狮泉河—永珠—嘉黎构造带中西段硅质岩地球化学特征及其沉积环境

放射虫硅质岩为蛇绿岩的上覆沉积盖层,其地球化学特征可以帮助探讨蛇绿岩的形成环境和沉积盆地的构造-古地理演化。对狮泉河—永珠—嘉黎构造带西段古昌蛇绿岩中的硅质岩进行了详细的地球化学分析,并与拉果错和果芒错硅质岩进行了对比研究,以探讨它们的成因及沉积环境。古昌硅质岩SiO₂含量较低,Al/(Al+Fe+Mn)比值为0.87~0.95,为非热液成因硅质岩,稀土配分模式整体为轻稀土略亏损的左倾曲线,具有明显的负Eu异常,基本无Ce异常,Y/Ho比值为21~24,Ti/V比值为220.67~862.02,V/Y比值为0.05~0.29,沉积于大陆边缘环境。对比分析发现,拉果错硅质岩具有与古昌硅质岩相似的特征,而果芒错硅质岩显示了一定大洋盆地硅质岩的特征,并且受到轻微热液作用影响,可能形成于一个更加成熟开阔的洋盆之中。综合蛇绿岩其他端员的研究成果,狮泉河—永珠—嘉黎构造带中西段可能代表了一个靠近大陆边缘的古洋盆,东部果芒错地区发育较为开阔,往西洋盆规模逐渐变小。

Geochemical characteristics and sedimentary environments of siliceous in the middle and western parts of the Shiquanhe-Yongzhu-Jiali tectonic belt

The geochemistry of radiolarian siliceous, as sedimentary cover of ophiolite, plays a role in investigating the formation environment of ophiolite and tectonic-paleogeographic evolution of the sedimentary basin. In this paper, we studied the petrology and whole-rock geochemistry of Guchang siliceous in the western part of the Shiquanhe-Yongzhu-Jiali tectonic belt (SYJTB), and carried out a comparative analysis involving Laguoco and Guomangco siliceous to determine the origin and sedimentary environments of siliceous in these areas. The low SiO₂ content and Al/(Al+Fe+Mn) ratio (0.87-0.95) indicate a non-hydrothermal origin of Guchang siliceous. The rare earth element (REE) patterns of most of these rocks show left-leaning types with slight light REE (LREE) and obvious Eu depletion and no Ce anomalies. The Y/Ho, Ti/V and V/Y ratios in Guchang siliceous are 21-24, 220.67-862.02 and 0.05-0.29, respectively. These results suggest that Guchang siliceous is formed in a continental margin environment. The characteristics of Laguoco and Guchang siliceous were similar. However, Guomangco siliceous has some oceanic basin siliceous characteristics affected slightly by hydrothermal fluids, indicating its probable formation in a more mature oceanic basin. Combining with the ophiolite data, the middle and western part of the SYJTB likely represent an ancient oceanic basin near the continental margin, and the oceanic basin in Guomangco is more mature and wider than that those in Guchang and Laguoco.