Geochronology and petrogenesis of Middle Permian S-type granitoid in southeastern Guangxi Province, South China: Implications for closure of the eastern Paleo-Tethys

Abstract

The Bangxi-Chenxing suture zone is an important region to address the history of closure of the eastern Paleo-Tethys ocean. Among the widespread S-type granite batholiths in the SE Guangxi Province adjacent to this suture is the Nali granodiorite. Here we report zircon U–Pb ages, major and trace element geochemistry, and Sr–Nd–Hf isotope geochemistry of a newly found Middle Permian S-type granite in this region. LA-ICP-MS zircon U–Pb geochronology yields emplacement ages of 265±2 to 262±2Ma for the Nali granodiorite. Both the inner and outer phases are characterized by variable SiO2 (66.91–71.39wt.%), high Al2O3 (12.99–14.04wt.%), K2O+Na2O (4.78–5.98wt.%), and A/CNK values (1.11–1.50), resembling those of typical S-type granites. The rocks are enriched in Rb, Th, U and light rare earth elements with weak negative Eu anomalies, and show depletion in Nb, Ta, Ti, with typical arc-like affinity. They have relatively high (87Sr/86Sr)i ratios of 0.7228 to 0.7331, low εNd(t) values of −13.6 to −11.9, and low zircon εHf(t) values (−21.9 to −8.8). High whole-rock Nd isotopic model ages (2.00–2.02Ga) and zircon Hf isotopic model ages (1.59–2.20Ga), indicate that they might have been generated by partial melting of ancient lower crust with minor contribution from mantle sources. The granodiorite shows continental arc affinity and likely formed within a subduction setting. By combining previously extant data for Permo-Triassic arc-/collision-related magmatism in the Jinshajiang-Ailaoshan-Song Ma and Bangxi-Chenxing suture zones, we propose that the subduction of the eastern Paleo-Tethys might have started during Middle Permian triggered by the northward subduction of the Indochina Block (ICB) beneath the South China Block (SCB) and closure during Middle Triassic.