Isotopic Geochronological Evidence for the Caledonian Xiongdian Eclogite in the Northwestern Dabie Mountains, China

A typical HP/MT (high pressure/medium temperature) eclogite from Xiongdian, northwestern 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 crystallization age of 399.5 ± 1.6 Ma. 40Ar–39Ar dating on amphibole gives a plateau age of 399.2 ± 4 Ma, 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 ± 1.6 Ma, 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 ± 2 Ma, reflecting a later retrogression event. An age profile of post–eclogite metamorphism is documented, including amphibolite facies metamorphism at 399.2 Ma shortly after eclogitization and later retrogressive metamorphism at 301 Ma. Sm–Nd mineral isochron of garnet+omphacite gives an unreasonable age of 544 ± 23 Ma, which is quite older than that of prismatic zircon and may be attributed to isotopic disturbance by retrogression.