early proterozoic postcollisional granitoids of the biryusa block of the siberian craton

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Early Proterozoic postcollisional granitoids of the Biryusa blockof the Siberian craton

T.V. Donskaya a,*, D.P. Gladkochub a, A.M. Mazukabzov a, M.T.D. Wingate b

a Institute of the Earth’s Crust, Siberian Branch of the Russian Academy of Sciences, ul. Lermontova 128, Irkutsk, 664033, Russia

b Geological Survey of Western Australia, East Perth, WA 6004, Australia

Received 25 December 2012; accepted 23 May 2013

Abstract

Comprehensive geochemical and geochronological studies were carried out for two-mica granites of the Biryusa block of the Siberiancraton basement. U–Pb zircon dating of the granites yielded an age of 1874 ± 14 Ma. The rocks of the Biryusa massif correspond in chemicalcomposition to normally alkaline and moderately alkaline high-alumina leucogranites. By mineral and petrogeochemical compositions, theyare assigned to S-type granites. The low CaO/Na2O ratios (<0.3), K2O ≈ 5 wt.%, CaO < 1 wt.%, and high Rb/Ba (0.7–1.9) and Rb/Sr (3.9–6.8)ratios indicate that the two-mica granites resulted from the melting of a metapelitic source (possibly, the Archean metasedimentary rocks ofthe Biryusa block, similar to the granites in εNd(t) value) in the absence of an additional fluid phase. The granite formation proceeded at740–800 ºC (zircon saturation temperature). The age of the S-type two-mica granites agrees with the estimated ages of I- and A-type granitoidspresent in the Biryusa block. Altogether, these granitoids form a magmatic belt stretching along the zone of junction of the Biryusa blockwith the Paleoproterozoic Urik–Iya terrane and Tunguska superterrane. The granitoids are high-temperature rocks, which evidences that theyformed within a high-temperature collision structure. It is admitted that the intrusion of granitoids took place within the thickened crust incollision setting at the stage of postcollisional extension in the Paleoproterozoic. This geodynamic setting was the result of the unification ofthe Neoarchean Biryusa continental block, Paleoproterozoic Urik–Iya terrane, and Archean Tunguska superterrane into the Siberian craton.

Keywords: granites; U–Pb zircon age; geochemistry; Paleoproterozoic; Siberian craton

Introduction

Within the southern marginal salients of the Siberian cratonbasement, there are abundant granitoids dated at 1.84–1.88 Ga, which intruded at the final stages of the cratonformation (Fig. 1). Larin et al. (2003) united these granitoidsand rocks of the North Baikal volcanoplutonic belt into theSouth Siberian postcollisional magmatic belt. Biotite–amphi-bole granites, assigned to A-granites by geochemical features,are the most widespread among postcollisional granitoids(Donskaya et al., 2002, 2003, 2005; Larin et al., 2000, 2009;Levitskii et al., 2002; Nozhkin et al., 2003; Savel’eva andBazarova, 2012; Turkina et al., 2003). They are present inalmost all marginal salients of the southern Siberian craton.Besides A-type biotite–amphibole granites, granitoids of othergeochemical types also formed within this area in the sameperiod (Larin et al., 2006; Turkina, 2005; Turkina et al., 2006).

These are, e.g., S-type two-mica granites (1846 ± 8 Ma) in theTonod salient of the Siberian craton basement (Larin et al.,2006). The greatest diversity of Paleoproterozoic postcolli-sional granites is observed within the Biryusa block of thebasement. Tonalites, I-type diorites, and A-type biotite–amphi-bole granites here were studied in detail (Turkina, 2005;Turkina et al., 2006). Levitskii et al. (2002) also examinedA-type biotite–amphibole granites and, in addition, revealedtwo-mica granites in the Biryusa block, which they assignedto the same rock complex as the A-type granites. Nevertheless,these two-mica granites, in contrast to the biotite–amphiboleones, tonalites, and diorites of the block, have been poorlystudied until recently.

In this work we present results of geochronological andgeochemical studies of two-mica granites of the Biryusa blockof the Siberian craton. In addition, we generalize our andliterature data on the Paleoproterozoic postcollisional grani-toids of the Buryusa block and analyze the possible causes ofthe nearly synchronous formation of granites of widely varyingcompositions within a small site of the Siberian cratonbasement.

Russian Geology and Geophysics 55 (2014) 812–823

* Corresponding author.E-mail address: [email protected] (T.V. Donskaya)

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Geologic position of the Biryusa block granitoids

Paleoproterozoic nonmetamorphosed granitoids are wide-spread within the cis-Sayan marginal salient of the Siberiancraton basement, including the Biryusa block and the zone ofits junction with the Urik–Iya terrane (Fig. 2).

The Biryusa block is composed of Archean rocks of theKhailama and Monkres Groups discordantly overlain byPaleoproterozoic rocks of the Neroi and Subluk Groups(Belichenko et al., 1988; Dmitrieva and Nozhkin, 2012;Turkina et al., 2006). The Khailama Group includes gneisses(biotite, garnet–biotite, garnet–cordierite, biotite–hornblende),amphibolites, two-pyroxene schists, granulites, and migma-tites. The age of the rock metamorphism is estimated at 1.9 Ga(207Pb/206Pb zircon dating of biotite gneisses) (Turkina et al.,2006). The Nd model age of the Khailama Group is 2.6–2.8 Ga(Turkina, 2005; Turkina et al., 2006). The Monkres Group iscomposed of mafic metavolcanics, amphibolites, gabbroidswith interbeds of felsic volcanics, and quartzites (Belichenkoet al., 1988). The Paleoproterozoic Neroi Group is formedmainly by metacarbonate-terrigenous deposits, and the SublukGroup, by metavolcanosedimentary deposits (Belichenko etal., 1988; Turkina et al., 2006). The Neroi Group rocks havea model Nd age of 1.9–2.7 Ga (Dmitrieva and Nozhkin, 2012).

The Paleoproterozoic nonmetamorposed granitoids intrudeinto the above-described Archean rocks and the Paleoprotero-zoic rocks of the Subluk Group and, together with them, areoverlain by Neoproterozoic sedimentary rocks (Fig. 2). Therelationships between the granitoids and the PaleoproterozoicNeroi Group rocks are still debatable. All Paleoproterozoicgranitoids are post-tectonic postfolding rocks and belong tothe Sayan complex subdivided into two phases. Levitskii etal. (2002) recognized the third phase composed of veinedrocks. According to these authors, the first phase includesgranodiorites, syenites, quartz syenites, granosyenites, quartzdiorites, and subordinate pyroxene diorites and tonalites; thesecond phase is formed by biotite, amphibole–biotite, andtwo-mica granites. On the geological map (scale 1:1,500,000)(Yanshin, 1983), granites, granodiorites, granosyenites, andgranite-gneisses are assigned to the first phase of the Sayancomplex, and granites, granosyenites, syenites, aplites, andpegmatites, to the second phase. The legends of somegeological maps (Rik et al., 1959) present the Sayan andBiryusa complexes, with the latter being regarded as a possibleindividual phase of the Sayan complex.

Earlier, Levitskii et al. (2002) proposed a geologicalscheme of the location of the Sayan complex granitoids, wherethey recognized a group of garnet-bearing hypersthene–biotitegranitoids composing the massifs along the Major Sayan Fault.

Fig. 1. Schematic geologic structure of the southern Siberian craton, modified after Didenko et al. (2003). 1, Central Asian Fold Belt; 2, Siberian Platform cover;3, Riphean deposits of subsided craton sites; 4–6, rocks of the marginal salients of the Siberian craton basement: 4, Paleoproterozoic postcollisional granitoids;5, Paleoproterozoic North Baikal volcanoplutonic belt; 6, united Archean–Paleoproterozoic metamorphic and igneous complexes; 7, major faults; 8, boundaries ofblocks within the cis-Sayan marginal salient of the basement. Inset schematically shows major tectonic elements of the Siberian craton, modified after Gladkochubet al. (2006) and Rosen (2003). 1, superterranes; 2, Paleoproterozoic fold belts; 3, basement salients; 4, suture zones. Encircled numerals: 1, Angara Fold Belt;2, Akitkan Fold Belt.

T.V. Donskaya et al. / Russian Geology and Geophysics 55 (2014) 812–823 813

On other geological schemes (Yanshin, 1983), however, thegranitoids of these massifs are considered Paleozoic (Fig. 2).

At present, there are several U–Pb zircon dates forgranitoids of the Sayan complex of the Biryusa block (Fig. 2).The age of biotite–amphibole granites of the Barbitai massifis 1858 ± 20 Ma (Levitskii et al., 2002); the age of tonalitesof the Podporog massif is 1869 ± 10 Ma (Turkina et al., 2003);and the age of quartz diorites of the Uda massif is 1859± 10 Ma (Turkina et al., 2006). Up to now, the age oftwo-mica granites of the Sayan complex has not beendetermined.

For study, we chose two-mica granites of the conditionallynamed Biryusa massif in the Biryusa block. The massifextends along the Biryusa River in its middle reaches (Fig. 3).The contacts of the massif granites with the host metamorphicrocks are overlain by Neoproterozoic sedimentary strata. Thestudied granites of the Biryusa massif are light gray medium-to coarse-grained rocks composed of plagioclase (35–38%),quartz (30–35%), K-feldspar(25–30%), biotite (3–4%), and

muscovite (1–2%). Muscovite occurs mainly as a primarymineral in the form of plates and flakes. Sericite pseudo-morphs after plagioclase and chlorite pseudomorphs afterbiotite are observed in some varieties. Accessory minerals inthe granites are zircon, apatite, and rutile.

Methods

The representative samples of two-mica granites from theBiryusa massif were analyzed for major oxides and trace andrare-earth elements. One sample was studied by U–Pb zircondating, and its Nd isotope composition was determined. Thelocalities of sampling for petrogeochemical and geochro-nological studies are shown in Fig. 3.

The contents of major oxides were determined by silicateanalysis at the Institute of the Earth’s Crust SB RAS, Irkutsk(analysts N.N. Ukhova and N.Yu. Tsareva). The contents ofRb, Sr, Y, Zr, Nb, and Ba were measured by the X-ray

Fig. 2. Schematic geologic structure of the cis-Sayan marginal salient of the Siberian craton, modified after Yanshin (1983). 1, Central Asian Fold Belt; 2, Phanerozoicdeposits of the Siberian Platform cover; 3, Devonian volcanosedimentary rocks of superposed troughs; 4, Neoproterozoic sedimentary rocks of the cis-Sayan trough;5, Paleozoic granitoids; 6, Paleoproterozoic postcollisional granitoids; 7, Paleoproterozoic rocks of the Urik–Iya terrane; 8, Paleoproterozoic–Archean rocks of theBiryusa block; 9, Archean rocks of the Sharyzhalgai salient of the Tunguska superterrane basement; 10, Major Sayan Fault; 11, Biryusa Fault (generalized line).

814 T.V. Donskaya et al. / Russian Geology and Geophysics 55 (2014) 812–823

fluorescent method at the Geological Institute SB RAS, UlanUde (analyst B.Zh. Zhalsaraev). The concents of rare-earthelements, Th, and U were determined by ICP MS on a VGPlasmaquad PQ-2 (VG Elemental, England) mass spectrome-ter, following the technique proposed by Garbe-Schonberg(1993), at the Center of Collective Use of the Irkutsk ScientificCenter SB RAS (analysts S.V. Panteeva and V.V. Markova).The mass spectrometer was calibrated against the G-2 andGSP-2 International standard samples. Chemical decomposi-tion of the samples for ICP MS analysis was performed bytheir fusion with LiBO2, following the technique described byPanteeva et al. (2003), which favored the total dissolution ofall minerals. The error of the ICP MS determination of traceand rare-earth elements was ≤5%.

The contents of Sm and Nd and the Nd isotope compositionof sample 02100 were determined on a Finnigan MAT-262(RPQ) mass spectrometer in the static regime at the GeologicalInstitute of the Kola Scientific Center SB RAS, Apatity. Thesamples were prepared for isotope analysis following Bay-anova’s (2004) technique. The blank sample contained 0.06 ngSm and 0.3 ng Nd. The errors of determination of Sm andNd concentrations and isotopic ratios were as follows: Smand Nd—±0.2% (2σ), 147Sm/144Nd—±0.2% (2σ), and 143Nd/144Nd—±0.003% (2σ). The measured 143Nd/144Nd valueswere normalized to 148Nd/144Nd = 0.241578, which corre-sponds to 146Nd/144Nd = 0.7219, and were reduced to143Nd/144Nd = 0.511860 in the La Jolla Nd-standard. Theweighted average 143Nd/144Nd value in the La Jolla Nd-stand-ard was 0.511833 ± 0.000014 (n = 11).

U-Pb zircon dating of two-mica granites from the Biryusamassif (sample 02100) was performed on a SHRIMP-II ionmicroprobe at the Curtin University of Technology, Perth(Australia). The manually selected zircon grains were im-planted into epoxy resin together with grains of the TEMORAzircon standard. Then, the preparation was polished andsubjected to gold spraying. The spot (crater) diameter was30 µm. The U/Pb ratios were normalized to the value of theTEMORA standard zircon (0.0668), corresponding to thezircon age of 416.75 Ma (Black and Kamo, 2003). Themeasured values were corrected for common lead based onthe measured 204Pb content and in accordance with the modelvalues (Stacey and Kramers, 1975). The age was calculatedusing the generally accepted uranium decay constants (Steigerand Jäger, 1977). The data obtained were processed using theSQUID (Ludwig, 2001) and ISOPLOT (Ludwig, 2003) pro-grams. A diagram with a concordia was constructed accordingto Tera and Wasserburg (1972).

Isotope and geochemical characteristics of granites

Two-mica granites of the Biryusa massif contain 71.9–74.5 wt.% SiO2 and 7.3–8.5 wt.% alkalies (K2O/Na2O ≥ 1.4)(Table 1). By composition, these are normally alkaline andmoderately alkaline leucogranites. According to the classifi-cation by Frost et al. (2001), the granites are both magnesianand ferroan (FeO*/(FeO* + MgO) = 0.73–0.86), mainly alkali-

calcic rocks (Na2O + K2O–CaO = 7.1–8.3). These are high-alumina granites, with ASI varying from 1.17 to 1.49 (Ta-ble 1). In the above petrogeochemical features they are similarto the world high-alumina leucogranites (Frost et al., 2001).

The granites have high contents of Rb (230–380 ppm) andTh (14–41 ppm), low contents of Sr (41–70 ppm), mediumcontents of Ba (160–390 ppm), Zr (60–150 ppm), Nb (10–24 ppm), and Y (8–27 ppm) (Table 1), and ΣREE = 95–288 ppm. They are characterized by a fractionated REEpattern ((La/Yb)n = 5.6–26.5) and a distinct negative Euanomaly (Eu/Eu* = 0.16–0.37) (Fig. 4).

By mineral and petrogeochemical compositions, the Bi-ryusa massif granites are assigned to S-type granites accordingto the classification of Chappell and White (1974, 1992).

Sm–Nd isotope studies were performed for two-micagranite from the Biryusa massif (sample 02100). The sampleis characterized by εNd(1874 Ma) = –6.4 and a Neoarcheanmodel age—tNdDM = 2.74 Ga (Table 2).

Results of geochronological U–Pb studies

For dating granite from the Biryusa massif we used sample02100. The sampling locality is shown in Fig. 3. The separatedaccessory zircon has fine euhedral and subeuhedral crystals

Fig. 3. Schematic geologic structure of the region in the middle reaches of theBiryusa River, modified after (State..., 2011). 1, Phanerozoic deposits of theSiberian Platform cover; 2, Neoproterozoic sedimentary rocks of the cis-Sayantrough; 3, Neoproterozoic dolerite dikes; 4, Paleoproterozoic postcollisionalgranitoids; 5, Early Precambrian rocks of the Biryusa block; 6, sampling locali-ties.


Table 1. Chemical composition of two-mica granites of the Biryusa massif

Component 01092 01093 01095 01096 01097 01098 01099 01100 01101 01102 01103 0298 0299 02100

SiO2, wt.% 74.53 71.86 72.84 73.22 71.97 73.59 73.60 74.38 73.72 73.56 74.24 73.84 73.47 72.92

TiO2 0.12 0.21 0.17 0.22 0.24 0.25 0.19 0.15 0.19 0.13 0.13 0.22 0.21 0.24

Al2O3 14.45 14.25 14.40 14.20 14.15 14.43 14.20 13.65 14.25 13.72 13.75 13.70 13.90 14.00

Fe2O3 0.31 0.21 0.38 N.f. 0.03 0.01 0.01 N.f. 0.06 0.06 0.00 0.28 0.42 0.36

FeO 1.11 2.20 1.28 2.46 2.78 2.33 2.56 1.80 1.77 2.53 1.95 1.64 1.65 1.77

MnO 0.03 0.05 0.05 0.01 0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01

MgO 0.51 0.45 0.51 0.63 0.69 0.53 0.42 0.40 0.63 0.67 0.45 0.45 0.66 0.53

CaO 0.19 0.81 0.43 0.29 0.26 0.24 0.22 0.07 0.07 0.10 0.16 0.62 0.42 0.47

Na2O 2.63 3.32 2.85 3.01 3.22 2.77 3.11 2.80 2.85 2.76 3.14 3.36 3.19 3.24

K2O 4.65 4.70 5.43 5.04 4.83 4.74 4.77 5.47 5.17 5.28 5.28 4.64 5.00 5.23

P2O5 0.07 0.10 0.09 0.09 0.08 0.25 0.09 0.06 0.09 0.05 0.08 0.06 0.07 0.07

H2O– 0.07 0.07 <0.01 0.15 0.25 0.05 0.23 0.22 0.18 0.03 0.13 <0.01 <0.01 0.20

LOI 1.22 1.24 1.22 1.18 1.41 1.26 1.09 1.00 1.05 1.43 0.87 0.77 1.33 1.27

CO2 0.06 0.11 <0.06 <0.06 <0.06 <0.06 <0.06 <0.06 <0.06 <0.06 <0.06 0.11 <0.06 <0.06

F 0.16 0.16 0.16 0.04 0.06 0.06 0.03 0.04 0.05 0.03 0.04 0.10 0.11 0.08

–O(F) 0.07 0.07 0.07 0.02 0.03 0.03 0.01 0.02 0.02 0.01 0.02 0.04 0.05 0.03

Total 100.04 99.67 99.74 100.52 99.96 100.49 100.51 100.02 100.06 100.34 100.20 99.75 100.38 100.35

Rb, ppm 260 350 380 310 290 320 310 270 270 230 230 280 280 280

Sr 54 62 57 58 60 54 64 70 67 58 57 59 41 41

Y 16 18 16 15 17 17 15 20 9 23 8 11 23 27

Zr 60 150 95 140 150 130 130 110 100 110 91 140 130 130

Nb 11 24 22 18 24 20 23 10 15 20 20 18 20 19

Ba 160 180 200 260 300 300 260 380 370 250 170 380 390 290

La 17.38 59.80 42.78 – 59.15 59.29 52.93 38.86 – – 24.17 – – 45.88

Ce 43.43 130.80 90.61 – 134.28 124.36 112.32 80.41 – – 55.57 – – 107.32

Pr 4.39 15.92 11.10 – 15.26 14.79 13.46 9.43 – – 6.01 – – 11.60

Nd 15.14 53.27 37.87 – 49.48 52.49 44.64 31.70 – – 20.77 – – 37.77

Sm 2.57 10.03 6.94 – 8.60 8.28 8.88 6.93 – – 4.88 – – 6.83

Eu 0.30 0.45 0.33 – 0.51 0.48 0.60 0.64 – – 0.47 – – 0.37

Gd 2.43 7.08 4.58 – 6.10 6.01 6.63 5.55 – – 4.11 – – 4.75

Tb 0.46 0.81 0.62 – 0.70 0.88 0.71 0.67 – – 0.51 – – 0.80

Dy 3.09 4.31 3.38 – 4.12 4.55 3.75 3.79 – – 2.76 – – 5.00

Ho 0.65 0.70 0.61 – 0.72 0.82 0.65 0.81 – – 0.51 – – 1.03

Er 1.98 1.99 1.61 – 2.03 2.13 1.62 2.32 – – 1.33 – – 2.85

Tm 0.34 0.27 0.21 – 0.29 0.24 0.20 0.30 – – 0.18 – – 0.53

Yb 2.08 2.02 1.12 – 2.09 1.50 1.38 2.23 – – 1.27 – – 2.86

Lu 0.36 0.24 0.20 – 0.29 0.25 0.21 0.32 – – 0.18 – – 0.40

Th 13.58 51.29 24.03 – 41.41 31.43 45.65 31.61 – – 35.51 – – 38.63

U 2.67 7.94 4.16 – 5.15 3.95 8.33 6.22 – – 8.71 – – 3.55

C, norm. 4.88 2.43 3.22 3.44 3.30 4.86 3.69 3.09 4.00 3.36 2.74 2.12 2.59 2.28

ASI 1.50 1.20 1.28 1.31 1.30 1.47 1.34 1.29 1.38 1.32 1.24 1.18 1.23 1.19

FeO*/(FeO* + MgO) 0.73 0.84 0.76 0.80 0.80 0.82 0.86 0.82 0.74 0.79 0.81 0.81 0.75 0.80

CaO/Na2O 0.07 0.24 0.15 0.10 0.08 0.09 0.07 0.03 0.02 0.04 0.05 0.18 0.13 0.15

Rb/Sr 4.8 5.6 6.7 5.3 4.8 5.9 4.8 3.9 4.0 4.0 4.0 4.7 6.8 6.8

Rb/Ba 1.6 1.9 1.9 1.2 1.0 1.1 1.2 0.7 0.7 0.9 1.4 0.7 0.7 1.0

Sr/Ba 0.3 0.3 0.3 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.3 0.2 0.1 0.1

Lan/Ybn 5.6 19.8 25.6 – 18.9 26.5 25.7 11.7 – – 12.7 – – 10.7

Eu/Eu* 0.37 0.16 0.18 – 0.22 0.21 0.24 0.32 – – 0.32 – – 0.20

Al2O3/TiO2 120 68 85 65 59 58 75 91 75 106 106 62 66 58

T, °C 737 793 761 796 800 797 792 775 772 776 756 788 784 781

Note. C, norm., Content of normative corundum; ASI(mol) = Al2O3/(CaO – 1.67 × P2O5 + Na2O + K2O); FeO* = FeO + 0.8998 × Fe2O3; Eu/Eu* =Eun/√(Smn x Gdn) ; n, chondrite-normalized (Sun and McDonough, 1989) values; T, °C, temperatures at the initial stages of melt crystallization (zircon saturationtemperatures) (Watson and Harrison, 1983). N.f., Not found. Dash, Not analysed.


50 to 130 µm in size, with elongation of 1:2 and 1:3.5. Thecathodoluminescence images show a distinct magmatic zoningin the zircon grains (Fig. 5). The results of ion microprobeanalysis (SHRIMP-II) of eight zircon grains are presented inTable 3 and in Fig. 6. The contents of U and Th are 118–491and 52–225 ppm, respectively. The 232Th/238U ratio variesfrom 0.46 to 0.83. On the U–Pb diagram with concordia(Fig. 6), five isotope composition points of the studied zirconlie on the concordia, and its concordant age is 1879 ± 8 Ma(MSWD = 0.45). The average 207Pb/206Pb age calculated oversix points of zircon isotope composition is 1874 ± 14 Ma(MSWD = 2.0). Isotope composition points 2.1, 3.1, and 8.1were ignored on the calculation of the concordant age, andpoints 3.1 and 8.1 were not involved in the calculation of theweighted average age (Table 3, Fig. 6). Exclusion of points3.1 and 8.1 from all calculations was due to the strongdiscordance of obtained values. In accordance with themorphology of zircon, pointing to its magmatic genesis, theweighted average age of 1874 ± 14 Ma can be interpreted asthe time of its crystallization and, correspondingly, as the ageof the Biryusa massif granites.

Discussion

Petrogenesis and formation conditions of two-micagranites of the Biryusa massif. The mineral composition oftwo-mica granites of the Biryusa massif and their geochemicalfeatures similar to those of S-type granites point to theirmetasedimentary nature. Experimental data showed that suchgranites can result from the melting of pelites or psammites(Harris and Inger, 1992; Miller, 1985; Patiño Douce and

Johnston, 1991; Skjerlie and Johnston, 1996). One of thecriteria for distinguishing between granites melted from com-positionally different sources is the CaO/Na2O ratio reflectingdifferent contents of plagioclase and clay minerals in pelitesand psammites (Sylvester at al., 1998). The studied Biryusamassif granites show low CaO/Na2O ratios (<0.3), whichindicates their pelitic source (Table 1, Fig. 7a) (Sylvester atal., 1998). The metapelitic composition of the source is alsoevidenced by the high-alumina composition of granites (nor-mative corundum amounts to 2.12–4.88), K2O ≈ 5 wt.%,CaO < 1 wt.%, and high values of Rb/Ba (0.7–1.9) and Rb/Sr(3.9–6.8) (Table 1, Fig. 7b) (Miller, 1985; Sylvester, 1998).This is also confirmed by the arrangement of the figurative

Table 2. Sm-Nd isotope data for granites of the Biryusa massif

Sample Age, Ma Content, ppm 147Sm/144Nd 143Nd/144Nd±2σ

εNd(t) tNd(DM), Ma tNd(DM-2st),Ma

Sm Nd

02100 1874 5.60 32.42 0.1045 0.511175 ± 14 –6.4 2745 2888

Fig. 4. Chondrite-normalized (Sun and McDonough, 1989) REE patterns oftwo-mica granites of the Biryusa massif.

Table 3. Results of U-Pb analysis of zircons from two-mica granite of the Biryusa massif (sample 02100)

Crystal,crater

238U, ppm 232Th, ppm Th/U f204, % Isotope ratios Age, Ma (± 1σ) D, %

238U/206Pb*(1)

± 1σ 207Pb*/206Pb*(1)

± 1σ 238U/206Pb*(1)

207Pb*/206Pb*(1)

1.1 206 130 0.66 0.070 2.976 0.034 0.11485 0.00055 1868 ± 19 1877 ± 9 0.5

2.1 195 91 0.48 0.190 2.986 0.035 0.11296 0.00064 1862 ± 19 1848 ± 10 –0.8

3.1 491 232 0.49 0.046 3.226 0.035 0.11324 0.00046 1741 ± 17 1852 ± 7 6.0

4.1 133 68 0.53 –0.016 2.988 0.036 0.11562 0.00063 1861 ± 19 1890 ± 10 1.5

5.1 161 77 0.50 –0.084 3.017 0.036 0.11494 0.00070 1845 ± 19 1879 ± 11 1.8

6.1 281 225 0.83 0.054 2.966 0.034 0.11484 0.00053 1873 ± 18 1877 ± 8 0.2

7.1 161 80 0.52 0.045 2.967 0.035 0.11450 0.00059 1872 ± 19 1872 ± 9 0.0

8.1 118 52 0.46 0.141 3.183 0.039 0.11285 0.00087 1761 ± 19 1846 ± 14 4.6

Note. f204, Portion of common 206Pb in total measured 206Pb. (1), corrected for common Pb, using measured 204Pb. Pb*, Radiogenic Pb. D, Discordance.


points of the analyzed granites on the Al2O3/(MgO + FeO*)–CaO/(MgO + FeO*) diagram (Altherr et al., 2000) (Fig. 7c).

The behavior of Rb, Ba, and Sr in the high-alumina granitespermits elucidation of not only the composition of their source(Fig. 7b) but also the crystallization conditions of the melt andthe main mineral phases controlling its generation, becausethese elements are predominant in the major minerals of thegranites: plagioclase, K-feldspar, and micas. According to themodel calculations by Harris and Inger (1992), leucogranitesresulting from the incongruent melting of muscovite (ormuscovite and biotite) in metapelites in the absence of a fluidphase can contain K-feldspar, plagioclase, quartz, garnet, andsillimanite among the restite phases. If only muscovite under-goes melting in the source, then the restite can contain biotiteas well. The melts thus produced show high Rb/Sr and lowSr/Ba ratios and a negative Eu anomaly. Melting in thepresence of fluid, on the contrary, leads to low Rb/Sr and highSr/Ba ratios, which is due to the absence of plagioclase andK-feldspar from the restite. Thus, the high Rb/Sr (3.8–6.8)(Table 1, Fig. 7b) and low Sr/Ba (0.1–0.3) (Table 1) valuesand negative Eu anomaly on the REE patterns of the granites(Fig. 4) evidence that the metapelitic-source melting pro-ceeded in the absence of an additional fluid phase (Harris andInger, 1992). Moreover, the above Rb/Sr and Sr/Ba ratiossuggest the presence of both plagioclase and K-feldspar in therestite during the melting-out of two-mica granites of theBiryusa massif (Harris and Inger, 1992). The elevated contentsof Y and HREE in the granites (Table 1, Fig. 4) indicate thatgarnet could not be the main restite phase during theirformation. The negative correlation between the SiO2 and(FeO* + MgO + TiO2) contents (Fig. 7d) suggests the pres-ence of biotite in the restite and, correspondingly, the meltingof only muscovite in the metapelitic source. To sum it up, thetwo-mica granites of the Biryusa massif resulted from the

dehydration melting of muscovite in a metapelitic source inequilibrium with the restite containing plagioclase, K-feldspar,biotite, quartz, and sillimanite.

The similar Nd isotope compositions of two-mica granitesof the Biryusa massif (εNd(1874 Ma) = –6.4, tNdDM = 2.74 Ga)and Archean gneisses of the Khailama Group of the Biryusablock (εNd(1874 Ma) = –3.9 to –6.8, tNdDM = 2.57–2.81 Ga(Turkina, 2005; Turkina et al., 2006)) suggest that the latterrocks were the source of the former.

The earlier viewpoint (Levitskii et al., 2002) that thetwo-mica granites belong to the second phase of the Sayangranitoid complex in the Biryusa block is not confirmed bythe available isotope-geochemical data. The amphibole andbiotite–amphibole granitoids assigned by the above authors tothe first phase of the complex cannot be even potential sourcesof the two-mica granites because they are characterized byεNd(t) = –0.7 to –2.0 (Kirnozova et al., 2003). Moreover, theBiryusa massif granites resulted from the melting of ametapelitic source; therefore, they cannot be geneticallyrelated to amphibole and biotite–amphibole granites of theSayan complex.

The low Al2O3/TiO2 ratio (58–120, average 78) in thetwo-mica granites of the Biryusa massif testifies to therelatively high temperature of granite melts (Table 1, Fig. 7a)(Sylvester, 1998). This is confirmed by the calculations madewith the zircon thermometer of Watson and Harrison (1983).It helps to estimate the degree of melt saturation with zircondepending on temperature and melt composition and theapproximate temperatures at the initial stages of granitoid meltcrystallization. The calculations yielded crystallization tem-peratures of 740–800 ºC (average 780 ºC) for the two-mica

Fig. 5. Cathodoluminescence images of zircon crystals from two-mica granite ofthe Biryusa massif.

Fig. 6. 207Pb*/206Pb*–238U/206Pb* diagram (Tera and Wasserburg, 1972) forzircons from two-mica granite of the Biryusa massif. Gray squares are theisotope composition points of zircons used for calculation of the weightedaverage age; black squares are the isotope composition points ignored on thecalculation.


granites of the Biryusa massif, which are close to those ofsome A-type granites (Donskaya et al., 2005). The highcontents of Y and Yb in the granites and, correspondingly,the absence of garnet as a restite phase indicate that thepressures during the formation of parental melts must havebeen <7 kbar (Patiño Douce and Johnston, 1991).

Diversity of Paleoproterozoic granitoids in the Biryusablock of the Siberian craton and tectonic consequences.Within the Biryusa block, there are granitoids of differentcompositions that intruded nearly at the same time (Fig. 2):I-type tonalites and diorites (Turkina, 2005; Turkina et al.,2006), S-type two-mica granites (this paper), and A-typebiotite–amphibole granites (Levitskii et al., 2002; Turkina etal., 2006). All these granites form a single magmatic beltstretching along the zone of junction of the Biryusa block withthe Paleoproterozoic Urik–Iya terrane and Tunguska superter-rane of the Siberian craton (Figs. 1 and 2).

In petrogeochemical features the granites of differentgeochemical types differ from each other. The S-type two-mica granites are ferroan and magnesian, mainly alkali-calcicrocks; I-type tonalites and diorites (Turkina, 2005; Turkina etal., 2006) are magnesian calcic or calc-alkalic rocks; and

A-type biotite–amphibole granites (Levitskii et al., 2002;Turkina et al., 2006) are ferroan alkali-calcic or alkalic rocks(Fig. 8a, b). The composition points of the Biryusa blockgranites are arranged on the FeO*/MgO–(Zr + Nb + Y + Ce)diagram (Whalen et al., 1987) in accordance with the geo-chemical types, forming individual fields (Fig. 9). Thus, threetypes of the Biryusa block granitoids are recognized bygeochemical features. Here questions arise: How could suchcompositionally diverse granites form within a single structurenearly at the same time? Are there any distinctive features intheir formation conditions? Let us dwell on the A-typebiotite–amphibole granites described by Levitskii et al. (2002)and Turkina et al. (2006). Despite their high Fe and alkalicontents (Fig. 8) and the localization of their compositionpoints in the field of A-type granites (Fig. 9), they havegeochemical features nonspecific for A-type granites (Whalenet al., 1987). In particular, they are similar to I-type granitesin high Sr and low Y and Nb contents but are richer in Zr,like A-type granites (Levitskii et al., 2002; Turkina et al.,2006). Note that the anorogenic A-type granites (1747 Ma) inthe Biryusa block are more similar to typical A-type granitesand are richer in Zr, Nb, and Y and poorer in Sr than the

Fig. 7. CaO/Na2O–Al2O3/TiO2 (Sylvester, 1998) (a), Rb/Ba–Rb/Sr (Sylvester, 1998) (b), Al2O3/(MgO + FeO*)–CaO/(MgO + FeO*) (Altherr et al., 2000) (c), and(FeO* + MgO + TiO2)–SiO2 (d) diagrams for two-mica granite of the Biryusa massif.


postcollisional granites (Turkina et al., 2006). In other words,the postcollisional biotite–amphibole granites of the Biryusablock should not be regarded as typical A-granites accordingto the classifications by Bonin (2007) and Whalen et al.(1987).

We should emphasize a few else facts that are crucial forreconstructing the mechanism of formation of Paleoprotero-zoic postcollisional granitoids of the Biryusa block. Turkina(2005) noted that the tonalites of the Podporog massif formedat high pressure (>10 kbar), i.e., in thickened continental crust.Later on, Turkina et al. (2006) showed that the Paleoprotero-zoic I- and A-type postcollisional granitoids of the Biryusa

block were generated from mixed mantle–crustal sources.Calculations yielded high temperatures (740–800 ºC) at theinitial stages of crystallization of two-mica granites (Table 1,Fig. 10). High temperatures were also estimated for the I-typetonalites and diorites studied by Turkina et al. (2006)(Fig. 10). A-type granites are characterized by high meltingtemperatures. The temperature ranges of the three recognizedgroups of the Biryusa block granites overlap (Fig. 10), i.e.,these are high-temperature granites. All this brings us to theconclusion that the Biryusa block postcollisional granitoidswere generated within a high-temperature collision structure(Sylvester, 1998).

Thus, high-temperature granitoids of different compositionswere intruded into the rocks of the Biryusa block of theSiberian craton during the postcollisional extension within thethickened crust in the Paleoproterozoic. Their geochemicalfeatures, permitting assigning them to I-, S-, and A-typegranites, suggest that all these granitoids formed in collisionsetting resulting from the unification of continental blocks andterranes of different geodynamic nature into a single structure.This diversity of coeval granitoids seems to be specific forareas of junction of several blocks (terranes). The unificationof blocks leads to a diversity of substrates undergoing melting,which is one of the key petrologic factors for the formationof granites of different compositions (Donskaya et al., 2005,2013; Turkina et al., 2006). We think that the crustalthickening in the region might have been caused by theunification of the Neoarchean Biryusa continental block(Turkina et al., 2007), Paleoproterozoic Urik–Iya terrane(Gladkochub et al., 2009), and Archean Tunguska superterrane(Gladkochub et al., 2009; Turkina, 2010; Turkina et al., 2007,2013; Urmantseva et al., 2012) into the single Siberian craton(Gladkochub et al., 2006; Rosen, 2003; Rosen et al., 1994).The high temperature of generation for all granites was, mostlikely, due to the inflow of mantle material to the crust

Fig. 9. FeO*/MgO–(Zr + Nb + Y + Ce) diagram (Whalen et al., 1987) for Pa-leoproterozoic postcollisional granitoids of the Biryusa block. A-type, field ofA-type granites; FG, field of fractionated granites; OGT, field of S- and I-typenonfractionated granites.

Fig. 8. FeO*/(FeO* + MgO)–SiO2 (a) and (Na2O+K2O–CaO)–SiO2 (b) diagrams (Frost et al., 2001) for Paleoproterozoic postcollisional granitoids of the Biryusablock. Circles, S-type two-mica granites (this work); triangles, I-type tonalites and diorites, chemical compositions are borrowed from Turkina (2005) and Turkina etal. (2006); squares, A-type biotite–amphibole granites, chemical compositions are borrowed from Levitskii et al. (2002) and Turkina et al. (2006).


basement (this is confirmed by the presence of granitoids withmixed mantle–crust characteristics). Formation of chemicallydifferent coeval granitoids evidences that basaltic melts werethe source of heat (e.g., for S-type two-mica granites of theBiryusa massif) and interacted with the crustal material duringthe formation of I- and A-type granitoids. That is, thecontribution of the mantle material to the granite formationwas one else factor responsible for the diversity of the Biryusablock granitoids. Moreover, we admit that the formation ofcompositionally different granites might have been related toshearing caused by the oblique collision of continental blocks,because it is shear systems that are usually formed bygranitoids of different compositions (Luchitskaya, 2012).During the collision of blocks, shears might have spread deepinto the crust, thus favoring the penetration of mantle magmasto higher levels and promoting the beginning of their melting.

The studied belt of Paleoproterozoic granitoids can beconsidered a sewing structure uniting several collided blocks.This collision within the Sharyzhalgai salient (southern salientof the Tunguska superterrane of the Siberian craton) ismanifested as large massifs of post-tectonic biotite–amphibolegranites of the Shumikha and Sayan complexes, coeval withthe Biryusa block granites (Didenko et al., 2003; Donskaya etal., 2002; Levitskii et al., 2002). Compared with the Biryusablock, the large massifs of post-tectonic granitoids in theSharyzhalgai salient are mainly biotite–amphibole graniteswith geochemical features of A-type granites. The Sharyzhal-gai salient granites are richer in indicator elements Zr, Nb,and Y than the A-type granites of the Biryusa block (Donskayaet al., 2002; Levitskii et al., 2002; Turkina et al., 2006). Thismight be explained by the fact that the Archean Tunguskasuperterrane (and, correspondingly, Sharyzhalgai salient withlarge fragments of Paleo-Archean crust) was an ancientcontinental block as compared with the Neoarchean Biryusablock and Paleoproterozoic Urik–Iya terrane. Collision of thelatter two young structures with the large thick Tunguskasuperterrane led to the formation of typical high-temperatureA-type granites in the latter.

Conclusions

Based on the results obtained, we have drawn the followingconclusions.

U–Pb zircon dating of the Biryusa massif granites yieldedan age of 1874 ± 14 Ma. This date agrees with the ageestimated for the biotite–amphibole granites, tonalites, andquartz diorites of the Biryusa block of the Siberian craton(Levitskii et al., 2002; Turkina et al., 2003, 2006).

Two-mica granites of the Biryusa massif correspond inchemical composition to normally alkaline and moderatelyalkaline high-alumina leucogranites. By mineral and petrogeo-chemical compositions, they are assigned to S-type granites.

The geochemical characteristics of two-mica granites of theBiryusa massif show their formation through the melting of ametapelitic source (possibly, the Archean metasedimentaryrocks of the Biryusa block, similar to the granites in εNd(t)

value) in the absence of an additional fluid phase. The graniteformation proceeded at 740–800 ºC (zircon saturation tem-perature).

The S-type two-mica granites together with the coevalI-type tonalites and diorites (Turkina, 2005; Turkina et al.,2006) and A-type biotite–amphibole granites (Levitskii et al.,2002; Turkina et al., 2006) form a magmatic belt stretchingalong the zone of junction of the Biryusa block with theUrik–Iya terrane and Tunguska superterrane. The granitoidsare high-temperature rocks, which evidences that they formedwithin a high-temperature collision structure (Sylvester, 1998).The intrusion of granitoids took place within the thickenedcrust in collision setting at the stage of postcollisionalextension in the Paleoproterozoic. This geodynamic settingwas the result of the unification of the Neoarchean Biryusacontinental block, Paleoproterozoic Urik–Iya terrane, andArchean Tunguska superterrane into the Siberian craton. Thecoexistence of nearly coeval granitoids of different geochemi-cal types within a local crust site is a distinctive feature ofgranite formation in thickened continental crust.

We thank T.B. Bayanova, Geological Institute of the KolaScientific Center RAS, Apatity, for isotope study of Nd. Thiswork was supported by grant 12-05-00749 from the RussianFoundation for Basic Research and by Basic Research Project79 from the Siberian Branch of the Russian Academy ofSciences.

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Editorial responsibility: A.E. Izokh


early proterozoic postcollisional granitoids of the biryusa block of the siberian craton

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