A refractory mantle protolith in younger continental crust, east-central China: age and composition of zircon in the Sulu ultrahigh-pressure peridotite

Zircons have been extracted from garnet peridotite and its wall rock (gneiss), which was intersected by the pre-pilot hole of the Chinese Continental Scientific Drilling project (CCSD-PP1) in the Sulu ultrahigh-pressure (UHP) metamorphic belt. The peridotitic zircons record early Mesozoic UHP metamorphism ([sup.206]Pb/[sup.238]U age of 223.5 [+ or -] 7.5 Ma), but their Hf isotope compositions indicate that the protolith of the peridotite is at least Mesoproterozoic in age (minimum depleted-mantle ages [[T.sub.DM]] of 1.4 Ga). Zircons from the gneiss also reflect the Mesozoic metamorphism, with a cluster of [sup.206]Pb/[sup.238]U ages at 224.5 [+ or -] 11.5 Ma; a trail of discordant grains indicates a protolith age older than 800 Ma, consistent with [T.sub.DM] model ages of younger than 1.2 Ga. The peridotitic zircons have trace-element patterns similar to kimberlitic and/or carbonatitic zircons, while those from the gneiss have affinities with zircons from syenites/monzonites. The differences suggest that the metasomatic agents that affected the peridotitic zircons were derived from the asthenospheric mantle rather than from subducted continental crust. The strong depletion of the CCSD-PP1 peridotite in basaltic components, and the relatively unradiogenic Hf isotopic compositions (e.g., -16.3 to ~-13.8 [[epsilon].sub.Hf]) of the peridotitic zircons, indicate that the peridotitic body is a fragment of refractory Archean mantle that experienced Mesoproterozoic metasomatism and represents a tectonic intrusion into younger crust. Keywords: peridotitic zircon, Hf isotope, trace element, U-Pb dating, Sulu UHP terrane, China.