Late Cretaceous alkaline sills of the south Tethyan suture zone, Pakistan: Initial melts of the Réunion hotspot?

Abstract

During the closure of the Tethyan Ocean, Triassic to early Jurassic marine sediments containing alkali dolerite and ultramafic lamprophyre sills were accreted to the margin of Greater India. The 69.7 ± 0.2 Ma weighted mean 40Ar- 39Ar plateau age of the sills, their location, and their incompatible element and Nd–Pb–Sr isotopic composition, which is similar to that of recent Réunion shield lavas, are consistent with their derivation from the Réunion hotspot. Mantle melt modelling suggests that the alkali dolerites are the result of < 3% melting of a moderately incompatible element-depleted, garnet-bearing source. However, some of the ultramafic lamprophyres have lower heavy rare-earth element, Nb and Zr contents that cannot be explained by melting of the same mantle source as the alkali dolerites. This unusual feature can be explained by small-degree melting of a metasomatised mantle source containing small amounts (< 0.02 vol.%) of residual zircon. Isotopic signatures of the ultramafic lamprophyres are similar to those of the alkali dolerites, implying that metasomatism was not significantly older than the melting event that formed the sills. The metasomatism may have been due to small-degree melting and re-solidification at the leading edge of the starting head of the Réunion plume. This alkaline magmatism may represent the arrival of the Réunion plume at the base of the Indian lithosphere several million years before the onset of the main phase of Deccan flood basalt volcanism.