When did crustal melting weaken the heart of the Himalaya?

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Major mountain belts are contortions of the Earth's crust, ravaged by gravity. Rocks buried in these zones soften, stretch and melt, with drastic consequences for their mechanical strength. Just a few percent of partial melt can dramatically weaken the continental crust1 and rapidly change the evolution of the mountain belt.In the Himalaya, research on granites has mainly focused on conspicuous, pale bodies of Miocene-aged granites (leucogranites). These magmas formed when fertile rocks were rapidly exhumed from the mid-crust, decompressed and melted. However, these melts were a symptom of that dramatic exhumation, not its cause. Clues to what triggered that exhumation in the Himalayan core must lie in earlier events.Sporadic evidence for earlier melting has been recognised along the entire Himalayan chain from Pakistan to Bhutan2 (Figure 1). These cryptic, deformed kyanite-bearing leucogranites and partly-molten gneisses (migmatites) crystallized during Paleogene prograde burial and heating. However, such evidence is commonly overlooked among rocks with textures heavily reworked during Neogene mountain-building.Understanding Paleogene crustal melting in these youthful mountains is therefore key for establishing the tipping point at which crustal thickening was overtaken by exhumation3. Moreover the spatial distribution of such melting will help fingerprint the underlying tectonic mechanism that drove the tectonic extrusion (critical taper, wedge tectonics or channel flow).This project aims to interrogate field relations and mineral assemblages to define melt reactions during heating in the crystalline core of the Himalaya. Results from the project will yield insights into viscosity changes in both the Paleogene Himalaya and older collisional orogens, providing critical constraints on thermomechanical models that attempt to explain how all mountain belts evolve.Methodology: The initial phase of this study will examine samples of Paleogene granites from the OU collection to identify the most appropriate field area for detailed study. Fieldwork in the Indian Himalaya will allow the spatial relationships of these granitic bodies and their deformational and metamorphic histories to be assessed. Migmatites, leucogranites and potential source rocks for granitic melt will be subjected to trace element and isotopic (Nd and Sr) whole-rock study, while melt accessory phases (monazite, zircon) will be dated via the U-Pb system and analysed for Hf isotopes to trace the history of the melts. Monazite and zircon ages will be linked to crystallization reactions by employing pseudosection modeling of metamorphic minerals4