柴北缘鱼卡地区达肯大坂岩群的地质特征与构造环境

Geological Characteristics and Tectonic Setting of the Dakendaba Group in Iqe Area,Northern Margin of Qaidam Basin

柴达木北缘鱼卡河地区的达肯大坂岩群可划分为斜长角闪岩岩组和片岩岩组。斜长角闪岩岩组主要由变质基性火山岩和碎屑岩组成,火山岩的地球化学特征指示为岛弧环境构造;片岩岩组分布在柴达木山西南侧,为一套陆源碎屑岩建造。该岩群遭受了三幕构造变形,前两幕褶皱变形是造山作用的产物,具有近似的北西-南东向或北北西向的褶皱枢纽,近共轴褶皱叠加的构造样式指示了北东-南西向挤压收缩的动力学背景。达肯大坂岩群遭受中压高绿片岩相-角闪岩相的变质,变质程度往北东方向递减,可与其南柴达木盆地一侧的高压-超高压变质带构成双变质带。结合最近从达肯大坂岩群中获得的锆石年龄,推断该岩群形成于大陆边缘的弧后盆地,时代为新元古代晚期-早古生代,是柴北缘早古生代造山带的重要组成部分。

Dakendaba Group occurred in Iqe area, northern margin of Qaidam basin, can be divided into amphibolite assemblage and schist assemblage. The amphibolite assemblage is chiefly composed of metamorphic basic-volcanic rocks and elastic rocks, and close with the Tanjianshan island-arc belt. The schist assemblage distributed on the southwest of Qaidam mountain consists of metamorphic sandstone and pelite. The Geochemical characteristics of the amphibolite indicate a tectonic setting related to island-arc. The Dakendaba Group underwent three-episodes of deformation, among which the most developmental is the second episode schistosity fold. The former two episodes fold related to orogeny have an accordant NW-SE or NNW hinge of fold, and the coaxial superposed pattern indicated a kinetics setting of compression-shortening deformation on NE-SW direction. Dakendaba Group underwent medium-pressure amphibolite - greenschist facies metamorphism, and the metamorphic grade took from amphibolite facies to greenschist facies from SW to NE. The MP medium-lower grade metamorphic belt of Dakendaba Group and the Yuqiahe HP-UHP metamorphic belt can constitute a pair of metamorphic belt structure. Combined with the zircon U-Pb ages obtained from Dakendaban Group recently, we conclude Dakendaba Group was an important part of early Paleozoic collisional orogenic belt in the northern margin of Qaidam basin, and formed in the back-arc basin of active continental margin at the terminal of Neoproterozoic to early Paleozoic, and underwent intense deformation and metamorphism during collisional orogeny at middle-late stage of the early Paleozoic.