AGES AND MAGMATIC PROCESSES OF QUATERNARY BASALTIC ANDESITES FROM LAOGUIPO (SE TIBETAN PLATEAU): U-TH-RA ISOTOPE AND ZIRCON HF-O ISOTOPE CONSTRAINTS

Rare Quaternary basaltic andesites on the Tibetan Plateau occur at Laoguipo volcano within the hydrothermally active Tengchong volcanic field from Southeast Tibetan Plateau. A low seismic velocity zone indicative of a likely magma body is detected 10-25 km below Laoguipo volcano. The relatively primitive volcanic rocks (SiO₂ = 52 to 55 wt.%) from Laoguipo provide important information about the early stage of magma evolution process for post-collisional magmas from Tibet. Reported K-Ar ages for Laoguipo rocks vary from Holocene to Middle Pleistocene (up to 239 ka, thousand years ago). The uncertainties in K-Ar ages may be attributed to excess or loss of argon. Here we use whole-rock ²³⁸U-²³⁰Th-²²⁶Ra isotope disequilibrium and zircon ²³⁸U-²³⁰Th isotope disequilibrium to constrain the ages of Laoguipo basaltic andesites. The Laoguipo rocks show ²³⁰Th-²²⁶Ra secular equilibrium, whereas the nearby Holocene volcanic rocks at Maanshan, Dayingshan and Heikongshan exhibit ²³⁰Th-²²⁶Ra disequilibrium. ²³⁰Th-²²⁶Ra equilibrium for Laoguipo rocks indicates their eruption age older than Holocene time, as any original ²³⁰Th-²²⁶Ra disequilibrium disappears after about 10,000 years (about six half-lives of ²²⁶Ra). Zircon ²³⁸U-²³⁰Th isochron ages for 2 Laoguipo rocks are 55 ± 5 ka (2σ) and 61 ± 10 ka (2σ), with a weighted average age of 56 ± 8 ka (2σ). We thus conclude that the Laoguipo rocks erupted in Late Pleistocene. Oxygen isotopes (δ¹⁸O) for 57 zircons range from 6.8 to 7.8 ‰, higher than mantle-derived zircon δ¹⁸O value of 5.3 ± 0.6 ‰ (2σ). Thus, all Laoguipo zircons are crustal zircons. Hafnium isotopic values (ε_Hf) for these 57 zircons are all negative, ranging from -13.9 to -3.7. Notably, zircon ε_Hf values and zircon δ¹⁸O values are negatively correlated, suggesting magma assimilation during fractional crystallization. In addition, decoupling of zircon/whole-rock Hf isotopes, a positive correlation between whole-rock ⁸⁷Sr/⁸⁶Sr and SiO₂, a negative correlation between whole-rock ¹⁴³Nd/¹⁴⁴Nd and SiO₂, and a negative correlation between whole-rock ε_Hf and SiO₂, are consistent with assimilation fractional crystallization process in an open-system magma chamber for these relatively primitive post-collisional volcanic rocks from SE Tibet.