Warren, C. J.; Grujic, D.; Kellet, D. A.; Cottle, J.; Jamieson, R. A. and Ghalley, K. S.
Probing the depths of the India-Asia collision: U-Th-Pb monazite chronology of granulites from NW Bhutan.
Tectonics, 30(TC2004) pp. 1–24.
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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 an associated 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 13.5 ± 0.5 Ma and core ages of ca. 550 Ma. Monazite associated with sillimanite-grade metamorphism in gneiss at structurally lower levels yields U-Pb rim ages of 21-17 Ma and core ages of 600-500 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 tectonic forcing over an incoming Indian crustal ramp is a viable mechanism for exhumation of lower crustal granulitised eclogites in Bhutan.
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