Pleistocene Melting and Rapid Exhumation of the Nanga Parbat Massif, Pakistan: Age and P–T Conditions of Accessory Mineral Growth in Migmatite and Leucogranite

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Rapid Pleistocene exhumation of the core of the Nanga Parbat massif (northwestern Himalayan syntaxis) is inferred by combining U-Pb dates from monazite, xenotime, and zircon from migmatitic rocks and a leucogranite dike with pressure estimates that are closely linked to dated events in the melting and crystallization history. Exhumation rates of 11–13 mm/a were calculated from (i) migmatitic rocks that were produced at 1.7 Ma and 5.0 kbar by dehydration melting of biotite on decompression and (ii) veins with garnet and cordierite that crystallized at 1.0 Ma and 3.5 kbar. Tourmaline-bearing leucogranitic dikes separated from the source and ascended to crystallize near their solidus at 0.7 Ma. Modeling of Th/U in the leucogranite magma based on Th/U and U-Pb data from monazite, xenotime, and zircon shows a decrease from 1.1 to 0.2 over a span of 0.15 Ma. The implied acceleration of exhumation at 1.7 Ma may be linked to mid-crust flow as the evolving thermal structure of the Neogene metamorphism encountered the biotite dehydration–melting reaction. The rapid exhumation may have resulted from significant lowering of the effective viscosity of mid-crustal rocks, leading to vertical channel flow into the core of the Nanga Parbat massif along bounding shear zones.