浙东南松阳县枫坪组沉积岩碎屑锆石年代学及地质意义

Detrital zircon geochronology for the sedimentary rock of Fengping Formation in southeastern Zhejiang and its geological significance

利用激光剥蚀电感耦合等离子质谱技术(LA-ICP-MS)对浙东南松阳县枫坪组地层底部的细砂岩进行了锆石U-Pb定年及稀土元素分析。样品中大部分碎屑锆石具有较好的振荡环带且Th/U值大于0. 1,表明其为岩浆成因。碎屑锆石中稀土元素含量变化范围较大,(La/Yb)_N值为0～0. 29,LREE/HREE值为0. 003×10^(-6)～0. 977×10^(-6),表明样品中锆石的轻、重稀土元素分异程度较小且重稀土元素相对富集,具有轻稀土元素含量低、重稀土元素含量高的左倾模式且具有负Eu异常、正Ce异常的特征,表明碎屑锆石主要以岩浆锆石为主,同时存在少量的变质锆石。碎屑锆石U-Pb测年结果表明,最年轻的锆石年龄为193 Ma,显示枫坪组沉积岩成岩年龄不晚于193 Ma。碎屑锆石年龄可分为4组,分别为2 461～1 743 Ma、887 Ma、435和422 Ma、330～193 Ma。碎屑锆石的主要年龄区间分别与已知的构造-岩浆热事件时间相对应(吕梁期、晋宁期、加里东期和印支-海西期),表明本区的构造岩浆活动与中国大地构造运动相一致,并具有幕式发展的特征。通过对枫坪组沉积岩碎屑锆石U-Pb年龄的系统分析并与可能物源区的年代学对比研究,认为枫坪组沉积岩物质来源主要是周边出露的中条期侵入岩体和浙东南出露的印支期花岗岩体。

The rare earth elements characteristics of detrital zircons and the U-Pb age of detrital zircons of finegrained sandstone from Fengping Formation were investigated by the LA-ICP-MS analytical technique. The detrital zircons have a good oscillatory zones and Th/U ratios of most zircons are higher than 0. 1,indicating that they are of magmatic origin. The content of REE varies in a large range. The ratios of( La/Yb)N( 0 ~ 0. 29) and LREE/HREE( 0. 003 × 10^-6~ 0. 977 × 10^-6) show the relative enrichment of HREE. The distribution pattern of rare earth elements in detrital zircons exhibits mainly low content of LREE and high content of HREE as well as Eu negative anomaly and Ce positive anomaly. The result shows that the majority of zircons originated from magmatic source,whereas some other zircons originated from metamorphic activity. The youngest U-Pb age of detrital zircon is 193 Ma,indicating that its diagenetic age is not later than 193 Ma. The data suggest that zircons are mainly composed of 4 age stages,including 2 461 ~ 1 743 Ma,887 Ma,435 and 422 Ma,330 ~ 193 Ma,respectively,corresponding to the known tectonic-magmatic thermal events( Lüliang Movement,Jinning Movement,Caledonian Movement,Indosinian-Hercynian Movement). The result reveals that the tectonic magmatism in this area is consistent with the tectonic activity in China,and it has the characteristics of continuous development. Through the systematic analysis of the U-Pb dating data of detrital zircons from the sedimentary rock of Fengping Formation and chronology of possible provenances,it is shown that the materials for sedimentation was from surrounding rock mass of Zhongtiao Movement and the Indosinian granite body in southeastern Zhejiang.