The Roots of Ash Flow Calderas in Western North America: Windows into the Tops of Granitic Batholiths

Abstract

Large-volume ash flow eruptions and associated caldera collapses provide a direct link with subvolcanic granitic plutons of batholithic dimensions. The eruptive history, structural features, and petrologic evolution of ash flow calderas provide data on early stages of the evolution of an associated subvolcanic magmatic system. Broadly cogenetic, erosionally unroofed granitic plutons provide a record mainly of the late stages of emplacement and crystallization of silicic magmas. This review summarizes features of well-studied calderas and ash flow volcanic fields in western North America, exposed at advantageous levels where both remnants of a volcanic sequence and upper parts of the cogenetic intrusion are preserved, in comparison with similar rocks elsewhere in the world. Primary examples include San Juan, Mogollon-Datil, Marysvale, Latir-Questa, Chiricahua-Turkey Creek, Challis, and Boulder Batholith-Elkhorn Mounfains. Most ash flows have erupted from sites of preceding volcanism that records shallow accumulation of caldera-related magma. Structural boundaries of calderas are single ring faults or composite ring fault zones that dip vertically to steeply inward' outward dipping boundary faults favored by some models have not been identified in North American calderas. The area and volume of caldera collapse are roughly proportional to the amount of erupted material. Pyroelastic eruptions of relatively small volume (less than 50-100 km 3) may cause incomplete hinged caldera subsidences or structural sags; larger systems are bounded by complete ring faults. Few ash flow vent structures have been related to major calderas' vent geometry, as determined by size analyses of pyroelastic materials, may shift complexly during 9aidera collapse. Scalloped topographic walls beyond the structural boundaries of most calderas are due to secondary gravitational slumping during subsidence. Most exposed floors are a structurally coherent plate or cylinder bounded by a ring fault or dike, indicating pistonlike caldera collapse' chaotically brecciated floors predicted by models of piecemeal collapse have not been identified. Deviations from circular shape commonly reflect influence of regional structures; some calderas in extensional terranes are elongate in the direction of extension. Large calderas (greater than 100 km 3 of erupted material) collapse concurrently with eruption, as indicated by thick intracaldera ash flow fill and interleaved collapse slide breccias. Volumes of intracaldera an d outflow tuff tend to be subequal' correlation between them is commonly complicated by contrasts in abundance and size of phenocrysts and lithie fragments, degree of welding, devitrification, alterati øn, and even chemical composition of magmatic material. Postcollapse volcanism may occur from varied vent geometries within ash flow calderas' ring vent eruptions are most common in resurgent calderas, reflecting renewed magmatic pressure. Large intrusions related to resurgence are exposed centrally within some calderas; ring dikes and other ...

Mecanismos eruptivos y procesos depositacionales del Grupo Choiyoi en el área de Las Caletas, Cordillera Frontal de San Juan, Argentina

“…En las tramos inferiores y medios de la unidad se reconocen cuatro 'episodios eruptivos' (sensu Fisher y Schminke, 1984): 1. volcanismo efusivo precolapso, representado por la Riolita El Aforo; 2. volcanismo explosivo freatomagmático, representado por la Toba Las Caletas; 3. volcanismo ignimbrítico y colapso volcanotectónico, coincidente con la Ignimbrita La Plateada; y 4. volcanismo efusivo poscolapso y sedimentación sineruptiva, materializados en el Complejo de La Junta. En el análisis estratigráfico de la Formación Horcajo pueden visualizarse los eventos de un 'ciclo caldérico' (Lipman, 1984) con muchos de los elementos litofaciales y estructurales que permiten reconocer este tipo de centros eruptivos FIG. 11.…”

Eruptive history, geochronology, and post-eruption structural evolution of the late Eocene Hall Creek Caldera, Toiyabe Range, Nevada

“…Key evidence for a caldera includes the great thickness (about 2,600 m) of intracaldera tuff, well-exposed caldera floor and walls, extensive deposits of meso-and megabreccia composed of Paleozoic caldera wallrock, and the overall lithic-rich character of the tuff. In map view, collapse calderas typically have an inner "structural" margin-the fault that accommodated caldera collapse-and an outer "topographic" margin or rim, formed by catastrophic erosion of the caldera during collapse (for example, Lipman, 1984Lipman, , 1997. In the following sections, the caldera wall or margin refers to the structural margin of the Hall Creek caldera, which is easy to locate in the field.…”

“…This topography is probably a combination of pre-caldera erosional topography and a topographic caldera rim that extended several kilometers outward from the structural margin of the caldera. Topographic caldera rims form as blocks of the caldera wall slump into the caldera during and immediately after ash-flow eruption and caldera subsidence (Lipman, 1984(Lipman, , 1997Best and others, 2013). The presence of abundant megabreccia in the Hall Creek caldera demonstrates that the caldera margin grew outward by this process, but distinguishing a topographic caldera rim from pre-caldera topography is difficult.…”

The link between volcanism and plutonism in epizonal magma systems; high-precision U–Pb zircon geochronology from the Organ Mountains caldera and batholith, New Mexico

“…Models for the generation of ignimbrites and shallow level plutonic systems can be broadly described as down-temperature magma crystallization models or up-temperature partial melting models (Bachmann and Bergantz 2008;Mills and Coleman 2013). In down-temperature models, rhyolitic magmas are formed by differentiation of mafic magmas in crustal chambers or mush zones (e.g., Bergantz 2004, 2008;Cooper and Kent 2014;Halliday et al 1991;Lipman 1984). The silicic magmas can erupt to form ignimbrites, and the residual solids are preserved as plutonic rocks.…”

“…The role of shallow (<8 km depth) plutons has been debated. Many studies have argued that silicic tuffs undergo fractionation in the upper-to mid-crust prior to eruption and that plutons represent the residual solids from this processes (e.g., Bergantz 2004, 2008;Bachmann et al 2007;Hildreth 1981Hildreth , 2004Lipman 1984Lipman , 2007Lipman and Bachmann 2015;Metcalf 2004;Smith 1979). On the other hand, two recent geochronologic studies have documented an offset between the timing of caldera-related volcanic eruptions and crystallization of spatially associated shallow plutons (Mills and Coleman 2013;Tappa et al 2011) .…”