AGE OF THE ALVORD PEAK BASALT, CENTRAL MOJAVE DESERT, CALIFORNIA

The Alvord Mountains in the Central Mojave Desert of California consist of Paleozoic and Mesozoic igneous and metamorphic basement rock unconformably overlain by Tertiary sedimentary and volcanic rocks, the latter of which are exposed in the eastern Alvord Mountains. Over the past 60 years, attention has been paid to Tertiary rocks within the Alvord Mountains as recorders of basin development associated with Miocene extension in the central Mojave. The Tertiary stratigraphy remains in part unresolved, however, due to the limited distribution of some formations and the sparse number of radiometric dates of others. This study aims to resolve the position of the Alvord Peak Basalt (APB) in the Tertiary stratigraphy, which has eluded geologists due to the fact that critical contacts are often poorly exposed where the APB crops out.

New ages based on ⁴⁰Ar/³⁹Ar laser incremental heating of groundmass from samples collected south of Spanish Canyon and near Alvord Peak yield plateau ages of 17.2 ± 0.12 Ma and 18.1 ± 0.14 Ma, respectively. These ages are considerably older than Fillmore’s (1993) interpretation that the APB be placed in the upper Barstow Formation (~13-14 Ma), but are more closely in agreement with Byers (1960) and Strona and Miller (2002), who argued for a position between the Clews and Spanish Canyon Formations. Our results suggest that the APB lies between Spanish Canyon and Barstow Formations, although it is possible that some of the upper APB flows are interbedded within the lower Barstow Formation. We prefer the former interpretation because APB flows are chemically distinct from two other basalt flows that clearly lie within the lower 100 meters of the Barstow Formation.

Dating of diabase from N30W trending dikes 1.7 km east of Spanish Canyon previously interpreted as feeder dikes of the APB (Byers, 1960) yield a plateau age of only 9.56 ±0.41 Ma, which disqualifies these features as the subvolcanic equivalent to the APB lava flows. Samples of the upper and lower basalt lava flows within the Spanish Canyon Formation were also dated, yielding plateau ages of 18.6 ±0.28 Ma and 19.1 ±0.20 Ma, respectively, which closely resemble the underlying Peach Springs Tuff, dated by Ferguson et al. (2013) at 18.78 ±0.02 Ma, exposed at the top of the Clews Formation (Byers, 1960; Fillmore, 1993).