Zircon U–Pb ages and geochemistry of deep-seated crustal xenoliths from Yangyuan: Implications for the evolution of lower crust beneath the Trans North China Orogen

Abstract

Deep-seated crustal xenoliths captured by volcanic rocks play a rare opportunity in understanding the composition and evolution of the lower continental crust. Here, we present mineral chemistry, whole-rock major and trace elements and Sr–Nd isotopes, and in-situ zircon U–Pb ages and Hf isotope data for a suite of granulite and biotite gneiss xenoliths entrained in the Oligocene basalts from the Yangyuan county in the central part of the Trans North China Orogen (TNCO) with aims to decipher the composition and evolutionary history of the lower crust beneath this area. Calculated equilibrium temperatures (841–933 °C) and pressures (9.7–14.3 kbar) suggest an origin of the Yangyuan granulites from the lower crust. The dominantly Archean–Proterozoic zircon ages and low Mg# values (29.0–68.4) of the Yangyuan crustal xenoliths indicate that they represent the ancient crust and the fractional crystallization plays a key role during the protolith formation. Furthermore, the zircon U–Pb–Hf isotopes data reveal multiple thermal events, including two main peaks of crustal growth at ∼2.7 Ga and ∼2.5 Ga, ∼1.85–1.80 Ga and the Phanerozoic accretion and reworking of the lower crust beneath Yangyuan. In addition, a comparison study of crustal xenoliths from Yangyuan, Nangaoya and Hannuoba areas demonstrates that the lower crust beneath the central–northern part of the TNCO experienced as a whole the Neoarchean crustal growth, Paleoproterozoic metamorphism and Phanerozoic magmatic underplating. More importantly, the absence of U–Pb age peak of ∼1.95 Ga and Hf model age peaks of ∼3.9–3.8 Ga, ∼2.9–2.8 Ga, ∼2.2–2.1 Ga and ∼700–600 Ma in Yangyuan shows a heterogeneous evolution of the lower crust beneath the TNCO, which could pave the way for a more comprehensive picture of the crustal evolutional history beneath the North China Craton.