Mount Dutton volcano, Alaska: Aleutian arc analog to Unzen volcano, Japan

Holocene eruptions from Mount Dutton, a small Late Quaternary volcano near the tip of the Alaska Peninsula, bear strong physical and petrologic simila rities to the 1990-1995 Unzen Fugendake eruption in Japan. The volcano had a protracted phase of effusive calcalkaline andesitic (54-59 wt.% SiO2) con e-building in the late Pleistocene followed by an abrupt switch to more sil icic (similar to 65 wt.% SiO2) lavas, emplaced as a central summit cluster of steep-sided domes beginning in the early Holocene. The flanks of the vol cano are mantled by pyroclastic flows, debris flows, and talus formed as a result of gravitational dome collapse. Disequilibrium mineral assemblages, including coexisting quartz and olivine in eruptive episodes ranging from t he initial cone-building basaltic andesite lavas to the latest Holocene dac ite domes, suggest extensive ma,oma mixing. In addition, up to meter-sized, pillow-like cognate mafic enclaves of hornblende + plagioclase + glass are common in the latest of the summit dacite domes, Mineralogical evidence an d bulk chemical data indicate the enclaves represent a high-alumina basalt parent with variable and subordinate reservoir contaminant, and the host la va is reservoir magma with variable and subordinate basaltic contaminant. M ount Dutton's history and petrology can be interpreted as reflecting the mo notonous repetitive intrusion of mantle-derived mafic magma into a silicic crystal-rich crustal reservoir. During the Holocene, these injections resul ted in the extrusion of partially crystallized, viscous, 'sticky' central d omes which typically failed by collapse resulting in small volume Merapi-ty pe flowage deposits. We speculate that slow introduction of mafic magma int o the silicic chamber leads both to enclave formation and to the effusive e ruption style. Mount Dutton volcano experienced severe shallow earthquake s warms in 1984, 1988, and to a lesser extent in 1991; although none of these swarms resulted in an eruption, their epicenter distribution and volcanic- tectonic character indicate they recorded the movement of magma at shallow depth beneath the volcano. The lessons of Unzen and Montserrat suggest that , despite limited tephra production, these small volcanic centers with cent ral dome complexes present a serious hazard to life and property. (C) 1999 Elsevier Science B.V. All rights reserved.