Ar-40-Ar-39 age of a national geological monument: The Gilbert Hill basalt, Deccan Traps, Bombay

The Deccan Trap geology of Bombay (Mumbai) differs from the main Deccan flo od basalt province in several ways. Very few geological, geochemical and ge ochronological studies exist on the Deccan geology of Bombay. The basalt of Gilbert Hill, Andheri occupies a special place in Bombay geology on accoun t of its spectacular columnar jointing, more than 50 m high, and for this h as been designated a National Geological Monument by the Geological Survey of India. We have obtained an Ar-40-Ar-39 plateau age of 60.5 +/- 1.2 Ma (2 sigma) for the Gilbert Hill basalt, and this basalt can be argued to have erupted considerably later (similar to 6 m.y.) than the lower part of the W estern Ghats lava pile, and the total duration of Deccan volcanism as a who le appears to have been at least similar to 8 m.y.