西昆仑大红柳滩东含锂辉石花岗伟晶岩脉年代学和地球化学特征及其地质意义

Geochronology,Geochemistry and Their Geological Significances of Spodumene Pegmatite Veins in the Dahongliutandong Deposit,Western Kunlun,China

西昆仑地区是中国重要的伟晶岩型锂铍成矿带,近年来在大红柳滩一带取得重大找矿突破,已形成大型矿产资源基地。笔者对西昆仑大红柳滩东锂辉石花岗伟晶岩进行了详细的岩相学、年代学和地球化学研究,能够为本区伟晶岩型锂矿成矿作用研究提供新的依据。利用LA−ICP−MS锆石U−Pb定年,获得大红柳滩东含锂辉石钠长花岗伟晶岩及含锂辉石电气石花岗伟晶岩年龄分别为(205.2±1.4)Ma和(205.0±2.6)Ma,形成时代为晚三叠世。岩石地球化学研究表明,大红柳滩东含锂辉石花岗伟晶岩以高Si、富Al、富Na、钙碱质,高分异,低K、Fe、Mg、Ca和Ti为特征,属强过铝质花岗伟晶岩。岩石明显富集Rb、U、Nb、Ta、Pb、P、Hf等元素,亏损Ba、Th、La、Ce、Pr、Sr、Nd、Sm、Ti等元素;稀土总量较低,ΣREE值为0.56×10^(-6)~3.34×10^(-6),具有弱−中等的负铕异常,δEu值为0.30~0.89。大红柳滩东伟晶岩均具有低且负的εHf(t)值(–4.6~0)和古老的二阶段Hf模式年龄TDM2(1497~1208 Ma),反映其源岩为古老地壳物质的部分熔融。结合西昆仑地区已有年代学资料和区域地质构造演化特征,认为大红柳滩一带伟晶岩矿床形成于南昆仑地体与甜水海地体后碰撞背景下。

West Kunlun is an important pegmatite−type Li−Be metallogenic belt in China.In recent years,a great breakthrough in prospecting has been made in Dahongliutan area,and a large mineral resource base has been formed.In this paper,detailed researches on petrography,geochronology and geochemistry of the spo-dumene granite pegmatites in Dahongliutandong,West Kunlun,can provide a new basis for the study of peg-matite−type lithium mineralization in this area.By using LA−ICP−MS zircon U−Pb dating,the ages of spo-dumene−bearing albite pegmatite and spodumene−bearing tourmaline granite pegmatite in the Dahongliutan-dong are 205.2±1.4Ma and 205.0±2.6Ma,respectively,in the Late Triassic.Geochemical study shows that spo-dumene−bearing granite pegmatites in the Dahongliutandong is characterized by high Si,rich Al,Na,calc−alka-li,high differentiation and low K,Fe,Mg,Ca and Ti,and belongs to strongly peraluminous granite pegmatite.The pegmatites are obviously rich in elements such as Rb,U,Nb,Ta,Pb,P,Hf,but depleting in elements such as Ba,Th,La,Ce,Pr,Sr,Nd,Sm and Ti.The total amount of rare earth is low,withΣREE of 0.56×10^(-6)~3.34×10^(-6),weak−medium negative Eu anomaly andδEu of 0.30~0.89.The pegmatites in the Da-hongliutandong have low and negativeεHf(t)values(-4.6~0)and the old two−stage Hf model age TDM2(1497~1208 Ma),which indicates that their source rocks are from partial melting of ancient crustal mate-rials.Based on the chronological data and the evolution characteristics of regional geological structure in West Kunlun,it is considered that pegmatite deposits in Dahongliutan area were formed in the background of post-collision between South Kunlun terrane and Tianshuihai terrane.