Serch for the Earth's ealiest life in Western Australia

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Intensive 1/5000 scale field mapping in the North Pole area in Pilbara coupled with indoor analyzes has clarified the following new aspects of the 3.5 Ga greenstone belt. 1) The predominant basaltic greenstone (previously called the Mount Ada Basalt, Towers Formation and Apex Basalt by Hickman, 1983) form imbricated thrust sheets (Unit I-IV ; each composed of 1-2 km thick greenstones and thin chert), suggesting a layr-parallel shortening after the primary volcanism and sedimentation. 2) The absence of coarse-grained terrigenous clastics in the greenstone-chert units suggests a mid-oceanic rather than continental setting. 3) Numerous silica dikes called T-chert penetrated pillowed greenstones (particularly of the Unit I) to their stratigraphic top, suggesting an extensional regime prior to the contraction. 4) The gradual transition from T-chert to overlying bedded chert demonstrates a feeder and deposits relationship for silica. Bedded chert up to 60 m thick is a sum consequence of inte … More rmittent normal faulting with T-chert intrusion in several generations. 5) Bedded chert has slumping-dewatering structures, indicating a rapid and syn-deformational sedimentation. 6) Filamentous microorganisms (bacteria) from the bedded chert resemble to the known oldest ones (Schopf, 1993). 7) Eu-positive REE patterns of both T-chert and bedded chert suggest a hydrothermal origin of silica. 8) Barite vein and bedded barite in chert are also in a feeder and deposits relationship. The alleged oldest stromatolite associated with barite is of non-photosynthetic but hydrothermal origin. The non-terrigenous environment, extensional tectonic regime, and hydrothermal activity concordantly suggest a mid-oceanic ridge oprigin of these Early Archean rocks and fossils. The Archean life habitat appears analogous to modern deep-sea hydrothermal sites along MORs, although the Archean MOR depth is unknown. The layr-parallel shortening was probably resulted from subduction-accretion along an Archean trench. Less