Genesis of post-hotspot, A-type rhyolite of the Eastern Snake River Plain volcanic field by extreme fractional crystallization of olivine tholeiite

Michael McCurry, Karl P. Hayden, Lee H. Morse, Stan Mertzman

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71 Scopus citations

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 (εNd=-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.

Original languageEnglish (US)
Pages (from-to)361-383
Number of pages23
JournalBulletin of Volcanology
Volume70
Issue number3
DOIs
StatePublished - Jan 2008

Bibliographical note

Funding Information:
Acknowledgements This work is supported by EG&G, Idaho Contract C84-110421 to M. McCurry. We thank Bill Hackett (ISU) and Mel Kuntz (USGS) for introducing us to the many wonders of Eastern Snake River Plain volcanic geology. We gratefully acknowledge and thank Hanna Nekvasil and Don Lindsley (SUNY Stony Brook), Bill Bonnichsen (IGS), Marty Godchaux, Dick Smith (INL), Carol Frost (UW), Bill Leeman (NSF), Anita Grunder (OSU), Dennis Geist (UI), Scott Hughes and Dave Rodgers (ISU) for many helpful and insightful discussions, Duane Champion (USGS) for assistance in drilling rock samples at Cedar Butte and for unpublished paleomagnetic data from Cedar Butte and CH-1, Scott Hughes for assistance in INAA analysis, Linda Davis (USGS) for assistance with core from the USGS core library, and Regan Grandy for preparation of heavy mineral separates from Cedar Butte. Mertzman would like to acknowledge the financial support of the NSF and Franklin and Marshall College in the purchase of the Panalytical 2404 X-ray spectrometer and to thank Karen and Julie Mertzman for their long hours of careful work in the lab. We also thank Keith Putirka, Anita Grunder and Eric Christiansen for reviews that substantially improved the manuscript.

Copyright:
Copyright 2008 Elsevier B.V., All rights reserved.

Keywords

  • A-type magma
  • Basalt differentiation
  • Bimodal volcanism
  • Fractional crystallization
  • Hotspot
  • Rhyolite genesis
  • Yellowstone-Snake river Plain

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