10.6084/m9.figshare.3453920.v1 Georg F. Zellmer Georg F. Zellmer Kenneth H. Rubin Kenneth H. Rubin Christian A. Miller Christian A. Miller J. Gregory Shellnutt J. Gregory Shellnutt Alexander Belousov Alexander Belousov Marina Belousova Marina Belousova Resolving discordant U–Th–Ra ages: constraints on petrogenetic processes of recent effusive eruptions at Tatun Volcano Group, northern Taiwan Geological Society of London 2016 Tatun Volcano Group Mt Cising equilibrium Ra Huangzuei volcano Shamao dome eruption isochron disequilibrium crystal Th lava Mt Cising sample Holocene mineral Geology 2016-06-21 11:46:09 Journal contribution https://geolsoc.figshare.com/articles/journal_contribution/Resolving_discordant_U_Th_Ra_ages_constraints_on_petrogenetic_processes_of_recent_effusive_eruptions_at_Tatun_Volcano_Group___northern_Taiwan/3453920 <p>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 <sup>226</sup>Ra–<sup>230</sup>Th disequilibrium in magnetite, and less than approximately 70 ka (but potentially Holocene) for a Huangzuei flow, based on <sup>238</sup>U–<sup>230</sup>Th disequilibrium in plagioclase. Discordant Ar–Ar, <sup>238</sup>U–<sup>230</sup>Th and <sup>226</sup>Ra–<sup>230</sup>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 <sup>234</sup>U–<sup>238</sup>U equilibrium. Minerals in the Mt Cising sample are in <sup>234</sup>U–<sup>238</sup>U disequilibrium, despite the <sup>234</sup>U–<sup>238</sup>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. </p>