New pressure-temperature-time constraints on initial India-Asia collision from eastern Himalayan syntaxis eclogite-facies metamorphic rocks

Abstract

Eclogite-facies rocks exposed within the Himalayan Range can be used to constrain the timing and characteristics of the India–Asia collision, which remain strongly debated. Many collision-related high- and ultrahigh-pressure lithologies are exposed around the western Himalaya; however, the rarity of similar units in the eastern Himalaya limits our understanding of the along-strike geodynamic evolution of India–Asia collision and the characteristics of the subducted India plate. Here, we document collision-related, low-temperature eclogite-facies metapelites from the easternmost Indus-Yarlung suture zone, which retain vital records of the timing of the initial India–Asia collision in this region. Petrology and phase diagram modeling indicate that the metapelite experienced cold subduction along a low thermal gradient (∼12 °C/km), reaching peak low-temperature eclogite-facies conditions (ca. 1.5 GPa and 590 °C), and was retrogressed to amphibolite-facies conditions during exhumation. Zircon and monazite U–Th–Pb chronology reveals new eclogite-facies ages of ca. 47 Ma, constraining collision of the Indian and Asian continents in the eastern Himalayan syntaxis at ca. 50 Ma, which is consistent with the timing in the western Himalaya and east-central Himalaya. This demonstrates that the initial India–Asia collision occurred quasi-synchronously along orogenic strike, although the maximum depth of subduction of continental material differed. Moreover, the subduction dynamics of continental crust within the collisional orogen exhibit pronounced lateral heterogeneity. These findings further demonstrate a significant change in slab dip angle, convergence velocity, and/or lithospheric strength along strike during closure of the Neo-Tethys Ocean, which must be considered in future geodynamic simulations of the evolution of the orogen.