Petrology
Sima Peighambari; Mahdieh Mohammadi; Hamid Ahmadipour
Abstract
Podiform chromitite bodies of various sizes associated with dunite envelopes found at several localities in the Ab-bid ultramafic massif in the southeast of the outer Zagros ophiolite belt. The chromitites occur as layered and lenticular bodies and veins which show different magmatic textures such as ...
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Podiform chromitite bodies of various sizes associated with dunite envelopes found at several localities in the Ab-bid ultramafic massif in the southeast of the outer Zagros ophiolite belt. The chromitites occur as layered and lenticular bodies and veins which show different magmatic textures such as massive, disseminated, and banded features. The Ab-Bid chromitites display a variation in Cr# from 65.36 to 58.43. The Al2O3 and TiO2 contents of chromites range from 18.03% to 22.58 % and 0.20 % to 0.39 %, respectively. The Al2O3, TiO2 and FeO/MgO values, calculated for parental melts of Ab-Bid chromitites, are within the range of melts which are similar to tholeiitic melts. Structural features such as sharp contacts between chromitites and their dunite envelope with host harzburgites, no correlation between chromitite size and dunite thickness, and various textures may be related to magmatic-metasomatic processes through interconnected dunitic channels. The mineralogical and chemical compositions of the chromitites as well as calculated parental melt compositions of Ab-Bid chromitites are consistent with the idea that the Ab-Bid massif was part of mantle section of an ophiolite from a suprasubduction zone. Harzburgites was influenced by hydrous partial melts; variable melt/rock interaction caused the formation of dunitic channels and allowed the chromitite parental melt to percolate through them. Middle partial melting degrees and other mineral chemistry of chromites could be representative of back-arc oceanic spreading environments. Therefore, Ab-Bid harzburgites are parts of mantle wedge above the subducting Neo-Tethys oceanic plate which is under a probably Upper Triassic-Cretaceous back-arc spreading centre.
