The Study of Petrogenesis of olivine gabbronorites from Cheshmeh-Ghassaban area (NW of Hamedan) by using chemistry of the minerals

Abstract

The Cheshmeh-ghassaban area plutonic rocks are located in NW of the Hamedan city and is a part of the Alvand plutonic complex. These bodies intruded in the Hamedan phyllites in Mesozoic. They range in composition from olivine gabbronorite, hornblende gabbro to biotite gabbro together with some quartz-diorite and granodiorite. The olivine gabbrosnorites are composed of orthopyroxene, clinopyroxene, plagioclases and olivines with corona texture. Chemical analyses of the minerals show that the olivine forsterite contents range from 66 to 77 and increase toward the crystal cores. The olivines also were replaced with orthopyroxene and hornblende in their rims due to reaction with the surrounding melt and its associated fluids that formed the corona texture. The clinopyroxenes are of diopside type and orthopyroxene have pigeonite composition. Moreover, MgO and Cr2O3 contents of the clinopyroxenes indicate that the parent magma of the olivine gabbronorite was tholeiitic and formed in a subduction zone that endured rifting and asthenospheric magma upwelling. Geothermobarometry calculations based on proxene composition show 900 to 1200 oC and 7 to 10 kbar for this rock. Also, petrological and mineralogical properties of these rocks are similar to those of the ultramafic-mafic Alaskan type complexes that form above subduction zones. The results of the present study indicate the role of mantle-originated magmas in formation of the Alvand complex.

Keywords

• olivine gabbronorite
• corona
• chemistry of minerals
• cheshmeh ghassaban
• alvand