LONG LIFESPANS OF IGNIMBRITE-CALDERA MAGMATIC SYSTEMS: EXAMPLES FROM THE SOUTHERN ROCKY MOUNTAIN VOLCANIC FIELD, CO-NM (Invited Presentation)

Caldera-forming ignimbrites have been much studied as geologically instantaneous samples that offer perspectives on processes in upper-crustal silicic magmatic systems. Less investigated have been the precursor volcanoes, which provide partial records for inception and growth of the ignimbrite magma bodies, or the postcaldera lavas and intrusions that record continued evolution and eventual waning of the overall trans-crustal system. The ~30 sizable ignimbrites (>100->5000 km³) and associated calderas in the mid-Cenozoic Southern Rocky Mountain volcanic field (SRMVF) provide diverse records of prolonged eruptive and intrusive processes involved in magma-body growth, commonly lasting several m.y. or more for areally restricted igneous loci. Clusters of early andesitic volcanoes record sites of initial magmatic focusing that led to assembly of sizable upper-crustal magma bodies capable of producing large silicic ignimbrites. Peak growth at precursor volcanoes in the SRMVF was commonly followed by extended durations (0.5->2 m.y.) of reduced eruptive activity and inferred prolonged maturation of the crustal magmatic system prior to generation of a voluminous melt-dominant lid and large ignimbrite eruption. Some ignimbrites erupted from a polycyclic caldera or caldera cluster, with recurrence intervals varying from several m.y. to as brief as 10-20 k.y. For short-duration recurrences, generation of ignimbrite-scale bodies of eruptible magma likely required high rates of mantle-derived mafic recharge. After each ignimbrite eruption, associated caldera depressions commonly filled rapidly (10s-100s k.y.) with compositionally diverse lavas or later-erupted ignimbrites, while exposed intrusions document continued magmatism at many caldera loci for several m.y. or longer (to 6-7 m.y. for some). The overall volcanic-plutonic record of magma-body growth from inception to waning, >10 m.y. at some SRMVF loci, records large-scale batholith assembly, sustained mantle input, and major compositional and structural modification of subvolcanic crust. Individual volcanic units and shallow plutons have briefer zircon crystallization records than deeper intrusions, but lifespans of SRMVF ignimbrite systems are comparable to those at nested granitoid complexes in continental-arc batholiths.