Pressure, temperature and fluid composition in the Kalaybar aureole, NW Iran; Mineral equilibria in metapelitic and calc-silicate rocks

Intrusion of the Kalaybar nepheline syenite into the Cretaceous pelitic and calcareous sediments has produced a thermal aureole up to 1km thick. Six mineralogical zones are developed in the pelitic rocks with minerals such as cordierite, K feldspar, sillimanite and corundum. Chemically suitable rocks are partially melted in response to the heat from the pluton. Five mineralogical zones are developed in the calc-silicate rocks with minerals such as tremolite, diopside, garnet, scapolite, wollastonie and vesuvianite. A petrogenetic grid was used for pelitic rocks to put constraints on pressure of metamorphism. Intersection of the reaction curves on the grid indicates a pressure of ca. 3 kbar for the contact metamorphism in the Kalaybar aureole. Calculation of the positions of the reaction curves for the calc-silicate rocks assuming unit activity for all phases, and plotting them on T-XCO2 diagrams shows that the XCO2 is about 0.2 and it is almost independent of pressure. Calculation of the position of the reaction curves assuming dilution of the phases due to solid-solution gives the similar results indicating that the actual composition of the fluid had a XCO2 about 0.2 during peak metamorphic condition. The temperature was 700-750°C for sillimanite zone in pelites and 600-700°C for wollastonite-vesuvianite zone in the calc-silicates.