西藏果芒错蛇绿混杂岩中硅质岩的地球化学特征及其形成环境

Geochemical characteristics and sedimentary environments of siliceous rocks in Guomangco ophioliticmélange of Tibet

果芒错蛇绿混杂岩位于狮泉河—永珠—嘉黎蛇绿混杂岩带中段,是该带中保存较好的一套蛇绿混杂岩,其形成环境是确定狮泉河—永珠—嘉黎蛇绿混杂岩带构造属性的重要依据。对果芒错蛇绿混杂岩中的硅质岩进行了地球化学分析,为判断蛇绿混杂岩的形成环境提供新的约束条件。硅质岩通常呈几十厘米夹层产于玄武岩中,含有大量晚三叠世—白垩纪放射虫化石。硅质岩SiO2含量为71.38%～77.67%,Al2O3含量为8.62%～11.51%,MnO/TiO2值为0.28～0.35,（Ce/Ce＊）SN值为0.92～0.94,（La/Ce）SN值为1.13～1.17,反映了陆源物质的影响,而V、Ni、Cu和V/Y值高于大陆边缘硅质岩,与洋中脊和大洋盆地硅质岩相似,说明果芒错硅质岩可能形成于受陆源物质影响且与大陆边缘有一定距离的环境中。结合变质橄榄岩、镁铁质岩墙和玄武岩的地球化学特征,初步认为果芒错蛇绿混杂岩的形成环境为靠近大陆边缘的弧后盆地。

The Guongmangco ophiolitic mélange is well developed in the middle part of Shiquanhe-Yongzhu-Jiali ophiolitic mélange belt, and its formational environment can provide important evidence for investigating the tectonics of the Shiquanhe-Yongzhu-Jiali ophiolitic mélange belt. In this paper, the geochemistry of siliceous rocks was studied to judge the formational environment of the Guomangco ophiolitic mélange. Siliceous rocks occur as intercalated beds in basalt and have abundant Late Triassic-Early Cretaceous radiolarite. The siliceous rocks have SiO2 content of 71.38%～77.67%, high Al2O3 content of 8.62%～11.51%, MnO/TiO2 ratios of 0.28～0.35,（Ce/Ce＊）SNratios of 0.92～0.94 and（La/Ce）SNratios of 1.13～1.17, implying the influence of the input of terrigenous components. However, they have higher V, Ni and Cu content and V/Y ratios than the siliceous rocks in epicontinental settings, similar to things of siliceous rocks in ocean mid-ridge and basin. Guomangco siliceous rocks were probably formed in an ocean basin relatively far away from the continent and mainly affected by the input of terrigenous components. In combination with the geochemistry of metamorphic peridotite, mafic dike and basalt, the authors hold that the Guomangco ophiolitic mélange was probably formed in a back-arc basin close to the continental margin.