Archean Tectonics of the Pilbara craton, Western Australia

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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