西藏冈底斯带花岗岩的时空分布特征及地壳生长演化信息

Spatial and Temporal Distribution and Characteristics of Granitoids in the Gangdese,Tibet and Implication for Crustal Growth and Evolution

花岗岩是大陆特有的重要组成部分,同时与矿产资源有密切的关系。西藏花岗岩约占西藏自治区面积的12%,其中,80%分布在冈底斯岩浆岩带。在空间上,冈底斯花岗岩带大致可以分为3个亚带:北带、中带和南带;在时间上,以印度—亚洲大陆碰撞事件为标尺,可将青藏高原构造-岩浆事件划分为碰撞前(>65Ma)、碰撞期(65~45Ma)、后碰撞(<45Ma)3大阶段。在这3个阶段中,冈底斯带都产生了具有各自特点的花岗岩构造-岩浆类型。冈底斯花岗岩类的形成演化与新特提斯班公湖-怒江洋及雅鲁藏布洋的形成演化,有密切的关系。因此,其岩石的Nd,Sr同位素资料,对地壳生长与演化有重要的指示意义。冈底斯南带的大部分地区的花岗岩,均具有εNd(t)(+)值(+1.64^+5.21),模式年龄tDM也很年青(<500Ma),具有初生(juvenile)地壳的特征。在花岗岩成因中地幔物质有重要的贡献。而冈底斯中带、北带及南带西段的花岗岩类以εNd(t)(-)值为特征(-5.3^-17.3),模式年龄tDM有两组值1.2Ga及2.0~2.5Ga,暗示这些地区的地壳具有古元古代—中元古代基底在花岗岩成因中,地壳组分具有主要贡献。

Granites are important components in the continents, and also closely related to mineral resources. Granites take c. 12% of the total area of Tibetan territory and about 80% （in area） of them are distributed in the Gangdese magmatic belt. Spatially, the Gangdese granitoid belt consists of three zones, i. e. , north, middle and south zones, as a result of the evolution of Neo-Tethyan Bangong Co-Nujiang ocean and Yarlung Zangbo Ocean. Temporally, the tectono-magmatie events in the Tibetan Plateau and the Gangdese can be divided into three stages： Pre-collisional （ 〉65 Ma）, syn-collisional （65 -45 Ma） and post-collisional （ 〈 45Ma）. Nd and Sr isotopes of granitoids in the Gangdese provide a hint for understanding the crust growth and evolution. Most of granites in southern Gangdese have positive values of εNd （t） （ ＋ 1. 64 - ＋ 5.21 ） and young Nd model ages tDM （ 〈500 Ma）, showing characteristics of the juvenile crust. It implies that the mantle material played a significant role in granite petrogenesis. Granites in middle and northern Gangdese, however, are characterized by negative εNd（t） （ -5.3 - - 17.3） and relatively older tDM, 1. 2 Ga and 2.0 - 2.5 Ga, implying a Neo- and Paleo-Proterozoic basement. Crustal material played a major role in granite petrogenesis for the latter case.