香山群狼嘴子组硅质岩地球化学特征及形成环境

香山群自建群以来,时代归属一直存在争议,通过区域地质调查,把香山群地质时代限定为中晚寒武世。为深入探讨鄂尔多斯西南边缘香山群形成的沉积环境和构造背景,对香山群狼嘴子组硅质岩岩石学、地球化学特征进行了综合分析。分析结果表明硅质岩SiO2含量为82.44%～98.13%,Al/(Fe Mn Al)比值为0.46～0.69,Al2O3/(Al2O3 Fe2O3)比值为0.53～0.77;LREE/HREE为3.57～8.45,轻重稀土分异作用强,LaN/CeN值0.66～1.15,Ce/Ce*值0.66～0.93,为大陆边缘型硅质岩,成岩过程包含热水沉积和正常沉积作用。同时,硅质岩建造中碎屑岩和产出的辉绿岩床的地球化学特征表明:碎屑岩沉积背景也属被动大陆边缘区,辉绿岩属于伸展作用构造背景下的大洋拉斑玄武岩系列。综合地质资料认为,在中晚寒武世,阿拉善地块南缘与鄂尔多斯地块西南缘具有离散型大陆边缘性质。

Geochemical characteristics and tectonic setting of cherts in the Langzuizi Formation of the Xiangshan Group

The age assignment of the Xiangshan Group has long been controversial since its establishment. Regional geological data indicate that the Xiangshan Group is Middle-Late Cambrian in age. In order to discuss the sedimentary environments and tectonic setting of the Xiangshan Group on the southwestern margin of the Ordos basin, the petrological and geochemical characteristics of cherts in the Langzuizi Formation of the Xiangshan Group are analyzed. The analysis of cherts suggests that: the SiO2 content is 82.44%-98.13%, the Al/(Fe Mn Al) ratio ranges from 0.46 to 0.69 and Al2O3/(Al2O3 Fe2O3) is 0.53-0.77; the fractionation between LREE and HREE is strong with LREE/HREE=3.57-8.45, LaN/CeN is 0.66-1.15 and Ce/Ce is 0.66-0.93. These characteristics show that the cherts formed in a continental-margin environment and that the diagenetic processes included hydrothermal deposition and normal deposition. The geochemical characteristics of clastic rocks and diabase sills in cherts indicate that: clastic rocks were also deposited on the passive continental margins and diabase belongs to the oceanic tholeiite series formed in an extensional setting. An integration of geological data suggests that an environment of divergent continental-margin nature occurred on the southwestern margin of the Ordos block and the southern margin of the Alxa block during the Mid-Late Cambrian.