Origin and emplacement of the andesite of Burroughs Mountain,a zoned,large-volume lava flow at Mount Rainier,Washington, USA |
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| Institution: | 1. United States Geological Survey, Astrogeology Science Center, Flagstaff, AZ, USA;2. Sagan Center at the SETI Institute, Mountain View, CA, USA;3. NASA Ames Research Center, Moffett Field, CA, USA;4. Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA;5. Planetary Science Institute, Tucson, AZ, USA;1. U.S. Geological Survey, Integrated Information Dissemination Division, Middleton, WI, United States;2. Universidade do Estado de Minas Gerais (UEMG), Unidade Divinópolis, Belo Horizonte, Minas Gerais, Brazil;1. Laboratoire Magmas et Volcans, Université Blaise Pascal–CNRS–IRD, OPGC, 5 rue Kessler, 63038 Clermont Ferrand, France;2. Observatoire Volcanologique du Piton de la Fournaise, Institut de Physique du Globe de Paris, Sorbonne Paris Cité, Univ. Paris Diderot, CNRS, F-97418 La Plaine des Cafres, La Réunion, France;3. ISTerre, CNRS, Université de Savoie, Le Bourget-du-Lac, France;4. Institut de Recherche pour le Développement, Université de la Nouvelle-Calédonie, BPR4-98851 Nouméa Cedex, France;5. Vanuatu Meteorological and Geohazards Department, Port-Vila, Vanuatu |
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| Abstract: | Burroughs Mountain, situated at the northeast foot of Mount Rainier, WA, exposes a large-volume (3.4 km3) andesitic lava flow, up to 350 m thick and extending 11 km in length. Two sampling traverses from flow base to eroded top, over vertical sections of 245 and 300 m, show that the flow consists of a felsic lower unit (100 m thick) overlain sharply by a more mafic upper unit. The mafic upper unit is chemically zoned, becoming slightly more evolved upward; the lower unit is heterogeneous and unzoned. The lower unit is also more phenocryst-rich and locally contains inclusions of quenched basaltic andesite magma that are absent from the upper unit. Widespread, vuggy, gabbronorite-to-diorite inclusions may be fragments of shallow cumulates, exhumed from the Mount Rainier magmatic system. Chemically heterogeneous block-and-ash-flow deposits that conformably underlie the lava flow were the earliest products of the eruptive episode. The felsic–mafic–felsic progression in lava composition resulted from partial evacuation of a vertically-zoned magma reservoir, in which either (1) average depth of withdrawal increased, then decreased, during eruption, perhaps due to variations in effusion rate, or (2) magmatic recharge stimulated ascent of a plume that brought less evolved magma to shallow levels at an intermediate stage of the eruption. Pre-eruptive zonation resulted from combined crystallization–differentiation and intrusion(s) of less evolved magma into the partly crystallized resident magma body. The zoned lava flow at Burroughs Mountain shows that, at times, Mount Rainier’s magmatic system has developed relatively large, shallow reservoirs that, despite complex recharge events, were capable of developing a felsic-upward compositional zonation similar to that inferred from large ash-flow sheets and other zoned lava flows. |
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