Eruption age and duration of the ∼9 km3 Burney Mountain dacite dome complex, northern California, USA

Abstract

Abstract At ∼9 km3, the six dacite domes (db1–db6) of Burney Mountain, northern California, USA, constitute the most voluminous Quaternary dome complex in the Cascades volcanic arc. Whole-rock geochemistry, electron microprobe, and petrographic data indicate that the domes are magmatically related, which when integrated with geomorphology and stratigraphic superposition, indicate early (db1, db2, and db3) and late (db4, db5, and db6) erupted groups. We present 40Ar/39Ar ages of 271.9 ± 4.6 ka (db1), 280.8 ± 8.2 and 281.7 ± 6.8 ka (db2), and 290.2 ± 6.0 ka (db3) along with a previous age of 280 ± 12 ka (db1). These ages scatter over 20 k.y., whereas remanent magnetic directions are similar between 53.3–59.0° inclination and 352.7–355.9° declination. The latter data set indicates that the dacite domes were emplaced over a geologically brief time interval, not thousands of years. Crystal-size distribution patterns of plagioclase were used to calculate residence times, which we use to infer the duration over which the eruptions likely occurred. Three slopes represent three populations of plagioclase crystals (fine-grained groundmass, coarse-grained groundmass, and phenocrysts). A commonly used growth rate for plagioclase in dacitic magmas (10−10 mm/s) yields 9–10 yr of growth for the coarse-grained groundmass (early erupted domes of db1, db2, and db3), whereas plagioclase in the fine-grained groundmass (late erupted domes of db4, db5, and db6) grew over 4–5 yr. All plagioclase phenocrysts have apparent residence times of 26–36 yr; however, they contain high anorthite (An)>70 resorbed cores with sieve textures, which have euhedral, lower An<65 overgrowth rims. Similarities in chemistry between groundmass plagioclase and phenocryst overgrowth rims indicate that they grew concurrently, and we therefore propose that both have similar residence times. Thus, the Burney Mountain dacite dome complex was emplaced during a single eruptive episode over the course of years to decades at 281.1 ± 4.8 ka (weighted mean age).