NEW GEOCHEMICAL AND THERMOCHRONOLOGIC CONSTRAINTS ON THE TECTONIC AFFINITY, COOLING HISTORY, AND TIMING OF OBDUCTION OF THE SPONGTANG OPHIOLITE, NORTHWEST INDIA

The Spongtang ophiolite, located in northwest India, is a fragment of oceanic lithosphere emplaced during a collision between the Indian and Asian plates. The structure was previously sampled and studied by Reuber, who argued that Spongtang formed at a slow spreading center near a transform fault. Here, we report new major and trace element data for a peridotite (SPO-25) and a sample from the metamorphic sole (SPO-26). Our objectives are to gather insights into the tectonic setting in which the structure formed, and to characterize the ophiolite’s cooling history and the timing of its formation and emplacement. Amphibole, clinopyroxene, olivine, and spinel mineral compositions were measured using an Electron Microprobe. SPO-26 amphibole is zoned hornblende, with relatively Al2O3 rich, FeO poor rims and flat REE patterns (~35 × chondrite). SPO-25 is a spinel harzburgite. Spinel Cr# [100×(Cr/(Cr+Al)), in moles] provides information regarding the degree of partial melting experienced by a sample and is a useful proxy for tectonic setting. With a spinel Cr# of 30 and an olivine Mg# of 90 [100×(Mg/(Mg+Fe)), in moles], the sample falls within the range of abyssal peridotites and ophiolites with mid-ocean ridge affinity, consistent with the structural interpretations of Reuber. Clinopyroxene are depleted in light rare earth elements and have flat mid-heavy chondrite normalized REE patterns. Abundances range from 0.1×chondrite for Sm to 4×chondrite for mid-heavy REEs. Using the REE-in-two-pyroxene thermometer of Liang et al. (2013), a temperature of 1060ºC was calculated and from the major element based two-pyroxene thermometer of Brey and Köhler (1990), a temperature of 876ºC was calculated. The temperature discrepancy suggests the ophiolite cooled slowly, on the order of 10s of degrees / Myr. Additional work is underway to determine ages of zircons from the metamorphic sole and a diabase dike that cross cut the ophiolite. Using an SEM, zircon crystals have been located which will be analyzed by SIMS to constrain the timing of metamorphism and/or emplacement. The timing and sequence of plate- and arc-collisional events from the ophiolite will significantly improve our understanding of the paleogeography of the Neo-Tethyan Ocean and the formation of the Himalayan Mountains.