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The magmatic response above subducting ocean lithosphere can range from weak to vigorous and from a narrow zone to widely distributed. The small and young Cascade Arc, riding on the margin of the tectonically active North American plate, has expressed nearly this entire range of volcanic activity. This allows an unusually good examination of arc initiation and early growth. We review the tectonic controls of Cascade-related magmatism from its inception to the present, with new considerations on the influences of tectonic stress and strain on volcanic activity. The Cascade Arc was created after accretion of the Siletzia oceanic plateau at ~ 50 Ma ended a period of flat-slab subduction. This (1) initiated dipping-slab subduction beneath most of the northern arc (beneath Washington and Oregon) and (2) enabled the more southerly subducting flat slab (beneath Nevada) to roll back toward California. As the abandoned flat slab fragmented and foundered beneath Oregon and Washington, vigorous extension and volcanism ensued throughout the northwest USA; in Nevada the subducting flat slab rolled back toward California. Early signs of the Cascade Arc were evident by ~ 45 Ma and the ancestral Cascade Arc was well established by ~ 35 Ma. Thus, from ~ 55–35 Ma subduction-related magmatism evolved from nearly amagmatic to regional flare-up to a clearly established volcanic arc in two different tectonic settings. The modern Cascades structure initiated ~ 7 Ma when a change in Pacific plate motion caused partial entrainment of the Sierra Nevada/Klamath block. This block pushes north and west on the Oregon Coast Ranges block, breaking the arc into three segments: a southern extensional arc, a central transitional arc, and a northern compressional arc. Extension enhances mafic volcanism in the southern arc, promoting basalt decompression melts from depleted mantle (low-K tholeiites) that are subequal in volume to subduction fluxed calcalkaline basalts. Compression restricts volcanic activity in the north; volcanism is dominantly silicic and intra-plate-like basalts cluster close to the main arc volcanoes. The transitional central arc accommodates dextral shear deformation, resulting in a wide volcanic arc with distributed basaltic vents of diverse affinities and no clear arc axis.

The late Neogene Deschutes Formation of central Oregon preserves a remarkable volcanic and sedimentary record of the initial stages of High Cascades activity following an eastward shift in the locus of volcanism at ~ 7.5 Ma. Numerous ignimbrite and tephra-fall units are contained within the formation, and since equivalent deposits are relatively rare for the Quaternary Cascades, the eruptions of the earliest High Cascade volcanoes were likely more explosive than those of the Quaternary arc. In this study, the timing and frequency of eruptions which produced 14 laterally extensive marker ignimbrites within the Deschutes Formation are established using 40Ar/39Ar geochronology. Plagioclase 40Ar/39Ar ages for the lowermost (6.25 ± 0.07 Ma) and uppermost (5.45 ± 0.04 Ma) marker ignimbrites indicate that all major explosive eruptions within the Deschutes Formation occurred within a period of 800 ± 54 k.y. (95% confidence interval). Minimum estimates for the volumes of the 14 ignimbrites, using an ArcGIS-based method, range from 1.0 to 9.4 km3 and have a total volume of 62.5 km3. Taken over the 50 km of arc length, the explosive volcanic production rate of the central Oregon High Cascades during Deschutes Formation time was a minimum of 1.8 km3/m.y./km of arc length. By including estimates of the volumes of tephra-fall components, as well as ignimbrites that may have traveled west, we estimate a total volume range, for these 14 eruptions alone, of 188 to 363 km3 (~ 121 to 227 km3 DRE), a rate of 4.7–9.1 km3/m.y./km arc length. This explosive volcanic production rate is much higher than the average Quaternary eruption rates, of all compositions, estimated for the entire Cascade arc (1.5–2.5), Alaska Peninsula segment of the Aleutian arc (0.6–1.0), and the Andean southern volcanic zone (1.1–2.0). We suggest that this atypical explosive pulse may result from the onset of regional extension and migration of the magmatic arc, which had the combined effect of increasing magmatic flux and temporarily enhancing melting of more fusible crust.

The dangerous juxtaposition of human populations and active volcanic arcs worldwide makes elucidating the history of explosive eruptions of these arcs a crucial endeavor. The well-preserved Deschutes Formation, which forms part of the Deschutes Basin of central Oregon, USA, contains a distal record of a period of unusually frequent explosive eruptions between 5.45 and 6.25 Ma, which is atypical of the Cascades arc as a whole. However, no previous study has established the chronology, frequency, or total volume of all of the ignimbrites and tephra-fall units exposed within the Formation in order to compare to baseline activity of the arc. We use multivariate statistics on major and trace element compositions of pumice glass to correlate pyroclastic units across the Deschutes Basin, and combine these with recently published 40Ar/39Ar dates to establish, for the first time, a detailed and comprehensive tephrochronology of the Deschutes Formation. Our results suggest that at least 78 statistically distinct explosive eruptions occurred between 6.25 and 5.45 Ma. Eruption frequency is not constant throughout the 800 kyr of pyroclastic volcanism; a maximum rate of 1.7 to 3.0 explosive eruptions per 10 kyr occurred between 5.76 and 5.68 Ma and decreased for 340 kyr prior to graben formation. We estimate that the volume for 51 ignimbrite eruptions is 290 to 565 km3 (170 to 280 km3 DRE), assuming an equal westward flow component and a fall:flow ratio of 0.5:1 to 1.9:1. Using a single isopach method, we also estimate that a minimum volume of 110 km3 (45 km3 DRE) was deposited as 27 pumice-fall units which have no correlated ignimbrites. Thus, we estimate that a cumulative pyroclastic volume of 400 to 675 km3 (210 to 330 km3 DRE) was erupted in the central Oregon Cascades during this time. The volumetric rate (2.7 to 4.1 km3/10 kyr, DRE) and average frequency of ignimbrite eruptions (1.0 to 1.5 eruptions/ 10kyr) are more than a factor of ten greater than the Quaternary Cascades arc. Thus, the Deschutes Formation records North America's most recent arc-sourced ignimbrite flare-up. Although smaller in volume than most well-recognized flare-up events, this represents an important category of flare-ups that may be more common in arcs than previously recognized. We hypothesize that a heightened flux of basalt, possibly induced by slab-rollback, was focused beneath the arc and into the shallow crust by minor amounts of crustal extension. This extension allowed for the high flux of basalt to be stored at shallow levels beneath a new arc locus within fertile crust, resulting in an unusually silicic and explosive period of Cascade arc history.

[1] Multiple episodes of Oligocene and younger silicic volcanism are represented in the high lava plateau of central and southeastern Oregon. From 12 Ma to Recent, volcanism is strongly bimodal with nearly equal volumes of basalt and rhyolite. It is characterized by moderate to high silica (SiO2 >72 wt. %) rhyolitic tuffs and domes that are younger to the west, and widespread, tholeiitic basalts that show no temporal pattern. We report 18 new 40Ar/39Ar incremental heating ages on rhyolites, and establish that the timing of the age-progressive rhyolites is decoupled from basaltic pulses. This work expands on that of previous workers by clearly linking the High Lava Plains (HLP) and northwestern-most Basin and Range (NWBR) rhyolite volcanism into a single age-progressive trend. The spatial-temporal relationship of the rhyolite outcrops and regional tectonics indicate that subsidence due to increasingly dense crust creates large, primarily sediment-filled basins within the more volcanically active HLP. The west-northwest age progression in rhyolitic volcanism is counter to the trend expected for a quasi-stationary mantle upwelling relative to North American plate motion. We attribute the rhyolitic age progression to mantle upwelling in response to slab rollback and steepening, and this is consistent with mantle anisotropy under the region and analog slab rollback models. This removes the necessity of deep mantle plume involvement. Laboratory experimental studies indicate that the geometry of the downgoing slab can focus upwelling or asthenospheric counterflow into a constricted band, resulting in greater volcanic volumes in the HLP as compared to the NWBR.

The rhenium–osmium isotopic system has the potential to track the maturation of arc crust at lower crustal levels because the 187 Os/ 188 Os in magmas is sensitive to the age and composition of crust with which they interact. We here present Re and Os isotopic data for a suite of samples from the extensional central Oregon Cascade arc that includes diverse, primitive (high Mg) basalts and low- and medium-K basaltic andesites from North Sister and Little Brother volcanoes. The basaltic andesites contain higher 187 Os/ 188 Os than the two most common basalt types, calc-alkaline basalt and low-K tholeiites (0.17–0.19 vs. 0.14– 0.15), indicating interaction with mafic crust. The evolution of 187 Os/ 188 Os in the deep crust depends on Re partitioning that in turn depends on sulfide and magnetite stability and oxygen fugacity. We estimate bulk mineral-melt partition coefficients for Re to be � 4a tfO2 � QFM and � 10 at fO2 � +1 to +3 DQFM in keeping with near liquidus phase equilibria at deep crustal conditions (Mercer and Johnston, 2008) and on fO2 of central Oregon primitive magmas (Rowe et al., 2009). Modeling fractional crystallization of the central Oregon primitive basalts indicates that the deep arc crust is likely to have 187 Re/ 188 Os similar to the range measured in North Sister basaltic andesites (� 55–260). The age of the mafic crustal contaminant in the central Oregon Cascades is then estimated to be 9–29 Ma and is younger than the start of Cascade arc volcanism at � 45 Ma in central Oregon. The range in 187 Os/ 188 Os at the Cascade volcanic centers Mt. Lassen and Mt. Adams is greater than is observed for central Oregon arc magmas. We estimate a range in mafic crustal contaminant ages of 8–340 Ma for Mt. Lassen and 0–70 Ma for Mt. Adams and are older than suggested by previous studies (Hart et al., 2002, 2003). Mafic contaminant age ranges reflect the degree to which the crust is homogenized, or the extent to which the composition of the crust is a product of recent arc magmatism vs. pre-existing protoliths. Intra-arc extension in central Oregon facilitates homogenization and leads to assimilation of relatively young arc crust. Where extension is less narrowly focused, the range in Re and Os isotopes is greater and reflects isotopic heterogeneities, including variably aged crust at Mt. Lassen or variable enrichment of Re and Os in the lithosphere beneath Mt. Adams. 2013 Elsevier Ltd. All rights reserved.

Phenocryst assemblages of lavas from the long-lived Aucanquilcha Volcanic Cluster (AVC) have been probed to assess pressure and temperature conditions of pre-eruptive arc magmas. Andesite to dacite lavas of the AVC erupted throughout an 11-million-year, arc magmatic cycle in the central Andes in northern Chile. Phases targeted for thermobarometry include amphibole, plagioclase, pyroxenes, and Fe–Ti oxides. Overall, crystallization is documented over 1–7.5 kbar (~25 km) of pressure and ~680–1,110 °C of temperature. Pressure estimates range from ~1 to 5 kbar for amphiboles and from ~3 to 7.5 kbar for pyroxenes. Pyroxene temperatures are tightly clustered from ~1,000–1,100 °C, Fe–Ti oxide temperatures range from ~750–1,000 °C, and amphibole temperatures range from ~780–1,050 °C. Although slightly higher, these temperatures correspond well with previously published zircon temperatures ranging from ~670–900 °C. Two different Fe–Ti oxide thermometers (Andersen and Lindsley 1985; Ghiorso and Evans 2008) are compared and agree well. We also compare amphibole and amphibole–plagioclase thermobarometers (Ridolfi et al. 2010; Holland and Blundy 1994; Anderson and Smith 1995), the solutions from which do not agree well. In samples where we employ multiple thermometers, pyroxene temperature estimates are always highest, zircon temperature estimates are lowest, and Fe–Ti oxide and amphibole temperature estimates fall in between. Maximum Fe–Ti oxide and zircon temperatures are observed during the middle stage of AVC activity (~5–3 Ma), a time associated with increased eruption rates. Amphibole temperatures during this time are relatively restricted (~850–1,000 °C). The crystal record presented here offers a time-transgressive view of an evolving, multi-tiered subvolcanic reservoir. Some crystals in AVC lavas are likely to be true phenocrysts, but the diversity of crystallization temperatures and pressures recorded by phases in individual AVC lavas suggests erupting magma extensively reams and accumulates crystals from disparate levels of the middle to upper crust.

New data indicate that northeast-directed extensional faulting characterizes slip across the Brothers fault zone (BFZ), which marks the northern limit of the northwestern Basin and Range (NWBR) extensional province in southeastern Oregon. Structural separation across individual north-northeast striking NWBR faults decreases to zero south of the BFZ. Field relationships and cross-sections demonstrate limited kinematic linkage and independent evolution of the two fault systems since ∼7 Ma. West-directed extension accumulated on NWBR faults at 0.01 mm/yr and lengthened northward after 7.05 Ma. BFZ faults accumulated northeast-directed extension at rates of 0.01 mm/yr since 5.68 Ma. Deformation coincides with periods of heightened basaltic magmatism in the High Lava Plains, implying that volcanism weakened the crust and promoted extension in the BFZ. In a new model, we reconcile the observed northward diminishing rate and clockwise motion of the modern NWBR deformation field with regional geology. The BFZ defines a small circle about the pole of rotation and separates a stable block to the NE from the extending region to the south. Faults to the south are growing northward, consistent with the northward decrease in rate and magnitude of extension in the NWBR.

Clusters of high-sulfidation epithermal deposits containing more than 50 Moz of gold are hosted by advanced argillic-altered Miocene volcanic rocks in the Yanacocha district, northern Peru (lat. 6°59'30" S, long. 78°30'45" W). We describe the nature of the volcanism and its relation to the gold ores on the basis of new district-scale geologic mapping, 69 40Ar/39Ar ages on igneous rocks and hydrothermal alunite, and petrologic and geochemical investigations. Volcanic rocks of the Calipuy Group are the oldest Cenozoic rocks at Yanacocha, and include the Huambo Cancha andesite, andesitic lahars of Tual and Chaupiloma (19.5–15.9 Ma), and the dacitic Cerro Fraile pyroclastics (15.5–15.1 Ma). The younger Yanacocha Volcanics (14.5–8.4 Ma) form a cogenetic series of lavas and pyroclastic rocks with a cumulative volume of ~88 km3 that represents eruption from a moderate-size volcanic center. Early pyroxene>hornblende-bearing lavas of the Atazaico Andesite (14.5–13.3 Ma) erupted from small stratovolcanoes progressively younger from southwest to northeast. Dacitic dikes followed that are spatially associated with gold deposits at Quilish and Cerro Negro (~7 Moz Au) and stage 1 alunite (13.6–12.6 Ma). The Colorado Pyroclastics erupted in the center of the district and include the hornblende- and biotite-bearing andesitic to dacitic Cori Coshpa (12.6 Ma) and Maqui Maqui (12.5–12.4 Ma) ignimbrites. The Colorado Pyroclastics are overlain by hornblende>pyroxene-bearing andesitic to dacitic lavas, flow-domes, and minor pyroclastic rocks of the Azufre Andesite (12.1–11.6 Ma). Widespread stage 2 alunite (11.5 Ma) and minor gold deposition (~0.5 Moz) closely follow. The San Jose Ignimbrite (11.5–11.2 Ma) overlies the Azufre Andesite and stage 2 alunite and includes three members of hornblende-pyroxene (biotite) dacite and andesite that erupted in the center of the district and flowed southward. Mineralogically similar domes were emplaced into the inferred vents. The Coriwachay Dacite (10.8–8.4 Ma) forms the youngest and most silica rich igneous rocks in the district, and includes intrusions and flow domes of dacite to rhyolite at Corimayo (10.8 Ma), Cerro Yanacocha (9.9 Ma), and Yanacocha Lake (8.4 Ma). Most of the gold (>47 Moz) was deposited at Yanacocha during intrusion of the late Coriwachay Dacite. These late dacites are volumetrically smallest (~2% of the total volume of erupted magma) and are temporally associated with stage 3 to 5 alunites. Stage 3 alunite (11.0–10.7 Ma) developed along a northeast trend for 9 km that includes the gold deposits of Corimayo, San Jose, Carachugo, and Maqui Maqui. The deeper Kupfertal Cu-Au porphyry has an age of 10.7 Ma on hydrothermal biotite and underlies zones of stage 4 and 5 quartz-alunite alternation that are 0.8 and 1.5 m.y. younger, respectively. Stage 4 alunite ranges in age from 10.2 to 10.3 Ma at the Tapado and Chaquicocha Sur gold deposits to Cerro Sugares east of the Maqui Maqui deposit. Stage 4 also includes a younger alunite age of 9.9 Ma from the San Jose gold deposit. Stage 5 alunite ranges from ~9.3 to ~8.2 Ma at Cerro Yanacocha, the largest gold deposit in the district. All these deposits contain massive and vuggy quartz, quartz-alunite, and quartz-pyrophyllite alteration associated with pyrite±enargite-tennantite-covellite. Magmatism in the Yanacocha district lasted for ~11 m.y. The Yanacocha Volcanics spanned the last ~6 m.y. of this period and were associated with long-lived magmatic-hydrothermal activity and episodic gold mineralization. The Yanacocha calc-alkaline suite was oxidized, water and sulfate rich, and evolved from early pyroxene>hornblende andesite to late titanite-bearing dacite and minor rhyolite. Several dacites contain populations of both high- and low-aluminum hornblendes that crystallized in the middle and upper crust, respectively. The variation of Mg, Ti, P, Sr, and Ba contents in these rocks is consistent with a complex magmatic origin via both cooling, fractional crystallization, periodic recharge of deeply derived hydrous basaltic or andesitic melts, and mixing with silicic melts derived by crustal melting. Low eruption rates, high phenocryst contents of the volcanic rocks, and widespread hydrothermal alteration are consistent with the hypothesis that most of the magmas at Yanacocha crystallized in shallow chambers as granitoids that passively degassed ore fluids. The compositional diversity of the volcanic rocks together with an extended magmatic-hydrothermal history reflect complex magmatic processes that were optimum for producing the world-class gold deposits at Yanacocha.

Age and trace element data from zircons constrain the progressive stages of development of the magmatic underpinnings of the long-lived, dacite-dominant Aucanquilcha volcanic cluster in northern Chile. Large U/Pb age spectra (

We report new He, Nd and Sr isotope results for basalts from the northwestern United States. The new 3 He/ 4 He results for olivine phenocrysts in basalts from the eastern Snake River Plain (SRP), the Owyhee Plateau (OP) and the Oregon High Lava Plains (HLP), together with published He isotope data for Yellowstone and the Cascades volcanic arc, delineate distinct mantle sources for each of these sub-provinces. All basalts from the eastern SRP (8 Quaternary localities plus 1 Miocene locality) have 3 He/ 4 He ratios higher than observed in normal mid-ocean ridge basalts, but overlapping with ranges observed in hotspot-related oceanic islands. For a lateral distance of some 400 km along the SRP, 3 He/ 4 He ranges from ∼11 R A in the west to >19 R A adjacent to Yellowstone. Such high ratios have not been observed elsewhere in the western U.S., and are consistent with the presence of a mantle plume. The lateral gradient in 3 He/ 4 He suggests that the proportion of plume-derived He decreases westward, but this interpretation is complicated by possible addition of crustal helium during open-system crystal fractionation in some SRP basaltic magmas. Although crustal contamination may modulate 3 He/ 4 He in basalts along the SRP, the effect is not strong and it does not obscure the elevated 3 He/ 4 He mantle source signature. In contrast, young basalts from the HLP and the OP have 3 He/ 4 He values of 8.8-9.3 R A , within the range for mid-ocean ridge basalts; these data reflect a shallow asthenospheric source with no discernible influence from the Yellowstone hotspot. Basalts from Newberry volcano have slightly lower 3 He/ 4 He (7.6―8.3 R A ), within the range for other Cascades arc lavas (7.0-8.4 R A ). Three alternative explanations are possible for the origin of the high 3 He/ 4 He signature along the SRP: (1) multi-component mixing of (a) magmas and/or CO 2 -rich fluids derived from plume mantle having high 3 He/ 4 He, (b) continental lithosphere having low 3 He/ 4 He, and (c) shallow asthenospheric mantle (MORB source); (2) a mantle plume beneath Yellowstone that has an unusual combination of He, Nd and Sr isotope characteristics; or (3) a continental lithospheric mantle that experienced ancient enrichment of 3 He relative to (U+Th). The isotope relations between He-Nd and He-Sr, along with other considerations, generally favor the first explanation, but the other possibilities cannot be ruled out at the present time.

Along with magma supply rates, glacial ice and local tectonics influence the eruptive style and preservation of arc volcanoes. We present the results of geologic mapping and 20 new 40 Ar/ 39 Ar dates for the mafic composite North Sister Volcano, the adjoining Little Brother shield volcano, and Matthieu Lakes Fissure in the central Oregon Cascade Arc, along with major-and trace-element analyses. North Sister is the oldest and most mafic of the Three Sisters Volcanoes and has produced dike-fed, low-K basaltic andesitic (52.5%–55% SiO 2 ) agglutinated lava, scoria, and tuffs over its 3 volume of North Sister was produced during the first two stages. The >11-km-long, north-trending Matthieu Lakes Fissure (75–11 ka) transects North Sister and erupted in three magmatic pulses, yielding a series of thick lavas, scoria cones, and subglacially erupted flow-dominated tuyas. Glacial ice has been present during much of the eruptive history of North Sister. Palagonitic tuffs, tuyas, and ice-contact lava flows as well as major unconformities between eruptive stages correlate with the δ 18 O of seawater global climate record at ca. 250 ka, 180–130 ka, and 80–18 ka. Time-integrated eruption rates at North Sister appear to have slowed from 0.18 to 0.12 km 3 per k.y. between 3 per k.y. between ca. 100 ka and 50 ka. The waning of volcanism at North Sister coincides with a decrease in Ni and MgO concentrations, indicating that olivine fractionation dominated the later stages. Also, toward the end of volcanism at North Sister, dikes changed from a radial pattern to a N-S pattern, parallel to faults and vent alignments associated with E-W extension of the High Cascades graben. The Matthieu Lakes Fissure represents the final overprinting of the edifice and distribution of magma supply away from the North Sister center.

In the central Oregon Cascades, extension of the arc has promoted eruption of primitive basalts that are of three types, calcalkaline (CAB), low K tholeiitic (LKT) and rare absarokitic (ABS) in the forearc. Based on a comparison with the distribution of primitive magma types and their 87Sr/86Sr and 143Nd/144Nd isotopic signature in the Cascades, we divide the arc into four segments that correspond to distinct tectonic settings and reflect mantle domains and melting regimes at depth. The segments are: 1) the North Segment from Mt. Meager to Glacier Peak; 2) the Columbia Segment from Mt. Rainier to Mt. Jefferson; 3) the Central Segment from the Three Sisters to Medicine Lake, and 4) the South Segment from Mt. Shasta to Lassen Peak. Calcalkaline basalts (CABs) are found all along the arc axis and are produced by fluxing of variable mantle domains by subduction-derived fluid. In the South Segment, the degree of fluxing and melting is greatest as indicated by high 87Sr/86Sr and Ba/Ce of CABs relative to other types of ambient basalt and is consistent with the greater abundance of high-Mg basaltic andesite, relative to other segments. High flux and abundant melt is enhanced by the presence of a slab window and subduction of the altered and deformed Gorda Plate. In the northern part of the arc, small degrees of flux melting are coupled with the presence of an enriched mantle component to yield abundant high-field strength element-enriched (HFSE-rich) basalts. Extension and higher heat flow favors the production of abundant low potassium tholeiites LKT in the Central Segment. A distinct shift in 87Sr/86Sr of low LKTs occurs between the Columbia and Central Segments (0.7028 vs. 0.7034, respectively), which we interpret as juxtaposition of mantle of accreted oceanic terranes, including the enriched large igneous province Siletz Terrane, with encroaching mantle related to the adjacent Basin and Range Province. The latter, although depleted, carries an enrichment signature from an older subduction history. The segmentation presented here for the Cascade Arc provides a framework for testing the relative influences of the downgoing slab, mantle heterogeneity, and the tectonics and make up of the upper place.

Volcan Aucanquilcha, northern Chile, has produced ∼37 km3 of dacite (63–66 wt% silica), mainly as lavas with ubiquitous magmatic inclusions (59–62 wt% silica) over the last ∼1 million years. A pyroclastic flow deposit related to dome collapse occurs on the western side of the edifice and a debris avalanche deposit occurs on the eastern side. The >6,000-m high edifice defines a 9-km E–W ridge and lies at the center of a cluster of more than 15 volcanoes, the Aucanquilcha Volcanic Cluster, that has been active for at least the past 11 million years. The E–W alignment of vents is nearly orthogonal to the arc axis. A majority of Volcan Aucanquilcha was constructed during the first 200,000 years of eruption, whereas the last 800,000 years have added little additional volume. The peak eruptive rate during the edifice-building phases was ∼0.16 km3/ka and the later eruptive rate was ∼0.02 km3/ka. Comparable dacite volcanoes elsewhere show a similar pattern of high volcanic productivity during the early stages and punctuated rather than continuous activity. Volcan Aucanquilcha lavas are dominated by phenocrysts of plagioclase, accompanied by two populations of amphibole, biotite, clinopyroxene, Fe–Ti oxides and (or) orthopyroxene. Accessory phases include zircon, apatite and rare quartz and sanidine. One amphibole population is pargasite and the other is hornblende. The homogeneity of dacite lava from Volcan Aucanquilcha contrasts with the heterogeneity (52–66 wt% silica) at nearby Volcan Ollague, which has been active over roughly the same period of time. We attribute this homogeneity at Aucanquilcha to the thermal development of the crust underneath the volcano resulting from protracted magmatism there, whereas Volcan Ollague lacks this magmatic legacy.

We consider the origin of rhyolites associated with tholeiitic basalt in bimodal provinces, as exemplified by the Rattlesnake Tuff of the High Lava Plains of eastern Oregon, in comparison to rhyolites associated with calcalkaline suites in light of recent models of extraction of rhyolite from crystal mush (Hildreth, J Volcanol Geotherm Res, 136:169–198, 2004; Bachmann and Bergantz, J Petrol, 45:1565–1582, 2004). The High Lava Plains encompass a strongly bimodal, tholeiite-rhyolite suite, spatially and compositionally related to the Snake River Plain and Yellowstone Plateau. In our assessment we draw the distinction between fractionation dominated processes to make rhyolites from rhyolites and processes required to make the parental rhyolite melt. New isotopic data and compositional zoning profiles in phenocrysts confirm that crystal fractionation dominated the generation of progressively more evolved, discrete rhyolites in the zoned Rattlesnake Tuff and are consistent with an origin of the least evolved high-silica rhyolites by partial melting of a mafic crust. While the most evolved rhyolites are compositionally virtually indistinguishable from those of calcalkaline suites, the parental rhyolites from bimodal suites are more Fe-rich than their calcalkaline counterparts. Oxygen isotope thermometry yields pre-eruptive temperatures of 860°C, in keeping with 800–880°C zircon saturation temperatures. High magmatic temperatures are common among rhyolites of bimodal suites, distinguishing them from cooler rhyolites of calcalkaline suites. Extraction of interstitial melt from a granodioritic mush cannot produce compositions of the Rattlesnake Tuff on the basis of major and trace element arguments (especially Fe, Ba, Sr, and Eu) and on the basis of temperature considerations. Chemically viable parental crystal mushes are syenite and alkali (A-type) granites for the production of all more evolved Rattlesnake Tuff rhyolites; ferro-dacitic mush is required for production of the least-evolved, parental Rattlesnake Tuff rhyolite. Paucity of such ferro-dacitic compositions in tholeiitic bimodal suites, especially compared to the abundance of dacitic (granodioritic) compositions in calcalkaline suites, argues against the mush extraction model for the parental rhyolite. Furthermore, rhyolites of bimodal suites lack associated voluminous eruptions of crystal-rich ignimbrite that might represent a parental mush, as exemplified by the “monotonous intermediate” Fish Canyon Tuff in calcalkaline suites. We conclude that extensive fractionation is common among rhyolites and may obscure their ancestry. Fe-rich parental rhyolites common in bimodal tholeiitic suites, as represented by Rattlesnake Tuff, may often be the result of partial melting of mafic to intermediate crust, in contrast to calcalkaline high-silica rhyolites that are related to voluminous suites of intermediate intrusive rocks where the pre-plutonic mush-extraction model works better.

The arid climate of the Altiplano has preserved a volcanic history of ∼11 million years at the Aucanquilcha Volcanic Cluster (AVC), northern Chile, which is built on thick continental crust. The AVC has a systematic temporal, spatial, compositional and mineralogical development shared by other long-lived volcanic complexes, indicating a common pattern in continental magmatism with implications for the development of underlying plutonic complexes, that in turn create batholiths.The AVC is a ∼700-km2, Tertiary to Recent cluster of at least 19 volcanoes that have erupted andesite and dacite lavas (∼55 to 68 wt.% SiO2) and a small ash-flow tuff, totalling 327 ± 20 km3. Forty 40Ar/39Ar ages for the AVC range from 10·97 ± 0·35 to 0·24 ± 0·05 Ma and define three major 1·5 to 3 million-year pulses of volcanism followed by the present pulse expressed as Volcán Aucanquilcha. The first stage of activity (∼11–8 Ma, Alconcha Group) produced seven volcanoes and the 2-km3 Ujina ignimbrite and is a crudely bimodal suite of pyroxene andesite and dacite. After a possible two million year hiatus, the second stage of volcanism (∼6–4 Ma, Gordo Group) produced at least five volcanoes ranging from pyroxene andesite to dacite. The third stage (∼4–2 Ma, Polan Group) represents a voluminous pulse of activity, with eruption of at least another five volcanoes, broadly distributed in the centre of the AVC, and composed dominantly of biotite amphibole dacite; andesites at this stage occur as magmatic inclusions. The most recent activity (1 Ma to recent) is in the centre of the AVC at Volcán Aucanquilcha, a potentially active composite volcano made of biotite-amphibole dacite with andesite and dacite magmatic inclusions.These successive eruptive groups describe (1) a spatial pattern of volcanism from peripheral to central, (2) a corresponding change from compositionally diverse andesite-dacite volcanism to compositionally increasingly restricted and increasingly silicic dacite, (3) a change from early anhydrous mafic silicate assemblages (pyroxene dominant) to later biotite amphibole dacite, (4) an abrupt increase in eruption rate; and (5) the onset of pervasive hydrothermal alteration.The evolutionary succession of the 327-km3 AVC is similar to other long-lived intermediate volcanic complexes of very different volumes, e.g., eastern Nevada (thousands of km3, Gans et al. 1989; Grunder 1995), Yanacocha, Perú (tens of km3, Longo 2005), and the San Juan Volcanic System (tens of thousands of km3, Lipman 2007) and finds an analogue in the 10-m. y. history and incremental growth of the Cretaceous Tuolumne Intrusive Suite (Coleman et al. 2004; Glazner et al. 2004). The present authors interpret the AVC to reflect episodic sampling of the protracted and fitful development of an integrated and silicic middle to upper crustal magma reservoir over a period of at least 11 million years.

The abrupt changes in character of variably welded pyroclastic deposits have invited decades of investigation and classification. We conducted two series of experiments using ash from the nonwelded base of the rhyolitic Rattlesnake Tuff of Oregon, USA, to examine conditions of welding. One series of experiments was conducted at atmospheric pressure (1 At) in a muffle furnace with variable run times and temperature and another series was conducted at 5 MPa and 600 °C in a cold seal apparatus with variable run times and water contents. We compared the results to a suite of incipiently to densely welded, natural samples of the Rattlesnake Tuff. Experiments at 1 At required a temperature above 900 °C to produce welding, which is in excess of the estimated pre-eruptive magmatic temperature of the tuff. The experiments also yielded globular clast textures unlike the natural tuff. During the cold-seal experiments, the gold sample capsules collapsed in response to sample densification. Textures and densities that closely mimic the natural suite were produced at 5 MPa, 600 °C and 0.4 wt.% H2O, over run durations of hours to 2 days. Clast deformation and development of foliation in 2-week runs were greater than in natural samples. Both more and less water reduced the degree of welding at otherwise constant run conditions. For 5 MPa experiments, changes in the degree of foliation of shards and of axial ratios of bubble shards and non-bubble (mainly platy) shards, are consistent with early densification related to compaction and partial rotation of shards into a foliation. Subsequent densification was associated with viscous deformation as indicated by more sintered contacts and deformation of shards. Sintering (local fusion of shard–shard contacts) was increasingly important with longer run times, higher temperatures, and greater pressures. During runs with high water concentrations, sintering was rare and adhesion between clasts was dominated by precipitation of sublimates in pore spaces. A few tenths wt.% H2O in the rhyolite glass promote the development of welding by sharp reduction of glass viscosity. Large amounts of water inhibit welding by creating surface sublimates that interfere with sintering and may exert fluid pressure counter to lithostatic load if sintering and vapor-phase sublimates seal permeability in the tuff.

This volume is a collection of ten papers largely derived from a special session, Welding Processes in Volcanology, organized for the 2003 meeting of EUG-EGS-AGU in Nice, France, and brings together field, experimental, and modeling studies. We briefly review advances in understanding welding as represented by this volume and point to future lines of inquiry, with an emphasis on understanding the timescales of welding, the metrics of strain, and feedback relationships between compaction, porosity, permeability loss, fluid transport and rheology of the deposit. Our analysis suggests that, within the realm of field and textural studies, more systematic and quantitative data are necessary to evaluate strain histories related to welding and the timing of welding relative to other processes during eruption and emplacement. A new generation of experiments is needed in which load, strain rate and volatile pressure can be controlled. Numerical models are needed to further explore the relationships and feedbacks between processes that contribute to welding. D 2004 Elsevier B.V. All rights reserved.

Columbia River Basalt Group dikes invade biotite---hornblende tonalite to granodiorite rocks of the Wallowa Mountains. Most dikes are strongly quenched against wallrock, but rare dike segments have preserved zones of partial melt in adjacent wallrock and provide an opportunity to examine shallow crustal melting. At Maxwell Lake, the 4 m thick wallrock partial melt zone contains as much as 47 vol. % melt (glass plus quench crystals) around mineral reaction sites and along quartz--feldspar boundaries. Bulk compositional data indicate that melting took place under closed conditions (excepting volatiles). With progressive melting, hornblende, biotite, and orthoclase were consumed but plagioclase, quartz, and magnetite persisted in the restite. Clinopyroxene, orthopyroxene, plagioclase, and Fe---Ti oxides were produced during dehydration-melting reactions involving hornblende and biotite. Reacting phases became more heterogeneous with progressive melting; crystallizing phases were relatively homogeneous. Progressive melting produced an early clear glass, followed by light (high-K) and dark (high-Ca) brown glass domains overprinted by devitrification. Melts were metaluminous and granitic to granodioritic. Thermal modeling of the partial melt zone suggests that melting took place over a period of about 4 years. Thus, rare dikes with melted margins represent long-lived portions of the Columbia River Basalt dike system and may have sustained large flows.

The 10.4-Ma Duck Butte Eruptive Center (DBEC) of southeastern Oregon is anomalous within the High Lava Plains bimodal basalt/high-silica rhyolite province in that it produced mainly intermediate composition lavas. At Duck Butte, eruptions successively tapped a compositionally zoned system that became less silicic and volumetrically smaller in time. The first eruptions produced rhyodacite lava with andesite inclusions, followed by dacite lava with basaltic andesite inclusions, and finally andesite and basaltic andesite lavas. High-silica rhyolite domes, more typical of the province, erupted before and after Duck Butte, immediately to the east and west, respectively. Evidence from mingling and mixing textures and mixing calculations indicate that successive eruptions at Duck Butte represent mixing of residual magma in the system. Eruption rate outpaced the rate of silicic magma production. The mixing end-members are rhyodacite, which was produced by small degrees of crustal melting, and basaltic andesite. The mixing-dominated history at Duck Butte is in contrast to high-silica rhyolite at nearby Indian Creek Butte (ICB), which was derived by crystal fractionation from rhyodacitic crustal melt. We attribute the petrogenetic process that produced intermediate composition magma to disruption of the Duck Butte magma system by faulting, which caused high eruption rates and stirring of the magma. Magmatism at Duck Butte was related to faulting as indicated by both the alignment of rhyodacite vents parallel to a major regional basin–range fault and by faults that cut through early flows, whereas later flows drape the escarpment and are also offset by the fault. In contrast, tectonic quiescence at neighboring centers allowed for protracted differentiation of crustal melt to produce high-silica rhyolite. The material inputs to both the bimodal and dominantly intermediate centers are the same, but the contrast in tectonic influence and eruption rate strongly affects the erupted compositions.

A variety of cognate basalt to basaltic andesite inclusions and dacite pumices occur in the 7-Ma Rattlesnake Tuff of eastern Oregon. The tuff represents ∼280 km3 of high-silica rhyolite magma zoned from highly differentiated rhyolite near the roof to less evolved rhyolite at deeper levels. The mafic inclusions provide a window into the processes acting beneath a large silicic chamber. Quenched basaltic andesite inclusions are substantially enriched in incompatible trace elements compared to regional primitive high-alumina olivine tholeiite (HAOT) lavas, but continuous chemical and mineralogical trends indicate a genetic relationship between them. Basaltic andesite evolved from primitive basalt mainly through protracted crystal fractionation and multiple cycles (≥10) of mafic recharge, which enriched incompatible elements while maintaining a mafic bulk composition. The crystal fractionation history is partially preserved in the mineralogy of crystal-rich inclusions (olivine, plagioclase ± clinopyroxene) and the recharge history is supported by the presence of mafic inclusions containing olivines of Fo80. Small amounts of assimilation (∼2%) of high-silica rhyolite magma improves the calculated fit between observed and modeled enrichments in basaltic andesite and reduces the number of fractionation and recharge cycles needed. The composition of dacite pumices is consistent with mixing of equal proportions of basaltic andesite and least-evolved, high-silica rhyolite. In support of the mixing model, most dacite pumices have a bimodal mineral assemblage with crystals of rhyolitic and basaltic parentage. Equilibrium dacite phenocrysts are rare. Dacites are mainly the product of mingling of basaltic andesite and rhyolite before or during eruption and to a lesser extent of equilibration between the two. The Rattlesnake magma column illustrates the feedback between mafic and silicic magmas that drives differentiation in both. Low-density rhyolite traps basalts and induces extensive fractionation and recharge that causes incompatible element enrichment relative to the primitive input. The basaltic root zone, in turn, thermally maintains the rhyolitic magma chamber and promotes compositional zonation.

The Rattlesnake Tuff of eastern Oregon comprises >99% of high-silica rhyolite glass shards and pumices representing ∼280 km³ of magma. Glassy, crystal-poor, high-silica rhyolite pumices and glass shards cluster in five chemical groups that range in color from white to dark gray with increasing Fe concentration. Compositional clusters are defined by Fe, Ti, LREE, Ba, Eu, Rb, Zr, Hf, Ta, and Th. Progressive changes with increasing degree of evolution of the magma occur in modal mineralogy, mineral composition, and partition coefficients. Partition coefficients are reported for alkali feldspar, clinopyroxene, and titanomagnetite. Models of modal crystal fractionation, assimilation, successive partial melting, and mixing of end members cannot account for the chemical variations among rhyolite compositions. On the other hand, ∼50% fractionation of observed phenocryst compositions in non-modal proportions agrees with chemical variations among rhyolite compositions. Such non-modal fractionation might occur along the roof and margins of a magma chamber and would yield compositions of removed solids ranging from syenitic to granitic. A differentiation sequence is proposed by which each more evolved composition is derived from the previous, less evolved liquid by fractionation and accumulation, occurring mainly along the roof of a slab-like magma chamber. As a layer of derivative magma reaches a critical thickness, a new layer is formed, generating a compositionally and density stratified magma chamber.

Socorro Island, Mexico, is an alkaline and peralkaline volcanic island located in the eastern Pacific Ocean on a mid-ocean ridge spreading center that was abandoned at ∼3.5 Ma. Silicic peralkaline rocks comprise up to 80% of the surface of the island, rendering Socorro virtually unique in the Pacific Ocean. Precise, replicate 40Ar/39Ar ages of 21 peralkaline trachytes and rhyolites reveal a history of episodic volcanic activity from ∼540 to 370 ka that may have culminated with caldera formation; repose periods between these episodes may have had maximum duration of ∼30 kyr. After up to 200 kyr of quiescence, 40Ar/39Ar ages indicate that postcaldera silicic peralkaline activity commenced by 180 ka, forming the Cerro Evermann Formation. Postcaldera mafic alkaline lavas of the Lomas Coloradas Formation erupted dominantly between 70 and 150 ka based upon relative age relations. The dominant lithology of precaldera and syncaldera silicic peralkaline deposits on Socorro is nonfragmental and nonvesicular and lacks lithic fragments and fiamme; despite this, numerous lines of evidence including welding zonation, presence of a proximal ignimbrite or co-ignimbrite deposit, association with a caldera, and compositional heterogeneity within eruptive units suggest that they are dominantly ash flow tuffs. A change in eruptive style, from predominantly explosive to predominantly effusive, followed caldera formation and suggests that a change in the efficacy of magma degassing may be linked to caldera formation. On the basis of the presence of a caldera, the magma chamber associated with Socorro Island is shallow and probably resides within the upper oceanic crust or the edifice. This together with a prolonged history of silicic magmatism indicates that intrusion of mafic magma maintained thermal viability of the magmatic plumbing system. The minimum calculated growth rate for the entire volcanic edifice (7 × 10−4 km3/yr) exceeds those of nonhotspot off-axis volcanoes in the Pacific by almost an order of magnitude. Eruption rates for subaerial phases on Socorro may be several orders of magnitude smaller than this growth rate and are comparable to subaerial eruption rates of isolated ocean islands related to mantle plumes.

The 7.05 Ma Rattlesnake Tuff covers ca. 9000 km², but the reconstructed original coverage was between 30000 and 40000 km². Thicknesses are remarkably uniform, ranging between 15 and 30 m for the most complete sections. Only 13% of the area is covered with tuff thicker than 30 m, to a maximum of 70 m. The present day estimated tuff volume is 130 km³ and the reconstructed magma volume of the outflow is 280 km³ DRE (dense rock equivalent). The source area of the tuff is inferred to be in the western Harney Basin, near the center of the tuff distribution, based mainly on a radial exponential decrease in average pumice size, and is consistent with a general radial decrease in welding and degree of post-emplacement crystallization. Rheomorphic tuff is found to a radius of 40–60 km from the inferred source. Four facies of welding and four of post-emplacement crystallization are distinguishable. They are: non-welded, incipiently welded, partially welded and densely welded zones; and vapor phase, pervasively devitrified, spherulite and lithophysae zones. The vapor phase, pervasively devitrified and lithophysae zones are divided into macroscopically distinguishable subzones. At constant thickness (20±3 m), and over a distance of 1–3 km, nonrheomorphic sections can cary between two extremes: (a) entirely vitric sections grading from nonwelded to incipiently welded; and (b) highly zoned sections. Highly zoned sections have a basal non- to densely welded vitric tuff overlain by a spherulite zone that grades upward through a lithophysae-dominated zone to a zone of pervasive devitrification, which, in turn, is overlain by a zone of vapor-phase crystallization and is capped by partially welded vitric tuff. A three-dimensional welding and crystallization model has been developed based on integrating local and regional variations of 85 measured sections. Strong local variations are interpreted to be the result of threshold-governed welding and crystallization controlled by residence time above a critical temperature, which is achieved through differences in thickness and accumulation rate.

The “PLUTONS: Investigating the Relationship between Pluton Growth and Volcanism in the Central Andes” themed issue of Geosphere is dedicated to the memory of Todd Christian Feeley, our friend, colleague, and mentor. He distinguished his career with a long string of insightful papers that integrated the geology and petrology of volcanic rocks. His work was always soundly based in field geology, and his coworkers remember him for his outstanding skills in the field.

[1] Strongly bimodal, basalt-rhyolite volcanism of the High Lava Plains Province of Oregon followed the Middle Miocene flood basalts of the Pacific Northwest and extends to recent time. During the 8 m.y. of volcanism recorded in the central High Lava Plains, in western Harney Basin, three distinct mafic magmatic trends originate from primitive high-alumina olivine tholeiites (HAOT); they are tholeiitic, calcalkaline and ferro-trachytic. Tholeiitic basalts occur throughout the history and their compositions are derived by crystal fractionation while traversing the crust and mixing with evolved mafic magmas. Scavenging of apatite from crustal rocks and minor contamination with felsic melts accounts for P, incompatible element enrichments and increasing tilts of incompatible element patterns with differentiation. The calcalkaline mafic suite occurs in temporal association with abundant silicic volcanism and is the only suite with Fe decreasing with Mg. Calcalkaline compositions are derived from evolved tholeiitic basalt by crystal fractionation coupled with assimilation of felsic crust or crustal melts. The ferro-trachytic suite occurs mainly late, is highly enriched in incompatible element with patterns parallel to tholeiites from which it is derived by protracted fractionation and recharge. The three suites primarily reflect changes in magma flux and crustal interactions in time. High magma flux promotes crustal melting and contamination of tholeiite to make the calcalkaline suite. On the other hand, ferrotrachytic magmas erupted mainly late in the sequence, during magmatic waning and after significant basaltification of the crust.

Thousands of cubic kilometers of dominantly intermediate composition, metaluminous magma erupted at approximately 35 Ma in eastern Nevada. Two stages of crustal contamination are inferred from detailed study of the earliest vocanic rocks. Fine-grained mafic rocks and rocks with mixed textures, ranging from basalt to rhyolite, were contaminated with a crustal component rich in Nd but poor in Sr. Overlying plagioclaserich andesites and dacites have greater Sr and 87Sr/86Sr, but less Nd and lower ɛNd and are interpreted to have been contaminated by a crustal component with the opposite elemental signature (i.e., poor in Nd but rich in Sr). The first contaminant represents a partial melt of the crust with plagioclase as a residual phase and the second contaminant is the residue of the partial melting event, or bulk crust. The net effect is bulk crustal assimilation, but in two “bites”. The separation of the crustal souree into two elementally distinct contaminants causes divergent trends with respect to the variation of 87Sr/ 86Sr versus eNd that could be misinterpreted to indicate the existence of isotopically distinct crustal reservoirs. Comparison of the calculated contaminants to melting relationships in pelites is consistent with the two contammants representing melt and residue at about 30% melting. The model age of the bulk crust is approximately 2.2 Ga, consistent with an early Proterozoic crustal province inferred by other workers.

The oxygen isotope systematics of Tertiary volcanic rocks of east-central Nevada and of plutonic and metamorphic rocks of the Ruby Mountains-East Humboldt Range core complex provide complementary evidence for major18O-depletion and 18O/16O homogenization of mid-crustal rocks during metamorphism and magmatism. The δ18O value of crustal source material for silicic volcanic rocks decreased from between +9 and +11‰ to between +7 and +8‰ over ∼ 5 Ma. Mid-crustal metasedimentary and granitic rocks in the East Humboldt Range have δ18O values very similar to the volcanic rocks and values are lower and more homogeneous at deeper structural levels. Exchange with deep-seated mantle-derived igneous rocks, or fluids derived therefrom, is the most plausible18O-depletion mechanism. Intrusion of these mafic magmas promoted crustal melting and fluid migration. Homogenization of 18O/16O resulted from migration of high-temperature fluids and melts at mid-crustal levels, and was less effective at higher structural levels where the crust was dominated by less permeable carbonate rocks.

The Egan Range volcanic complex lies 30 km northwest of Ely, on the edge of a highly extended domain in east-central Nevada. It consists mainly of lavas with subordinate tuffs and sedimentary rocks. The rocks are divided into three stratigraphic and lithologic groups that correlated with widespread middle Tertiary volcanic rocks associated with early stages of extension in the region. Volcanic rocks of the early group are predominantly two-pyroxene dacite and andesite lavas, all of which contain quenched, mafic inclusions and have compositions indicating they were derived by mixing between a contaminated mantle melt and a rhyodacitic crustal component. Rocks of the middle group are relatively homogeneous biotite, hornblende dacite and rhyodacite lavas. Elevated compatible and incompatible element concentrations and straight-line correlations of compositional data in the early and middle groups support a simple mixing model. Minor fractionation of clinopyroxene is required to explain some low Cr concentrations. Major element variations of the late group can be successfully modeled by crystal fractionation of observed phenocrysts accompanied by moderate assimilation of a crustal component to account for elevated Rb, Th, U, and light rare earth element concentrations. Rocks of all three groups appear to be related to a common primary magma type, the composition of which can be modeled from the mafic inclusions in the early group. Low Ni and Mg contents in the inclusions indicate that olivine was fractionated prior to their participation in mixing of early group magmas. Based on estimated volumes of volcanic rocks in the Egan Range volcanic complex and in the region, and on the petrologic models for each group, a significant amount of basalt must have been added to the crust during this middle Tertiary magmatic episode.

[1] The High Lava Plains province (HLP) is a late Cenozoic bimodal volcanic field at the northern margin of the Basin and Range province in southeastern Oregon that hosts a westward younging trend of silicic volcanism that crudely mirrors northeastward migration of silicic volcanism along the Yellowstone–Snake River Plain (YSRP) trend. We present 40Ar/39Ar ages for 19 rhyolite domes, 5 rhyolite ash flow tuffs, and 34 basaltic lavas from the HLP. The previously identified trend of westward migration of HLP rhyolites is confirmed. The rate of propagation is ∼33 km/m.y. from 10 to 5 Ma, slowing to ∼13 km/m.y. after 5 Ma. The duration of silicic volcanism at any locus is ∼2 m.y. Three older HLP dacite domes yielded ages of ∼15.5 Ma. Basalts are not age progressive. We identify several episodes of increased basaltic activity at 7.5–7.8, 5.3–5.9, and 2–3 Ma, with the younger episode likely continuing into the Recent. The HLP and YSRP trends emerged from the axis of middle Miocene basaltic volcanism of the Columbia River and Steens basalts. We propose a model in which (1) Miocene flood basalts and widespread silicic rocks are the result of emplacement of a plume head near the craton margin, enhanced by flow up a topographic gradient along the base of the lithosphere at the craton margin; (2) the HLP trend is the result of westward flow originating at the craton margin; and (3) the YSRP trend is the trace of the motion of the North American plate over the tail of the plume.

Following seismic activity in late September 2004, the current eruption of Mount St. Helens began with an explosive steam and ash emission on 1 October 2004, with hot dacite emerging from the crater floor on 11 October 2004. Nearly two years later, with more than 80 million cubic meters of erupted dacite, accompanied by rare explosions and predominantly shallow seismicity questions still remain about what initiated and what is sustaining the eruption. The U.S. Geological Survey's (USGS) Cascade Volcano Observatory (CVO) hosted the 2006 Mount St. Helens Petrology Workshop in Vancouver, Wash., on 27–30 August 2006. With many of the more than 40 workshop participants finalizing contributions to a USGS Professional Paper on the current Mount St. Helens eruption, the workshop was a timely opportunity to share results, reconcile interpretations, and plan future research.