Barren Island (India) is a relatively little studied, little known active volcano in the Andaman Sea, and the northernmost active volcano of the great Indonesian arc. The volcano is built of prehistoric (possibly late Pleistocene) lava flows (dominantly basalt and basaltic andesite, with minor andesite) intercalated with volcaniclastic deposits (tuff breccias, and ash beds deposited by pyroclastic falls and surges), which are exposed along a roughly circular caldera wall. There are indications of a complete phreatomagmatic tephra ring around the exposed base of the volcano. A polygenetic cinder cone has existed at the centre of the caldera and produced basalt-basaltic andesite aa and blocky aa lava flows, as well as tephra, during historic eruptions (1787-1832) and three recent eruptions (1991,. The recent aa flows include a toothpaste aa flow, with tilted and overturned crustal slabs carried atop an aa core, as well as locally developed tumuli-like elliptical uplifts having corrugated crusts. Based on various evidence we infer that it belongs to either the 1991 or the 1994-95 eruptions. The volcano has recently (2008) begun yet another eruption, so far only of tephra. We make significantly different interpretations of several features of the volcano than previous workers. This study of the volcanology and eruptive styles of the Barren Island volcano lays the ground for detailed geochemical-isotopic and petrogenetic work, and provides clues to what the volcano can be expected to do in the future.

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The felsic volcanics (rhyolites and rhyodacites) of the St. Mary's Islands (SMI), southern India (V13³N), were originally interpreted as a distant outlier of the V65 Ma Deccan volcanic province of west^central India, comprising dominantly flood basalts. Later the SMI volcanics were dated at V93 Ma by the K^Ar technique. However, this K^Ar age' was dubious, being merely an average of five out of six widely varying dates and arbitrary data selectivity being involved in this averaging. Our first 40 Ar^3 9 Ar dating of the SMI volcanics yields excellent plateau and isochron ages, and their weighted mean isochron age is 85.6 þ 0.9 Ma (2c). Interestingly, the southern Indian Precambrian terrain is intruded by numerous mafic^doleritic dyke swarms ranging in age from Proterozoic to the latest Cretaceous (69^65 Ma, Deccan-related), and indeed, two regional dykes (a leucograbbro and a felsite) from the Kerala region of southwestern India remain previously dated at V85 Ma, but again with the K^Ar technique. However, this age for the SMI volcanics also corresponds excellently with 40 Ar^3 9 Ar ages of V89^85 Ma (weighted mean isochron age 87.6 þ 1.2 Ma, 2c: equivalent to 88.1 þ 1.2 Ma corresponding to MMhb-1 age of 523.1 þ 2.6 Ma) for the Madagascar flood basalt province. Together, therefore, the Madagascar flood basalt province, the SMI volcanics, and possibly the Kerala dykes could represent volcanic activity associated with the break-up of Greater India (India plus Seychelles) and Madagascar, thought to have occurred in the Upper Cretaceous at V88 Ma. ß

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