Bonanza, an "Extreme" Resurgent Ignimbrite-Caldera Cycle in the San Juan Mountains, Colorado

Among large calderas associated with ignimbrite super-eruptions, the 33.2 Ma Bonanza caldera of the Southern Rocky Mountain volcanic field displays compositional and structural features that provide near-endmember examples of ignimbrite eruptive processes. Bonanza, source of a compositionally complex regional ignimbrite sheet erupted at 33.19±0.04 Ma, is a subsequent resurgently domed structure ~20 km diameter that subsided >3 km during eruption of ~1,000 km³ of ignimbrite. Among its exceptional features: (1) extreme compositional gradients in the associated Bonanza Tuff (mafic dacite to silicic rhyolite; 59-76.5% SiO₂); (2) multiple alternations of mafic and silicic zones, rather than simple upward gradient from silicic to mafic; (3) similarly large compositional diversity among postcollapse caldera-fill lavas and exposed roof zones of resurgent intrusions; (4) compositional contrasts among outflow sectors (mainly rhyolite to east, dacite to west); (5) brief time span for the entire caldera cycle (33.2-32.9 Ma); (6) a uniquely steep-sided resurgent dome (dips of 40-50^o on west and 70-80^o on east flanks); (7) unique exposure levels due to later structural tilting and rugged present-day topography--from postcollapse lavas, to thickly ponded intracaldera ignimbrite and interleaved landslide breccia, down through precaldera volcanic floor, into underlying Paleozoic and Precambrian basement that are intruded by resurgent plutons. Some near-original caldera morphology remains defined by present-day landforms (western topographic rim, resurgent core, ring-fault valley), while tilting and deep erosion provide exceptional three-dimensional exposures of fill, floor, and resurgent structures. An ~2.5-km-thick section of intracaldera ignimbrite on the western flank of the resurgent dome is complexly compositionally zoned (up to nine gradational alternations of rhyolite and dacite, interleaved with collapse-breccia lenses), underlain by caldera-floor intermediate-composition volcanics, and overlain by caldera-filling andesite to rhyolite lavas. Caldera-fill ignimbrite has been largely stripped from the southern and eastern flank of the dome, exposing large area of caldera-floor as a structurally coherent domed plate, bounded by ring faults with locations that are geometrically closely constrained even though largely concealed beneath valleys. Floor rocks are intensely shattered within ~100 m of ring faults, and upper levels of the floor are locally penetrated by dike-like crack fills of intracaldera ignimbrite.