The Cretaceous of the Eastern Bangong-Nujiang Suture Zone (Tibet):Tectono-Sedimentation

1IntroductionThe Tibet plateau consists of terranes which were accreted in a southward-younging succession from Late Permian to Mid-Eocene (Dewey etal., 1988; Mattern etal., 1998; Mattern and Schneider, 2000). The Songpan-Ganzi terrane was accreted to the Kunlun during Late Permian along Kunlun-Qin-ling suture. The Qiangtang block was added onto the Songpan-Ganzi terrane at the Jinsha suture dur-ing Late Triassic-Early Jurassic. The Lhasa block became attached to the Qia…     

青藏高原新特提斯蛇绿岩的地质特征及其构造演化

西藏雅鲁藏布江缝合带（YZSZ）和班公湖-怒江缝合带（BNSZ）蛇绿岩代表了新特提斯洋壳和岩石圈地幔残余，是我国铬铁矿和蛇绿岩型金刚石的重要原产地，目前这两条蛇绿岩带的成因和相互关系还存在着争论。本文总结了YZSZ、BNSZ、狮泉河-纳木错蛇绿混杂岩带（SNMZ）和松多缝合带蛇绿岩的时空分布、组成和构造背景，归纳了拉萨地块晚古生以来的岩浆岩分布，获得以下主要认识：（1）Panjal地幔柱活动可能促使怒江洋和雅江西洋在早二叠世空谷期（283~272Ma）打开；（2）雅江东洋由于松多洋的南向俯冲在晚三叠世打开，与雅江西洋以萨嘎-措勤为界，并形成冈底斯东部245~200Ma岩浆热事件；（3）~140Ma班怒洋闭合以及南羌塘与北拉萨地块碰撞，导致雅江洋扩张速率加快而引发了北向拉萨地块的平板俯冲，进而导致班怒洋的再次裂解形成133~104Ma"红海型"小洋盆；（4）YZSZ缝合带西段南带蛇绿岩为北带的逆冲推覆体；（5）BNSZ和SNMZ蛇绿岩隶属于一个洋盆，后者代表了班怒洋成熟洋盆扩张脊的残余。     

西藏拉萨地块阿翁错北二长花岗岩成因：锆石U-Pb年代学、岩石地球化学及Hf同位素制约

西藏拉萨地块阿翁错—盐湖岩浆弧的成因是解决班公湖—怒江特提斯洋俯冲极性和时限的关键。本文选取阿翁错北岩体中的二长花岗岩进行岩相学、锆石U-Pb年代学、岩石地球化学与Hf同位素研究。结果表明：阿翁错北二长花岗岩的锆石²⁰⁶Pb/²³⁸U加权平均年龄为(107.0±0.5) Ma,MSWD＝2.6,属早白垩世晚期。样品表现为高硅、富钾的高钾钙碱性岩石系列,A/CNK值介于1.006～1.019之间,属弱过铝质;微量元素富集大离子亲石元素Rb、K、U、Th及轻稀土元素（LREE）,亏损Nb、Ta、P、Ti等高场强元素,具中等至弱的负Eu异常（δEu=0.55~0.78）,属弱过铝质未分异的I型花岗岩。二长花岗岩样品锆石初始Hf同位素ε_Hf(t)值除1颗锆石达11.1外,其他17颗锆石介于3.7~6.3之间,平均值5.0,Hf同位素二阶段模式年龄变化于928~765 Ma之间。基于同位素以及岩石地球化学数据,阿翁错北岩体很可能是新生地壳熔融产生的长英质岩浆与镁铁质岩浆发生不均一混合作用形成,并有少量幔源物质的参与。结合拉萨地块中北部岩浆岩Hf同位素研究分析,阿翁错—盐湖岩浆弧形成于班公湖—怒江特提斯洋后退式俯冲的构造体制下,阿翁错北岩体的形成时代（107~104 Ma）代表了由断离板片俯冲末期向碰撞环境转化的时限。     

Petrology,geochemistry and geochronology of the Zhongcang ophiolite,northern Tibet: implications for the evolution of the Bangong-Nujiang Ocean

Ophiolites are widespread along the Bangong-Nujiang suture zone, northern Tibet. However, it is still debated on the formation ages and tectonic evolution process of these ophiolites. The Zhongcang ophiolite is a typical ophiolite in the western part of the Bangong-Nujiang suture zone. It is composed of serpentinized peridotite, cumulate and isotropic gabbros, massive and pillow basalts, basaltic volcanic breccia, and minor red chert. Zircon SHRIMP Ue Pb dating for the isotropic gabbro yielded weighted mean age of 163.4 ± 1.8 Ma. Positive zircon ε Hf(t) values(+15.0 to +20.2) and mantle-like σ~(18)O values(5.29 ±0.21)% indicate that the isotropic gabbros were derived from a long-term depleted mantle source. The isotropic gabbros have normal mid-ocean ridge basalt(N-MORB) like immobile element patterns with high Mg O, low TiO_2 and moderate rare earth element(REE) abundances, and negative Nb,Ti, Zr and Hf anomalies. Basalts show typical oceanic island basalt(OIB) geochemical features, and they are similar to those of OIB-type rocks of the Early Cretaceous Zhongcang oceanic plateau within the Bangong-Nujiang Ocean. Together with these data, we suggest that the Zhongcang ophiolite was probably formed by the subduction of the Bangong-Nujiang Ocean during the Middle Jurassic. The subduction of the Bangong-Nujiang Tethyan Ocean could begin in the Earlye Middle Jurassic and continue to the Early Cretaceous, and finally continental collision between the Lhasa and Qiangtang terranes at the west Bangong-Nujiang suture zone probably has taken place later than the Early Cretaceous(ca. 110 Ma).     

西藏班公湖-怒江缝合带中段塔仁本洋岛型玄武岩的发现及地质意义

塔仁本洋岛型玄武岩是中特提斯洋古洋壳的重要组成部分,是班公湖怒江缝合带内构造混杂岩中局部有序部分的特殊洋壳残片。它以具有海底爆发相与远洋放射虫硅质岩相整合和大洋板内碱性玄武岩的构造环境为主要特征。它的发现丰富了班怒带的研究内容,证明当时中特提斯洋曾经有发育完好的古洋壳存在     

班公湖-怒江结合带北侧陆缘火山-岩浆弧带的厘定及其意义

在班公湖-怒江结合带西段北侧的拉热拉新岩体东、西两侧,新发现了一些早白垩世岩体、相伴的陆缘火山岩组合和矿(化)点,侵入岩和火山岩的岩石化学特征均显示其成因与中特提斯洋向北俯冲消减密切相关。本文将班- 怒带北侧的火山- 侵入岩带厘定为五峰尖拉热拉新晚侏罗世—早白垩世陆缘火山岩浆弧带,同时讨论了陆缘火山- 岩浆弧带的厘定在分析中特提斯构造演化方面的研究意义。