Serch for the Earth's ealiest life in Western Australia

在Pilbara北极地区进行的1/5000比例尺野外制图结合室内分析，明确了3.5 Ga绿岩带的以下新方面。1)主要的玄武岩绿岩（Hickman， 1983年称其为阿达山玄武岩、塔组和顶玄武岩）形成叠瓦状逆冲片（第I-IV单元，每一单元由1-2 km厚的绿岩和薄燧石组成），表明在初级火山作用和沉积作用之后发生了层平行缩短。2)绿岩-燧石岩单元中未发现粗粒陆源碎屑，表明其背景为中大洋而非大陆。3)大量被称为t -燧石的硅质岩脉穿过枕状绿岩（特别是第一单元的绿岩）到达地层顶部，表明在收缩之前存在伸展状态。4)从t型燧石到上覆层状燧石的逐渐转变表明了二氧化硅的喂料和沉积关系。厚达60米的层状燧石岩是几代以来频繁的正断层与t型燧石侵入的综合结果。5)层状燧石具有滑塌-脱水构造，表明其沉积速度快，属于同变形沉积。6)层状燧石中的丝状微生物（细菌）与已知的最古老的微生物相似（Schopf, 1993）。7) t型燧石和层状燧石的Eu-positive REE模式表明石英的热液成因。(8)燧石中的重晶石脉和层状重晶石也具有喂矿和沉积关系。与重晶石有关的所谓最古老的叠层石是非光合作用的，但起源于热液。非陆源环境、伸展构造制度和热液活动一致表明，这些早太古代岩石和化石的起源为洋中脊。太古宙的生命栖息地似乎类似于现代深海热液遗址，尽管太古宙的深度尚不清楚。层平行缩短可能是太古宙海沟俯冲-增生的结果。少

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