Magmatic characteristics and crystallization condition of the Gasht- Masuleh gabbros (North of Iran) based on clinopyroxene and orthopyroxene composition

The Alborz Range, as a part of the Alpine-Himalayan orogeny, formed during the Cimmerian orogeny due to northward movements of the Central Iran micro-plate towards Eurasia. Subsequently, it underwent different compressional and extensional geological events from the Late Triassic collision to the recent ongoing contraction and volcanism. The gabbros in the Gasht-Masuleh area are small kilometer-sized cumulate and isotropic gabbro bodies that intruded the Paleozoic metasediments and Mesozoic sediments in the three localities including Chapul, Gilvandehrud, and Zudel. Clinopyroxene chemistry suggests that the calc-alkaline parent magma formed in a transitional environment between arc and extension related tectonic settings. Diopside clinopyroxene composition is Wo47-51 En43-49 Fs4-9 and enstatite orthopyroxene has Wo0-3 En71-79 Fs20-28 composition. The low TiO2 content of clinopyroxene indicates a depletion of parental magma source during old subduction in the north of Iran. Using different thermometers, based on clinopyroxene and orthopyroxene composition suggest that the crystallization temperature of pyroxene was low (~800 ℃). Al2O3, Na2O and Cr2O3 contents of orthopyroxene and clinopyroxene are low indicating the crystallization of the parent magma at low pressure at upper crustal levels.