40AR/39AR GEOCHRONOLOGY OF THE OLIGOCENE-MIOCENE GROSVENOR HILLS AND THE MIOCENE NOGALES FORMATION, UPPER SANTA CRUZ BASIN, SOUTHERN ARIZONA
Previous ages reported for the Miocene Nogales Formation range from 12 to 18 Ma. Our results generally support these ages, but suggest that the formation is as young as 11 Ma and as old as 20 Ma. A basalt in the upper part of the formation near Nogales, Arizona, along the international border yielded an 40Ar/39Ar plateau age of 11.67 ± 0.09 Ma. In addition, 40Ar/39Ar ages of other basalts in the Nogales Formation in the Agua Fria Canyon area, about 10 km north of the international border, were slightly older, with 40Ar/39Ar plateau ages ranging from 13.23 ± 0.10 to 13.64 ± 0.11 Ma, indicating these basalts were emplaced in the middle to upper part of the formation. Sanidine and plagioclase separated from sandstones in the lower and middle parts of the formation in Nogales, Arizona have single crystal 40Ar/39Ar laser fusion ages mostly ranging from 23 to 28 Ma. These ages suggest detritus originated from the Grosvenor Hills volcanics, or some equivalent rocks in the Nogales, Sonora area. Sanidine and plagioclase single crystal 40Ar/39Ar laser fusion ages from a rhyolite tuff in Agua Fria Canyon, near the basal part of the Nogales Formation, yielded a mixed population of ages; an older population of sanidine originating from the Grosvenor volcanics and a younger population of plagioclase grains between 16 and 20 Ma with a mean age of 18.4 ± 0.38 Ma, which represents the oldest age reported for the Nogales Formation.