40Ar- 39Ar 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 (̃13°N), were originally interpreted as a distant outlier of the ̃65 Ma Deccan volcanic province of west-central India, comprising dominantly flood basalts. Later the SMI volcanics were dated at ̃93 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 40Ar- 39Ar dating of the SMI volcanics yields excellent plateau and isochron ages, and their weighted mean isochron age is 85.6±0.9 Ma (2 σ). 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 ̃85 Ma, but again with the K-Ar technique. However, this age for the SMI volcanics also corresponds excellently with 40Ar- 39Ar ages of ̃89-85 Ma (weighted mean isochron age 87.6±1.2 Ma, 2 σ: equivalent to 88.1±1.2 Ma corresponding to MMhb-1 age of 523.1±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 ̃88 Ma.