Metamorphic Evolution of the Garnet Amphibolite Schist from Mawat Ophiolite, Kurdistan Region, Northeast Iraq: Geochemistry, Mineral Chemistry and Thermodynamic Approach

Abstract

The garnet amphibolite schist is associated with the gabbroic unit of the Mawat ophiolite, Kurdistan region, Northeast Iraq, in the imbricated zone of the Zagros orogenic belt. Field observation, petrographic study, mineral chemistry, geochemical data, and thermodynamic modeling were used to demonstrate mineral identification, predict the observed stable mineral assemblages, and validate the protolith. The mineral assemblages of prograde are garnet (pyrope-almandine), chlorite (clinochlore), amphibole (grunerite), plagioclase (anorthite), iron oxides while the garnet (almandine-pyrope), chlorite (chamosite), amphibole (cummingtonite), and iron oxide represent retrograde assemblage. The relict pod of metagabbro in the center of the garnet amphibolite schist body consists mainly of plagioclase, amphibole, pyroxene, and iron oxide. Geochemical data reveals that the rock is depleted in SiO2 and CaO and enriched in refractory elements compared to the metagabbro. This may be attributed to the generation of silicic melts by partial melting and producing nearby plagiogranite. The rare earth elements (REE) patterns show that the garnet amphibolite schist is depleted in light rare earth elements (LREE) and gradually enriched in heavy rare earth elements (HREE). Thermodynamic modeling, field observation, combined with petrographic study constrains two metamorphic stages and one possible anticlockwise P-T-t path for garnet amphibolite schist, with increasing pressure and temperature of gabbroic rocks units of Mawat ophiolite. The rocks reached peak metamorphic conditions at more than 800 °C and 8–10 kbar, which partially melted the rock produced prograde resitite and melt. Later, the resitite rock retrograded to the lower stage at temperatures ranging from 620 to 650 °C and pressures ranging from 6.5 to 6.8 kbar. Anticlockwise P-T-t paths suggested based on geochemical and thermodynamic data for garnet amphibolite schist of the Mawat ophiolite. Moreover, the gabbro is considered to be the protolith of garnet amphibolite schist, which have been undergone eclogite facies metamorphism later overprinted by amphibolite facies.