This contribution provides in-situ LA-ICP-MS U-Pb ages and trace element determinations of zircons from dacitic to rhyolitic lavas, ignimbrites and intrusions in the Southern Rocky Mountain Volcanic Field (SRMVF) in Colorado, USA. The data record a period of intense magmatic activity in the Oligocene-early Miocene (∼37-22 Ma) which gave rise to some of the largest explosive ignimbrites in the geological record (e.g. the Fish Canyon Tuff). Age data are drift corrected, but not corrected for radiation dosage or Th disequilibrium, in order to allow users to apply their own algorithms. Xenocrysts (much older crystals up to 2 Ga from the Proterozoic basement) are included in this record.
Abstract Clusters of early central volcanoes in the mid-Cenozoic Southern Rocky Mountain volcanic field (SRMVF; southwestern Colorado, USA) record sites of initial magmatic focusing that led to assembly of sizable upper-crustal magma bodies capable of generating large ignimbrites. Peak growth at precursor andesitic volcanoes was followed by extended periods (0.5 to >2 m.y.) of reduced eruptive activity during inferred prolonged incubation of the crustal reservoir prior to eruption of ignimbrites at the San Juan magmatic locus, as exemplified by the 5000 km3 Fish Canyon Tuff and associated La Garita caldera. After a magma system became thermally mature and compositionally evolved, additional large ignimbrites could erupt more rapidly from polycyclic calderas. In contrast, incubation times for smaller ignimbrite magmas, as at Crater Lake, Oregon, were briefer than for San Juan systems. Plutonic counterparts to the temporal-compositional assembly of arc-ignimbrite magmas are exemplified by incrementally emplaced granitoid intrusions like the Mesozoic Tuolumne complex in the Sierra Nevada.
Hawaiian volcanoes are exceptional examples of intraplate hotspot volcanism. Hotspot volcanoes, which frequently host large eruptions and related earthquakes, flank‐failure landslides, and associated tsunamis, can present severe hazards to populated regions. Many studies have focused on subaerial parts of Hawaiian volcanoes, but the deep‐water flanks of the edifices, which can reach 5700 m below sea level, remain poorly understood because they are so inaccessible. In 1998 a collaborative program between Japan and the United States was initiated to explore the evolution of Hawaiian volcanoes, including their growth and degradation.
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 worjd. Primary examples include San Juan, Mogollon‐Datil, Marysvale, Latir‐Questa, Chiricahua‐Turkey Creek, Challis, and Boulder Batholith‐Elkhorn Mountains. 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. Pyroclastic 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 pyroclastic materials, may shift complexly during caldera 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 and outflow tuff tend to be subequal; correlation between them is commonly complicated by contrasts in abundance and size of phenocrysts and lithic fragments, degree of welding, devitrification, alteration, and even chemical composition of magmatie 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 intrusions along bounding ring fractures are especially common in alkalic igneous systems in extensional environments. Subvolcanic magma chambers of calc‐alkaline affinities associated with plate‐convergent tectonic settings may rise to such high levels that deep cauldron subsidence structures are obliterated. Resurgence within calderas may result in a symmetrical dome or more geometrically complex forms; resurgence is most common in large calderas (greater than 10‐km diameter) in cratonic crust and is associated with large silicic intrusions. In addition to resurgence within single calderas, broader magmatic uplift occurs widely within silicic volcanic fields, reflecting isostatic adjustment to emplacement of associated subvolcanic batholiths. Much additional space for shallow batholith emplacement is probably accommodated by gravitationally driven down warping of wall rocks at lower structural levels. Hydrothermal activity and mineralization accompany all stages of ash flow magmatism, becoming dominant late during caldera evolution. Much rich mineralization is millions of years later than caldera collapse, where the caldera served primarily as a structural control for genetically unrelated intrusions and associated hydrothermal systems.
This chapter contains sections titled: Introduction Emplacement and Inflation of Flow-Lobe Tumuli And Pahoehoe Lobes Lobe Emplacement And Lava Supply Rates Discussion Conclusion
Recent inference that Mesozoic Cordilleran plutons grew incrementally during >106 yr intervals, without the presence of voluminous eruptible magma at any stage, minimizes close associations with large ignimbrite calderas. Alternatively, Tertiary ignimbrites in the Rocky Mountains and elsewhere, with volumes of 1–5 × 103 km3, record multistage histories of magma accumulation, fractionation, and solidification in upper parts of large subvolcanic plutons that were sufficiently liquid to erupt. Individual calderas, up to 75 km across with 2–5 km subsidence, are direct evidence for shallow magma bodies comparable to the largest granitic plutons. As exemplified by the composite Southern Rocky Mountain volcanic field (here summarized comprehensively for the first time), which is comparable in areal extent, magma composition, eruptive volume, and duration to continental-margin volcanism of the central Andes, nested calderas that erupted compositionally diverse tuffs document deep composite subsidence and rapid evolution in subvolcanic magma bodies. Spacing of Tertiary calderas at distances of tens to hundreds of kilometers is comparable to Mesozoic Cordilleran pluton spacing. Downwind ash in eastern Cordilleran sediments records large-scale explosive volcanism concurrent with Mesozoic batholith growth. Mineral fabrics and gradients indicate unified flow-age of many pluton interiors before complete solidification, and some plutons contain ring dikes or other textural evidence for roof subsidence. Geophysical data show that low-density upper-crustal rocks, inferred to be plutons, are 10 km or more thick beneath many calderas. Most ignimbrites are more evolved than associated plutons; evidence that the subcaldera chambers retained voluminous residua from fractionation. Initial incremental pluton growth in the upper crust was likely recorded by modest eruptions from central volcanoes; preparation for caldera-scale ignimbrite eruption involved recurrent magma input and homogenization high in the chamber. Some eroded calderas expose shallow granites of similar age and composition to tuffs, recording sustained postcaldera magmatism.
