Clinopyroxene Compositions of the Basalts from the Ashin Ophiolite, Central Iran: New Evidence on Its Supra-Subduction Zone Setting

The Ashin ophiolitic melange is located along the curve of the Doruneh fault system in the western corner of the Central Iranian microplate. The rock units of the ophiolitic massive due to tectonic events are located in the serpentinite matrix. The crustal rocks of oceainic crust include tectonic slices of sheeted dykes, pillow lavas, and massive basalts covered by radiolarian chert, rodingite, and globotroncana limestone. Microprobe analysis results indicate that mineral association of pillow lavas are clinopyroxene (augite), hornblende, plagioclase, calcite, ilmenite, and magnetite; massive basalt consists of clinopyroxene (augite), hornblende, plagioclase (albite), ilmenite and magnetite; dyke is composed of clinopyroxene (diopside and augite), plagioclase (albite), chlorite and prehnite. Microscope studies reveal porphyritic and intersertal as the most common textures of this rocks. Barometry indicates these rocks formed in medium to low pressure (2 to 5 Kbar) during their ascending, also, thermometry calculations indicate temperatures of 1100 degrees C to 1200 degrees C for clinopyroxene crystallization. Clinopyroxene compositions are similar to those of both boninites and island-arc tholeiites. The occurrence of high-Mg and low-Ti magma constrains the formation of these rocks in a supra-subduction environment. The water content in the crystallization of pyroxenes is more than 10% and those are formed in high oxygen fugacity. The clinopyroxenes of lava and associated dyke of the Ashin ophiolitic melange have similar chemical compositions to those of the other supra-subduction zone type Eastern Mediterranean ophiolites that show island arc affinity.