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Geology and complex collapse mechanisms of the 3.72 Ma Hannegan caldera, North Cascades, Washington, USA

David Tucker^,1, Wes Hildreth², Tom Ullrich³ and Richard Friedman³

¹ Geology Department, Western Washington University, Bellingham, Washington 98225, USA
² U.S. Geological Survey, MS 910, Menlo Park, California 94025, USA
³ Pacific Centre for Isotopic and Geochemical Research, University of British Columbia, Vancouver, British Columbia V6T 1Z4, Canada

Contiguous ring faults of the 8 x 3.5 km Hannegan caldera enclose the Hannegan volcanics in the Cascade arc of northern Washington. The caldera collapsed in two phases, which each erupted rhyolitic ignimbrite (72.3%–75.2% SiO₂). The first collapse phase, probably trap-door style, erupted the 900-m-thick ignimbrite of Hannegan Peak at 3.722 ± 0.020 Ma. This single cooling unit, generally welded, has an uppermost facies of nonwelded ignimbrite and fine ash. A short period of localized sedimentation followed. Eruption of the ignimbrite of Ruth Mountain then led to a second trap-door collapse as the first-phase partial ring fault propagated to the south to completely enclose the caldera. Wall-rock breccias are intercalated as lenses and megabreccia blocks in both ignimbrites. The minimum intracaldera volume is 55–60 km³. No base is exposed, nor are outflow sheets preserved. Caldera collapse and glacial erosion have removed precaldera volcanic rocks, which survive only as intracaldera breccias. Rhyolite dikes and pods, one of which yielded a ⁴⁰Ar/³⁹Ar age of 3.72 ± 0.34 Ma, intrude the ring fault and caldera fill. Daciteandesite domes, dikes, and lava flows were emplaced subsequently; one lava flow gives a ⁴⁰Ar/³⁹Ar age of 2.96 ± 0.20 Ma. The quartz diorite of Icy Peak and the granite of Nooksack Cirque (plutons with ²⁰⁶Pb/²³⁸U zircon ages of 3.42 ± 0.10 Ma and 3.36 ± 0.20 Ma, respectively) intrude caldera fill and basement rocks on the southwest margin of the caldera. Both plutons are now exceptionally well exposed on high, glacially sculpted peaks within the caldera, indicating erosion of at least 1 km of intracaldera fill. Hannegan caldera anchors the northeast end of a linear NE-SW age-progressive migration of magmatic focus from the Chilliwack batholith to the active Mount Baker volcano.

Key Words: Hannegan • caldera • volcanology • geochronology • collapse mechanism • trap door