Detrital Zircon U–Pb Geochronological Records and Geological Significance of the Cambrian–Devonian Strata in the Southern Part of South China

Abstract

The southern part of South China records an unconformity between the Devonian and the underlying Cambrian strata, which responded to the Caledonian Movement during the Early Palaeozoic period. Zircon U–Pb dating and in situ Lu–Hf isotope analyses were conducted on four samples from the Cambrian and Early Devonian strata near the unconformity on the west side of Qin-Fang Belt, and their significance for palaeogeographic evolution was discussed followed by regional provenance comparisons. The results show that the Early Devonian and Cambrian samples share the similar detrital zircon age spectra with two prominent peaks at ~985 and ~ 530 Ma, respectively. Based on comparisons of the detrital zircon age spectra and ε _Hf(t) values with contemporary igneous zircons inside and outside of South China, the provenances of the Cambrian strata are deduced to be mainly supplied by the old strata in the East Gondwana margins. But the ultimate provenances were mainly from the East Ghats-Rayner orogenic belt and South Indian Granulite Terrane, and partly from Musgrave Province. The provenances of the Early Devonian strata were mainly derived from the recycling of the underlying Cambrian strata. Both sides of the Qin-Fang Belt share the same detrital zircon age spectra in the Cambrian period, indicating that there existed not the South China Residual Ocean. The Early Palaeozoic (460–430 Ma) detrital zircons are absent in the Early Devonian strata on the Qin-Fang Belt and its west side, but are widely distributed in the Middle Devonian strata on the Qin-Fang Belt and eastern margin of the Yunkai Massif, indicating that the most of the Yunkai Massif was submerged below the sea level in the Early Devonian period and emerged above the sea level in the Middle Devonian period.