The geochemistry of sedimentary rocks from the Fig Tree Group, Barberton greenstone belt: Implications for tectonic, hydrothermal and surface processes during mid-Archaean times
The geochemistry of sedimentary rocks from the Fig Tree Group, Barberton greenstone belt: Implications for tectonic, hydrothermal and surface processes during mid-Archaean times
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DOI：
10.1016/j.precamres.2005.09.005
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发表时间：
2005-12
影响因子：
3.8
通讯作者：
A. Hofmann
中科院分区：
文献类型：
--
作者：
A. Hofmann
CA。巴伯顿绿岩带南部3.25 Ga的无花果树群由深水-浅水页岩、杂砂岩、碧玉质条带状铁建造（BIF）和碳质硅质岩组成。该层序厚度超过1200米，在构造上是重复的，但在地层上是不同的构造-地层单位。Fig Tree地层源区的化学风化作用较小。相反，热液交代事件影响了研究区的沉积岩，导致碱土元素的亏损和钾交代。利用稀土元素和微量元素比值的物源建模表明，从超镁铁质到镁铁质绿岩、TTG和HREE未耗尽花岗岩的贡献各不相同。地层对成分的明显控制，表现为微量金属含量增加和LaN/YbN比值降低，反映了源体内超镁铁质和镁铁质火山岩的逐渐增加。条带状铁建造显示REE-Y模式与其在海洋环境中从富Eu海水中沉淀一致。无花果树群底部的碳质硅质岩并不代表来自热大洋的化学沉淀物，而是代表在海底低温热液流体喷发过程中硅化的碳质沉积物。
The ca. 3.25Ga old Fig Tree Group in the southern part of the Barberton greenstone belt consists of deep- to shallow-water shale, greywacke, jaspilitic banded iron formation (BIF), and carbonaceous chert. The sequence is more than 1200m thick and crops out as tectonically duplicated, but stratigraphically distinct tectono-stratigraphic units. Chemical weathering of the source terrain of Fig Tree strata was minor. Instead, hydrothermal-metasomatic events affected the sedimentary rocks in the study area, resulting in the depletion of alkaline earth elements and K metasomatism. Provenance modelling using REEs and trace element ratios indicate varying contributions from ultramafic to mafic greenstones, TTGs and HREE-undepleted granites. A clear stratigraphic control on the composition, in the form of increasing trace metal contents and decreasing LaN/YbNratios, reflects progressive increase of ultramafic and mafic volcanic rocks in the source. Banded iron formation shows REE-Y patterns consistent with its precipitation in a marine environment from Eu-enriched seawater. Carbonaceous cherts at the base of the Fig Tree Group do not represent chemical precipitates out of a hot Archaean ocean, but represent carbonaceous sediments that were silicified during low-temperature hydrothermal fluid emanations on the seafloor.