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,
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