40Ar/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

mikheev@igm.nsc.ru

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.

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