独居石电子探针定年及其在新疆东天山变质作用研究中的应用  

Electron Microprobe Dating of Monazite and Its Application to Metamorphism in Eastern Tianshan, Xinjiang

作　　者：党青宁[1] 刘树文[1] 舒桂明[1] 李秋根[1] 

机构地区：[1]北京大学造山带与地壳演化教育部重点实验室,地球与空间科学学院北京100871

出　　处：《高校地质学报》2004年第4期578-585,共8页Geological Journal of China Universities

基　　金：国家自然科学基金资助项目 (批准号 :4 0 0 72 0 6 5 )

摘　　要：以化学法独居石电子探针定年的原理以及前人改良过的年龄计算公式为基础 ,利用全微分和最大误差原理 ,确定了年龄误差计算新方法。并用VisualC语言在Windows平台下编制出了计算年龄和年龄误差的计算机程序。运用此程序对前人公开发表的分析资料和计算的年龄以及年龄误差数据进行了重新计算 ,验证了给出的误差公式的可靠性。同时 ,利用JXA 810 0电子探针仪对新疆东天山变质沉积岩的两个样品 (KM2 12 7 5 ,DK2 10 7 2 )中的独居石进行了电子探针微区U -Th -Pb成分分析。结果表明 ,样品DK2 10 7 2记录了两期变质作用 ,峰期年龄分别是 341.0±3.9Ma和 2 5 5 .2± 3.3Ma ,其中最主要的变质峰期年龄为 341.0± 3.9Ma ,该期的矿物组合为Ky +St+Bt+Pl+Q +/ -Or;而 2 5 5 .2± 3.3Ma和样品KM2 12 7 5记录的变质峰期年龄 2 6 2 .3± 4 .4Ma为次要变质峰期年龄 ,该期矿物组合是Cord +Bt +Pl+Or +Q。分析结果与前人用4 0 Ar- 39Ar法得到的结果相吻合 ,表明独居石电子探针定年技术是一种可靠有效的测年方法。Based on chemical method of electron microprobe dating of monazite and previous improved calculating formula of ages and age uncertainties, we put forward a new formula dealing with age error on the basis of general differentiation and wrote a Visual C computer program on the Windows 2000 for calculations of age and age error. Using the new computer program, we recalculated previously published data of known ages and age errors, demonstrating the feasibility of the new formula of age error. The monazites from two metamorphic sedimentary rock samples (KM2127-5, DK2107-2) from the Eastern Tianshan were analyzed for U-Th-Pb compositions. All analyses were carried out using the JXA-8100 electron microprobe. The monazites from sample DK2107-2 record two peaks of metamorphic ages which are 341.0±(3.9) Ma and 255.2±3.3 Ma, respectively. The metamorphic age of 341.0±3.9Ma is the most important period, forming an early mineral assemblage of Ky+St+Bt+Pl+Q±Or. The monazites from sample KM2127-5 display one peak of ages, 262.3±4.4 Ma. Both the 255.2±3.3 Ma and 262.3±4.4 Ma data represent the second peak period of metamorphism, during which a mineral assemblage of Cord+Bt+Pl+Or+Q were produced. The analytical results of monazite dating are well consistent with the previous ^(40)Ar-^(39)Ar ages, indicating that the electron microprobe dating technique for monazite and the new formula estimating age uncertainties are acceptable.

关 键 词：独居石 U—Th—Pb 电子探针定年 计算程序 变质作用 东天山 

分 类 号：P575.1[天文地球—矿物学] P578.92[天文地球—地质学]