FROM HELL TO THE HIMALAYA: 4.4 BILLION YEARS OF TERRESTRIAL FELSIC MAGMATISM

Detrital zircons as old as nearly 4.4 Ga offer unprecedented insights into the earliest moments of Earth history. Investigations using these ancient zircons have revealed their formation under low temperatures (~680°C) and geotherms (~25°C/km) but at relatively high fO2, their incorporation of hydrous S- and I-type inclusion assemblages, δ¹⁸O enrichments in some grains, Lu-Hf crustal extraction ages as old as 4.5 Ga, and continental trace element signatures. These observations have been interpreted to reflect continental-like crust forming by 4.5 Ga, a mature, water-mediated sedimentary cycle operating by ~4.3 Ga, active Hadean recycling between crust and mantle, and meta- and peraluminous granitoid melts forming by fluid fluxing into fertile ~700°C rocks in a convergent plate boundary setting. While this sounds remarkably similar to modern granitoid genesis, the fact is that the Hadean zircon spectrum is offset to significantly lower temperatures than Phanerozoic magmatic zircons, including the vast majority of Himalayan leucogranites known independently to have formed via dehydration melting. This could reflect the far shallower depths at which minimum melting would be expected on a much hotter, early Earth than today and thus the more proximal position of the source to surface water. However, the ca. 20 Ma Arunachal leucogranites (AL) are an exception to this generality as they show LILE covariance trends indicative of wet basement melting together with a Gaussian distribution of magmatic zircon crystallization temperatures averaging 660°C. This seemingly unique character may reflect an origin more akin to that of the Hadean zircons. Along strike extrapolation of field measurements in the Siang window, eastern Himalayan syntaxis, indicates that Cenozoic foreland sediments were thrust beneath the source of the AL during the Early Miocene. As these molasse deposits had not experienced Eo-himalayan or earlier dehydration events (as had, for example, the Lesser Himalayan Formations), they represent a possible source of water with which to flux anatexis in the Arunachal Greater Himalayan Crystallines. The inference of water-saturated minimum melting during underthrusting in a ca. 25°C/km gradient both at 4.3 and 0.02 Ga suggests the AL as a possible analogue for Hadean felsic magmatism.