Study of Deccan Basalts.

德干高原玄武岩是世界上最大的大陆溢流玄武岩之一，其成因和侵位方式尚不清楚。我们研究的目的是在岩石学、地质学和地球化学研究的基础上阐明其成因。1991年1月，我们对德干洪水玄武岩进行了初步的野外考察。今年的目标区域是位于高原东北部的Jabalpur地块，以及纳尔马达河穿过德干玄武岩的印多尔地段。我们还对西加尔特山脉的一条路线进行了初步调查，这里是德干玄武岩形成的2000m高的悬崖，被认为是喷发的中心，由于贾巴尔普尔地块是德干高原的东北端，因此认为熔岩堆的厚度很薄。在本次调查中，我们采集了17个熔岩序列的样品。超过17个流量的熔岩堆的存在表明…的总厚度此外，该地区的高原玄武岩并不那么薄。考虑到印度地质调查局团队的重力测量结果，我们可以预计，在纳尔马达河，贾巴尔普尔地块的厚度可能最大，并且向北和向南变薄。在地块的东部和西部识别出东西走向的岩墙群，那里的基底岩石因侵蚀而裸露。这些证据可能表明，Jabalpur地块是通过纳尔玛达河流经的墓穴中的裂隙喷发形成的，熔岩流是向北和向南流动的，填充了墓地。该地块中的熔岩流动主要是简单流动，在这次考察中也发现了一些复合流动。该地块中的复合流是在厚的简单流的边缘或顶部观察到的，这表明熔岩流的侵位方式不同于西加特地区常见的厚厚的复合流堆积。我们还发现了枕状熔岩和透明碎屑岩，它们表明在德干玄武岩形成时存在湖泊或大河流。我们还认识到，一些以巨大的斜长石斑晶为特征的熔岩流可追溯到70多公里的单一流，它们根本没有任何基底折皱。这可能表明这些熔岩在流动时是相当流畅的。仔细观察巨型斑晶，我们得出的结论是，它们可能是快速生长而不是缓慢结晶形成的。从这些观察中，我们得出了一个可行的假设，即岩浆在喷发时过热，当岩浆溢出地表时，巨大的斜长石斑晶迅速结晶。假设过热的流体岩浆喷发，可以理解熔岩流分布相当广泛的存在。这一工作假说必须通过详细的地质学和岩石学研究来验证。我们从印多尔地区25多个连续熔岩流中收集了样本。此外，还从单一熔岩流中采集了几个样品，以研究熔岩流内主体成分、岩石学和矿物学的变化。这类研究对于建立基于整体化学的远程熔岩流动对比方法是必要的。在这次考察中，还从暴露在印多尔以南的岩墙群中获得了几个样品。其中一些岩脉含有可能来自中地壳的包体，它们可能对岩浆上升的机制以及岩浆与地壳物质之间的化学反应产生一些制约。我们正在分析这次考察中采集的样品。由于我们今年研究的地区的玄武岩还没有被表征，所以结果将给出更准确的德干玄武岩性质的图像。较少

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