Geochemical and zircon U–Pb–Hf–O isotopic evidence for a coherent Paleoproterozoic basement beneath the Yangtze Block, South China

Abstract

The early history of the Yangtze Block is not well constrained owing to the scarce outcrops of the Archean to Paleoproterozoic rocks. Here, we report an integrated study of zircon U–Pb and Hf–O isotopes, as well as whole rock major and trace elements and Nd isotopes for the orthogneisses of the Dongling Complex in the eastern Yangtze Block. LA-ICP-MS zircon U–Pb dating yields a 207Pb/206Pb age of ca. 1.85Ga for the protolith age of the orthogneiss, revealing a reliable Paleoproterozoic basement in the eastern Yangtze Block. Zircons from the orthogneiss have εHf(t) values ranging from −13.4 to −9.0, corresponding to THfDM2 model ages of 3.03–2.80Ga. The zircon δ18O values range between 6.5‰ and 8.8‰ with an average of 7.8‰. In addition, the whole rock εNd(t) values of the orthogneisses range from −9.3 to −8.0 and TNdDM2 model ages from 2.98Ga to 3.05Ga. The orthogneisses have relatively high SiO2, K2O+Na2O, Rb, Ga, Zr, Nb, Y and Ce contents and show low CaO, Eu, Ba and Sr concentrations. Their 10,000*Ga/Al ratios range between 3.0 and 3.5. All these geochemical and Nd–Hf–O isotopic signatures suggest A-type granite protolith for orthogneisses in the Dongling Complex. They were generated by partial melting of Archean crustal materials in a post-collisional tectonic setting. When our data is combined with the existing geochronological data in other parts of the Yangtze Block, it demonstrates a coherent Paleoproterozoic basement beneath the Yangtze Block, representing an important part of the Columbia supercontinent. The ca. 1.85Ga A-type magmatism may have led to craton stabilization over all of the Yangtze Block. In addition, a potential Precambrian dual basement structure may exist in the eastern Yangtze Block, which is supported by Paleoproterozoic metamorphic basement and overlaying Neoproterozoic sedimentary cover.