Geochemistry and Implications of Clastic Sedimentary Rocks from the Northern Margin of Yangtze Craton
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摘要: 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εNd(t), 分别为+2.72和+0.69, TDM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εNd(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.Abstract: 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 εNd(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 εNd(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 εNd(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.
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图 1 扬子克拉通北缘西乡-碑坝地区区域地质略图(高山, 1989)
1.西乡群; 2.后河群、火地娅群; 3.震旦纪-显生宙沉积岩; 4.基性-超基性岩; 5.英云闪长岩、奥长花岗岩; 6.偏碱性或碱性花岗岩类; 7.断层
Fig. 1. Generalized geological map of Xixiang-Beiba subunit
图 2 扬子北缘西乡-碑坝小区沉积岩Al2O3- (CaO*+Na2O) -K2O三角图(Nesbitt and Young, 1989)
图中a线表示风化趋势; b线表示钾质交代趋势; c线表示风化线; 黑色圆点表示平均页岩组成; 斜线花圆点表示上地壳组成
Fig. 2. Ternary plot of Al2O3- (CaO*+Na2O) -K2O for the Xixiang-Beiba metasediments
图 3 西乡-碑坝小区中晚元古代(a) 和显生宙(b) 碎屑沉积岩稀土元素配分曲线
Fig. 3. Chondrite-normalized REE diagram of clastic sediments for the Middle-Upper Proterozoic (a) and the Phanerozoic (b) from Xixiang-Beiba subunit
图 4 扬子克拉通西乡-碑坝小区沉积岩εNd (t) 随时间(t) 演化图
图中新元古火山岩包括二郎坪群火山岩(张宗清等, 1994), 耀岭河群火山岩(黄萱和吴利仁, 1990), 西乡群火山岩, 毛堂群(待刊); 佛坪群和秦岭群包括佛坪群黑云石榴片麻岩(王居里和张国伟, 1999), 秦岭群蛇尾黑云石榴片麻岩和斜长透灰石岩(张宗清等, 1994), 陡岭群变质碎屑沉积岩(沈洁等, 1997); 后河群包括奥长花岗岩和英云闪长岩(凌文黎, 1994), 鱼洞子群黑云斜长片麻岩(张宏飞等, 1997); 崆岭群为奥长花岗岩和英云闪长岩(马大铨等, 1997; Gao et al., 2001); 其余为本文数据, 根据沉积岩地层年龄计算样品的εNd (t), 由于部分地层还没有准确的年龄限制, 因此本文所用沉积年龄是近似值, 计算参数: (143Nd/144Nd) CHUR (0) =0.512 638, (147Sm/144Nd) CHUR (0) =0.196 7
Fig. 4. εNd (t) versus t for the sediments from Xixiang-Beiba subunit, Yangtze craton
图 5 扬子克拉通北缘西乡-碑坝小区沉积岩与秦岭群杂砂岩微量元素对比
图中秦岭群杂砂岩据高山(1989);西乡-碑坝小区碎屑沉积岩据表 1
Fig. 5. Comparison of average trace elements contents of sediments between the Xixiang-Beiba subunit and Qinling terrain
表 1 扬子克拉通西乡-碑坝小区和高川小区碎屑沉积岩主量元素(%) 和微量元素含量(10-6)
Table 1. Concentration of trace elements of clastic sediment from Xixiang-Beiba subunit
表 2 南秦岭和扬子克拉通各小区碎屑沉积岩及部分火山岩Sm-Nd同位素数据
Table 2. Sm-Nd isotopic composition of clastic sediment from Xixiang-Beiba subunit
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