Finding a Pulse: Melt Formation and Timing in the Garhwal Himalaya

The most significant consequence of prograde metamorphism for orogenic evolution is the melting of high-grade metamorphic rocks, resulting in a dramatic decrease in their mechanical strength, the activation of shear zones and consequent exhumation. Granitic bodies emplaced within the highest metamorphic grades of the Himalayan orogen form by the melting of amphibolite-grade pelitic rocks, either due to the presence of aqueous fluid or through the dehydration of hydrous phases such as muscovite. Across the Himalayas, these granites, and partially melted source migmatites, are found in the Greater Himalayan Sequence (GHS), bounded by the Main Central Thrust (MCT) and the South Tibetan Detachment (STD). Many of these granites formed during the Miocene when decompression of the unit during rapid exhumation triggered melting; however, exact timings and reaction pathways appear to vary laterally across the orogen. The timescales of anatexis, amalgamation, migration, and emplacement are the focus of active research and have implications for orogenic tectonic development. Recent studies of granite pluton formation suggest a series of pulsed melting events with protracted periods of crystallisation under low melt-fraction conditions. These studies show that grain-scale variations in age can be linked with trace element data in both monazite and zircon, spanning millions of years of crystallisation. It is, therefore, important to recognise the geochemical signatures that these processes leave in granites, migmatites, and melt-extracted restite and to delineate more precisely the relevant processes and timescales leading to magma genesis. We present a preliminary dataset that aims to constrain the source, melt reactions, and timescales of melting episodes that form the migmatites and leucogranites of the upper GHS. We sampled leucogranites, migmatites, and their host metasediments along the Rishi Ganga (Badrinath) and Alaknanda valleys in the Garhwal region of the Indian Himalaya. Zircon from these samples were analysed for their crystallisation age (U-Pb), Hf-isotopic ratios, oxygen isotope and trace element composition using LA-ICPMS.  Rim domains identified using cathodoluminescence (CL) imaging were preferentially targeted, with the aim of collecting data that related to Himalayan melting processes. Preliminary findings suggest that the leucogranites crystallised from 22 Ma to ~13 Ma, with punctuated zircon crystallisation occurring throughout this timespan. Zircon rim ages from migmatites are generally older, ranging from 34 Ma to ~15 Ma. Integration of Hf-isotopic and trace elemental data, combined with petrographic observations allow mineral age data to be linked to changes in geological processes.