Abstract

Any initial crust forming process such as island arc, active continental margin formation etc. should finish with continental collision to form a recurrent supercontinent. The main petrologic mark of that collision is a granite melt generation. Three depth levels of granite process in the thickened collisional crust are demonstrated in this paper: 1- the uppermost granite intrusions into the folded collisional complex, 2- the middle level, represented by granite-gneiss layer, which crops out after erosion of the collision highland to form the upper crust of shield areas, and 3- the granite source level in the high metamorphosed lower crust, which is depleted with lithophile elements after granite melt out. On that level granites present as relics to form the migmatite leucosomes and veins.

In the recent collisional thickened crust of Himalayas (Fig.1) and Caucasus (Fig.2) the granite melt layer of approximately 10 km thickness is situated at 10-15 km depth. The Caucasus collision is a result of underthrusting northwards of the Trans-Caucasian plate below the Scythian plate. The Tyrnauz granite, intruded into the the uppermost folded complex 10 km above the granite layer, came from the 15 % partial melting of the Mesozoic island arc volcanics of the Trans-Caucasian plate probably of average andesitic composition. Separation of that granite is assumed to result to depletion of the source in the lower crust (Table.1, Fig.3).

The granite melt layer above appears to crop out in the upper crust, thickness of 10 km, of the Caledonian collision system in the western Baikal region (Figs. 4-8). It was formed under collisional thrusting of the Paleozoic island arc volcanic terranes. Granite subliquids substance together with granite veins formed cupolas under intralayer convection. The process ended in shear deformation and oblique thrusting onto the Siberian craton.

The north-eastern Siberian craton appears to be a result of the amalgamation of Archean granulite-gneiss and granite-greenstone terranes (Tables 2,3, Fig.13) joined together through collisional shear zones (Figs.9-12). These zones demonstrate two local metamorphic and granite generation events at 1.9 and 1.8 Ga. The both are accompanied with the areal granulite grade metamorphic events of the same ages in the terranes occurring nearby. The terranes ant collisional shear zones in the Siberian craton demonstrate a surface of a level of the lower crust. That level was cropped out at 1.65 Ga after erosion of a collisional mountain building and the upper crust beneath. This upper crust eroded out is assumed to be composed probably of granites that melted out from the collisional shear zones above.

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O.Rosen, V.Fedorovsky