High-Cr chromitites of the Nidar Ophiolite Complex, northern India: Petrogenesis and tectonic implications

Abstract The Nidar Ophiolite Complex (NOC) in the Ladakh Himalaya region of northern India is well-preserved and exposed along the Indus Yarlung–Zangbo suture zone. This sequence has been thrusted onto the Indus Formation to the north and to the Zildat ophiolitic melange in the south. Chromitite mineralization in the NOC occurs as massive, semi-massive, and disseminated ore bodies hosted by dunite. The chromite in these ore bodies contains inclusions of isolated silicates, base metal sulfides, and platinum-group minerals. Little variation in the [Cr/(Cr+Al)] (i.e., Cr#) (0.78–0.86) is observed, in contrast to Mg# ([Mg/(Mg+Fe2+)]), which is more variable (0.56–0.73). The NOC chromitites are high in Cr (Cr# > 0.78), suggesting a possible boninitic parent magma. The calculated (Al2O3)melt and (FeO/MgO)melt range from 9.29–11.50 and 0.49–0.10, respectively. The trace element compositions of these chromitites (Ga: 14–27 ppm; Ni: 440–1301 ppm; V: 612–802 ppm; Sc: 2–6 ppm; Co: 207–542 ppm) are comparable to those hosted in the mantle sections of other ophiolite complexes. The overall chemical composition, deduced from the Al2O3 content and FeO/MgO ratio of the primitive parent magma of high-Cr chromitites, is consistent with island arc tholeiitic melts of boninitic affinity. This parent magma would have been in equilibrium with podiform chromitites and associated ultramafic rocks. Further, the distributions of minor and trace elements in NOC chromites also indicate a boninitic magma source. The field relations, petrographic observations, and major and trace element compositions of the studied chromitites suggest that they have undergone melt–rock and melt–melt interactions, which led to the precipitation of high-Cr chromitites from Cr-rich and Al- and Ti-poor boninitic melts. The occurrence of high-Cr chromitites and the boninitic nature of the parent magma indicate that the chromitite layers of the NOC likely formed in an island arc supra-subduction zone.