Petrology of granites from Gasht metamorphic complex (W Rasht)

Gash metamorphic complex is cropped out in the Talesh Mountain (Western Alborz). According to the field and microscopic evidences, there are two generations of granite in the Gash metamorphic complex. The first generation of granite (G1) is medium grained and light gray and displays mylonitic fabric. The second generation (G2), contains coarse leucogranites that has been injected into the first generation of granite and metapelitic rocks. Quartz, plagioclase, K-feldspar and biotite are the rock forming minerals of the first generation granite. The main minerals of the second generation granite are quartz, K-feldspar and plagioclase. Tourmaline, garnet and sillimanite are accessory minerals. Geochemical and mineralogical evidences indicating that the precursor of first and second generation of granites are I- and S-type, respectively. There is considerable compositional similarity particularly between the second generation granite and leucosome of the adjoining migmatites and Himalayan leucogranites that are indicator of collision setting. The prolonged geological history of Paleo-Tethys closure episode was resulted in successive generations of granitization in the continental active margin from Carboniferous to Upper Triassic. Probably, during subduction and prior to continental collision stage, the first generation of granites have been formed in the edge of Turan block. The final stage of Paleo-Tethys vanishing has taken place as a consequence of collision between Alborz block and southern margin of Eurasia during Eo-Cimmerian orogeny and terminated with the formation of metapelitic of Gash complex in the middle to lower depth of the continental collision setting and production of the second generation of granitization.