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Geology and complex collapse mechanisms of the 3.72 Ma Hannegan caldera, North Cascades, Washington, USA
Abstract
Contiguous ring faults of the 8 x 3.5 krn Hannegan caldera enclose the Hannegan volcanics in the Cascade are of northern Washington. The caldera collapsed in two phases, which each erupted rhyolitic ignimbrite (72.3%-75.2% SiO2). The first collapse phase, probably trap-door style, erupted the >= 900-m-thick ignimbrite of Hannegan Peak at 3.722 +/- 0.020 Ma. This single cooling unit, generally welded, has an uppermost facies of nonwelded ignimbrite and fine ash. A short period of localized sedimentation followed. Eruption of the ignimbrite of Ruth Mountain then led to a second trap-door collapse as the first-phase partial ring fault propagated to the south to completely enclose the caldera. Wallrock breccias are intercalated as lenses and megabreccia blocks in both ignimbrites. The minimum intracaldera volume is 55-60 km(3). No base is exposed, nor are outflow sheets preserved. Caldera collapse and glacial erosion have removed precaldera volcanic rocks, which survive only as intracaldera breccias. Rhyolite dikes and pods, one of which yielded a Ar-40/Ar-39 age of 3.72 +/- 0.34 Ma, intrude the ring fault and caldera fill. Dacite-andesite domes, dikes, and lava flows were emplaced subsequently; one lava flow gives a Ar-40/Ar-39 age of 2.96 +/- 0.20 Ma. The quartz diorite of Icy Peak and the granite of Nooksack Cirque (plutons with Pb-206/U-238 zircon ages of 3.42 +/- 0.10 Ma and 3.36 +/- 0.20 Ma, respectively) intrude caldera fill and basement rocks on the southwest margin of the caldera. Both plutons are now exceptionally well exposed on high, glacially sculpted peaks within the caldera, indicating erosion of at least 1 km of intracaldera fill. Hannegan caldera anchors the northeast end of a linear NE-SW age-progressive migration of magmatic focus from the Chilliwack batholith to the active Mount Baker volcano.
... The Chilliwack batholith is compositionally similar to the Mount Baker volcanic field (Mullen et al., 2018 and references therein), which provides a unique opportunity to study both intrusive and extrusive rocks of the same magmatic system. Although erosion during glaciations severely affected the pre-Pliocene units Mullen et al., 2018), removing most of the extrusive equivalents of the batholiths in the area, about a kilometer of Oligocene tuff remains around Mount Rahm (Tucker et al., 2007). Also volcanic rocks of Big Bosom Buttes and Pioneer Ridge (dacite to less common andesite and basalt) are probably Oligocene in age (Tabor et al., 2003). ...
... Mount Baker is the most magmatically productive volcano in the North Cascades arc (Mullen and McCallum, 2014) and has been active since at least 1.3 Ma , but since 3.72 Ma when taking into account the Hannegan caldera (Tucker et al., 2007). Mount Baker is likely capable of an eruption with a size of VEI 7 (Newhall et al., 2018), based on the fact that many VEI 7 eruptions are repetitions of previous VEI 7 eruptions from the same center, after a period of recharge of several thousands of years. ...
... Mount Baker has had its last caldera-forming eruption at 1.165 ± 0.013 Ma (Mullen et al., 2018), which was likely a VEI 7, with the eruption of 100 km 3 rhyodacite (Hildreth et al., 2004;Newhall and Self, 1982). Although less well-constrained, the eruption of the Hannegan caldera was likely also very large, with the eruptive volume estimated at ~140 km 3 (Tucker et al., 2007). During the Oligocene, there were possibly another two very large eruptions, as both Big Bosom and Mount Rahm are possible calderas, with estimated tuff thicknesses of one kilometer (Tucker et al., 2007). ...
Proposal I wrote for a Mendenhall fellowship, unsuccessful.
... Hildreth et al. (2003) carried out a comprehensive 40 Ar/ 39 Ar dating study showing that the Mount Baker volcanic field has been active since ϳ1.3 Ma, culminating in the ϳ40 ka to recent andesitic Mount Baker stratocone. Additional 40 Ar/ 39 Ar, K-Ar, and isotope dilution -thermal ionization mass spectrometry (ID-TIMS) U-Pb zircon ages Tucker et al. 2007) extend the magmatic record intermittently back to the 3.722 Ma Hannegan caldera, which is embedded within older Chilliwack plutons and intruded by younger ones (Fig. 2). Advancement of the mag-matic focus from the Hannegan caldera to the modern Mount Baker entailed ϳ25 km of progressive southwesterly migration of the magmatic focus . ...
... Vertical dashed line at 4 Ma distinguishes the modern from the ancestral Cascades. Other radiometric age determinations for Cascade Arc volcanic rocks (yellow diamonds) and plutonic rocks (pink circles) are shown with 2 error bars where reported and are from Armstrong (1980), du Bray et al. (2011), Green et al. (1988), Hildreth et al. (2003), Hildreth and Lanphere (1994), Jicha et al. (2009), Jutzeler et al. (2014, Mathews et al. (1981), Mattinson (1977), Ray (1986), Reiners et al. (2000), Richards and McTaggart (1976), Tucker et al. (2007), Wanless et al. (1978), Evarts et al. (1987), Tabor et al. (2003Tabor et al. ( , 2002Tabor et al. ( , 2000Tabor et al. ( , 1993, Schasse (1987), Phillips (1987), and du Bray and John (2011) ...
... 2 error bars are smaller than symbols. Previous age determinations are shown as gray filled circles with corresponding 2 error bars(Tabor et al. 2003 and references therein;Hildreth et al. 2003;Tucker et al. (a) Stacked histogram of the areas (km 2 ) defined by Northern Cascades intrusive rocks, binned into 1 million year intervals. Pluton areas are tabulated in ...
We present thirty new laser ablation inductively coupled plasma mass spectrometry U–Pb zircon dates for intermediate to silicic plutons of the Northern Cascade Arc with emphasis on the Chilliwack batholith – Mount Baker magmatic focus, located in southwestern British Columbia and northern Washington. Chilliwack magmatism commenced at ~35 Ma in southwestern British Columbia and the most voluminous plutons define a cluster at ~32–29 Ma, documenting an early flare-up. During the same interval, the Index, Squire Creek, and Cascade Pass intrusions were emplaced south of the Chilliwack batholith. North of the Chilliwack, maximum pluton ages become progressively younger northward, tracking the northerly migration of the edge of the Farallon–Juan de Fuca–Explorer plate system relative to North America. Chilliwack magmatism continued from ~29 Ma to 22 Ma at a slightly reduced flux, followed by a lull from 22 to 11 Ma during which magmatism shifted north to the Mount Barr batholith (18 Ma). Chilliwack magmatism resumed by 11 Ma but was intermittent and the intrusive flux was significantly lower. The temporal decrease in intrusive flux displayed by the Chilliwack batholith correlates with the declining convergence rate of the Juan de Fuca plate since arc inception. The 11 Ma-to-present magmatism extends a pattern of southwesterly migration of the magmatic focus previously identified from ~4 Ma (Hannegan caldera) to the modern Mount Baker volcanic field. Crustal rotation accounts for the rate of the first ~7 million years of migration. However, the migration rate more than doubled at ~4 Ma, coinciding with separation of the Explorer plate and initiation of Juan de Fuca plate rollback.
... The 8 × 3.5 km Pliocene Hannegan caldera (Tucker, 2004;Tucker et al., 2007) lies in deeply dissected terrain of the North Cascades, 15 km south of the Washington-British Columbia border, and 25 km northeast of the Holocene Mount Baker volcano. Geologic units of the caldera were mapped at 1:24,000 scale. ...
... Geologic units of the caldera were mapped at 1:24,000 scale. For a detailed discussion of the Hannegan caldera, chemical analyses, and geochronology data, see Tucker et al. (2007). ...
... Major element geochemical analyses were obtained by X-ray fl uorescence (XRF). Data tables for geochemistry and age determinations are in Tucker et al. (2007). Yellow stars indicate locations of dated samples on the map. ...
INTRODUCTION The 8 × 3.5 km Pliocene Hannegan caldera (Tucker, 2004; Tucker et al., 2007) lies in deeply dissected terrain of the North Cascades, 15 km south of the Washington–British Columbia border, and 25 km northeast of the Holocene Mount Baker volcano. Geologic units of the caldera were mapped at 1:24,000 scale. For a detailed discussion of the Hannegan cal-dera, chemical analyses, and geochronology data, see Tucker et al. (2007). Caldera collapse occurred in two phases. The fi rst col-lapse, probably trap-door style, followed eruption of the ≥900-m-thick ignimbrite of Hannegan Peak (map unit Thh) at 3.722 ± 0.020 Ma (Hildreth et al., 2003). This single cooling unit, generally welded, has an uppermost facies of non-welded ignimbrite and fi ne co-ignimbrite ash. No base is exposed. Eruption of the ignimbrite of Ruth Mountain (map unit Thr) led to a second collapse, likely also trap-door, as the fi rst-phase partial ring fault propagated to the south and closed. Ruth Mountain ignimbrite, also ≥900-m thick but nonwelded, was deposited on top of the Hannegan Peak ignimbrite. The two rhyolitic (72.3%–75.2% SiO 2) intracaldera ash-fl ow tuffs have a combined volume of ~125 km 3 and are entirely bounded by a continuous ring fault. No ignimbrite outfl ow sheets are preserved. Precaldera basement rocks are schists of the Cretaceous Darrington phyllite member of the Easton Metamorphic Suite (Tabor et al., 2003; map unit Ked) and Ter-tiary arc plutons of the Chilliwack composite batholith (Tabor et al., 2003). Wall-rock breccias are intercalated as lenses and isolated megabreccia blocks in both ignimbrites. Volcaniclas-tic sedimentary rocks, nowhere more than 30 m thick, locally separate the two ignimbrites. Rhyolite intruded the ring fault and caldera fi ll. One of these, the rhyolite of Hells Gorge (map unit rhg), yielded a 40 Ar/ 39 Ar age of 3.72 ± 0.34 Ma. Dacite-andesite domes and lava fl ows followed. The andesite lava of Chilliwack Pass (map unit acp) yielded a 40 Ar/ 39 Ar age of 2.96 ± 0.20 Ma. Nearly all of the intracaldera units are cut by generally northwest-to-northeast–striking dikes, which have compositions ranging from basaltic andesite to dacite. Only representatives of the hundreds of dikes observed are shown on the geologic map. Two plutons intruded the intracaldera fi ll and adjoining base-ment in the southwest quadrant of the caldera. First described by Tabor et al. (2003), these are the quartz diorite of Icy Peak (map unit Tcid) and the granite of Nooksack Cirque (map unit Tcnm). Zircons from these plutons yielded 206 Pb/ 238 U ages of 3.42 ± 0.1 Ma and 3.36 ± 0.2 Ma, respectively. The plutons are now exposed on high, glacially sculpted peaks within the caldera. Based on the medium grained texture of these plutons, it is esti-mated that at least 1 km of the intracaldera fi ll, including some of the two plutons, has been stripped. This estimate does not include the 1000 m of relief imposed by erosion on the modern landscape surrounding Hannegan caldera. DESCRIPTION OF MAP UNITS Surfi cial units were generally not mapped, although an extensive cover of talus, glacial deposits and alluvium is present. Vegetation, ranging from old growth coniferous forests to subal-pine meadows, covers much of the map area. Major element geochemical analyses were obtained by X-ray fl uorescence (XRF). Data tables for geochemistry and age determinations are in Tucker et al. (2007). Yellow stars indicate locations of dated samples on the map. Plateau ages are given when 40 Ar/ 39 Ar ages are stated.
... The enforced axisymmetric geometry and the absence of two-way coupling between the moving fluid and the subsiding block significantly oversimplify subsidence and fluid flow. Indeed analyses of natural examples of caldera-forming eruptions show that magma chamber roofs sink with significant components of tilting and rotation 16,17 , and that dyke widths vary spatially [18][19][20][21] . These observations are also apparent in experiments on caldera subsidence 12 , implying that the coupled problem of flow and subsidence is intrinsically three dimensional. ...
... We base our initial Ti (Ti 0 ) and Su (Su 0 ) on field observations of ring dykes and calderas [3][4][5][17][18][19][20][21] . The initial Ti 0 is 0.08 and the initial Su 0 is 2.12 (that is, a natural caldera with a shallow, sill-like magma chamber, d = 12 km, liquid magma chamber thickness (H ) = 2.83 km, depth of chamber (h) = 4 km and w = 0.32 km) (Fig. 1c). ...
Caldera-forming eruptions can be explosive and lead to the eruption of phenomenal volumes of magma that can devastate the global environment. Such eruptions involve ground subsidence related to catastrophic sinking of a magma chamber roof, accompanied by buoyant flow of magma through a ring conduit around the sinking roof. Previous work points to a feedback between subsidence and eruption: eruption initiates subsidence of the chamber roof, which in turn drives the ongoing eruption. Although subsidence-driven eruption dominates caldera evolution, the coupled dynamics of subsidence and magma flow are poorly understood. Here, we use analogue models to show that, under most conditions, caldera subsidence is spatially and temporally variable, leading to complicated and vigorous magma stirring and mixing. On the basis of the experimental results and a scaling analysis, we construct a regime diagram that helps demonstrate how the coupled flow and subsidence are influenced by the fluid dynamics and geometry of the system. The vigorous stirring we infer can considerably modify the style of subsidence and can explain textural, petrological and geochemical variation in deposits that have been related to caldera-forming eruptions.
... Many partially eroded calderas allow direct information on the deeper structure, as at Gran Canaria (Canary Islands, Schmincke, 1967), Western United States (Lipman, 1984;Rytuba and McKee, 1984), Trans Pecos (Texas; Henry and Price, 1984), Sierra Madre Occidental (Mexico; Swanson and McDowell, 1984), Tavua (Fiji; Setterfield et al., 1991), Scafell (England;Branney and Kokelaar, 1994), Dorobu and Kumano (Japan; Miura and Tamai, 1998;Miura, 1999;Miura, 2005), Glencoe and Rum (Scotland; Moore and Kokelaar, 1998;Troll et al., 2000), Hannegan (Tucker et al., 2007) and the Archean Hunter Mine Group (Canada; Mueller and Mortensen, 2002). Among these, Kumano and Hannegan reveal trapdoor collapses (Miura, 2005;Tucker et al., 2007), and Scafell and Glencoe a piecemeal-like structure, consisting Fig. 1. ...
... Many partially eroded calderas allow direct information on the deeper structure, as at Gran Canaria (Canary Islands, Schmincke, 1967), Western United States (Lipman, 1984;Rytuba and McKee, 1984), Trans Pecos (Texas; Henry and Price, 1984), Sierra Madre Occidental (Mexico; Swanson and McDowell, 1984), Tavua (Fiji; Setterfield et al., 1991), Scafell (England;Branney and Kokelaar, 1994), Dorobu and Kumano (Japan; Miura and Tamai, 1998;Miura, 1999;Miura, 2005), Glencoe and Rum (Scotland; Moore and Kokelaar, 1998;Troll et al., 2000), Hannegan (Tucker et al., 2007) and the Archean Hunter Mine Group (Canada; Mueller and Mortensen, 2002). Among these, Kumano and Hannegan reveal trapdoor collapses (Miura, 2005;Tucker et al., 2007), and Scafell and Glencoe a piecemeal-like structure, consisting Fig. 1. Surface structure of some of the best exposed active calderas on Earth: (a) Erta Ale, Afar, Ethiopia; (b) detail of the SW rim of Kilauea; (c) Rano Kau caldera; Easter Island, Chile; (d) N rim of Bromo caldera, Indonesia. ...
Understanding the structure and development of calderas is crucial for predicting their behaviour during periods of unrest and to plan geothermal and ore exploitation. Geological data, including that from analysis of deeply eroded examples, allow the overall surface setting of calderas to be defined, whereas deep drillings and geophysical investigations provide insights on their subsurface structure. Collation of this information from calderas worldwide has resulted in the recent literature in five main caldera types (downsag, piston, funnel, piecemeal, trapdoor), being viewed as end-members. Despite its importance, such a classification does not adequately examine: (a) the structure of calderas (particularly the nature of the caldera's bounding faults); and (b) how this is achieved (including the genetic relationships among the five caldera types). Various sets of analogue models, specifically devoted to study caldera architecture and development, have been recently performed, under different conditions (apparatus, materials, scaling parameters, stress conditions).
... Two Holocene magmatic eruptions are known: 9.8 ka (a scoria cone and lava flow on the volcano's south flank) and 6.5 ka (a subplinan ash eruption from the volcano's summit (Hildreth 2007)). A phreatic eruption was reported from Sherman Crater, just south of the volcano's summit, in 1843 (Tucker et al. 2007). ...
Experience during historical time throughout the Cascade arc and the lack of deep-seated deformation prior to the two most recent eruptions of Mount St. Helens might lead one to infer that Cascade volcanoes are generally quiescent and, specifically, show no signs of geodetic change until they are about to erupt. Several decades of geodetic data, however, tell a different story. Ground- and space-based deformation studies have identified surface displacements at five of the 13 major Cascade arc volcanoes that lie in the USA (Mount Baker, Mount St. Helens, South Sister, Medicine Lake, and Lassen volcanic center). No deformation has been detected at five volcanoes (Mount Rainier, Mount Hood, Newberry Volcano, Crater Lake, and Mount Shasta), and there are not sufficient data at the remaining three (Glacier Peak, Mount Adams, and Mount Jefferson) for a rigorous assessment. In addition, gravity change has been measured at two of the three locations where surveys have been repeated (Mount St. Helens and Mount Baker show changes, while South Sister does not). Broad deformation patterns associated with heavily forested and ice-clad Cascade volcanoes are generally characterized by low displacement rates, in the range of millimeters to a few centimeters per year, and are overprinted by larger tectonic motions of several centimeters per year. Continuous GPS is therefore the best means of tracking temporal changes in deformation of Cascade volcanoes and also for characterizing tectonic signals so that they may be distinguished from volcanic sources. Better spatial resolution of volcano deformation can be obtained through the use of campaign GPS, semipermanent GPS, and interferometric synthetic aperture radar observations, which leverage the accumulation of displacements over time to improve signal to noise. Deformation source mechanisms in the Cascades are diverse and include magma accumulation and withdrawal, post-emplacement cooling of recent volcanic deposits, magmatic-tectonic interactions, and loss of volatiles plus densification of magma. The Cascade Range thus offers an outstanding opportunity for investigating a wide range of volcanic processes. Indeed, there may be areas of geodetic change that have yet to be discovered, and there is good potential for addressing a number of important questions about how arc volcanoes work before, during, and after eruptions by continuing geodetic research in the Cascade Range.
... At this time big blocks of granitoids (the host rock) fell gravitationally onto the collapsed floor, covering the Luingo I intracaldera Ignimbrite, and emitting the Luingo II Ignimbrite. The collapse breccia (Luingo Breccia) indicates a catastrophic topographic change, possibly associated with an increase in the conduit size (c.f., Wilson et al., 1978;Lipman, 1976;Tucker et al., 2007). The conduit enlargement was accompanied by the eruption of great volumes of magma and the deposition of a thick intracaldera ignimbrite (Luingo II) and its outflow equivalent, the Pucarilla Ignimbrite. ...
This article identifies the Pucarilla–Cerro Tipillas Volcanic Complex and its major eruptive source, the Luingo caldera (26° 10′S–66° 40′W). Detailed geological mapping, stratigraphic sections, facies analysis and correlations, including the identification of typical caldera components, allow us to infer the position of a collapse caldera, elongated at N65° and with a diameter of 19km×13km, which is responsible for an eruption of 135km3 (DRE) of magma. The high-crystal contents of the associated ignimbrites, combined with its tectonic setting, indicate that regional and local tectonic structures played a crucial role in the formation of the caldera.The Luingo caldera is located on the south-eastern border of the Puna, and is the south-easternmost recognised caldera of the Altiplano–Puna plateau. The age of the caldera and its products is 12.1 to 13.5Ma. Based on its location near the Cerro Galán Complex (2 to 6.5Ma), we can imply that volcanism existed in the area for about 10Ma. The caldera morphology and product distribution account for a middle Miocene paleao-topography similar to the present one.
The Shiratakiyama Formation of the Abu Group, southwest Japan, is part of a dissected caldera within a complex of Cretaceous volcanic and plutonic rocks. The formation contains the products of rhyolitic and andesitic magmas emplaced in a back-arc region. It is important to understand the genetic relationship between the volcaniclastic ejecta and structural constraints on these rocks in order to determine the evolution of caldera volcanism.
The orientations of bedding planes within the Shiratakiyama Formation suggest the occurrence of a buried asymmetric structure within basement rocks. The depth of the basement surface increases toward the center of the caldera in the northern part of the Shiratakiyama Formation, dipping at 40° to 70°, whereas in the southern half of the caldera the surface dips at 20° or less. This asymmetric basement surface is also discordant with the orientation of basement rocks themselves. In addition, the formation is bound by intersecting high-angle normal faults and/or intrusive rocks. These observations suggest the presence of a small (6×4 km) cauldron, here named the Shiratakiyama cauldron.
The Shiratakiyama Formation is divided into two members, here named the Futanoigawa rhyolite ash-flow tuff and the overlying Tenjougatake andesite lava. The formation also contains many associated intrusive rocks, such as porphyrites, felsites, granite porphyry, and intrusive breccias. Thick and voluminous ash-flow tuff is the dominant rock within the cauldron interior. The total volume of ash-flow tuff is ≥ 9.6 km³, and it is locally intercalated with lacustrine rocks, andesite lavas, and volcaniclastic rocks, which represent cooling units. Caldera-collapse meso-breccias occur in the lower part of the ash-flow tuff sequence.
These findings suggest that the deeper structure of the Shiratakiyama cauldron was formed by asymmetric piecemeal collapse rather than by coherent trapdoor subsidence.
In the area located between Ghío lake and sierra Colorada, in Cordillera Patagónica Austral, the Late Jurassic El Quemado Complex is represented by vent-facies of pyroclastic and lava origin. A reconstruction of the volcanic architecture has been carried out based on the integrated study of the lithofacies and the structures. Four ignimbritic units and one rhyolitic lava unit have been recognized, mainly controlled by NNW trending transtensional faults. The evolution of the La Peligrosa Caldera is modellized in three stages:1) pre-collapse, when a precursory downsag-piecemeal subsidence took place, related to a dilatational zone which become the caldera 2) collapse, when the emplacement of large volume crystal-rich ignimbrites and megabreccias occurred under a progressive subsidence controlled by a transtensional regime with a NE direction of extension and 3) post-collapse, when the lava flows and associated domes were emplaced, controlled by oblique extension conditions with a NW direction of extension. The caldera development was accompanied by a remarkable change from transtension to oblique extension, which may represent an important variation in the deformation conditions during Jurassic time. The La Peligrosa Caldera may be considered as a key event to understand the eruptive mechanisms of the flare-up volcanism in the Silicic Chon Aike Province.
Multistage histories of incremental accumulation, fractionation, and solidification during construction of large subvolcanic magma bodies that remained sufficiently liquidto erupt are recorded by Tertiary ignimbrites, source calderas, and granitoid intrusions associated with large gravity lows at the Southern Rocky Mountain volcanic field (SRMVF). Geophysical data combined with geological constraints and comparisons with tilted plutons and magmatic-arc sections elsewhere are consistent with the presence of vertically extensive (>20km) intermediate to silicic batholiths (with intrusive:extrusive ratios of 10:1 or greater) beneath the major SRMVF volcanic loci (Sawatch, San Juan, Questa-Latir). Isotopic data require involvement of voluminous mantle-derived mafic magmas on a scale equal to or greater than that of the intermediate to silicic volcanic and plutonic rocks. Early waxing-stage intrusions (35-30 Ma) that fed intermediate-composition central volcanoes of the San Juan locus are more widespread than the geophysically defined batholith; these likely heated and processed the crust, preparatory for ignimbrite volcanism (32-27 Ma) and largescale upper-crustal batholith growth. Age and compositional similarities indicate that SRMVF ignimbrites and granitic intrusions are closely related, but the extent to which the plutons record remnants of former magma reservoirs that lost melt to volcanic eruptions has been controversial. Published Ar/Arfeldspar and U-Pb-zircon ages for plutons spatially associated with ignimbrite calderas document final crystallization of granitoid intrusions at times indistinguishable from the tuff to ages several million years younger. These ages also show that SRMVF calderarelated intrusions cooled and solidified soon after zircon crystallization, as magma supply waned. Some researchers interpret these results as recording pluton assembly in small increments that crystallized rapidly, leading to temporal disconnects between ignimbrite eruption and intrusion growth. Alternatively, crystallization ages of the granitic rocks are here inferred to record late solidification, after protracted open-system evolution involving voluminous mantle input, lengthy residence (105-106yr) as near-solidus crystal mush, and intermittent separation of liquid to supply volcanic eruptions. The compositions of the least-evolved ignimbrite magmas tend to merge with those of caldera-related plutons, suggesting that the plutons record nonerupted parts of long-lived cogenetic magmatic systems, variably modified prior to final solidification. Precambrian-source zircons are scarce in caldera plutons, in contrast to their abundance in some peripheral waning-stage intrusions of the SRMVF, implying dissolution of inherited crustal zircon during lengthy magma assembly for the ignimbrite eruptions and construction of a subvolcanic batholith. Broad age spans of zircons (to several million years) from individual samples of some ignimbrites and intrusions, commonly averaged and interpreted as "intrusion-emplacement age," alternatively provide an incomplete record of intermittent crystallization during protracted incremental magma-body assembly, with final solidification only when the system began to wane. Analyses of whole zircons cannot resolve late stages of crystal growth, and early growth in a long-lived magmatic system may be poorly recorded due to periods of zircon dissolution. Overall, construction of a batholith can take longer than recorded by zircon-crystallization ages, while the time interval for separation and shallow assembly of eruptible magma may be much shorter. Magma-supply estimates (from ages and volcano-plutonic volumes) yield focused intrusion-assembly rates sufficient to generate ignimbrite-scale volumes of eruptible magma, based on published thermal models. Mid-Tertiary processes of batholith assembly associated with the SRMVF caused drastic chemical and physical reconstruction of the entire lithosphere, probably accompanied by asthenospheric input.
The subsurface structures of caldera ring faults are often inferred from numerical and analog models as well as from geophysical studies. All of these inferred structures need to be compared with actual ring faults so as to test the model implications. Here, we present field evidence of magma channeling into a caldera ring fault as exhibited at Hafnarfjall, a deeply eroded and well-exposed 5-Ma extinct volcano in western Iceland. At the time of collapse caldera formation, over 200 m of vertical displacement was accommodated along a ring fault, which is exceptionally well exposed at a depth of approximately 1.2 km below the original surface of the volcano. There are abrupt changes in the ring fault attitude with depth, but its overall dip is steeply inward. Several inclined sheets within the caldera became arrested at the ring fault; other sheets became deflected up along the fault to form a multiple ring dike. We present numerical models showing stress fields that encourage sheet deflection into the subvertical ring fault. Our findings provide an alternative mechanical explanation for magma channeling along caldera ring faults, which is a process likely to be fundamental in controlling the location of post-caldera volcanism.
Tunnicliffe, J., Church, M. & Enkin, R. J. 2012 (January): Postglacial sediment yield to Chilliwack Lake, British Columbia, Canada. Boreas, Vol. 41, pp. 84–101. 10.1111/j.1502-3885.2011.00219.x. ISSN 0300-9483.
Seismic records and evidence from sediment cores at Chilliwack Lake provide the basis for a long-term (postglacial) sediment budget for a 324-km2 Cordilleran catchment. Chilliwack Lake (11.8 km2 surface area), situated in the North Cascade Mountains, near Chilliwack, British Columbia, was formed behind a valley-wide recessional moraine in the final phase of post-Fraser alpine glaciation. Seismic surveys highlight the postglacial lacustrine record, which is underlain by a thick layer of sediments related to deglacial sedimentation. Sediment cores provide details of grain-size fining from the delta to the distal lake basin. The cores also show a record of intermittent fire and debris flows. Magnetic measurements of lake sediments provide information on grain size, as well as a dating framework. The total postglacial lake-floor deposit volume is estimated to be 397 ± 27 × 106 m3. Including estimates of fan and delta deposition, the specific postglacial yield to the lake is calculated to be ∼86 ± 13 Mg km2 a−1. The sediment volume in the uppermost (Holocene) lacustrine layer is 128 ± 9 × 106 m3, representing ∼41 ± 4 Mg km2 a−1 in the Holocene. Compared with other Cordilleran lakes of similar size, particularly those with glacial cover in the watershed, Chilliwack Lake has experienced relatively modest rates of sediment accumulation. This study provides an important contribution to a growing database of long-term (postglacial) sediment yield data for major Cordilleran lakes, essential for advancing our understanding of the pace of landscape evolution in formerly glaciated mountainous regions.
Pyroclastic and lava vent-facies, from the Late Jurassic El Quemado Complex, are described at the southern Lake Ghío, in the Cordillera Patagónica Austral. Based on the comprehensive study of lithology and structures, the reconstruction of the volcanic architecture has been carried out. Four ignimbrites and one rhyolitic lava unit, affected by oblique-slip normal faults have been recognized. The evolution of La Peligrosa Caldera has been modeled in three different stages:1) initial collapse, consisting of a precursory downsag subsidence, related to a dilatational zone, which controlled location of the caldera, 2) main collapse, with the emplacement of large volume crystal-rich ignimbrites and megabreccias, under a progressive subsidence controlled by a pull-apart structure related to a transtensional regime and 3) post-collapse, in which lava flows and associated domes were emplaced under an oblique-extensional regime. The caldera records a remarkable change from transtension to oblique extension, which may represent an important variation in regional deformation conditions during Jurassic times. La Peligrosa Caldera may be considered a key event to understand the eruptive mechanisms of the flare-up volcanism in the Chon Aike Silicic Province.
At Mount Baker, elevated gas and heat fluxes from fumaroles in Sherman Crater indicate the presence of a degassing magma reservoir. Campaign Global Positioning System (GPS) surveys in 2006 and 2007 provide slope distance measurements of 19 trilateration lines and provide baseline positions for future GPS study on Mount Baker. Comparison of slope distance measurements acquired in 1981 and 1983 with electronic distance meters (EDM) indicates that significant surface deformation has occurred on Mount Baker during the past quarter century. Slope distances have predominantly shortened around the edifice at rates
The intracaldera Hannegan volcanics were erupted during two collapse episodes of the Hannegan caldera in the North Cascade mountains of Washington State. The first eruption yielded a down-to-the-north trapdoor style collapse at 3.722 ± 0.020 Ma (40Ar/39Ar) that is bounded by a horseshoe-shaped ring fault. The second collapse, most probably also trapdoor style, followed a short period of sedimentation, and completed the elliptical ring fault around the southern margin of the caldera. Post caldera plutons, with U-Pb ages of 3.42 ± 0.10 and 3.36 ± 0.20 Ma, intruded the intracaldera ignimbrite.
The Lassen volcanic center is the most recent of several long-lived volcanic centers in the southernmost Cascade Range. These centers have erupted products ranging from basaltic andesite to rhyolite and are superimposed on a background of regional basaltic to andesitic volcanism. The evolution of the Lassen volcanic center is described in three stages. Stages I and II comprise the Brokeoff volcano, and 80 km³ andesitic stratocone, active from 600 to 400 ka. Brokeoff volcano is compositionally equivalent to the regional basaltic andesite to andesite volcanism in the Lassen region and is the result of structurally controlled focusing of the diffuse regional mafic magmatism. Stage III comprises a silicic dome field and adjacent area of hybrid andesites and has a total volume of about 100 km³. Volcanism during stage III was episodic and is subdivided into four sequences of lithologically and temporally distinct lavas. Stage III began at 400 ka with a rhyolitic, caldera-forming pyroclastic eruption and chemically related lavas. -from Author
We conducted scaled analogue sandbox models of caldera formation in order to understand the effects of chamber depth and orientation on the spatial and temporal development of calderas. Dry sand contained in a 1-m-diameter cylinder served as a crustal rock analogue, and a water-filled 0.6-m-diameter rubber bladder served as an analogue magma chamber. Scaling parameters included a length ratio (L*) of 2.5 x 10(-5) and a stress ratio (sigma*) of 1.8-2.4 x 10(-5). In contrast to some previous analogue models, the viscosity of the fluid in the chamber and its withdrawal rate were properly scaled. Generally, deformation began with broad sagging, followed by an arcuate or linear outward-dipping fault that formed on one side of the caldera. This fault propagated laterally around the caldera in both directions, sometimes joining other faults, and typically forming an overall polygonal structure. As subsidence continued, the caldera grew incrementally outward and progressively formed a series of concentric outward-dipping faults. Lastly, a peripheral zone of extension and pronounced sagging, and commonly an inward-dipping outer fault related to extension, developed at the surface. As the depth of the chamber increased, (1) the area of faulting decreased, (2) the symmetry of the caldera was affected, and (3) the coherence of the subsiding block decreased. Tilting the chamber caused highly asymmetric subsidence to occur. In this case, faults formed first where the bladder was shallowest. Subsidence then shifted rapidly to where the bladder was deepest, producing an elongate trapdoor caldera that was deepest where the bladder was deepest. Our experiments highlight the roles of sagging and faulting during caldera subsidence. Surface fault patterns both in our experiments and at natural calderas are frequently not circular. The aspect ratio of the block above the magma chamber controls the shape of the caldera, which is frequently polygonal. The faults at natural calderas determine locations and migration of eruptive vents, the degree of subsidence, the style of postcaldera resurgent magmatism, and the extent of hydrothermal circulation. Our experiments reveal details of how calderas grow outward incrementally and demonstrate that asymmetric subsidence along linear and arcuate faults is common to many calderas.
Scaled experiments have been carried out on caldera collapse mechanisms, using silicone as analogue magma and dry sand as analogue rock. Experiments were carried out in two and three dimensions using a range of roof aspect ratios (thickness/width 0.2 to 4.5) appropriate for caldera collapse. They reveal a general mechanism of collapse, only weakly dependent on the shape of the reservoir. For low roof aspect ratios (1), multiple reverse faults break up the roof into large pieces, and subsidence occurred as a series of nested wedges (2-D) or cones (3-D). The extensional zone dominates the surface depression. In the case where preexisting regional faults do not play a major role, the collapse mechanics of calderas probably depends strongly on the roof aspect ratio. Calderas with low roof aspect ratios are predicted to collapse as coherent pistons along reverse faults. The annular extensional zone might be the source of the large landslides that generate intracaldera megabreccias. Collapse into magma reservoirs with high roof aspect ratios may be the origin of some funnel calderas where explosive reaming is not dominant.
Features of ash flow calderas are reviewed in relation to silicic
volcanic fields and the accompanying magmatic system. The study
concentrated on caldera internal features at relatively deep levels and
the intrusive rocks that are remnants of the ash flow magma chamber. An
inventory of published descriptions of major North American caldera
structures is proved, e.g., walls, structural boundaries, floor
characteristics, and the stratigraphy of deposits. Data which have aided
the identifications of evolutionary stages of the variously sized
caldera are reviewed, noting the discovery that rich mineralization has
often occurred millions of years after caldera collapse. The caldera
then acted as a structural control for genetically unrelated inclusions
and accompanying hydrothermal systems.
Crater Lake was surveyed nearly to its shoreline by high-resolution multibeam echo sounding in order to define its geologic history and provide an accurate base map for research and monitoring surveys. The bathymetry and acoustic backscatter reveal the character of landforms and lead to a chronology for the concurrent filling of the lake and volcanism within the ca. 7700 calibrated yr B.P. caldera. The andesitic Wizard Island and central-plattform volcanoes are composed of sequences of lava deltas that record former lake levels and demonstrate simultaneous activity at the two vents. Wizard Island eruptions ceased when the lake was ~80 m lower than at present. Lava streams from prominent channels on the surface of the central platform descended to feed extensive subaqueous flow fields on the caldera floor. The Wizard Island and central-platform volcanoes, andesitic Merriam Cone, and a newly discovered probable lava flow on the eastern floor of the lake apparently date from within a few hundred years of caldera collapse, whereas a small rhydacite dome was emplaced on the flank of Wizard Island at ca. 4800 cal. yr B.P. Bedrock outcrops on the submerged caldera walls are shown in detail and, in some cases, can be correlated with exposed geologic units of Mount Mazama. Fragmental debris making up the walls elsewhere consists of narrow talus cones forming a dendritic pattern that leads to fewer, wider ridges downslope. Hummocky topography and scattered blocks up to ~280 m long below many of the embayments in the caldera wall mark debris-avalanche deposits that probably formed in single events and commonly are affected by secondary failures. The flat-floored, deep basins contain relatively fine-grained sediment transported from the debris aprons by sheet-flow turbidity currents. Crater Lake apparently filled rapidly (ca. 400-750 yr) until reaching a permeable layer above glaciated lava identified by the new survey in the northeast caldera wall at ~1845 m elevation. Thereafter, a gradual, climatically modulated rise in lake level to the present 1883 m produced a series of beaches culminating in a modern wave-cut platform, commonly ~40 m wide, where suitable material is present. The new survey reveals landforms that result from intermediate-composition volcanism in rising water, delineates mass wasting and sediment transport into a restricted basin, and yields a more accurate postcaldera history leading to improved assessment of volcanic hazards.
This paper describes the reasoning that lies behind the construction of the field boundaries in the Total Alkali-Silica diagram (TAS) for the chemical classification of volcanic rocks. It shows that by utilizing nomenclature in common use by petrologists in combination with a large computer database of geochemical analytical information derived from the published literature, it was possible to construct a Total Alkali-Silica diagram for volcanic rocks of only 15 fields, which has straight-line boundaries and necessitates the definition of only 11 points to construct the diagram. One of the principal constraints on the positioning of the boundaries between the various named fields was to minimize the degree of overlap between adjacent fields.Diese Arbeit beschreibt die berlegungen die zur Konstruktion des Total Alkali-SilicaDiagramms (TAS) zur chemischen Klassifikation der Vulkanite fhrten. Es wird gezeigt, da durch die Kombination der gelufigen petrographischen Nomenklatur mit der geochemischen Information einer grossen computerisierten Datensammlung publizierter Gesteinsanalysen ein Alkali-Kieselsurediagramm mit nur 15 Namensfeldern fr vulkanische Gesteine konstruiert werden konnte. Die Felder haben gerade Begrenzungslinien und bentigen nur 11 definierte Punkte zur Einteilung. Eine der
Diverse subsidence geometries and collapse processes for ash-flow calderas are inferred to reflect varying sizes, roof geometries,
and depths of the source magma chambers, in combination with prior volcanic and regional tectonic influences. Based largely
on a review of features at eroded pre-Quaternary calderas, a continuum of geometries and subsidence styles is inferred to
exist, in both island-arc and continental settings, between small funnel calderas and larger plate (piston) subsidences bounded
by arcuate faults. Within most ring-fault calderas, the subsided block is variably disrupted, due to differential movement
during ash-flow eruptions and postcollapse magmatism, but highly chaotic piecemeal subsidence appears to be uncommon for large-diameter
calderas. Small-scale downsag structures and accompanying extensional fractures develop along margins of most calderas during
early stages of subsidence, but downsag is dominant only at calderas that have not subsided deeply. Calderas that are loci
for multicyclic ash-flow eruption and subsidence cycles have the most complex internal structures. Large calderas have flared
inner topographic walls due to landsliding of unstable slopes, and the resulting slide debris can constitute large proportions
of caldera fill. Because the slide debris is concentrated near caldera walls, models from geophysical data can suggest a funnel
geometry, even for large plate-subsidence calderas bounded by ring faults. Simple geometric models indicate that many large
calderas have subsided 3–5 km, greater than the depth of most naturally exposed sections of intracaldera deposits. Many ring-fault
plate-subsidence calderas and intrusive ring complexes have been recognized in the western U.S., Japan, and elsewhere, but
no well-documented examples of exposed eroded calderas have large-scale funnel geometry or chaotically disrupted caldera floors.
Reported ignimbrite "shields" in the central Andes, where large-volume ash-flows are inferred to have erupted without caldera
collapse, seem alternatively interpretable as more conventional calderas that were filled to overflow by younger lavas and
tuffs. Some exposed subcaldera intrusions provide insights concerning subsidence processes, but such intrusions may continue
to evolve in volume, roof geometry, depth, and composition after formation of associated calderas.
The age of the Rockland tephra, which includes an ash-flow tuff south and west of Lassen Peak in northern California and a widespread ash-fall deposit that produced a distinct stratigraphic marker in western North America, is constrained to 565,000 to 610,000 yr by 40Ar/39Ar and U–Pb dating. 40Ar/39Ar ages on plagioclase from pumice in the Rockland have a weighted mean age of 609,000 ± 7000 yr. Isotopic ages of spots on individual zircon crystals, analyzed by the SHRIMP-RG ion microprobe, range from ∼500,000 to ∼800,000 yr; a subpopulation representing crystal rims yielded a weighted-mean age of 573,000 ± 19,000 yr. Overall stratigraphic constraints on the age are provided by two volcanic units, including the underlying tephra of the Lava Creek Tuff erupted within Yellowstone National Park that has an age of 639,000 ± 2000 yr. The basaltic andesite of Hootman Ranch stratigraphically overlies the Rockland in the Lassen Peak area and has 40Ar/39Ar ages of 565,000 ± 29,000 and 565,000 ± 12,000 yr for plagioclase and groundmass, respectively. Identification of Rockland tephra in ODP core 1018 offshore of central California is an important stratigraphic age that also constrains the eruption age to between 580,000 and 600,000 yr.
In four large Oligocene calderas in the western San Juan Mountains - Lake City, Silverton, San Juan, and Uncompahgre - spectacular breccias are intermixed with thick intracaldera ash-flow tuffs that accumulated during caldera collapse. These breccias are divided into two intergradational types: (1) mesobreccia in which numerous small clasts are visible within single outcrops and (2) megabreccia in which many clasts are so large that the fragmental nature of the deposit is obscure in many individual outcrops. In general, mesobreccia occurs as thin tabular deposits locally interlayered with upper parts of the intracaldera ash-flow accumulations; it is readily interpretable as resulting from small- to medium-sized rock falls and rock slides from the caldera walls. In contrast, megabreccia is dominant in the lower part of the caldera-filling sequence and contains only minor intermixed ash-flow material. Megabreccia is difficult to distinguish from pre-collapse caldera floor in places, but local lenses of welded tuff near the deepest stratigraphic levels exposed within the calderas indicate that these rocks are mostly megabreccia that resulted from major slumping and caving of caldera walls during the initial stages of caldera collapse. An especially large megabreccia unit within the San Juan and Uncompahgre calderas is here named the Picayune Megabreccia Member of the Sapinero Mesa Tuff. Megabreccias similar to those in the western San Juan calderas occur in other eroded collapse structures in the western United States, and the presence of such deposits may be useful guides to the roots of caldera structures in deeply eroded, highly altered, or structurally complex volcanic terranes.
Small (<5 km2), lithologically diverse gabbro and diorite stocks make up ∼ 2% of the 34 to 2 Ma Chilliwack batholith, and overlap in age with associated calc-alkaline granitoids. These mafic plutons are similar to those in other I-type bath-oliths, and represent basaltic magmas present during batholith formation. Objectives of this study are: (1) to examine the origins of both interpluton and intrapluton petrologic diversity, and (2) to compare chemical and Sr-Nd isotopic traits of these gabbros with those of Cascade are basalts. Mafic rocks in the Chilliwack are divided into a medium-K series (MKS) and a low-K series (LKS). The former contain 0.7–2.4 wt% K 2 0 and are similar in composition to calc-alkaline basalts and basaltic andesites. Inverse REE modeling supports derivation of the MKS by 9–27% melting of a garnet-free, LREE-enriched source (La/γb N ∼ 2). Chilliwack LKS gabbros have chemical characteristics of low-K olivine tho-eiites, including low K 2 O (0.3–0.5 wt%) and La/γb N (1.7–3.4), and high CaO (8.8–11.3 wt%) and Na 2 O/K 2 O (6–22). These traits suggest a source with more clinopyroxene and lower La/γb N than the MKS source. Differences in ∈ Nd(O) between MKS and LKS gabbros suggest that lower Nd/Sm is a long-lived LKS source characteristic. Lithologic variation within composite plutons of both series resulted primarily from multiple intrusion of related magmas, in some cases differentiates of a common parent. Two contrasting examples were studied in detail. At Mt Sefrit, MKS variation (gabbronorite-quartz diorite) is modeled by low-pressure fractionation (ol + plag + cpx), accompanied by ∼10% wallrock assimilation. In contrast, chemical and Sr-Nd isotopic variation among LKS gabbro-quartz diorite at Copper Lake points to crystallization dominated by clinopyroxene+plagioclase�Cr-spinel, indicative of differentiation at pressures ≥10 kbar, although the assimilant in this case is poorly constrained. Chemical and isotopic similarities between these mafic plutons and Quaternary Cascade lavas indicate that mafic magmas present during the production of Chilli-wack granitoids were low-and medium-K are basalts.
The Shuksan metamorphic suite and correlative Easton schist occur on the W flank of the north Cascades of Washington as large fault-bounded fragments (10s of km long) imbricated with other rock units in a N-S belt extending >180 km. The Shuksan suite is dominated by greenschist, blueschist, quartzose carbonaceous phyllite and quartzo-feldspathic semi-schist. A Jurassic, oceanic, near-arc tectonic site of deposition is hypothesized. Metamorphism began with production of late Jurassic high-P amphibolites in a contact aureole localized near peridotite, followed by regional blueschist-facies metamorphism until the early Cretaceous. The protolith rock type markedly controlled the development of the metamorphic index minerals. T of regional metamorphism ranged approx 330-400oC; P were perhaps 7-9 kbar. Deformation during blueschist metamorphism was directed normal to the continental margin. Uplift and imbrication of the Shuksan suite with other units occurred by fault motion parallel to the continental margin. The metamorphic and uplift events are correlated with periods of high-angle and low-angle convergence respectively between the Farallon and North America plates.-J.M.H.
Synopsis
Intermittently exposed around the margins of the Ben Nevis volcanic pile is a thin (< 5m) rhyolite with a well developed magmatic flow foliation that includes rock previously described as ‘flinty crush rock’. Previous models for the development of this ‘flinty crush rock’ suggested that it developed as a result of intense mechanical shearing during subsidence of the volcanic pile into the underlying Inner Granite. New field, petrographic and geochemical evidence presented here, indicate that the ‘flinty crush rock’ is magmatic and part of a larger body of flow-foliated rhyolite that represents the remains of an ignimbrite conduit formed during caldera collapse. Post-eruption closure of the conduit imposed a well developed flow-foliation on the rhyolite destroying any primary pyroclastic textures. The rhyolite is compositionally identical to a small outcrop of amphibole-biotite Inner Granite, suggesting an intimate and probably co-genetic relationship between flow-foliated rhyolite and Inner Granite magmas. It is recommended that the term ‘flinty crush rock’ be abandoned and replaced by ‘Ben Nevis Intrusive Ring Tuff. This unit affords a rare opportunity to examine a ring fracture conduit associated with a caldera-forming eruption.
Mount Baker, a steaming, ice-mantled, andesitic stratovolcano, is the most conspicuous component of a multivent Quaternary volcanic field active almost continuously since 1.3 Ma. More than 70 packages of lava flows and ∼110 dikes have been mapped, ∼500 samples chemically analyzed, and ∼80 K-Ar and 40Ar/39Ar ages determined. Principal components are (1) the ignimbrite-filled Kulshan caldera (1.15 Ma) and its precaldera and postcaldera rhyodacite lavas and dikes (1.29-0.99 Ma); (2) ∼60 intracaldera, hydrothermally altered, andesite-dacite dikes and pods-remnants of a substantial early-postcaldera volcanic center (1.1-0.6 Ma); (3) unaltered intracaldera andesite lavas and dikes, including those capping Ptarmigan and Lasiocarpa Ridges and Table Mountain (0.5-0.2 Ma); (4) the long-lived Chowder Ridge focus (1.29-0.1 Ma)-an andesite to rhyodacite eruptive complex now glacially reduced to ∼50 dikes and remnants of ∼10 lava flows; (5) Black Buttes stratocone, basaltic to dacitic, and several contemporaneous peripheral volcanoes (0.5-0.2 Ma); and (6) Mount Baker stratocone and contemporaneous peripheral volcanoes (0.1 Ma to Holocene). Glacial ice has influenced eruptions and amplified erosion throughout the lifetime of the volcanic field. Although more than half the material erupted has been eroded, liberal and conservative volume estimates for 77 increments of known age yield cumulative curves of volume erupted vs. time that indicate eruption rates in the range 0.17-0.43 km3/k.y. for major episodes and longterm background rates of 0.02-0.07 km3/k.y. Andesite and rhyodacite each make up nearly half of the 161 ± 56 km3 of products erupted, whereas basalt and dacite represent only a few cubic kilometers, each representing 1%-3% the total. During the past 4 m.y., the principal magmatic focus has migrated stepwise 25 km southwestward, from the edge of the Chilliwack batholith to present-day Mount Baker.
The Cenozoic Garibaldi belt comprises 6 volcanic fields spaced at irregular intervals along an axis extending 240 km north-northwest from the head of Howe Sound to the Bridge River area; 2 additional fields, the Franklin Glacier and Silvert lie 140 and 190 km west of the north end of the main volcanic belt. The volcanoes erupted lavas ranging in composition from augite-olivine basalt, through hypersthene andesite, hornblende andesite, and hornblende-biotite andesite, to biotite rhyodacite. Many of the volcanic complexes are characterized by geomorphic features which indicate complex interactions between volcanism and the Pleistocene ice sheets, but preglacial and postglacial phases are also present. Whole-rock samples from 18 volcanic complexes have been dated by the K-Ar method. Most of the results are internally consistent with stratigraphic relationships and with limited 14C and paleomaguetic data. They suggest that volcanic activity was episodic; most of the analyzed andesitic and dacitic lavas were erupted in the intervals 2.3 to 1.7 Ma and 1.1 Ma to present in the northern part and in the intervals 1.4 to 1.0 Ma and 0.7 Ma to present in the southern part of the belt. Basaltic volcanism occurred only during the past 0.15 m.y., except in the Salal Glacier area where hawaiite and alkali-olivine basalt, which are perhaps an "edge effect" related to subduction of the Juan de Fuca plate, were erupted as early as 0.97 Ma. The timing of Garibaldi belt volcanism provides information bearing on the distribution of pre-Wisconsin glaciers in southwestern British Columbia and constrains interpretations of late Cenozoic changes in Explorer-Juan de Fuca-North American plate configuration along the continental margin.
Calderas that collapse during large pyroclastic eruptions are anomalously rare in the Cascade arc. Recognition of the early Pleistocene 4.5 × 8 km Kulshan caldera, filled with rhyodacite ignimbrite at the northeast foot of Mount Baker, brings to only three the Quaternary calderas identified in the Cascades. A near-vertical ring fault cut in basement rocks of the North Cascades encloses 30 km2 of intracaldera ignimbrite (and intermixed collapse breccia) >1 km thick but with no floor exposed. The Lake Tapps tephra in the Puget lowland is the correlative fallout; 200 km from the source, it is as thick as 30 cm. Features of the distal ash fall and the intracaldera tuff suggest large-scale phreatomagmatism during an eruption that may have started subglacially. Several advances of the Cordilleran ice sheet subsequently obliterated the topographic rim, removed every vestige of extracaldera ignimbrite and proximal fallout, and stripped any precaldera extrusive rocks - the former existence of which is suggested only by a few silicic intrusions that cut the circumcaldera basement. Although the caldera is not structurally resurgent, several early intracaldera rhyodacite lavas intrude and rest directly on ignimbrite or on ashy caldera-lake sediments reworked from the eruption products. Subsidence areas, pumice compositions, and volumes of magma erupted (>50 km3) are similar for the Kulshan, Rockland, and Crater Lake (Mazama) events, the three Quaternary caldera-forming eruptions now recognized in the Cascades.
The ash flow is the basic unit of most pyroclastic deposits known as welded tuffs, tuff flows, pumice flows, or ignimbrites. Other useful units are the cooling unit, both simple and compound, the composite sheet, and the ash-flow field. The deposit of one ash flow is considered analogous to, but not necessarily the same as, the deposit produced by the passage of one nuée ardente. The geological evidence favors gas-emitting particulate flows as the agents of transport. The ash-flow is an efficient heat-conserving mechanism, and, because many ash-flow fields contain cooling units believed to be derived from common magma, but clearly emplaced at different temperatures, a cooling mechanism must be invoked. Such a mechanism may be the vertical eruption column which influences the final properties of many ash flows. Deposits are grouped in seven orders of magnitude ranging from 0.001 to 10,000 km3. Orders 1 to 3 include deposits erupted from domes. Those from craters rarely exceed 10 km3, order 4. Deposits of orders 5 to 7 are associated with subsidence structures in all examples where the source area is known. Welded tuffs occur in fields of all volumes, but they are common among the fissure-erupted deposits of larger volume. Some ash-flow fields distributed from a common source area reach areal dimensions of more than 12,000 square miles and volumes of about 500 cubic miles; some multiple-source fields are known to have volumes of more than 2000 cubic miles. Deposits of single-eruption cycles having volumes of more than a few cubic miles are thought to be related to subsidence structures. Sorting data suggest that most welded tuffs contain more than 70 per cent by weight of materials less than 4 mm in diameter. Welding and crystallization depend largely on relative temperature and thickness of the cooling units, and most of the textures in welded ash flows can be explained in terms of these variables. Welding begins with incipient cohesion of glass shards and fragments and continues with decreasing pore space and deformation to complete welding which results in a dense black glass. Crystallization is superimposed on the welded material at any stage, but the degree of welding may influence the type of crystallization. The degrees of welding and of crystallization are zonal and permit the distinction between simple and compound cooling. Incipient welding may take place below 535° C., but complete welding depends on load pressure and time and is thus influenced by the cooling history of individual cooling units. Deposits emplaced at low temperatures such as the Crater Lake, Oregon, "pumice flows" are essentially nonwelded in thick deposits. The high-temperature welded tuffs of southeastern Idaho are densely welded in very thin sheets. Most welded tuffs are probably emplaced at temperatures intermediate between these limits. Some deposits record a systematic change in temperature during the eruption cycle; others record a change in magmatic composition. Some ash flows are thought to be genetically related to near-surface plutons, some of which are large and complex.
Kulshan caldera (4.5×8 km), at the northeast foot of Mount Baker, is filled with rhyodacite ignimbrite (1.15 Ma) and postcaldera lavas and is only the third Quaternary caldera identified in the Cascade arc. A gravity traverse across the caldera yields a steep-sided, symmetrical, complete Bouguer anomaly of −16 mGal centered over the caldera. Density considerations suggest that the caldera fill, which is incised to an observed thickness of 1 km, may be about 1.5 km thick and is flat-floored, overlying a cylindrical piston of subsided metamorphic rocks. Outflow sheets have been stripped by advances of the Cordilleran Ice Sheet, but the climactic fallout (Lake Tapps tephra) is as thick as 30 cm some 200 km south of the caldera. Ten precaldera units, which range in 40Ar/39Ar age from 1.29 to 1.15 Ma, are dikes and erosional scraps that probably never amounted to a large edifice. A dozen postcaldera rhyodacite lavas and dikes range in age from 1.15 to 0.99 Ma; rhyodacites have subsequently been absent, the silicic reservoir having finally crystallized. At least 60 early Pleistocene intermediate dikes next intruded the caldera fill, helping energize an acid–sulfate hydrothermal system and constituting the main surviving record of an early postcaldera andesite–dacite pile presumed to have been large. Most of the pre- and postcaldera rhyodacites were dated by 40Ar/39Ar or K–Ar methods, and 13 were drilled for remanent magnetic directions. In agreement with the radiometric ages, the paleomagnetic data indicate that eruptions took place before, during, and after the Jaramillo Normal Polarity Subchron, and that one rhyodacite with transitional polarity may represent the termination of the Jaramillo. Most of the biotite–hornblende–orthopyroxene–plagioclase rhyodacite lavas, dikes, and tuffs are in the range 68–73% SiO2, but there were large compositional fluctuations during the 300-kyr duration of the rhyodacite episode. The rhyodacitic magma reservoir was wider (11 km) than the caldera that collapsed into it (8 km).
At the 41×23 km Miocene Kumano caldera in southwestern Japan, a large arcuate pyroclastic breccia unit is interpreted as the dissected conduit for voluminous explosive eruptions. The pyroclastic breccia and associated granite porphyry occur along the southern margin of the caldera. These rocks, collectively as much as 22 km long and 800 m wide, intrude the arcuate fault bounding the southern caldera margin. The breccia is interpreted as the vent facies of ash-flow tuff adjacent to the caldera, based on similar lithologies, phenocryst modes of juvenile clasts, and matrix character. Cataclasites along the major faults include both country rocks and pyroclastic breccia, yet some cataclasite blocks are contained within the pyroclastic breccia. These geometric and textural relations suggest that the faulting took place during caldera-forming eruption. Prevolcanic sedimentary rocks enclosed by the arcuate faults are interpreted as the dissected coherent floor of the caldera, which were subsided several hundreds to a thousand meters. No large-scale piecemeal disruption of the caldera floor is evident. Orientations of striations with the cataclasites and altitudes of the base of the tuff suggest the asymmetric trap-door subsidence of this caldera. The overall caldera geometry is the nested trap-door piston-cylinder subsidence, each 21×15 and 25×23 km in diameter, in contrast to previous interpretation of a funnel shape.
Large ignimbrite dykes continuous with an overlying sheet of rhyolitic ignimbrite have been found at two localities in the centrally-subsided block of the Sabaloka cauldron. There is good evidence that these dykes fed ash-flow eruptions. Other possible feeder vents occur along the marginal fracture zone of the cauldron but evidence for the origin of some of these structures is ambiguous. Ignimbrites within the dyke-shaped feeders contain a very strong eutaxitic foliation oriented parallel to the contacts and this feature is thought to result from inwardly-directed pressures exerted by the dyke walls during a collapse which followed eruption of the ash-flows. Vents of this type are believed to originate by permissive intrusion through the roof of a shallow-seated magma chamber, in contrast to forcefully injected diatreme vents. A broad genetic classification of these bodies is suggested, based on their mode of emplacement.
Calc-alkaline granitoid rocks of the Oligocene-Pliocene Chilliwack batholith, North Cascades, range from quartz diorites to granites (57–78% SiO2), and are coeval with small gabbroic stocks. Modeling of major element, trace element, and isotopic data for granitoid and mafic rocks suggests that: (1) the granitoids were derived from amphibolitic lower crust having REE (rare-earth-element) and Sr-Nd isotopic characteristics of the exposed gabbros; (2) lithologic diversity among the granitoids is primarily the result of variable water fugacity during melting. The main effect of fH
2
O variation is to change the relative proportions of plagioclase and amphibole in the residuum. The REE data for intermediate granitoids (quartz diorite-granodiorite; Eu/Eu*=0.84–0.50) are modeled by melting with fH
2
O* =1.0–0.54) require residual amphibole in the source and are modeled by melting with fH
2
O=2–3 kbar. Consistent with this model, isotopic data for the granitoids show no systematic variation with rock type (87Sr/86Sri =0.7033–0.7043; Nd(0)=+3.3 to +5.5) and overlap significantly with data for the gabbroic rocks (87Sr/86Sri =0.7034–0.7040; Nd(0)=+3.3 to +6.9). The fH
2
O variations during melting may reflect additions of H2O to the lower crust from crystallizing basaltic magmas having a range of H2O contents; Chillwack gabbros document the existence of such basalts. One-dimensional conductive heat transfer calculations indicate that underplating of basaltic magmas can provide the heat required for large-scale melting of amphibolitic lower crust, provided that ambient wallrock temperatures exceed 800C. Based on lithologic and geochemical similarities, this model may be applicable to other Cordilleran batholiths.
Comparative UPb dating of zircon, xenotime and monazite from two different samples of the Himalayan “Makalu” granite shows the two U decay series to be in disequilibrium, particularly in monazite. This disequilibrium is due to excess or deficit amounts of radiogenic206Pb which originate from an excess or deficit of230Th, respectively, occurring initially in the mineral. Such an initial disequilibrium is caused by UTh fractionation between the crystallising mineral and the magma. Therefore, the UPb ages of Th-rich minerals such as monazite (and allanite) have to be corrected for excess206Pb due to excess230Th, whereas Th-poor minerals such as zircon and xenotime require a correction for a deficit of206Pb due to deficiency of230Th. The extent of this correction depends on the degree of ThU fractionation and on the age of the rock. For the two monazite populations analysed here, these excess amounts of206Pb were, with reference to the amount of radiogenic206Pb, 8–10% and 15–20% respectively, and less than 1% for zircon and xenotime. The varying degrees of Th enrichment relative to U in monazite show that the ThU partition coefficients for this mineral are not constant within a single granite. Furthermore, for monazite there is evidence for excess amounts of radiogenic207Pb originating from the decay of initial excess231Pa, also enriched during crystal growth.The very low Th/U ratios of 0.196 and 0.167, determined for thetwo whole rocks from which the minerals have been extracted, substantiate the view that granite formation is a fundamental mechanism for ThU fractionation in continental crust.The different ages of 21.9 ± 0.2m.y. and24.0 ± 0.4m.y., obtained by averaging the corrected238U206Pb ages of the monazites, suggest that the apparently homogeneous Makalu granite was generated over a period of at least 2 m.y.
New investigations of the geology of Crater Lake National Park necessitate a reinterpretation of the eruptive history of Mount Mazama and of the formation of Crater Lake caldera. Mount Mazama consisted of a glaciated complex of overlapping shields and stratovolcanoes, each of which was probably active for a comparatively short interval. All the Mazama magmas apparently evolved within thermally and compositionally zoned crustal magma reservoirs, which reached their maximum volume and degree of differentiation in the climactic magma chamber ∼ 7000 yr B.P.