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首页> 外文期刊>Earth and Planetary Science Letters: A Letter Journal Devoted to the Development in Time of the Earth and Planetary System >Gravity and magnetic signatures of volcanic plugs related to Deccan volcanism in Saurashtra, India and their physical and geochemical properties
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Gravity and magnetic signatures of volcanic plugs related to Deccan volcanism in Saurashtra, India and their physical and geochemical properties

机译：与印度绍拉什特拉德干火山活动有关的火山岩塞的重力和磁性特征及其物理和地球化学性质

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摘要

The Bouguer anomaly and the total intensity magnetic maps of Saurashtra have delineated six circular gravity highs and magnetic anomalies of 40-60 mGal (10~(-5)m/s~2)and 800-1000 nT, respectively. Three of them in western Saurashtra coincide with known volcanic plugs associated with Deccan Volcanic Province (DVP), while the other three in SE Saurashtra coincide with rather concealed plugs exposed partially. The DVP represents different phases of eruption during 65.5 ± 2.5 Ma from the Reunion plume. The geochemical data of the exposed rock samples from these plugs exhibit a wide variation in source composition, which varies from ultramafic/mafic to felsic composition of volcanic plugs in western Saurashtra and an alkaline composition for those in SE Saurashtra. Detailed studies of granophyres and alkaline rocks from these volcanic plugs reveal a calc-alkaline differentiation trend and a continental tectonic setting of emplacement. The alkaline plugs of SE Saurashtra are associated with NE-SW oriented structural trends, related to the Gulf of Cambay and the Cambay rift basin along the track of the Reunion plume. This indicates a deeper source for these plugs compared to those in the western part and may represent the primary source magma. The Junagadh plug with differentiated ring complexes in western Saurashtra shows well defined centers of magnetic anomaly while the magnetic anomalies due to other plugs are diffused though of the same amplitude. This implies that other plugs are also associated with mafic/ultramafic components, which may not be differentiated and may be present at subsurface levels. Paleomagnetic measurements on surface rock samples from DVP in Saurashtra suggest a susceptibility of 5.5 * 10~(-2) SI units with an average Koenigsberger ratio (Q_n) of almost one and average direction of remanent magnetization of D = 147.4°and I = +56.1°. The virtual geomagnetic pole (VGP) position computed from the mean direction of magnetization for the volcanic plugs and Deccan basalt of Saurashtra is 30°N and 74°W, which is close to the VGP position corresponding to the early phases of Deccan eruption. Modeling of gravity and magnetic anomalies along two representative profiles across Junagadh and Barda volcanic plugs suggest a bulk density of 2900 and 2880 kg/m~3, respectively and susceptibility of 3.14 * 10~(-2) SI units with a Q_n ratio of 0.56 which are within the range of their values obtained from laboratory measurements on exposed rock samples. The same order of gravity and magnetic anomalies observed over the volcanic plugs of Saurashtra indicates almost similar bulk physical properties for them. The inferred directions of magnetization from magnetic anomalies, however, are D = 337°and 340°and I = -38°and -50°which represent the bulk direction of magnetization and also indicate a reversal of the magnetic field during the eruption of these plugs. Some of these plugs are associated with seismic activities of magnitude ≤4 at their contacts. Based on this analysis, other circular/semi-circular gravity highs of NW India can be qualitatively attributed to similar subsurface volcanic plugs.
机译：Saurashtra的布格异常和总强度磁图分别描绘了六个圆形重力高点和40-60 mGal（10〜（-5）m / s〜2）和800-1000 nT的磁异常。索拉什特拉西部的其中三个与已知的德干火山省（DVP）相关的火山岩相吻合，而东南索拉什特拉的其他三个与部分隐藏的相当隐蔽的相吻合。 DVP代表来自团聚羽的65.5±2.5 Ma内喷发的不同阶段。来自这些岩塞的裸露岩石样品的地球化学数据显示，其源组成变化很大，从萨澳拉什特拉西部的火山岩塞的超镁铁质/镁铁质到长英质成分，而东南苏拉什特拉的则为碱性成分。对来自这些火山岩层的花岗岩和碱性岩石的详细研究揭示了钙碱性的分化趋势和大陆构造的位置。 SE Saurashtra的碱性栓塞与NE-SW导向的结构趋势有关，与沿着团聚流的轨迹的坎贝湾和坎贝裂谷盆地有关。这表明与西部地区相比，这些岩浆的来源更深，可能代表了主要岩浆。在索拉什特拉西部具有不同环复合物的Junagadh塞显示出定义良好的磁异常中心，而其他塞子引起的磁异常则以相同的幅度扩散。这意味着其他塞子也与铁镁质/超音波组分相关联，这些组分可能没有区别，并且可能存在于地下层。在Saurashtra DVP的地表岩石样品上进行的古磁测量表明，磁化率为5.5 * 10〜（-2）SI单位，平均Koenigsberger比（Q_n）几乎为1，平均剩余磁化方向为D = 147.4°和I = + 56.1°。根据火山塞和索拉什特拉的Deccan玄武岩的平均磁化方向计算出的虚拟地磁极（VGP）位置为30°N和74°W，该位置接近与Deccan爆发早期阶段相对应的VGP位置。沿Junagadh和Barda火山岩塞的两个代表性剖面的重力和磁异常模型表明，堆密度分别为2900和2880 kg / m〜3，磁化率为3.14 * 10〜（-2）SI单位，Q_n比为0.56其值在实验室对裸露岩石样品的测量值的范围内。在Saurashtra火山塞上观察到的重力和磁异常的顺序相同，表明它们几乎具有相似的整体物理性质。但是，从磁异常推断出的磁化方向为D = 337°和340°，而I = -38°和-50°代表了磁化的整体方向，并且还指出了在这些异常喷发过程中磁场的反转插头。这些插头中的一些与它们的触点处的≤4级地震活动有关。根据此分析，印度西北地区的其他圆形/半圆形重力高点可以定性地归因于类似的地下火山岩塞。

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