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Episodes 2020; 43(4): 1017-1039

Published online December 1, 2020

https://doi.org/10.18814/epiiugs/2020/020065

Copyright © International Union of Geological Sciences.

Genesis and geotectonic setting of podiform chromitites from the Zhob Valley Ophiolite, Pakistan: inferences from chromite composition

by Muhammad Ayoub Khan1*, Thomas Ulrich2, Muhammad Ishaq Kakar1, Recep Melih Akmaz3, Rehanul Haq Siddiqui4, and Liaqat Ali5

1 Centre of Excellence in Mineralogy, University of Balochistan, Quetta, Pakistan
2 Department of Geoscience, Aarhus University, Høegh-Guldbergs Gade 2, DK-8000 Aarhus C, Denmark
3 Department of Geological Engineering, Bülent Ecevit University, 67100 Zonguldak, Turkey
4 Balochistan University of Information Technology, Engineering and Management Sciences, Quetta, Pakistan
5 National Centre of Excellence in Geology, University of Peshawar, Peshawar, Pakistan

Correspondence to:*E-mail: ayoub.khan57@yahoo.com

Received: January 16, 2020; Revised: May 9, 2020; Accepted: May 9, 2020

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/4.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

Zhob valley ophiolite comprises Khanozai, Muslim Bagh, and Zhob chromitite occurrences located in Pakistan. These occurrences occur in massive, disseminated and nodular which exhibit both magmatic and deformational texture. The Muslim Bagh and Khanozai chromitites are classified into high-Cr chromitite (Cr#=0.66–0.85) while Zhob chromitite falls into high-Al chromitite (Cr#=0.53–0.58) composition. The calculated values of Al2O3 (wt%) and TiO2 (wt%) for parental melts of high-Cr chromitites show similarities with boninitic melts, whereas those of the high-Al ones exhibit MORB melt affinity. Minor and trace element contents (Ti, V, Ni, Zn, and Ga (ppm)) are higher in high-Al chromitites than high-Cr chromitites and contrasting geochemical behavior of these elements with Cr# might be due to magma fractionation. MORB normalized major and trace element patterns of high-Cr chromitite show similar patterns with boninite while high-Al chromitite differs from boninite. Field and textural investigations, major and trace element geochemistry, and the presence of hydrous silicate inclusions, indicate that high-Cr chromitite were generated in equilibrium with boninitic melt, possibly in supra subduction zone. In contrast, high-Al chromitite crystallized from MORB-like melt during reaction with depleted mantle and probably originated in a back arc basin tectonic environment.