Recurrent Early Cretaceous, Indo-Madagascar (89–86 Ma) and Deccan (66 Ma) alkaline magmatism in the Sarnu-Dandali complex, Rajasthan: 40Ar/39Ar age evidence and geodynamic significance

Abstract

The Sarnu-Dandali alkaline complex in Rajasthan, northwestern India, is considered to represent early, pre-flood basalt magmatism in the Deccan Traps province, based on a single 40Ar/39Ar age of 68.57Ma. Rhyolites found in the complex are considered to be 750Ma Malani basement. Our new 40Ar/39Ar ages of 88.9–86.8Ma (for syenites, nephelinite, phonolite and rhyolite) and 66.3±0.4Ma (2σ, melanephelinite) provide clear evidence that whereas the complex has Deccan-age (66Ma) components, it is dominantly an older (by ~20millionyears) alkaline complex, with rhyolites included. Basalt is also known to underlie the Early Cretaceous Sarnu Sandstone. Sarnu-Dandali is thus a periodically rejuvenated alkaline igneous centre, active twice in the Late Cretaceous and also earlier. Many such centres with recurrent continental alkaline magmatism (sometimes over hundreds of millions of years) are known worldwide. The 88.9–86.8Ma 40Ar/39Ar ages for Sarnu-Dandali rocks fully overlap with those for the Indo-Madagascar flood basalt province formed during continental breakup between India (plus Seychelles) and Madagascar. Recent 40Ar/39Ar work on the Mundwara alkaline complex in Rajasthan, 120km southeast of Sarnu-Dandali, has also shown polychronous emplacement (over ≥45millionyears), and 84–80Ma ages obtained from Mundwara also arguably represent post-breakup stages of the Indo-Madagascar flood basalt volcanism. Remnants of the Indo-Madagascar province are known from several localities in southern India but hitherto unknown from northwestern India 2000km away. Additional equivalents buried under the vast Deccan Traps are highly likely.