Petrology and Geochemistry of Early Precambrian Graywackes from the Fig Tree Group, South Africa
Petrology and Geochemistry of Early Precambrian Graywackes from the Fig Tree Group, South Africa
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南非无花果树群早前寒武纪杂砂岩的岩石学和地球化学
DOI：
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发表时间：
1970
期刊：
影响因子：
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通讯作者：
T. Reimer
中科院分区：
文献类型：
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作者：
K. Condie;John E Macke;T. Reimer
南非无花果树群的灰岩来自一个主要由燧石、火山岩、花岗岩-变质岩和一些超镁铁岩组成的不同源区。燧石和石英的假象后，碎片状的碎片和良好的斜长石和钾长石晶体在一些graywacks表明，一些碎屑来自当代火山作用。不成熟的结构表明，温和的源区风化和快速的侵蚀，沉积和埋藏。异常大量的白云岩中的白云岩可能是沉积的富含碳酸盐的孔隙流体在成岩作用。一个相对的Sr亏损（相比，K，Rb，Ca和Ba）在graywackes的下部无花果树组（示巴组）解释在丰富的Sr亏损火成岩在其源区。大量的镍在所有无花果树杂砂岩和页岩似乎已来自超镁铁质源岩。在灰岩中，花岗岩成分的地层逐渐增加，表明花岗岩变质岩的逐渐去顶，最初被一层厚的翁韦尔瓦赫特或类似昂弗瓦赫特的火山岩覆盖。这一来源似乎位于斯威士兰中部。杂砂岩中相对丰富的花岗质碎屑间接记录了一个或多个前无花果树（3.4-4.0 b.y.）南部非洲的花岗岩形成事件。
Graywackes of the Fig Tree Group in South Africa were derived from a diverse source area composed principally of chert, volcanics, granitic-metamorphic rocks, and some ultramafics. Chert and quarts pseudomorphs after shard-like fragments and well-formed plagioclase and K-feldspar crystals in some graywackes indicate that some detritus was derived from contemporary volcanism. Immature textures indicate mild source-area weathering and rapid erosion, deposition, and burial. An unusually large amount of dolomite in the graywackes was probably deposited by carbonate-rich pore fluids during diagenesis. A relative Sr depletion (compared to K, Rb, Ca, and Ba) in graywackes of the lower Fig Tree Group (Sheba Formation) is interpreted in terms of an abundance of Sr-depleted igneous rocks in their source area. Large amounts of Ni in all Fig Tree graywackes and shales appear to have been derived from ultramafic source rocks. A progressive stratigraphic increase in granitic components in the graywackes suggests progressive unroofing of a granitic metamorphic terrane which was initially covered by a thick sequence of Onverwacht or Onverwacht-like volcanic rocks. This source appears to have been located in central Swaziland. The relatively abundant granitic detritus in the graywackes indirectly records one or more pre-Fig Tree (3.4-4.0 b.y.) granite-forming events in southern Africa.