西藏雅鲁藏布江缝合带西段巴尔地幔橄榄岩成因及构造意义

摘要

The Baer ophiolite crops out in the western segment of the Yarlung-Zangbo suture zone ( YZSZ), about 1200km west of Lhasa. It extends more than 40km, in an E-W and is 3 ~ 4km wide. The Baer peridotites consist of cpx-harzburgites and minor lherzolites. According to the texture and structure characteristics in peridotites, the mineral assemblage can be divided into three generations: ( 1 ) The first generation, the mineral assemblage of the residual mantle, includes olivine, orthopyroxene and clinopyroxene porphyroclast; (2) The second generation includes olivine, orthopyroxene, clinopyroxene and spinel. They are always along the first generations mineral porphyroclasts. They are the products of partial melting and melt-peridotites reaction in the peridotites. (3) The third generation is amphibole, which are the products of mantle metasomatism. Three generation mineral assemblages are interpreted to suggest that the Baer peridotites experienced a two-stage evolution. At first, they were formed at a MOR (mid-ocean ridge) setting and subsequently entered a SSZ ( super-subduction zone) setting. Compared to the characteristics of the YZSZ peridotites, we argue that there are two type ophiolites in the YZSZ. The peridotites in the middle section in the YZSZ may be formed in a complex SSZ setting, where the peridotites in the west and east section maybe formed in MOR setting but reconstructed by SSZ. Such tectonic evolutions features maybe attribute to the heterogeneity and complexity of tectonic evolution along the Neo-Tethys Ocean.%巴尔蛇绿岩属于雅鲁藏布江缝合带的西延部分,距拉萨约1200km,主要由地幔橄榄岩、少量的橄长堆晶岩和玄武岩组成.地幔橄榄岩主体为合单辉方辉橄榄岩,少量为二辉橄榄岩.根据巴尔蛇绿岩地幔橄榄岩的结构构造特征,将矿物组合划分为3个世代,第一世代残余地幔矿物组合:橄榄石+斜方辉石+单斜辉石；第二世代部分熔融及熔-岩反应矿物组合:橄榄石+斜方辉石+单斜辉石+尖晶石；第三世代地幔交代作用矿物组合,主要为含水矿物角闪石.将3个世代的矿物组合归并为2个演化阶段:第一阶段,包括第一世代和第二世代矿物组合,形成于MOR(mid-ocean ridge)构造环境下的洋脊扩张阶段；第二阶段,为第三世代矿物角闪石,形成于SSZ(super-subduction zone)环境下的俯冲阶段.对比雅鲁藏布江缝合带不同区段蛇绿岩中地幔橄榄岩的特征,发现雅鲁藏布江缝合带存在MOR和SSZ两种类型的蛇绿岩,其中中段的蛇绿岩主要以典型的SSZ型地幔橄榄岩为主,而东、西段则以受到不同程度SSZ环境改造的MOR型地幔橄榄岩为主,认为雅鲁藏布江缝合带蛇绿岩地幔橄榄岩演化分段性的特征,与新特提斯洋沿弧方向上板块活动的动力学机制的不均一有关.