The Petrogenesis of Kyanite Migmatites in Eastern Bhutan

Phillips, Stacy (2021). The Petrogenesis of Kyanite Migmatites in Eastern Bhutan. PhD thesis The Open University.



Understanding orogenic processes is crucial for evaluating tectonic models of mountain belts such as the Himalaya. The channel flow model predicts that the Himalayan metamorphic core is exhumed by the buoyant flow of melt-bearing rocks in the mid-crust. Such melts, generated in the kyanite stability field during prograde metamorphism, may herald the transition from burial to exhumation tectonics. This thesis investigates the evidence for this prediction by examining the petrogenesis of kyanite migmatites, through detailed petrography, high-resolution mineral-scale geochemistry (including the first LA-ICP-MS trace element mapping of kyanite) and geochronology.

In Eastern Bhutan, melting of metapelites from the lowermost parts of the Greater Himalayan Sequence (GHS) generated ‘in-source’ kyanite migmatites by low-volume, fluid-present muscovite melting. Much of the kyanite in the leucosomes is xenocrystic, having been entrained from the metapelitic source rocks. Kyanite growth continued in the melt, producing both distinctive new rims observed in cathodoluminescence images, and fresh, Ge-enriched crystals. Back-reaction of kyanite with melt produced muscovite rims, also with elevated Ge concentrations. Peritectic kyanite growth is limited but is prevalent in structurally higher migmatites. Variations in kyanite Cr/V composition reflect disequilibrium melt production and changing melt compositions.

Zircon U-Pb geochronology suggests melt formed episodically between ~34 and 12 Ma in response to periodic water availability. Pulses of zircon growth at ~21 Ma and ~14 Ma may relate to movement on the Main Central Thrust, which would facilitate fluid percolation into the overlying rocks, triggering melting. This is later than predicted by the classic channel flow model but consistent with recent composite models that propose segmentation of the GHS into tectonic slices. As the lowermost GHS is the final slice to be exhumed, prograde Miocene melting in the kyanite field could still have driven the change from burial to exhumation in this crustal section.

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