Petrogenesis and tectonic setting of Late Triassic granite in western Yunnan, China: insights from whole rock geochemistry, zircon U-Pb and Hf isotopes

A-type granites have distinctive geochemical characteristics indicative of an extensional tectonic background, thereby providing critical constraints on magma sources and tectonic processes, especially playing a crucial role in reconstructing orogenic processes. In this study, we conducted systematic zircon U-Pb dating, whole-rock geochemistry and Hf isotope analyses on the newly identified granites in the Muchang area of the southern Baoshan block to investigate their petrogenesis and tectonic setting. Granites at Muchang contain K‑feldspar, quartz, biotite, minor plagioclase, and alkaline mafic minerals dominated by riebeckite, with accessory apatite and zircon. The zircon U-Pb dating results show that the crystallization ages of granites are 231.0 ± 4.0 Ma and 235.5 ± 1.3 Ma, respectively, indicating that they were formed in the Late Triassic. Geochemically, these rocks have high SiO2 (64.04–72.80 wt.%) and total alkali (Na2O + K2O = 7.60–10.11 wt.%) contents, low MgO (0.22–0.35 wt.%), CaO (0.21–0.80 wt%), and P2O5 (0.02–0.05 wt.%) contents, with negative anomalies of Ta, Sr, Ba, P, and Ti elements. They have high FeOT/MgO ratios, Zr + Nb + Ce + Y contents (284.2–655.4 ppm), and 10,000 × Ga/Al ratio (3.92–5.15), which are consistent with the characteristics of typical A-type granites. Furthermore, they exhibit low Y/Nb ratios (<1.20), indicating an A1-type geochemical affinity. Their high K2O/Na2O ratios, low Ce/Nb and Y/Nb ratios, combined with positive zircon εHf(t) values (5.04 to 8.54) and TDM2 ages (942 to 716 Ma), are consistent with derivation from partial melting of the alkali-rich mafic lower crust, possibly in response to mantle-derived magma upwelling. Based on regional geological and geochronological evidence, the A-type granites are considered to have been formed in a post-collisional extensional setting after the closure of the Paleo-Tethys Ocean.