Zircon SHRIMP U–Pb dating for gneisses in northern Dabie high T/P metamorphic zone, central China: Implications for decoupling within subducted continental crust

Zircon SHRIMP U–Pb ages and cathodoluminescence (CL) images reveal that most zircon separated from two tonalitic gneiss samples in the northern Dabie high T/P metamorphic zone (NDZ) have four different domains: (1) inherited core, with clear oscillatory zoning, low-P mineral inclusions and high Th/U ratios, indicating the Neoproterozoic age of the protolith; (2) inner-mantle, with homogeneous CL intensity, low Th/U ratios of ≤ 0.09 and occasionally ultrahigh pressure metamorphic (UHPM) mineral inclusions such as diamond, garnet and rutile, which defined a weighted mean 206Pb/238U age of 218 ± 3 Ma, corresponding to the age of ultrahigh-pressure metamorphism; (3) outer-mantle with lower Th/U ratio and retrograde mineral inclusions, which yields a retrogressive age of 191 ± 5 Ma; and (4) rim, which is black luminescence, with relatively high Th/U ratios of 0.12–0.40 and a weighted mean 206Pb/238U age of 126 ± 5 Ma, indicating an overprint of the Early Cretaceous migmatization. Consequently, four discrete and meaningful age groups have been identified. Remarkably, the U–Pb ages of both UHPM zircon and retrograded metamorphic zircon from the NDZ are significantly younger than the U–Pb ages of 238 ± 3 Ma–230 ± 4 Ma for UHPM zircon and U–Th–Pb ages of 218 ± 1.2 Ma–209 ± 3 Ma for rutile and monazite overgrowths (representing the cooling or retrograded metamorphic time corresponding to amphibolite-facies) from the southern Dabie UHPM zone (SDZ), respectively. Combined with reported ages for UHPM rocks from the NDZ, SDZ and Huangzhen low-T eclogite zone (HZ), we found that the metamorphic ages of these three UHPM units in the Dabie orogen gradually decrease from south to north. This age distribution suggests that the three UHPM units represent 3 exhumed crustal slices, which are decoupled from each other and have different subduction and exhumation histories. Firstly, the subducted Huangzhen crustal slice was detached from the underlying subducted continental lithosphere, and then exhumed because of the buoyancy. In the meantime, subduction of underlying continental plate continued. After the initiation of this process, lithologic separations or decoupling within subducted continental crust occurred repeatedly several times to form an imbricate megastructure in the Dabie UHPM belt.