PALEOMAGNETISM AND AR-36 AR-40 VS AR-39/AR-40 ISOTOPE CORRELATION AGES OF DYKE SWARMS IN CENTRAL KERALA, INDIA - TECTONIC IMPLICATIONS/

Palaeomagnetic results and whole-rock Ar-36/Ar-40 vs. Ar-39/Ar-40 isot ope correlation ages are presented for dyke swarms occurring within th e granulite terrain in central Kerala (west coast of India). Palaeomag netic data show that the NNW-SSE leucogabbro dykes sampled are normall y magnetised (D = 307 and I = -57), whereas most NW-SE dolerite dykes are reversely magnetised(D = 163 and I = 61). One dolerite dyke shows normal polarity. The present Ar-40/Ar-39 isotope ages for dolerite dyk es and previously reported K-Ar ages for leucogabbros suggest 81 +/- 2 Ma for acquisition of the leucogabbro directions and 69 +/- 1 Ma for acquisition of the reverse magnetic directions within the dolerites. B ased on palaeomagnetic and isotopic age data, the dolerite dykes appea rs to represent feeder systems for the Deccan volcanic sequences. We s uggest that the Deccan eruptions were widespread, and probably extende d southward at least into central Kerala. The entire magmatic episode may have occurred between ca. 70 and 65 Ma (during the 31R period prob ably corresponding to the Cretaceous/Tertiary boundary). The leucogabb ros may correspond to the 34N superchron and represent a plume-related magmatic episode precursory to the main volcanic event. These data su ggest that an early intraplate plume tectonic setting (since ca. 80 Ma ) preceded lithospheric thinning, asthenospheric upwelling and consequ ent massive flood volcanism during the period 70-65 Ma. This culminate d in ocean-floor spreading between the Seychelles-Mascarene and contin ental India.