RESOLVING DISCORDANT U–TH–RA AGES: CONSTRAINTS ON PETROGENETICPROCESSES OF RECENT EFFUSIVE ERUPTIONS AT TATUN VOLCANO GROUP,NORTHERN TAIWAN

Abstract

U– Th–Ra isotope analyses of whole rocks and mineral separates were conducted in order to perform isochron dating of three morphologically young lavas from Tatun volcano, northern Taiwan (from Mt Cising, the Shamao dome and the Huangzuei volcano). The data do not yield tight U–Th isochrons, indicating open-system magmatic processes. However, crystallization ages of two samples can be constrained: namely, less than about 1370 years for the Shamao dome, based on 226 Ra– 230 Th disequilibrium in magnetite, and less than approximately 70 ka (but potentially Holocene) for a Huangzuei flow, based on 238 U– 230 Th disequilibrium in plagioclase. Discordant Ar–Ar, 238 U– 230 Th and 226 Ra– 230 Th ages are best explained by young lavas having inherited some crystals from older lithologies (crystal mushes or rocks), and indicate that the above ages represent maxima. Our study provides the first evidence of effusive volcanism at the Tatun Volcano Group in Late Holocene times. All separates from the Shamao dome and Huangzuei volcano are in 234 U– 238 U equilibrium. Minerals in the Mt Cising sample are in 234 U– 238 U disequilibrium, despite the 234 U– 238 U equilibrium of the whole rock. We interpret this as uptake of a hydrothermally altered, old crystal cargo into fresh melt prior to eruption. A different dating approach will thus be required to constrain the eruption age of Mt Cising.