(a) The newborn toe of a compound pahoehoe basalt flow that has emerged from under the solidified lava crust, as a “breakout”. Map of the Dediapada dyke swarm in the Narmada-Satpura region of the Deccan (after Krishnamacharlu, 1970).The front is about 1 m from the camera and 0.5 m wide. This is how I believe the compound pahoehoe flows of the Deccan were emplaced. This is one of the large, spectacular oriented dyke swarms of the Deccan Traps.Nevertheless, the following observations and deductions suggest that the plume model is not valid for the Deccan (, 1999a,b, 2005a). Map showing the approximate boundaries of the Precambrian cratons making up the Indian shield (e.g., Pandey & Agrawal, 1999; Naqvi & Rogers, 1987), the granulite terrain, the Precambrian structural trends (heavy broken lines), rift zones crossing peninsular India (e.g., Biswas, 1987), and the present outcrop areas of the Deccan and Rajmahal flood basalts.Inset shows the breakup of the Seychelles microcontinent, situated along the northern tip of the Mascarene Plateau (black), from India, soon after the Deccan flood basalt episode (after Norton and Sclater, 1979; Mahoney, 1988).(2002), as also the modelled hotspot track showing expected ages in Ma.Note the rift zones underlying the Deccan, and the absence of any triple junction.
The overall appearance of the Deccan, with its roughly circular outcrop, and the linear Laccadives-Chagos (more correctly, Lakshadweep-Chagos) Ridge to the south of India, looks very much like what is expected for a spherical plume head and a narrow plume tail (Figures 2 & 3).The Deccan flood basalt sequence is best developed in the Western Ghats region with ~3 km of stratigraphic thickness (Figures 1-3), and picritic basalts are found there, but these are enriched in cumulus olivine and clinopyroxene and do not represent liquid compositions.The parental melts of these picrites are estimated to have contained only ~ 9-10% Mg O ( Figure 4. Note the complete absence of compositions other than basalt and basaltic andesite, and the nearly exclusive subalkalic (tholeiitic) nature.(a) A plot of 624 samples of Deccan basalts of the Western Ghats (data of Beane, 1988; courtesy J. Dividing lines between alkalic and subalkalic fields proposed by Macdonald & Katsura (1964) and Irvine & Baragar (1971) are also shown.(b) Plot of the same samples on the familiar AFM diagram, showing the Fe enrichment trend typical of tholeiitic basalts.