Geological Study of Deccan Flood Basalts

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Field surveys at Deccan Plateau were carried out in three seasons ; January, 1993, October, 1993 and December, 1993 to January, 1994. During the first field season, east-west traverse of the region of Deccan flood basalt was carried out, and the basaltic agglutinates were recognized in the eastern portion of the plateau. This indicates that the eruption center of the flood basalt is not restricted at the western portion of the Deccan plateau. During the second field season, plateau basalt lava was found near Panna area, where no basaltic activity have not been described. This indicates that the distribution of the Deccan flood basalt should be revised to the further north and the erupted volume could be much larger than ever estimated.During the third field season, two different trends of dikes were recognized ; E-W and N-S. They sometimes cut each other, indicating the two different fissure systems were available during the Deccan plateau volcanism.More than 300 samples were analyzed with XRF. Most striking feature is that the magnesian basalt which contains more than 9wt % of MgO contains accumulated olivine phenocrysts and that the liquid composition should be less magnesian ; less than 7wt % of MgO. This indicates that the picrite primary magma cannot be the parental magma for the Deccan flood basalt. Also, the chemical trend within the Deccan flood basalt cannot be explained by simple fractional crystallization, but can be explained several different basaltic parental magmas with different alkali and titanium contents. Some of the primitive basaltic rocks were found to show chemical feature which resemble the partial melt of MORBs at high pressure. This may suggest that the source materials for the Deccan flood basalt may not be mantle peridotite, but MORBs which had subducted into the mantle in the much older time.