古气候对西非加纳南部古生代页岩组成的控制作用

D. Asiedu, D. Atta‐Peters, E. Hegner, A. Hegner, T. Shibata
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引用次数: 4

摘要

加纳南部晚奥陶世至早白垩世Sekondian群由7个主要沉积地层组成。Sekondian群的一个重要特征是在最低的两个地层(即Ajua页岩和Elmina砂岩)和最高的地层(即Essikado砂岩)中有大量相当新鲜的斜长石;但在集团的其他成员中却几乎不见踪影。Sekondian群的基底部分被解释为冰川起源,而上部地层被认为是在较温暖的阶段形成的。因此,在最低的两个地层中,丰富的斜长石可能是由于不太普遍的化学风化,而不是由于一个独特的来源的侵蚀。本文利用Ajua页岩和Takoradi页岩(位于Elmina砂岩之上)的细粒沉积岩地球化学资料,研究了古风化作用对Sekondian群组成的影响。两组样品中过渡微量金属Cr、Ni、V和Cu的含量以及Cr/V-、Zr/Y-和Ba/ co的比值相似,表明它们主要来源于侵入Birimian表壳岩石的花岗岩类。而Takoradi页岩样品中Rb、Cs、Th、U、Nb元素相对于Ajua页岩样品富集,这可能与烃源岩风化程度的差异有关。Takoradi页岩样品具有较高的蚀变指数(CIA)值和Rb/Sr比值(CIA > 75);Rb/Sr > 1),表明烃源岩风化程度高。相比之下,Ajua页岩样品的CIA值和Rb/Sr比值(CIA)较低
本文章由计算机程序翻译,如有差异,请以英文原文为准。
Palaeoclimatic Control on the Composition of Palaeozoic Shales from Southern Ghana, West Africa
The Late Ordovician to Early Cretaceous Sekondian Group of southern Ghana is made up of seven major sedimentary formations. One important feature of the Sekondian Group is the abundance of fairly fresh plagioclase in the lowest two formations, i.e., the Ajua Shale and Elmina Sandstone, and at the uppermost formation (i.e., Essikado Sandstone); but a virtual absence throughout the rest of the Group. The basal part of the Sekondian Group has been interpreted as of glacial origin whereas the upper formations are thought to have formed in a warmer stage. The abundant plagioclase in the lowest two formations could, therefore, be due to less pervasive chemical weathering rather than erosion of a distinct source. In this paper, geochemical data for fine-grained sedimentary rocks of the Ajua Shale and the Takoradi Shale (that overlie the Elmina Sandstone) formations are used to investigate the influence of palaeoweathering on the composition of the Sekondian Group. The concentrations of the transition trace metals Cr, Ni, V, and Cu, and Cr/V-, Zr/Y-, and Ba/Co-ratios are similar in the samples from both formations and suggest that they were largely derived from the granitoids that intrude the Birimian supracrastal rocks. However, the Takoradi Shale samples are enriched in Rb, Cs, Th, U, and Nb relative to the Ajua Shale samples, and this may be related to differences in the degree of weathering in the source rocks. The Takoradi Shale samples are characterized by high chemical index of alteration (CIA) values and Rb/Sr ratios (CIA > 75; Rb/Sr > 1) indicating high degrees of weathering of their source rocks. In contrast, the Ajua Shale samples have low CIA values and Rb/Sr ratios (CIA
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