Pre-Deccan and Deccan Magmatism in Kutch, India: Implications of New 40 Ar/ 39 Ar Ages of Intrusions

The Deccan Traps are of global interest for their possible links to Cretaceous-Tertiary (K-T) mass extinction event and global climate change. Radiometric dating of Deccan Trap lavas and intrusions have shown that bulk of the magmatic activities occurred 65 (±1) Ma. Isotopic similarity between Deccan and Reunion lavas have been used as an evidence to suggest that Deccan magmas were supplied by the Deccan-Reunion plume-head. Kutch is located in northwest India where, based on the trajectory that Indian plate took while crossing over the Deccan-Reunion plume, the earliest Deccan lavas would be expected to erupt. Contrary to this expectation, published 40 Ar/ 39 Ar dates of igneous activities suggested that Kutch volcanics are as old as the rest of the Deccan province, even with a small "tail" eruption occurring as late as ca. 61 Ma. Kutch's geology is complicated by the fact that it is marked by old rifting events and significant neotectonic activities. Here we present four new 40 Ar/ 39 Ar ages of igneous intrusions from Kutch. Three such intrusions occur in the so-called 'Island Belt' that juts out of the great salt flats (Great Rann of Kutch) in northern Kutch. The fourth intrusion is a gabbroic body that occurs at Dhar Dongar in southern Kutch. Two of the Island Belt intrusions (Sadara Sill and Nir Wandh gabbro) give ages of ca. 75 Ma. A lamprophyre dike from the Island Belt gave an age of ca. 67 Ma. The fourth intrusion from Kutch Mainland yielded the youngest age found in Kutch of ca. 61 Ma. These ages, combined with those determined in previous studies (65-67±1 Ma), suggest the following: Earliest rift-related magmatism occurred 75 Ma, and is clearly of pre-Deccan age. The rest of the igneous activities occurred in two episodes - the most voluminous episode coincided with Deccan age (65-67 Ma) whereas a small volume igneous activity took place at ca. 61 Ma, following a 6 m.y. long hiatus. The 61 Ma episode appears to occur in many different locations within the Deccan. We suggest that the 75 Ma pre-Deccan rifting-magmatic event is a relict of magmatism that occurred during separation of Madagascar from India, which was caused by the Marion plume.