Study of Deccan Basalts.

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The origin and the mode of emplacement of the Deccan plateau basalt, which is one of the largest continental flood basalts in the world, are not yet understood well. The purpose of our research is to clarify its origin based on the petrological, geological and geochemical studies. We have conducted preliminary field survey on the Deccan flood basalt at January, 1991. Target area of this year were Jabalpur massif which is on the northeaster part of the plateau, and the Indore section where the Narmada river cut through the Deccan basalt. We also made preliminary survey on a route of the western Gharts where Deccan basalts form cliff of 2000m high and is believed the center of the eruption..Because the Jabalpur massif is the northeastern termination of the Deccan plateau, it has been believed that the thickness of the lava pile is quite thin. During the present survey, we collected samples from 17 lava sequences. Existence of lava piles exceeding 17 flows indicates the total thickness of … More the plateau basalt in this region is not so thin. Considering the results of the gravity survey made by the team of the Geological Survey of India, we can expect the thickness of the Jabalpur massif could be maximum at Narmada river, and it becomes thinner towards north and also towards south. Dike swarms trending E-W are recognized to the east and west of the massif, where basement rocks are exposed due to erosion. These lines of evidence may suggest that the Jabalpur massif was formed though the fissure eruptions in the graven where the Narmada river is running now, and that the lava flows were issued both northward and southward filling the graven.The lava flows in this massif are mainly simple flow, and some compound flows are also found through this expedition. The compound flows in this massif are observed at the edge or at the top of a thick simple flow, indicating the mode of emplacement of the lava flow is different from the Western Gharts where thick piles of compound flows are common. We also recognized pillow lavas and hyaloclastites, which indicate the existence of lakes or large rivers while the Deccan basalts are formed.We also realized that some lava flows which are characterized by giant phenocryst of plagioclase could be traced more than 70 km as single flow and that they do not have any basal crinkers at all. This may indicate those lavas are quite fluidal while they were flowing. Careful observation of the giant phenocrysts led us to the conclusion that they could be formed by rapid growth rather than slow crystallization. From these observations, we came to the working hypothesis that the magma was super-heated at its eruption and that the giant plagioclase phenocryst were crystallized rapidly while the magma flooded over the surface. Assuming the eruption of the super-heated fluidal magma, the existence of the quite wide distribution of lava flows can be understood. This working hypothesis must be tested through detailed geological and petrological studies.We have collected samples from more than 25 sequential lava flows at the Indore section. Also several samples were collected from a single lava flow in order to study the within-flow variations of bulk compositions, petrography and mineralogy. This kind of study is necessary to establish the method correlating remote lava flows based on the bulk chemistry.Several samples from the dike swarms, which are exposed to the south of Indore, were also obtained during this expedition. Some of the dikes contain xenoliths probably derived from middle crust, and they could give some constraints on the mechanism of the magma ascent and the chemical reactions between magma and crustal materials.We are now analyzing the samples collected during this expedition. Because the basalts from the region we studied this year have not been characterized at all, the results will give more precise image on the nature of the Deccan basalts. Less