Coupled trace element and Hf-isotope measurements of Hadean through Paleoarchean zircons from the Singhbhum Craton indicate derivation from a long-lived, mantle-derived protocrust

Heather Kirkpatrick, Emily Stoll, Nadja Drabon

Due to the dearth of rock records during the Hadean, little is known about early crustal chemistry and geodynamics. Here, we present zircon trace and rare earth element and Lu-Hf measurements of zircons ~ 3.3 Ga to ~ 4.2 Ga from the Older Metamorphic Tonalitic Gneiss in the Singhbhum Craton to better understand geodynamic changes during the Hadean and Archean. We find decreasing, subchondritic zircon εHfT among zircons > 3.8 Ga and no indication of addition of new crustal material during the Hadean after initial formation of this protocrust. Trace and rare earth element analyses of >3.8 Ga grains indicate derivation from a source heavily influenced by the enriched mantle with little evidence for flux melting, and no clear evidence for deep melting. At ~ 3.8 Ga, we observe an average εHfT increase indicating new additions of juvenile material. These zircons still show predominantly mantle-like trace elements, but with the appearance of some arc-like trace element zircon signatures and evidence for crustal thickening. This period of crustal reorganization speaks towards a shift in geodynamic regime characterized by the onset of communication between the mantle and the crust from which the zircons formed at this location. The presence of long-lived, mantle-derived protocrust in both India and South Africa appears to suggest the possibility of a stagnant lid, albeit more data is necessary to confirm this. The presence of a transition of εHfT data globally may point towards an important period of crustal reorganization 3.9 – 3.6 Ga ago.