HR: 10:45h
AN: V22A-02    [Abstracts]
TI: Eruptive and Intrusive History of Mount Mazama and the Crater Lake Region Explains Growth of a Young Silicic Magma Body
AU: * Bacon, C R
EM: cbacon@usgs.gov
AF: US Geological Survey, 345 Middlefield Road, MS 910, Menlo Park, CA 94025 United States
AU: Lanphere, M A
EM: alder@usgs.gov
AF: US Geological Survey, 345 Middlefield Road, MS 910, Menlo Park, CA 94025 United States
AU: Lowenstern, J B
EM: jlrnstrn@usgs.gov
AF: US Geological Survey, 345 Middlefield Road, MS 910, Menlo Park, CA 94025 United States
AB: Crater Lake caldera (subsidence area >25 km²) collapsed into a shallow long-lived magmatic system during the ~7.7 ka climactic eruption of Mount Mazama that ejected ~50 km³ of magma dominated by homogeneous crystal-poor rhyodacite pumice. Geologic mapping, Ar and U-Th geochronology, and geochemistry provide volume-time-composition information on the growth and evolution of the Mazama system. Granodiorite and related plutonic rocks in the climactic ejecta give a window into the intrusive component of that system. We interpret the eruptive and intrusive history in terms of the competition between (1) crystallization driven by degassing and hydrothermal cooling and (2) thermal input from a regional magma flux focused at Mazama. Throughout its ~400 kyr history, the andesite-dacite volcano known as Mount Mazama, built astride an earlier silicic dome field, was accompanied by nearby basaltic to andesitic effusive volcanism representing a range of differentiation of melts of depleted mantle containing varied amounts of water and subduction-related fluid-mobile elements. Similar parent magmas appear to have differentiated to form the products of Mount Mazama. The Mazama edifice was constructed in many comparatively brief episodes, some of the more voluminous being approximately coeval with volcanic pulses in the surrounding region. Magmas as evolved as dacite erupted many times, commonly associated with or following voluminous andesite effusion. It was not until ~30 ka, after 400 kyr of preconditioning, that a shallow rhyodacite-dominated magma reservoir began to grow. Between ~30 ka and the climactic eruption, rhyodacitic magmas vented mainly north of the edifice, unaccompanied by other compositions. From ~35 ka into postglacial time unusually primitive lavas (magnesian basaltic andesite and tholeiitic basalt) erupted west of Mazama suggest increased thermal input. Reconstruction of the Mazama climactic magma chamber from preclimactic rhyodacites and their enclaves and from andesitic scoriae and gabbroic cumulates ejected near the end of the climactic eruption suggests incremental generation of an average of 2.5 km³/kyr of rhyodacite dominantly by crystallization differentiation of basaltic to andesitic magma that was repeatedly intruded between cumulate mush and overlying silicic derivative magma. The mineralogy and elemental and radiogenic isotopic compositions of the granodiorite blocks are similar to those of volcanic rocks of Mount Mazama. Zircon SHRIMP U-Th geochronology gives a range of crystallization ages for granodiorites that correspond to times of dacite effusion. Because erupted silicic magmas were zircon undersaturated, zircon apparently grew late when magmas were mostly crystallized. Variation in crystallization age of blocks from different localities around the caldera suggests a composite pluton underlies Mount Mazama at shallow depth. The granodiorite blocks commonly are partially melted up to 50%, showing that the climactic magma chamber was melting its walls. Retention of zircon antecrysts in pre-35-ka magmas of the granodiorites implies recycling of zircon into high-crystallinity magmas whereas climactic rhyodacite appears to have consumed virtually all zircon from any assimilated wallrock. We conclude that (1) the composite pluton formed from relatively small volumes of derivative magma that crystallized before the upper crust had been heated sufficiently to sustain voluminous convecting crystal-poor melt and (2) that a large volume of eruptible silicic magma accumulated because of heating associated with input of relatively primitive magma to the roots of the system.
DE: 1035 Geochronology
DE: 1040 Isotopic composition/chemistry
DE: 3640 Igneous petrology
DE: 3655 Major element composition
DE: 8439 Physics and chemistry of magma bodies
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2005 Joint Assembly