Ar-40/Ar-39 dating of mineral separates and whole rocks from the Western Ghats lava pile: further constraints on duration and age of the Deccan traps

There has been an ongoing debate for almost two decades on the age and dura tion of Deccan trap volcanism. Some of the best available ages have been de termined using the Ar-40/Ar-39 method, but differences in sample location, laboratory procedure, type of analyzed material, and ages of monitors may h ave been responsible for much of the apparent spread in the data. We have c ollected and analyzed samples from dose to the bottom and top of the main l ava pile in the Western Ghats near Bombay. We confirm previous findings tha t alteration and Ar recoil strongly affect these rocks, particularly measur ements made on whole rocks. When mineral separates (mostly plagioclase) are analyzed, the spread is much reduced. We find a mean age of 65.4 +/- 0.7 M a for five flows near the base of the section (Jahwar and Igatpuri formatio ns) and 65.2+/-0.4 Ma for a dyke cross-cutting the Poladpur formation, belo w the topmost Mahabaleshwar formation. This implies that at least 1800 m ou t of the 2500 m composite trap section were erupted in a short interval, le ss than 1 Ma long and close to 65.5 Ma, i.e. close to the Cretaceous-Tertia ry boundary. We find that older ages (near 67 Ma) found by several authors are essentially based on whole rock samples, which display disturbed spectr a (when this can be checked, i.e, when the spectra are published). (C) 2000 Elsevier Science B.V. All rights reserved.