秦皇岛地区奥陶纪硅质岩地球化学特征及沉积环境意义

Geochemistry Characteristic and the Environmental Implications of Chert from Ordovician in Qinhuangdao Area

秦皇岛地区位于华北地台东北缘,该区奥陶系亮甲山组和马家沟组主要由细晶白云质灰岩、细晶白云岩、豹皮状灰岩、砾屑灰岩组成,灰岩中普遍含有结核状、条带状和团块状硅质岩。XRD分析显示硅质岩主要矿物成分为石英、方解石和少量白云石,硅质岩显微镜下为蠕虫状、球状、针状玉髓集合体,个别样品可见交代结构。硅质岩贫Mn、Fe,Mn/TiO2小于0.5,Si/(Si+Al+Fe)比值在0.98～0.99之间,U/Th(0.422～1.225),REE总量低;硅质岩经北美页岩标准化的REE配分模式图上无明显Ce负异常,呈现平坦配分曲线;同时(La/Ce)N=(0.98～1.23),(La/Yb)N=(0.8～1.78),Ce/Ce*=0.92～1.02,反映奥陶纪亮甲山组和马家沟组中硅质岩团块和条带为生物化学成因,形成于离陆较近的浅海环境。

The Qinhuangdao area is located in the northeastern margin of the North China Block.The Liangjiashan and Majiagou Formation in this area are mainly composed of fine-grained dolomitic limestone,fine-grained dolomite,leopard-like limestone and calcirudite,which generally contain nodular,striped and lump cherts.XRD analyses show that the major minerals of the silicalite are quartz,calcite and some dolomite.The silicalite samples are worm-like,spherical and needle-like chalcedony aggregates with metasomatic structure in some of them under the microscope.The silicalites are poor in Mn and Fe,with Si /（Si ＋ Al ＋ Fe） ratios of 0.98-0.99,low ∑REE,and very weak negative Ce anomalies in the NASC-normalized flat REE patterns.The characteristics of Mn / TiO2（ 0.5）,U / Th（0.422-1.225）,Ce / Ce＊（0.92-1.02）,（La / Ce）N（0.98-1.23）,and（La/Yb）N（0.8-1.78） reflects a bio-chemical origin rather than hydrothermal origin formed in a setting of continental margin.