Evolution of the Amba Dongar Carbonatite Complex: Constraints from 40Ar-39Ar Chronologies of the Inner Basalt and an Alkaline Plug

Abstract

The Amba Dongar carbonatite-alkaline complex is one of several alkaline complexes present within the Chhota Udaipur subprovince of the Deccan flood basalt province, western India. Despite previous geochronological studies, the evolutionary history of this complex had remained uncertain due to lack of precise age data for the tholeiitic basalts that are exposed within and outside the ring dike of the complex. Also, ages of many alkaline plugs of the subprovince had remained unknown. Here we report results of 40Ar-39Ar dating of the basalt exposed inside the ring dike at Amba Dongar and phonolite from an alkaline plug located at Tawa at the northernmost edge of the subprovince; the analyses yield plateau ages of 68.5 ± 0.9 and 65.2 ± 0.7 Ma, respectively. These results not only suggest that the basalts present inside the Amba Dongar complex predate the complex itself, but also confirm the earlier view that all the alkaline activity in the Chhota Udaipur subprovince was contemporaneous. This in turn has significant implications for the contributions of CO2 and SO2 into the atmosphere at the time of the Cretaceous-Tertiary boundary events.