Zircon U–Pb geochronology and geochemistry of two types of Paleoproterozoic granitoids from the southeastern margin of the North China Craton: Constraints on petrogenesis and tectonic significance

Abstract

Two types of Paleoproterozoic granitoids, i.e., K-feldspar/albite granites and granitic gneisses, were recognized from the Wuhe Complex of the Precambrian basement and neighboring areas in the southeastern margin of the North China Craton (NCC). They share the identical 207Pb/206Pb ages of 2096 ± 9 Ma and 2096 ± 8 Ma defined by zircon SHRIMP U–Pb dating. These granitic rocks are commonly enriched in HREE, Zr, Nb, Ga and Y, and depleted in V, Cr and Ni, most of which have high TFeO/MgO ratios and Zr-saturation temperatures over 850 °C, suggestive of an affinity with A-type granite. In addition, both have consistent whole-rock εNd(t) values (−3.0 to −0.9) and zircon Hf isotopic compositions (εHf (t) = −3.3 to +1.9). The latter are similar with ones of the Neoarchean TTG gneisses in the Eastern Block of the NCC. Thus these ∼2.1 Ga granitoids in the region were most likely derived from the partial melting of ∼2.5 Ga TTG gneisses with minor involvement of coeval mafic crustal materials. The zircon grains from the granitic gneisses can be categorized into core, mantle and rim domains with distinct CL images and trace-element characteristics. The grey mantles exhibit steep REE patterns, low LREE, Th, Pb contents and remarkably low Th/U ratios (<0.1), and define mean U/Pb ages of ∼1.85 Ga. The dark rims display relatively higher LREE, U, Th, Pb contents and Th/U values (0.1–0.3) with slightly younger ages of 1.81–1.83 Ga than mantles. Accordingly, the zircon mantles and rims are interpreted as metamorphic zircons formed during granulite facies metamorphism and anatectic zircons crystallized from limited melts, respectively. The comparable 2.1 Ga magmatism, ∼1.85 Ga high grade metamorphism and subsequent anataxis between the granitic rocks in this study and the Jiao-Liao-Ji Belt (JLJB) suggest that the Wuhe Complex and the JLJB share the same tectonic evolutional processes. These two types of granitoids both formed at an extensional background at ∼2.1 Ga, perhaps in a subduction-related rifting environment, subsequently suffered from granulite-facies metamorphism and selected partial melting during the Paleoproterozoic collisional orogeny.