Geochronology and petrogenesis of basaltic trachyandesites from Laoguipo volcano (SE Tibetan Plateau): U-series and Sr-Nd-Hf-O isotope constraints

Late Quaternary basaltic trachyandesites (SiO₂ = 52.1–55.6 wt%) on the Tibetan Plateau are present at Laoguipo volcano, situated within the hydrothermally active Tengchong volcanic field (TVF) in the southeastern Tibetan Plateau. A low seismic velocity zone likely representing a magma body as the heat source for the geothermal system is detected 10–25 km below the Laoguipo volcano area and 80 % of seismic activities in TVF occur below Laoguipo and nearby Maanshan. Previously reported K-Ar ages for Laoguipo vary from Holocene to Middle Pleistocene (9 ka to 239 ka, thousand years ago) with large uncertainties. Here we use whole-rock ²³⁰Th-²²⁶Ra isotope disequilibrium and zircon ²³⁸U-²³⁰Th isotope disequilibrium to constrain the ages of Laoguipo basaltic trachyandesites. The eruption age for Laoguipo is 10.5 ± 2.5 ka. Zircon ²³⁸U-²³⁰Th isochron ages for 2 Laoguipo rocks are 55 ± 5 ka (2σ) and 61 ± 10 ka (2 σ), indicating a long magma residence time (∼45 ka) prior to eruption. Oxygen isotopes (δ¹⁸O) for Laoguipo zircons range from 6.8 to 7.8 ‰, higher than mantle-derived zircon δ¹⁸O value of 5.3 ± 0.6 ‰ (2σ). Hafnium isotopic values (ε_Hf) for Laoguipo zircons are all negative, ranging from −13.9 to −3.7. Notably, zircon ε_Hf values and zircon δ¹⁸O values are negatively correlated. Coupled with the sieve texture of plagioclase crystals, our data reveal that these relatively primitive mafic volcanic rocks at Laoguipo underwent an early-stage open-system magmatic processes. The young eruption age (10.5 ± 2.5 ka) and the active seismic activities suggest that Laoguipo is a potentially dangerous volcano in TVF, SE Tibet.