Geology of the saucer-shaped sill near Mahad, western Deccan Traps, India, and its significance to the Flood Basalt Model
Geology of the saucer-shaped sill near Mahad, western Deccan Traps, India, and its significance to the Flood Basalt Model
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DOI：
10.1007/s00445-013-0731-4
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发表时间：
2013-06
影响因子：
3.5
通讯作者：
R. Duraiswami;Tahira N. Shaikh
中科院分区：
文献类型：
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作者：
R. Duraiswami;Tahira N. Shaikh
马哈德附近有一个厚约22米的碟形岩床，在德干圈闭内以曲线形微型山脊的形式出露。窗台有不同的倾角（42-55°）。长轴7.1公里，短轴5.3公里（纵横比1.4），比南非金谷岩床复合体的MV岩床和澳大利亚潘顿岩床都要大。岩床有明显的玻璃状的上部和下部冷缘，具有粗粒的高度节理化的核心。由于后期蚀变作用，边缘的样品无一例外地是破碎的和铁染的。岩床出露的岩石为斜长斑状玄武岩。至少有三个厚的窗台状隆起从主窗台的底部发出。由于弯曲和变薄，骨突改变方向，从顶部的水平整合片变为不整合的倾斜形式，再次弯曲进入下部的水平整合片。我们解释这样的功能作为“新生的碟形窗台”，没有膨胀形成嵌套窗台。地球化学上，岩床由低分化拉斑玄武岩，具有有限的地球化学范围。临界微量元素比值和原始地幔标准化的微量元素和稀土元素模式表明，岩床的地球化学亲合性的Poladpur化学类型和pahoehoe流，他们侵入属于布什组。计算的岩浆超压在岩床侵位范围为8.4至11.3 MPa（杨氏模量E = 5 GPa）和16.7至22.5 MPa（为E =10 GPa）和岩浆房的深度范围为8.5至11.5 km（E= 5 GPa）和17.1至22.9 km（E= 10 GPa），与岩石学和重力模拟一致。马哈德岩床的体积约为276 km 3，且无论主岩杨氏模量值的变化如何，其体积都是恒定的。1980年，考克斯（J Petrol 21：629-650，1980）提出了卡鲁循环流化床下方壳幔剖面的概念模型，该模型被认为是溢流玄武岩火山作用的基本模型。我们的论文证实了存在一个窗台加上推断的子结构下面的马哈德是兼容的预测模型。在LIPS中，碟形岩床形成于经历伸展构造的区域，其中分层沉积岩或熔岩流之间的Cook-Gordon脱层和Dundurs弹性伸展错配等过程导致堤坝偏转成岩床。设想用类似的方法形成马哈德岩床。
An ∼22-m-thick saucer-shaped sill occurs near Mahad and is exposed as a curvilinear, miniature ridge within the Deccan Traps. The sill has variable dips (42–55°). It has a 7.1-km long axis and 5.3 km short axis (aspect ratio of 1.4) and is larger than the MV sill of the Golden Valley sill complex, South Africa and the Panton sill, Australia. The sill has distinct glassy upper and lower chilled margins with a coarse-grained highly jointed core. The samples from the margin are invariably fractured and iron stained because of deuteric alteration. The rock from the sill is plagioclase-phyric basalt. At least three thick sill-like apophyses emanate from the base of the main sill. The apophyses change direction because of bending and thinning from a horizontal concordant sheet at the top to a discordant inclined form that bends again to pass into a lower horizontal concordant sheet. We interpret such features as ‘nascent saucer-shaped sills’ that did not inflate to form nested sills. Geochemically, the sill consists of poorly differentiated tholeiitic basalt that has a restricted geochemical range. Critical trace element ratios and primitive mantle normalised trace and REE patterns indicate that the sills have geochemical affinities to the Poladpur chemical type and that the pahoehoe flow they intrude belongs to the Bushe Formation. Calculated magmatic overpressures during sill emplacement range from 8.4 to 11.3 MPa (for Young’s modulusE= 5 GPa) and 16.7 to 22.5 MPa (forE=10 GPa) and depth to magma chamber ranges from 8.5 to 11.5 km (E= 5 GPa) and 17.1 to 22.9 km (E= 10 GPa), consistent with petrological and gravity modelling. The volume of the Mahad sill is approximately 276 km3and is constant irrespective of the variations in the values of host-rock Young’s modulus. In 1980, Cox (J Petrol 21:629–650, 1980) proposed a conceptual model of the crust–mantle section beneath the Karoo CFB which is considered as the fundamental model for flood basalt volcanism. Our paper confirms the presence of a sill plus the inferred substructure beneath Mahad that are compatible with predictions of that model. In LIPS, saucer-shaped sills are formed in areas experiencing extensional tectonics where processes such as the Cook–Gordon delamination and Dundurs elastic extensional mismatch between layered sedimentary rocks or lava flows are responsible for the deflection of dykes into sills. A similar process is envisaged for the formation of the Mahad sill.