A Post K–T Boundary (Early Palaeocene) Age for Deccan-type Feeder Dykes, Goa, India

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Abstract
Basaltic dykes exposed along the coast of Goa represent the youngest phase of a number of different mafic suites intruding the complex Precambrian terrain that forms the pre-Deccan basement of peninsular India. These dykes crop out ∼50–80 km beyond the southern limits of the Deccan continental flood basalt (CFB) province. Four of seven sampled dykes were dated using the whole-rock 40Ar/39Ar step-heating technique. Three 40Ar/39Ar determinations yield Palaeocene plateau and isochron ages that clearly indicate an early post-Cretaceous (i.e. Danian) age and, as a weighted mean average of all four samples yields an age of 62·8 ± 0·2 Ma, these intrusions are clearly significantly younger than the current estimates for the age range of Deccan Traps volcanism (∼65–69 Ma). The trace element signatures, rare earth element patterns and 87Sr/86Sr ratios of these dykes are consistent with those of the nearby Deccan basalts. More specifically, their geochemical signatures are similar to those of the basalt flows composing the volumetrically important Ambenali–Mahabaleshwar Formations, which form the uppermost succession (Wai Subgroup) of the SW Deccan. These geochemical and age data demonstrate that Deccan-type magmatism continued to affect western India for at least 1–2 my after the K–T boundary (65·0 ± 0·1 Ma) and, therefore, their emplacement places important constraints upon the timing, duration and evolution of this much-debated CFB province. The importance of these dykes is examined within the context of the Deccan eruptions and the associated rifting of the western Indian margin.