扬子克拉通北缘碎屑沉积岩地球化学特征及意义

通过系统研究扬子克拉通北缘西乡-碑坝小区后太古代变质沉积岩的主量元素、微量元素和Nd同位素组成, 得出如下结论: (1) 扬子北缘中、晚元古代三花石组杂砂岩的ε_Nd(t), 分别为+2.72和+0.69, T_DM分别为1.40Ga和1.57Ga, 与扬子北缘同时期西乡群火山岩具有相近的Nd同位素组成, 扬子克拉通北缘新元古代存在的岛弧物质控制着该区中、晚元古代沉积物源的地球化学组成, 后河群碎屑物质对火地垭群沉积岩有一定的影响; (2) 扬子克拉通北缘古生代沉积物源区以再旋回沉积物源为主, 并随时间变化对再旋回沉积物源的接受逐渐加强, 沉积源区的长英质物质逐渐增多, 分异程度越来越高; 西乡-碑坝小区从寒武纪-志留纪沉积物源很可能是扬子克拉通基底崆岭群和基性火山岩等风化剥蚀后混合的产物, 本区从晚泥盆世开始, 其沉积物源区与秦岭群片麻岩的风化剥蚀有明显的关系; (3) 扬子北缘西乡-碑坝小区从晚二叠世到早侏罗世碎屑沉积岩的ε_Nd(t) 比早古生代碎屑沉积岩的明显偏大, 结合微量元素数据分析, 这与晚二叠世峨眉山玄武岩的多次喷发(火山灰) 和风化剥蚀作用有关. 

Geochemistry and Implications of Clastic Sedimentary Rocks from the Northern Margin of Yangtze Craton

This paper presents Sm-Nd and geochemical data on fine-grained sediments of the north margin from the Yangtze craton, China. Systematic research leads to the following conclusions: (1) Nd isotope composition of the clastic sedimentary rocks from the Middle-Upper Proterozoic has the ε_Nd(t) values of +2.72 to +0.69, with the Nd model ages of 1.40 Ga and 1.57 Ga, which corresponds to that of the contemporaneous volcanic rocks from Xixiang Group. It indicates that the arc materials from the Proterozoic control the geochemical composition of main provenances of the Middle-Upper Proterozoic sedimentary rocks. (2) The probable provenances of the Paleozoic sedimentary rocks are dominantly the recycled products of Proterozoic crust as indicated by the decrease of the ε_Nd(t) with the time. The sedimentary rocks from the Cambrian-Silurian were derived ultimately from a polycyclic mixture of the old continental crust materials from the Kongling strata and arc materials from the Proterozoic volcanic in the north margin of the Yangtze craton. The derivation is markedly of sources relatively more felsic extreme weathering. The geochemical and isotopic data indicate that the provenance of Devonian terrain has changed with the progressively increasing proportions of materials from the Qinling complex by erosion. (3) The more prominent increase in the ε_Nd(t) values of the Upper Permian formation probably reflects the significant incorporation of the mantle-derived materials. The comparison between the data of trace element and the data of EMS indicates that the dust of volcano from EMS was related to the Late Permian strata during the Late Permian when the volcanism, represented by Emeishan flood basalts (251-258) in the south-western margin of the Yangtze block was extremely active.