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.

DOI: https://doi.org/10.21954/ou.ro.00010e20

Abstract

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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