What drove partial melting of Earth's youngest exposed migmatites? Insights from monazite petrochronology of the Western Himalayan Syntaxis

The Nanga Parbat-Haramosh Massif (NPHM) comprises the western syntaxis of the Himalayan orogenic belt, Earth's youngest exposed partially melted metamorphic rocks been exhumed to the surface at extreme rates. As such, the NPHM has served as a natural laboratory to study interactions and feedbacks between surficial and deep crustal processes. Pleistocene melt crystallization ages have been documented in the NPHM, but the timing and timescales of partial melting have remained ambiguous. This information is crucial for assessing whether surface erosion can initiate partial melting and exhumation of the lower crust, as suggested by previous studies. Here we present texturally and chemically constrained Th-Pb dates of monazite to constrain the timing and duration of prograde metamorphism and partial melting in the NPHM core. Monazite in a restitic pelite shows a decrease in HREE+Y at ~2.2 Ma, concurrent with an increase in Sr, Ca, and Th. These data, combined with microstructures and chemical zonation of garnet and K-feldspar, suggest a Pleistocene initiation of biotite/apatite breakdown melting at ~2.2 Ma. High-Y, low-Sr monazite grew at ~1 Ma, potentially constraining the timing of melt crystallization and garnet breakdown. Another rock (migmatitic granitoid) records a different history. Garnet crystals in this rock are resorbed and surrounded by cordierite coronae; thermodynamic modelling suggests these textures formed during near-isothermal decompression from ~0.6 to 0.3 GPa, concomitant with melt crystallization. Monazite analyses show increasing HREE+Y from ~3.5 to ~1.5 Ma, which record the timing of decompression, garnet breakdown, and melt crystallization. In both rocks, the Oligo-Miocene record of Himalayan orogenesis has largely been erased from monazite, likely due to pre-Pliocene fluid dissolution and subsequent growth during Plio-Pleistocene apatite and garnet breakdown.

The two samples analyzed in this study record disparate histories: melt in one rock was being produced while melt in another rock was crystallizing during Pleistocene decompression. These data suggest that while rapid surface erosion may contribute to partial melting of orogenic crust, an additional deep crustal mechanism (i.e. anomalous heat and/or mass input) may be necessary to initiate such feedbacks.