Michael McCurry

Contrasting origins of Cenozoic silicic volcanic rocks from the western Cordillera of the United States

Two fundamentally different types of silicic volcanic rocks formed during the Cenozoic of the western Cordillera of the United States. Large volumes of dacite and rhyolite, mostly ignimbrites, erupted in the Oligocene in what is now the Great Basin and contrast with rhyolites erupted along the Snake River Plain during the Late Cenozoic. The Great Basin dacites and rhyolites are generally calc-alkaline, magnesian, oxidized, wet, cool (<850°C), Sr-and Al-rich, and Fe-poor. These silicic rocks are interpreted to have been derived from mafic parent magmas generated by dehydration of oceanic lithosphere and melting in the mantle wedge above a subduction zone. Plagioclase fractionation was minimized by the high water fugacity and oxide precipitation was enhanced by high oxygen fugacity. This resulted in the formation of Si-, Al-, and Sr-rich differentiates with low Fe/Mg ratios, relatively low temperatures, and declining densities. Magma mixing, large proportions of crustal assimilation, and polybaric crystal fractionation were all important processes in generating this Oligocene suite. In contrast, most of the rhyolites of the Snake River Plain are alkaline to calc-alkaline, ferroan, reduced, dry, hot (830–1,050°C), Sr-and Al-poor, and Nb-and Fe-rich. They are part of a distinctly bimodal sequence with tholeiitic basalt. These characteristics were largely imposed by their derivation from parental basalt (with low fH2O and low fO2) which formed by partial melting in or above a mantle plume. The differences in intensive parameters caused early precipitation of plagioclase and retarded crystallization of Fe–Ti oxides. Fractionation led to higher density magmas and mid-crustal entrapment. Renewed intrusion of mafic magma caused partial melting of the intrusive complex. Varying degrees of partial melting, fractionation, and minor assimilation of older crust led to the array of rhyolite compositions. Only very small volumes of distinctive rhyolite were derived by fractional crystallization of Fe-rich intermediate magmas like those of the Craters of the Moon-Cedar Butte trend.

'Snake River (SR)-type' volcanism at the Yellowstone hotspot track: Distinctive products from unusual, high-temperature silicic super-eruptions

A new category of large-scale volcanism, here termed Snake River (SR)-type volcanism, is defined with reference to a distinctive volcanic facies association displayed by Miocene rocks in the central Snake River Plain area of southern Idaho and northern Nevada, USA. The facies association contrasts with those typical of silicic volcanism elsewhere and records unusual, voluminous and particularly environmentally devastating styles of eruption that remain poorly understood. It includes: (1) large-volume, lithic-poor rhyolitic ignimbrites with scarce pumice lapilli; (2) extensive, parallel-laminated, medium to coarse-grained ashfall deposits with large cuspate shards, crystals and a paucity of pumice lapilli; many are fused to black vitrophyre; (3) unusually extensive, large-volume rhyolite lavas; (4) unusually intense welding, rheomorphism, and widespread development of lava-like facies in the ignimbrites; (5) extensive, fines-rich ash deposits with abundant ash aggregates (pellets and accretionary lapilli); (6) the ashfall layers and ignimbrites contain abundant clasts of dense obsidian and vitrophyre; (7) a bimodal association between the rhyolitic rocks and numerous, coalescing low-profile basalt lava shields; and (8) widespread evidence of emplacement in lacustrine-alluvial environments, as revealed by intercalated lake sediments, ignimbrite peperites, rhyolitic and basaltic hyaloclastites, basalt pillow-lava deltas, rhyolitic and basaltic phreatomagmatic tuffs, alluvial sands and palaeosols. Many rhyolitic eruptions were high mass-flux, large volume and explosive (VEI 6–8), and involved H2O-poor, low-δ18O, metaluminous rhyolite magmas with unusually low viscosities, partly due to high magmatic temperatures (900–1,050°C). SR-type volcanism contrasts with silicic volcanism at many other volcanic fields, where the fall deposits are typically Plinian with pumice lapilli, the ignimbrites are low to medium grade (non-welded to eutaxitic) with abundant pumice lapilli or fiamme, and the rhyolite extrusions are small volume silicic domes and coulées. SR-type volcanism seems to have occurred at numerous times in Earth history, because elements of the facies association occur within some other volcanic fields, including Trans-Pecos Texas, Etendeka-Paraná, Lebombo, the English Lake District, the Proterozoic Keewanawan volcanics of Minnesota and the Yardea Dacite of Australia.

The evolution of a silicic magma system: isotopic and chemical evidence from the Woods Mountains volcanic center, eastern California

The isotopic compositions of Nd and Sr and concentrations of major and trace elements were measured in flows and tuffs of the Woods Mountains volcanic center of eastern California to assess the relative roles of mantle versus crustal magma sources and of fractional crystallization in the evolution of silicic magmatic systems. This site was chosen because the contrast in isotopic composition between Precambrian-to-Mesozoic country rocks and the underlying mantle make the isotope ratios sensitive indicators of the proportions of crustal- and mantle-derived magma. The major eruptive unit is the Wild Horse Mesa tuff (15.8 m.y. old), a compositionally zoned rhyolite ignimbrite. Trachyte pumice fragments in the ash-flow deposits provide information on intermediate composition magma types. Crustal xenoliths and younger flows of basalt and andesite (10 m.y. old) provide opportunities to confirm the isotopic compositions of potential mantle and crustal magma sources inferred from regional patterns. The trachyte and rhyolite have ɛNd values of -6.2 to -7.5 and initial 87Sr/86Sr ratios mostly between 0.7086 and 0.7113. These magmas cannot have been melted directly from the continental basement because the ɛNd values are too high. They also cannot have formed by closed system fractional crystallization of basalt because the 87Sr/86Sr ratios are higher than likely values for parental basalt. Both major and trace element variations indicate that crystal fractionation was an important process. These results require that the silicic magmas are end products of the evolution of mantle-derived basalt that underwent extensive fractional crystallization accompanied by assimilation of crustal rock. The mass fraction of crustal components in the trachyte and rhyolite is estimated to be between 10% and 40%, with the lower end of the range considered more likely. The generation of magmas with SiO2 contents greater than 60% appears to be dominated by crystal fractionation with minimal assimilation of upper crustal rocks.

Paleogeographic implications of non-North American sediment in the Mesoproterozoic upper Belt Supergroup and Lemhi Group, Idaho and Montana, USA

A non-North American provenance for the lower Belt Supergroup of North America has been used to support various pre-Rodinian paleogeographic reconstructions. Unlike the lower Belt Supergroup, most upper Belt Supergroup provenance studies have inferred Laurentian sediment sources. We test this hypothesis by analyzing U-Pb and Lu-Hf isotopes on detrital zircons, and whole-rock Nd isotopes from the Missoula (upper Belt Supergroup) and Lemhi Groups, and comparing to possible Laurentian sources. Detrital zircons from 11 sandstones analyzed show dominant ages between 1680 and 1820 Ma. These zircons are predominantly magmatic in paragenesis. Belt Supergroup–aged (1400–1470 Ma) and 2400–2700 Ma populations represent minor components. Lu-Hf isotopic analyses for 1675–1780 Ma Missoula Group and Lemhi Group detrital zircons range from eHf(i) +9 to –12 and +8 and –7, respectively. Belt Supergroup–aged grains from the Bonner Formation, Missoula Group, have eHf(i) values between +5 and –9, exceeding coeval ranges from the Mojave and Yavapai terranes [eHf(i) between +5 and 0]. Whole-rock Nd isotopes from Lemhi Group argillites yield a range in eNd(1400) between +1.1 and –5.9. Immature feldspathic sediment, nearly unimodal detrital zircon spectra, and dissimilar Belt Supergroup–aged zircon Hf signatures suggest that distal portions of the Yavapai and Mojave terranes intruded by A-type magmas were not the source for the Missoula and Lemhi Groups. Instead, a slightly modified Mesoproterozoic proto-SWEAT (southwestern United States and East Antarctica) model can best account for the sedimentologic and isotopic characteristics of the Missoula and Lemhi Groups. An alternative model with a source from southeastern Siberia and the Okhotsk Massif is less preferred.

Isotopic evidence on the origin of compositional layering in an epizonal magma body

A detailed isotopic study of the Oligocene age (36 Ma), alkaline composition Organ Needle pluton in south-central New Mexico was undertaken to test models for the generation of compositional layering in silicic, epizonal magma bodies. The pluton is isotopically heterogeneous with its alkali feldspar granite composition cap (73–76% SiO2) having lower initial ϵNd and higher 87Sr86Sr ratio than the underlying main syenite (−5 vs. −2 and ∼ 0.709 vs. ∼ 0.706, respectively). Both lithologies have isotopic compositions significantly different from those of the Precambrian granite wall-rock (ϵNd ∼ −12.1 and 87Sr86Sr ∼ 0.784 at 36 Ma). The isotope data indicate that none of the lithologies of the pluton represent the products solely of roof or wall-rock melting, and that the capping granite could not have been derived in a closed-system from differentiation of the underlying syenitic magma. However, the capping granite has isotopic compositions similar to those of a chemically heterogeneous inequigranular syenite found along the margin of the pluton at its deepest exposed level (4–6 km paleodepth). Field observations, and new 40Ar39Ar age determinations, confirm that this lithology was comagmatic with the remainder of the pluton. We conclude that the capping granitic magma was derived from buoyant ϵNd ∼ −5 magma rising along the margins of the magma chamber and that the inequigranular syenite preserves a remnant of this sidewall magma. The main syenite body was encapsulated in this marginal, lower ϵNd, magma, while undergoing closed-system differentiation, most likely from a mafic progenitor. The ϵNd = −5 magma may represent mafic magma initially equivalent isotopically to the main syenite but which subsequently assimilated Precambrian wall-rock (∼ 10% by mass) at the base of the magma system.

Mid-Miocene record of large-scale Snake River–type explosive volcanism and associated subsidence on the Yellowstone hotspot track: The Cassia Formation of Idaho, USA

The 1.95-km-thick Cassia Formation, defined in the Cassia Hills at the southern margin of the Snake River Plain, Idaho, consists of 12 refined and newly described rhyolitic members, each with distinctive field, geochemical, mineralogical, geochronological, and paleomagnetic characteristics. It records voluminous high-temperature, Snake River−type explosive eruptions between ca. 11.3 Ma and ca. 8.1 Ma that emplaced intensely welded rheomorphic ignimbrites and associated ash-fall layers. One ignimbrite records the ca. 8.1 Ma Castleford Crossing eruption, which was of supereruption magnitude (∼1900 km 3 ). It correlates regionally and exceeds 1.35 km thickness within a subsided, proximal caldera-like depocenter. Major- and trace-element data define three successive temporal trends toward less-evolved rhyolitic compositions, separated by abrupt returns to more-evolved compositions. These cycles are thought to reflect increasing mantle-derived basaltic intraplating and hybridization of a midcrustal region, coupled with shallower fractionation in upper-crustal magma reservoirs. The onset of each new cycle is thought to record renewed intraplating at an adjacent region of crust, possibly as the North American plate migrated westward over the Yellowstone hotspot. A regional NE-trending monocline, here termed the Cassia monocline, was formed by synvolcanic deformation and subsidence of the intracontinental Snake River basin. Its structural and topographic evolution is reconstructed using thickness variations, offlap relations, and rheomorphic transport indicators in the successive dated ignimbrites. The subsidence is thought to have occurred in response to incremental loading and modification of the crust by the mantle-derived basaltic magmas. During this time, the area also underwent NW-trending faulting related to opening of the western Snake River rift and E-W Basin and Range extension. The large eruptions probably had different source locations, all within the subsiding basin. The proximal Miocene topography was thus in marked contrast to the more elevated present-day Yellowstone plateau.

Complex magma mixing, mingling, and withdrawal associated with an intra-Plinian ignimbrite eruption at a large silicic caldera volcano: Los Humeros of central Mexico

Los Humeros is the largest caldera volcano in the Mexican volcanic belt. Its second largest caldera-forming eruption, the ca. 0.1 Ma Zaragoza eruption, is recorded by two Plinian pumice-fall layers and a zoned intra-Plinian ignimbrite. Diverse pumice types within the ignimbrite provide insights about the way that different magmas within a single magmatic system interact, and the way in which this can give rise to a major explosive ignimbrite-forming eruption. Normal-and-reverse compositional zoning in the ignimbrite is defined by vertical variations in the relative abundance of rhyodacitic (69–71 wt%% SiO 2 ) and andesitic (54–63 wt% SiO 2 ) pumice lapilli: Lower parts are dominated by rhyodacite and pass gradationally up into a central part with andesitic and rhyodacite pumice, and this passes up into a rhyodacitic uppermost part, with no andesite. Petrographic and microprobe analyses of coexisting glass and phenocrysts provide mixed evidence of equilibrium and disequilibrium conditions in the magmas at the time of eruption. The Fe-Ti oxides record magma temperatures of ∼850 °C (andesite) and 780 °C (rhyodacite). The andesitic pumice contains euhedral labradorite (∼An 60 ), and orthopyroxene and clinopyroxene, in a dacitic glass groundmass, which yield equilibrium Na-Ca K d pl/liq and Fe-Mg K d pl/liq ratios. It also contains highly calcic plagioclase (to An 82 ) that in some cases is highly resorbed and mantled by the more sodic plagioclase, which may record early mixing between andesitic and plagioclase-bearing basaltic magmas, followed by equilibrium crystallization within the hybrid magma. The rhyodacite contains euhedral crystals of more-evolved plagioclase (∼An 30-40 ) and euhedral pyroxenes in a rhyolitic glass groundmass (74–75 wt% SiO 2 ). The pyroxenes yield disequilibrium Fe-Mg K d pl/liq ratios and indicate formation from a liquid that was more mafic than the liquid that formed the glass groundmass of the dacitic pumice. Subordinate pumices with interbanded rhyodacite and a scarcity of intermediate-composition pumices indicate that the magmas remained separate for most of the time, and mingled only immediately prior to, and during eruptive quenching. Rather than a simple density-stratified magma chamber, the Zaragoza eruption may have occurred in response to intrusion of a hybridized andesitic magma into a rhyodacitic magma reservoir, possibly arranged as semiconnected high-melt lenses or zones within a partially consolidated crystal mush. However, contrary to assumptions of simple replenishment, tapping, and fractionation-type systems, the Zaragoza magmas contain no record of previously erupted highly evolved rhyolites that developed when zircon joined the fractionating assemblage. This absence indicates that the highly evolved rhyolites had either been completely tapped or solidified prior to the Zaragoza eruption eruption, or that interaction was prevented by contrasting magma densities and viscosities.

Identification of mineral phases on basalt surfaces by imaging SIMS

A method for the identification of mineral phases on basalt surfaces utilizing secondary ion mass spectrometry (SIMS) with imaging capability is described. The goal of this work is to establish the use of imaging SIMS for characterization of the surface of basalt. The basalt surfaces were examined by interrogating the intact basalt (heterogeneous mix of mineral phases) as well as mineral phases that have been separated from the basalt samples. Mineral separates from the basalt were used to establish reference spectra for the specific mineral phases. Electron microprobe and X-ray photoelectron spectroscopy were used as supplemental techniques for providing additional characterization of the basalt. Mineral phases that make up the composition of the basalt were identified from single-ion images which were statistically grouped. The statistical grouping is performed by utilizing a program that employs a generalized learning vector quantization technique. Identification of the mineral phases on the basalt surface is achieved by comparing the mass spectra from the statistically grouped regions of the basalt to the mass spectral results from the mineral separates. The results of this work illustrate the potential for using imaging SIMS to study adsorption chemistry at the top surface of heterogeneous mineral samples.

Petrogenesis and volcanology of anorogenic rhyolites: a special issue dedicated to Bill Bonnichsen

The occurrence of large volume, high-temperature silicic volcanic rocks is apparently confined to continental regions and is a manifestation of significant energy input into the continental crust over extended time periods. In the western United States, such magmatism occurs predominantly in extensional settings in the Cenozoic Great Basin and TransPecos (west Texas) volcanic provinces. It is particularly prolific in the late Cenozoic Snake River Plain-Yellowstone province that forms a time-transgressive ‘hot spot’ track across southern Idaho, where rhyolites were extruded as large lava flows and even larger ignimbrites. Volumes of individual eruptions range up to at least 100 km 3 for the lava flows and to as much as 3,000 km 3 for the ignimbrites. Cumulative volumes of silicic magma produced likely exceed 10 5 km 3 , making this province one of the most prolific rhyolite outpourings on Earth. These rhyolites are characterized by high magmatic temperatures (sometimes exceeding 1,000°C), anhydrous phenocryst assemblages, low oxygen fugacities, and A-type trace element chemistry. In all these respects the “anorogenic” rhyolites contrast sharply with silicic magmas from “orogenenic” settings (i.e., convergent margins and mid-Cenozoic calcalkaline silicic magmas of the Great Basin and Colorado Plateau including