Alteration and geochemical patterns in the 3.7–3.8 Ga Isua greenstone belt, West Greenland
Alteration and geochemical patterns in the 3.7–3.8 Ga Isua greenstone belt, West Greenland
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DOI：
10.1016/s0301-9268(03)00095-0
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发表时间：
2003-10
影响因子：
3.8
通讯作者：
A. Polat;A. Hofmann
中科院分区：
文献类型：
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作者：
A. Polat;A. Hofmann
3.7-3.8Ga Isua绿岩带的火山岩经历了非均质、多期次的岩浆后蚀变，导致LREE和Th等多种元素的活化。Nb、Ta、Ti、Zr、Y和HREE在蚀变过程中相对不动。元素的流动性程度似乎已控制的化学成分，原生火山结构，碳酸盐和二氧化硅富集，并接近超镁铁质单位和剪切带。根据蚀变最小的样品的地球化学特征，在结构上分离的岩石构造序列中识别出两种不同的组合。低HFSE组合（TiO_2 =0.20-0.40wt.%; Zr=12- 30 ppm，Nb=0.13-0.80ppm，Y=6- 14 ppm），类似于第三纪玻安岩; Zr=34-77ppm; Nb=1.2-2.7ppm; Y=11- 28 ppm），与中生代岛弧玄武岩-苦橄质火山岩相当。这两个地球化学上不同的序列很可能已经并列的结果，在地球早期的板块构造过程中运作的恐龙式。Isua火山岩的特征是Nb/Ta（10-16）比值为亚南极。可以排除岩浆期后蚀变是造成这些低比值的原因。相反，这些比值反映了古太古代地幔源的组成特征。亚南极Nb/Ta比值可能是由于Nb作为弱亲铁元素优先下沉到核中，或在俯冲带成岩过程中优先保留在俯冲洋壳中。或者，地球的组成部分并不是由类似CI的球粒陨石材料组成的。
Volcanic rocks of the 3.7–3.8Ga Isua greenstone belt have undergone heterogeneous, poly-phase post-magmatic alteration, resulting in mobilisation of many elements including LREE and Th. In contrast, Nb, Ta, Ti, Zr, Y and HREE were relatively immobile during alteration processes. The degree of element mobility appears to have been controlled by chemical composition, primary volcanic structures, carbonate and silica enrichment, and proximity to ultramafic units and shear zones. On the basis of the geochemical characteristics of the least altered samples, two distinct associations have been recognised in structurally separated lithotectonic sequences. These magmatic associations are: a low-HFSE association (TiO2=0.20–0.40wt.%; Zr=12–30ppm; Nb=0.13–0.80ppm; Y=6–14ppm) in the central arc sequence, resembling Tertiary boninites; and a high-HFSE association (TiO2=0.50–1.14wt.%; Zr=34–77ppm; Nb=1.2–2.7ppm; Y=11–28ppm) in the outer and inner arc sequences, comparable to Phanerozoic island arc basaltic to picritic volcanic rocks. These two geochemically distinct sequences were likely to have been juxtaposed as a consequence of Phanerozoic-style plate tectonic processes operating in the early Earth. The Isua volcanic rocks are characterised by sub-chondritic Nb/Ta (10–16) ratios. Post-magmatic alteration can be ruled out as the cause of these low ratios. Rather, these ratios reflect a compositional characteristic of the Paleoarchean mantle source(s). The sub-chondritic Nb/Ta ratios may have resulted from either preferential sink of Nb into the core as a weakly siderophile element, or preferential retention of Nb in the subducted oceanic crust during subduction zone petrogenetic processes. Alternatively, the building blocks of the Earth were not composed of CI-like chondritic material.