Serch for the Earth's ealiest life in Western Australia

在皮尔巴拉北极地区进行了密集的1/5000比例尺野外测绘，结合室内分析，阐明了3.5Ga绿岩带的以下新方面。1)主要的玄武质绿岩(以前被希克曼称为阿达山玄武岩、塔楼建造和尖端玄武岩，1983)形成叠瓦式逆冲断层(I-IV单元，每个单元由1-2公里厚的绿岩和薄的硅质岩组成)，表明在原生火山作用和沉积之后发生了平行的层状缩短。2)绿岩-硅质岩单元中没有粗粒陆源碎屑，表明其处于大洋中部而不是大陆环境。3)大量被称为T-硅质岩的硅质岩脉穿透枕状绿岩(特别是单元I)至其地层顶部，表明在收缩之前处于伸展状态。(4)T-硅质岩向上覆层状硅质岩的逐渐过渡显示了硅质的供矿和沉积关系。厚达60米的层状硅质岩是Inte…的综合结果在几代人中，伴随着T-chert侵入体的更频繁的正常断层。5)层状硅质岩具有滑塌-脱水结构，指示快速、同变形的沉积作用。6)来自层状硅质岩的丝状微生物(细菌)类似于已知的最古老的微生物(Schopf，1993)。7)T-硅质岩和层状硅质岩的Eu-Eu正稀土配分模式均表明硅质为热液成因。8)硅质岩中的重晶石脉状重晶石和层状重晶石也是一种供矿关系。据称与重晶石伴生的最古老的叠层石是非光合作用的热液成因。非陆源环境、伸展构造体制和热液活动一致地暗示了这些太古宙早期岩石和化石的大洋中脊喷发。太古代生物栖息地看起来类似于莫尔斯沿岸的现代深海热液遗址，尽管太古宙莫尔深度尚不清楚。层状缩短可能是太古宙海沟俯冲-吸积作用的结果。较少

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