Early Mesozoic granites in the Nanling Belt, South China: Implications for intracontinental tectonics associated with stress regime transformation

Abstract

The link between two tectono-magmatic events during Early and Late Mesozoic periods in the Nanling Belt of South China remains debated. Here we present zircon U–Pb geochronology, Hf isotopes and whole-rock geochemistry of granitic intrusions from Zhuguangshan in the Nanling Belt. The zircon data exhibit two magmatic episodes with 206Pb/238U ages of ca. 230Ma and 150Ma, representing the Indosinian (Early–Middle Triassic period) and Early Yanshanian (Jurassic–Cretaceous period) events, respectively. The Indosinian granites are characterized by strongly peraluminous nature with high A/CNK values (>1.1), whereas the Early Yanshanian granites are weakly peraluminous (average A/CNK value of 1.06). Although the Early Yanshanian granites bear higher HREE contents and lower LREE/HREE ratios, the two-phase granites show enrichment in LREE, Rb, Th, U and Pb, depletion in Eu and negative Ba, Sr, Nb and Ti anomalies. All samples show variably negative εHf(t) values (−5 to −16) with two-stage Hf model ages clustered around 1.8–2.1Ga. Our data indicate that the two-phase granitic magmas were generated from the partial melting of early Paleoproterozoic basement rocks with no input of mantle material. Combined with other geological evidence, we infer that the Indosinian tectono-magmatic event was closely related to intracontinental orogeny triggered by collisions along the boundaries of South China Craton, in relation to processes associated with the E–W trending Tethys tectonic domain. We envisage that the Early Yanshanian event might be a response to the back-arc extension of NW-directed paleo-Pacific plate subduction. The Nanling Belt is a critical zone that records the transformation from Tethys to paleo-Pacific tectonic regimes.