Record of emergent continental crust ∼3.5 billion years ago in the Pilbara craton of Australia
Record of emergent continental crust ∼3.5 billion years ago in the Pilbara craton of Australia
复制标题
澳大利亚皮尔巴拉克拉通约 35 亿年前出现大陆地壳的记录
作者：
R. Buick;J. R. Thornett;N. McNaughton;J. B. Smith;M. Barley;M. Savage
地球最古老岩石的同位素数据17表明，在早白垩世（>3.0 Gyr ago）存在着相当大的大陆地壳，但不知道这些地壳何时开始形成新兴大陆。沉积地球化学证实，直到太古代晚期，大陆裸露的面积可以忽略不计，这一论点得到了以下事实的支持：到目前为止，所有被证明沉积在侵蚀大陆基底上的绿岩表壳火山岩和沉积岩系列的年龄都为3.0 Gyr。在这里，我们报告的角度不整合（一个古老的侵蚀面）的岩石下的3.46-Gyr Warrawoona集团在澳大利亚的皮尔巴拉克拉通，目前已知的最古老的保存完好的绿岩继承。在不整合面之下，年龄大于3.5 Gyr的低级绿岩在侵蚀前被大量花岗岩侵入。由于上覆的Warrawoona岩石仅发生轻度变质、轻微变形，并且沉积在海平面附近，我们推断它们积聚在已经坚硬、凉爽和有浮力的地壳上。因此，在3.46 Gyr前，Warrawoona岩石形成了一个大陆地壳的新兴块体，这是已知的最古老的。
ISOTOPIC data for the Earth's oldest rocks17 imply that a considerable volume of continental crust existed during the early Archaean aeon (>3.0 Gyr ago), but it is not known when this crust first began to form emergent landmasses. Sedimentary geochemistry suggests8,9 that the area of exposed continent was negligible until late in the Archaean10, a contention supported by the fact that, until now, all greenstone supracrustal volcanic and sedimentary successions shown to have been deposited on eroded continental basement have yielded ages of ≲3.0 Gyr. Here we report the discovery of an angular unconformity (an ancient erosion surface) beneath rocks of the 3.46-Gyr Warrawoona Group in the Pilbara craton of Australia, currently the oldest known well-preserved greenstone succession. Below the unconformity, low-grade greenstones older than 3.5 Gyr were intruded by voluminous granitoids before erosion. As the overlying Warrawoona rocks are only mildly metamorphosed, slightly deformed and were deposited near sea level, we infer that they accumulated on crust that was already rigid, cool and buoyant. Thus, by 3.46 Gyr ago, the sub-Warrawoona rocks formed an emergent block of continental crust, the most ancient known.