湖北峡东地区奥陶系南津关组灰岩中硅质岩地球化学特征及其沉积环境意义

湖北峡东地区位于华南板块上扬子地块分区,其奥陶系南津关组灰岩中发育大量条带状和结核状硅质岩。在详细野外调查的基础上,对这些硅质岩进行岩相学和地球化学分析,探讨其成因及沉积环境意义。结果表明:硅质岩主要由石英、玉髓组成; 硅质岩贫Mn、Fe,其A1/(Al+Fe+Mn)值为0.27～0.84,平均值为0.64,Mn/TiO₂值小于0.5,Th/U值为0.02～0.86; 硅质岩具有低的稀土元素含量(质量分数,下同),稀土元素总含量为(0.67～27.79)×10^-6,经北美页岩标准化后呈现相对平缓右倾的稀土元素配分模式和弱的Ce异常,同时(La/Yb)_N平均值为1.31; 硅质岩寄主灰岩的δ¹³C值为-2.37‰～2.30‰,平均值为-0.006 4‰,δ¹⁸O值为-9.04‰～-6.49‰,平均值为-7.61‰,计算的古环境温度为4 ℃～21 ℃。这些结果反映湖北峡东地区奥陶系南津关组灰岩中的硅质岩主要为生物成因,形成于温暖缺氧的大陆边缘环境。

Geochemistry Characteristics of Cherts from Ordovician Nanjinguan Formation Limestone in the Eastern Yangtze Gorges Area of Hubei,China and Their Implications for Sedimentary Environment

The eastern Yangtze Gorges area in Hubei belongs to the Upper Yangtze block of South China plate. The cherts from Ordovician Nanjinguan Formation limestone in this area are mostly developed as nodules and strips. Based on the detailed field geological survey, the petrography and geochemistry characteristics of these cherts were analysed, and the genesis and sedimentary environment were discussed. The results show that the main mineral compositions of cherts are quartz and chalcedony. Cherts are poor in Mn and Fe, the ratios of A1/(Al+Fe+Mn)range from 0.27 to 0.84 with an average of 0.64, which is closer to that of the biogenic cherts. The ratio of MnO/TiO₂ is much less than 0.5, indicating cherts in this area form in the shallow sea sedimentary environment near the edge of the land. The ratios of Th/U range from 0.02 to 0.86 with an average of 0.24. Cherts have low REE content, the total REE content is(0.67-27.79)×10^-6. The NASC-normalized REE pattern shows a relatively gentle right-leaning trend. The δCe ranges from 0.97 to 1.19 with an average of 1.11, and the average value of(La/Yb)_N is 1.31, reflecting a bio-chemical origin rather than hydrothermal origin, and cherts form in a setting of continental margin. The carbon and oxygen isotope information of the host limestone of cherts in Nanjinguan Formation shows that the δ¹³C ranges from -2.37‰ to 2.30‰ with an average of -0.006 4‰, the δ¹⁸O ranges from -9.04‰ to -6.49‰ with an average of -7.61‰. The paleoenvironmental temperature varies from 4 ℃ to 21 ℃. The geochemistry characteristics of cherts in Nanjinguan Formation limestone indicate that cherts are biogenic origin and form in a warm and anoxic continental marginal tectonic settings.