变质岩同位素年代学:Rb-Sr和Sm-Nd体系  

Isotope geochronology of metamorphic rocks:Rb-Sr and Sm-Nd systematics

作　　者：李曙光[1,2,3] 安诗超 

机构地区：[1]中国地质大学(北京)科学研究院地质过程与矿产资源国家重点实验室,北京100083 [2]中国科学院壳幔物质与环境重点实验室 [3]中国科学技术大学地球空间科学学院,安徽合肥230026

出　　处：《地学前缘》2014年第3期246-255,共10页Earth Science Frontiers

基　　金：国家自然科学基金项目(41230209;41090372)

摘　　要：对变质岩经历的进变质和退变质作用定年并构建其p-T-t轨迹是观测地壳运动过程的重要途径。全岩等时线和矿物等时线是变质岩Rb-Sr和Sm-Nd定年的两个基本方法。在变质过程中同位素均一化尺度是影响全岩等时线定年的主要因素。在一般情况下,变质过程中Rb-Sr同位素体系的均一化尺度远大于Sm-Nd体系,从而Rb-Sr全岩等时线可以给出有意义的变质年龄,而Sm-Nd数据不能。然而,对于低级变质作用,因其较高级变质作用有更丰富的流体,其Nd同位素均一化尺度可能变,从而使得一些全岩Sm-Nd等时线给出和Rb-Sr年龄一致的有意义变质年龄。对于矿物等时线定年,在变质作用时矿物之间能否达到同位素平衡则是关键。已经证明,超高压变质(UHPM)岩的退变质作用是开放体系,然而UHPM矿物的Sr-Nd同位素体系仍保持封闭。已观测到UHPM矿物和退变质矿物之间的Sr-Nd同位素不平衡,因此,高压矿物(如石榴石、多硅白云母)和退变质矿物或全岩的连线将会给出没有意义的偏老的Sm-Nd年龄和偏年轻的Rb-Sr年龄。由3个以上很好分开的矿物确定的等时线的良好线性、不同定年方法获得的年龄的一致性以及确定等时线矿物之间的氧同位素平衡可用于判定矿物间Nd同位素达到平衡。由于石榴石具有高Sm/Nd和Lu/Hf比,因此石榴石是榴辉岩或石榴辉石岩Sm-Nd或Lu-Hf定年最重要的矿物。然而由于石榴石非常宽的p-T稳定范围,石榴石可以在高级变质岩的前进变质和退变质作用中生长,从而具有复杂的环带结构。因此,如何从具有复杂结构的石榴石不同部位取样和分析并判断其成因就成为榴辉岩或石榴辉石岩Sm-Nd或LuHf矿物等时线定年的一个挑战。这需要今后做更进一步的研究。Timing of prograde and retrograde metamorphisms undergone by metamorphic rocks to construct their p-T-t path is an important way to observe crustal movement process. Whole rock isochron and mineral isochron are two basic methods of Rb-Sr and Sm-Nd isotopic dating of metamorphic rocks. Isotopic homogeneous length during metamorphism is a major factor affecting whole rock isochron dating. In general, the homogeneous length of Rb-Sr isotope system is much larger than that of Sm-Nd isotope system during metamorphism so that whole rock Rb-Sr isochron gives a meaningful metamorphic age but Sm-Nd whole rock data do not. For lower grade metamorphism in which metamorphic fluid is more abundant than higher grade metamorphism, however, the Nd isotopic homogeneous scale could be large so that some Sm-Nd whole rock isochrons can yield a meaningful metamorphic ages similar to Rb-Sr isochron age. Whether isotopic equilibrium is achieved between minerals is a key for mineral isochron dating. It is demonstrated that the retrograde metamorphism of ultrahigh pressure metamorphic （UHPM） rocks occurred in an open chemical system, whereas the Nd and Sr isotopic systematics of UHPM minerals may have remained closed. Nd and Sr isotopic disequilibrium between UHPM minerals and retrograde metamorphic minerals has been observed~ therefore, the tie line of high-P mineral （such as garnet or phengite） and retrograde metamorphic minerals or whole rock gives an old Sm-Nd age and a young Rb-Sr age, respectively, with no geologic significance. Good linearity of isochron defined by more than 3 well separated minerals and a consistence between ages obtained using different isotopic dating method as well as O isotope equilibrium between minerals have been applied to diagnose Nd isotopic equilibrium between minerals which defined an isochron. Garnet is an important mineral for Sm-Nd and Lu-Hf isotopic dating of eclogite or garnet pyroxenite because of its high Sm/Nd and Im/Hf ratios. However, since the P-T stability field of garnet is large s

关 键 词：变质岩 年代学 Rb-Sr和Sm-Nd体系 同位素不平衡 

分 类 号：P588.3[天文地球—岩石学] P597.3[天文地球—地质学]