Co-operative study of the oldest rocks of the Earth : essentials for the whole Earth history

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Strain analysis of the conglomerate showed that axial ratios of the strain ellipsoid is 3.73 and 1.23 in the XZ and YZ planes, respectively. The conglomerate before the deformation was restored by the de-straining technique, and found to be a pebbly mudstone. The deposition of the conglomerate is presumed to have taken place in deeper water environment than that previously considered. Five different lithologies are discriminated in the banded cherts : green, white, black gray and brown layrs. Anomalously large amount of lr was detected in a black layr by the INAA technique. Such high lr content suggests that anomalously large amount of extraterrestrial materials were accumulated in the early Archaean ocean. The banded iron formations were divided into five different lithologies : black, red, white, green and gray layrs, They commonly consists of quartz, magnetite, amphibole and carbonate. Their component are different from layr to layr. The depositional rhythm has not yet found out from the banded iron formation. Metamorphic zonation in the lsua region was proposed ; the region is divided into three zones on the basis of metamorphic mineral assemblages : amphibolite, amphibolite/greenschist and greenschist facies in the ascending order of metamorphic grade. Pyroxenes in the peridotite is found to be linearly arranged and boudinaged. These structures strongly indicate that the mantle convection in the early Archaean was much faster that that in Phanerozoic. The pillow lave conformably overlain by cherts has been discovered by the Japanese group. The pillow lava-chert assemblage is the same as the top of the present-day oceanic crust. This discovery suggests that the tectonic processes in the early Archaean is very similar to the plate tectonics active in the phanerozoic.