Mineral inclusions in zircon domains and geological significance of SHRIMP U-Pb dating for coesite-bearing zircons of paragneiss in Sulu terrane, eastern China

Abstract

Laser Raman spectroscopy and cathodoluminescence (CL) image reveal that zircons separated from paragneisses in the southwestern Sulu terrane (eastern China) preserve multi-stage mineral assemblages in different zircon domains. In the same paragneiss zircon sample, some zircon grains retain inherited (detrital) cores with abundant low-pressure mineral inclusions of Qtz+Phe+Ap+impurities and Qtz+Phe+impurities. The ultrahigh-pressure (UHP) metamorphic overgrowths mantles of these zircons preserve Coe, Coe+Phe and other UHP mineral inclusions, indicating that these inherited (detrital) zircons from protoliths experienced metamorphic recrystallization during the Sulu UHP metamorphic event. However, other zircon grains preserve UHP mineral inclusions of Coe, Coe+Ap and Coe+Phe in the cores and mantles, whereas the outmost rims contain quartz (Qtz) and other low-pressure mineral inclusions. These phenomena prove that the second group zircons were crystallized at UHP meta-morphic stage and overprinted by the late retrogressive metamorphism related to the Sulu ter-rane exhumation. SHRIMP U-Pb dating reveals that zircons separated from the paragneisses recorded complicated geochronological traces. Some zircons retain inherited cores with the 206Pb/238U ages of 284–754 Ma, indicating the complexity of the detrital zircons in protoliths. Coesite-bearing zircon domains recorded 238–266 Ma (with the average age of 245±14 Ma) for the UHP metamorphic event. The retrogressive rims of zircons recorded 213–223 Ma (with the average age of 217±15 Ma) for the late-stage amphibolite facies metamorphic event, indicating that the rapid exhumation time for the Sulu UHP terrane is about 220 Ma.