∼2.7 Ga crust growth in the North China craton

Abstract

Numerous U–Pb zircon geochronological data show that rocks of the Precambrian basement from the North China craton (NCC) have two age peaks at 1.8–1.9 and ∼2.5 Ga, respectively. Whether the most prominent peak at ∼2.5 Ga really registered a period of major crustal growth in the NCC thus making it different from the global major crustal growth period of ∼2.7 Ga is uncertain. In this paper, we carried out in situ U–Pb and Hf isotopes of zircons and whole rock chemical and Sr–Nd isotopic analyses for six Precambrian samples from the NCC in order to understand crustal growth and reworking. The samples have U–Pb zircon ages of 1.8–1.9, ∼2.5 and ∼2.7 Ga, respectively. The two samples with U–Pb zircon ages of ∼2.7 Ga have zircon Hf model ages (T Hf DM) and/or whole rock Nd model ages (T Nd DM) close to their U–Pb zircon ages. It is clear that they represent juvenile crust generation, thus providing direct evidence that ∼2.7 Ga was a period of new crustal addition. The samples with U–Pb zircon ages of ∼1.8 and ∼2.5 Ga also have T Hf DM and/or T Nd DM similar to those of the ∼2.7 Ga samples, indicating that these samples also have a mantle extraction time of 2.7 ± 0.1 Ga and their ∼1.8 and ∼2.5 Ga ages may represent periods of reworking of the 2.7 ± 0.1 -Ga crust rather than juvenile crust generation. The reworking is manifested either as metamorphism or as lower crustal-derived magmatism. Specifically, the ∼2.5-Ga tonalitic gneiss sample is thought to be derived by melting of the ∼2.7 Ga lower crust. Since many ∼2.5-Ga tonalite–trondhjemite–granodiorites (TTGs) and calc–alkaline granitoids in the NCC contain ∼2.7-Ga xenocrystic and inherited zircons, we postulate that they are probably also derived mainly from the 2.7 ± 0.1-Ga crust. Our data imply that the quantity of ∼2.7 Ga continental crust in the NCC is probably greater in extent than that exposed today.