Complex magma mixing, mingling, and withdrawal associated with an intra-Plinian ignimbrite eruption at a large silicic caldera volcano: Los Humeros of central Mexico

Research Article| November 01, 2012 Complex magma mixing, mingling, and withdrawal associated with an intra-Plinian ignimbrite eruption at a large silicic caldera volcano: Los Humeros of central Mexico Gerardo Carrasco-Núñez; Gerardo Carrasco-Núñez † 1Centro de Geociencias, Universidad Nacional Autónoma de México, Campus Juriquilla, Boulevard Juriquilla 3001, Querétaro, 76230, Mexico †E-mail: gerardoc@geociencias.unam.mx; mccumich@isu.edu; mjb26@leicester.ac.uk Search for other works by this author on: GSW Google Scholar Michael McCurry; Michael McCurry † 2Department of Geology, Idaho State University, Pocatello, Idaho 83209-8072, USA †E-mail: gerardoc@geociencias.unam.mx; mccumich@isu.edu; mjb26@leicester.ac.uk Search for other works by this author on: GSW Google Scholar Michael J. Branney; Michael J. Branney † 3Department of Geology, University of Leicester, Leicester LE1 7RH, England, UK †E-mail: gerardoc@geociencias.unam.mx; mccumich@isu.edu; mjb26@leicester.ac.uk Search for other works by this author on: GSW Google Scholar Michael Norry; Michael Norry 3Department of Geology, University of Leicester, Leicester LE1 7RH, England, UK Search for other works by this author on: GSW Google Scholar Christopher Willcox Christopher Willcox 3Department of Geology, University of Leicester, Leicester LE1 7RH, England, UK Search for other works by this author on: GSW Google Scholar GSA Bulletin (2012) 124 (11-12): 1793–1809. https://doi.org/10.1130/B30501.1 Article history received: 27 Feb 2011 rev-recd: 23 Jan 2012 accepted: 24 Jan 2012 first online: 08 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Gerardo Carrasco-Núñez, Michael McCurry, Michael J. Branney, Michael Norry, Christopher Willcox; Complex magma mixing, mingling, and withdrawal associated with an intra-Plinian ignimbrite eruption at a large silicic caldera volcano: Los Humeros of central Mexico. GSA Bulletin 2012;; 124 (11-12): 1793–1809. doi: https://doi.org/10.1130/B30501.1 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 Los Humeros is the largest caldera volcano in the Mexican volcanic belt. Its second largest caldera-forming eruption, the ca. 0.1 Ma Zaragoza eruption, is recorded by two Plinian pumice-fall layers and a zoned intra-Plinian ignimbrite. Diverse pumice types within the ignimbrite provide insights about the way that different magmas within a single magmatic system interact, and the way in which this can give rise to a major explosive ignimbrite-forming eruption. Normal-and-reverse compositional zoning in the ignimbrite is defined by vertical variations in the relative abundance of rhyodacitic (69–71 wt% SiO2) and andesitic (54–63 wt% SiO2) pumice lapilli: Lower parts are dominated by rhyodacite and pass gradationally up into a central part with andesitic and rhyodacite pumice, and this passes up into a rhyodacitic uppermost part, with no andesite. Petrographic and microprobe analyses of coexisting glass and phenocrysts provide mixed evidence of equilibrium and disequilibrium conditions in the magmas at the time of eruption. The Fe-Ti oxides record magma temperatures of ∼850°C (andesite) and 780°C (rhyodacite). The andesitic pumice contains euhedral labradorite (∼An60), and orthopyroxene and clinopyroxene, in a dacitic glass groundmass, which yield equilibrium Na-CaKdpl/liq and Fe-MgKdpl/liq ratios. It also contains highly calcic plagioclase (to An82) that in some cases is highly resorbed and mantled by the more sodic plagioclase, which may record early mixing between andesitic and plagioclase-bearing basaltic magmas, followed by equilibrium crystallization within the hybrid magma. The rhyodacite contains euhedral crystals of more-evolved plagioclase (∼An30–40) and euhedral pyroxenes in a rhyolitic glass groundmass (74–75 wt% SiO2). The pyroxenes yield disequilibrium Fe-MgKdpl/liq ratios and indicate formation from a liquid that was more mafic than the liquid that formed the glass groundmass of the dacitic pumice. Subordinate pumices with interbanded rhyodacite and a scarcity of intermediate-composition pumices indicate that the magmas remained separate for most of the time, and mingled only immediately prior to, and during eruptive quenching. Rather than a simple density-stratified magma chamber, the Zaragoza eruption may have occurred in response to intrusion of a hybridized andesitic magma into a rhyodacitic magma reservoir, possibly arranged as semiconnected high-melt lenses or zones within a partially consolidated crystal mush. However, contrary to assumptions of simple replenishment, tapping, and fractionation-type systems, the Zaragoza magmas contain no record of previously erupted highly evolved rhyolites that developed when zircon joined the fractionating assemblage. This absence indicates that the highly evolved rhyolites had either been completely tapped or solidified prior to the Zaragoza eruption, or that interaction was prevented by contrasting magma densities and viscosities. 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