Relict zircon U-Pb age and O isotope evidence for reworking of Neoproterozoic crustal rocks in the origin of Triassic S-type granites in South China

Abstract

Granites derived from partial melting of sedimentary rocks are generally characterized by high δ18O values and abundant relict zircons. Such relict zircons are valuable in tracing the source rocks of granites and the history of crustal anatexis. Here we report in-situ U-Pb ages, O isotopes and trace elements in zircons from Triassic granites in the Zhuguangshan and Jiuzhou regions, which are located in the Nanling Range and the Darongshan area, respectively, in South China. Zircon U-Pb dating yields magma crystallization ages of 236±2Ma for the Zhuguangshan granites and 246±2Ma to 252±3Ma for the Jiuzhou granites. The Triassic syn-magmatic zircons are characterized by high δ18O values of 10.1–11.9‰ in Zhuguangshan and 8.5–13.5‰ in Jiuzhou. The relict zircons show a wide range of U-Pb ages from 315 to 2185Ma in Zhuguangshan and from 304 to 3121Ma in Jiuzhou. Nevertheless, a dominant age peak of 700–1000Ma is prominent in both occurrences, demonstrating that their source rocks were dominated by detrital sediments weathered from Neoproterozoic magmatic rocks. Taking previous results for regional granites together, Neoproterozoic relict zircons show δ18O values in a small range from 5 to 8‰ for the Nanling granites but a large range from 5 to 11‰ for the Darongshan granites. In addition, relict zircons of Paleozoic U-Pb age occur in the two granitic plutons. They exhibit consistently high δ18O values similar to the Triassic syn-magmatic zircons in the host granites. These Paleozoic relict zircons are interpreted as the peritectic product during transient melting of the metasedimentary rocks in response to the intracontinental orogenesis in South China. Therefore, the relict zircons of Neoproterozoic age are directly inherited from the source rocks of S-type granites, and those of Paleozoic age record the transient melting of metasedimentary rocks before intensive melting for granitic magmatism in the Triassic.