Genesis of post-hotspot,A-type rhyolite of the Eastern Snake River Plain volcanic field by extreme fractional crystallization of olivine tholeiite |
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| Authors: | Michael McCurry Karl P Hayden Lee H Morse Stan Mertzman |
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| Institution: | (1) Department of Geosciences, Idaho State University, Pocatello, ID 83209, USA;(2) Department of Earth and Environment, Franklin & Marshall College, Lancaster, PA 17604-3003, USA |
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| Abstract: | Rhyolites occur as a subordinate component of the basalt-dominated Eastern Snake River Plain volcanic field. The basalt-dominated
volcanic field spatially overlaps and post-dates voluminous late Miocene to Pliocene rhyolites of the Yellowstone–Snake River
Plain hotspot track. In some areas the basalt lavas are intruded, interlayered or overlain by ~15 km3 of cryptodomes, domes and flows of high-silica rhyolite. These post-hotspot rhyolites have distinctive A-type geochemical
signatures including high whole-rock FeOtot/(FeOtot+MgO), high Rb/Sr, low Sr (0.5–10 ppm) and are either aphyric, or contain an anhydrous phenocryst assemblage of sodic sanidine
± plagioclase + quartz > fayalite + ferroaugite > magnetite > ilmenite + accessory zircon + apatite + chevkinite. Nd- and
Sr-isotopic compositions overlap with coeval olivine tholeiites (ɛNd = −4 to −6; 87Sr/86Sri = 0.7080–0.7102) and contrast markedly with isotopically evolved Archean country rocks. In at least two cases, the rhyolite
lavas occur as cogenetic parts of compositionally zoned (~55–75% SiO2) shield volcanoes. Both consist dominantly of intermediate composition lavas and have cumulative volumes of several 10’s
of km3 each. They exhibit two distinct, systematic and continuous types of compositional trends: (1) At Cedar Butte (0.4 Ma) the
volcanic rocks are characterized by prominent curvilinear patterns of whole-rock chemical covariation. Whole-rock compositions
correlate systematically with changes in phenocryst compositions and assemblages. (2) At Unnamed Butte (1.4 Ma) the lavas
are dominated by linear patterns of whole-rock chemical covariation, disequilibrium phenocryst assemblages, and magmatic enclaves.
Intermediate compositions in this group resulted from variable amounts of mixing and hybridization of olivine tholeiite and
rhyolite parent magmas. Interestingly, models of rhyolite genesis that involve large degrees of melting of Archean crust or
previously consolidated mafic or silicic Tertiary intrusions do not produce observed ranges of Nd- and Sr-isotopes, extreme
depletions in Sr-concentration, and cogenetic spectra of intermediate rock compositions for both groups. Instead, least-squares
mass-balance, energy-constrained assimilation and fractional crystallization modeling, and mineral thermobarometry can explain
rhyolite production by 77% low-pressure fractional crystallization of a basaltic trachyandesite parent magma (~55% SiO2), accompanied by minor (0.03–7%) assimilation of Archean upper crust. We present a physical model that links the rhyolites
and parental intermediate magmas to primitive olivine tholeiite by fractional crystallization. Assimilation, recharge, mixing
and fractional melting occur to limited degrees, but are not essential parts of the rhyolite formation process.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.
This paper constitutes part of a special issue dedicated to Bill Bonnichsen on the petrogenesis and volcanology of anorogenic
rhyolites. |
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| Keywords: | Yellowstone– Snake River Plain Hotspot Rhyolite genesis Bimodal volcanism A-type magma Basalt differentiation Fractional crystallization |
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