Petrogenesis of 2.7–2.65 Ga TTGs in the Wutai Complex: Constraints on the Neoarchean crustal evolution of the North China Craton

Abstract

The Wutai complex, located on the boundary between the Eastern and Western Blocks of the North China Craton (NCC), offers valuable insight into early crustal growth and evolution of the NCC. In this study, a suite of 2.7–2.65 Ga TTG gneisses has been identified in the Wutai complex. Zircon grains from eight samples indicate that their protoliths were emplaced between 2706 Ma and 2640 Ma. The trondhjemites have low K2O/Na2O, high La/Yb and Sr/Y ratios, and positive Eu anomalies. The observed trend of decreasing Dy/Yb and Dy/Dy* ratios with increasing La/Yb ratios support for an amphibole-dominated fractional crystallization process during the TTG formation. Zircon Hf and oxygen isotope analyses (εHf(t) = 3.5–7.0, TDM = 2.86–2.71 Ga; δ18O = 5.5–6.3 ‰) indicate that the parental magma may originate from juvenile crustal sources altered by surface water at low temperature. When combined with the previous studies of volcanic rocks and TTGs formed around 2.5 Ga, it becomes evident that the Wutai complex experienced significant crustal growth during the period from 2.7 to 2.5 Ga. The identification of 2.7–2.65 Ga magmatic activity as well as the older detrital/xenocryst zircons up to 3.6 Ga, provide increasing evidence that the Wutai complex has a much longer and nearly continuous geological history. Combined with geological evidence, zircon data compilations and the Re-depletion model age distribution of the mantle xenoliths of the NCC, it can be inferred that the period from 2.6 to 2.5 Ga was likely the most significant for the crustal generation within the NCC's geological history.