Palaeomagnetism and ³⁶Ar/⁴⁰Ar vs.³⁹Ar/⁴⁰Ar isotope correlation ages of dyke swarms in central Kerala, India: tectonic implications

Abstract

Palaeomagnetic results and whole-rock ³⁶Ar/⁴⁰Ar vs. ³⁹Ar/⁴⁰Ar isotope correlation ages are presented for dyke swarms occurring within the granulite terrain in central Kerala (west coast of India). Palaeomagnetic data show that the NNW-SSE leucogabbro dykes sampled are normally 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 ⁴⁰Ar/³⁹Ar isotope ages for dolerite dykes 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. Based on palaeomagnetic and isotopic age data, the dolerite dykes appears to represent feeder systems for the Deccan volcanic sequences. We suggest that the Deccan eruptions were widespread, and probably extended southward at least into central Kerala. The entire magmatic episode may have occurred between ca. 70 and 65 Ma (during the 31R period probably corresponding to the Cretaceous/Tertiary boundary). The leucogabbros may correspond to the 34N superchron and represent a plume-related magmatic episode precursory to the main volcanic event. These data suggest that an early intraplate plume tectonic setting (since ca. 80 Ma) preceded lithospheric thinning, asthenospheric upwelling and consequent massive flood volcanism during the period 70-65 Ma. This culminated in ocean-floor spreading between the Seychelles-Mascarene and continental India.