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Eruption of the Grande Ronde Basalt lavas, Columbia River Basalt Group: Results of numerical modeling

By
Sedelia Rodriguez
Sedelia Rodriguez
Department of Earth Sciences, Florida International University, Miami, Florida 33199, USA
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Gautam Sen
Gautam Sen
Department of Earth Sciences, Florida International University, Miami, Florida 33199, USA
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Published:
August 2013

The Grande Ronde Basalt lavas constitute ~63% of the Columbia River Basalt Group, a large igneous province in the NW United States. The lavas are aphyric or contain less than 5% phenocrysts of plagioclase, augite, pigeonite, and olivine (altered). Plagioclase hygrometry shows that the erupted lavas generally contained less than 0.3% dissolved H2O; however, the presence of rare disequilibrium An96 plagioclase phenocrysts suggests that some magmas may have originally had 4.5 wt% dissolved H2O at depth, but they all degassed during ascent and eruption. The size of plagioclase phenocrysts suggests an average plagioclase phenocryst residence time in the magmas of 160 yr. Ignoring hiatuses between eruptions, we estimate that the ~110 flows of the Grande Ronde Basalt erupted over a cumulative time of 17,600 yr, with an average eruption rate of ~8.6 km3/yr. The average interval between eruptions is estimated to be 3658 yr. It is envisaged that a shallow intrusive dike-sill complex, rather than large kilometer-sized magma chamber(s), fed the Grande Ronde basalt lavas.

We performed model simulations using the COMAGMAT software to retrace the pre-eruption histories of the Grande Ronde Basalt lavas. Based on such simulations and petrological reasoning, we find that the primary melts could have been generated from a spinel peridotite source at 1.5 GPa, perhaps under hydrous conditions. Extensive melting of lithospheric eclogite may have played an important role as well; however, this is not constrained by our simulations. All lavas were contaminated by the crust, and they were last processed (mixing, crystallization) during their short residence within shallow (6 km) intrusive rocks prior to eruption. Our petrologic conclusions lead us to present a petrotectonic model that supports the hypothesis that the Columbia River Basalt Group magma generation was greatly aided by a thinned lithosphere and H2O that may have come off the asthenospheric wedge.

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GSA Special Papers

The Columbia River Flood Basalt Province

Stephen P. Reidel
Stephen P. Reidel
School of the Environment, Washington State University-Tri-Cities, 2710 Crimson Way, Richland, Washington 99354, USA
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Victor E. Camp
Victor E. Camp
Department of Geological Sciences, San Diego State University, 5500 Campanile Drive, San Diego, California 92182, USA
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Martin E. Ross
Martin E. Ross
Department of Marine and Environmental Sciences, Northeastern University, Boston, Massachusetts 02115, USA
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John A. Wolff
John A. Wolff
School of the Environment, Washington State University, Pullman, Washington 99164, USA
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Barton S. Martin
Barton S. Martin
Department of Geology and Geography, Ohio Wesleyan University, 61 South Sandusky Street, Delaware, Ohio 43015, USA
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Terry L. Tolan
Terry L. Tolan
Department of Geology, Portland State University, Portland, Oregon 97201, USA
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Ray E. Wells
Ray E. Wells
U.S. Geological Survey, 345 Middlefield Road, MS 973, Menlo Park, California 94025, USA
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Geological Society of America
Volume
497
ISBN print:
9780813724973
Publication date:
August 01, 2013

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