U–Pb zircon ages and Nd, Sr, and Pb isotopes of lower crustal xenoliths from North China Craton: insights on evolution of lower continental crust

Abstract

Granulite xenoliths sampled from the Hannuoba basalts, North China craton, allow recognition of three types of ancient lower crustal components. The first type is represented by felsic granulite xenoliths. They have upper intercept zircon ages of 1.75–1.89 Ga and lower intercept ages of 105–122 Ma, and are characterized by variable radiogenic Sr isotopic compositions ( 87Sr/ 86Sr=0.707 to 0.746). The second type has an intermediate composition, and is characterized by remarkably unradiogenic Nd ( ɛNd(0)=−26.8 to −29.5) and Pb ( 206Pb/ 204Pb=16.14 to 16.38) and relatively radiogenic Sr ( 87Sr/ 86Sr=0.709 to 0.723) isotopic compositions. Zircon dating yields upper intercept ages of ∼2.5 Ga and lower intercept ages of 88 Ma for this type of xenoliths. The third type is more mafic and shows relatively radiogenic Nd ( ɛNd(0)=−12.6 to −19.3), Pb ( 206Pb/ 204Pb=16.43 to 17.85), and less radiogenic Sr ( 87Sr/ 86Sr=0.707 to 0.709) isotopic compositions. Together with pyroxenites, they lie on an apparent mixing trend between Mesozoic basalt and preexisting lower crust (e.g., DMP-27) in the ɛNd- 87Sr/ 86Sr figure, which can be well modeled by an AFC process. Zircons from the third type contains two distinct phases of old relict core surrounded by new metamorphic overgrowth rim, and yields two age populations of 160–140 and 140–80 Ma. We thus suggest that they are products of ∼160–140 Ma basaltic underplating and ∼140–80 Ma granulite-facies metamorphism. Oscillatory zonation and high Th/U ratios (0.34–1.53) of 97–158 Ma zircons in one olivine pyroxenite indicate an origin from fractional crystallization of a mafic magma, and thus point to basaltic underplating in the late Mesozoic. 206Pb/ 238U ages of granulite xenoliths, regardless of lithology, show an age range of 80–150 Ma, suggesting an important episode of granulite-facies metamorphism. The overlapping timing for underplating and granulite-facies metamorphism indicates that the latter was induced by the former, which was related to the latest Jurassic to Cretaceous lithospheric extension.