Ar/39Ar isotopic age of Svyatoy Nos Peninsula (Transbaikalia) granulites and problem of its geodynamic interpretation

Mikheev E.I.^1,2, Yudin D.S.², Malyshev A.V.³

¹Novosibirsk State University, Russia; ² V.S. Sobolev Institute of Geology and Mineralogy SB RAS, Novosibirsk, Russia; ³ Geological Institute SB RAS, Ulan-Ude, Russia

[e-mail]

Metamorphic rocks of Svyatoy Nos Peninsula (Transbaikalia) are basic gneisses, graphitic marbles, diopside plagioschists and quartzites. The metamorphic rocks are intruded by granitoids of Barguzinskiy complex of the Angara-Vitim batholith (280-298 Ma) [1,2]. A specific feature is occurrence of marble melange zones with inclusions of metabasic rocks. The basic gneisses are intruded by abundant veins of garnetbearing potash granites and granitic pegmatites.

The most common rocks in the granulitic complex in Svyatoy Nos Peninsula are Grt-Bt-Amph, Bt-Grt-Scp-CPx-Amph and Grt-Amph-Bt-Cpx gneisses. It is supposed that protolith of the gneisses was island-arc volcanic rocks.

For ⁴⁰Ar/³⁹Ar studies were selected biotite and amphibole from clinopyroxene-amphibole gneisses (sample CH-1/5-09, CH-1/2a-09). In the amphibole ⁴⁰Ar/³⁹Ar spectrum of the sample CH-1/5-09 allocated two sites - one with the age of 263.7 ± 2.4 Ma, corresponding to 50% of the selected ³⁹Ar, and second - 251.9 ± 2.5 Ma, corresponding to 32% of the selected ³⁹Ar. At the age spectrum of biotite from the sample is allocated CH-1/5-09 plateau age 182.7 ± 1.6 Ma, corresponding to 98% of the selected ³⁹Ar. In the amphibole ⁴⁰Ar/³⁹Ar spectrum of the sample CH-1/2a-09 is allocated plateau age of 250.2 ± 2.2 Ma, corresponding to 95% of the selected ³⁹Ar. In the spectrum of biotite from the same sample is observed with a plateau age of 168.8 ± 1.6 Ma, which corresponds to 65% of the selected ³⁹Ar.

Detailed petrological and geochemical studies of granulites in the Chernorudskaya zone (Western Baikal region) and Svyatoy Nos Peninsula indicate the proximity of the geological structure, chemical composition and metamorphic PT-parameters [3]. The first results of the Ar-Ar isotopic dating of Bt and Amph of Svyatoy Nos Peninsula granulites showed a surprising result (Fig). Initially it was assumed that one can expect reinstalling K-Ar isotopic system in Amph and Bt, subjected to thermal exposure of the Angara-Vitim batholith, but they were significantly younger (270-170 Ma, see Figure). In comparison with the results of Ar-Ar isotopic dating of granulites Olkhon region [4-6], which are placed in regular thermochronological trends, Svyatoy Nos Peninsula granulites have younger thermal peak, whose formation is likely to be associated with Mesozoic tectonic activation of the Baikal region. For the final solution of this question is necessary to conduct U-Pb isotopic dating of magmatic and metamorphic complexes of Svyatoy Nos Peninsula.

Authors thank D.A. Orsoev (GIN SB RAS, Ulan-Ude) and A.S. Mekhonoshin (IGC SB RAS, Irkutsk) for help in organizing field works and discussion of scientific issues.

The sponsorship is integration project №10.3 of the Earth sciences department.

References:

1. Makrygina V.A., Petrova Z.I., Koneva A.A., Suvorova L.F. Composition, PT-parameters and metasomatic transformation of basic shales of Svyatoy Nos Peninsula (Baikal region) // Geochemistry. - 2008. - № 2. - P. 167-182.

2. Cygankov A.A., Matukov D.I., Berezhnaya N.G., Larionov A.N., Posokhov V.F.,Cyrenov B.C., Khromov A.A., Sergeev S.A. Sources of magmas and the stages in the Late Paleozoic granitoids of Western Transbaikalia // Geology and geophysics. – 2007. – V. 48. - № 1. – P. 156-180.

3. Mikheev E.I. Correlation of earlycaledonian granulitic complexes of Baikal region (for example, Olkhon region and Svyatoy Nos Peninsula) // Proceedings of 50th International Students Scientific Conference «Students and Progress in Science and Technology», april, 16–20. GEOLOGY. Novosibirsk, 2011.

4. Volkova N.I., Travin A.V., Yudin D.S., Khromykh S.V., Mekhonoshin A.S., Vladimirov A.G.The first results of ⁴⁰Ar/³⁹Ar dating of metamorphic rocks of the Olkhon region (Western Baikal region) // Doklady of RAS. - 2008. - V. 420. - № 4. - P. 512-515.

5. Travin A.V., Yudin D.S., Vladimirov A.G., Khromykh S.V., Volkova N.I., Mekhonoshin A.S., Kolotilina T.B. Thermochronology of Chernorudskaya granulitic area (Olkhon region, Western Baikal region) // Geochemistry. - 2009. - № 11. - P. 1181-1199.

6. Vladimirov A.G., Volkova N.I., Mekhonoshin A.S., Travin A.V., Vladimirov V.G., Khromykh S.V., Yudin D.S., Kolotilina T.B. Geodynamic model of the early caledonides of Olkhon region (Western Baikal region) // Doklady of RAS. – 2011. – V. 435, № 6. – P. 1-7.

Investigation of polyphase and fluid inclusions in clinopyroxene from diamond-grade calc-silicate rocks of Kokchetav Massif (Northern Kazakhstan)

Mikhno A.O.

V.S. Sobolev Institute of Geology and Mineralogy SB RAS, Novosibirsk, Russia

[e-mail]

By far the discovery of melt inclusions associated with diamond in carbonate rocks of Kokchetav massif renewed interest of scientists to the nature of melt in UHPM rocks [De Corte, 1998; Korsakov & Hermann, 2006]. These findings provided the strong evidence that presence of melts promotes diamond growth in deeply subducted crust. Previous study of fluid and polyphase inclusions was mainly focused on diamond-bearing rocks.

In this paper we present preliminary results on phase and chemical composition of polyphase inclusions from clinopyroxenes of diamond-free calc-silicate rocks from Kokchetav massif. Lens and interlayers of garnet-clinopyroxene (calc-silicate) rocks occur among biotite gneisses and schists [Shatsky et al., 1995]. Calc-silicate rocks are composed by garnet, clinopyroxene, calcite, quartz, plagioclase, K-feldspar. Titanite, zircon and pyrrotine occur as accessory minerals. Samples contain a compositional layering consisting of different amounts of coarse- to medium- grained clinopyroxene (30-80 vol. %) and garnet (20-50 vol. %) in calcite matrix. Garnets are characterized by following composition: Alm₁₀Sps₃Pyr₁Grs₇₇Andr₉. Clinopyroxenes shows high diopside content. Cores of large porphyroblasts of clinopyroxene (up to 4 mm in size) contain abundant lamellas of KAlSi3O8 and coexisting polyphase and fluid inclusions, whereas the rims are free of lamellas and inclusions. Isolated inclusions showing the shape of negative crystal are primary inclusions. Raman-imaging combining with scanning electron microscopy allow to identify following phases in the representative inclusions: KAlSi₃O₈ (kokchetavite), calcite, muscovite/phengite, H₂O-liquid.

According to obtained data we assume that originally polyphase inclusions were trapped as a melt. During clinopyroxene growth there were two immiscible liquids: fluid and melt. It confirms by findings of coexisted fluid and polyphase inclusions within the same growth zone of clinopyroxene porphyroblast. Crystallization daughter phase calcite and mica from these melt inclusions during retrograde stage liberates liquid H₂O. The differences between Raman spectra for clinopyroxene in direct proximity to the polyphase inclusion and host-clinopyroxene indicate that there are differences in chemical composition within this thin zone. Formation of this zone can be related either to interaction of melt/fluid with host-clinopyroxene or to precipitation of newly formed clinopyroxene on the wall of melt/fluid inclusions.

This study was supported by the RFBR grants (10-05-00616-а).

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