Probing the depths of the India‐Asia collision: U‐Th‐Pb monazite chronology of granulites from NW Bhutan

Abstract

Rocks metamorphosed to high temperatures and/or high pressures are rare across the Himalayan orogen, where peak metamorphic conditions recorded in the exposed metamorphic core, the Greater Himalayan Sequence (GHS), are generally at middle to upper amphibolite facies. However, mafic garnet‐clinopyroxene assemblages exposed at the highest structural levels in Bhutan, eastern Himalaya, preserve patchy textural evidence for early eclogite‐facies conditions, overprinted by granulite‐facies conditions. Monazite hosted within the leucosome of neighboring granulite‐facies orthopyroxene‐bearing felsic gneiss yields LA‐MC‐ICP‐MS U‐Th‐Pb ages of 13.9 ± 0.3 Ma. Monazite associated with sillimanite‐grade metamorphism in granulite‐hosting migmatitic gneisses yields U‐Th‐Pb rim ages between 15.4 ± 0.8 Ma and 13.4 ± 0.5 Ma. Monazite associated with sillimanite‐grade metamorphism in gneiss at structurally lower levels yields U‐Pb rim ages of 21–17 Ma. These data are consistent with Miocene exhumation of GHS material from a variety of crustal depths at different times along the Himalayan orogen. We propose that these granulitized eclogites represent lower crustal material exhumed by tectonic forcing over an incoming Indian crustal ramp and that they formed in a different tectonic regime to the ultrahigh‐pressure eclogites in the western Himalaya. Their formation and exhumation in the Miocene therefore do not require diachroneity in the timing of the initial India‐Asia collision.