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 km 3 of cryptodomes, domes and flows of high-silica rhyolite. These post-hotspot rhyolites have distinctive A-type geochemical signatures including high whole-rock FeO tot /(FeO tot +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 (ɛ Nd =−4 to −6; 87 Sr/ 86 Sr i =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% SiO 2 ) shield volcanoes. Both consist dominantly of intermediate composition lavas and have cumulative volumes of several 10's of km 3 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 wholerock 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, energyconstrained 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% SiO 2 ), 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.
We characterize and interpret three-dimensional spatial patterns of alteration in basalt from core from five deep boreholes located across the Idaho National Engineering and Environmental Laboratory. The basalts range in age from ca. 0.5 to 2.5 Ma and host the eastern Snake River Plain aquifer.Consistent patterns of alteration occur. Basalts in the upper parts of wells are remarkably fresh, aside from minor caliche or drusy calcite deposits in vesicles. At depths ranging from 320 to 508 m there are pronounced increases in the intensity of authigenic pore mineralization. Changes from largely unaltered to moderately to strongly altered basalt occur over narrow vertical intervals in at least two of the wells; in all cases these transitions correlate closely with sharp inflections in the temperature gradients in these wells. Many pores and fractures are partially or completely filled by nontronite ؓ saponite ؓ calcite; intersertal glass and olivine are partially to completely altered to nontronite ؓ saponite. Alteration of the basalts increases in intensity downward from the temperature-gradient inflections and appears to have been largely isochemical.Pronounced downward transitions from unaltered to altered basalts do not correlate systematically with depth, age of the basalts, prevailing temperatures, or stratigraphic features. We propose that the alteration is produced by transient thermal inputs into the base of the aquifer from a deep-seated geothermal source and that the coincidence of alteration and the temperature-gradient inflections can be used to identify the effective base of the aquifer with a high degree of confidence.
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