Journal of Earth System Science ( IF 1.9 ) Pub Date : 2021-08-24 , DOI: 10.1007/s12040-021-01673-4 Priyanka Mazumdar 1 , Ananya Mukhopadhyay 1 , Alono Thorie 1 , Tithi Banerjee 1 , Santosh Kumar Rai 2
Research highlights
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This is the first geochemical work ever done for the shales of Neoproterozoic Chhaosa Formation of Simla Group, Lesser Himalaya.
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K2O/Na2O vs. SiO2/Al2O3, TiO2 vs. Al2O3, and Zr vs. TiO2 binary diagrams of the studied shales indicate felsic source rock.
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High CIA, CIW, A–CN–K and Al2O3 values of Chhaosa shales support intermediate to strong weathering of source rock.
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K2O/Na2O vs. SiO2 plot, K2O vs. Na2O, plot and CaO–K2O–Na2O ternary plot represent deposition of Chhaosa deposits in a passive margin setting.
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Values of Ni/Co, Cu/Zn, V/Cr, authigenic U and U/Th, represent a well-oxygenated condition during the deposition of the Chhaosa delta system. This is consistent with the concept of an increase in oxygen levels during the Neoproterozoic era is favoured by many workers.
Abstract
Neoproterozoic Chhaosa shales of Simla Group, Lesser Himalaya, were considered for major and trace element analysis to delineate palaeo-weathering, palaeo-oxygenation, tectonic setting, sediment maturity, and provenance. The shales exhibit a significant proportion of SiO2, Al2O3, K2O, MgO, Fe2O3, Zr, Zn, Rb, V, Cr, Sr, Y, Co, Ga, Th, Nb, Sc, and U. Source rocks are primarily of granitic (acidic) origin, as indicated by Al2O3 wt.% vs. TiO2 wt.% and Cr (ppm) vs. Ni (ppm) bivariate plots. Th/Cr, Cr/Th, Th/Sc, and Th/Co values of studied samples have been correlated with post-Archaean Australian average shale (PAAS) and upper continental crust (UCC) values, which indicate that the source rock is felsic. Palaeo-weathering condition of the source is delineated by average CIA, chemical index of weathering (CIW), and A–CN–K triangular plot values. The shales depict intermediate to strong chemical weathering and semi-arid to humid climatic conditions. V/Cr, Cu/Zn, Ni/Co, and U/Th values reveal sedimentation of Chhaosa Formation under oxic atmospheric conditions, which corroborates with the inferred oxic conditions of the Neoproterozoic. SiO2 vs. K2O/Na2O discriminant diagram exhibits deposition along a passive margin field. ICV values suggest deposition of compositionally mature sediments under tectonically stable conditions. Deposition of Chhaosa delta system occurred during a relatively quiescent stage of tectonism between intermittent phases of rifting related to the Rodinia Supercontinent.
中文翻译：
新元古代 Chhaosa 页岩的地球化学，Simla Group，小喜马拉雅山：其对物源和构造的影响
研究亮点
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这是对小喜马拉雅山西姆拉群新元古代Chhaosa组页岩的首次地球化学工作。
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所研究页岩的K 2 O/Na 2 O vs. SiO 2 /Al 2 O 3、TiO 2 vs. Al 2 O 3和Zr vs. TiO 2二元图表明长英质烃源岩。
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Chhaosa 页岩的高 CIA、CIW、A-CN-K 和 Al 2 O 3值支持烃源岩的中强风化。
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K 2 O/Na 2 O vs. SiO 2图、K 2 O vs. Na 2 O 图和CaO–K 2 O–Na 2 O 三元图表示在被动边缘环境中Chhaosa 矿床的沉积。
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Ni/Co、Cu/Zn、V/Cr、自生 U 和 U/Th 的值代表 Chhaosa delta 系统沉积过程中的充氧条件。这与新元古代时期氧含量增加的概念是一致的，受到许多工人的青睐。
抽象的
考虑对小喜马拉雅山 Simla Group 的新元古代 Chhaosa 页岩进行主量和微量元素分析，以描绘古风化、古氧化作用、构造环境、沉积物成熟度和来源。页岩表现出显着比例的 SiO 2、Al 2 O 3、K 2 O、MgO、Fe 2 O 3、Zr、Zn、Rb、V、Cr、Sr、Y、Co、Ga、Th、Nb、Sc、和 U。烃源岩主要是花岗岩（酸性）成因，如 Al 2 O 3 wt.% vs. TiO 2 wt.% 和 Cr (ppm) vs. TiO 2 wt.% 所示。Ni (ppm) 二元图。研究样品的 Th/Cr、Cr/Th、Th/Sc 和 Th/Co 值与澳大利亚太古代后平均页岩 (PAAS) 和上大陆地壳 (UCC) 值相关，表明烃源岩为长英质. 源的古风化条件由平均 CIA、化学风化指数 (CIW) 和 A-CN-K 三角图值描述。页岩表现出中等至强烈的化学风化作用和半干旱至潮湿的气候条件。V/Cr、Cu/Zn、Ni/Co 和 U/Th 值揭示了在含氧大气条件下 Chhaosa 组的沉积，这证实了新元古代的含氧条件的推断。SiO 2与K 2 O/Na 2O 判别图展示了沿被动边缘场的沉积。ICV 值表明在构造稳定的条件下成分成熟的沉积物的沉积。Chhaosa delta 系统的沉积发生在与 Rodinia 超大陆有关的间歇性裂谷阶段之间的相对静止的构造活动阶段。
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