Fluid regime of initial granite-forming processes in metamorphic complexes of various pressure: A case study of the western Baikal metamorphic belt.

Syncollisional granite generated in zonal metamorphic complexes at 500-700°C and various pressure and in the presence of fluid of various composition are proved to differ in proportions of granite material generated by metamorphic, metasomatic, and magmatic processes. Monometamorphic low-pressure complexes devoid of the influx of foreign solutions are characterized by the most intense metamorphic differentiation: the dissolution and redeposition of quartz and plagioclase into lower pressure zones and the development of plagiomigmatites. The process starts with deformations of the schists at 500-550°C. The main factors (at a constant total composition of the rock) is an uneven pressure distribution and metamorphic solutions. This stage is followed by metasomatic granitization, which is initiated if shear motions and decompression are associated with influx of silicic alkaline solutions into the rocks and their transformation into migmatite (up to granite-gneiss). Granitization during this stage is controlled by an increase in the permeability of the rocks and the introduction of perfectly mobile Si, Na, and K and removal of Mg, Ca, and Fe, which modifies the composition of the rocks and makes them similar to granite. Metasomatic granitization may take place in low-pressure polymetamorphic complexes. Under a high lithostatic pressure and in the presence of water-carbon dioxide fluids (kyanite-sillimanite facies series of metamorphism), the magmatic process begins at higher temperatures. The early stages are dominated by metamorphic differentiation and metasomatic sodic migmatization. The rocks modified by these processes (their salic and mafic minerals are segregated) are affected by anatexis at 650-700°C (partial melting of the granite eutectic) and, at higher temperatures, by diatexis, which generates granite melts able to migrate. These processes are variably pronounced in all metamorphic complexes. [ABSTRACT FROM AUTHOR]