The Orange River Group: a Major Proterozoic Calcalkaline Volcanic Belt in the Western Namaqua Province, Southern Africa
The Orange River Group: a Major Proterozoic Calcalkaline Volcanic Belt in the Western Namaqua Province, Southern Africa
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奥兰治河群：南部非洲纳马夸省西部的一个主要元古代钙碱性火山带
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发表时间：
1987
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通讯作者：
A. Moyes
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作者：
D. Reid;A. J. Erlank;H. Welke;A. Moyes
橙子河群（ORG）由一个厚的火山沉积岩堆（> 8公里）组成，作为南部非洲纳马夸省的一个次省保存下来。火山活动在沉积的ORG主要是陆上的，并涉及喷发的安山岩为主的calcalkaline套件，从玄武岩到流纹岩。Rb-Sr、Th-Pb、Pb-Pb和Sm-Nd等时线年龄的一致性表明，ORG的挤压作用发生在2000 Ma左右。与现代弧套相比，ORG以高K、Mg及相关微量元素（Rb、Sr、Ba、U、Th、LREE; Ni、Cr）为特征。基性熔岩与现代钙碱性熔岩不同，其A1含量较低，类似拉斑玄武岩。当标准化为N型MORB时，ORG基性熔岩显示出K、Rb、Ba、Th和LREE的富集，而Nb、Ta、Zr、Hf、Ti、Y和HREE的亏损）。变质叠印与1000马Namaqua构造事件是明显的，在不同程度上在整个次省，并排除了精确的监测在喷发时的初始同位素比。来自受变质叠加影响最小的岩石的证据表明，在这个特定的区域，整个玄武岩到流纹岩套具有共同的初始Sr、Pb和Nd同位素组成，这将支持分离结晶模型，并排除了老西太平洋地壳主要参与产生大量安山岩的可能性。初始同位素数据转换为Sr（T）+10），Nd（T）+10），μ值明显较高，结合前面描述的地球化学特征，表明ORG的基性母岩浆源区显示出与俯冲带上的地幔楔相似的模式。因此，钙碱性ORG火山岩套可能代表了2000 Ma古老的元古代弧杂岩的残余。高钾特征、同源岩浆花岗岩类的密切组合、铜矿斑岩的发育以及缺乏下伏陆壳的直接或间接证据都表明该地区为成熟的岛弧环境。活动大陆边，如安第斯山脉，显示出类似的地球化学和同位素特征，但有人认为，这种边缘的高海拔将导致它们优先受到侵蚀，在古代地形中缺乏代表性。
Summary The Orange River Group (ORG) consists of a thick volcano-sedimentary pile (> 8 km) preserved as a subprovince within the Namaqua Province of southern Africa. Volcanic activity during deposition of the ORG was predominantly subaerial, and involved the eruption of an andesite dominated calcalkaline suite ranging from basalt through to rhyolite. Agreement between Rb-Sr, Th-Pb, Pb-Pb and Sm-Nd isochron ages indicate extrusion of the ORG around 2000 Ma ago. Compared with modern arc suites, the ORG is characterized by high K and Mg, together with related trace elements (Rb, Sr, Ba, U, Th, LREE; Ni, Cr). Basic lavas differ from many modern calcalkaline types in having lower A1 contents reminiscent of tholeiites. When normalized to N-type MORB, the ORG basic lavas show enrichments in K, Rb, Ba, Th, and the LREE, and depletions in Nb, Ta, Zr, Hf, Ti, Y and the HREE). Metamorphic overprinting associated with the ∼ 1000 Ma Namaqua tectonothermal event are evident to varying degrees throughout the subprovince, and precludes precise monitoring of initial isotope ratios at time of eruption. Evidence from the rocks least affected by metamorphic overprinting suggests common initial Sr, Pb and Nd isotopic compositions for the entire basalt to rhyolite suite in this particular area, which would support a fractional crystallization model and rule out major involvement of old Archaean crust to produce the voluminous andesites. Initial isotopic data translate to ɛsr(T) + 10), ɛNd(T)∼0, and an apparent high μ value, which, when taken together with the geochemical features described previously, indicate that the source region to the mafic parental magmas of the ORG shows patterns similar to those in the mantle wedge overlying subduction zones. The calcalkaline ORG volcanic suite may therefore represent the remnants of a 2000 Ma old Proterozoic arc complex. The high K character, the intimate association of comagmatic granitoids, the development of copper porphyries, and the lack of any direct or indirect evidence for underlying Archaean crust all point to a mature island arc environment. Active continental margins such as the Andes show similar geochemical and isotopic features, but it is argued that the high elevation of such margins will cause them to be preferentially eroded and lack representation in ancient terrains.