Graduate Thesis Or Dissertation
 

Petrology of selected volcanic rocks of the Harney Basin, Oregon

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  • The volcanic rocks of the Harney Basin comprise a bimodal assemblage of rhyolites (SiO₂ greater than 72%) and basalts (SiO₂ less than 52%). Tholeiitic basalts account for 15 rocks analyzed and alkalic basalts for three. Most basalts contain phenocrysts of olivine and plagioclase. Pyroxene phenocrysts were not found. Rhyolite flows are peraluminous and contain phenocrysts of hornblende, biotite, plagioclase, quartz, and sanidine. Rhyolite ash-flows are subaluminous and contain phenocrysts of quartz, alkali-spar, clinopyroxene, and magnetite. Volcanism was apparently episodic with basalt and rhyolite erupted contemporaneously. Age dates of basalts are 2.6, 2.8, 7.0, and 8.9 m. y. and one andesite flow, 5. 8 m. y. Rhyolite ages are 2.7, 5.6, 6. 6, 8. 2, 8. 4, and 8.6 m. y. Basaltic volcanism has occurred during historic time. Strontium isotope initial ratios of rhyolites (0. 7035 and 0.7038) are similar to basalts (0. 7033 and 0. 7036) and both are as low as island arc rocks and distinctly higher than unaltered oceanic basalt. Strontium isotope initial ratios, Rb, K, and Sr data indicate the magmas were derived from a depleted mantle and perhaps contaminated (less than 15%) by crustal material. The correlation of the Rattlesnake Ignimbrite Tongue over 50,000 km2 in eastern Oregon is firmly established and the ash flow volume, excluding the airfall component, is estimated to be 930 km³ magmatic volume. The Rattlesnake Ignimbrite Tongue is 6.6 ± 0.2 m. y. old and issued from fissure type vents in the Buzzard Creek-Alkali Lake area. The top of the magma chamber is estimated to be 6 km in depth and the eruption did not produce a caldera. The Rattlesnake Ignimbrite Tongue is a multiple flow simple cooling unit which is composed of at least two and possibly four flow units. Laminar flow occurred where thick accumulations of hot plastic ashflow material were faulted soon after the eruption. The various colors of shards and pumice of the Rattlesnake Ignimbrite Tongue and the ash-flow tuff of Devine Canyon are chemically distinct and the result of contamination which took place when basalt magma was introduced into the rhyolite magma chamber. This basalt "intrusion" produced boiling and eruption of the rhyolite magma.
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