南岭金鸡岭岩体锆石U-Pb年龄、地球化学特征及地质意义

Zircon U-Pb Ages, Geochemical Characteristics and Geological Significance of Jinjiling Pluton in Nanling

对南岭九嶷山地区金鸡岭岩体开展LA-ICP-MS锆石U-Pb年代学研究,结果显示主体粗中粒似斑状正长花岗岩和补体细粒斑状二长花岗岩的成岩年龄分别为156.40±0.66 Ma、153.00±2.20 Ma;结合已有资料将二者的形成时代约束在(156.40±0.66)~(159.00±0.45)Ma与(146.00±0.86)~(153.0±2.2)Ma之间;确定其成岩年代同为燕山早期,也明确了两者的形成顺序.岩石地球化学研究表明,金鸡岭岩体整体具有富硅、碱,贫钙、镁,准铝-过铝质(A/KNC=0.97~1.14)等特点;主体岩性稀土含量较补体明显偏高,分别为488.63×10^(-6)~571.67×10^(-6)和166.33×10^(-6)~275.51×10^(-6),(La/Yb)_(N)分别为10.27~15.84、0.38~1.13;前者稀土元素配分曲线呈右倾轻稀土富集型,而后者则无明显轻、重稀土分馏;两类花岗岩的δEu值都很低,分别为0.049~0.063、0.003~0.007,为铕亏损型;分异指数(DI)为88.76~89.20、90.66~94.06,表明补体发生了更强的岩浆分异作用.金鸡岭岩体富集Rb、K、Th、U、Nd、Hf等大离子高场强元素,亏损Ba、Sr、P、Nb、Ti等元素;Ga/Al(×10^(4))比值为3.32~5.02,平均3.50;Zr+Nb+Ce+Y为255.5×10^(-6)~554.9×10^(-6),平均422.37×10^(-6);锆石饱和温度T_(Zr)=780.31~820.67℃,平均803.77℃;显示A型花岗岩地球化学属性.Sr、Nd、Hf同位素显示金鸡岭岩体具较高Sr同位素初始值(I_(Sr)=0.712 58~0.732 51),较低的ε_(Nd)(t)(-6.2~-7.0)、ε_(Hf)(t)(-4.2~-5.5)值特点;示踪其源区以地壳物质为主,无地幔/新生陆壳物质贡献;钕、铪二阶段模式年龄十分接近,分别为1 465~1 566 Ma和1 459~1 541 Ma,揭示其源岩从地幔储库中脱离的时间为中元古代.结合南岭地区地质演化史,推测金鸡岭岩体形成的大地构造背景为太平洋板块俯冲引起的陆内伸展环境.

A study of LA-ICP-MS zircon U-Pb dating for Jinjiling pluton from Jiuyishan area in Nanling granite belt was carried out.The results show that the diagenetic age of the main medium grained porphyritic syenite and the complement fine-grained porphyritic monzonite are 156.40±0.66 Ma and 153.00±2.20 Ma,respectively.Combined with the existing data,their ages are restricted between (156.40±0.66)-(159.00±0.45) Ma and (146.00±0.86)-(153.00±2.20) Ma,which indicates that the Jinjiling pluton was formed during the early Yanshanian.Geochemical data show that the Jinjiling pluton is characterized by enriched silicone-alkali,depleted calcium-magnesium and metal uminous-peraluminous (A/KNC=0.97-1.14).The content of rare earth elements of the main granite are significantly higher than those of the complement granites,which is 488.63×10^(-6)-571.67×10^(-6)and 166.33×10^(-6)-275.51×10^(-6),respectively.The main granite is enriched in light rare earth elements (LREE),with the complement granite having no obvious fractionation,both of which show negative Eu anomalies and (La/Yb)Nvalues ranging from10.27 to 15.84 and 0.38 to 1.13,respectively.The differentiation index (DI) are 88.76-89.20,90.66-94.06,which indicate that the complement granite has undergone stronger magmatic differentiation.The Jinjiling pluton is enriched in large ion lithophile elements (LILE,e.g.,Rb,K,Th,U,Nd and Hf) and relatively depleted in high field strength elements (HFSEs,e.g.,Ba,Sr,P,Nb and Ti),with Ga/Al(10^(4)) ratios of 3.32-5.02 (average 3.50),Zr+Nb+Ce+Y of 255.5×10^(-6)-554.9×10^(-6)(average422.37×10^(-6)) and zircon saturation temperatures T_(Zr)of 780.31-820.67℃(average 803.77℃),similar to geochemical features of A-type granites.The Jinjiling pluton has higher initial Sr isotope values of 0.712 58 to 0.732 51,lower ε_(Nd)(t) values of-6.2 to-7.0 and ε_(Hf)(t) values of-4.2 to-5.5,revealing that the source area of the Jinjiling pluton is mainly composed of crustal materials,without any contributions from mantle or new continental crust materials.The model ages of Nd and Hf are relatively close,with 1 465-1 566 Ma and 1 459-1 541 Ma,respectively,suggesting that the source rocks were separated from the mantle reservoir during the Mesoproterozoic.Combined with the geological evolution of Nanling area,it is inferred that the tectonic background of Jinjiling pluton is an intracontinental extensional environment caused by subduction of Pacific plate.