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

Research Article| March 01, 2007 Geology and complex collapse mechanisms of the 3.72 Ma Hannegan caldera, North Cascades, Washington, USA David Tucker; David Tucker 1Geology Department, Western Washington University, Bellingham, Washington 98225, USA Search for other works by this author on: GSW Google Scholar Wes Hildreth; Wes Hildreth 2U.S. Geological Survey, MS 910, Menlo Park, California 94025, USA Search for other works by this author on: GSW Google Scholar Tom Ullrich; Tom Ullrich 3Pacific Centre for Isotopic and Geochemical Research, University of British Columbia, Vancouver, British Columbia V6T 1Z4, Canada Search for other works by this author on: GSW Google Scholar Richard Friedman Richard Friedman 3Pacific Centre for Isotopic and Geochemical Research, University of British Columbia, Vancouver, British Columbia V6T 1Z4, Canada Search for other works by this author on: GSW Google Scholar Author and Article Information David Tucker 1Geology Department, Western Washington University, Bellingham, Washington 98225, USA Wes Hildreth 2U.S. Geological Survey, MS 910, Menlo Park, California 94025, USA Tom Ullrich 3Pacific Centre for Isotopic and Geochemical Research, University of British Columbia, Vancouver, British Columbia V6T 1Z4, Canada Richard Friedman 3Pacific Centre for Isotopic and Geochemical Research, University of British Columbia, Vancouver, British Columbia V6T 1Z4, Canada Publisher: Geological Society of America Received: 30 Sep 2005 Revision Received: 10 Aug 2006 Accepted: 22 Aug 2006 First Online: 08 Mar 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 GEOLOGICAL SOCIETY OF AMERICA GSA Bulletin (2007) 119 (3-4): 329–342. https://doi.org/10.1130/B25904.1 Article history Received: 30 Sep 2005 Revision Received: 10 Aug 2006 Accepted: 22 Aug 2006 First Online: 08 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn Email Permissions Search Site Citation David Tucker, Wes Hildreth, Tom Ullrich, Richard Friedman; Geology and complex collapse mechanisms of the 3.72 Ma Hannegan caldera, North Cascades, Washington, USA. GSA Bulletin 2007;; 119 (3-4): 329–342. doi: https://doi.org/10.1130/B25904.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Contiguous ring faults of the 8 × 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. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

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