【摘要】 电子探针独居石Th -U -Pb化学法定年是近年来发展起来新的定年方法。它的实施前提是独居石中基本没有普通铅存在 ,除了Th和U衰变过程以外 ,其它因素基本不会改变独居石内的Th/U/Pb之间的比值关系。因此通过独居石内母、子元素定量测量和相关处理后 ,便可以计算出相应的年龄。选用国际标准的ThO2 、金属U ,PbCrO3和YAG分别作为Th ,U ,Pb和Y 4种元素的标样 ,在JEOL公司生产的JXA 810 0电子探针仪上进行了系统误差和条件试验 ,建立了实验方法。并对已经有较好年龄约束的五台群金刚库组变质泥质岩进行了试用检验 ,取得了满意的效果 ,说明此方法可以应用于变质变形定年研究 ,并具有明显的优势更多还原

【Abstract】 Electron-microprobe Th-U-Pb chemical age determination of monazite is a new in-situ geochronological method developed over the last few years. Its basic premises are that monazite contains a negligible amount of common lead and that the Th/U/Pb values are not affected by any processes excepting Th and U radioactive decay. Therefore, Th-U-total Pb chemical age of monazite can be obtained by high-precision determination of mother and daughter elements and statistical analysis. Using the international standards of ThO₂, U metal, PbCrO₄ and YAG as standards, we carried out systematic evaluation of experimental conditions and analyses for Th-U-total Pb chemical dating of monazite on a JEOL JXA-8100 electron microprobe. Application of this electron-microprobe chemical dating of monazite from a metasedimentary rock of the Jingangkou formation, Wutai Group, Wutai Mountains yielded three distinct isochron ages: 2 172 ± 37 Ma （n=4）, 1 922 ± 24 Ma （n=40）, and 1 847 ± 62 Ma （n=6）. The 2 172 ± 37 Ma age, in agreement with SHRIMP U-Pb zircon ages for granitoid magmatism in the Wutai group, appears to represent an early thermotectonic event. The 1 922±24 Ma age is within the analytical uncertainty of the metamorphic age of 1 911 ±38 Ma, determined by an internal Sm-Nd isochron of whole-rock and metamorphic minerals. Also, the 1 847 ± 62 Ma age is similar to the ¹ 850 Ma SHRIMP metamorphic zircon ages in neighbouring Hengshan and Fuping complexes. These geologically significant results from the Wutai Group indicate that this electron-microprobe Th-U-total Pb chemical dating of monazite has obvious advantages over other geochronological methods.更多还原