Graduate Thesis Or Dissertation
 

The geochemistry and geochronology of the Eocene Absaroka volcanic province northern Wyoming and southwest Montana, USA

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/9019s609b

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  • The Absaroka volcanic province is the largest of Eocene volcanic fields in the northern Cordillera of the western U.S., and consists of 25,000 km2 of lava flows, shallow intrusions, ash-flow tuffs and volcaniclastic deposits. It is aligned with northwest-trending Precambrian lineaments, and includes the remains of at least ten volcanic centers. This study presents 40Ar/39Ar geochronological data, and major, trace element and isotopic compositional data which are the result of detailed mapping and field sampling of four representative volcanic centers, peripheral lava flows, intrusions, and ash-flow tuffs. Age data show that volcanism occurred between 53 and 43 Ma in a general northwest to southeast age progression, and have allowed significant revisions in regional correlations across the volcanic province. Local dike orientations from one volcanic center suggest that volcanism occurred during extensional faulting. Geochemical and Nd, Sr and Pb isotopic data show that mafic lavas are enriched in incompatible elements derived from an ancient source. Mafic rocks (<53% SiO2, >5% MgO) are characteristically potassic, and are typical products of early eruptions at each of the Hyalite, Crandall, Ishawooa, and Rampart volcanic centers, and not spatially restricted to any one region. Although products of volcanism are broadly similar, volcanic centers exhibit distinct mineralogic, compositional and isotopic characteristics. Least squares calculations based on mineralogical data indicate that shoshonites can be produced from mafic samples by fractionation of olivine and augite t plagioclase. More silicic samples have petrographic features and compositions which indicate they are derived from a mixture of sources including continental crust. Changing chemical and isotopic compositions suggest that early eruptions contain a lithospheric mantle component, with an increasing melt contribution from crustal sources with time. The last eruptions include rhyolite from the southern Absaroka volcanic province and basanite with an asthenospheric isotopic composition sampled from a peripheral lava flow in the northern Absaroka volcanic province.
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