新疆阿尔泰沙依肯布拉克铍矿区片麻状花岗闪长岩年代学、地球化学及其意义

Geochronology, geochemistry and significance of the gneissic granodiorite from Shayikenbulake Be deposit in Altay, Xinjiang

沙依肯布拉克片麻状花岗闪长岩出露于中阿尔泰,为铍矿化伟晶岩的直接围岩,是研究其与稀有金属矿化关系、区域构造环境和中阿尔泰是否存在古老基底的理想对象。本文采用LA-ICP-MS锆石U-Pb定年法,获得其加权平均年龄为405.6±3.9Ma,属早泥盆世,明显早于阿尔泰稀有金属成矿高峰期。岩石具有高硅(SiO2=73.8%~74.7%)、富铝(Al2O3=13.3%~14.6%)、相对富钠(Na2O/K2O=1.28~3.13)的特点,A/CNK和A/NK值均大于1,属低钾钙碱性过铝质-强过铝质花岗岩。微量元素表现出Ba、Sr、P、Nb、Ti、Ce的负异常和Rb、Th、Ta、Pb、Nd、Sm、Hf的相对正异常,稀土元素显示轻稀土弱富集、重稀土平缓及较弱的负Eu异常(δEu=0.70~0.82)。岩石的εNd(t)和二阶段Nd模式年龄较为均一,分别为-1.76^-1.22和1.19~1.21Ga。综合以上地球化学特征,结合区域构造-演化,本文认为中阿尔泰存在古老基底,沙依肯布拉克花岗闪长岩可能来源于中元古代地壳物质在早泥盆世俯冲消减环境下的部分熔融,在形成过程中混入了部分幔源物质。

The Shayikenbulake gneissic granodiorite located in the Central Altay is the direct wall rock of Be-mineralized pegmatite,and it is an ideal object to study its relationship with rare metal mineralization,regional tectonic environment and whether the ancient basement exist in the Central Altay.Zircons yielded weighted mean LA-ICP-MS U-Pb age of 405.6±3.9Ma for the granodiorite,indicating that it was emplaced in the Early Devonian and is obviously earlier than the peak of rare metal mineralization in Altay.The granodiorites are high in the concentrations of SiO2(73.77%~74.69%),Al2O3(13.32%~14.57%)and Na2O/K2O(1.28~3.13)with A/CNK and A/NK ratios>1,which are defined as low-K calc-alkaline peraluminous-strongly peraluminous granites.Trace elements of the granodiorite show negative anomalies for Ba,Sr,P,Nb,Ti,Ce and relative positive anomalies for Rb,Th,Ta,Pb,Nd,Sm,Hf.REE show weak enrichment of LREE,gentle HREE,and weak negative Eu anomalies(δEu=0.70~0.82).TheεNd(t)values and two-stage model ages t2DM(Nd)range from-1.76 to-1.22 and from 1.19Ga to 1.21Ga,respectively.Combined with regional tectonic evolution and geochemistry characteristics presented here,this paper suggest that the Central Altay contains ancient basement,the Shayikenbulake granodiorite was probably derived from partial melt of Mesoproterozoic crustal material in the Early Devonian subduction environment,and mixed with mantle-derived components in the formation process.