福建紫金山矿田黑云母花岗岩锆石U-Pb年代学和Hf同位素组成

Zircon Geochronology and Hf Isotope Compositions of Biotite Granite in Southeast Ore Section of Zijinshan Ore Field,Fujian Province

紫金山矿田位于华南褶皱系东部,闽西南凹陷西南部,是我国大型-超大型浅成低温热液铜金矿床,区内发现多个与岩浆活动密切相关的金、银、铜等金属矿床.对紫金山矿田东南矿段与成矿密切相关的细粒黑云母花岗岩开展了详细野外地质调查、岩相学和锆石稀土元素、U-Pb年代学和Hf同位素研究.LA-ICP-MS法获得细粒黑云母花岗岩中岩浆结晶锆石^(206)Pb/^(238)U-^(207)Pb/^(235)U谐和年龄为109.5±1.9Ma(MSWD=0.74,N=16),^(206)Pb/^(238)U加权平均年龄为107.44±0.94Ma(MSWD=1.06,N=16),二者在误差范围内结果一致,结合锆石稀土元素和岩浆振荡环带特征及Th/U比值,上述年龄结果可代表岩石的结晶年龄,表明细粒黑云母花岗岩侵位于燕山期早白垩世.细粒黑云母花岗岩锆石Hf同位素初始比值ε_(Hf)(t)均为负,介于-4.99～-1.06(平均值为-2.99);两阶段Hf模式年龄(tDM2)介于1 233.7～1 485.4Ma(平均值为1 362.4Ma).样品的ε_(Hf)(t)、Hf同位素地壳模式年龄分布范围变化较小,暗示岩体的岩浆来源具有较为均一的锆石Hf同位素组成.紫金山矿田东南矿段早白垩世花岗岩体的锆石U-Pb年龄和Hf同位素特征,反映了闽西南早白垩世的岩浆成矿活动时间和源区特征,其成因与中国东南部岩石圈伸展减薄和壳源物质参与岩浆形成演化密切相关.

Zijinshan ore field is located in the eastern part of South China fold system,belonging to the Southwest Fujian depression,where develops many large and super-large epithermal Au-Cu-Ag deposits which are closely related with magmatic activities.The study in this paper is based on a detailed research of the fine-grained biotite granite in the southeastern ore section including field geological survey and petrography,zircon rare earth elements,in-situ zircon U-Pb chronology and Hf isotope et al..The LA-ICP-MS zircon U-Pb dating shows that the 206Pb/238U-207Pb/235U correlated age of the biotite granite is 109.5±1.9 Ma(MSWD=0.74,N=16),and 206Pb/238U weighed age is 107.44±0.94 Ma(MSWD=1.06,N=16),and the two ages are within the error range.According to zircon REE and magmatic oscillatory zoning characteristics and the Th/U ratios,the age can represent the rock crystallization age,which suggests fine-grained biotite granite emplaced in the Early Cretaceous of the Yanshanian epoch.The Hf isotope initial ratioεHf(t)values are negative,ranging from-4.99 to-1.06(mean value is-2.99).Two stage Hf model ages(tDM2)range from 1 233.7 Ma to 1 485.4 Ma(mean value is 1 362.4 Ma).TheεHf(t)and Hf values show small variations in distribution,which implies that magmatic rocks having a more uniform zircon Hf isotope composition.The in-situ zircon U-Pb chronology and Hf isotope show time and source characteristics of Early Cretaceous magmatic activity in south-western part of Fujian Province,which results from the evolution of the extension and thinning,and crustderived magma substance involved from lithosphere are closely related.In this paper,it provides new evidence on the mineralization and tectonic evolution of the Zijinshan ore field.