Protoliths and metamorphism of the central Himalayan eclogites: Zircon/titanite U-Pb geochronology, Hf isotope and geochemistry

The high-pressure (HP) eclogites in the central Himalaya provide insights into the metamorphism and exhumation history of crustal material beneath the Tibetan plateau. Due to the paucity of exposure, the nature and timing of the protolith and metamorphism of the eclogites remain poorly known. Here we report zircon and titanite U-Pb ages, bulk-rock and mineral compositions and zircon Hf isotope data on the eclogites from the Thongmon and Kharta areas in the central Himalaya. The eclogites record peak HP eclogite-facies metamorphism at >1.6 GPa, high-temperature to ultrahigh-temperature (HT-UHT) granulite-facies overprinting (ca. 0.88-0.99 GPa and 875-920 degrees C), and subsequent decompression-cooling retrograde metamorphism (ca. 0.42-0.62 GPa and 800-820 degrees C). Geochemical data suggest that the eclogite protoliths are most consistent with being seafloor tholeiitic basalts with an E-MORB signature. Inherited magmatic zircon cores from the eclogites give a protolith age of similar to 450 Ma and epsilon(Hf)(t) values of + 3.2 to + 7.0. Metamorphic overgrown mantles or some rims of zircon record an early decompression granulite-facies metamorphic stage (similar to 17.9-15.3 Ma); other metamorphic overgrown rims and metamorphic growth zircons reflect later decompression-cooling retrograde metamorphic stage (similar to 14.8-13.3 Ma). The titanite U-Pb age (similar to 15.4 Ma) further indicates the beginning of the later retrograde metamorphism. All these data and observations allow the proposal that in contrast to the ultrahigh-pressure (UHP) eclogites in the western Himalaya, the HP eclogites in the central Himalaya record the long-lived burial and relatively slow exhumation of the Indian continental crust since initial Indo-Asia collision at similar to 55 Ma. (C) 2021 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.

Automated summary

The high-pressure eclogites in the central Himalaya provide insights into the metamorphism and exhumation history of crustal material beneath the Tibetan plateau. Through the analysis of zircon and titanite U-Pb ages, bulk-rock and mineral compositions, and zircon Hf isotope data, the study reveals the evolution of these eclogites, from peak HP eclogite-facies metamorphism to later retrograde metamorphism. The findings suggest a long-lived burial and slow exhumation process of the Indian continental crust since the initial Indo-Asia collision at around 55 Ma.