西藏班公湖-怒江缝合带西段去申拉组泥质硅质岩的发现及其地质意义

The discovery of Qushenla Formation argillaceous cherts in the western part of the Bangong Co-Nujiang suture zone,Tibet and its significance

首次在班公湖-怒江缝合带西段去申拉组中发现了泥质硅质岩,呈2个层位产出。为探讨泥质硅质岩的沉积环境、成因及与班-怒特提斯洋西段构造演化的关系,进行岩石学和地球化学分析,结果显示,第一层位泥质硅质岩Al_2O_3/(Al_2O_3+Fe_2O_3)、Ce/Ce~?、(La/Ce)_N、V/(Ni+V)、Ce/La、Ce_(anom)、Eu_(anom)平均值分别为0.60、0.80、1.24、0.72、1.84、-0.08、0.01,第二层位泥质硅质岩相应比值平均值分别为0.65、0.83、1.16、0.77、1.97、-0.07、0.02。结合泥质硅质岩的Fe_2O_3/TiO2-Al_2O_3/(Al_2O_3+Fe_2O_3)、(La/Ce)N-Al_2O_3/(Al_2O_3+Fe_2O_3)、Hf/3-Th-Ta关系图解,表明第一、二层位泥质硅质岩形成于活动大陆边缘,沉积时水-岩界面为水体分层不强烈的厌氧环境。U-Th、Zn-Ni-Co、La-Ce、La/Yb-REE关系图解和稀土元素特征指示了第一、二层位泥质硅质岩为热水成因,热水活动与玄武岩岩浆活动有关,第二层位泥质硅质岩沉积时热水活动更强烈。去申拉组泥质硅质岩的岩石学、地球化学特征表明,狮泉河地区班-怒特提斯洋至少在早白垩世仍具有一定规模的洋盆,其闭合时间应晚于约109Ma,进一步限定了洋盆的闭合时间。

The Qushenla Formation argillaceous cherts, which occur along two horizons, were found for the first time in the western part of the Bangong Co-Nujiang suture. In order to discuss depositional environment and origin of argillaceous cherts and infer the closure of the western part of Bangong Co-Nujiang Tethys Ocean, the authors made a systematic analysis of petrology and geochemistry of argillaceous cherts in this study. The ratios of Al2O3/(Al2O3+Fe2O3), Ce/Ce*,(La/Ce)N, V/(Ni+V), Ce/La, Ceanom,Euanomof argillaceous cherts in the first horizon are 0.60, 0.80, 1.24, 0.72, 1.84,-0.08, 0.01 respectively, while their ratios in the second horizon are 0.65, 0.83, 1.16, 0.77, 1.97,-0.07, 0.02 respectively. Considering the diagrams of Fe2O3/TiO2 vs. Al2O3/(Al2O3+Fe2O3),(La/Ce)N vs. Al2O3/(Al2O3+ Fe2O3)and Hf/3-Th-Ta,the authors hold that these characteristics indicate that all of argillaceous cherts tended to occur in an active continental margin setting and an anoxic environment. The diagrams of U-Th, ZnNi-Co, La-Ce and La/Yb-REE and the characteristics of REE demonstrates that the argillaceous cherts of the two horizons originated from hydrothermal sediments, closely related to basalt magmatism event, and the activities of hot water might have been more pronounced in the second horizon. These data provide petrologic and geochemical evidence for the evolution of Bangong CoNujiang Tethys Ocean. In Shiquanhe area there might have existed a certain scale ocean basin in the Early Cretaceous, which was closed at a time later than 109 Ma.