40Ar/39Ar dating of the Rajahmundry Traps, Eastern India and their relationship to the Deccan Traps

Rajahmundry lava flows exposed along India’s eastern coast have long been suggested to be outliers of the more massive Deccan Traps despite the distance of more than 400 km separating their present-day erosional remnants. Recent rock quarries in Rajahmundry Trap lavas of basaltic composition have exposed fresh surfaces available for sampling displaying clear stratigraphic relationships within the typical basalt–sediment–basalt stratigraphy. 40Ar/39Ar ages for plagioclase separates from eight Rajahmundry lavas both above (Upper Trap) and below (Lower Trap) the continuous sedimentary interlayer reveal a mean age of 64.7±0.5 Ma. Plagioclase Ca/K values are high (20–350), requiring use of finer grain sizes (63–125 μm) and well-known interference corrections for Ar isotopes produced during sample irradiation to produce the best possible ages. Individual ages confirm the rapid eruption (<2 Myr) of the Upper and Lower Rajahmundry lavas. These data indicate a lengthy hiatus between volcanism and subsequent deposition of Eocene sandstones, which locally overly the Rajahmundry sequences. These age data also place the eruption of the Rajahmundry Traps coincident with late-stage Deccan Trap volcanism. This synchronicity is consistent with new geochemical data and published paleomagnetic evidence. An unequivocal temporal connection between the Rajahmundry Traps and Deccan Traps may necessitate revision of transport mechanisms of melt and/or flow distances in the Deccan Traps.