Determination of P-T Conditions of Metamorphism of Mahneshan Complex NW Iran

Mahneshan Complex in the northwest of Iran was affected by regional and contact metamorphism. Microstructural and petrographical features as well as field relations show that Mahneshan Metamorphic Complex has been affected by four episodes of metamorphism (M1 to M4) and at least two deformational phases (D1 and D2). The M2 metamorphic stage is characterized by a strong preferential orientation of minerals (S2) and development of a peak metamorphic assemblage. This metamorphism is temporally associated with D2 deformational phase. The M3 metamorphism was contact metamorphism and M4 metamorphism is retrograde. The mineral assemblages of peak metamorphism M2 are muscovite, biotite, garnet, staurolite, andalusite and sillimanite. Pressure and temperature of metamorphism in the Mahneshan Complex were estimated by multiple equilibria calculations, cation exchange reaction thermometry and net transfer reaction in order to determine the geothermal gradients and type of metamorphism. The temperature of M1 metamorphism is estimated 420-450ºC and pressure of 3-4 kbar. M2 (peak metamorphism) temperature is 600-620ºC and pressure of 5-7 kbar. The temperature of M3 metamorphism is 520-560 ºC and pressure of 2.-3.5 kbar. The Geothermal gradients for the peak of metamorphism show high value for the upper crust (33° C/ km) indicating a Barrovian type of metamorphism for the study area. Tectonic setting of metamorphism is related to continental crust and magmatic arc.