独居石成因特征与U-Th-Pb定年及三江特提斯构造演化研究例析

Genetic feature of monazite and its U-Th-Pb dating:Critical considerations on the tectonic evolution of Sanjiang Tethys

独居石U-Th-Pb定年在地质年代学研究中占有重要地位,但其成因类型的识别和年龄值的合理地质解释始终是研究的难点。论文通过对比分析岩浆、热液和沉积成因独居石的内部结构、矿物组合以及稀土和微量元素地球化学特征,厘定了不同成因类型独居石的鉴别标志;并以三江特提斯构造演化研究为例,探讨了独居石的U-Th-Pb定年及其合理地质应用。不同地质环境中形成的独居石具有不同的内部结构和矿物组合及元素组成:岩浆独居石可能存在较宽且平直的岩浆环带,常与磷钙钍石共生;相对富集重稀土,具有较高的Y、Sc、Th、U、Sm、Gd等含量和Th/U比值,较低的La、Ce含量和稀土总量,显示为Eu强烈亏损的右倾斜分布模式。而热液独居石可能存在与流体作用有关的扇形环带,常与斜钍石共生;沉积独居石形态多不规则,与自生矿物相伴生。它们具有典型的四分组稀土分布模式,稀土总量较高,相对富集轻稀土(如La、Ce),贫Y、Sc、Th、U、Sm、Gd等,Th/U比值较低。独居石U-Th-Pb定年限定了松潘-甘孜造山带204～190Ma的早期变质作用和168～158Ma的局部热干扰,约140～130Ma冈底斯浅色花岗岩的形成与地壳的缩短加厚及快速隆升作用,约33～22Ma红河断裂带与伸展作用有关的岩体侵位和左行走滑运动年代(且北部的去顶剥蚀作用要比中部早9Ma),约8～11Myr的东喜马拉雅构造结"挤出"构造变形时限。

Monazite U-Th-Pb dating plays an important role in isotopic geochronology study.Identification of the genesis and interpretation of the age for monazites are the points.The internal texture,mineral assemblage,REE and trace element features are compared in detail in the paper to determine the identification of magmatic,hydrothermal and sedimentary monazites.Study case of monazite U-Th-Pb geochronology on the tectonic evolution of Sanjiang Tethys gets further discussed.Monazites formed in different geological environments generally display distinguished internal textures,mineral assemblages and elemental compositions：though the zonings developed in monazites are not as perfect as those in zircons,magmatic monazites having paragenesis with brabantite show typical oscillatory zoning possibly which are broad and straight,as well as are characterized by high trace elements（Y,Sc,Th,U） content,Th/U ratio which has positive correlation with that in zircon,enrichment in heavy rare earth elements and Sm,Gd,relatively depleted in ∑REE and La and Ce,and display intensely depleted rightward REE distribution pattern.Hydrothermal monazites having paragenesis with huttonite display sector zoning probably which reflect the activity of fluid,sedimentary monazites with irregular morphology usually are associated with authigenic mineral.In addition,they are both distinguished from magmatic monazites by their low trace elements content,Th/U ratio which has no stable correlation with that in zircon,enrichment in ∑REE and La,Ce,relatively poor in light rare earth elements and Sm,and Gd,and display typical tetrad effect REE distribution pattern.The monazites U-Th-Pb ages constrain the earlier Barrovian metamorphism in the interval ca.204~190Ma and the local thermal perturbation at ca.168~158Ma throughout the Songpan-Ganzi Orogenic Belt,the emplacement of Gangdese leucogranite as well as the crust shortening and thickening with tectonic uplifting at ca.140~130Ma,magmatism and left-lateral strike-slip movements in relation to extension occurring coevally from 33Ma to 22Ma along the Red River shear belt（unroofing in the north took place almost 9 million years earlier than that in the central gneiss segments of the shear）,and the extrusion structure within the eastern Himalayan syntaxis at ca.8~11Myr.