Geology and Geochemistry of Juniper Ridge, Horsehead Mountain and Burns Butte: Implications for the Petrogenesis of Silicic Magma on the High Lava Plains, Southeastern Oregon

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 volcnism 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 +/- 0.2 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, prophyritic 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% rhyolite. Unuasually 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 teastern and western Juniper Ridge have lower rare-earth element concentrations than the intermediate fractionates at eastern JUniper Ridge, thus precluding an origin by crystal frractionation 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 tuffs of the 5-10 Ma portion of the age-progressive 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 tuffs include the Devine Canyon Tuff, the Prater Creek Tuff, the Rattlesnake Tuff and the tuff 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 rhyolites, and is instead pa

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