浙江庆元离子吸附型稀土矿成矿母岩锆石U-Pb测年与Hf同位素特征

U-Pb dating and Hf isotope characteristics of zircons from ore-forming parent rocks in the Qingyuan ion-adsorbed REE deposit, Zhejiang Province, China

近年来,浙南地区首次探获了大型离子吸附型稀土矿,进一步拓宽了我国南方该类型稀土矿空间分布范围。为深入研究浙南地区离子吸附型稀土矿的成矿特征,本文对南岭造山带东段浙江庆元地区离子吸附型稀土矿成矿母岩黑云母二长花岗斑岩进行同位素年代学和Hf同位素测试分析。研究结果表明,采自岩体地表样品锆石年龄为(135.18±0.18)Ma(n=19,MSWD=0.25),εHf(t)范围为-15.19^-8.84,Hf的二阶段模式年龄(TDM2)为2423~2991 Ma;采自钻孔岩体样品锆石的年龄为(135.46±0.16)Ma(n=18,MSWD=2.0),εHf(t)范围为-15.21^-9.75,Hf的二阶段模式年龄(TDM2)为2504~2993Ma,二者均显示该岩体成岩年龄为早白垩世。其次,εHf(t)<0,TDM2>>t(U-Pb),t-εHf(t)图解分布于上地壳线以上,表明岩浆来源以地壳重熔为主。综合研究表明,浙江庆元离子吸附型稀土矿成矿母岩形成于浙闽交界地区燕山晚期构造活动时期,主要由古太平洋板块向华南地块下俯冲,玄武质岩浆底侵,引起上地壳部分熔融、再循环而形成的。

In recent years,a large-scale ion-absorption type REE deposit has been discovered for the first time in the southern Zhejiang.This has further broaden the spatial distribution area of this type of REE deposits in the southern China.In order to intensively study the metallogenic characteristics of the Qingyuan ion-adsorption type REE deposit in the southern Zhejiang,eastern part of the Nanling orogenic belt,the U-Pb dating and Hf isotope analysis of zircons from the biotite monzogranite porphyry of the deposit have been undertaken in this paper.The results suggest that zircons of biotite monzogranite porphyry from the surface sample ZJLT have an age of 135.18±0.18 Ma(n=19,MSWD=0.25),εHf(t)values varying from-15.19 to-8.84,and model ages of two stages(TDM2)ranging from 2423 Ma to 2991 Ma;The zircons of biotite monzogranite porphyry from the core sample ZJLTZK havean age of 135.46±0.16 Ma(n=18,MSWD=2.0),εHf(t)values varying from-15.21 to-9.75,and model ages of two stages(TDM2)ranging from 2504 Ma to 2993 Ma.These indicate that the biotite monzogranite porphyry was formed in the early Cretaceous.The features ofεHf(t)<0,TDM2>>t(U-Pb),and plotting dots are above the crust line in the t-εHf(t)diagram for zircons of the biotite monzogranite porphyry indicate that the magma could be mainly derived from the partial melting of ancient upper crust materials in the south China.Combined with geological setting and study results,it is a considered that the biotite monzogranite porphyry,parent rock of the Qingyuan ion-adsorption type REE deposit,could be formed in the border area between Zhejiang and Fujian provinces during the late Yanshanian tectonic active movement via the recycling of magma derived from the partial melting of upper crust caused by the underplating of the basaltic magma due to the subduction of the Pacific plate into the underneath of the South China block.