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

Abstract

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

Crustal velocity structure of the Neoarchean convergence zone between the eastern and western blocks of Dharwar Craton, India from seismic wide-angle studies

“…Other examples include: the volcanics of St Mary's island and dykes along the western coast of India (Figs. 1 and 2) (Kumar et al, 2001;Pande et al, 2001;Radhakrishna and Joseph, 2012). In Madagascar, late Cretaceous volcanics and intrusives outcrop along ∼1500 km stretch on the east coast which suggests a rifted continental margin .…”

Sedimentation record in the Konkan–Kerala Basin: implications for the evolution of the Western Ghats and the Western Indian passive margin

“…Ma (Pande et al, 2001), and hence are contemporaneous with Marion hot-spot magmatism (Joseph & Nambiar, 1996). Accordingly, basin initiation is likely to have occurred at 88 Ma, the time of IndiaMadagascar rifting and during the peak of Marion hotspot volcanism (Storey, 1995); nevertheless, the bulk of sediments in the Konkan-Kerala basin appear to have been deposited during the Cenozoic.…”

Were the Deccan Flood Basalts Derived in Part from Ancient Oceanic Crust Within the Indian Continental Lithosphere?

“…Anil Kumar et al, 2001;Pande et al, 2001). Still earlier, the Rajmahal-Sylhet CFB formed at ~116-120 Ma, about 10 million years after the breakup of eastern Greater India from Australia (e.g., Kent et al, 1997).…”