Geological Study of Deccan Flood Basalts

1993年1月、1993年10月和1993年12月至1994年1月在德干高原进行了三个季节的野外调查。在第一个野外季节，对德干泛滥玄武岩区域进行了东西向的穿越，并在高原的东部发现了玄武岩胶结物。这表明溢流玄武岩的喷发中心并不局限于德干高原西部。在第二个野外季节，在潘纳地区附近发现了高原玄武岩熔岩，那里没有玄武岩活动。这表明，德干溢流玄武岩的分布应修订到更北的地方，喷发量可能比以往估计的要大得多。在第三个野外季节，识别出两种不同的岩脉趋势：E-W和N-S。它们有时会相互切割，表明在德干高原火山活动期间，两种不同的裂缝系统是可用的。最显著的特征是，含MgO大于9wt%的镁玄武岩含有积累的橄榄石斑晶，并且液体成分应较少含镁; MgO小于7wt%。这表明苦橄岩原生岩浆不可能是德干溢流玄武岩的母岩浆。此外，内的德干洪水玄武岩的化学趋势不能解释简单的分离结晶，但可以解释几个不同的玄武岩母岩浆具有不同的碱和钛含量。一些原始玄武质岩石显示出类似于MORB在高压下部分熔融的化学特征。这可能表明，德干溢流玄武岩的源区物质可能不是地幔橄榄岩，而是在更古老的时间内俯冲到地幔中的MORB。

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.