Mineralogy and geochemistry of peridotites and chromitites in the Jijal Complex ophiolite along the Main Mantle Thrust (MMT or Indus Suture Zone) North Pakistan

Abstract

The Jijal Complex, one of the largest Neo-Tethyan ophiolites (ca. 150 km2) in Pakistan, occupies a deep-level section of the Cretaceous Kohistan Arc that was obducted along the Main Mantle Thrust or Indus Suture Zone. Peridotites of the Jijal Complex consist mainly of harzburgite and dunite with numerous podiform chromitites. In this paper, we present new mineral compositions, whole-rock major elements, trace elements, rare earth elements (REEs) and platinum group elements (PGEs) to evaluate the melt evolution and petrogenesis of these rocks. Chromian spinel in the chromitites is fairly uniform in composition with Cr#s ranging from 75 to 81, Mg#s from 59 to 63 and TiO2 < 0.2 wt%, which suggest crystallization from boninitic magmas. Chromian spinel in the peridotites has relatively lower Cr#s of 71–74, Mg#s of 38–48 and very low TiO2 contents (averaging 0.09 wt%). The Fe3+#s of chromian spinels both in the chromitites and peridotites are very low (maximum 0.02), reflecting crystallization under low oxygen fugacity (fO2). The harzburgites and dunites have low average CaO, Al2O3 and REE values and contain Al-poor clinopyroxene and high-Cr spinels. Mantle-normalized trace element patterns of the harzburgite, dunites and chromitites reflect high-degrees of partial melting (25%–30%). The high-Cr chromitites all show similar chondrite-normalized platinum group element patterns characterized by enrichment in Os, Ir, Ru and Rh relative to Pt and Pd. The mineral chemistry, whole-rock compositions and PGE geochemistry indicate that the chromitites in the Jijal Complex were generated from a boninitic parental magma generated under low to high fO2 in a supra-subduction zone environment. The calculated compositions of the parental melts of the Jijal chromitites are consistent with differentiation of arc-related magmas.