台湾海峡西岸闽江口和九龙江口沉积物中碎屑锆石铀-铅定年及物源意义

碎屑锆石能较好保存源区岩石组成的信息,其铀-铅年龄是沉积物源区示踪有效的方法。利用激光剥蚀等离子体质谱和多接收器等离子体质谱联机(LA-MC-ICPMS)技术,对台湾海峡西部闽江口和九龙江口沉积物中的碎屑锆石进行原位铀-铅测年,同时获得锆石的微量元素。研究表明大部分碎屑锆石的钍与铀含量之比值大于0.1,稀土元素球粒陨石标准化配分曲线呈左倾型,并具有明显的铈含量正异常和铕含量负异常,结合阴极发光图像表明这些锆石绝大部分为岩浆成因。闽江口和九龙江口的碎屑锆石具有相同的印支期(峰值为236 Ma)、燕山早期(峰值为155 Ma)和燕山晚期(峰值为110 Ma)等3个峰值,这三期碎屑锆石占主要部分,反映闽江口和九龙江口碎屑沉积物主要来自各流域上游印支期和中下游燕山期岩体,而闽江口碎屑锆石具有明显的加里东期峰(峰值为465 Ma)和前寒武纪(553~2 765 Ma)较多的碎屑锆石,这些碎屑物质可能来自上游武夷山地区加里东期和前寒武纪基底的物质。对比闽江口碎屑锆石铀-铅年龄与台湾岛西部海滨沙滩(苗栗-新竹、嘉义-台南)中的独居石年龄具有相似的谱峰特征,表明台湾岛西部海滨沙滩碎屑物质可能有部分来自闽江。闽江口碎屑物质明显区别于福建的九龙江和浙江的瓯江,这为判别台湾海峡沉积物的物质来源提供可靠的依据。

Uranium-lead dating of detrial zircons from the Minjiang and Jiulong Estuaries in the western coast of the Taiwan Strait: implication for its provenance

The source information of sediments was preserved in detrital zircons,which could sufficiently interpret the provenance of sediments uranium-lead ages and trace elements in detrital zircons from the Minjiang and Jiulong Estuaries was analyzed by in situ laser ablation-multiple collector-inductively coupled plasma mass spectrometry (LA-MC-ICP-MS). In most zircons the ratio of thorium content to uranium is higher than 0.1,indicating that zorcons have HREE-enriched feature with the positive anomaly of cerium content and the negative anomaly of europium content in chondrite-normalized REE patterns,a feature of magmatic origin. The uranium-lead ages of zircon from the Minjiang and Jiulong Estuaries can be divided into three groups: Indosinian (ca.236 Ma),Early Yanshanian (ca.155 Ma) and Late Yanshanian (ca.110 Ma). These age ranges are consistent with the ages of granitic rocks outcropped in the middle and lower reaches of the rivers. However, the uranium-lead ages of zircon from the Minjiang Estuary show distinctly Caledonian (ca.465 Ma) and Precambrian ages (553~2765Ma),which are possibly derived from the Caledonian and Precambrian rocks from upstream. In regard of similar age ranges of detrital zircons from the Minjiang Estuary and monazite from beach sands of the western Taiwan Island(Miaoli—Hsinchu,Chiayi—Tainan),the sediments of the western Taiwan Island are very likely derived from the Minjiang River.