Tourmaline genesis in the metamorphic and magmatic rocks of Gasht-e- Rudkhan district (Gasht metamorphic Complex, West of Rasht)

Gasht metamorphic complex crops out in the Talesh Mountains (Western Alborz). This complex mainly consists of metapelitic and granitic rocks. Tourmaline is prevalent as an accessory phase within these rocks. In schist samples, dravitic tourmaline is abundant as nucleation component and fine size, and displays compositional zoning. Dravitic tourmaline of gneissic samples are coarser and homogenous. Tourmaline of the migmatites are restricted to both leucosome and melanosome parts. Brown tourmalines of garnet-bearing melanosome are magnesiofoitite in composition and similar to the other mafic phases of melanosome, indicate a residuum nature. However, dravitic tourmalines of the garnet – free melanosome are coarse crystalline and have grown over biotite with green pleochroism. The composition of the leucogranite tourmalines are schorlitic and they become dravitic toward rims. Different types of replacement are observed in the tourmaline composition of Gasht complex. According to the compositional signatures, metapelites tourmaline were formed mainly under metamorphic conditions, while the one of the leucogranites have been formed in the magmatic conditions. Probably B-bearing fluids of Gasht complex in the high grade metamorphism caused lowering of the temperature of water-bearing solidus of granite. This resulted in facilitating of migmatitization process. Furthermore, boron element caused decreasing of leucosome viscosity, mobilization and easier fractionation of melt from melanosome and creation of adjacent leucogranites.