| Abstract |
- The Juniper Ridge volcanic complex is located in the High Lava Plains Province of
southeastern Oregon, a wide zone of bimodal volcanism and faulting that marks the northern
limit of widespread Basin and Range-style faulting in the northern Great Basin Province. Rhyolite
dome complexes are progressively younger to the northwest along the High Lava Plains,
providing a mirror-image to age-progressive silicic volcanism on the Snake River Plain in
southern Idaho. 40Ar-39Ar dating of rocks from western and eastern Juniper Ridge (5.70 ± 0.02
Ma and 6.87 ± 0.02 Ma, respectively) and Burns Butte (7.75 ± 0.06 Ma) confirms the overall age
progression, and shows that age relations within the Juniper Ridge complex are consistent with
the trend. Horsehead Mountain (15.54 ± 0.03 Ma) predates the age progression altogether.
Rocks at both eastern Juniper Ridge and western Juniper Ridge are overlain by
diktytaxitic olivine basalt flows similar to regional high-alumina olivine tholeiites (HAOTs). At
western Juniper Ridge, high-silica rhyolite flows overlie lower-silica rhyolite and hybrid andesite
flows. At eastern Juniper Ridge, a suite of intermediate rocks ranging from basaltic andesite to
dacite overlies a series of rhyolite flows. Rocks at Burns Butte consist of high-silica rhyolite,
porphyritic dacite and rhyodacite, and andesite.
Field and petrographic observations, including mixing textures, inclusions, and
xenocrysts, along with straight-line relationships on chemical diagrams, show that the
intermediate rocks at western Juniper Ridge formed by mixing between high-silica rhyolite
magma and slightly evolved HAOT, probably at the base of a large silicic magma chamber. In
contrast, curvilinear geochemical trends and an internally consistent four-stage major and trace
element model suggest that intermediate rocks at eastern Juniper Ridge evolved in small,
unconnected magma chambers by removal of olivine, plagioclase, clinopyroxene, and magnetite
from a primitive HAOT parent, accompanied by contamination by up to 22 percent rhyolite.
Unusually elevated concentrations of incompatible trace elements in the Squaw Butte basaltic
andesite were produced by either zone refining or combined fractionation and recharge.
The least-evolved rhyolites at eastern and western Juniper Ridge have lower rare-earth
element concentrations than the intermediate fractionates at eastern Juniper Ridge, thus
precluding an origin by crystal fractionation from the observed intermediate rocks, but have
major element compositions close to those of experimental dehydration melts of amphibolite.
The rhyolites probably originated as dehydration melts of an amphibolite lower crust, which were
subsequently modified by removal of quartz, sanidine, plagioclase, clinopyroxene and zircon. At
western Juniper Ridge, the fractionating assemblage probably included trace allanite.
Rhyolites from dome complexes and ash-flow tufts of the 5- to 10-Ma portion of the ageprogressive
trend show systematic trace element variations with position along the trend. Dome
complexes include Burns Butte, Palomino Butte, eastern and western Juniper Ridge and Glass
Buttes; ash-flow tufts include the Devine Canyon Tuft, the Prater Creek Tuft, the Rattlesnake Tuff
and the tuft of Buckaroo Lake. From east to west, Y/Nb and Yb/Ta increase, and Ce/Yb
decreases in the least-evolved rhyolite in each suite. These variations can be accounted for by a
systematic increase in degree of garnet-residual partial melting of amphibolite crust from east to
west, correlating either with degree of extension with time along the Brothers fault zone, or with
distance from the main axis of faulting.
Porphyritic, calc-alkaline andesites and dacites erupted from at least six vents at
Horsehead Mountain. Field relationships indicate a general decrease in age from southwest to
northeast within the complex. At 15 Ma, calc-alkaline volcanism at Horsehead Mountain predates the age-progressive rhyolftes, and is instead part of an earlier phase of volcanism that
produced the Steens and Columbia River flood basalts, along with several other caic-alkaline
intermediate centers that are unrelated to subduction. Overlapping rare-earth element patterns
of Horsehead Mountain intermediate rocks with those of Steens basalts precludes their
derivation by fractionation from the broadly contemporaneous flood basalts.
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