Liu Fulai

Ultrahigh-pressure mineral inclusions in zircons from gneissic core samples of the Chinese Continental Scientific Drilling Site in eastern China

The Chinese drillhole CCSD-PP2 penetrated to a depth of 1028.28 m near Donghai in the southwestern Sulu terrane. Core samples consist mainly of para- and orthogneisses with minor intercalated layers of amphibolite and eclogite. Coesite and coesite-bearing ultrahigh-pressure (UHP) mineral assemblages were identified using Raman spectroscopy and electron microprobe analysis as inclusions in zircon separates from para- and orthogneiss samples. Matrix minerals in the coesite-bearing para- and orthogneiss samples are amphibolite-facies assemblages of quartz + feldspar + epidote + biotite ± amphibole ± garnet ± titanite. In most cases, zircons, however, preserve multi-stage assemblages in different domains; the parageneses and compositions of these included minerals yielded P-T paths consistent with those deduced previously from eclogite and garnet peridotite outcrops in Donghai. In generally, quartz inclusions were identified in zircon cores and rims, whereas coesite and other UHP mineral inclusions occur in the mantles (intermediate zircon growth zone between core and rim) of the same zircons. The occurrence of coesite inclusions in zircon indicates that eclogites together with their country rock gneisses experienced UHP metamorphism and retrogression. Coesite inclusions are ubiquitous in paragneiss and orthogneiss core samples in CCSD-PP2 drillhole and in outcrop samples from the southwestern Sulu terrane, indicating that coesite-bearing lithologies are widespread in the Sulu terrane, requiring subduction of a large terrane to mantle depths. Cathodoluminescence images of zircons from gneissic rocks display distinct zoning from core to rim, that can be genetically correlated with inherited (especially in the orthogneisses) or prograde, UHP, and retrograde mineral inclusion assemblages. These images reveal irregular zoning patterns and various thickness of cores, mantles, and rims. The abundance of inclusions complicates conventional U-Pb age dating, therefore the SHRIMP microspot U-Pb analyses are essential for protolith and metamorphic age dating.

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

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.