OPEN-SYSTEM MAGMATIC PROCESSES AND CONDITIONS FOR ASHISHAN AND WULUKESHAN VOLCANOES FROM NORTHWESTERN TIBETAN PLATEAU

The Ashishan and Wulukeshan volcanos are two of the most prominent volcanos of the Quaternary Ashikule Volcanic Cluster from northwestern Tibetan Plateau, both in size and recent activity. Compositionally, both Ashishan and Wulukeshan are potassium-rich trachyandesites. Zircon ²³⁸U-²³⁰Th disequilibrium isotopes have yielded ages of 0.070 ± 0.006 Ma (2σ) for Ashishan and 0.113 ± 0.034 Ma (2σ) for Wulukeshan and thus firmly demonstrated their young ages. However, magmatic processes and conditions remain to be investigated for these two volcanoes. Here we use plagioclase Sr isotope compositions and zircon U-Pb age data to evaluate open-system magmatic processes and use the compositions of orthopyroxenes and clinopyroxenes to constrain magma temperature and pressure conditions. ⁸⁷Sr/⁸⁶Sr data for plagioclase crystals from Ashishan volcano range between 0.710361- 0.715863. About 50% plagioclase crystals have clearly higher ⁸⁷Sr/⁸⁶Sr ratios than corresponding whole-rock values of 0.71010-0.71021. U-Pb ages of zircons from Wulukeshan range from 0.33 ± 0.32 Ma (1σ) to 2695 ± 10 Ma (1σ). About 90% of the zircon U-Pb ages are undoubtedly older than the Wulukeshan young zircon U-Th disequilibrium ages (0.113 Ma), indicating that the vast majority of Wulukeshan zircons are assimilated xenocrysts from the basement and country rocks. Thus, both plagioclase Sr isotopes and zircon U-Pb ages indicate magma contaminations. Regarding magmatic conditions, the pairs of clinopyroxene and orthopyroxenes yield temperature ranging from 1081-1104℃, with an average of 1094 ± 12℃, and pressures ranging from 5.6-7.1 kbars, with an average of 6.4 ± 0.8 kbars. Extrapolated magma chamber depths range between 18.5-23.4 km, with an average of 21.1 ± 2.5 km.