Late-Neoarchean magmatism and metamorphism at the southeastern margin of the North China Craton and their tectonic implications

Abstract

In the present study, two gneiss xenoliths from the Mesozoic Jiagou intrusion at the southeastern margin of the North China Craton (NCC) are examined for in situ zircon SHRIMP U–Pb dating in combination with zircon trace element and Hf isotope analyses. Cathodoluminescence (CL) images and trace element data reveal that most zircons from the dated samples display distinct core–rim structures, i.e., magmatic cores and metamorphic rims. The cores show typical igneous characteristics with oscillatory growth zoning and high rare earth element (REE) contents, whereas the metamorphic rims are characterized by spherical to oval shape, and high and homogeneous luminescent intensity. Zircon SHRIMP U–Pb dating results suggest that the xenoliths formed at 2.55–2.64Ga and experienced high-grade metamorphism at 2.48–2.49Ga. The lower HREE contents, negative Eu anomalies, high Th/U ratios (generally >0.5) and high Ti-in-zircon temperatures (>800°C) of metamorphic zircon rims suggest that the 2.48–2.49Ga event represents an episode of granulite facies metamorphism. These results demonstrate that the lower-crustal rocks in the studied area experienced late-Neoarchean magmatism and subsequent metamorphism. Positive ɛHf(t) values of +2 to +5, with model ages younger than 3.0Ga and formation ages of 2.55–2.64Ga for igneous zircons from the xenoliths indicate that they were derived from juvenile crustal sources and strongly suggest the existence of significant late-Neoarchean crustal growth in the region.