Late Pleistocene granodiorite beneath Crater Lake caldera, Oregon, dated by ion microprobe

Variably melted granodiorite blocks ejected during the Holocene caldera-for ming eruption of Mount Mazama were plucked from the walls of the climactic magma chamber at similar to 5 km depth. Ion-microprobe U-Pb dating of zirco ns from two unmelted granodiorite blocks with SHRIMP RG (sensitive high-res olution ion microprobe-reverse geometry) gives a nominal U-238/Pb-206 age o f 101(-80)(+78) ka, or 174(-115)(+89) ka when adjusted for an initial Th-23 0 deficit. SHRIMP RC U-Th measurements on a subset of the zircons yield a T h-230/U-238 isochron age of 112 +/- 24 ka, considered to be the best estima te of the time of solidification of the pluton. These results suggest that the granodiorite is related to andesite and dacite of Mount Mazama and not to magmas of the climactic eruption. The unexposed granodiorite has an area of at least 28 km(2). This young, shallow pluton was emplaced in virtually the same location where a similarly large magma body accumulated and power ed violent explosive eruptions similar to 7700 yr ago, resulting in collaps e of Crater Lake caldera.