Using Clinopyroxene Composition in the Petrogenesis of the Gabbros from Misho Mountain in SW Marand-Northwestern Iran

The Misho gabbroic massif with an east- west trend, 30 Km length and 7 Km wıdth is penetrated in the Infra-Cambrian Kahar formation. Themicro-gabbroic dykes have cut this intrusive massif. Gabbros have homogenous compositions and major phases are plagioclase and pyroxene and minor minerals are olivine, amphibole, biotite, K-feldespar and opaque minerals. Their texture is granular. The results of EMP analysis of clinopyroxenes show that they have diopside to augite compositions. In different diagrams, gabbros mostly show tholiitic composition with MORB affinities in tectonic setting. Pyroxenes generally have high SiO2 (50.02-52.69 wt %), high Mg number [Mg/ (Mg+Fe2+)] (78.59- 82.42wt %), variable Al2O3 (1.8–5.09 wt %), low TiO2 (0.53-1.33 wt %), low Na2O 0.21-0.47 wt %) contents and low Al [6]/Al [4] ratios (0-0.39), suggesting relatively low pressure crystallization conditions of the magma. The temperature calculated for the clinopyroxenes in gabbros is 1027-1306oC and pressure is less than 8 Kbars. The investigations of variation trends ofclinopyroxene compositions don’t show any zoning in clinopyroxenes and clinopyroxenes are in chemical equilibrium with other minerals in the rocks. The obtained textural and mineral chemical data, suggest that the clinopyroxenes may have crystallized from a tholiitic magma derived from a depleted lithospheric mantle. This is confirmed by the whole-rock chemistry. The gabbroic massif intrusion in a tectonic setting of extensional rifting followed by the opening of Paleotethyan oceanic crust in early Carboniferous in NW Iran.