ISOTOPE-GEOCHEMICAL EVIDENCE OF THE NATURE OF THE PROTOLITHS OF DIAMONDIFEROUS ROCKS OF THE KOKCHETAV SUBDUCTION-COLLISION ZONE
V.S. Shatsky1,2,3, A.L. Ragozin1,3, S.Yu. Skuzovatov2,3, O.A. Koz'menko1, E. Yagoutz4
1V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences, pr. Akademika Koptyuga 3, Novosibirsk, 630090, Russia
2A.P. Vinogradov Institute of Geochemistry, Siberian Branch of the Russian Academy of Sciences, ul. Favorskogo 1а, POB 9, Irkutsk, 650033, Russia
3Novosibirsk State University, ul. Pirogova 1, Novosibirsk, 630090, Russia
4Max-Planck-Institut fur Chemie, Hahn-Meitner-Weg 1, 55128 Mainz, Germany
Keywords: Sm-Nd isotope data, continental subduction, UHP metamorphism, continental collision, Kokchetav subduction-collision zone
Abstract
The isotope-geochemical features of diamondiferous metamorphic rocks of the Kokchetav subduction-collision zone (KSCZ) show that both the basement rocks and the sediments of the Kokchetav massif were their protoliths. A whole-rock Sm-Nd isochron from the diamondiferous calc-silicate and garnet-pyroxene rocks and migmatized granite-gneisses of the western block of the KSCZ yielded an age of 1116 ± 14 Ma, while an age of 1.2-1.1 Ga was obtained by U-Pb dating of zircons from the granite-gneiss basement of the Kokchetav microcontinent. Based on these data, we assume that the protoliths of the calc-silicate and garnet-pyroxene rocks and the granite-gneisses of the KSCZ were the basement rocks sharing an initially single Nd source, which was not influenced by high- to ultrahigh-pressure metamorphism (~530 Ma). Therefore, their geochemical features are probably not directly related to ultrahigh-pressure metamorphism. The corresponding rock associations lack isotope-geochemical evidence of partial melting that would occur during ultrahigh-pressure metamorphism, which suggests that they were metamorphosed under granulite facies conditions. At the same time, the high-alumina diamondiferous rocks of the Barchi area (garnet-kyanite-mica schists and granofelses), which were depleted to different degrees in light rare-earth elements (REE) and K, have yielded a Sm-Nd whole-rock isochron age of 507 ± 10 Ma indicating partial melting of these rocks during their exhumation. The close ɛNd(1100) values of the basement rocks and garnet-kyanite-mica schist with geochemical characteristics arguing against its depletion during high-pressure metamorphism indicate that the basement rocks were a crustal source for high-alumina sediments.
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