Study on U-Pb age and Hf isotope of zircons in the Bixiling peridotites, the Dabie ultrahigh-pressure, metamorphic belt

Abstract

On the basis of the form and internal structure of the Bixiling peridotitic zircons from the southern Dabie Orogen, in situ U-Pb ages and Hf isotope of them were analyzed. The zircons are mainly recrystalizated/metamorphic growth (210-220Ma) with minor relics of magmatism. One grain with high Th/U ratio gives a nearly concordant age of 646Ma, representing the minimum age estimate of magmatism. The intrusion age can be obtained by the upper intercept of 745Ma. The ages of 210-220Ma record the growth of zircons during exhumation of the Dabie ultrahigh-pressure belt. The Bixiling peridotitic zircons, except few grains with negative εHf(e.g. -2.9), have positive εHf(high up to + 8.1) and Neoproterozoic depleted mantle model ages (mean 0.8Ga). The data suggest that the initial materials of the Bixiling peridotites are Neoproterozoic cumulates beneath the northern margin of the Yangtze craton. The magma derived from the depleted asthenospheric mantle, erupted at the lower part of the continental crust (e.g. underplating) and assimilated a slight proportions of pre-existing crust. The protolith of the Bixiling peridotites experienced complex processes accompanying with subduction of the Yangtze lithosphere, collision with the North China craton, and exhumation of the Dabie-Sulu orogen. The metasomatic growth or recrystalization of zircons (< 230Ma) in the Bixiling peridotites may mainly record the exhumation event of the Dabie orogen following the collision.