Detrital Zircons Reveal Evidence of Hadean Crust in the Singhbhum Craton, India

Abstract

The Singhbhum craton is one of five Archean cratons constituting the Indian subcontinent. It consists of four major lithotectonic units with broadly defined ages from Eoarchean to Neoarchean: the Older Metamorphic Group (3.7– 3.2 Ga), Older Metamorphic Tonalite Gneisses (3.8–3.1 Ga), Singhbhum Granite (3.5–3.0 Ga), and Iron Ore Group (3.51–2.55 Ga). In this study, 270 zircons were separated from modern sediment of the Baitarani River, which is wholly contained within the craton. Zircons were analyzed with laser ablation ICP-MS for their U-Pb systematics; 150% were less than 5% discordant. Three primary age groupings account for ∼98% of analyses: 3.62–3.55 Ga (5%), 3.50– 3.22 Ga (87%), and 3.10–3.06 Ga (6%). The preponderance of 3.50–3.22 Ga zircons is consistent with the local basement that includes a 3.47 Ga tonalite gneiss enclave within a 3.35–3.30 Ga outcrop of the Singhbhum Granite near Keonjhar. Lu-Hf systematics of zircons yielded 67% with positive initial εHf scattered above and below the mantle growth curve and 33% with negative initial εHf, indicating contributions from both depleted mantle and older crustal sources. Singlestage model ages range from 4.29 to 3.10 Ga. Of note is a single zircon with a 207Pb/206Pb age of 401559 Ma (1.3% discordant), which is the first Hadean zircon documented from any of the Indian cratons. This grain yielded an initial εHf of 25.30, which indicates an episode of Hadean felsic crust formation in the Singhbhum craton comparable to that proposed for the Jack Hills of the Yilgarn craton (Australia).