SHRIMP U-Pb dating on coesite-bearing zircon: Evidence for Indosinian ultrahigh-pressure metamorphism in Su-Lu, East China

The argument about the Su-Lu UHPM belt formed in the Indosinian or Neoproterozoic time has been continued. A combined study of Laser raman spectrometer (LR), cathodoluminescence imagine (CL) and ion probe U-Pb in-situ dating (SHRIMP) provided an excellent solution to obtain the real UHPM age of the rocks collected from Jiaonan, NE of the Su-Lu UHPM belt. By using the LR the authors found coesite and other UHP minerals enclosed in zircon separated from ultramafic rock, eclogite and gneiss (a country rock of eclogite). The CL was used to distinguish the zoning structure of these zircons with mineral inclusions, and finally, the SHRIMP U-Pb in-situ dating was performed on specific spots to get the ages of different geological events. Three folds of results are obtained: (1) The SHRIMP U-Pb dating gave 221 ± 12Ma of coesite-bearing zircons from the ultramafic rock, whereas the eclogite yielded a 228 ± 29 Ma age of the lower intercept on an concordia plot. These indicated that the UHP metamorphism in the Jiaodong region occurred in the Indosinian time. (2) The emplacement age of ultramafic complex in the east of Weihai is 581 ± 44 Ma, i.e., the Neoproterozoic. The zircons in the complex may also record another magmatic event around 400 Ma. (3) The protoliths of eclogite formed in the middle Proterozoic (1821 ± 19Ma), which is close to the age of gneissic country rocks of the eclogite (1744 - 1847Ma). It suggests that the protolith of the eclogite is a component of an old metamorphic complex, and the Triassic subduction occurred in situ.