Two-Stage Exhumation of Ultrahigh-Pressure Metamorphic Rocks from the Western Dabie Orogen, Central China

Abstract

Zircon trace element, U-Pb age, and Lu-Hf isotope composition were determined for two eclogite samples from the Xinxian ultrahigh-pressure (UHP) unit in the western Dabie orogen to constrain its exhumation processes. Cathodoluminescence imaging reveals that most zircons are metamorphic, while some have inherited magmatic cores in one sample. The magmatic cores have high Th/U and Lu/Hf ratios, high trace element contents, and a clear negative Eu anomaly, consistent with their igneous genesis. They yield an upper intercept age of Ma and a positive εHf(t) value of 8.0, arguing for reworking of early Mesoproterozoic crust during the middle Neoproterozoic in the western Dabie orogen. The metamorphic zircons contain mineral inclusions of garnet, omphacite, rutile, and quartz. They are characterized by low Nb, Ta, Y, and heavy rare earth elements (HREE) contents, a nearly flat HREE pattern, and an insignificant negative Eu anomaly. These indicate that the metamorphic zircons formed under high-pressure (HP) quartz eclogite-facies metamorphic conditions. The metamorphic zircons in the two samples have Ti-in-zircon temperatures of and C, respectively, suggesting that they formed in different HP eclogite-facies metamorphic stages. The metamorphic zircons in the two samples yield weighted mean U-Pb ages of and Ma, which may date the initial exhumation to ∼90 km and ∼700°C and a subsequent HP quartz eclogite-facies retrogression at ∼50 km and ∼600°C for ultrahigh-pressure (UHP) rocks in the western Dabie orogen. This yields a maximum exhumation and average cooling rates of ∼0.33 cm/yr and 8°C/Ma, respectively. Based on the published Rb-Sr and Ar-Ar ages of ∼212 Ma for the UHP rocks in the area, the subsequent stage of exhumation of UHP rocks has a maximum exhumation and average cooling rates of 0.67 cm/yr and 65°C/Ma, respectively. This two-stage cooling process may be common for HP-UHP metamorphic terranes in continental collision zones.