Tso Morari Eclogites, Eastern Ladakh: Isotopic and Elemental Constraints on Their Protolith, Genesis, and Tectonic Setting

Abstract

The Tso Morari Crystalline Complex (TMC), eastern Ladakh, is marked by the presence of eclogites as boudins and lenses within the Puga Formation. These eclogites are composed of garnet, omphacite, amphibole, phengite, glaucophane, quartz, and iron oxide, with rare coesite inclusions in garnet reflecting ultrahigh-pressure metamorphic characteristics. Geochemically, TMC eclogites have high Fe-Ti basaltic compositions and classify as subalkaline tholeiites. Rare earth element and multielement diagrams display enriched patterns similar to enriched mid-ocean ridge basalt coupled with perturbed large ion lithophile elements and higher whole-rock (87Sr/86Sr) ratios (0.70884 to 0.72721) reflecting the possible influence of postcrystallization processes rather than variable interaction with host granite gneisses (87Sr/86Sr ratio: ∼0.73901). To evaluate the existing protolith possibilities, we calculated εNd(t=289Ma) values (+1.9 to +9.5) and εNd(t=140Ma) values (+1.1 to +8.9) of TMC eclogites; both indicate their derivation from depleted-mantle sources. The εNd(t=289Ma) values of the early Permian enriched Panjal volcanics of Kashmir Valley (−5.3 to +1.3) and Phe volcanics of Zanskar Himalaya (−7.4 to −1.1) are very different from TMC eclogites. However, the εNd(t=289Ma) values of TMC eclogites are similar to the depleted Panjal volcanics (+0.3 to +4.3). Similarly, the εNd(t=140Ma) values of the TMC eclogites closely resemble those of the adjoining Ladakh ophiolites, such as the Nidar-Spongtang-Shergol-Dras ophiolitic mafic rocks (+5.1 to +9.9). These observations partly negate the existing hypothesis of enriched Panjal and Phe volcanics for being the protolith for the TMC eclogites. Thus, we propose that the protolith for the TMC eclogites could be represented by the subducted portion of the early Permian depleted Panjal volcanics and Late Jurassic to Early Cretaceous Ladakh ophiolitic mafic rocks, subducted to eclogite-grade metamorphism (around ~53 Ma) and were subsequently tectonically accreted to the obducting Indian continental crust during their exhumation.