Unusual Hf contents in metamorphic zircon from coesite-bearing eclogites of the Dabie Mountains, east-central China: Implications for the dating of ultrahigh-pressure metamorphism

Metamorphic zircon from coesite-bearing eclogites in the Dabie Mountains encloses high-P phases, and may have formed at the peak of ultrahigh-pressure (UHP) metamorphism. Morphologically, the metamorphic zircon typically occurs as small, multi-faceted, near-spherical grains with homogeneous internal structure and weak backscattered electron (BSE) luminescence. Geochemically, it is characterized by extremely high and relatively constant contents of hafnium (Hf) and very low contents of Y, U and Th, reflecting the contraction of the zircon lattice under the UHP conditions. High contents of Hf may be characteristic of zircon formed during UHP metamorphism, which has important consequences for interpretation of geochronological results. We propose that the metamorphic zircon extremely enriched in Hf may be used to date the peak of UHP metamorphism that produced the coesite-bearing eclogites in the Dabie Mountains, and potentially in other UHP terranes.