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Journal ArticleDOI

Volcanic History of the San Juan Mountains, Colorado, as Indicated by Potassium–Argon Dating

01 Aug 1970-Geological Society of America Bulletin (Geological Society of America)-Vol. 81, Iss: 8, pp 2329-2352
TL;DR: In this paper, the gross petrologic evolution throughout the San Juan remnant of this field was relatively simple, with initial intermediate lavas and breccias, followed closely in time by more silicicic ash-flow tuffs, and ending with a bimodal association of basalt and rhyolite.
Abstract: Volcanic rocks in the San Juan Mountains constitute the largest erosional remnant of a once nearly continuous volcanic field that extended over much of the southern Rocky Mountains and adjacent areas in Oligocene and later time. Recent regional studies have shown that the gross petrologic evolution throughout the San Juan remnant of this field was relatively simple, with initial intermediate lavas and breccias, followed closely in time by more silicic ash-flow tuffs, and ending with a bimodal association of basalt and rhyolite. More limited data from other remnants of the original field indicate a similar evolution. In the San Juan field, voluminous early lavas and breccias—mainly alkali andesite, rhyodacite, and mafic quartz latite—were erupted from numerous scattered central volcanoes onto an eroded tectonically stable terrane. They formed mostly during the interval 35 to 30 m.y. ago, but some probably were erupted earlier and others up to several million years later. About 30 m.y. ago, major volcanic activity changed to explosive ash-flow eruptions of quartz latite and low-silica rhyolite that persisted until about 26 m.y. ago. Source areas for the ash flows are marked by large calderas in the central and western San Juan Mountains. Two groups of lavas and associated rocks of intermediate composition intertongue with the ash-flow sequence: (1) quartz latitic lavas that were erupted in and adjacent to caldera structures and are genetically related to the ash-flow activity; and (2) other, generally more mafic lavas and related rocks that are widely distributed without evident structural relation to the ash-flow eruptive centers. The second group apparently represents a continuation of the early intermediate activity into the period of major ash-flow eruption. In the early Miocene the character of volcanism changed notably. Whereas the Oligocene volcanics are predominantly intermediate lavas and related silicic differentiates, the younger rocks are largely a bimodal association of basalt and high-silica alkali rhyolite. Basalt and minor rhyolite were erupted intermittently through the Miocene and Pliocene, and at one time formed a widespread thin veneer over the older volcanic terrane. The marked contrast between the Oligocene intermediate to low-silica rhyolitic magmas and the later basaltic and rhyolitic magmas implies either different conditions of magma generation or processes of differentiation for the two suites. This petrologic change coincides approximately in time with nearby development of the Rio Grande depression, a major rift that is the local expression of widespread late Cenozoic crustal extension. Whatever the cause of the petrologic change, the progression from predominantly intermediate to bimodal basalt-rhyolite volcanism, approximately concurrent with initiation of late Tertiary crustal extension, appears characteristic of Cenozoic volcanism for much of the western interior United States.
Citations
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Journal ArticleDOI
TL;DR: In this paper, a compilation of about one hundred estimates of volumetric rates of magma emplacement and volcanic output that are average rates for durations of igneous activity greater than 300 yrs.

772 citations

Book
01 Jan 2001
TL;DR: Oversby and others as mentioned in this paper pointed out that these large linear blocks are not all necessarily related to specific eruptive events but are more complex synvolcanic subsided blocks, and they regarded the blocks as first-order volcano-tectonic subsidence features that include individual cauldrons, which they considered to be second-order volcanic subsidence structures.
Abstract: Early Permian volcanic sections preserved in north-central Queensland to lie within volcanic subsidence structures; many of these features clearly are volcanic cauldrons or subvolcanic ring structures, but some are quite large graben-like structures. Oversby and others (1980) later pointed out that these large linear blocks are not all necessarily related to specific eruptive events but are more complex synvolcanic subsided blocks. They regarded the blocks as first-order volcano-tectonic subsidence features that include the individual cauldrons, which they considered to be second-order volcanic subsidence structures. Mackenzie (1987; 1993; Mackenzie and others, 1993) further emphasized the extensional tectonic setting of the volcanism and the range of styles of associated subsidence—graben-like, sag-like, and ring-faulted. The volcanic history and structural pattern of the Yellowstone-Snake River Plain region suggests the further possibility that some of the large linear blocks of northern Queensland, largely filled with volcanic deposits, may be extensional tectonic features analogous to the downdropped fault blocks of the region around Yellowstone and the eastern Snake River Plain, in which are preserved sections of welded tuffs erupted from the associated ring and cauldron complexes and emplaced as ash flows into adjacent tectonic fault basins. Some of the larger subsided blocks might have been synchronous with volcanism but related to regional tectonic stresses rather than to subsidence resulting directly from the transfer of magma from the crust to the surface. The associated cauldron complexes would represent the subvolcanic parts of calderas related to individual volcanic cycles climaxed by major ashflow eruptions. Furthermore, some of the associated granites might be cogenetic late-stage discordant plutons such as I have suggested to be associated with the polybatholithic complex beneath Yellowstone.

429 citations

Journal ArticleDOI
TL;DR: In this paper, the authors compare a variety of growth and resorption histories of the Fish Canyon Tuff and Nutras Creek Dacite (a volumetrically minor lava).
Abstract: from >5·5–6 to 7·7–8·5 wt % Al2O3). Homogeneity in magma More than 5000 km of nearly compositionally homogeneous crystalcomposition at the chamber-wide scale, contrasting with extreme rich dacite (>68 wt % SiO2: >45% Pl + Kfs + Qtz + textural and chemical complexities at the centimeter–millimeter scale, Hbl + Bt + Spn + Mag + Ilm + Ap + Zrn + Po) is consistent with a dynamic environment, wherein crystals with a erupted from the Fish Canyon magma body during three phases: (1) variety of growth and resorption histories were juxtaposed shortly the pre-caldera Pagosa Peak Dacite (an unusual poorly fragmented before eruption by convective currents. pyroclastic deposit, >200 km); (2) the syn-collapse Fish Canyon Tuff (one of the largest known ignimbrites, >5000 km); (3) the post-collapse Nutras Creek Dacite (a volumetrically minor lava). The late evolution of the Fish Canyon magma is characterized by

374 citations

Journal ArticleDOI
TL;DR: In this paper, it was shown that low-density upper-crustal rocks, inferred to be plutons, are 10 km or more thick beneath many calderas.
Abstract: Recent inference that Mesozoic Cordilleran plutons grew incrementally during >10 6 yr intervals, without the presence of voluminous eruptible magma at any stage, minimizes close associations with large ignimbrite calderas. Alternatively, Tertiary ignimbrites in the Rocky Mountains and elsewhere, with volumes of 1–5 × 10 3 km 3 , record multistage histories of magma accumulation, fractionation, and solidifi cation in upper parts of large subvolcanic plutons that were suffi ciently liquid to erupt. Individual calderas, up to 75 km across with 2–5 km subsidence, are direct evidence for shallow magma bodies comparable to the largest granitic plutons. As exemplifi ed by the composite Southern Rocky Mountain volcanic fi eld (here summarized comprehensively for the fi rst time), which is comparable in areal extent, magma composition, eruptive volume, and duration to continental-margin volcanism of the central Andes, nested calderas that erupted compositionally diverse tuffs document deep composite subsidence and rapid evolution in subvolcanic magma bodies. Spacing of Tertiary calderas at distances of tens to hundreds of kilometers is comparable to Mesozoic Cordilleran pluton spacing. Downwind ash in eastern Cordilleran sediments records large-scale explosive volcanism concurrent with Mesozoic batholith growth. Mineral fabrics and gradients indicate unifi ed fl owage of many pluton interiors before complete solidifi cation, and some plutons contain ring dikes or other textural evidence for roof subsidence. Geophysical data show that low-density upper-crustal rocks, inferred to be plutons, are 10 km or more thick beneath many calderas. Most ignimbrites are more evolved than associated plutons; evidence that the subcaldera chambers retained voluminous residua from fractionation. Initial incremental pluton growth in the upper crust was likely recorded by modest eruptions from central volcanoes; preparation for calderascale ignimbrite eruption involved recurrent magma input and homogenization high in the chamber. Some eroded calderas expose shallow granites of similar age and composition to tuffs, recording sustained postcaldera magmatism. Plutons thus provide an integrated record of prolonged magmatic evolution, while volcanism offers snapshots of conditions at early stages. Growth of subvolcanic batholiths involved sustained multistage opensystem processes. These commonly involved ignimbrite eruptions at times of peak power input, but assembly and consolidation processes continued at diminishing rates long after peak volcanism. Some evidence cited for early incremental pluton assembly more likely records late events during or after volcanism. Contrasts between relatively primitive arc systems dominated by andesitic compositions and small upper-crustal plutons versus more silicic volcanic fi elds and associated batholiths probably refl ect intertwined contrasts in crustal thickness and magmatic power input. Lower power input would lead to a Cascade- or Aleutian-type arc system, where intermediate-composition magma erupts directly from middle- and lowercrustal storage without development of large shallow plutons. Andean and southern Rocky Mountain–type systems begin similarly with intermediate-composition volcanism, but increasing magma production, perhaps triggered by abrupt changes in plate boundaries, leads to development of larger upper-crustal reservoirs, more silicic compositions, large ignimbrites, and batholiths. Lack of geophysical evidence for voluminous eruptible magma beneath young calderas suggests that near-solidus plutons can be rejuvenated rapidly by high-temperature mafi c recharge, potentially causing large explosive eruptions with only brief precursors.

357 citations


Cites background from "Volcanic History of the San Juan Mo..."

  • ...…Range trend, widely scattered intermediate-composition centers (dominantly andesite, lesser volumes of dacite, and minor rhyolite) erupted lava fl ows and fl anking volcaniclastic breccias in the San Juan region starting at ca. 35 Ma (Lipman et al., 1970; McIntosh and Lipman, unpublished data)....

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  • ...…from https://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/3/1/42/853201/i1553-040x-3-1-42.pdf by guest on 28 September 2019 46 Geosphere, February 2007 and peak volcanism largely preceded the bulk of extension (Lipman et al., 1970; Coney and Reynolds, 1977; Lipman, 1992; Chapin et al., 2004)....

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  • ...Composite volumes of the early-intermediate volcanoes are large—in the San Juan region, stratigraphic sequences commonly are more than a kilometer thick, and total volume, estimated at 25,000 km3 (Lipman et al., 1970), is roughly double that of the later-erupted ignimbrites....

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  • ...…involves initial eruption of intermediatecomposition lavas from central volcanoes, followed by one or more large ignimbrites of more silicic composition; concurrent caldera subsidence is central within the area of prior lava vents (e.g., Lipman et al., 1970; Elston, 1984; Steven et al., 1984)....

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Journal ArticleDOI
TL;DR: The inception of basaltic, alkalic, or bimodal volcanism and associated regional extension of inland areas appears to date the times at which plate-tectonic boundaries between North America and two Pacific plates underwent drastic changes.
Abstract: A major change in volcanic associations and their tectonic settings occurred in much of the Western United States during late Cenozoic time. Where this volcano-tectonic transition can be documented, an earlier orogenic and post-orogenic association of predominantly calc-alkalic andesitic rocks typical of circum-Pacific continental margin and island arcs was succeeded by fundamentally basaltic volcamsm which accompanied regional normal and strike-slip faulting. The igneous fields regarded here as fundamentally basaltic include: (1) basaltic fields, (2) alkalic fields in which differentiated igneous series commonly can be related to alkali-basaltic parent magmas, and (3) bimodal associations of mafic and silicic rocks, generally basalts and high-silica rhyolites. Similar igneous fields occur in other regions of the world characterized by tectonic extension. The nature and timing of the late Genozoic volcano-tectonic transition in various areas of the Western United States are documented from published references. The transition began in the southeastern part of the region in latest Oligocene time and moved northwestward through Miocene, Pliocene, and Quaternary time. The inception of basaltic, alkalic, or bimodal volcanism and associated regional extension of inland areas appears to date the times at which plate-tectonic boundaries between North America and two Pacific plates underwent drastic changes. These changes resulted from collision of the East Pacific Rise with a mid-Tertiary continental-margin trench and resulting direct contact of the American and western Pacific plates along a right-lateral transform fault system. These platetectonic interactions have evolved continuously and have determined the volcanic and tectonic evolution of the Western United States for the last 30 million years.

353 citations