Resolving discordant U-Th-Ra ages; constraints on petrogenetic processes of recent effusive eruptions at Tatun Volcano Group, northern Taiwan

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 (super 226) Ra- (super 230) Th disequilibrium in magnetite, and less than approximately 70 ka (but potentially Holocene) for a Huangzuei flow, based on (super 238) U- (super 230) Th disequilibrium in plagioclase. Discordant Ar-Ar, (super 238) U- (super 230) Th and (super 226) Ra- (super 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 (super 234) U- (super 238) U equilibrium. Minerals in the Mt Cising sample are in (super 234) U- (super 238) U disequilibrium, despite the (super 234) U- (super 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.