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.
A. Imamalipour
Abstract
The Khoy ophiolite complex in northwest Iran hosts several podiform chromitite bodies. The chromitite deposits of Aland area in this ophiolite have lenticular, tabular and irregular vein shapes and emplaced in depleted mantle hurzburgite. Chromitites have different textures including disseminated, massive, ...
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The Khoy ophiolite complex in northwest Iran hosts several podiform chromitite bodies. The chromitite deposits of Aland area in this ophiolite have lenticular, tabular and irregular vein shapes and emplaced in depleted mantle hurzburgite. Chromitites have different textures including disseminated, massive, nodular, banded and cataclastic textures. The ore bodies are surrounded by dunitic envelopes of variable thickness. Mineral chemistry of chromite grains indicate that they are recognized by #Cr [Cr/(Cr+Al)] values range between 0.66 and 0.88, #Mg [Mg/( Mg +Fe+2)] values range between 0.42 and 0.66. Cr2O3, Al2O3 and MgO values range between 45.89 and 54.95 %, 8.53-17.18 % and 8.38-14.29% respectively. There is a negative correlation among #Cr and #Mg of chromites, typical of podiform chromitites. The composition of chromites including #Cr, #Mg, their contents of Cr2O3, Al2O3, Fe2O3, MgO and TiO2 and also correlation coefficients between different components of them have conformity with total composition of podiform chromitites. Compositions of chromites in Aland area correspond to Cr-rich chromites with #Cr>0.6 and Al2O3<25%. Their lower content of TiO2 accompanied with their compositional location in TiO2 vs. Cr/(Cr+Al) and TiO2 vs. Al2O3 diagrams may reflect the crystallization of chromites from boninitic magmas in supra-subduction geotectonic setting. The bulk rock analyses of chromitites indicate that their Cr2O3 values have widespread range, 19-51.6 wt %. Compositional variations in their Cr2O3, Al2O3, Fe2O3, SiO2 and MgO components are reflections of their textural characteristics.
