西大别超高压榴辉岩中石英脉锆石年龄和Hf同位素组成  

Zircon U-Pb age and Hf isotope composition of a quartz vein in ultrahigh-pressure eclogite from the western Dabie orogen

作　　者：刘小驰[1] 吴元保[1,2] 弓虎军[2] 杨赛红[1] 汪晶[1] 彭敏[1] 焦文放[1] 

机构地区：[1]中国地质大学地球科学学院,地质过程与矿产资源国家重点实验室,武汉430074 [2]西北大学地质学系,大陆动力学国家重点实验室,西安710069

出　　处：《科学通报》2009年第10期1449-1454,共6页Chinese Science Bulletin

基　　金：国家自然科学基金(批准号:40772042,90714010和40521001);教育部项目(编号:IRT0441,B07039和NCET-06-0659);西北大学大陆动力学国家重点实验室开放基金资助项目

摘　　要：对西大别红安地区超高压榴辉岩中石英脉的锆石进行了U-Pb年代学和Hf同位素组成的研究.结果提供了超高压变质岩石折返阶段流体活动的时间、条件和流体来源等方面的信息.石英脉中的锆石具有非常完好的晶型,振荡环带或弱分带结构,表明它们是从石英脉形成过程中的流体中结晶出来的.它们的206Pb/238U加权平均年龄为(224.7±1.3)Ma,是石英脉形成时间的最佳估计值,记录了超高压岩石早期榴辉岩相折返阶段流体的活动.热液锆石具有非常低的Lu/Hf同位素比值,与它们形成于榴辉岩相变质作用阶段相一致.这些锆石的Hf同位素组成与寄主榴辉岩中的锆石一致,指示石英脉形成过程中只有小规模的流体活动,流体为内部来源.因此,热液锆石的Lu-Hf同位素体系可以为流体的形成条件、性质以及来源提供有效的制约.In this paper, we report U-Pb age and Hf isotope composition of hydrothermal zircons from a quartz vein within an UHP eclogite outcrop from the Hong’an area, western Dabie orogen. These data are used to decipher the age, formation conditions and source of fluid flow during the exhumation of UHP rock. Zircons from the vein have perfect euhedral shape, oscillatory zoning or weak zoning, and very low Th/U ratios (0.03―0.07), indicating that they precipitated from the aqueous fluid responsible for the vein formation. The weighted mean 206Pb/238U age of 224.7 ± 1.3 Ma of these zircons is taken as the best estimated age of the quartz vein formation, and records aqueous fluid flow during the early exhumation stage of the UHP rocks. The zircons in the quartz vein show low Lu/Hf ratios and have similar Hf compositions to those in the host eclogite, indicating an internal source and small-scale fluid transport for veining. Therefore, the Hf isotope composition of hydrothermal zircon from quartz vein can constrain on the nature, forming condition, and origin of high-pressure metamorphic fluids.

关 键 词：热液锆石 LU-HF同位素 U-PB年龄 石英脉 西大别 

分 类 号：P597[天文地球—地球化学]