Metamorphosed Mélange in the Eastern Himalayan Syntaxis: Constraint for the India–Asia Collision in the Eocene

Tectonic mélanges are crucial for deciphering collision processes in orogenic belts. This study investigates the nature of the metamorphosed mélange in the Milin area of the eastern Himalayan syntaxis to shed light on the India–Asia collision in the region. The mélange exhibits a ‘block-in-matrix’ fabric in the lower and upper parts and a ‘lenticular-thrust-slices’ fabric in the middle. Quartzite blocks in the lower part are proposed to be Silurian or younger quartz arenites with a Tethyan affinity. Metacherts and amphibolites in the middle and upper parts may have originated from hydrothermal or terrestrial cherts and Nb-enriched basalts in the Asian forearc or syncollisional basin at ∼48–45 Ma. The diverse blocks/slices of the mélange, sourced from both the subducting and overriding plates, are suggested as having formed during Indian plate subduction. The protoliths of the amphibolite–metachert slice (∼45 Ma) in the mélange are distinctly different from the contemporary shallow sea (post-collisional) deposits but similar to the deep-water (initial collisional or pre-collisional) sediments in other parts of the suture zone. This implies that the amphibolite–metachert slice may represent the product of the initial India–Asia collision or even pre-collision and that (hard) collision likely lagged by ∼10 Myr in the eastern Himalaya syntaxis. Supplementary material: Analytical methods, Figures S1–S5 showcasing field occurrences, cathodoluminescence images of zircons, and geochemical characteristics, Table S1 – geochronology data, Table S2 – zircon Lu–Hf isotopic data, and Table S3 – whole-rock major and trace element results are available at https://doi.org/10.6084/m9.figshare.c.6896942 Thematic collection: This article is part of the Ophiolites, melanges and blueschists collection available at: https://www.lyellcollection.org/topic/collections/ophiolites-melanges-and-blueschists