Alteration of primary Cr-spinel mineral composition from the Suru Valley ophiolitic peridotites, Ladakh Himalaya: Their low-temperature metamorphic implications

Abstract

The present study focuses on the chemical modification of a Cr-spinel from the Suru Valley ophiolitic peridotites exposed near Trespone Village of Kargil district, Ladakh Himalaya. The Suru Valley peridotite is partially serpentinised with the preservation of the spinel and relics of olivine and pyroxene. These peridotites contain characteristic red-brown spinel grains of corroded grain boundaries. While analysing these grains for mineral chemistry, compositional variation was observed with Cr-rich cores rimmed by Cr-poor compositions. Secondary spinel compositions, i.e., ferritchromite and magnetite were observed along the margins and cracks of primary Cr-spinel grains. The primary Cr-spinel cores are identified as Cr-rich and are characterised by higher values of Cr3+# (0.5–0.6) and lower values of Al3+# (0.42–0.54). From primary spinel cores to altered rims it was observed that Cr3+# and Fe3+# increase while Mg2+# decreases due to Mg2+–Fe2+ and Al3+–Fe3+ exchange with surrounding silicates of host peridotite during alteration. On the basis of present spinel mineral chemistry, metamorphic alteration conditions were transitional between greenschist and lower amphibolite similar to most of the Neo-Tethyan ophiolite peridotites.