Abstract

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 (206Pb/238U age of 223.5 ± 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 [TDM] of 1.4 Ga). Zircons from the gneiss also reflect the Mesozoic metamorphism, with a cluster of 206Pb/238U ages at 224.5 ± 11.5 Ma; a trail of discordant grains indicates a protolith age older than 800 Ma, consistent with TDM 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 εHf) 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.

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