Resolving the timing of late pleistocene dome emplacement at Mono Craters, California, from 238U--230Th and 40Ar/39Ar dating

The Mono Craters chain in eastern California is one of the youngest sites of rhyolitic volcanism in North America and comprises at least 28 overlapping lava domes, flows, and tephra rings of mostly Holocene age. New U-series and 40Ar/39Ar geochronological data presented here extend the age of the Mono Craters into the Late Pleistocene. Ion microprobe 238U--230Th isochron dating of unpolished rims of allanite and zircon and 40Ar/39Ar laser fusion and step-heating dating of sanidine from the porphyritic biotite-bearing dome lavas of the Mono Craters yield Late Pleistocene ages, but the two techniques yield discordant results. The 238U--230Th isochron method gives ages of 26 +/- 1.2 ka, 38 +/- 1.2 ka, and 42 +/- 1.1 ka for domes 31 (newly recognized), 24, and 19, respectively, whereas the corresponding 40Ar/39Ar sanidine ages are all older by an amount that exceed analytical errors. The anomalously older 40Ar/39Ar sanidine ages are attributed to excess argon from incompletely degassed antecrysts and/or melt inclusions trapped in juvenile phenocrysts. Explosive eruptions preceded dome emplacement and produced tephra layers in the Wilson Creek formation of ancestral Mono Lake. The independently dated tephra layers can be correlated to the domes via titanomagnetite geochemistry. Correlation of specific tephras to the domes verifies that the 238U--230Th isochron rim ages of euhedral zircon and allanite provide the best estimates of eruption ages for the Mono Craters.