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 (2σ). 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, 2σ: 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.