Chromite chemistry as an approach to the origin of the north-east of Hadji Abad ultramafic massif, Hormozgan province

The Hadji-Abad ultramafic complex is located at the north of Hormozgan province and represents a part of Orzuieh-Dowlat Abad colouredmelange in the northern edge of Zagros thrust. The complex contains harzburgites, lherzolites, dunites and chromitites. Harzburgite is the dominant rock type. Evidence such as lobateboundaries, elongation of chromianspinels and pyroxenes, evidence for incongruent melting of orthopyroxenes and exsolution lamellae of clinopyroxenes show that the studied peridotites, experienced high temperature deformation in the upper mantle, and then emplaced in the crust. Different amounts and various shapes of chromite grains occur in all of the studied lithological units. In the harzburgites and lherzolites, disseminated brown chromites are seen as either euhedral isolated crystals or anhedral interstitial ones which crystallized between the other minerals. In the dunites, chromites appear as disseminated black euhedral and subhedral crystal grains which formed within or at the boundaries of olivine grains. In the high grade chromitites, the coarse black euhedralchromites show smooth and triple junction boundaries with cumulative textures, while in the low grade types, disseminated euhedralchromites set in a silicate matrix. Chemical analysis of disseminated chromites in the studied rocks show that maximum amounts of Cr# belong to those exist in the high grade chromitites (80-84) and the minimum are for those in the lherzolites (45-52). Tectonic discrimination diagrams reveal that chemical compositions of chrome spinels from the harzburgites and lherzolites are similar to those exist in the suprasubduction zone mantle peridotites. The host rocks as a part of ophiolites in this environment, suffered 15 to 20% partial melting. Disseminated chrome spinels from the dunite and also the chromitites have been crystallized from boninitic type melts in the same tectonic setting. These evidence show that probably, Hadji-Abad peridotites and their host ophiolitemelange belong to suprasubduction zone upper mantle and the evolutions related to this environment such as melt-peridotite reaction and partial melting have been recorded in their mineral chemistry and textures.