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Titre du document / Document title

The Oligocene Lund Tuff, Great Basin, USA : a very large volume monotonous intermediate

Auteur(s) / Author(s)

MAUGHAN Larissa L. (1) ; CHRISTIANSEN Eric H. (1) ; BEST Myron G. (1) ; GROMME C. Sherman (2) ; DEINO Alan L. (3) ; TINGEY David G. (1) ;

Affiliation(s) du ou des auteurs / Author(s) Affiliation(s)

(1) Department of Geology, Brigham Young University, Provo, UT 84602, ETATS-UNIS
(2) U.S. Geological Survey, Menlo Park, CA 94025, ETATS-UNIS
(3) Berkeley Geochronology Center, Berkeley, CA 94709, ETATS-UNIS

Résumé / Abstract

Unusual monotonous intermediate ignimbrites consist of phenocryst-rich dacite that occurs as very large volume (> 1000 km3) deposits that lack systematic compositional zonation, comagmatic rhyolite precursors, and underlying plinian beds. They are distinct from countless, usually smaller volume, zoned rhyolite-dacite-andesite deposits that are conventionally believed to have erupted from magma chambers in which thermal and compositional gradients were established because of sidewall crystallization and associated convective fractionation. Despite their great volume, or because of it, monotonous intermediates have received little attention. Documentation of the stratigraphy, composition, and geologic setting of the Lund Tuff - one of four monotonous intermediate tuffs in the middle-Tertiary Great Basin ignimbrite province - provides insight into its unusual origin and, by implication, the origin of other similar monotonous intermediates. The Lund Tuff is a single cooling unit with normal magnetic polarity whose volume likely exceeded 3000 km3. It was emplaced 29.02 ± 0.04 Ma in and around the coeval White Rock caldera which has an unextended north-south diameter of about 50 km. The tuff is monotonous in that its phenocryst assemblage is virtually uniform throughout the deposit: plagioclase > quartz hornblende > biotite > Fe-Ti oxides sanidine > titanite, zircon, and apatite. However, ratios of phenocrysts vary by as much as an order of magnitude in a manner consistent with progressive crystallization in the pre-eruption chamber. A significant range in whole-rock chemical composition (e.g., 63-71 wt% SiO2) is poorly correlated with phenocryst abundance. These compositional attributes cannot have been caused wholly by winnowing of glass from phenocrysts during eruption, as has been suggested for the monotonous intermediate Fish Canyon Tuff. Pumice fragments are also crystal-rich, and chemically and mineralogically indistinguishable from bulk tuff. We postulate that convective mixing in a sill-like magma chamber precluded development of a zoned chamber with a rhyolitic top or of a zoned pyroclastic deposit. Chemical variations in the Lund Tuff are consistent with equilibrium crystallization of a parental dacitic magma followed by eruptive mixing of compositionally diverse crystals and high-silica rhyolite vitroclasts during evacuation and emplacement.

Revue / Journal Title

Journal of volcanology and geothermal research    ISSN  0377-0273   CODEN JVGRDQ 

Source / Source

2002, vol. 113, no1-2, pp. 129-157 (2 p.1/5)

Langue / Language

Anglais

Editeur / Publisher

Elsevier, Oxford, ROYAUME-UNI  (1976) (Revue)

Mots-clés anglais / English Keywords

Phanerozoic

;

Cenozoic

;

Tertiary

;

Paleogene

;

igneous rocks

;

volcanic rocks

;

pyroclastics

;

North America

;

Basin and Range Province

;

Ar-40/Ar-39

;

dating

;

mineral assemblages

;

calderas

;

electron probe data

;

magnetic polarity

;

cooling

;

chemical analysis

;

chemical composition

;

stratigraphy

;

magmatic differentiation

;

fractional crystallization

;

magma chambers

;

andesites

;

plinian-type eruptions

;

precursors

;

rhyolites

;

zoning

;

dacites

;

phenocrysts

;

ignimbrite

;

volume

;

ash-flow tuff

;

Oligocene

;

paleomagnetism

;

mixing

;

tuff

;

Great Basin

;

Mots-clés français / French Keywords

Phanérozoïque

;

Cénozoïque

;

Tertiaire

;

Paléogène

;

Roche ignée

;

Roche volcanique

;

Pyroclastite

;

Amérique du Nord

;

Province Basin and Range

;

Ar 40-Ar 39

;

Datation

;

Association minérale

;

Caldeira

;

Donnée microsonde électronique

;

Polarité magnétique

;

Refroidissement

;

Analyse chimique

;

Composition chimique

;

Stratigraphie

;

Différenciation magmatique

;

Cristallisation fractionnée

;

Chambre magmatique

;

Andésite

;

Eruption type plinien

;

Phénomène précurseur

;

Rhyolite

;

Zonation

;

Dacite

;

Phénocristal

;

Ignimbrite

;

Volume

;

Tuf coulée cendre

;

Oligocène

;

Paléomagnétisme

;

Mixage

;

Tuf volcanique

;

Great Basin

;

Mots-clés espagnols / Spanish Keywords

Fanerozoico

;

Cenozoico

;

Terciario

;

Paleógeno

;

Roca ignea

;

Roca volcánica

;

America del norte

;

Provincia Basin and Range

;

Fechado

;

Asociación mineral

;

Caldera

;

Dato sonda electrónica

;

Enfriamiento

;

Análisis químico

;

Estratigrafía

;

Diferenciación magmática

;

Cristalización fraccionada

;

Cámara magmática

;

Andesita

;

Fenómeno precursor

;

Riolita

;

Zonalidad

;

Dacita

;

Fenocristal

;

Ignimbrita

;

Volumen

;

Oligoceno

;

Paleomagnetismo

;

Mezcla

;

Toba volcánica

;

Localisation / Location

INIST-CNRS, Cote INIST : 17236, 35400010152008.0090

Nº notice refdoc (ud4) : 13686551



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