High-potassium rocks of Sayan complex of South-Eastern part of Main Sayan Fault zone (interfluvial of Irkut and Kitoy rivers)

Bazarova E.P.

Institute of the Earth Crust sb ras, Irkutsk, Russia

bazarova@crust.irk.ru

Early Proterozoic intrusive complex of Sayan is determined among the magmatic formation of Pre-Sayan border ledge of Siberian platform foundation and belongs to postcollision South-Siberian magmatic zone [3]. This comples includes very large spectrum of rocks - from diorites, granodiorites, granosyenite to biotite-muscovite granites. The massifs of Sayan complex granitoids are usually related to tectonic borders of geological blocks.

The authors were carried out the geochemical and mineralogical investigations, there were determined the temperature of crystallization and the age of earlier insufficiently known formations of Sayan complex at South-Eastern part of Main Sayan Fault zone (interfluvial of Irkut and Kitoy rivers, the headstreams of B. Bil’chir and Shara-Zalga). This rocks were compared with the rocks of Shumikha massif, which was earlier referred to Sayan complex, and with rocks of Barbitaiskiy massif of Sayan complex. For the comparison were used the results of analyses presented in publications of T.V. Donskaya and joint authors [2] and V.I. Levitskiy and joint authors [4].

At the country between two rivers of Irkut and Kitoi the studied rocks are forming the massif of a cigar-shaped form with the extension of nearly 12 km and the width of to 1 km, directed from North-West to South-East, and having the intrusive contacts with the rocks of Archaean Zidoiskiy strata series and tectonic contact o along the Main Sayan fault zone with the rocks of early Paleozoic Sludyansky crystalline complex.

In composition of the massif the coarse-grained corniferous- biotite and biotite monzonites and syenites, quite often porphyrous (with the impregnations of alkaline feldspar), represents the first intrusive phase of the massif are predominate. In Main Sayan Fault zone these rocks are strongly dynamometamorphized up to ultra-mylonites forming on them. The main rockformative (ing) minerals are potassium feldspar (usually microperthitouse), the plagioclase is met in subordinate quantity. Dark-colored minerals are presented by biotite and amphibole (hornblende); main accessory and ore minerals are presented by zircon, apatite, ilmenite, magnetite. Among the secondary changes the plagioclase sericitization, the biotite chloritization, the titanite edging formation around the ilmenite are observed. These rocks are bursting as fine - medium-grained bifeldspar biotite granites, related to second phase.

According to classified geochemical diagram SiO₂ – K₂O+Na₂O [9] the rocks of the first phase, containing less than 60% SiO₂ are referred to monzonite, and the rocks with the SiO₂ content more than 60% - to syenite and quartz monzonite. The second phase rocks are get to the granite fields - the granodiorite. In general the SiO₂ content for the first phase rocks changes from 53,45 to 73,8 mass %, for the second phase rocks - from 68,3 to 79,1 mass %. The rocks of Shumikha and Barbitaiskiy massifs occupies the fields of quartz monzonite – the granites. In accordance with the classification [6], the primary phase rocks are mainly belong to ferriferous formations, and the second phase - to magnesia formations. At that, the rocks of Shumikha and Barbitayskyi massifs are different by increased ferruginosity in comparison with the rocks studied. On the diagram (Na₂O+K₂O-CaO) – SiO₂ [6] the points of contents are mainly set the area of alkaline series, the second phase sets the areas of calc-alkali and alkali-lime series, and the rocks of Shumikha and Barbitayskyi massifs sets between the points of first and second phases. By the ratio K₂O and SiO₂ [10], the first phase rocks mainly refers to shoshonite series, and the second phase – mainly to high-potassium calc-alkali series, as the rocks of Shumikha and Barbitayskyi massifs. On the classification diagram [8] all concerned rocks sets the area of meta-aluminous and peralumina formation (Shend^,s Index value ASI is changing from 0,85 to 1,22 for the first phase and from 1,08 to 1.43 for granitoids of second phase).

For the rocks of the first phase the increased contents of (average, g/t) F (1900), Ва (4600), Sr (1000), Zr (830), Rb (290), Y (47), V (78), Cr (63), Ni (16), Co (10), РЗЭ (990) and low Nb (8), Li (9) are typical. Spectra of REE are sharply bended to the right (La/Yb)_N=79-88, with the very poor Eu- minimum (Eu/Eu*=0.87). In second phase rocks the contents of F (<300 г/т), Ва (950 г/т), Sr (205 г/т), Zr (107 г/т), Y (11 г/т)and also Th, Zn, Co, V are sharply decreased.

For the rocks of both phases are observed the sharply bended to the right spectra REE with poorly marked Eu-minimum (Eu/Eu* от 0,79 до 0,97) for the rocks of first phase and with thestrongly marked Eu-minimum in the spectrum of second phase rocks. In general for both phases is observed the strong depletion of heavy REE (LaN/YbN from 50,68 to 88,44 for the rocks of first phase and 76,74 for the rocks of second phase). It is observed the strong fractionating of light lanthanide (La_N/Sm_N from 5,13 to 9,69 for the rocks of first phase, 13,67 – for the rocks of second phase) and temperate fractionating of heavy lanthanide (Gd_N/Yb_N from 4,2 to 5,31 for the first phase rocks and 3,74 – for the second phase rocks.) On spider-diagrams for the rocks of both phases the negative anomalies for Sr, P, Ti, Nb and U are observed, which is probably connected with fractionating of mineral phases, contains these elements: plagioclase (Sr), apatite (Р), sphene (Ti), rutile (Nb) [5]. At that, the content of rare elements in granosyenite of secondary phase is for an order lower than in the rocks of primary phase.

The biotite in the rocks investigated is of an intermediate content between annite and phlogopite (f=41-58%, Al^IV=1.04-1.3 on formula unit); for these mineral high content of Cl (to 0.65%) and temperate content of F (to1.4%) are typical for it. Biotites from the rocks of Shumikha massif are notable for its increased ferruginosity and sets the area of annite ( no data on the biotite content from Barbitaiskiy massif formation are available). On diagram, shown at publication [Мarkushev, Tatarin, 1965] for biotites from the granites of different alkaline groups, the biotites from monzonities and syenites of primary phases are mainly sets the area of normal and increased alkalinity, secondary phase sets the area of decreased alkalinity, and the biotites of Shumikha massif sets the area of normal alkalinity. It is observed the increase of aluminous with the extension of ferruginosity for the biotite from investigated rocks, in contrast to biotites of Shumikha massif, in which under the increase of ferruginosity, the aluminous is stable. The amphibole is presented by cornifer rock with the Al₂O₃ content of 8.9-10.3%. The calculations for amphibole geobarometer [7] are shown that the crystallization of melt were under Р=3.2-4.5 кbar.

The authors were carried out the calculations of the temperature of melt crystallization by the zirconium geobarometer [11], which revealed that under the greatest temperature are crystallizes the syenites of primary phases (904⁰). The crystallization of monzonites of first phase was under the temperature of 892⁰, for the rocks of second phase -734⁰, for the rocks of Shumikha and Barbiraiskiy massifs - 814⁰.

The zircons from the first phase granites sampled at the upper B. Bil’chir river were dated by local U-Pb method (SHRIMP-II). The grains of zircons are mainly idiomorphic, sized of less 0,25 mm, pink, transparent, dipyramidal shape. U-Pb dating were done with use of ionic microprobe SHRIMP-II at Centre of Isotopic Researches VSEGEI by Presnyakov S.L. It was done 12 analyses in 12 zircon crystals in total, which are resulted as 1844±8,2 Myr. This value is agreed with earlier data received by A.N. Didenko and the others [1] for the age of the granites of Sayan complex, sampled at the middle stream of Tosuisk river, within the limits of the measurement error close to the age of the granites of Barbitaiskiy massif of Sayan complex (1858±20 Myr [4]) and this value is younger then the age received for the granites of shumikhinskiy comlex of Pre Sayan lenge border (1861±1 Myr [2]). The comparison of the analyses of Sayan complex rocks at upper B. Bil’chir river and Shara-Zhalga with granitoides of Shumikha and Barbitaiskiy massif revealed that the granitoids investigated are close to the rocks of Barbitaiskiy massif by petro- and geochemical characteristics while the granitoids of Shumikha massif are differ for its increase contents of Y, Nb и Th, decreased index ASI and concentrations of Na₂O and K₂O. Moreover, the biotite content of the studied rocks of primary phase is different for increased alkalinity, and the calculations of zirconium geobarometer demonstrates that monzonites and syenites of primary phase were crystallized under the greatest temperature.

The work is supported by RFBR grant 10-05-00289-а

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