The making of intermediate composition magma in a bimodal suite: Duck Butte Eruptive Center, Oregon, USA

The 10.4-Ma Duck Butte Eruptive Center (DBEC) of southeastern Oregon is anomalous within the High Lava Plains bimodal basalt/high-silica rhyolite province in that it produced mainly intermediate composition lavas. At Duck Butte, eruptions successively tapped a compositionally zoned system that became less silicic and volumetrically smaller in time. The first eruptions produced rhyodacite lava with andesite inclusions, followed by dacite lava with basaltic andesite inclusions, and finally andesite and basaltic andesite lavas. High-silica rhyolite domes, more typical of the province, erupted before and after Duck Butts, immediately to the east and west, respectively. Evidence from mingling and mixing textures and mixing calculations indicate that successive eruptions at Duck Butte represent mixing of residual magma in the system. Eruption rate outpaced the rate of silicic magma production. The mixing end-members are rhyodacite, which was produced by small degrees of crustal melting, and basaltic andesite. The mixing-dominated history at Duel; Butte is in contrast to high-silica rhyolite at nearby Indian Creek Butts (ICB), which was derived by crystal fractionation from rhyodacitic crustal melt. We attribute the petrogenetic process that produced intermediate composition magma to disruption of the Duck Butte magma system by faulting, which caused high eruption rates and stirring of the magma. Magmatism at Duck Butte was related to faulting as indicated by both the alignment of rhyodacite vents parallel to a major regional basin-range fault and by faults that cut through early flows, whereas later flows drape the escarpment and are also offset by the fault. In contrast, tectonic quiescence at neighboring centers allowed for protracted differentiation of crustal melt to produce high-silica rhyolite, The material inputs to both the bimodal and dominantly intermediate centers are the same, but the contrast in tectonic influence and eruption rate strongly affects the erupted compositions. (C) 2000 Elsevier Science B.V. All rights reserved.