The Deccan volcanic rocks with an eruptive age of around 65 Ma occupy a large area spanning longitude 710E to 810E in western India. Mafic dykes, common in the western and central part, are rare in the eastern part of the province (EVP). In the eastern part of the province, the thickness of the Deccan lava pile is much less compared to that in the western part (Mahabaleshwar, for example). Six lava flows with an aggregate thickness of 170 m occur in Amarkantak section in the eastern part of the province. In the adjacent Umaria section sixteen lava flows have been identified that include vesicular and massive flows. Individual flows range in thickness from 15 to 40 m. We present here petrological and geochemical data for representative samples of the mafic flows and dykes from EVP to explore the inter-relationship.
The tholeiitic basalts are fine-grained porphyritic with micro-phenocrysts of plagioclase set in a groundmass of plagioclase, clinopyroxene and glass. The dolerite dykes are coarser grained porphyritic. Olivine is present in some samples.
Both the flows and the dykes have a restricted chemical composition; SiO2 ranges from 47.1 to 50.5 per cent and TiO2 ranges from1.8 to 3.5 per cent. In a total alkali-silica diagram, the flow and the dyke samples fall in the basalt field. The dykes show fractionated chondrite normalized rare earth element patterns with small or no Eu anomalies. (La/Yb)n varies from 2.9 to 5.1 in the dykes and from 3.3 to 4.7 in the flows. La/Yb ratios show a scatter on a plot against MgO for all the flows as well as for the dykes .CaO, Cr and Ni show a positive correlation, but Al2O3, K2O, Na2O and ΣREE show negative correlation with MgO. These suggest fractionation of olivine and clinopyroxene.
In a plot of Ba/Y vs. Zr/Nb, the flows and the dykes plot near the field of Réunion and close to the MORB indicating a depleted source. The primitive mantle normalised trace element patterns for all the samples show positive Ba, Ta, Nd, and Zr spikes but negative Hf, Zr and Sr spikes. The incompatible elements are enriched compared to the primitive mantle.
87Sr/86Sr ratios in EVP basalt and dyke samples vary from 0.704344 to 0.807390. In general, the dykes have higher 87Sr/86Sr ratios than the flows. The highest 87Sr/86Sr ratio of 0.807390 is obtained in a dyke sample. There is no stratigraphic control on the 87Sr/86Sr ratio but a positive correlation with SiO2 exists particularly for the flows. The strontium isotopic composition is likely to be the result of mixing between a R¨Ļunion like composition and another radiogenic 87Sr/86Sr crustal material.