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地质学报  2011 

鄂西长阳南华系地球化学特征的气候指示意义及地层对比

, PP. 576-585

Keywords: 化学蚀变化指数(CIA),SiO2/Al2O3,FeO/Fe2O3,气候,南华系,鄂西长阳

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Abstract:

鄂西长阳南华系出露良好、发育完整,主要由莲沱组、南沱组构成,少数地段(长阳古城)包括古城组和大塘坡组。莲沱组为河流-河口湾相的碎屑岩组合,区域上南厚北薄,岩性三分性明显。古城组为典型的冰碛岩,对应于国际上Sturtian冰期。大塘坡组为间冰期海陆过渡相沉积。南沱组为研究区内最发育、最重要的冰碛地层,对应于国际上Marinoan冰期,区域上岩性变化不大,厚度具西南厚北东薄之趋势。长阳流溪剖面上,仅发育莲沱组和南沱组。南沱组CIA值介于52.52~55.80范围内,反映寒冷干燥气候条件、低等化学风化程度的沉积环境。莲沱组底部CIA值为68.14,为温暖湿润气候条件、中等化学风化程度的沉积;下部CIA值介于61.61~64.57之间,为温暖干燥气候条件、低等化学风化程度的沉积;上部CIA值介于55.03~59.80之间,为寒冷干燥气候条件、低等化学风化程度的沉积。莲沱组SiO2/Al2O3值介于2.13~5.10之间,变化较大,但总体明显低于南沱组SiO2/Al2O3值4.47~5.16,莲沱组FeO/Fe2O3值介于0.16~0.49之间,远低于南沱组FeO/Fe2O3值1.96~3.11,也就是说南沱组形成时期气候寒冷干燥、化学风化作用较弱,而莲沱组沉积时期气候相对温暖、化学风化作用相对较强。CIA值和SiO2/Al2O3、FeO/Fe2O3比值表明研究区南华系经历了“莲沱间冰期”温暖湿润-温暖干燥气候、低-中等化学风化程度,至“莲沱间冰期”寒冷干燥气候、低等化学风化程度,再到“南沱冰期”寒冷干燥气候、低等化学风化的沉积环境变化过程。

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