雅鲁藏布江缝合带中部硅岩地球化学特征及构造环境制约  

Sedimentary Environments of Siliceous Rock in the Western Yarlung Zangbo Suture Zone, Southern Tibet

作　　者：孙立新[1] 万晓樵[1] 贾建称[1] 胡华斌[1] 

机构地区：[1]中国地质大学岩石圈构造,深部过程及探测技术实验室,北京100083

出　　处：《地质学报》2004年第3期380-389,共10页Acta Geologica Sinica

基　　金：中国地质调查局 1: 2 5万区域地质调查项目 (编号 2 0 0 0 13 0 0 0 0 9181);国家自然科学基金项目 (编号 40 2 72 0 0 7)联合资助成果

摘　　要：硅岩在雅鲁藏布江缝合带广泛发育。在 1∶ 2 5万萨嘎县幅、桑桑区幅区域地质调查中 ,在缝合带及其南侧宗卓组中识别出 3种硅岩沉积组合 :1嘎学群内与玄武岩伴生的硅岩 ;2构造混杂带内夹于页岩中的硅岩 ;3宗卓组内夹于页岩—岩屑石英杂砂岩的硅岩。在硅岩中分离出的放射虫组合时代为白垩纪。运用地球化学研究手段 ,对硅岩岩石组合、岩石化学、微量元素和稀土元素特征进行了综合分析 ,与玄武岩伴生的硅岩 Al/ (Al+Fe+Mn)为 0 .5 33～ 0 .5 4 6 <0 .6 19(除 9号样外 ) ,Ce/ Ce* 为 0 .6 7(平均 ) ,L an/ Ybn为 0 .83～ 1.37和 L an/ Cen为0 .82～ 2 .2 3;夹于页岩中的硅岩 Al/ (Al+Fe+Mn)为 0 .5 5 7～ 0 .6 19,Ce/ Ce*为 0 .995～ 1.35 0 ,L an/ Ybn为 0 .71～ 1.0 3和 L an/ Cen为 0 .6 8～ 0 .98;宗卓组内的硅岩 Al/ (Al+Fe+Mn)为 0 .6 15～ 0 .70 7>0 .6 19,Ce/ Ce* 为 0 .94～1.14 ,L an/ Ybn 为 1.4 4～ 1.6 6和 L an/ Cen 为 0 .85～ 1.0 7。研究证实 ,3种背景的硅岩均反映非热水或生物成因。嘎学群内与玄武岩伴生的硅岩具深水盆地沉积特征 ;宗卓组中的硅岩具典型大陆边缘沉积环境特征 ;Siliceous rocks are widespread in the western Yarlung Zangbo Suture Zone, and assigned to Cretaceous based on radiolarian fossils. Three principal lithologic associations have been recognized: (1) alternating pillow-basalt/siliceous rock association; (2) mélange chert association; and (3) clastic/chert association. Geochemically, siliceous rock element ratios of Al/ (Al+Fe+Mn), MnO/TiO 2, and Ce/Ce * anomaly reveal sedimentary features. Siliceous rocks from the alternating pillow-basalt/chert association have ratios of Al/(Al+Fe+Mn)=0.533～0.546 (<0.619), Ce/Ce *=0.67 (on average), (La/Yb) n=0.83～1.37 and (La/Ce) n=0.82～2.23. Siliceous rocks from the mélange have ratios of Al/(Al+Fe+Mn) = 0.557～0.619 (<0.619), Ce/Ce * = 0.995～1.35, (La/Yb) n=0.71～1.03 and (La/Ce) n=0.68～0.985. Chert from the clastic/chert association of the Zongzhuo Formation have ratios of Al/(Al+Fe+Mn)=0.615～0.707, Ce/Ce *=0.94～1.14, (La/Yb) n =1.44～1.66 and (La/Ce) n= 0.85～1.07. In the Al-Fe-Mn diagram the siliceous rock samples fall into the biogenesis field. A biogenesis of the three principal lithologic associations is supported. Each association represents a distinct envirment, i.e. alternating pillow basalt/chert association deposited in an oceanic basin, clastic/chert turbidite graywacke-chert deposited in a deep-water environment of continental margin, and mélange chert-shale deposited in the transition belt between oceanic basin and continental margin in the western Yarlung Zangbo Suture Zone.

关 键 词：地球化学 构造环境 沉积 硅岩 白垩纪 

分 类 号：P588.2[天文地球—岩石学]