藏北湖区拉弄蛇绿岩枕状玄武岩地球化学特征及其成因

Geochemical Characteristics and Genesis of Pillow Basalts from the Lanong Ophiolite in Tibet,China

微量元素分析结果表明,藏北湖区拉弄蛇绿岩中枕状玄武岩富集大离子亲石元素Sr、Ba和放射性生热元素Th,且Rb变化混乱;Nb、Ta、Ti亏损,显示出TNT(Ta、Nb和Ti)的负异常现象,具有岛弧型火山岩的特点;而在稀土元素球粒陨石标准化配分图中显示平坦型曲线,玄武岩的(La/Yb)N和(Ce/Yb)N比值同N-MORB接近,表现出N-MORB的特征。通过对其构造环境的判别,并结合蛇绿岩与相邻地质体的关系,提出拉弄蛇绿岩可能是北侧的新特提斯洋在中晚侏罗世向南俯冲消减过程中,在其后缘诱发拉张作用引起次级弧后扩张,形成了新的俯冲带之上的弧后盆地大洋岩石圈,并在后来的拼贴过程中蛇绿岩构造就位于此。

The Lanong ophiolite,located in Jiangcuo,Bange,Tibet,is a key element within the Baila-Lanong ophiolite belt.Characterized by well preserved pillow lavas,the Lanong ophiolite consists of blocks of peridotites,gabbros,diabases and pillow lavas.Trace elements analysis indicates that the pillow lavas are enriched in Sr,Ba and Th,delpleted in Nb,Ta and Ti,clearly displaying compositional characteristics of the island-arc volcanic rocks.On the Chondrite-normalized trace element diagram,all lava samples show flat patterns,with （La/Yb）N and （Ce/Yb）N ratios similar to those of N-MORB.In conclusion,the Lanong ophiolite is of SSZ affinity,and should be the relics of a back arc basin.Combining the terranes＇ distribution with the discrimination results of geochemical diagrams,a simple tectonic evolution model is suggested：the Neotethys oceanic crust to the north began to southwardly subduct in middle-late Jurrasic,then a spreading sub-centre was formed on the obducting slab south of the subdction zone,which developed into a back arc basin subsequently.As the subdction going on,the back arc basin closed and only portions of the back arc crust were preserved by accretion.