Geochronology, Geochemistry and Tectonic Implications of Triassic A-Type Granites in Pingtian Area, Northern Guangdong

Pingtian pluton is located in the central part of South China, providing an ideal window for understanding the Triassic tectonic-magmatic activities and the geodynamic setting in South China. Systematic whole-rock geochemistry, zircon U-Pb dating and zircon in situ Hf isotope studies were carried out, taking the Pingtian granite group as the object of study. The results show that the Pingtian pluton consists of coarse-grained porphyritic biotite granite, coarse-grained porphyritic potassium feldspar granite and medium-grained monzonite, with diagenetic ages ranging from 238 to 239 Ma, and it was formed in the Middle Triassic. Geochemical characteristics show that the rocks are enriched in light rare earth elements with obvious europium negative anomalies (δEu average=0.42). It is enriched in Zr, Hf, Y and Ce, and significantly depleted in Sr, P and Ti. It belongs to the metaluminous to weakly peraluminous alkaline granite type, and is A-type granite. The zircon εHf(t) values range from -37.7 to -5.0, and the tDM2 two-stage model ages range from 1 578 to 3 597 Ma. Combined with the whole-rock geochemical characteristics, it reveals that the magmas were derived mainly from the partial melting of felsic material in the crust under low-temperature and high-pressure environment, probably mixed with old crustal material, and experienced crystalline differentiation, and was formed in the post-collisional extensional background. Integrating the geochemical characteristics and spatial distribution of A-type granites and alkaline syenite in South China, we propose that the tectonic evolution of the Triassic was mainly controlled by the collisional interaction of the South China block with the Indochina block and the North China block, and that the tectonic setting changed at around 238 Ma, from a collisional extrusion environment in the Early Triassic to a transition to a post-collisional extensional environment in the Middle to Late Triassic. © 2024 China University of Geosciences. All rights reserved.