SHRIMP U-Pb dating of zircons of a dark eclogite and a garnet-bearing gneissic granitic rock from Bixiling, eastern Dabie area, Anhui Province: Isotope chronological evidence of neoproterozoic UHP metamorphism

The paper reports SHRIMP U-Pb zircon data of a dark eclogite and a post-ecl ogite garnet-bearing gneissic granitic rock from the Bixiling area, Yuexi C ounty, Anhui Province, in the eastern Dabie Mountains. The eclogite, which is metamorphosed basic tuff, contains very scarce zircons in omphacite or g arnet, but more zircons in quartz. They usually exhibit a double-layered te xture, as shown clearly in cathodoluminescence images. Their inner main par ts give a Pb-206/U-238 age of 757+/-7 Ma, representing the approximate age of the high-pressure (HP)- ultrahigh-pressure (UHP) metamorphic event durin g 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 inner main parts of the anatectic zircons with oscillatory zoning give a Pb-206/U-238 age of 727+/-15 Ma for the approximate age of the emplacemen t of the granitic rock, and their outer parts, an age of 219+/-3 Ma for a s imilar or even the same fluid event. It is thus suggested that the HP-UHP m etamorphism of the Bixiling eclogite facies rocks took place during the Neo proterozoic Jinningian, and the Indosinian age values may only represent a late event in the nature of fluid activity.