 235 430 210 25 186 275 281 170 255 60 50
Zn 246 180 100 73 145 150
Pb 0.5 8 20 2 18 1 3
Cu 7 2 62 60 80 56 14 80
Cr 240 5 39 150 150 210 1300 80
V 240 7 290 80 80 170 120 400
Ni 150 9 73 100 100 120 390 80
Co 45 0.5 37 20 20 36 65 5
Sc 43 2 50 20 2 37 16 30
Note: Here and in the tables that follow the number of analyses used to calculate the mean values is given in the parentheses. 1) metagabbro anorthosite; 2) sillimanite-biotite quartzitic gneiss; 3) biotite-muscovite quartzite; 4) calcite marble; 5) apoandesite plagiogneiss with biotite, amphibole, and pyroxene; 6) biotite plagiogneiss; 7) melanocratic biotite plagiogneiss; 8) garnet-biotite plagiogneiss; 9) plagiogneiss with sillimanite, cordierite, and biotite; 10) sillimanite gneiss; 11) two-pyroxene plagioschist with amphibole.
ries of isochrons with ages ranging between 2.6 and 2.2 Ga was derived for the different types of the migmatites and autochthonous and allochthonous granitoids of the Kitoi Series [Sandimirova et al., 1993].
The rocks of the postultrametamorphic phase are
represented mainly by amphibole- and to a lesser extent by scapolite-, biotite-, epidote-, and zoisite-bearing rock assemblages. They make up bodies of an irregular and a vein form, are often restricted to contacts between contrasting rocks, and commonly trace fault zones.
The East Sayan granite-greenstone region borders the Baikal granulite-gneiss region along fault zones. The discovery of plagiogranites with an age of 3.25 Ga along
the Onot River [Bibikova et al, 1982] was the first evidence which justified the idea of the wide development of tonalite-trondjemite associations and greenstone belts in this region. Later, the East Sayan Superbelt was distinguished on the basis of geological and structural data [Evolution..., 1988]. Recently the term “East Sayan granite-greenstone region” became very popular [Nozhkin et al., 1995]. Based on the sum total of the geostructural, geochronological, petrological, and geochemical data, this region includes: (1) the rocks of an infrastructure-the oldest tonalite-trondjemite associations of the basement complex and (2) the rocks of a supras-tructure, which compose the Onot, Targozoi, Monkres, and other extensive greenstone belts which differ in the collection and ratio of their rock associations (Figure 1 and 2).
Figure 3. Distributions of rare earth elements, chondrite-normalized.
Basement complex (a): (1) apotonalite amphibole-biotite plagiogneiss, (2) plagioclasite, (3) K-feldspar granite-pegmatite, (4) amphibolite, (5) K-feldspar leucocratic granite, (6) trondjemite. Onot greenstone belt (b): (1-2) biotite-amphibole plagiogneiss, (3) apodacite biotite plagiogneiss, (4) biotite granite, (5) biotite-garnet-amphibole rock of the postultrametamorphic stage, (6) amphibolite of Kamchadal Suite, (7) amphibolite of Maloiret Suite. Kitoi Series (c): (1) shlieric K-feldspar migmatite, (2) biotite granite, (3) leucocratic granite, (4) amphibolite. Shumikha Complex (d): (1, 2) amphibole-biotite granodiorite of phase 1, (3) granosyenite and (4) granite porphyry of phase 3, (5) granite-pegmatite.
The gray-gneiss complex of the basement is represented by metatonalite biotite-amphibole plagiogneisses with minor lenticular amphibolite inclusions. There are early banded trondjemites (type 1), which occur as massifs ranging between 1-5 and 20-28 km in length and are traceable from the Onot R. to the Savina R., and late crosscutting bodies of massive trondjemite and tonalite (type 2). The compositions of these rocks are presented in Table 3. The age of their emplacement, derived earlier from the
trondjemites of types 1 and 2, was found to be 3.711±0.26 Ga with (87Sr/86Sr)o = 0.698±0.001 [Sandimirova et al, 1992]. Based on the data available for the metatonalite plagiogneiss and trondjemite of type 1, the age of the rocks was found to be 3.113±0.0039 Ga with (87Sr/86Sr)0 = 0.7004±0.0005 (our data, in press). The petrography, petrology, and geochemistry of the tonalite-trondjemite associations were described earlier [Nozhkin et al, 1995; Sandimirova et al, 1992]. Plagiogneisses and trondjemites from the basement of
Table 2. Chemical compositions (wt.%) and the contents of minor and rare-earth elements (ppm) in the ultrametamorphic rocks of the Kitoi Series
no. 1(1) 2(1) 3(1) 4(2) 5(4) 6(7) 7(4) 8(3) 9(4) 10(4) 11(7) 12(1)
Si02 75.15 77.14 71.75 55.19 66.79 74.03 73.14 74.29 75.28 75.39 75.27 73.00
Ti02 0.03 0.20 0.36 1.05 0.61 0.29 0.19 0.08 0.26 0.13 0.06 0.02
AI2O3 11.50 11.96 14.26 12.21 14.80 13.08 14.14 13.98 11.79 12.48 13.16 14.63
FeO 4.30 2.44 3.04 10.57 5.99 2.64 1.74 0.44 3.50 2.41 1.28 0.60
MnO 0.09 0.02 0.04 0.18 0.07 0.04 0.03 0.17 0.03 0.03 0.02 0.01
MgO 0.80 0.07 0.89 5.82 2.32 0.42 0.37 0.13 0.22 0.21 0.14 0.07
CaO 1.50 0.98 1.54 8.00 3.79 1.33 1.12 0.63 0.74 0.52 0.49 0.34
P2O5 0.02 0.02 0.06 0.11 0.12 0.07 0.06 0.04 0.04 0.04 0.04 0.02
K20 2.40 2.23 3.58 1.33 1.56 3.97 5.43 7.18 4.86 5.41 6.10 8.76
Na20 2.77 4.68 3.39 4.35 3.54 3.39 3.23 2.70 2.78 2.84 3.04 2.27
LOI 0.89 0.16 0.84 1.305 0.97 0.54 0.42 0.41 0.4 0.475 0.35 0.14
Li 54 1 10 10 14 14 10 5 11 6 4 3
Rb 104 65 12 45 70 130 150 175 183 189 194 240
Cs 0.10 0.50 2.00 2.00 0.40 1.05 1.30 1.75 1.02 2.00
Ba 590 460 1395 230 325 827 605 1400 745 515 569 740
Sr 100 20 325 215 245 184 240 100 35 53 101 250
B 7 15 14 12 8 12 13 13
Be 2.20 2.00 5.35 1.60 2.55 1.65 1.18 0.79
F 400 1300 280 483 240 390 169
Mo 0.50 1.50 0.50 3.67 0.35 0.75 0.83
Sn 4.40 2.00 4.80 2.40 5.03 3.45 1.05 1.35
La 120 120 25 62 79 64 13 190 7
Ce 140 220 55 120 141 117 31 320 18
Nd 80 80 26 25 54 59 9 120 6
Yb 13.00 1.60 4.85 1.70 4.85 2.65 1.35 6.20 1.69
Y 77 18 35 21 43 29 14 47 12
Zr 440 330 325 185 145 266 118 63 355 223 91 10
Zn 107 60 100 50 160 54 2 8
Pb 27 2 10 28 36 32 47 55
Cu 11 1 100 18 9 4 2 14 11
Cr 32 15 74 49 19 11 11 20 6
V 23 30 310 95 9 7 4 26 3
Ni 30 10 38 18 12 4 8 12 5
Co 6 5 54 18 3 4 1 10 3
Sc 17 4 7 5 2 8 2
Note: 1) biotite-cordierite plagiomigmatite; 2) biotite-amphibole plagiomigmatite; 3) schlieric garnet-biotite migmatite; 4) amphi-bole and pyroxene plagiomigmatites; 5) schlieric plagiomigmatite with biotite and amphibole; 6) K-feldspar migmatite with biotite, amphibole, and cordierite; 7) schlieric K-feldspar biotite migmatite; 8) paraautochthonous granite with biotite and amphibole; 9) autochthonous granite with biotite and amphibole; 10) paraautochthonous granite with biotite and garnet; 11) leucocratic allochthonous granite; 12) pegmatite. The primary rocks for (1-2), (6-7), and (9-10) were pelitic gneisses; plagiogneiss for (3), schist for (4-5, 8), migmatite for (11), and granite for (12).
the Onot greenstone belt, and from the other regions of the world [Trondjemites, Dacites..., 1983] show abnormally low mantle ratios (87Sr/86Sr)o and positive Eu anomalies (Figure 3a). As follows from the regional maps of magmatism and correlation, the tonalite-trondjemite associations with and without K-feldspar correlate with the plagiogranites and plagiogneisses of the Onot Complex (Figure 2).
The rocks of the ultrametamorphic phase occur as crosscutting veins, nests, or zones of biotite and amphibole-biotite plagioclase-K-feldspar and K-feldspar migmatites, and autochthonous, paraauthochthonous, and allochthonous,
usually leucocratic granites. Less common are veins of coarse-grained and pegmatoid rocks consisting largely or even solely of plagioclase, known as plagioclasite, and also K-feldspar or plagioclase pegmatites. The bodies of relict tonalite blocks in the migmatites and granites range between (1x3) and (100x1000) m in size. They differ from the original rocks of the tonalite-trondjemite association by their higher contents of Si02, AI2O3, K2O, Rb, Ba, Cs, Zr, Pb, and light REE (Figure 3a) and by their lower contents of F, MgO, CaO, Li, Yb, Y, Cu, V, Ni, Co, Sc, and in some cases of Na20 (Table 3).
Table 3. Chemical compositions (wt.%) and the contents of minor and rare-earth elements (ppm) in the rocks of the basement complex (1-5) and in the ultrametamorphic rocks developed after them (6-12)
no. 1 (4) 2(14) 3(1) 4(8) 5(5) 6(10) 7(6) 8(6) 9(5) 10(5) 11(9) 12(1)
Si02 49.01 68.29 68.65 73.85 71.58 72.28 72.04 74.39 73.46 73.14 74.09 59.81
Ti02 1.60 0.40 0.62 0.09 0.25 0.21 0.32 0.11 0.03 0.02 0.02 0.16
A1203 13.93 16.06 13.49 15.13 15.16 14.76 14.25 13.83 14.70 14.54 14.51 21.47
FeO 15.44 3.79 6.01 1.29 2.73 2.20 2.52 1.52 1.09 0.45 0.97 2.16
MnO 0.22 0.06 0.11 0.02 0.05 0.03 0.03 0.03 0.03 0.01 0.04 0.09
MgO 6.31 1.29 2.59 0.42 0.83 0.64 0.57 0.20 0.17 0.11 0.11 0.69
CaO 9.96 3.02 3.10 2.21 2.56 1.92 1.55 0.95 1.10 0.41 0.45 7.80
P2O5 0.11 0.09 0.12 0.04 0.06 0.06 0.10 0.03 0.03 0.02 0.09 1.07
K20 0.80 1.64 2.30 1.09 1.17 3.11 4.52 4.70 5.48 8.28 4.92 0.84
Na20 2.36 4.68 1.92 5.33 4.96 4.24 3.71 3.91 3.44 2.36 4.25 5.08
LOI 0.60 0.45 1.07 0.49 0.82 0.35 0.29 0.65 0.32 0.57 0.52 0.68
Li 19 46 40 5 20 19 29 9 3 2 11 18
Rb 41 78 100 24 49 87 161 184 139 170 295 32
Cs 1.03 2.28 5.00 0.28 2.53 2.29 3.67 3.20 2.50 1.00 11.38 2.00
Ba 154 330 540 163 273 467 665 352 826 1200 115 280
Sr 139 372 170 306 350 343 253 110 209 215 59 370
B 33 6 14 3 20 6 10 5 1 33
Be 0.65 0.88 1.16 2.42 1.15 0.90 0.75 0.60 0.30 1.08
F 1400 595 160 190 210 250 120 127 73 158
Mo 9.60 0.77 0.66 0.78 1.93 0.20 0.60 0.85 2.60 1.47
Sn 3.20 2.07 0.66 1.14 1.80 2.90 2.55 2.80 0.90 1.27
La 6.6 23.3 9.4 31.3 9.7 42 3.3 2 2 7 25
Ce 17 40.5 16.07 65.4 14.5 57 9.5 12 58
Nd 13.35 13.8 5.7 12.9 8 34 3.3 2 6.3 29
Yb 5.65 0.77 0.28 0.42 0.63 2.00 0.36 1.80 1.08 4.10
Y 26 5 2 4 2 22 2 8 0 9 46
Zr 96 146 78 118 98 225 121 43 34 37 10
Hf 3.90 3.35 1.43 4.20 4.20 2.50 2.30 1.10 0.20
Ta 0.15 0.55 0.33 0.30 0.45 0.15 0.20
Nb 4.00 4.24 1.28 2.13 6.00 4.10 2.00 1.20 2.45 0.50
Zn 220 57 22 47 35 45 20 20 5 13
Pb 6 7 10 9 13 27 21 73 89 72
Cu 120 27 14 35 16 14 3 10 7 29
Cr 180 19 6 7 8 7 6 11 7
V 280 52 15 30 8 13 4 3 7
Ni 80 13 5 5 7 6 4 5 4
Co 53 9 4 5 4 5 2 2 3
Sc 42 7 2 5 2 3 1
Ag 0.01 0.017 0.017 0.025 0.01 0.04 0.09 0.01 0.03
Note: 1) amphibolite; 2) biotite plagiogneiss with amphibole; 3) biotite plagiogneiss; 4) trondjemite of type 1; 5) trondjemite of type 2; 6) plagioclase-K-feldspar migmatite; 7) K-feldspar migmatite; 8) leucocratic autochthonous granite; 9) pegmatoid paraautochthonous granite; 10) allochthonous granite-pegmatite; 11) K-feldspar and plagioclase pegmatite; 12) pegmatoid plagioclasite. The primary rocks were basalt for (1), plagiogneiss (tonalite) for (2, 4-6) and (8-10), pelite for (3), gneiss and amphibolite for (7), and migmatite and granite for (11, 12).
The rocks of the ultrametamorphic phase show both a positive and a negative Eu anomaly (Figure 3). The age of the K-feldspar migmatites and granitoids in the basement and in the Onot greenstone belt was found to be 2.237 Ga [.Sandimirova et al., 1993].
The rocks of the Onot greenstone belt, metamorphosed in the conditions of the amphibolite and epidote-amphibolite facies, occur as a belt, pinching out in places,
solely in the rocks of the tonalite-trondjemite association in the Baikal granulite-gneiss region (Figure 1). The belt coincides with the boundaries of the Onot Graben [Shames, 1962]. The rocks of the belt are locally covered by the high-grade metamorphic rocks of the Kitoi Series.
The rocks of the belt were classified into (upward) the Burukhtui, Maloiret, Kamchadal, and Sosnovyi Baits suites (Figure 2). The Burukhtui Suite includes apobasaltoid am-phibolites, amphibole-biotite schists, quartzites, aporhy-
Table 4. Chemical compositions (wt.%) and the contents of minor and rare-earth elements (ppm) in the rocks of the metamorphic stage of the Onot greenstone belt
no. 1(2) 2(13) 3(4) 4(2) 5(2) 6(2) 7(4) 8(3) 9(10) 10(1) 11(16)
Si02 54.50 49.37 75.43 68.08 54.85 90.42 44.02 43.77 0.83 1.00 1.61
Ti02 0.83 0.96 0.34 0.52 0.71 0.09 0.02 0.02 0.03 0.01
A1203 11.66 14.48 10.92 14.63 16.11 1.94 0.18 0.86 0.10 0.05 0.08
FeO 12.40 13.02 4.49 5.00 14.81 1.91 54.71 55.96 0.79 1.57 1.18
MnO 0.22 0.20 0.08 0.07 0.10 0.03 0.03 0.01 0.29 0.49 0.25
MgO 8.23 7.66 0.36 1.77 5.26 1.43 1.65 0.37 20.99 20.30 45.56
CaO 8.37 10.09 1.03 2.23 3.02 1.38 0.50 0.20 30.47 29.60 0.90
P2O5 0.09 0.08 0.05 0.11 0.09 0.02 0.05 0.09 0.01 0.01 0.01
K20 0.51 0.73 3.75 2.85 2.15 0.41 0.04 0.05 0.02 0.03 0.01
Na20 2.63 2.45 2.80 3.83 0.56 0.27 0.11 0.11 0.02 0.03 0.01
LOI 0.49 1.00 0.52 0.77 2.46 3.56 1.02 0.53 46.49 46.25 50.4
Li 5 8 5 25 33 2 0 3 tr
Rb 16 18 119 120 83 22 2 2 tr 1
Cs 0.5 0.6 6.4 2.0 5.0 tr tr tr
Ba 110 125 785 1210 385 18 24 26 13 20 9
Sr 138 121 107 225 108 23 10 12 22 23 5
B 1 13 16 23 6 5 11 22 4 1 4
Be 1.60 0.50 1.90 1.70 1.80 0.10 1.05 1.03 0.18 0.15 0.26
F 650 739 375 630 360 100 243 143 134 110 291
Mo 0.40 0.80 2.00 0.75 0.10 1.00 1.00 0.15 0.10 0.23
Sn 1.40 1.71 4.90 2.35 2.70 0.80 5.30 8.10 0.76 1.10 0.25
La 24 7 79 87 31 16 2 2 2
Ce 33 5 123 141 52 30 2.5 2 2
Nd 17 4 63 60 24 15 2 2 2
Yb 1.90 3.33 7.35 3.60 2.70 0.70 0.20 0.15 0.10 0.30
Y 20 27 68 28 28 2 1 1 0.7 7
Zr 127 101 377 265 123 68 8 31 11 5 5
Zn 109 168 166 60 74 5 54 52 5 5 7
Pb 14 5 20 18 3 2 0 1 1 0 0
Cu 34 146 78 28 6 9 20 7 2 2 3
Cr 725 263 7 76 190 6 2 1 1 1 2
V 246 403 13 62 110 2 12 51 9 11 10
Ni 208 260 6 38 96 3 9 12 3 3 1
Co 33 48 4 16 29 2 11 11 2 3 0.5
Sc 32 124 7 11 22 3 3 1
Ag 0.01 0.032 0.02 0.025 0.01 0.057
Au 0.01 0.01 0.04
Note: 1) amphibolite (metabasaltic andesite); 2) amphibolite (metabasalt); 3) gneiss with biotite, garnet, and amphibole (metarhy-olite); 4) garnet-biotite plagiogneiss (metadacite); 5) biotite-garnet plagiogneiss (metapelite); 6) quartzite; 7) magnetite quartzite; 8) hematite quartzite; 9-10) dolomite marble; 11) magnesite marble. The samples were collected from the rocks of the following suites: Maloiret (1); Kamchadal, Burukhtui, and Sosnovyi Baits (2, 4, 6, 9); Burukhtui and Maloiret (3); Kamchadal and Sosnovyi Baits (5), Kamchadal (7, 10, 11), and Sosnovyi Baits (8).
olite and apopelite garnet-biotite plagiogneisses and pla-gioschists, quartzites, and marmorized limestones. The Maloiret Suite includes aporhyolite and apodacite biotite and biotite-garnet plagiogneisses, apopellite amphibole-biotite (locally with garnet) and biotite microgneisses, and apobasaltic andesite amphibolites. The Kamchadal Suite includes marbles, in which a magnesite variety dominates over the dolomite and calcite varieties. The marble beds are interstratified with amphibolites, monomineral and iron quartzites, amphibole, garnet-amphibole, biotite and garnet-biotite schists, and gneisses. The Sos-
novyi Baits Suite is dominated by amphibolites and biotite-garnet gneisses, which are thinly (in a flyschlike manner) interlayered by hematite-magnetite, hematite, monomineral, and sillimanite quartzites. The compositions of the rocks of the metamorphic phase are presented in Table 4, and their REE distribution patterns, in Figure 3b. A series of isochrons with ages ranging between 2.675±0.095 Ga with (87Sr/86Sr)0 = 0.701 and 2.786±0.059 Ga with (87Sr/86Sr)0 = 0.702 was derived by a Rb-Sr method using amphibolites (metabasaltoids) and biotite-garnet gneisses (metarhyolites) of different suites.
Table 5. Chemical compositions (wt.%) and the contents of minor and rare-earth elements (ppm) in the rocks of the ultrametamorphic stage of the Onot greenstone belt
no. 1 (2) 2 (2) 3(5) 4(1) 5 (1) 6 (1) 7(1) 8 (2) 9 (2) 10 (3)
Si02 72.39 70.27 75.53 59.18 67.64 49.33 41.50 42.47 50.27 49.37
Ti02 0.24 0.31 0.04 1.30 0.02 0.55 0.15 0.03 0.06 1.90
AI2O3 14.16 15.01 13.54 13.77 0.38 10.85 4.99 0.98 0.49 13.83
FeO 2.64 2.78 1.02 12.24 29.05 32.58 40.47 57.23 2.93 16.53
MnO 0.04 0.04 0.02 0.16 0.13 0.21 0.28 0.01 0.62 0.24
MgO 0.67 1.03 0.23 3.24 1.25 4.25 6.41 2.40 16.40 5.65
CaO 1.62 1.42 0.85 4.84 1.04 3.09 7.77 1.56 23.75 9.20
P2O5 0.04 0.10 0.03 0.14 0.02 0.02 0.03 0.03 0.01 0.21
K20 3.28 4.46 4.34 1.51 0.02 0.04 0.05 0.06 0.14 0.52
Na20 4.46 3.47 3.50 2.38 0.01 0.24 0.76 0.28 0.06 1.93
LOI 0.44 0.71 0.46 1.36 0.38 0.24 0.96 0.22 5.50 1.01
Li 16 17 3 6 2 2 2 8.5 7
Rb 103 130 171 130 4 2 2 8
Cs 3 2.5 4.25 22 1
Ba 515 930 248 360 20 5 120 27 33 167
Sr 86 330 106 120 5 60 20 17 25 117
B 6 22 12 36 10 10 8 16 10 4
Be 1.3 2 0.3 6.3 0.5 0.5 1.95 1.6 0.12 0.5
F 440 470 130 500 200 120 230
Mo 0.5 2.4 1.8 1 2 0.2 0.5 0.5 0.5 2.5
Sn 4.05 3.9 2.2 28 1 0.6 0.5 0.2 3.2 4.5
La 43 31 30 5 4 5
Ce 80 48 60 5
Nd 30 16 29 4
Yb 0.9 0.39 2 0.5 1 0.25 1.5
Y 6 4 26 3 15 2 20
Zr 97 185 59 150 10 80 22 12 15 127
Zn 53 33 5 280 100 200 170 150 37 250
Pb 29 51 30 24 1 1 0 0 14 13
Cu 5 37 11 13 50 8 520 9 3 128
Cr 7 1 8 35 5 200 69 1 6 80
V 9 30 7 250 5 100 66 11 15 225
Ni 4 10 6 65 10 300 23 11 4 120
Co 4 2 1 45 4 20 90 5 12 50
Sc 28 1 30 20
Ag 0.01 0.02 0.05 0.01
Note: 1) schlieric biotite plagiomigmatite; 2) K-feldspar migmatite; 3) leucocratic granite; 4) garnet-quartz-amphibole rock; 5) pyroxene-magnetite rock; 6) ferrosilite-amphibole-quartz-garnet rock; 7) cummingtonite-magnetite rock; 8) ferrosilite rock; 9) pyroxene skarn; 10) garnet-amphibole rock with biotite. The primary rocks were amphibolite for (1, 10), gneiss and amphibolite for (2, 3), iron quartzite for (4-8), and dolomite marble for (9).
The rocks of the ultrametamorphic stage contain relicts of the metamorphic rocks and are represented by plagiomigma-tites, K-feldspar and schlieric K-feldspar migmatites, granites, and also by garnet-amphibole biotite-bearing basic rocks in the gneisses and amphibolites; by pyroxene skarns in the dolomite marble, and by skarns with enstatite, forsterite, and spinel in the magnesite marble; and by garnet-quartz-amphibole, pyroxene-magnetite, ferrisilite-amphibole-quartz-garnet, cummingtonite-magnetite, and ferrisilite metasomatic rocks in the iron quartzites. The chemical compositions of these rocks are given in Table 5. The ultrametamorphic rocks developed after the alumosili-cate rocks are enriched in SiCb, K2O, Na20, Rb, Cs, Ba, Sr,
B, Mo, Sn, light REE, Zr, Pb, Ag, and Au, and are depleted in iron, CaO, MgO, F, Yb, Y, Zn, Cu, Cr, V, Ni, Co, and Sc.
The replacement of the dolomite and magnesite marbles by skarn increased the contents of Si02, AI2O3, iron, alkalis, and most of trace elements and diminished CaO and (or) MgO. The iron quarzites show the removal of Si02 and iron and the concentration of AI2O3, CaO, MgO, alkalies, and most of the trace elements (Tables 4 and 5; Figure 3b).
The rocks of the post-ultrametamorphic phase are developed in the basement complex and in the greenstone belt. They occur as elongated lenticular bodies, ovals, sheets, and pockets and have a distinct or poorly expressed zonal struc-
Table 6. Chemical compositions (wt.%) and the contents of minor and rare-earth elements (ppm) in the rocks of the post ultrametamorphic stage of the Kitoi Series and the Onot and Arban complexes
no. 1(8) 2(15) 3(1) 4(1) 5(10) 6(5) 7(3) 8(1) 9(11) 10(1) 11(6) 12(1) 13(1) 14(2)
Si02 51.77 49.36 56.50 60.82 49.15 48.62 52.18 73.15 57.75 57.48 56.38 43.40 79.67 30.19
Ti02 1.36 1.38 1.19 1.10 0.91 0.86 1.31 0.58 0.93 1.13 0.42 0.11 0.36 0.35
A1203 13.64 15.37 14.17 10.16 14.15 14.89 15.46 10.41 18.52 18.03 17.11 20.57 9.91 7.51
FeO 14.64 13.47 11.25 21.28 12.36 11.28 16.12 6.90 10.76 5.51 9.09 4.02 3.15 7.01
MnO 0.20 0.20 0.13 0.19 0.20 0.15 0.17 0.07 0.15 0.07 0.15 0.11 0.02 0.64
MgO 5.75 6.24 3.88 3.83 7.75 10.08 6.48 2.46 4.20 5.59 3.89 0.86 0.59 2.21
CaO 8.30 9.50 7.20 1.57 10.75 4.82 2.92 1.91 1.37 1.29 6.51 18.41 0.50 28.14
P2O5 0.15 0.12 0.15 0.12 0.08 0.06 0.09 0.07 0.08 0.32 0.12 0.22 0.08 0.05
K20 1.20 0.92 1.73 0.33 1.13 1.33 1.47 1.32 2.97 4.41 2.37 1.88 3.48 1.34
Na20 2.79 2.41 2.35 0.03 1.81 1.50 1.93 1.67 1.67 2.34 1.67 2.69 1.40 0.42
LOI 1.231 1.248 1.51 0.58 1.746 1.97 1.61 1.39 2.00 3.77 2.295 7.74 0.76 21.37
Li 12 5 13 14 6 12 34 9 20 60 29 12 15 16
Rb 26 23 42 18 24 31 59 31 78 210 54 58 96 31
Cs 0.58 0.55 0.10 2.00 0.10 0.55 3.28 0.10 5.17 2.00 6.00 2.00 0.10
Ba 276 195 390 100 187 331 344 295 573 820 478 310 770 265
Sr 167 187 400 20 116 159 95 95 109 320 203 280 140 95
B 14 13 5 95 30 45 26 1 84 22 25 23 4
Be 2.08 1.26 0.40 1.50 0.45 0.83 3.00 2.74 1.65 1.30 1.90
F 1508 925 375 255 590 760 2600 590 500
Mo 5.75 0.53 2.20 0.50 0.60 0.50 3.00 2.87 0.30 0.35 0.50
Sn 3.72 2.03 2.00 3.40 0.30 2.40 2.53 2.00 3.94 2.20 3.50 2.00 1.00
La 33 16 5 15 2 4 20 37 65 30 4
Ce 50 35 5 30 6 13 50 73 140 70 20
Nd 28 13 5 5 9 29 54 40 10
Yb 3.86 3.47 3.20 3.00 1.97 3.50 2.00 2.13 2.00 2.20 1.60
Y 28 33 28 30 19 30 20 26 20 21 15
Zr 131 95 150 110 55 73 114 150 139 170 110 14 140 115
Zn 168 145 80 250 200 97 158 100 164 75 100 36 35
Pb 9 5 6 10 3 9 9 15 24 4 14 9 10
Cu 90 93 100 29 150 141 110 80 68 24 48 11 15
Cr 105 121 5 100 160 454 101 200 322 240 275 22 125
V 347 307 200 200 335 210 200 100 142 150 104 170 20
Ni 73 78 40 80 120 128 120 100 101 110 66 17 40
Co 50 41 40 40 47 51 50 15 27 34 28 14 40
Sc 45 55 42 38 15 43 26
Ag 14.05 0.03 0.10 0.03 0.06 0.06 0.01 0.06
Pd 0.006 0.006 0.05 0.006 0.001
Note: 1-2) and 5-6) are amphibole rocks; 3) biotite-amphibole rock; 4) amphibole-garnet-quartz rock; 7) biotite-plagioclase-garnet-amphibole rock; 8) garnet-biotite-plagioclase-quartz rock; 9) substantially biotite rock with staurolite, garnet, amphibole, and plagioclase; 10) quartz-plagioclase-amphibole rock with muscovite; 11) garnet-plagioclase-staurolite-biotite-quartz rock; 12) zoisite-epidote-amphibole-plagioclase rock; 13) muscovite-biotite-plagioclase-quartz rock; 14) carbonate-bearing rock with garnet, biotite, and amphibole. The rock samples were collected from the following suites: Kitoi (1), Kamchadal Suite and the basement (2), Kamchadal (3, 8-12, 14), Sosnovyi Baits (4, 6, 7), Arban (5), and the basement (13). The primary rocks were gabbro-anorthosite for (1), gabbro and basic schist for (2), schist for (3), amphibolite for (4, 7-9, 14), gabbro for (5-6), granite for (10), gneiss for (11), amphibolite for (12), and tonalite for (13).
ture. Their characteristic feature is the areally spread scatter of minerals as disseminated particles. The most widely developed are apogabbro and schistose amphibole, biotite-amphibole, amphibole-garnet-quartz, biotite-plagioclase-garnet-amphibole, quartz-garnet-biotite-plagioclase-quartz and quartz-garnet (often with disthene), substantially biotite (with staurolite, garnet, amphibole, and plagioclase), quartz-plagioclase-amphibole, apodolomite quartz-hematite-amphibole-graphite, apogneiss garnet-plagioclase-
staurolite-disthene-biotite-quartz, zoisite-epidote-amphi-bole-plagioclase, muscovite-biotite-plagioclase-quartz, and carbonate-bearing (with garnet, chlorite, and amphibole) metasomatic rocks. Some of them are of the high-pressure kyanite-sillimonite type. The specific features of their composition are the elevated, relative to the source material, contents of K2O, MnO, Li, B, Be, Sn, Mo, F, Zr, Ag, Au, and Pd (Table 6). The post-ultrametamorphic biotite-garnet-quartz-plagioclase metasomatites with silimonite,
Table 7. Chemical compositions (wt.%) and the contents of minor and rare-earth elements (ppm) in the gabbroids, metaultrabasics, and granitoids
no. 1 (9) 2 (2) 3 (27) 4(14) 5 (21) 6 (9)
Si02 48.60 45.14 68.24 73.73 73.93 74.09
Ti02 1.35 0.45 0.83 0.40 0.25 0.02
A1203 13.70 4.14 13.42 12.65 12.88 14.51
FeO 14.01 13.56 5.30 2.89 2.52 0.97
MnO 0.21 0.18 0.07 0.04 0.04 0.04
MgO 7.47 28.77 0.97 0.40 0.37 0.11
CaO 11.08 4.94 2.43 1.21 0.92 0.45
P2O5 0.11 0.05 0.27 0.07 0.06 0.09
K20 0.55 0.05 4.53 5.30 5.62 4.92
Na20 2.09 0.23 2.98 2.82 2.91 4.25
LOI 0.96 0.79 0.70 0.5 0.31 0.52
Li 7 2 15 8 5 11
Rb 13 1 138 171 183 295
Cs 0.3 0.1 1.9 2.0 2.4 11.4
Ba 149 75 1488 997 732 115
Sr 123 88 279 170 139 59
B 14 8 7 11 33
Be 0.69 2.06 1.95 1.88 1.08
F 463 1100 666 303 158
Mo 0.73 1.70 1.11 1.10 1.47
Sn 1.80 5.38 6.19 7.13 1.27
La 16 100 76 39 7
Ce 43 131 111 65 12
Nd 27 77 48 33 6
Yb 2.22 3.83 5.52 4.33 1.08
Y 21 35 45 32 9
Zr 64 43 328 304 238 37
Hf 1.60 10.32 7.75 5.47 1.10
Ta 0.10 1.10 1.15 4.05 0.15
Nb 1.55 9.00 10.74 15.40 26.47 2.45
Zn 173 76 43 47 13
Pb 3 32 28 37 72
Cu 138 10 6 9 29
Cr 206 15 9 8 7
V 314 59 31 15 7
Ni 107 9 6 4 4
Co 47 6 5 4 3
Sc 25 15 7 9 1
Ag 0.05 0.02 0.10 0.02 0.03
Note: 1) gabbro of the Arban Complex; 2) metaultrabasic rock of the Ilchir Complex; 3-6) granitoids of the Shumikha Complex: biotite-amphibole granodiorite of phase 1 (3), leucocratic granite of phase 2 with biotite and amphibole (4), granite- and granosyenite porphyry and aplite-like granite of phase 3 (5), and K-feldspar and plagioclase pegmatite (6).
staurolite, and muscovite yielded isochrons ranging between 1.994±0.012 Ga with (87Sr/86Sr)0 = 0.709±0.0007 and 2.117±0.0145 Ga with (87Sr/86Sr)0 = 0.717±0.0008. It appears that the formation of high-pressure rocks in the Arban Massif [Sharkov et al, 1996] should be dated by the same period of time.
Arban gabbroids and Ilchir motaultramafics occur as a number of massifs, ranging from a few to hundreds of meters (rarely tens of kilometers), localized in the rocks of the basement, of the Kitoi Series, and of all suites of the
Onot greenstone belt. The chemical compositions of these rocks are given in Table 7 (columns 1 and 2). The fact that these gabbroids and ultramafics were not involved in the ul-trametamorphic transformations, but were actively replaced by the postultrametamorphic rocks suggests that their formation might have taken place in the time interval of 2.18-
2.2 Ga.
Shumikha granitoid complex is definitely restricted to a zone of junction of the highly metamorphozed rocks of the Sharyzhalgai and Kitoi series with the rocks of the Onot
greenstone belt and is traceable both in them and also in the gray gneisses of the basement over a distance of 250-300 km (Figure 1). As an independent complex, these granitoids were mapped during the geological surveys of the Irkutskge-ologiya Association conducted in the last decade. Earlier, most of these rocks had been included into the Sayan Complex. The rocks occur as one- or multiphase plutons ranging from tens of meters to 10-15 km in size. The rocks of the first phase are massive and porphyry-like amphibole, amphibole-biotite, and biotite granodiorites (often with hypersthene). The rocks of the second phase are massive biotite granites, and those of the late phases are vein aplite, granodiorite-, granosyenite-, and granite porphyry, and leucogranites. The compositions of these rocks are listed in Table 7 (columns 4-6), and their REE distribution patterns are displayed in Figure 3d.
The time of the granitoid formation determined by a Rb-Sr method for the amphibole and amphibole-biotite granodiorites and granite porphyry of the Onot Massif was found to be 1.983±0.048 Ga with (87Sr/86Sr)0 = 070633±0.00045. Similar granites, attributed to the Sayan Complex (Barbi-tai Massif in the NW Sayan region), were dated by a U-Pb method using zircons and found to be 1.848±0.018 Ga old with MSWD = 6.6 [Kirnozova et al., 2000].
Pegmatites and granite pegmatites are widely developed in the rocks of the Kitoi Series and gray-gneiss complex and are much more rare in the belt itself. They do not show any clearly expressed zoning. They are dominated by plagioclase and K-feldspar varieties with tourmaline (schorl), garnet, muscovite, and orthite (Table 7, column 6). The K-feldspar varieties were found to be abnormally high in Li, Rb, and Cs. These rocks were dated 1.86±0.004 Ga with (87Sr/86Sr)0 = 0.738±0.0003.
The metasomatites of deep fault zones are restricted to the zones of the Dabad (Kitoi-Zalari), Alagni-Kholomkha (Savinskii), Onot-Khartagninskii, and other faults. The alumosilicate rocks are dominated by albite, quartz-microcline-chlorite (with biotite, muscovite, and amphibole), and chlorite or serpentine-chlorite rocks; the early skarn and magnesite marble are dominated by talc-bearing associations. Much rare are low-temperature metasomatites with amphibole, K-feldspar, and biotite. The compositions of these rocks are listed in Table 8 and were discussed earlier [Levitskii, 1994]. The age of the rocks is 633±7 Ga with (87Sr/86Sr)0 = 1.2255±0.0063.
Discussion of Results
The geochronological, geological, and petrological data suggest the following sequence of the rock formation: the tonalites, trondjemites, and amphibolites of the basement— the granulite-facies metamorphic rocks of the Kitoi Series— the rocks of the Onot greenstone belt—the ultrametamor-phic rocks—the gabbroids of the Arban Complex—the rocks of the post-ultrametamorphic phase—the metasomatic rocks of the deep fault zones. As a rule, the rocks have tectonic
contacts which host abundant metasomatic rocks of various types. The main structural, petrographic, isotopic, and geochronological characteristics of the rocks are summarized in Table 9. The rocks have their own distinct fields in the AFM diagram of Figure 4.
As follows from their petrogeochemical, geochronological and isotopic characteristics, the rocks of the tonalite-trondjemite composition are similar to the trondjemite gneisses of Amitsok and Nuk, Greenland [Mac-Greyor, 1983], to the low-K gneisses of Swaziland and the tonalites of the Tispruit Pluton, South African Republic [Gollerson and Bridgewater, 1983], and to the Waiwak-1 tonalite-trondjemite gneisses of Labrador, Canada [Gollerson and Bridgewater, 1983]. Earlier, based on their structural and textural features, mineral composition, their contents of pet-rogenic and trace elements, their K/Rb, Rb/Sr, Sr/Ba and Ba/Rb ratios, their REE distribution patterns, their positive Eu anomaly (Figure 3a), and also on their abnormally high mantle 87Sr/86Sr ratios, Sandimirova et al. [1992] and Nozhkin et al. [1995] concluded that the rocks of the tonalite-trondjemite composition were similar to the oldest granitoids of the Earth [Trondjemites, Daeites..., 1983]. Based on a number of their properties, Condie and Banter [1976], Banter [1983], and Banter et al. [1978] believed them to be most close to the trondjemites of Swaziland and to the trondjemites of the Tispruit diapiric pluton from the Barberton greenstone complex (SAR). Considering the whole set of data, these rocks originated under continental conditions. Earlier, Petrova and Levitskii [1984] proved that the initial rocks of the Sharyzhalgai Complex developed SW of Lake Baikal were oceanic formations with an age of 3,1-3.7 Ga [Gornova and Petrova, 1999; Mekhonoshin et al., 1987; Sandimirova et al., 1979]. Therefore, it can be assumed that the basement of the marginal part of the Siberian Craton included both Early Archean sialic continental crust and mafic oceanic crust, both having low (0.700-0.701) primary 87Sr/86Sr ratios and similar ages (3.1-3.7 Ga), the age of the high- and low-grade metamorphic protolith (Table 9).
The mineral composition and petrogeochemical properties of the rocks of the Kitoi Series, such as the variations and high contents of Si02, AI2O3, CaO, K2O, Li, Ba, Rb, B, Zr, Hf, Nb, Cr, and Ni (Table 1), as well as the high (87Sr/86Sr)o ratios, suggest that a substantial contribution to the composition of the Kitoi rocks was made by the products of the disintegration, weathering, and chemical differentiation of the earlier continental (basement complex) and oceanic (Sharyzhalgai Complex) rocks. The metavolcanics are scarce and belong to a calc-alkalic series (Figure 4, Table 1). The new geochronological and petrogeochemical data justify the interpretation of the Kitoi Series as an independent stratigraphic unit of the Sharyzhalgai Complex.
The rocks of the Onot Belt accumulated in a paleorift, where bimodal volcanic rocks, with a growing amount of basaltoids and tuff, were succeeded first by terrigenous (clastic) sediments and then by chemogenic carbonate (both la-goonal and deep-sea) sediments. The volcanic rocks vary from basalts to rhyolites (Table 4; Figure 3b). The existence of one mantle source, which controlled the mechanism of petrogenesis during a long period of time, is proved by the low (87Sr/86Sr)o values obtained for the basement rocks
Table 8. Chemical compositions (wt.%) and the contents of minor and rare-earth elements (ppm) of metasomatic rocks from deep-fault zones
no. 1(5) 2(2) 3(4) 4(1) 5(3) 6(11) 7(12) 8(1) 9(1) 10(11) 11(18) 12(3) 13(1)
Si02 72.16 58.30 61.67 53.93 36.70 34.57 45.90 46.30 36.42 20.88 59.61 51.58 42.98
Ti02 0.18 1.09 0.16 0.87 1.06 0.01 0.23 0.01 1.97 0.14 0.02 0.02 0.01
A1203 14.41 17.09 18.21 18.43 17.17 0.36 6.94 0.07 14.29 3.35 0.45 0.30 0.47
FeO 1.57 6.22 2.05 9.28 7.54 0.76 3.27 1.49 3.36 9.73 1.05 0.89 1.40
MnO 0.01 0.07 0.02 0.09 0.08 0.05 0.07 0.10 0.02 0.21 0.05 0.07 0.03
MgO 0.51 5.39 8.36 3.91 25.83 26.44 25.55 35.62 32.09 29.46 30.82 10.90 40.80
CaO 0.42 1.59 0.19 3.45 0.21 20.01 2.86 0.31 0.14 5.98 1.25 14.29 0.06
P2O5 0.07 0.44 0.09 0.14 0.06 0.01 0.04 0.05 0.03 0.02 0.01 0.03
K20 7.49 3.48 3.74 1.66 0.70 0.03 0.67 0.01 0.02 0.04 0.02 0.06 0.01
Na20 2.32 2.66 0.02 6.48 0.08 0.08 0.02 0.02 0.32 0.09 0.05 0.11 0.32
LOI 0.763 3.65 5.53 1.77 10.61 17.83 13.86 16.1 11.36 29.89 6.75 20.68 13.93
Li 4 57 23 14 57 5 23 2 91 16 4 10 2
Rb 355 160 55 90 21 1 15 2 2
Cs 4.25 2.00 3.25 7.00 1.75 0.38 1.05 1.00
Ba 818 920 47 190 27 38 25 22 23 14 20 10
Sr 145 360 17 120 8 55 9 5 5 9 8 15
B 10 25 58 28 134 15 26 16 5 8 6
Be 1.85 2.15 1.84 1.18 0.25 0.81 0.05 2.40 0.86 0.22 0.42
F 338 2000 518 472 472 542 280 500 353 817 300
Mo 0.30 0.30 0.50 0.69 0.55 0.47 0.65 0.73
Sn 44.13 3.20 1.45 2.35 0.78 1.30 0.50 3.10 2.42 0.69 0.75
La 12 100 3 19 2 7 12 3 5
Ce 32 180 41 2
Nd 11 67 12 2 5 19
Yb 1.58 2.10 5.30 2.51 0.13 0.87 3.80 7.00 7.40
Y 12.5 23 47 13.6 0.37 10.42 35 37 48
Zr 141 190 46 160 55 6 30 5 120 59 6 5
Zn 9 73 16 67 18 27 5 5 47 5 5
Pb 22 8 2 0 5 0 0 0 3 2 0
Cu 526 20 2 10 3 4 3 1 37513 2 4
Cr 13 180 3 217 2 61 1 22 34 5 1
V 13 150 16 251 8 43 2 360 58 3 12
Ni 7 110 6 138 8 34 1 22 32 11 2
Co 2 25 7 43 2 8 1 16 94 4 0
Ag 3.23 0.26 0.03
Au 0.01 0.004
Pd 0.001 0.009
Note: 1) albite-quartz-microcline rock with muscovite and chlorite; 2) chlorite-muscovite rock; 3) chlorite-quartz-sericite rock; 4) epidote-plagioclase-quartz rock; 5) chlorite rock with serpentine, sericite, and talc; 6) serpentine rock (ophicalcite); 7) chlorite-serpentine-talc rock with quartz; 8) talc-serpentine rock; 9) hematite-chlorite-talc rock; 10) sulfide-bearing rock; 11) substantially talc rock; 12) quartz-dolomite rock; 13) asbestos rock. The primary rocks were granite for (1-4); granite, amphibolite, and migmatite for (5); dolomite marble for (6); apomagnesite skarn for (7); magnesite marble for (8); schist for (9); gneiss, schist, and marble and metasomatic rocks after them (10); magnesite marble and magnesian skarn for (11); serpentine rock for (12). The suites involved were Kamchadal (1-5 and 7-13) and Burukhtui (6).
and for the apobasalt amphibolites and aporhyolite garnet-biotite gneisses (Table 9). This fact justifies the interpretation of the granite-greenstone regions as independent and most important structural elements in the structure of the Precambrian continental crust.
The facts of the destruction of the early rocks and the profound differentiation of the weathering products are proved by the presence of marbles and monomineral iron and alumina quartzites produced by the accumulation of Si02, Fe,
MnO, CaO, MgO, and trace elements (Table 5). The common chemogenic conditions of carbonate formation are indicated by the absence of Si02 and AI2O3 in the dolomite, magnesite, and calcite marbles and by the elevated contents of MnO and iron in the rocks of the Onot Belt and the Kitoi Series (Table 1, column 4; Table 4, columns 9-12). These and other data suggest that the rocks of the Kitoi Series accumulated as a result of the area disintegration and weathering of rocks under terrigenous-chemogenic conditions, and
Table 9. Rock-structural and isotope-geochronological characteristics of rocks that originated successively in the SW Baikal and SE Sayan regions
Rock- structural type Series, complex Method of study and site Sample material Age, Ga 87Sr/86Sr Reference
Sharyzhalgai Series Rb/Sr Rb/Sr Rb/Sr Basic two-pyroxene, plagioclase schists; SW coastal area of Lake Baikal, Oringol iron ore district 3.7 3.49 3.104 0.701 0.7016 0.7021 [.Sandimirova et al, 1979] [Mekhonoshin et al, 1987] [Gornova and Petrova, 1999]
Baikal granulite- gneiss region Kitoi Series Rb/Sr Plagiogneisses, Onot R. and Biboi R. drainage areas 2.827 0.7055 [Levitskii et al, 1995]
U/Pb for Zr Charnockite-series enderbite, plagiogranite, garnet-cordierite and hypersthene migmatites, granite 2.4-2.3; 2.2-2.4; 2.4; 2.5; 1.9 [Bibikova et al, 1990]
Sharyzhalgai Series U/Pb for Zr Charnoenderbite, garnet gneiss; SW coast of L. Baikal 2.46±0.01; 1.96±0.01; 1.75-1,85; 2.85±0.07; 2.78±0.048 [Aftalion et al, 1991]
Rb/Sr Sm/Nd Garnet gneiss, migmatites; SW coast of L. Baikal 1.96±0.16; 2.275; 2.42; 2.34 [Aftalion et al, 1991]
U/Pb for Zr Plagiogneiss granite; Onot R. 3.25 [Bibikova et al, 1982]
East Rocks of Onot Belt basement Rb/Sr Trondjemite; Onot, Savina, and M. Belaya rivers 3.711 0.6984± 0.0015 [Sandimirova et al, 1992]
Sayan granite- greenstone Rb/Sr Trondjemite and tonalite; Onot and M. Belaya rivers 3.113± 0.039 0.7004± 0.0005 Our data (in press)
region Onot Belt, Maloiret and Kamchadal suites Rb/Sr Apobasalt amphibolite, aporhyolite biotite-garnet gneiss; Onot R. 2.675 2.786 0.7016 0.7018 [Levitskii et al, 1995]
those of the Onot Belt accumulated as a result of extensive associations in the tonalite-trondjemite basement, the Ki-redeposition only in synform linear zones in the same period toi Series, and the lower levels of the Onot greenstone belt of time. (lower levels of the Maloiret Suite) to the upper levels of
The evolution of the metabasaltic rocks from the early the Kamchadal Suite is imprinted in the replacement of the
Table 9. (continued)
Rock-structural type Series, complex Method of study and site Sample material Age, Ga 87Sr/86Sr Reference
Rocks of ultrametamorphic Basement Complex, Kitoi Series, Rb/Sr Plagiomigmatites, K-feldspar migmatites, granitoids; 2.237 0.7041 [Sandimirova et al, 1993]
stage Onot greenstone belt Onot, Savina, and Malaya Belaya drainage areas 2.40 2.60 0.704 0.703 [Levitskii et al, 1995]
Rocks of postultrametamorphic tage Basement Complex, Kitoi Series, Onot Belt, ultrametamorphic rocks Rb/Sr Amphibole and garnet-biotite-plagioclase-quartz rocks with staurolite and disthene; Savina, Onot, and M. Belaya drainage basins 1.994 2.128 0.7089 0.7168 [Levitskii et al, 1995]
Shumikha granitoid complex Rb/Sr Amphibole and amphibole-biotite granodiorite, aplite, and garnet porphyry; Onot R. 1.983 0.7063 [Levitskii et al, 1995]
Blocks of the Sayan marginal basement salient Sayan granitoid complex U/Pb for zircons Amphibole-biotite granodiorite with hypersthene; Barbitai R. 1.848 [Kirnozova et al, 2000]
Pegmatites Rb/Sr Plagioclase and K-feldspar pegmatites and granite-pegmatites; Onot R. 1.86 0.738 [Sandimirova et al, 1993]
Deep fault zones Dabad and Alagna-Kholomkha faults Rb/Sr Chlorite rocks or same with serpentine, sericite, and talc; Onot R. 0.633 1.225 [Sandimirova et al, 1993]
calc-alkalic differentiation trend by the dominating tholei-itic trend, close to NMORB (Figure 9). Based on the absence of the contiguous series of basic and ultrabasic rocks and on the presence of basic, intermediate, and acid vol-canics, which sometimes occur as bimodal series with comparable (87Sr/86Sr)o values, the Onot Belt can be classified as a secondary greenstone belt of the calc-alkalic type [Condie, 1983], which originated on the early sialic tonalite-trondjemite crust. Its apobasalt and apobasaltic andesite amphibolites from the base of the sequence include varieties similar to the Archean differentiated basalts of the TH2 type with TH1 basalts dominating greatly in the top [Condie, 1983]. The metarhyolite and metaandesite gneisses are similar to F2-type gneisses [Condie, 1983], characterized by their REE fractionation (Figure 3b). A distinctive feature of the
Onot Belt is the presence of carbonate rocks and the predominance of magnesite, known from the Kalar greenstone belt of India [Monin, 1987]. It is worth mentioning the fact that the rocks of the lower Maloiret Suite showed an older age (2.786 Ga) than the age of the rocks from the middle and top of the Kamchadal Suite (2.675 Ga), where various types of marble, gneiss, and quartzite dominate over the metavolcanics. This suggests the age and isotopic specifics of this belt’s rock accumulation history and calls for more geochronological, geological, and geochemical investigations to prove the accumulation sequence of various suites.
The processes of ultrametamorphism (granitization) were especially active in the junction zone between the East Sayan granite-greenstone and the Baikal granulite-gneiss regions. They facilitated the homogenization of the rocks of the base-
Figure 4. AFM diagram for the mean compositions of rocks.
(1-2) rocks of the Kitoi Series of the metamorphic (1) and ultrametamorphic (2) stages; (3-4) rocks of the basement complex of the metamorphic (3) and ultrametamorphic (4) stages; (5-7) rocks of the Onot greenstone belt of the metamorphic (5), ultrametamorphic (6), and postultrametamorphic (7) stages; (8-9) rocks of the Targazoi greenstone belt of the metamorphic (8) and ultrametamorphic (9) stages; (10) metasomatic rocks from deep fault zones developed after different primary rocks; (11) granitoids of the Shumikha Complex; (12) rocks of the Arban, Ilchir, and Nerchinsk complexes.
ment, the Kitoi Series, and the Onot greenstone belt, the obliteration of contacts between them, and the formation of one granite-metamorphic layer of the Earth’s crust, in which its high- or low-metamorphic substrate can be distinguished in very rare cases. At the early stages these processes were marked in the alumosilicate rocks by the formation of various migmatites, at the late stages, by the formation of granites and skarn after the marble. Skarn with spinel, forsterite, and enstatite was formed after the magnesite. In its turn, this skarn served as a source rock for the formation of commercial talcite deposits. The iron quartzite served as a source material for producing metasomatites with garnet, ortho- and clinopyroxene, amphibole, and quartz. In all cases one can
trace the superimposed character of transformations over all types of rocks and the effect of the source rocks on the compositions of the newly formed materials. The result of these processes was the fact that the rocks of the ultrametamorphic phase, developed after the amphibolites (on a moderate scale) and after the high-A1 gneisses, are higher in Si02, K2O, Rb, Ba, Zr, Pb, and light REE and lower in Fe, MgO, CaO, and, in some cases, in Na20, Li, Be, F, Mo, Sn, Yb, Y, Zn, Cu, Cr, V, Ni, Co, Sc, and Ag, as compared to the source material (Tables 1-6). The migmatites after the tonalite and trondjemite are slightly lower in Si02 and Na20 (Table 3). The metasomatites after iron quartzites are lower in Si02 and iron and higher in CaO and MgO. Where skarn devel-
oped after the marble, the contents of these elements are lower, but their Si02 and AI2O3 contents are higher. On the whole, the rocks of the ultrametamorphic phase showed, compared to the source material, the accumulation of light REE and the removal of heavy REE, as can be seen from the steeper curves in Figure 3c, and also the higher initial 87Sr/86Sr values in the rocks of the basement, the Kitoi Series, and the Onot greenstone belt (Table 9).
The petrogeochemical specific features of the rocks of the postultrametamorphic phase were controlled by the following factors: (1) the compositions of the replaced rock; (2) the chemical trends of the transformation processes accompanied by the redistribution of elements under the action of solutions enriched in H2O, F, Cl, CO2, and S; (3) the general physico-chemical conditions [Levitskii, 2000; Petrova and Levitskii, 1984]. These factors were responsible for the fact that the rocks of this group are extremely diverse in mineral and chemical compositions. They have highly variable and rather high (87Sr/86Sr)o values, indicative of a complex interaction between the crust and mantle materials and, probably, of isotope fractionation in zoned bodies. The early associations are represented by high-temperature and high-pressure assemblages, the late, by medium- and low-temperature and high-pressure rocks. Compared with the initial material, the rocks of the back zones are enriched in Si02 and (or) in AI2O3, and those from the marginal zones, in CaO and MgO. As the temperature of metasomatism declined (some temperature subclasses were replaced by others), the rocks were depleted in bases, alkalis, F, and Cl and enriched in Si02, H2O, CO2, and S. Generally the processes of postultrametamorphic transformations were accompanied by the redistribution of most petrogenic and trace elements.
The AFM diagram displayed in Figure 4 shows the mean compositions of the rocks from the Onot and Targazoi greenstone belts. They have similar characteristics: a calc-alkalic and a tholeiitic trend of the differentiation of the basic vol-canogenic rocks and a growth of alkali metals and silica in the rocks of the ultrametamorphic phase.
In terms of their alkalis contents, the domination of K over Na, and Fe over Mg, the REE contents and distribution pattern (Figure 3), and their (87Sr/86Sr)o values, the granitoids of the Shumikha Complex resemble rapakivi granites, especially the well-known rapakivi-like granites of the Primorskii Complex [Levitskii et al., 1997a]. This affinity is supported by the high Fe contents of their biotites (64-86%) and am-phiboles (77-88%), and also by the elevated content of K2O (0.9-2.3%) in amphiboles and of AI2O3 (13-16%) in biotites.
The rocks of the ultrametamorphic and post-ultrameta-morphic phases and the granitoids of the Shumikha Complex showed similar petrogeochemical features: elevated K, Ba, Sr, Zr, Nb, TR, Pb, and Sn contents, enrichment in light and depletion in heavy REE (Figure 3), and the (87Sr/86Sr)o values higher than in the source rocks. These features suggest their genetic association with the same mantle sources. This seems to have controlled a change from the substantially Na-mafic specifics of the previously formed oceanic and continental crust to a K-alumosilicate specifics which was responsible for the formation of the garnet-metamorphic layer.
The compositions of metasomatic rocks from the zones of deep faults in the rocks of the Onot greenstone belt, similar
to those of the postultrametamorphic phase, were controlled by the physicochemical conditions of their formation. A specific feature of the formation of the metasomatic rocks in the deep fault zones was the redistribution of petrogenic and trace elements, and also their removal and accumulation under more favorable conditions. For instance, the formation of the alumosilicate metasomatites after gneisses (granites, migmatites) was accompanied by the removal of Si02, alkalis, iron (after amphibolites), and almost all trace elements, which accumulated in the zones of the formation of apocar-bonate metasomatites, and also of metalliferous apoamphi-bolite, apomigmatite, and apogranitoid rocks with Co, Ni, Cr, Au, Pd, Sn, and Be. Generally, the metasomatic rocks of the deep fault zones are much higher, compared with the source rocks, in F, S, B, and Zr and in some cases in Sn, Ta, Be, and Hf, this fact suggesting their addition in the course of the petrogenesis. These rocks usually have an abnormally high (87Sr/86Sr)o value. Of fundamental importance is also the fact that the formation of the Onot greenstone belt and the development of the metasomatic rocks in it took place at different times and were not related genetically.
The structural and petrogenic features of rocks and the formation mechanisms of greenstone belts and rifts are known to be similar in many respects [Grachev, 1977; Grachev and Fedorovskii, 1970; and many other authors]. A hot discussion in the 1980s [Grachev and Fedorovskii, 1970; Keller et al., 1983; Upton and Blundell, 1978; to name but a few] on the topic of whether greenstone belts evolved from rift zones or island arcs resulted in the fact that at the present time most of investigators admit, though with significant reservations, the rift origin of greenstone belts, in general, [Bozhko, 1986; Khain and Bozhko, 1988; Mi-lanovskii, 1983; and others], and of the Onot greenstone belt, in particular [Mekhanoshin, 1999; and others].
In the last decade some geoscientists succeeded in developing alternative models for the origin and evolution of greenstone belts from the standpoints of plate and plume tectonics [Borukaev, 1996; Condie, 1992; Dobretsov and Kirdyashkin, 1994, 1995; Kroner, 1991; Sleep, 1992; and others]. These models provide a more complete explanation of the main features of the structure, evolution, and composition of all observed rock complexes that superseded one another over a period of almost 3 Ga. During the early 3.1-3.7 Ga period of time a differentiated oceanic (metatholeiite) crust, represented by the rocks of the Sharyzhalgai Series, and a continental sialic tonalite-trondjemite crust existed in the region. It was only the continental crust that experienced active extension and sagging [after Milanovskii, 1983] and later (2.6-2.7 Ga) the formation of a suprastructure—the Onot, Targazoi, Monkres, and Urik-Iya greenstone belts with the greatly varying proportions and compositions of sedimentary and volcanic rocks, bordering the margins of the Sayan Salient of the Siberian Craton. That period of time was dominated by plastic deformations during the formation of troughs at the early stages of the history. The accumulation of the rocks of that complex occurred at the expense of both the intrusion of bimodal series and the destruction and disintegration of the sialic (tonalite-trondjemite) and mafic [essentially tholeiite; Petrova and Levitskii, 1984] materials. The rocks of the Kitoi Series, represented mainly
by medium- and high-Al gneisses, marbles, and insignificant metabasalts, accumulated at the expense of the destruction of the Sharyzhalgai rocks. Later, the rocks of both series underwent granulite-facies metamorphism. The marginal parts of the structures tracing the junction zone between the greenstone belt and the basement rocks experienced, within the belt, intensive isochemical metamorphism (possibly to a granulite facies), and allochemical ultrametamorphism. Those were syncollision processes which operated during the interaction and collision of different, now consolidated blocks under the combination of extension and compression in different parts of the blocks and terminated the cratonization of the crust. These zones experienced intensive development of postultrametamorphic high-pressure meta-somatites and postkinematic rapakivi-like A-type granites in the time interval of 2.0-1.8 Ga. Their development reflected the high alkali-potassic specifics of ancient rift-like systems. The latest rocks (633 Ma) are the low-T metasomatic rocks in the zone of the Main Sayan Fault. Its trend coincides with the trend of the junction zone between the granulite-gneiss and granite-greenstone regions, and also with the trend of the Cenozoic and Neogene basalts in the Tunkinskii Rift [Grachev, 1977]. This suggests paragenetic relations of pet-rogenesis in this region with mantle sources, possibly with long-lived low-density mantle diapirs [after Bozhko, 1986] in ancient and young rift-related structures.
Conclusion
1. The Early Archean period of the region’s history was marked by the coexistence of the continental sialic crust represented by the tonalite-trondjemite rock associations in the basement of the Onot greenstone belt and of the oceanic (mafic) crust consisting of the rocks of the Sharyzhalgai Complex, which were later metamorphosed in the conditions of the granulite facies. The continental crust in the south of the Siberian Craton basement is composed of the rocks of a tonalite-trondjemite complex, the high-grade metamorphic rocks of the Kitoi Series, the rocks of the Onot greenstone belt, the rocks of the ultrametamorphic stage, the gabbroids of the Arban Complex, the metamorphosed ultramafics of the Ilchir Complex, the rocks of the post-ultrametamorphic stage, and the metasomatic rocks of the deep fault zones.
2. The Onot greenstone belt originated on the early sialic tonalite-trondjemite crust. Its base consists of calc-alkalic rocks ranging from rhyolites to basalts. Its middle interval includes tholeiitic metabasalts, clastic sediments, and carbonate facies which accumulated in shallow-sea areas and lagoons. The top of the sequence is highly dominated by clastic rocks. The rocks of the Kitoi Series accumulated simultaneously with the emplacement of the rocks of the Onot greenstone belt as a result of the disintegration and redeposition of the rocks of the Sharyzhalgai Complex.
3. The processes of the ultrametamorphic and post-ultra-metamorphic transformations were of a superimposed allochemical character and made a significant contribution to the formation of the granite-metamorphic layer of the continental crust. The rocks of the post-ultrametamorphic stage
accumulated under high-pressure conditions in the zones where geological structures of different age, metamorphism, and genesis contacted one another. The metasomatic rocks of deep fault zones and the ores they contain were not associated genetically with the formation of the Onot greenstone belt.
4. The rocks of the Shumikha Complex, classified here as rapakivi-like granites, are restricted to a contact between the Baikal granulite-gneiss region and the East Sayan granite-greenstone region. They are similar to the granites of the Primorskii Complex developed in the West Baikal region.
5. The contact zones between the East Sayan granite-greenstone and the Baikal granulite-gneiss regions show the linear bedding of the Onot rocks and the subconcordant alignment along these trends of the maximum development of the ultrametamorphic and postultrametamorphic rocks, Shumikha granitoids, and deep fault-zone metasomatites. This suggests the deep origin of these rocks and their genetic association with mantle sources.
Acknowledgments. This work was supported by the Russian Foundation for Basic Research, grants 00-05-64216, 99-05-64892, and 00-15-985760.
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(Received August 10, 2001)
