Kharazmi Journal of Earth Sciences

The north of Shahpasand Mountain Pluton, with the age of late Eocene – Oligocene, is located in the northwest of Saveh. Plutonic rocks of Shahpasand Mountain show a various composition of gabbrodiorite, diorite, monzonite, quartzmonzonite, granodiorite and granite. These rocks have granular, intergranular, porphyroid and poikilitic textures.The main minerals are plagioclase, pyroxene, amphibole, biotite, K- feldspar and quartz. The accessory minerals are zircon, apatite, sphene and opaque. From geochemistry, tectonic setting and petrogenesis point of view, the north of Shahpasand Mountain Pluton is compared with Silijerd pluton that is located in the northwest of Saveh. Spider diagrams show that a slight enrichment of LREE to HREE and also higher LILE relation to HFSE. Negative anomalies of Nb, Ti, P show the plutons belong to subduction zone. Geochemical data, suggest that the parent magmas of the studied rocks might be generated by crystal fractionation of arc basaltic magma in magma chambers, coupled with some crustal assimilation. The EPMA analyses on the plagioclase, biotite and pyroxene minerals in granodiorite and gabbrodiorite of the north Shahpasand Mountain Pluton show that plagioclase compositions in granodiorite are in the range of oligoclase to andesine and in the gabbrodiorite samples are from andesine to bytownite. The EPMA analyses of the pyroxene minerals show the existence of two types of pyroxene in the gabbrodiorite samples (orthopyroxene with hyperesthene composition and clinopyroxene with augite – diopside compositions), while in granodiorite clinopyroxene is augite – diopside composition. On the basis of geothermobarometry, crystallization temperature of pyroxenes are estimated to be about 800° C to 970° C and crystallization pressure 7 to 8 kbar for the pyroxenes.