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

Abstract

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 Thongmön 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 °C), and subsequent decompression-cooling retrograde metamorphism (ca. 0.42–0.62 GPa and 800–820 °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 ∼450 Ma and εHf(t) values of + 3.2 to + 7.0. Metamorphic overgrown mantles or some rims of zircon record an early decompression granulite-facies metamorphic stage (∼17.9–15.3 Ma); other metamorphic overgrown rims and metamorphic growth zircons reflect later decompression-cooling retrograde metamorphic stage (∼14.8–13.3 Ma). The titanite U–Pb age (∼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 ∼55 Ma.