Petrographical Study of Zircons and SHRIMP Dating of the Caledonian Xiongdian Eclogite, Northwestern Dabie Mountains

The Xiongdian eclogite at Luoshan County, Henan Province, occurs in the Sujiahe tectonic melange in the western Dabie Mountains. Zircons from the rock occur in garnet and other metamorphic minerals with clear sharp boundaries and exhibit textures growing under metamorphic conditions. They are multifaceted under the scanning electron microscope, thus indicating their growth in a solid medium. SHRIMP analyses give 206Pb/238U ages ranging from 335 - 424 Ma for the zircons, showing a certain degree of radiogenic lead loss; so the age of 424±5 Ma reflects the minimum age of the metamorphic zircons. From the above data Combined with the previous Sm-Nd, 40Ar-39Ar,U-Pb and 207Pb/206Pb data, it is suggested that the peak metamorphism of the Xiongdian eclogite is between 424-480 Ma. The present study confirms that there do exist a Caledonian HP-UHP metamorphic event in the western part of the Dabie orogenic belt.