Petrology and geochemistry of the I-type calc-alkaline Qorveh Granitoid Complex, Sanandaj-Sirjan Zone, western Iran

摘要

This study examines the geochemistry of the metaluminous Eocene-Oligocene I-type calc-alkaline Qorveh Granitoid Complex (QGC), western Iran. The complex was emplaced in a Sanandaj-Sirjan Zone (SSZ) convergent setting which resulted from subduction of Neo-Tethyian oceanic crust below Central Iran. Three main units have been identified within the QGC based on field observations, mineralogical and geochemical characteristics. Mafic intrusions consist of diorites, followed by felsic units that include granodiorites and granites. The diorites are characterized by SiO_2 contents between 48 and 54 wt percent, low abundances of incompatible elements (Ba, Nb, La and Th) relative to enriched mantle, but consistent with values for average middle crust. In addition, they have Al_2O_3/(MgO + FeO~T) ratios from 0.98 to 1.54 and molar CaO/(MgO + FeO~T) ratios from 0.59 to 0.71 and were probably derived from a mafic crustal source. The geochemical features combined with the high volume of the granitoid rocks are inconsistent with an origin via differentiation of mantle-derived basaltic parent magma and assimilation. The granodioritic and the granitic rocks show moderate values of molar Al_2O_3/(MgO + FeO~T) and molar CaO/(MgO + FeO~T) suggesting an origin involving dehydration melting of a metagreywacke source. Geochemical data on REEs, Y, Rb and Sr in the latter units indicate that amphibole and plagioclase were the major fractionating phases during magma segregation.