Middle Tertiary volcanic rocks of the central Chiricahua Mountains in southeast Arizona are the westernmost constituents of the Eocene-Oligocene Boot Heel volcanic field of southwestern New Mexico and southeastern Arizona. About two dozen volumetric ally and stratigraphically significant volcanic units are present in this area. These include large-volume, regionally distributed ash-flow tuffs and smaller volume, locally distributed lava flows. The most voluminous of these units is the Rhyolite Canyon Tuff, which erupted 26.9 million years ago from the Turkey Creek caldera in the central Chiricahua Mountains. The Rhyolite Canyon Tuff consists of 500-1,000 cubic kilometers of rhyolite that was erupted from a normally zoned reservoir. The tuff represents sequential eruptions, which became systematically less geochemically evolved with time, from progressively deeper levels of the source reservoir. Like the Rhyolite Canyon Tuff, other ashflow tuffs preserved in the central Chiricahua Mountains have equivalents in nearby, though isolated mountain ranges. However, correlation of these other tuffs, from range to range, has been hindered by stratigraphic discontinuity, structural complexity, and various lithologic similarities and ambiguities. New geochemical and geochronologic data presented here enable correlation of these units between their occurrences in the central Chiricahua Mountains and the remainder of the Boot Heel volcanic field.
Volcanic rocks in the central Chiricahua Mountains are composed dominantly of weakly peraluminous, high-silica rhyolite welded tuff and rhyolite lavas of the high-potassium and shoshonitic series. Trace-element, and to a lesser extent, major-oxide abundances are distinct for most of the units studied. Geochemical and geochronologic data depict a time and spatial transgression from subduction to within-plate and extensional tectonic settings. Compositions of the lavas tend to be relatively homogeneous within particular units. In contrast, compositions of the ash-flow tuffs, including the Rhyolite Canyon Tuff, vary significantly owing to eruption from compositionally zoned reservoirs. Reservoir zonation is consistent with fractional crystallization of observed phenocryst phases and resulting residual liquid compositional evolution. Rhyolite lavas preserved in the moat of the Turkey Creek caldera depict compositional zonation that is the reverse of that expected of magma extraction from progressively deeper parts of a normally zoned reservoir. Presuming that the source reservoir was sequentially tapped from its top downward, development of reverse zonation in the rhyolite lava sequence may indicate that later erupted, more evolved magma contains systematically less wallrock contamination derived from the geochemically primitive margins of its incompletely mixed reservoir.
New 40Ar/39Ar geochronology data indicate that the principal middle Tertiary volcanic rocks in the central Chiricahua Mountains were erupted between about 34.2 and 26.2 Ma, and that the 5.2 m.y. period between 33.3 and 28.1 Ma was amagmatic. The initial phase of eruptive activity in the central Chiricahua Mountains, between 34.2 and 33.3 Ma, was associated with a regional tectonic regime dominated by subduction along the west edge of North America. We infer that the magmatic hiatus, nearly simultaneous with a hiatus of similar duration in parts of the Boot Heel volcanic field east of the central Chiricahua Mountains, is related to a period of more rapid convergence and therefore shallower subduction that may have displaced subduction-related magmatic activity to a position east of the present-day Boot Heel volcanic field. The hiatus also coincides with a major plate tectonic reorganization along the west edge of North America that resulted in cessation of subduction and initiation of transform faulting along the San Andreas fault. The final period of magmatism in the central Chiricahua Mountains, between 28.1 and 23.2 Ma, ap