U-SERIES CRYSTAL AGES FROM THE ROCK MESA ERUPTION AT SOUTH SISTER VOLCANO, OREGON SUGGEST COMPLEX ORIGIN FOR RHYOLITE

We discuss U-Th ages of zircon and U-Th-Ra ages of plagioclase from the Rock Mesa lava flow at South Sister volcano, Oregon Cascades, which will be used to (1) test if zircon and plagioclase are recording the same events within the magma system and (2) investigate the method of rhyolite genesis at South Sister. Rock Mesa is a small (~0.5 km³) rhyolite dome on the south flank of South Sister volcano that erupted 2300 ¹⁴Cyr.BP (Scott 1987 – GSA SP v212). We present 10 new zircon U-Th spot ages for the Rock Mesa lava flow. Nine of the ages range from 9.4 ka ^+5.1_-4.9 to 53.3 ka ^+9.3_-8.5 which coincides with periods of andesitic to rhyolitic volcanism at South Sister (Hildreth 2007 – USGS PP 1744). The zircon saturation temperature for the Rock Mesa rhyolite is 792^oC (using the method of Watson and Harrison , 1983, EPSL v64), lower than the eruption temperature of ~877^oC, and a maximum dissolution time of 1050 yrs was calculated for these zircons using the method of Watson (1996 – GSA SP v315). The low saturation temperature and short dissolution time coupled with the fact that no zircon ages are within error of eruption suggests the zircons are antecrystic and were incorporated into the Rock Mesa magma shortly before eruption. Trace element traverses performed on the zircon grains show significant zonation in Y, Yb, and Hf suggesting these grains had a complex history. One zircon U-Th age is within error of secular equilibrium, implying a xenocrystic origin since volcanic activity did not being at South Sister until 178±1 ka (Hildreth 2007). These data suggest complex storage and assimilation ongoing at South Sister over the last few 100 k.y. U-Th-Ra dating of four size fractions of plagioclase is underway, which will compliment the age data provided by the zircon. If the plagioclase is related to the zircon and therefore antecrystic then plagioclase Th-Ra ages in secular equilibrium and U-Th ages similar to the zircon ages would be expected. Alternatively, if crystal/liquid separation more efficiently incorporates zircons while leaving behind plagioclase, then plagioclase would be expected to be young (within a few thousand years of eruption age). More complex scenarios are possible, but combining U-Th zircon ages and U-Th-Ra plagioclase ages will allow an effective evaluation of the source of the crystals and method of rhyolite genesis at South Sister.