The Metamorphic Evolution of Eastern Himalayan Lower Crust

Wood, Eleni May (2019). The Metamorphic Evolution of Eastern Himalayan Lower Crust. PhD thesis The Open University.



The Himalayan mountain range is the perfect natural laboratory to ground truth tectonic models of continental collision due to its young age. Evidence for late-stage exhumation of deep (>50 km) orogenic crust in the eastern Himalaya suggests that tectonic processes can rapidly mobilise the lower crust. This thesis presents a detailed metamorphic and petrochronological study of the high-pressure granulite-facies Masang Kang Terrane (MKT) in NW Bhutan. In this study, two different units have been identified within the MKT based on their P-T-t histories. In the northern MKT, the timing of eclogite-facies metamorphism (>17 kbar, ~650°C) is constrained by new zircon and monazite U-Pb ages between ~21-16 Ma. New observations of allanite textures in metabasitic ‘granulitised eclogites’ are used in conjunction with zircon to constrain the timing of the granulite overprint and decompression (750-800°C, 7-9 kbar) at ~15.5 Ma. The first occurrence of a sapphirine-bearing sample in the Himalaya yields a complementary P-T-t history, which supports high-temperature exhumation in the north MKT. Furthermore, inherited monazite & mixed garnet ages provide evidence for Bhimpedian-aged metamorphism at ca. ~500 Ma. In the southern MKT, monazite and garnet in sillimanite-grade metasediments reveal a protracted history between ~40-21 Ma, that includes isobaric heating to granulite-facies in the lower midcrust (~800°C, ~8 kbar). The contrasting P-T-t histories for the two units suggest the presence of a secondary ‘Lumgu’ thrust that juxtaposed the deep- and mid-crust, before final exhumation along the Laya thrust at their base. Similarities between P-T-t histories for the southern MKT and the neighbouring Jomolhari massif suggests that further work needs to be done to constrain the architecture of these terranes.

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