大别山西部熊店加里东期榴辉岩——同位素地质年代学的证据

ISOTOPIC GEOCHRONOLOGICAL EVIDENCE FOR THE CALEDONIAN XIONGDIAN ECLOGITE IN THE WESTERN DABIE MOUNTAINS,CHINA

以UP-Pb单颗粒锆石同位素稀释法、角闪石^(40)Ar—^(39)Ar坪年龄和Sm-Nd全岩及矿物等时线相结合的研究方法,系统地研究了大别造山带西部熊店榴辉岩的峰变质年龄,可能的原岩年龄和退变质年龄,建立了这一榴辉岩的构造-变质时间序列,提出了大别地区存在加里东期榴辉岩的确凿证据。

A typical HP/MT (high pressure/medium temperature) eclogite from Xiongdian, Western Dabie Mountains, has been geochronologically studied using the single zircon U-Pb,40Ar -39Ar and Sm-Nd methods. Prismatic zircons occurring as inclusions within garnets define a minimum crytallization age of 399. 5±1. 6Ma. 40Ar -39Ar dating on hornblende gives a plateau age of 399. 2±4Ma, which is interpreted as a retrogression age of amphibolite facies. This integrated study enables us to conclude that the age of high pressure metamorphism is older than 399. 5 ± 1. 6Ma , suggesting Caledonian collision between the North China and Yangtze plates. Round zircon within the aggregate of quartz and muscovite gives a concordant age of 301±2Ma, reflecting a later retrogression event. An age profile of post-eclogite metamorphism is documented, including amphibolite facies metamorphism at 399. 2Ma shortly after eclogitization, and later retrogressive metamorphism at 301Ma. Sm-Nd internal isochron of garnet+omphacite gives an unreasonalbe age of 544±23Ma, which is quite older than that of prismatic zircon and may be attributed to isotopic disturbance by retrogression.