First evidence of eclogites overprinted by ultrahigh temperature metamorphism in Everest East, Himalaya: Implications for collisional tectonics on early Earth

Modern-style plate tectonics, often characterised by subduction, is a fundamental dynamic process for planet Earth. Subduction related eclogites are widely used to indicate initiation of plate tectonics or whether different tectonic regimes dominated Earth history. However, such markers are commonly overprinted in ancient metamorphic terranes and rarely preserved even in most Phanerozoic mountain belts. This study tries to reveal the detailed burial and exhumation processes that formed granulitized eclogites in the Everest east region, central Himalaya, so as to explore the tectonic regimes recorded by similar rocks on early Earth. Robust Pressure-Temperature-time paths were achieved by studying the mineral relicts (Omp, Jd similar to 29%), high-temperature mineral textures (Sil-Crd-Qz-Spl-Mesoperthite assemblage, rutile exsolution in biotite), and multiple thermobarometry and petrochronology of eclogites and metapelites. Results show that these eclogites underwent eclogitization at conditions of 730-770 degrees C and similar to 20 kbar (similar to 11 degrees C/km) at similar to 30 Ma and were overprinted by a heating and decompression path to ultrahigh temperature (UHT) conditions of 6-11 kbar and 900-970 degrees C (similar to 40 degrees C/km) during 25-15 Ma. The resulting exhumation rate (2-3 mm/yr) is slow and prolonged (10-15 Myr) (U)HT favoured reequilibration of the eclogitic mineral assemblage and textures. The obtained UHT conditions, the first time ever reported for the Himalaya, were induced by combined effects of over-thickened (similar to 60 km) radioactive felsic crust and thinning of lithosphere to <90 km. This case study provides a critical example to understand the heat sources and timescale of UHT condition during continental collision. By comparing with the western Himalaya eclogites, we suggest that formation of cold vs. granulitized continental eclogites during the Himalayan orogeny is caused by different crustal thickness (normal similar to 30 km vs. over-thickened similar to 60 km) due to different collisional stages (infant vs. mature). In a wider perspective, ancient eclogites were commonly granulitized by stacking into the over-thickened orogenic crust during mature continental collision. According to similar granulitized eclogites preserved on early Earth, Himalaya-type continental subduction/collision should have become a global pattern during the Paleoproterozoic (2.0-1.8 Ga). (C) 2021 Elsevier B.V. All rights reserved.

Automated summary

This study investigates the burial and exhumation processes of granulitized eclogites in the Everest east region, central Himalaya. The results show that the eclogites underwent eclogitization and ultra-high temperature conditions induced by over-thickened felsic crust and lithosphere thinning. This case study provides insights into the UHT conditions during continental collision and suggests a global pattern of Himalaya-type continental subduction/collision during the Paleoproterozoic.