TECTONIC POSITION AND ORIGIN OF HT-LP GRANULITES IN THE WESTERN SANGILEN (SOUTH-EASTERN TUVA)

The system of brittle-plastic shear zones was defined in the Western Sangilen, which spatially coincides with outcrops of HT-LP granulites. Today there are two points of view on it's tectonic position. According to the first model (Kozakov et al., 1999), the rock was metamorphized in granulitic facies conditions at the moment of maximum tectonic deformation. During the posttectonic stage these rocks undergone diaphthoresis at amphibolitic facies of andalusite-sillimanite type. The second model (Kargopolov, 1991, 1997) is based on the supposition about isobaric metamorphism. In this case the tectonic deformations are only playing the role of the superheated fluid conductors, temperature influence of which have led to the formation of linear zones of HT-LP granulites. The last investigations suggest to conduct the isothermic model, which supposes sintectonic origin of the HT-LP granulites. This model corresponds to the "clockwise P-T evolution" with isothermic decompression. In the HT-LP granulites the metamorphic reactions Bt+Sil+Qtz->Crd+Spl+Kfs+H₂O, Bt+Sil+Qtz->Crd+Kfs+H₂O were observed. Moreover, aggregations of granoblastic cordierite grains in shear-bands and around solute minerals (coronite structure) were observed. The neogenic potassium feldspar fills unoccupied intergrain space. In conclusion we could say, that metamorphic reactions and formation of shear-bands occured synchronously. The fact that initiation and evolution of the shear zones causes abrupt decompression, and observed reaction proceed with increasing temperature or with decreasing pressure admits to suppose the formation of HT-LP granulites can be in direct connection with the origin and evolution of shear zones. The assumed mantle plum (Vladimirov et al., 1999) under Sangilen could be the sufficient heat source for the formation of high-gradient HT-LP metamorphic rocks. Associated global region heating gave the thermal gradient about 50-100 grad/km and provided the necessary prerequisites and conditions for origin and evolutions shear zones. We gratefully acknowledge the financial support of the Russian Foundation for Fundamental Research (Grants N00-05-65308, 99-05-64727).