Detailed mapping in a 30 sq. km area southwest of Union Pass, AZ, has revealed the existence of a thick and varied sequence of volcanic and shallow intrusive rocks on the east-tilted flank within a major segment of the Black Mountains accommodation zone, northern Colorado River extensional corridor. Field relations within this part of the Secret Pass Canyon volcanic center (SPC) indicate a transition from intermediate, trachyandesitic to trachydacitic magmatism to rhyolitic magmatism, and then a rapid return to trachydacitic magmatism. New 40Ar/39Ar ages tightly constrain the timing of the evolution of this center as follows: (a) initial voluminous, intermediate magmatism occurred at 18.56 Ma; (b) thick dome-related rhyolitic breccias and flows were emplaced at 17.68 Ma; (c) a trachydacitic plug intruded at 17.46 Ma (similar widespread dikes and sills are probably the same age).

The nearly 1 m.y. hiatus between intermediate and rhyolitic magmatism is marked by an ~150 m thick sequence of thin, felsic airfall and ashflow tuffs and tuffaceous sediments. This section contains a single 20 m thick ignimbrite (dating in progress) that may correlate with the 18.5 Ma Peach Springs Tuff. Initial reconnaissance suggests that the SPC may extend more than 10 km to the south, to the vicinity of Oatman, AZ, where the sequence loses its coherence and is highly disrupted by faults suggesting the location of a possible caldera. This may be the site of the Peach Springs Tuff caldera, which has been speculated to lie in the southern Black Mountains; it is plausible that the early intermediate magmas of the SPC may represent early stages of the magma system, and that the overlying ignimbrite could be Peach Springs Tuff.

The SPC may be related to the Spirit Mountain pluton, which lies in the footwall of the Newberry detachment fault ~20 km to the west. The ~17 Ma U-Pb date on the Spirit Mountain pluton (Howard, et al., 1996) is reasonably compatible with this hypothesis, especially for the later SPC trachydacites and rhyolites. Also consistent with this hypothesis is the inferred east-west slip direction on the Newberry detachment fault.