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The Oligocene Lund Tuff, Great Basin, USA : a very large volume monotonous intermediate
MAUGHAN Larissa L. ; CHRISTIANSEN Eric H. ; BEST Myron G. ; ...
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 consis
Elsevier; Elsevier; Elsevier
ISSN : 0377-0273 CODEN : JVGRDQ
Journal of volcanology and geothermal research A. 2002, vol. 113, n° 1-2, pp. 129-157 [Illustrations : Tableau] [bibl. : 2 p.1/5]
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