CUMULATE AND TECTONITE DUNITE FROM MAWAT OPHIOLITE, KURDISTAN REGION, NORTHEASTERN IRAQ: FIELD EVIDENCE AND MINERAL CHEMICAL CONSTRAINTS

  • Yousif O. Mohammad Department of Geology, College of Science, University of Sulaimani
Keywords: Cumulate dunite, Tectonite dunite, Black olivine, Mawat Ophiolite Complex, Kurdistan, Iraq

Abstract

The Mawat ophiolite Complex, with an exposed surface area of ~235 Km2, is one of the largest ophiolitic segments in the Zagros Imbricate Zone in Iraq of the main Zagros ophiolite belt. The ultramafic portion is studied in this paper to verify petrogenetic nature, occurrences and mineral constitutes of the dunites in the Mawat Ophiolite Complex. The dunites in Mawat ophiolite can be grouped into cumulate and tectonite based on the field observations, petrographic study and mineral chemical data of the constituents and stratigraphic succession. The lower tectonite member of the Mawat ophiolite is mainly composed of harzburgite-dunite association and the upper cumulate member is composed of dunite, cross cut by clinopyroxenite veins. The lower tectonite dunite shows typical characteristics of alpine-type II dunite which is characterized by metamorphic deformed rocks with a strong deformed fabric. These rocks have been subjected tosepentinization, spherical weathering, spinel alteration and chloritization of the dominant mineral phases. The upper cumulate dunite comprises the top summit of Mawat Ophiolite Complex. The rock is coarse crystalline massive highly fractured in the outcrop and shows granular mosaic texture in thin sections. The petrographic study shows that the cumulate dunite consists of primary mineral assemblages of olivine (black and green) + pyroxene (cpx and opx) + chromian spinel. No evidence of secondary alteration and metamorphism observed in hand samples and thin section, which may be attributed to crystallization in hydrous magmatic condition at crust-mantle boundary. As it was crystallized in equilibrium conditions with H2O rich fluid, thus would be stable during emplacement and exposure on the earth surface and unsusceptible to water to convert it to serpentine. Chromian spinels in dunites from Mawat ophiolite retain the peculiar major element signature of boninite melt and upper mantle peridotite, which are involved in the formation of cumulate and tectonite dunites in Mawat ophiolite respectively. Olivine and spinel from cumulates dunites have compositions in the range Fo 90 – 92 and Cr#0.81 – 0.82, whereas the same minerals in tectonite dunintes have compositions Fo 91 and Cr# 0.62 – 0.63. The black colored olivine in cumulate dunite may attribute to occurrences of minute inclusions of spinel + diopside, in addition to low Fo 90 – 91, MnO and NiO as compared to green olivine generation.

Published
2020-05-24
Section
Articles