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首页> 外文期刊>South African Journal of Geology: Being the Transaction of the Geological Society of South African: Syndie die Verhandelinge van die Geologiese Vereniging van Suid-Afrika >PROVENANCE OF THE NEOPROTEROZOIC ROCKS OF THE GIFBERG GROUP (WESTERN SOUTH AFRICA)
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PROVENANCE OF THE NEOPROTEROZOIC ROCKS OF THE GIFBERG GROUP (WESTERN SOUTH AFRICA)

机译：Gifberg群（南非西部）的新元古代岩石的起源

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摘要

New petrographic and lithogeochemical data are presented for siliciclastic units of the Gifberg Group (western South Africa), a succession rarely studied in the past. The main detrital source for the oldest succession of the Gifberg Group, the Karoetjes Kop Formation, has been identified as sourced by local metamorphic rocks with a felsic geochemical composition. Nearly unweathered detritus (Chemical index of alteration: 50 to 60; K/Cs 5000) occurs in the metadiamictite of this formation and is interpreted as palaeovalley infill, deposited only a short distance from its sources. The provenance signature of the overlying Aties Formation indicates significant mafic and/or intermediate sources. Possible sources are rocks of the Bushmanland Group and an unknown unmetamorphosed mafic to intermediate rock succession, which might have been exposed during the deposition of the Aties Formation. The overlying Bloupoort Formation is characterised by a geochemically homogenous metadiamictite (Swartleikrans Bed) with only slightly weathered detritus (K/Cs >10000) and overlying mature silica-rich metarenites and metaconglomerates. It is proposed that the Gifberg basin formed as a small-scale rift basin and then widened through time with subsequent longer sedimentary transport of the detritus which have caused the higher compositional maturity. The predominance of detrital input from local sources in the Karoetjes Kop Formation hinders correlation based on its mineralogical and geochemical content alone. A glacial origin for the Swartleikrans Bed was proposed previously based on the occurrence of possible dropstones. Certain criteria for a glacial depositional environment as outlined by Arnaud and Etienne (2011) were met in our study and include poor sorting, rapid lateral facies changes, crude stratification, variable thicknesses, compositional immaturity, and a gradational contact plus a chaotic texture with large boulders. While these observations point to a 'rainouf deposit, there is still a lack of evidence for glacial transport on grain surfaces or in the form of faceted clasts and pebbles.
机译：给出了Gifberg组（南非西部）的硅质碎屑岩单元的新岩石学和岩化化学数据，这是过去很少研究的。 Gifberg组最古老演替的主要碎屑源Karoetjes Kop地层，已被确定为由具有复杂地球化学成分的局部变质岩提供。几乎未风化的碎屑（化学变化指数：50至60; K / Cs 5000）发生在该地层的成岩岩中，被解释为古河谷填充物，距其来源仅一小段距离。上覆的Aties组的物源特征表明存在重要的镁铁质和/或中间物。可能的来源是布什曼兰群的岩石和未知的未变质的镁铁质岩体到中层岩性演替，这些岩性可能是在阿蒂斯组沉积期间暴露出来的。上覆的布卢普特组的特征是地球化学均质的变质岩（Swartleikrans床），只有轻微风化的碎屑（K / Cs> 10000）和上覆的成熟的富含二氧化硅的变质岩和变砾岩。建议将吉夫贝格盆地形成为小规模的裂谷盆地，然后随着时间的推移而扩大，随后碎屑的沉积时间更长，这导致了较高的组成成熟度。仅从其矿物学和地球化学含量来看，Karoetjes Kop组中来自本地的碎屑输入占主导地位。 Swartleikrans床的冰川起源先前是根据可能出现的落石提出的。我们的研究满足了Arnaud和Etienne（2011）概述的冰川沉积环境的某些标准，其中包括较差的分选，快速的侧相变化，原油分层，厚度变化，成分不成熟和渐变接触以及具有大尺寸的混沌纹理。巨石。尽管这些观察结果表明存在“雨状沉积物”，但仍缺乏证据表明冰川在谷物表面或以切面碎屑和小卵石的形式运输。

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