Petrogenesis of Ophiolitic Rocks from Indus Suture Zone, Ladakh Himalaya: Insights for Depleted Mantle Beneath an Intra-Oceanic Island Arc Complex

Abstract

The Indus Suture Zone (ISZ) preserves the ophiolitic remnants of the eastern part of the Neo-Tethys Ocean. Part of these ophiolitic remnants are exposed along the Khangral-Chiktan and Dras-Kargil road sections of western Ladakh. The observed rock types include mantle peridotites, ultramafic cumulates, gabbros and basalts emplaced as faulted blocks over the Mesozoic Dras arc complex. Geochemically, the studied rock types show sub-alkaline tholeiitic characteristics with basalt, basaltic-andesite to dacite compositional variation. On the basis of modal mineralogy, the mantle peridotites classify as dunites, ultramafic cumulates as wehrlite to olivine-websterite while as gabbros as hornblende-gabbro to olivine-norite. The mantle peridotites depict depleted chondrite normalized REE-patterns [(La/Yb) N = 0.6–1.1] while as ultramafic cumulates depict depleted to enriched REE-patterns [(La/Yb) N = 0.6–3.2]. Similarly, fractionated patterns are observed in gabbros [(La/Yb) N = 1.6–4.1] and basalts [(La/Yb) N = 1.0–12.3]. However, basalt samples depict flat and enriched chondrite normalized REE-patterns similar to NMORB, EMORB and OIB geochemical characteristics. The multi-element patterns of the studied rock types overall depict subduction-related geochemical characteristics such as enriched LILE and depleted HFSE compared to primitive mantle. Presence of Mg-rich olivines, orthopyroxenes and clinopyroxenes while Ti-poor clinopyroxenes in ultramafic cumulates and gabbros reflect their derivation from previously depleted mantle sources at high pressure and temperature comparable to base of the modern intra-oceanic island arc tholeiitic sequences. The present mineralogical and geochemical study suggest that the studied rock types exhibit close similarity to intra-oceanic subduction system contemporaneous to Dras-Suru-Thasgam ophiolitic slices of western Ladakh. The latter were responsible for the closure of the eastern part of Neo-Tethys Ocean during Late Jurassic to Early Cretaceous.