Zircon U–Pb and Lu–Hf isotopic constraints on Archean crustal evolution in the Liaonan Complex of northeast China

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The Changhai area is located in the southeastern part of the Liaonan Complex within the Eastern Block of the North China Craton (NCC) in China. Early Precambrian rocks in this area consist mainly of various types of gneisses and schists. We present cathodoluminescence images, U–Pb dates, and trace element and Hf isotope data for zircons from these rocks in order to constrain the formation and evolution of continental crust in the Liaonan Complex. Our results show that the magmatic precursors of the biotite monzonitic gneiss, granitic gneiss, garnet-bearing monzogranitic gneiss, and biotite granodioritic gneiss, that is, relatively sodium-rich gneisses in the area, were emplaced at ca. 2543Ma. Muscovite-bearing granitic gneiss, monzogranitic gneiss, and biotite granitic gneiss, that is, relatively potassium-rich gneisses in the area, were emplaced at ca. 2516Ma, whereas the sedimentary protolith(s) of the associated schists along the gneissose fabric of the gneisses were deposited after 2518Ma and metamorphosed at ca. 1885Ma. The latter age may represent the time of metamorphism of the South Liaohe Group in the Jiao–Liao–Ji Belt. Large numbers of inherited zircons with ages of 2596–2842Ma, coupled with the results of previous studies, suggest that multiple Meso- to Neoarchean magmatic events took place in the Liaonan Complex at ca. 2.84, 2.75, 2.67–2.61, 2.54, and 2.51Ga. Lu–Hf isotopic results for magmatic zircon grains with ages of 2.61–2.84Ga have positive εHf values (+2.4 to +11.3) and average continental crust model ages (TC DM) of 2.63–3.05Ga with a peak at 2.76–2.98Ga. Of the zircons with ages of ca. 2.54Ga and 2.51Ga, 94% have positive εHf values (+0.13 to +8.57, average=+3.1) and TC DM model ages of 2.53–3.06Ga, similar to those of zircons with ages of ca. 2.61–2.84Ga, suggesting that both sets of zircons were derived from partial melting of juvenile crust. However, the remaining zircons (6%) with ages of 2.54–2.51Ga and all metamorphic zircon grains with ages of ca. 1885Ma have negative εHf values of −0.12 to −3.00 and −8.8 to −15.9, respectively, and similar older TC DM model ages of 3.11–3.56Ga and 3.08–3.23Ga, respectively. These data, combined with those from previous studies of the NCC, indicate that the basement rocks of the Liaonan Complex underwent two significant juvenile crustal growth events at ca. 2.71–2.99Ga and 2.54–2.51Ga, a local reworking event at ca. 2.51Ga, and a major reworking event at 1.88Ga.