EARLY MIOCENE FELSIC VOLCANISM ON THE SOUTHWEST EDGE OF THE COLORADO PLATEAU

On the southwest edge of the Colorado Plateau, just south of the I-40 corridor, the Aquarius Mountains are underlain by early Miocene felsic volcanics, ultramafic to intermediate volcanics, and crystalline Precambrian rocks. The pre-felsic volcanics form a low-relief paleotopography. Three cycles of eruption are represented in the felsic rocks, and all three have a characteristic sequence of eruptive products. The oldest cycle (Tf1), erupted near the west end of the range, is rhyolitic, and the youngest two (Tf2, Tf3), spatially coincident and located near the east end of the range, are rhyodacitic. All three cycles began with relatively thin laminated ashfall tuffs overlain by non-welded ashflow tuffs, massive tuffs, and breccias. Near the vents, the tuffs are correlated to agglomerates and relatively coarse breccias. Each cycle is capped by lava flows and relatively finer red lithic breccias. Near the vents, the lava flows are interpreted to have welded the upper parts of the underlying agglomerates and breccias, and to have collapsed into lahars that formed the distal red lithic breccias. Tf1 outcrops in a ~1-km area, and is otherwise buried by the eruptive products of Tf2 and Tf3; total thickness is estimated to be ~ 200 m. Tf2 and Tf3 outcrop in a large area measuring ~20 km east-west and ~15 km north-south. Tf2 and Tf3 each total as much as 120 m in thickness. Radiometric ages of Tf2 and a dike cutting Tf2 and Tf3 are both 21.7±0.03 Ma, indicating a short duration (300 ka) for the last two cycles. The 18.78±0.02 Ma Peach Spring Tuff overlies Tf3 in the western Aquarius Mountains and underlies the bulk of the 22 to 5 Ma Mohon Mountains and Hope Mountain volcanics. Reverse- followed by Basin-and-Range normal-faulting is interpreted to have offset volcanic rocks at the western end of the Aquarius Mountains.