Title:  Hidden arc, rising sheets: Neoproterozoic magmatism, UHP metamorphism, and rapid exhumation in the NW Himalaya

Abstract: The Himalayan nappe sequence comprises distinct tectonic units, with magmatic contributions found in the Paleoproterozoic (~1800 Ma) Lesser Himalayan Sequence (LHS) and the early Paleozoic (~500 Ma) Greater Himalayan Sequence (GHS). For my PhD research, I propose to investigate the magmatic history of the NW Himalaya to identify whether only two main magmatic events are present, or whether a Neoproterozoic age on a pluton in the Munsiari Group is more common and represents a third major magmatic event. I will address this question by measuring zircon U-Pb ages and trace element compositions on 14 samples from the GHS, LHS, and the structurally intervening Munsiari unit. Preliminary data reveal three magmatic episodes: ~500 Ma (restricted to the GHS), ~1800 Ma (restricted to the LHS), and 800-1000 Ma (restricted to the Munsiari). The Neoproterozoic ages and arc-related trace element compositions of the Munsiari unit suggest a continental arc setting associated with the Rodinia supercontinent. These rocks correlate with magmatic belts of the Aravalli-Delhi Belt, Seychelles Islands, Madagascar, and the Yangtze Block of central China. The presence of this Neoproterozoic Munsiari Arc implies that the nappe sequence in the NW Himalaya requires reclassification, with the Munsiari rocks as a distinct unit.
A key question for tectonics is how ultrahigh-pressure (UHP) terranes are subducted, reach UHP metamorphism, and are then exhumed, especially for the Tso Morari UHP nappe in northwestern India. Prior zircon and garnet ages lacked precision and were limited geographically. To define better the age of metamorphism and to identify possible variations in ages and temperatures across the nappe, I will carry out high-precision Lu-Hf and Sm-Nd garnet geochronology, combined with Zr-in-rutile thermometry, on rocks distributed throughout the ~100 x ~30 km complex. Preliminary analyses from eight eclogites yield Lu-Hf ages ranging from 50.8±0.3 to 47.0±2.0 Ma and Sm-Nd ages of 48.5±2.1 and 78.8±5.3 Ma, with most clustering between 48-50 Ma. These ages correspond to garnet growth during prograde to peak metamorphism. Core contributions to the total Lu range from 40-85%, so UHP metamorphism must be later than ~48 Ma. Garnets from one sample were subsampled into pre-UHP cores and UHP rims, which yield ages of 49.4±0.6 Ma and 47.0±2.0 Ma, respectively. These data likely indicate rapid burial to UHP conditions. The consistency of ages across the complex suggests simultaneous metamorphism of the Tso Morari nappe as a coherent unit. However, preliminary Zr-in-rutile thermometry from eight samples across the complex shows a temperature increase from ~570 °C (northwest) to ~770 °C (southeast). These are the first spatially distributed temperature data for the complex, and they suggest that southeastern rocks may have been metamorphosed at greater depths (assuming temperature increases downward).