Non-silicate needles and metals in peridotites from Himalayan ophiolite, Western Ladakh, India: evidence of deep Earth origin

Abstract

Peridotites in ophiolites exposed along Himalayan suture zone open window to study deep Earth processes. We report c.2 km thick well exposed mantle section of Shergol ophiolite in the Indus Suture Zone (ISZ). The mantle section comprises lherzolite, serpentinised peridotite with cumulate chromitites. The in situ evidences from micron-sized mineral exsolutions are studied using optical microscopy, scanning electron microscopy and micro-Raman spectroscopy. From the ISZ lherzolite, ilmenite mineral exsolution is noted in the olivine grain. The exsolved ilmenites are oriented topotactically following former {111} planes of magnetite, providing evidence of spinel precursor. In another observation, the disoriented 1–3 µm wide and 10–80 µm long ilmenite needles are hosted in the olivine, is characterised by Raman peaks at 685, 370 cm⁻¹ with low peaks at 308 cm⁻¹,523 cm⁻¹, 683 cm⁻¹, 693 cm⁻¹ and 731 cm⁻¹ point to mixed mineral phases of magnetite with Cr–Fe–Al. Based on morphologies, crystal-chemical structure and modal calculation of exsolutions, we infer, exsolved Fe–Ti phases in Shergol peridotite is sourced from deeper part of the upper mantle. With the Fe–Ti phases, tiny silicate inclusions in chrome spinel and accompanying opaque base metal sulphides are also observed in the same peridotite, attributes successive stages of partial melting and subsequent cooling of metal enriched mantle. These observations, challenge shallow intra-oceanic arc setting for Shergol ophiolite and proposes, part of exsolved mineral phases in ophiolite has deep Earth origin. These mineral phases would ascent at the shallow mantle level beneath Neotethyan spreading ridge aided by dunite channel.