GSA Annual Meeting in Phoenix, Arizona, USA - 2019

Paper No. 119-1
Presentation Time: 9:00 AM-6:30 PM


OLSEN, Telemak, Department of Geosciences, Skidmore College, 815 N Broadway, Saratoga Springs, NY 12866, GUEVARA, Victor E., Geology, Amherst College, Amherst, Amherst, MA 01002, WHALEN, Lisa M., Geoscience, Virginia Tech, Blacksburg, VA 24061, KYLANDER-CLARK, Andrew R.C., Earth Science, University of California Santa Barbara, 1006 Webb Hall, University of California, Santa Barbara, CA 93106, CADDICK, Mark J., Department of Geosciences, Virginia Tech, Blacksburg, VA 24061, SMYE, Andrew J., Department of Geosciences, Pennsylvania State University, 407 Deike, University Park, PA 16802, SEARLE, Michael P., Department of Earth Sciences, University of Oxford, Oxford, OX1 3AN, United Kingdom and WATERS, David J., Department of Earth Sciences, University of Oxford, South Parks Road, Oxford, OX1 3AN, United Kingdom

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.