S型花岗岩中锆石U-Pb同位素体系的多阶段演化及其年代学意义─—以桂北三防岩体为例  

MULTIPLE-STAGE EVOLUTION OF ZIRCON U-Pb ISOTOPIC SYSTEMATICS IN S-TYPE GRANITES AND ITS CHRONOLOGICAL SIGN1FICANCE:A CASE STUDY ON SANFANG GRANITE,NORTHERN GUANGXI

作　　者：李献华[1] 刘颖[1] 涂湘林[1] 刘海臣[1] 凌湘寿 

机构地区：[1]中国科学院广州地球化学研究所,广州510640

出　　处：《矿物学报》1996年第2期169-177,共9页Acta Mineralogica Sinica

基　　金：中国科学院院长基金

摘　　要：S型花岗岩中含有残留锆石并经历过后期Pb丢失事件是一个较普遍的现象，这使得其钻石U－Pb体系呈现出复杂的多阶段演化特征。虽然离子探针对各种复杂锆石U－Pb体系定年最为有效，但因其昂贵的成本使其应用受到限制，目前较为常用的仍是常规化学－质谱分析方法。在现有实验室Pb本底水平上恰当地选择单颗或多颗粒锆石进行分析，是获得高质量U－Pb年龄数据的关键。桂北三防岩体含有残留锆石并经历过后期Pb丢失事件，使其锆石U－Pb体系具有三阶段演化的特征。离子探讨和多颗粒锆石化学－质谱两种分析方法得出了一致的207Pb／206Pb年龄为820±9Ma，代表了岩体的形成年龄。而过去用常规微量锆石化学－质谱法得到的年龄（899Ma）实际上是一个无地质意义的“上交点年龄”。It is a common phenomenon that some S-type granites contain inherited zircons and have undergone subsequent disturbance, resulting in radiogenic Pb loss. Zircons in such granites are characterized by the multiple-stage evolution of the U-Pb isotopic system. Ion microprobe is con-sidered to be the most powerful tool to date complicated zircon populations, but its application is limited due to its huge price and high operating cost. In fact, the conventional chemical-TIMS technique is still widely used in zircon age determination. lt is worth to note that the selection of single-or multiple-grain zircon fraction for U-Pb analysis is highly dependent on the analytical Pb blank level.Inherited radiogenic Pb and suhsequent Pb loss in zircons from the Sanfang granite, northern Guangxi, were identified in this study. Hence, the zircon U-Pb system in the granite is character-ized by a three-stage evolution. A concordant 207Pb/206Pb age of 820+9 Ma, which is interpreted as the crystallization age of the granite, is obtained by means of both SHRIMP and conventional multiple-grain zircon U-Pb dating techniques- The previously reported zircon U-Pb age of 899 Ma is only a geologically meaningless" upper intercept age ".

关 键 词：花岗岩 锆石 铀同位素 铅同位素 

分 类 号：P588.121[天文地球—岩石学]