An ash flow caldera in cross section: Ongoing field and geochemical studies of the Mid‐Tertiary Turkey Creek Caldera, Chiricahua Mountains, SE Arizona

Abstract

Volcanic and shallow plutonic (hypabyssal) levels of the Turkey Creek caldera, located in southeast Arizona, are exposed as a consequence of uplift and erosion. The 40Ar/39Ar geochronology and paleomagnetic data indicate that the caldera cycle was relatively short lived and occurred at about 26.9 Ma, coincident with early phases of ductile extension in the southern Basin and Range. The caldera is a transitional calc‐alkali to alkalic magmatic system and is similar to other relatively small volcanic‐plutonic centers that formed after the main pulse of compressional calc‐alkalic magmatism in the Cordillera. Trace element ratios and elemental distribution patterns for Turkey Creek rocks are consistent with origin in a transitional subduction to within‐plate extensional setting. Field relations also suggest synextensional magmatism; regional northwest trending high‐angle faults offset early caldera rocks but are buried by late moat rhyolite. Caldera collapse accompanied eruption of more than 500 km3 of high‐silica rhyolite tuff (the Rhyolite Canyon Tuff). Eruption of the tuff was followed immediately by emplacement of a dacite porphyry intrusion, probably a thick laccolith, into the caldera fill and by extrusion of the dacite porphyry from ring‐fracture‐hosted feeders. Intracaldera tuff at the roof of the intrusion was metamorphosed and brecciated to produce low‐pressure shock‐metamorphic effects and was locally melted. Interpretation of the intrusion as an intracaldera laccolith readily explains a lack of floor rocks within the caldera and the absence of intracaldera equivalents of most of the outflow tuff. Intrusion of intracaldera laccoliths may represent a relatively common, though rarely recognized process within calderas. Stratigraphic relations, rare mingled rocks, and overlapping 40Ar/39Ar ages indicate that both high‐silica rhyolite magma (Rhyolite Canyon Tuff) and dacite porphyry magma were present in the source reservoir (rhyolite above dacite) and were separated by a sharp interface. Dacite porphyry magma from beneath the interface was drawn up into and erupted from vents that previously fed ash flow eruptions; some dacite porphyry was trapped in and beneath the vents and solidified to form a ring dike at depth. Following an erosional hiatus of ≤0.3 m.y., rhyolite was again erupted, filling the caldera moat with ∼135 km3 of mainly aphyric high‐silica rhyolite. Gradational contacts with underlying densely welded tuff, relatively large volumes, planiform aspect ratios, and superliquidus temperatures suggest that some of the laminated rhyolites are rheomorphic tuff. Eruption of moat rhyolites records generation of a voluminous new batch of mainly high‐silica rhyolite with a distinct geochemical signature.