Geochemistry of Archean shales from the Witwatersrand Supergroup, South Africa: source-area weathering and provenance
Geochemistry of Archean shales from the Witwatersrand Supergroup, South Africa: source-area weathering and provenance
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DOI：
10.1016/0016-7037(87)90293-6
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发表时间：
1987-09
影响因子：
5
通讯作者：
D. Wronkiewicz;K. Condie
中科院分区：
文献类型：
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作者：
D. Wronkiewicz;K. Condie
与显生宙页岩相比，南非太古宙Witwatersrand超群（~2800 Ma）页岩的Na、Ca、LILE、REE和HFSE含量较低，但也有少数例外。Witwatersrand页岩中Cr、Co、Ni富集，西兰德组（WRG）和中下兰德组（CRG）页岩中Fe、Mg富集。CRG和上部WRG的页岩相对于下部WRG的页岩富集Na、Al、LILE、REE、HFSE和过渡金属。Witwatersrand页岩球粒陨石归一化稀土模式均富集轻稀土元素，且呈现小到中等负Eu异常。页岩中稀土元素与al2o3含量呈正相关，表明稀土元素主要存在于粘土矿物中。相对于CRG页岩，WRG页岩表现出Na、Ca和Sr的消耗，这一特征可能反映了其烃源岩强烈的化学风化作用。Witwatersrand页岩的CIA指数是可变的（主要是70-98），即使在同一页岩单元内也是如此。这些变化主要反映了气候带的变化或烃源区构造隆升速率的变化，并可能与物源和变质作用期间的元素再活动有关。与现今的上大陆地壳相比，除Orange Grove、Roodepoort和K8页岩外，其余页岩似乎都来自大陆源，富集了LILE、REE和HFSE，并富集了过渡金属。基于6种相对不动元素（Th、Hf、Yb、La、Sc、Co）和4种烃源岩的计算机混合模型表明，在沉积物源区，花岗岩、玄武岩和科马长岩的相对比例随着时间的推移而增加，而tonalite的相对比例则减少。玄武岩和科马长岩的贡献在Booysens页岩沉积时期达到最大，花岗岩的贡献在K8页岩沉积时期达到最大，可能在Orange Grove页岩沉积时期也达到最大。
With a few exceptions, shales from the Archean Witwatersrand Supergroup (~2800 Ma) in South Africa are depleted in Na, Ca, LILE, REE and HFSE compared to Phanerozoic shales. Cr, Co and Ni are enriched in all Witwatersrand shales and Fe and Mg are high in shales from the West Rand Groups (WRG) and lower Central Rand Group (CRG). Shales from the CRG and uppermost WRG are enriched in Na, Al, LILE, REE, HFSE and transition metals relative to shales from the lower WRG. Chondrite-normalized REE patterns for all Witwatersrand shales are enriched in light-REE and exhibit small to moderate negative Eu anomalies. A positive correlation of REE and Al2O3contents in the shales suggests that REE are contained principally in clay minerals.Relative to shales from the CRG, shales from the WRG exhibit depletions of Na, Ca and Sr, a feature probably reflecting intense chemical weathering of their source rocks. CIA indices in Witwatersrand shales are variable (chiefly 70–98), even within the same shale unit. Such variations reflect chiefly variable climatic zones or rates of tectonic uplift in source areas with perhaps some contribution from provenance and element remobilization during metamorphism.Compared to present-day upper continental crust, all but the Orange Grove, Roodepoort, and K8 shales appear to have been derived from continental sources depleted in LILE, REE, and HFSE and enriched in transition metals. Computer mixing models based on six relatively immobile elements (Th, Hf, Yb, La, Sc, Co) and four source rocks indicate that the relative proportions of granite, basalt and komatiite increased with time in sediment source areas at the expense of tonalite. The contributions of basalt and komatiite appear to reach a maximum during deposition of the Booysens shale, and granite during deposition of the K8 shales and possibly during deposition of the Orange Grove shales.