TIMING OF OBDUCTION, TECTONIC AFFINITY, AND COOLING HISTORY OF THE SPONGTANG OPHIOLITE, NORTHWEST INDIA, HIMALAYA

The Spongtang ophiolite, located in northwest India, is a fragment of oceanic lithosphere emplaced during collision between the Indian and Asian plates. The structure was previously sampled and studied, and it has been argued that the Spongtang ophiolite formed at a slow spreading center near a transform fault. Here, we report new major and trace element data for a peridotite and a sample from the metamorphic sole. Zircon grains from a diorite dike were dated using Secondary Ion Mass Spectrometry (SIMS). Our objectives are to (1) gather insights into the tectonic setting in which the structure formed, (2) to characterize the ophiolite’s cooling history, and (3) time its formation and emplacement.

Amphibole, clinopyroxene, olivine, and spinel mineral compositions were measured using an electron microprobe. Incompatible trace elements were measured in orthopyroxene, clinopyroxene, and amphibole. With a spinel Cr# of 30 and an olivine Mg# of 90, the peridotite sample falls within the range of abyssal peridotites and ophiolites with mid-ocean ridge affinity, consistent with previous structural interpretations. A REE-in-two-pyroxene thermometer yields a temperature of 1060⁰C, whereas a major element-based two-pyroxene thermometer yields a temperature of 876⁰C. The discrepancy suggests the ophiolite cooled slowly, on the order of tens of degrees per million years. Zircon ages of 173.8 ± 3.0 Ma and 95.8 ± 2.7 Ma were found in a diorite dike from U-Pb dating using SIMS. Respectively, these ages are believed to represent the mid-ocean ridge spreading in the Neo-Tethys, which created the oceanic crust preserved in the Spongtang ophiolite, and the start of plate re-organization that initiated the subduction-related volcanism in the Neo-Tethys to form the Spong arc, causing the zircons to be metamorphosed. 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 this portion of the Himalayas.