藏北红脊山地区角闪岩相变质地层的划分及时代

Division and age of anogubikute-facies strata in the Hongjishan area of northern Tibet,China

关于藏北羌塘中北部地区的变质地层的划分及其时代归属一直有不同的认识。红脊山地区出露的猫耳山岩组是一套由变质碎屑岩夹变质基性火山岩、大理岩和少量硅质岩组成,遭受了低角闪岩相变质作用改造的地层体,泥质变质岩中出现了十字石、蓝晶石、石榴石、角闪石等变质矿物,是羌塘中北部地区出露面积较大,变形强变质较深的构造岩石地层单位。其岩性组合、变质特征以及形成的构造环境均与上石炭统擦蒙组、展金组有明显的区别,二者之间为断层接触关系。根据岩性组合特征及岩石地球化学特征研究表明,猫耳山岩组是弧后盆地沉积的产物。获得角闪片岩全岩Sm-Nd等时线年龄值401±18M a,可能代表了猫耳山岩组的沉积时代,也是红脊山地区早泥盆世或更早的地质时期发生了一次弧后盆地扩张作用与沉积演化的年代学记录。

Always there has been the different understanding about the division and age assignment of the metamorphic strata in north - central Qiangtang in northern Tibet. In the Hongjishan area, the Mao＇er shan rock formation with a tectonic terrain suffered low - grade amphibolite - facies metamorphism, with a composition of metamorphic basic volcanic rocks, marble and a little siliceous rocks, with pelitic metamorphic rocks having metamorphic mineral such as staurolite, cyanite, garnet, amphibole etc is a tectostratigraphic unit distributed in a larger area and suffered strong metamorphism and deformation in north - central Qiangtang in northern Tibet. The Mao＇er shan rock formation, in lithologic association, metamorphic features and forming structural setting is distinct with the Late Carboniferous of Cameng and Zhanjin formations, and the former is contacted with the later by fault. According to the study of character of lithologic association and geochemistry of lithogeochemistry, the Mao＇er shan rock formation is a sedimentary sequuence of a backarc basin. 401.18 Ma of age of the Mao＇er shan rock formation has been tested out by Sm - Nd whole - rock isotopic age of the hornblende schist, it is a probable representative to the sedimentary age of the Mao＇er shan rock formation as well as a geochronology records of the process of spread and sedimentary evolution of the backarc basin in the Early Devonian or earlier geologic age in the Hongjishan area.