Meteorite impact at Ramgarh, India: Petrographic and geochemical evidence, and new geochronological insights

Abstract

The impact origin of Ramgarh crater in India has been a topic of debate for several decades due to its conspicuous morphology, with recent studies confirming the structure as a complex impact crater. In this work, we present the results from an integrated petrographic and geochemical study to evaluate the impact origin of the crater, in conjunction with introduction of a new geochronological data for the crater through zircon U Pb systematics. Petrographic studies of the target Vindhyan sedimentary rocks show the presence of shock fabrics such as planar fractures, decorated planar deformation features, brecciation, melted grain boundaries, diaplectic glass, and vitrified matrix. Geochemically, the lithounits have high concentrations of Cr, Ni, Cu, and the corresponding chondrite normalized values show notably elevated levels than crustal average. The elevated levels of Cr, Ni and Cu in the target rocks indicate its interaction with an extra-terrestrial object. The positive correlation between Ni and Co, higher concentration of Cr (9–623.35 ppm), and more importantly the average Ni/Cr (~0.006- ~ 1.3), Ni/Co (~1.8 to ~22) and Cr/Co (~57–~368) ratios in the target lithounits of Ramgarh, point to a differentiated achondrite being the possible impactor. The zircon U Pb geochronological data from two samples in this study do not show significant Pb-loss and U enrichment. However, the relatively younger age of 528–395 Ma, depicted by quartzite sample (AN-4), along with the discordance and distinguishable Pb-loss, might hint that the impact event at Ramgarh have occurred between this timeframe. This age is considerably closer to the Precambrian-Cambrian Boundary (543 Ma) and, it also encompasses the age corresponding to the mid-Ordovician Meteor Event (OME) (467.5 ± 0.28 Ma). As this study infers that the potential impactor at Ramgarh is a differentiated achondrite and not a L-chondrite, the OME age can be ignored, further narrowing the age of Ramgarh impact structure. Our study also confirms the Ramgarh structure as an impact crater, and constrains the possible timing of the impact. • Tracking the impact event at Ramgarh Crater, NW India • Shock features in quartz grains mainly includes decorated PDFs, and melting. • Whole rock, trace and REE geochemistry reveals impact event. • Zircon U Pb ages of 528 to 395 Ma, which is the first convincing age for this crater