Nature and evolution of the Tethyan Mantle evidenced by the Dras peridotites, Ladakh Himalayas, India.

Abstract

Prominent exposures of mantle rocks in the form of peridotites are observed extending in the E-W direction having a maximum width of 1.5km at the Dras village, Ladakh, India. The Dras peridotites are mainly dunites bearing chromite mineralization with minor harzburgites and wehrlites. The peridotites are emplaced within the Dras volcanics along with gabbros and radiolarian cherts. They display protogranular textures grading into equigranular mosaic textures typical of mantle peridotites. In dunites, olivines exhibit straight boundaries meeting at 120⁰ indicating recrystallization. Spinels associated with dunites are disseminated as tiny inclusions and at places they are lodged on olivine triple junctions. Porphyroclastic olivine in the harzburgites display kink bands whereas orthopyroxenes (enstatite) in harzburgites are subhedral with exsolution lamellae of clinopyroxene (dioside). Overall textures suggest that the peridotites have undergone progressive deep-seated deformation and solid-state recrystallization. The chromite mineralization associated with dunites displays a variety of structures viz. banded, lenticular, pull-apart, schlieren, massive, disseminated etc. Magnesite veins forming an intricate network in dunites are observed.   Geochemically, the peridotites are relatively fresh (LOI - 0.1 wt % to 5 wt %) with Mg# between 89 and 91, comparable with residual oceanic peridotites. Major element chemistry of the peridotites indicates they are abyssal peridotites, however, depleted REEs, trace elemental concentrations along with enriched LILEs, especially Cs and Nb-Ta and Zr-Hf anomalies indicate formation in a subduction setting. Olivines contain Mg-Cr-rich and Fe-poor rims compared to the cores. Spinels in dunites are chromites with Cr# 68-82 and Mg# 34-48 whereas, in harzburgites, spinels are magnesio-chromites with Cr# 44-55 and Mg# 56-62. Spinel and olivine data suggest that dunites have undergone very high degrees of partial melting about 35% possibly in the supra-subduction zone (SSZ) setting and may have interacted with boninite-like melts.  Harzburgites, on the contrary, are formed by lower degrees of partial melting ranging from 20-25%. However, when remodelled using the clinopyroxene trace element concentrations, the clinopyroxenes from harzburgites suggest 15% to 23% degrees of partial melting. Temperature estimates calculated on select mineral pairs yield temperatures of 816⁰C to 1046⁰C for the peridotites. Distinct petrological and geochemical signatures displayed by the rocks in the present study indicate that the Dras samples show mixed affinities with harzburgites formed at MOR setting whereas dunites being ultra-depleted and refractory owing to higher degrees of partial melting were modified in an SSZ environment. Keywords: Dras, peridotites, mantle, dunite, chromite mineralization, Ladakh.