Ar-40-Ar-39 age of the St. Mary's Islands volcanics, southern India: record of India-Madagascar break-up on the Indian subcontinent

The felsic volcanics (rhyolites and rhyodacites) of the St. Mary's Islands (SMI), southern India (similar to 13 degreesN), were originally interpreted as a distant outlier of the similar to 65 Ma Deccan volcanic province of w est-central India, comprising dominantly flood basalts. Later the SMI volca nics were dated at similar to 93 Ma by the K-Ar technique. However, this K- Ar 'age' was dubious, being merely an average of five out of six widely var ying dates and arbitrary data selectivity being involved in this averaging. Our first Ar-40-Ar-39 dating of the SMI volcanics yields excellent plateau and isochron ages, and their weighted mean isochron age is 85.6 +/- 0.9 Ma (2 sigma), Interestingly, the southern Indian Precambrian terrain is intru ded by numerous mafic-doleritic dyke swarms ranging in age from Proterozoic to the latest Cretaceous (69-65 Ma, Deccan-related), and indeed, two regio nal dykes (a leucograbbro and a felsite) from the Kerala region of southwes tern India remain previously dated at similar to 85 Ma, but again with the K-Ar technique. However, this age for the SMI volcanics also corresponds ex cellently with Ar-40-Ar-39 ages of similar to 89-85 Ma (weighted mean isoch ron age 87.6 +/- 1.2 Ma, 2 sigma: equivalent to 88.1 +/- 1.2 Ma correspondi ng to MMhb-1 age of 523.1 +/- 2.6 Ma) for the Madagascar flood basalt provi nce. Together, therefore, the Madagascar flood basalt province, the SMI vol canics, and possibly the Kerala dykes could represent volcanic activity ass ociated with the break-up of Greater India (India plus Seychelles) and Mada gascar, thought to have occurred in the Upper Cretaceous at similar to 88 M a. (C) 2001 Elsevier Science B.V. All rights reserved.