DETRITAL ZIRCONS REVEAL EVIDENCE OF HADEAN CRUST IN THE SINGHBHUM CRATON, INDIA

With five distinct cratonic nuclei, India has perhaps the highest density of unique Archean-age crustal components on Earth. Despite this fact, much of the Indian subcontinent remains understudied compared to other ancient cratons in North America, Africa, and Australia. More specifically, India-focused petrologic and geochemical research rarely hypothesizes implications for early Earth’s crustal evolution. Here we report additional evidence for the timing of cratonization and new evidence for the antiquity of crustal differentiation in the Singhbhum craton, India, based on ²⁰⁷Pb/²⁰⁶Pb age up to 4.02 Ga and Eoarchean and Hadean Lu-Hf model ages of detrital zircons from modern river sediment.

The Singhbhum craton consists of four major lithotectonic units and supracrustal sequences with broadly defined ages from Eoarchean to Neoproterozoic. The major units include the Older Metamorphic Group (OMG), the Older Metamorphic Tonalite Gneisses (OMTG), the Singhbhum Granite, and the Iron Ore Group (IOG – divided into Older and Younger portions) with overlapping ages of 3.8-3.2 Ga, 3.8-3.2 Ga, 3.5-3.0 Ga (with a late intrusion at 0.9 Ga), 3.51 Ga, and 3.0-2.55 Ga, respectively. In this study, 194 zircons were laser ablated using multi-collector ICP-MS for ²⁰⁷Pb/²⁰⁶Pb ages resulting in <10% discordance in 68% of analyses and <5% discordance in 61% of analyses. Three primary groupings are evident with ages 3.62-3.55 Ga (7%), 3.50-3.22 Ga (83%), and 3.13-3.03 Ga (9%). The 3.50-3.22 Ga range includes ages proposed for Phase I & II intrusions of the Singhbhum granite. Lu-Hf measurements for the least discordant grains yielded 63% positive initial ϵHf values, indicating contributions from both a depleted mantle and crustal sources throughout this time. Of note is one zircon with a ²⁰⁷Pb/²⁰⁶Pb age of 4.015 Ga, which is the first zircon of Hadean age documented in India. This grain displays an ϵHf of -5.30, buttressing previous findings of earliest crust from the Jack Hills region, Australia, and suggesting another locale for earliest crust formation.