MSA ROEBLING MEDAL LECTURE: PETROCHEMISTRY AND GEOCHRONOLOGY OF THE DABIE-SULU ULTRAHIGH-PRESSURE METAMORPHIC TERRANE OF EAST-CENTRAL CHINA – A SUMMARY

The Triassic Dabie-Sulu collisional orogen in eastern Asia was created by northward subduction of the Yangtze plate beneath the Sino-Korean craton. Eclogites, garnet peridotites and the surrounding country rock gneisses + marbles were subjected to in situ UHP metamorphism, as indicated by the presence of widespread coesite inclusions in eclogitic minerals and in country rock zircons, as well as by virtually identical metamorphic ages (240-215 Ma) of the various UHP rocks. P-T estimates combined with investigations of mineral exsolution textures and high-P polymorphs indicate that recovered depths of continental subduction may have reached or exceeded 200 km. Mineral inclusions in zircon separates from more than 2000 UHP rocks from both surface exposures and cores (0-5 km depths) from the first Chinese Continental Scientific Drilling project (CCSD) were examined both by Raman spectroscopy and SHRIMP U-Pb dating. These zircons display distinct zones with inherited core, prograde, peak, and outermost retrograde domains. Low-P mineral inclusions (Qtz and Fsp) are common in zircon cores, whereas Qtz eclogite-facies inclusions occur in prograde domains, coesite eclogite-facies inclusion assemblages in UHP domains, and amphibolite-facies assemblages in the retrograde rims. Parageneses of inclusion minerals in zircons combined with U-Pb ages delineate a well-constrained P-T-time path, and suggest exhumation rates of 5-10 km/Myr. Oxygen isotopes of phases from many surface samples and the CCSD cores indicate that d¹⁸O depletion took place along the northern edge of the Yangtze craton in a volume of Neoproterozoic protoliths exceeding 100,000 km³. Evidently, passive-margin sediments and bimodal igneous rocks that had formed during breakup of the supercontinent Rodinia were subjected to pervasive meteoric water-rock interactions attending terminal Neoproterozoic Snowball Earth conditions. This hydrothermal alteration volatilized and depleted C from highly oxidized protoliths, accounting for the exceedingly rare occurrences of microdiamond and graphite in Dabie-Sulu UHP rocks. Most Sulu garnet peridotites were derived from a depleted, metasomatized mantle, representing the subcontinental lithosphere beneath the southern margin of the Sino-Korean craton.