4:00 PM - 5:00 PM
Detrital zircons as old as nearly 4.4 Ga offer insights into the earliest moments of Earth history. Results of geochemical investigations of these grains have been interpreted to indicate their formation in near-H2O saturated meta- and peraluminous magmas under a relatively low (15-30°C/km) geotherm. A key feature in pursuing a petrotectonic model that explains the full spectrum of these observations is their seeming contrast to most Phanerozoic magmatic zircons. The ~22 Ma Arunachal leucogranites of the eastern Himalaya appear, however, to be a rare exception to this generality. They show clear evidence of wet basement melting and are dominated by the same inclusion population and P-T conditions. We suggest that the Arunachal leucogranites originated in the hangingwall of a megathrust that carried H2O-rich foreland sediments to depths of >20 km whereupon de-watering reactions released fluids that fluxed hangingwall anatexis. Modeling suggests the thermal structure of this continental collision environment could have been broadly similar to a Hadean ocean-continent subduction zone. The similarity of these two environments, separated by over 4 Ga may explain the remarkably similar Hadean and Arunachal leucogranite zircons.