SHRIMP U-Pb Dating of Zircons of a Dark-coloured Eclogite and a Garnet-bearing Gneissic-granitic Rock from Bixiling, Eastern Dabie Area——Isotope Chronological Evidence of Neoproterozoic HP-UHP Metamorphism

The paper reports SHRIMP U-Pb zircon data of a dark-coloured eclogite and a post-eclogite garnet-bearing gneissic-granitic rock from the Bixiling area, Yuexi County, Anhui Province, in the eastern Dabie Mountains. The eclogite, which is metamorphosed basic tuff, contains very scarce zircons in omphacite or garnet, but more in quartz. They usually exhibit a double-layered texture clearly shown in cathodoluminescence images. Their main inner parts give a 206Pb/238U age of 757±7 Ma, representing the approximate age of the HP-UHP metamorphic event, during which the eclogite was formed. The outer peripheral parts of the zircons, which have been modified by late-stage fluids, give an age of 223±3 Ma. The granitic rock contains more zircons of anatectic origin found mostly in feldspar and quartz and usually also showing a similar composite texture. The main inner parts of the anatectic zircons with rhythmic zoning give a 206Pb/238U age of 727±15 Ma for the approximate age of the emplacement of the granitic rock, and their outer parts, an age of 219±3 Ma for a similar or even the same fluid transformation event. It is thus suggested that the HP-UHP metamorphism of the Bixiling eclogite facies rocks took place during the Neoproterozoic Jinningian, and the Indosinian age values may only represent a late event in the nature of fluid transformation.