Geodynamic Setting of the Bushveld Complex

该项目得到了地球科学部岩石与地球化学和教育与人力资源项目以及国际科学与工程办公室非洲、近东和南亚项目的支持。南非的布什维尔德复合体是一个化石岩浆房，大约20亿年前在地壳浅层凝固。地质学家们都知道这个矿体，因为它拥有世界上最大的铂和相关元素储量，也是铬和钒的主要来源。布什维尔德还因其大小而闻名，据估计其体积高达60万立方公里，而且其结构反映了漫长而复杂的凝固历史。尽管它非常巨大，岩石的地球化学特征表明，一些原始岩浆已经从岩浆室中逸出，因此从现在的岩浆体中消失了。这就提出了一个问题：这些额外岩浆的性质是什么？它们可能去了哪里？如果地质学家想要了解布什维尔德复合体是如何以及在什么条件下固化的，就必须回答这些问题。到目前为止，研究小组的研究使他们假设，消失的岩浆现在是由一组被称为罗伊伯格群的熔岩（加上它们的侵入物，拉肖普花岗岩）代表的，它们的年龄在不确定的估计范围内与布什维尔德年龄相同。目前的研究旨在验证这一假设。作为该项目的一部分，Mathez及其同事还将为20-25名南非地球科学教师组织一个为期一周的关于南非矿产资源的研讨会。讲习班将由Mathez， VanTongeren和两位非裔美国纽约市学校教师授课，他们将因此获得矿藏知识和跨文化教学的独特经验。更具体地说，这项研究试图验证一个假设，即罗伊伯格熔岩和伴生岩石起源于布什维尔德上带。这将通过对上带微量元素地球化学的详细研究来完成，以确定与堆积矿物平衡的液体的组成。然后将推导出的液体成分与Rooiberg和Rashoop岩石成分进行比较。Rooiberg群顶部以流纹岩为主，而Bushveld上带是侵入岩化学演化最严重的部分，以堆积岩为主，由斜长石、贫钙辉石、富钙辉石、磁铁矿、钛铁矿、富铁橄榄石和磷灰石等矿物组成。因此，流纹岩可能与这些相处于平衡状态的可能性涉及火成岩学中最基本和最具争议的问题之一，即基性岩浆是否向富铁或富硅成分分化。此外，如果Rooiberg和Rashoop岩石确实起源于Bushveld岩浆房，这意味着Bushveld侵入地壳的深度比通常认为的要浅得多，从而有助于确定侵入的地球动力学背景。

This project is supported by the Petrology & Geochemistry and the Education & Human Resources programs in the Division of Earth Sciences and by the Africa, Near East and South Asia program of the Office of International Science and Engineering. The Bushveld Complex of South Africa is a fossil magma chamber that solidified in the shallow crust about 2 billion years ago. The body is well-known to geologists because it holds the world's largest reserves of platinum and related elements and is a major source of chromium and vanadium. The Bushveld is also notable for its size, which has been estimated to be up to 600,000 cubic kilometers, and for the fact that its structure reflects a long and complex solidification history. Despite its enormity, geochemical characteristics of the rocks indicate that some amount of the original magma had escaped from the chamber and is thus missing from the present body. This raises the questions of what was the nature of this additional magma and where could it have gone. These questions need to be answered if geologists are ever to understand how and under what the conditions the Bushveld Complex solidified. Research to date by the team has led them to hypothesize that the missing magma is now represented by a sequence of lavas known as the Rooiberg Group (plus their intrusive equivalents, the Rashoop granophyres), the ages of which are identical to the Bushveld age within uncertainty estimates. The present research is directed at testing this hypothesis. As part of the project, Mathez and colleagues will also organize a week-long workshop for 20-25 South African earth science teachers on the mineral resources of South Africa. The workshop will be taught by Mathez, VanTongeren, and two African-American New York City school teachers, who will thus gain knowledge of both mineral deposits and unique experience in teaching across cultural divides.More specifically, the research seeks to test the hypothesis that the Rooiberg lavas and associated rocks originated from the Bushveld Upper Zone. This will be accomplished by a detailed study of the trace element geochemistry of the Upper Zone to determine the compositions of the liquids in equilibrium with the cumulate minerals. The derived liquid compositions will then be compared with Rooiberg and Rashoop rock compositions. The top members of the Rooiberg Group are dominated by rhyolite, whereas the Bushveld Upper Zone, which is the most chemically evolved part of the intrusion, is dominated by cumulate rocks consisting of assemblages with most or all of the minerals plagioclase, Ca-poor pyroxene, Ca-rich pyroxene, magnetite, ilmenite, Fe-rich olivine, and apatite. Therefore, the possibility that a rhyolite could be in equilibrium with these phases bears on one of the most fundamental and debated questions in igneous petrology, namely whether mafic magmas differentiate toward Fe-rich or Si-rich compositions. In addition, if the Rooiberg and Rashoop rocks indeed originated from the Bushveld magma chamber, this implies that the Bushveld intruded into a much shallower level of the crust than has been generally appreciated, thus helping to define the geodynamic setting of the intrusion.