QUATERNARY PARTIAL MELTING IN THE WESTERN HIMALAYAN SYNTAXIS: INSIGHTS FROM MONAZITE PETROCHRONOLOGY OF EARTH’S YOUNGEST MIGMATITES

The youngest exposed migmatites on Earth are found in the core of the Nanga Parbat Massif (NPM), which comprises the western syntaxis of the Himalayan orogen. Anomalously young melt crystallization ages (~1 Ma) have been documented by previous studies, but the timing and timescales of partial melting are less well-constrained. These data are necessary to determine the geodynamic mechanism(s) for partial melting in the NPM. Here we present texturally- and chemically-constrained Th-Pb monazite dates from partially melted rocks in the core of the NPM. Monazite in a restitic pelite shows high-Y, low-Th cores, low-Y/high-Th mantles, and thin, high-Y rims. Laser-ablation split-stream analyses of these distinct monazite zones show a decrease in HREE+Y at ~2.2 Ma, concurrent with an increase in Sr, Ca, and Th. These data, combined with microstructural analyses and chemical zonation in garnet and K-feldspar, suggest a Quaternary initiation of partial melting, garnet growth, and the breakdown of biotite, plagioclase, and apatite at ~2.2 Ma. High-Y, low-Sr monazite grew again at ~1 Ma, potentially constraining the timing of garnet breakdown and melt crystallization. Another sample (cordierite leucogranite) contains resorbed garnet porphyroblasts with cordierite coronae. Monazite from this sample shows increasing Y from ~3.5 to ~1.5 Ma, which constrains the timing of garnet breakdown, cordierite growth, and melt crystallization, likely during decompression. The two samples analyzed here thus record seemingly disparate histories: one rock was partially melting while another was undergoing melt crystallization. This suggests that the geodynamic mechanism(s) for partial melting and rapid exhumation in the NPM may be more complex than previous studies suggest.