Supra-subduction origin of the Nidar ophiolitic sequence, Indus Suture Zone, Ladakh, India: Evidence from mineral chemistry of upper mantle rocks

The ultramafic rocks of Nidar ophiolitic sequence in the eastern part of th e Ladakh (NW Himalaya) have been studied combining petrography and mineral chemistry data. The ultramafic rocks mainly consist oi spinel-harzburgite a nd spinel-dunite, that are intruded by spinel-bearing pyroxenites. The moda l and chemical composition of minerals in hazburgite indicates a strongly-d epleted nature consistent with a residual origin after the extraction of ba saltic melt by single or multiple partial melting events. In contrast, duni te was probably formed as a consequence of interaction between harzburgite and migrating melt(s), after the partial melting events. This is well corro borated by the petrographical and chemical evidence, e.g., the growth of se condary mineral aggregates and the large chromite component in the spinels. The mineral chemistry suggests that the melts percolating and interacting w ith harzburgite were rich in MgO and NiO and deficient in CaO, Al2O3, and T iO2. In particular, the low Ti content shown by clinopyroxenes and spinals which are cumulus phases in pyroxenites and secondary neoblastic phases in peridotites), in association with high Cr# (> 60) exhibited by ail the spin els, are consistent with parent melt of boninitic affinity. These melts are typical of island are environment. Therefore, this infers that the ultrama fic units of Nidar ophiolite underwent melt percolation in a supra-subducti on tectonic environment related to Fore-are setting.