Long-lived (>20 m.y.) prograde heating in the Eastern Himalaya in response to Neo-Tethys slab breakoff

The Himalaya is the most prominent collisional belt on Earth, characterized by pronounced spatiotemporal variations in thermal gradients. The Western Himalayan syntaxis (WHS) records evidence of deep subduction with low thermal gradients, whereas the Eastern Himalaya is characterized by synchronous high-temperature granulite overprinting. The geodynamic mechanism responsible for these along-strike thermo-metamorphic differences remains poorly constrained, particularly regarding the timing and duration of heating during the India-Asia collision. This study investigates high-pressure pelitic granulites from the Eastern Himalayan syntaxis (EHS) to constrain their metamorphic evolution and duration of metamorphic stages. Pelitic granulites record a clockwise isothermal decompression pressure-temperature (P−T) path with peak conditions of 12−13 kbar and 815−830 °C, accompanied by muscovite-biotite dehydration melting. Metamorphic ages range from 47.3 Ma to 12.6 Ma, with long-lived prograde heating spanning from 47.3 Ma to 23.0 Ma. During this interval, zircon and monazite show increasing Y contents and decreasing (Dy/Yb)N ratios with decreasing age due to the breakdown of accessory minerals (e.g., xenotime and apatite). In contrast, retrograde metamorphism occurs at ca. 23.0−12.6 Ma and shows a reverse trend. The EHS thus documents prolonged (>20 m.y.) prograde heating, contrasting with the rapid subduction-exhumation in the WHS. Integrating the presence of Eocene magmatism, different P−T−time paths, and metamorphic ages between the EHS and WHS, this study proposes that the along-strike metamorphic differences reflect the evolving dynamics of the subduction slab system. The Neo-Tethys breakoff occurs earlier (ca. 47−45 Ma) in the EHS, thus resulting in flat subduction of the Indian lithosphere, Eocene magmatic flare-up, and long-lived prograde heating in the EHS. Conversely, the WHS experienced continuous continental subduction, with Neo-Tethyan breakoff occurring later (ca. 35 Ma) during the transition from subduction to collision.