Archean Tectonics of the Pilbara craton, Western Australia

西澳大利亚皮尔巴拉克拉通太古宙（3.5- 2.5Ga）沉积岩被认为是在陆壳上就地堆积的扩张层序。然而，目前的研究项目已经阐明，这些太古代沉积杂岩是在俯冲带形成的古老的增生杂岩。在皮尔巴拉克拉通花岗岩/绿岩带的Cleaverville、North Pole、Marble Bar、Camel Creek和比斯利河地区进行了详细的野外研究，获得了以下新成果：双重构造和洋板块地层的识别证明了太古代增生杂岩的存在。这一发现表明，包括大洋板块俯冲在内的板块构造在3.5Ga（中太古代）就已经开始活动，太古代花岗岩/绿岩带代表了与俯冲有关的科迪勒拉型造山带。此外，这意味着其他地区的太古宙花岗岩/绿岩带可能都是 ...更多信息 另一方面，太古代增生杂岩的一些独特特征是：1）增生杂岩中太古代MORB比中生代的MORB厚，2）出现了中生代不存在的科马提质绿岩，3）含金属的远洋沉积物，包括重晶石矿床。还澄清了太古代增生杂岩的相当大的变化，包括科马蒂岩杂岩（可能是海洋高原的起源）占主导地位的复杂，和那些与重晶石单位相关的热液喷口强烈的蚀变。此外，还发现了所谓最古老（3.5Ga）的叠层石，是一种与重晶石伴生的深海热液矿床，上述研究成果在该领域都是新的，本项目组正在争取尽快在国际期刊上发表。感谢日本文部省对本项目的支持。少

Archean (3.5-2.5 Ga) sedimentary rocks in the Pilbara craton, Western Australia, have been regarded as succesive sequences accumulated in situ on continetal crust. Present research project has clarified, however, that these Archean sedimentary complexes are ancient accretionary complexes formed at subduction zones. Detailed field research was conducted in the Cleaverville, North Pole, Marble Bar, Camel Creek, and Beasley River areas in the granite/greenstone belts of the Pilbara craton, and following results were newly obtained.Recognition of duplex structure and oceanic plate stratigraphy of component rocktypes proved the occurrence of Archean acrretionary complex. This find indicates that plate tectonics including subduction of oceanic plate was working already at 3.5 Ga (mid-Archean time), and that the Archean granite/greenstone belt represents subduction-related Cordilleran-type orogenic belt. Furthermore, it implies that Archean granite/greensone belts in other cratons can be all … More re-interpreted as Cordilleran-type orogenic belts.On the other hand, some peculiar characteristics of Archean accretionary complex can be documented as follows ; 1) Archean MORB in accretionary complex is fairly thicker than those in Phanerozoic equivalents, 2) occurence of komatiitic greenstones which is absent in Phanerozoic example, and 3) metalliferous pelagic sediments including barite deposits. What was also clarified is a considerable variation in Archean accretionary complexes including komattitic grenstone (probably of oceanic plateau origin) -dominant complex, and those with strong alteration by hydrothermal venting associated with barite units. In addition, the so-called oldest (3.5 Ga) stromotolite was revealed to be a deep-sea hydrothermal deposits associated with barite.The above-mentioned results are all new to this study field, and the present project team is now trying to publish these in international journals as soon as possible. We would like to appreciate the Ministry of education and culture of Japan for the support on this project. Less