جدایش رستایت و واکنش نرمال ماگمایی در گرانیتوئید کاشمر بعنوان یک نمونه برجسته از انواع گرانیتهای I-type در حرارت پایین

Abstract

The Kashmar granitoid is the largest plutonic mass occurring in northeastern Central Iran. It is cropped out between the two major faults, Doruneh Fault to the south and Rivash Fault to the north, defining the boundaries of the Taknar Zone. It has intruded into andesitic lavas and pyroclastic rocks of Eocene age. It comprises several plutons of tonalite, granodiorite, granite and alkali feldspar granite. The first isotopic data (Rb/Sr) on biotite-whole rock pairs show an age of ۴۳.۵±۰.۴ to ۴۲.۴±۰.۴ Ma for the Kashmar granitoid. Using microscopic and microprobe data, different minerals of the Kashmar granitoid exhibit characteristics of those minerals occurring in metaluminous (ASI<۱) I-type granites that formed under low-temperature and high fO۲. Similarity in initial ۸۷Sr/۸۶Sr (۷۰۴۷۱-۰.۷۰۵۶۹) and negative values of ۱۱۰۸;Nd (-۰.۷۰ to –۱.۸۶) with a limited range in different samples indicate a homogenous source rock, undergone at least one cycle of fractionation and originated from lower crust or perhaps infra-crustal depths. Due to similarity in geochemical and isotopic features, these plutons can be assigned as a simple suite. This simple suite is high in Al۲O۳, Na۲O, K۲O, FeO and low in CaO, MnO, P۲O۵, TiO۲ and MgO contents. The composition of microgranular enclaves is similar to the composition of the host rocks. This together with linear variations on Harker plots indicate that fractionation and restite separation (unmelted residual source materials) are two major processes for controlling the variation in the chemical composition of the Kashmar granitoid. Using tectonic discrimination diagrams, this granitoid and other plutonic rocks of the north Doruneh Fault magmatic arc, plot in the areas of magmas originated in subduction-related zones of continental margins.