Eleni Wood, Clare Warren , Nick M. W. Roberts , Tom Argles, Barbara Kunz , Ian Millar , Sam J. Hammond, Alison Halton

During continental collision, crustal rocks are buried, deformed, transformed and exhumed. The rates, timescales and tectonic implications of these processes are determined by linking geochemical, geochronological and microstructural data from metamorphic rock-forming and accessory minerals. Exposures of lower orogenic crust provide important insights into orogenic evolution, but are rare in young continental collision belts such as the Himalaya. In NW Bhutan, eastern Himalaya, a high-grade metamorphic terrane provides a rare glimpse into the evolution and exhumation of the deep eastern Himalayan crust and a detailed case study for deciphering the rates and timescales of deep-crustal processes in orogenic settings. We have collected U-Pb isotope and trace element data from allanite, zircon and garnet from metabasite boudins exposed in the Masang Kang valley in NW Bhutan. Our observations and data suggest that allanite cores record growth under eclogite facies conditions (>17 kbar ~650°C) at ca. 19 Ma, zircon inner rims and garnet cores record growth during decompression under eclogite facies conditions at ca 17-15.5. Ma, and symplectitic allanite rims, garnet rims and zircon outer rims record growth under granulite facies conditions at ~9-6 kbar; >750°C at ca. 15-14.5 Ma. Allanite is generally considered unstable under granulite-facies conditions and we think that this is the first recorded example of such preservation, likely facilitated by rapid exhumation. Our new observations and petrochronological data show that the transition from eclogite to granulite facies conditions occurred within 4-5 Ma in the Eastern Himalaya. Our data indicate that the exhumation of lower crustal rocks across the Himalaya was diachronous and may have been facilitated by different tectonic mechanisms.