Origin and global tectonic significance of Early Archean cherts from the Marble Bar greenstone belt, Pilbara Craton, Western Australia
Origin and global tectonic significance of Early Archean cherts from the Marble Bar greenstone belt, Pilbara Craton, Western Australia
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DOI：
10.1016/s0301-9268(03)00043-3
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发表时间：
2003-08
影响因子：
3.8
通讯作者：
Y. Kato
中科院分区：
文献类型：
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作者：
Y. Kato
分析了皮尔巴拉省大理石坝绿岩带太古代Warrawoona群镁铁质-超镁铁质岩中的五个层状硅质岩剖面，以了解它们的沉积环境，并对早太古代构造提供一些约束。剖面根据其野外产状、矿物学和地球化学特征可分为两种类型：厚的剖面（A和B）覆盖富铁低钾拉斑玄武岩，薄的剖面（C1、C2和C3）覆盖科马提质玄武岩。最厚的铁质部分（A; 45米厚）是塔组的大理石坝燧石，被解释为源自大洋中脊（莫尔）的高温热液的原位沉淀物。在莫尔比下，这些地球化学特征与现代热液富铁沉积物相似：（i）P、V、Zn和Y与Fe呈正相关;（ii）Eu正异常（标准化为球粒陨石）随∑REE和LREE/HREE的增加从6.6减小到1.3，Ce负异常从0.6减小到1.0。Apex玄武岩中13 m厚的B剖面以SiO2为主，含有大量的Ba（高达4330 ppm），源自低温莫尔热液。这一部分的特点是与大量的黑色/灰色硅脉，在扩张中心的正断层带热液馈线协会。B剖面下的绿岩的地球化学证据表明它们是MORB成因的。与A、B剖面不同，C1、C2、C3剖面与顶玄武岩的科马提质玄武岩整合互层，厚度3- 6 m。这三个部分的地球化学特征表明，它们很可能是由低T，弱热液活动，可能已与热点火山作用。它们表现出强烈的富集Cr和Ni，反映了沉积过程中科马提质玄武质碎屑的重要贡献。在其最上层的火山岩燧石的存在表明减少热液二氧化硅沉淀由于减弱热液活动。在这三个剖面中，最上部（C3）相对于下部C1和C2剖面表现出更高的Zr、Nb、Hf和Th的富集，以及更高的Th/Sc和（La/Yb）N。这可能意味着C3的沉积环境相对更接近大陆或由花岗岩类和/或火山岩组成的岛弧。最上层碎屑岩（T剖面;全景组）的硅质泥岩和砂岩具有类似于长英质深成岩/火山岩的地球化学特征。高Th/Sc，负Eu异常，高∑REE在一些硅质泥岩从这个单位意味着分化的花岗岩类（或火山等价物）已经暴露（喷发），至少在当地，在皮尔巴拉在这个时候。这项研究表明，这些Warrawoona组硅质岩沉积在各种环境中，从洋中扩张中心到会聚板块边界通过热点。这种变化很可能是由于水平板块运动，从而支持了早太古代板块构造的运作。
Five sections of bedded chert in mafic-ultramafic rocks of the Archean Warrawoona Group in the Marble Bar greenstone belt, Pilbara Craton, were analyzed in order to understand their depositional environment and to provide some constraints on Early Archean tectonics. The sections are divisible into two types based on their field occurrence, mineralogy and geochemistry; thicker ones (A and B) that overlie Fe-rich, low-K tholeiites and thinner ones (C1, C2, and C3) overlying komatiitic basalts. The thickest, ferruginous section (A; 45m thick) is the Marble Bar Chert of the Towers Formation, and is interpreted to have been an in situ precipitate derived from a high-T hydrothermal solution emanating from a mid-oceanic ridge (MOR). The following geochemical features are similar to those of modern hydrothermal iron-rich sediments at a MOR: (i) P, V, Zn, and Y are positively correlated with Fe, (ii) a positive Eu anomaly (normalized to chondrite) decreases from 6.6 to 1.3 and the magnitude of a negative Ce anomaly decreases from 0.6 to 1.0 as ∑REE and LREE/HREE increase. The 13m thick B-section in the Apex Basalt, dominated by SiO2and containing significant amounts of Ba (up to 4330ppm), originated from a low-T MOR hydrothermal solution. This section is characterized by an association with massive black/gray silica veins that were hydrothermal feeders in normal fault zones in the spreading center. Geochemical evidence from the greenstones underlying the B-section indicates that they are of MORB origin. In contrast to A and B, C1-, C2-, and C3-sections are intercalated conformably with komatiitic basalts of the Apex Basalt and are 3–6m thick. The geochemical signatures of these three sections suggest that they were most likely formed by low-T, weak hydrothermal activity that may have been associated with hot-spot volcanism. They show strong enrichment of Cr and Ni, reflecting a significant contribution of komatiitic basaltic detritus during sedimentation. The presence of volcaniclastic chert in their uppermost beds indicates a decrease of hydrothermal silica precipitation due to waning hydrothermal activity. Among the three sections, the uppermost (C3) exhibits greater enrichment in Zr, Nb, Hf, and Th, and higher Th/Sc and (La/Yb)Nrelative to the lower C1- and C2-sections. This could mean that the depositional environment of C3 was relatively closer to a continent or to island arcs composed of granitoids and/or their volcanic equivalents. Siliceous mudstones and sandstones of the uppermost clastic rocks (T-section; Panorama Formation) have geochemical signatures analogous to those of felsic plutonic/volcanic rocks. High Th/Sc, negative Eu anomalies, and high ∑REE in some siliceous mudstones from this unit imply that differentiated granitoids (or volcanic equivalents) were already exposed (erupted), at least locally, in the Pilbara at this time. This study shows these Warrawoona Group cherts were deposited in a variety of environments ranging from a mid-oceanic spreading center to a convergent plate boundary via a hot-spot. This variation was most likely due to horizontal plate motions which accordingly support the operation of plate tectonics in the Early Archean.