独居石成因矿物学特征及其对U-Th-Pb年龄解释的制约

Genetic Mineralogy of Monazite and Constraints on Interpretation of U-Th-Pb Ages

独居石U-Pb定年在岩浆活动、变质作用和沉积作用等方面发挥着日益重要的作用,但是由于独居石成因复杂,因此从成因矿物学角度对不同成因独居石的特征进行总结将有助于解释独居石年代学数据.总结了不同成因独居石的矿物共生关系和组构特征、外部形貌-内部结构、化学元素特征,依托Th、U、Y、Ca、Pb、REE含量及其比值关系进行化学组成特点分析.结果表明:岩浆独居石边缘平直、有棱角或港湾状,存在振荡环带、扇形分区和均一的内部结构,常作为包裹体存在于长石、石英内.变质独居石分为高级变质独居石和低级变质独居石.高级变质独居石边缘平整,存在均一状、同心环状、补丁状和交叉状的内部结构,常与变质矿物(如石榴子石)共生.低级变质独居石颗粒分散,含有丰富的包裹体,常见骨架状结构.热液独居石分为两种类型,一种晶形较小(<50μm),呈团簇状多颗粒集合体形态存在,另一种颗粒致密,存在振荡环带、扇形分区的内部结构.从化学组成来说,由于富含轻稀土,独居石的稀土元素配分型式整体为右倾型.岩浆独居石具有最为强烈的Eu负异常,Th、U、Pb、Y、HREE含量相对较高;高级变质独居石存在明显的Eu负异常,Th、U含量高,HREE的含量和Y元素含量与其共生矿物相关;低级变质独居石的Eu负异常不明显,Th、U含量低.热液独居石的Eu负异常较弱,U含量极低,Th/U比值高.

Monazite U-Pb geochronology plays an important role in dating magmatism,metamorphism and sedimentation.Because of its complex genesis,characterizing different types of monazite in terms of genetic mineralogy and chemistry is of great significance to interpret geochronology data.In this paper,it summarizes the occurrence(mineral paragenesis),crystal morphology and mineral geochemistry of different monazite in origins.The results show that magmatic monazite has straight edges,angular or embayment shape,showing oscillatory zoning,sector zoning and homogeneous internal structure,generally existing as inclusions that coexisting with feldspar and quartz.Metamorphic monazite is divided into high-grade and low-grade types.High-grade metamorphic monazite has straight edges,with concentric zoning,patchy zoning and intergrowth-like zoning or unzoned,generally associated with metamorphic porphyroblast(such as garnet).Low-grade metamorphic monazite occurs as discrete crystals with numerous inclusions,commonly exhibiting skeletal texture.Hydrothermal monazite has two types:the first type occurs as a cluster of multiple small monazite grain(<50μm)and the other type is grains showing oscillatory zoning and sector zoning.The rare earth element pattern of monazite is commonly right-leaning.Magmatic monazite has the strongest negative Eu anomalies with high Th,U,Pb,Y and HREE contents.High-grade metamorphic monazite has moderate negative Eu anomalies with high Th,U;HREE and Y are correlated with its coexisting minerals,whereas low-grade metamorphic monazite has insignificant negative Eu with low Th and U.Hydrothermal monazite has weak negative Eu,extremely low U and high Th/U ratio.