40Ar/39Ar Geochronology of the Rajmahal Basalts, India, and their Relationship to the Kerguelen Plateau

Abstract

During the mid-Cretaceous, extensive magmatism occurred in the Indian Ocean to form volcanic portions of the southern and central Kerguelen Plateau, Elan Bank and Broken Ridge. Basalt was erupted also along the rifted margin of eastern India (Rajmahal). We investigated the ages of these Indian basalts using Ar-40/Ar-39 incremental-heating experiments on whole rocks. Our results are consistent with the hypothesis that the lava pile of similar to230 m thickness in the Rajmahal Hills, Jharkhand, and alkalic basalts in the Bengal Basin were emplaced at similar to118 Ma. Dykes intruded to the SW of the Rajmahal Hills appear to be 2-3 Myr younger than these lavas. Magmatic activity in eastern India therefore was contemporaneous with the final stage of volcanism at Ocean Drilling Program Site 1136 on the Southern Kerguelen Plateau (119-118 Ma), but older than final magmatism at Sites 749 and 750 on the Southern Kerguelen Plateau (112-110 Ma), Site 1137 on Elan Bank (108 Ma) and Site 1138 on the Central Kerguelen Plateau (100 Ma). By combining these age data with plate reconstructions that take into account the motion of hotspots in a convecting mantle, we suggest that eruption of the Rajmahal basalts, formation of the Southern Kerguelen Plateau, and Elan Bank's separation from India are best explained by the presence of the Kerguelen hotspot close to the eastern Indian margin just after 120 Ma.