Metamorphism of Late Neoproterozoic-Early Cambrian Schists in Southwest of Zanjan from the Soltanieh Belt in Northwest of Iran

Late Neoproterozoic-Early Cambrian schists have been occurred in southwest of Zanjan city from the Soltanieh belt. The Soltanieh belt in northwest of Iran is uplifted basement of Precambrian-Paleozoic in main central Iran zone and includes outcrops of Precambrian, Paleozoic and Mesozoic Formations. Late Neoproterozoic-Early Cambrian schists, the oldest stratigraphy unit in the region, consist of phyllite, chlorite schist, mica schist and staurolite schist. Field and microstructural studies show several deformational phases in which the first phase (D1) is well characterized by S1 schistosity subparallel to the axial planes of F1 fold. D2 is a post D1-phase of deformation recognized by kink-like mesoscopic folds that fold S1 and the third phase (D3) is characterized by strongly developed crenulation cleavage (S3). Considering textural relationship, microstructural domain and mineral chemistry two main metamorphic events (M1 and M2) have been recognized. Geothermobarometry of the M2 assemblage containing white mica, chlorite, biotite, plagioclase and quartz in the KNCMFASH system considering water saturated condition, yields pressure of 6.3Kbar and temperature of 690°C and the P-T estimates for the M1 assemblage containing white mica, chlorite, biotite, plagioclase, staurolite and quartz yield pressure of 8.5Kbar and temperature of 580°C. The obtained clockwise P-T path shows an early intermediate pressure Barrovian type metamorphism (M1) following by a low pressure Buchan type metamorphism (M2). The deduced clockwise P-T path is characteristic of the metamorphic evolution of orogenic belts and probably originated through the convergence of the Gondwana and Eurasian plates during the Pan-African orogeny.