40AR/39AR GEOCHRONOLOGY OF THE OLIGOCENE-MIOCENE GROSVENOR HILLS AND THE MIOCENE NOGALES FORMATION, UPPER SANTA CRUZ BASIN, SOUTHERN ARIZONA

New mapping and ⁴⁰Ar/³⁹Ar geochronology of Tertiary rocks in the upper Santa Cruz basin of southern Arizona requires stratigraphic revision of the Oligocene-Miocene Grosvenor Hills and the Miocene Nogales Formation. Previous mapping and K-Ar dating of volcanic rocks in the Grosvenor Hills indicated the unit was about 27-28 Ma, but high-precison ⁴⁰Ar/³⁹Ar geochronology in the southern part of the Grosvenor Hills indicates that the formation extends from 23-28 Ma. New ⁴⁰Ar/³⁹Ar ages from a rhyolite tuff of the upper rhyodacite member of the formation range from 23.5 ± 0.1 to 24.8 ± 0.1 Ma (sanidine) and another sample ranged from 26.3 ± 0.7 (amphibole) to 26.9 ± 0.2 Ma (biotite). A sample of rhyolitic to dacitic intrusive rock in the middle rhyolite member yielded ages of 27.6 ± 0.4 (biotite) to 27.8 ± 0.3 Ma (amphibole), similar to the original ages reported for the formation.

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.