Geology and complex collapse mechanisms of the 3.72 Ma HanneganCaldera, north Cascades, Washington, USA

Abstract

Contiguous ring faults of the 8 x 3.5 km Hannegan caldera enclose theHannegan volcanics in the Cascade arc of northern Washington. Thecaldera collapsed in two phases, which each erupted rhyoliticignimbrite (72.3%-75.2% SiO (sub 2) ). The first collapse phase,probably trap-door style, erupted the > or =900-m-thick ignimbriteof Hannegan Peak at 3.722+ or -0.020 Ma. This single cooling unit,generally welded, has an uppermost facies of nonwelded ignimbrite andfine ash. A short period of localized sedimentation followed. Eruptionof the ignimbrite of Ruth Mountain then led to a second trap-doorcollapse as the first-phase partial ring fault propagated to the southto completely enclose the caldera. Wall-rock breccias are intercalatedas lenses and megabreccia blocks in both ignimbrites. The minimumintracaldera volume is 55-60 km (super 3) . No base is exposed, norare outflow sheets preserved. Caldera collapse and glacial erosionhave removed precaldera volcanic rocks, which survive only asintracaldera breccias. Rhyolite dikes and pods, one of which yielded a(super 40) Ar/ (super 39) Ar age of 3.72+ or -0.34 Ma, intrude thering fault and caldera fill. Dacite-andesite domes, dikes, and lavaflows were emplaced subsequently; one lava flow gives a (super 40) Ar/(super 39) Ar age of 2.96+ or -0.20 Ma. The quartz diorite of Icy Peakand the granite of Nooksack Cirque (plutons with (super 206) Pb/(super 238) U zircon ages of 3.42+ or -0.10 Ma and 3.36+ or -0.20 Ma,respectively) intrude caldera fill and basement rocks on the southwestmargin of the caldera. Both plutons are now exceptionally well exposedon high, glacially sculpted peaks within the caldera, indicatingerosion of at least 1 km of intracaldera fill. Hannegan calderaanchors the northeast end of a linear NE-SW age-progressive migrationof magmatic focus from the Chilliwack batholith to the active MountBaker volcano.

19 samples from this reference