Anatomy of a Recharge Magma: Hornblende Dacite Pumice from the rhyolitic Tshirege Member of the Bandelier Tuff, Valles C
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ORIGINAL PAPER
Anatomy of a Recharge Magma: Hornblende Dacite Pumice from the rhyolitic Tshirege Member of the Bandelier Tuff, Valles Caldera, New Mexico, USA Joseph R. Boro1 · John A. Wolff1 · Owen K. Neill2 Received: 30 January 2020 / Accepted: 5 August 2020 © Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract The 1.26 Ma Tshirege Member of the Bandelier Tuff is the second of two major (~ 400 k m3, dense rock equivalent) compositionally zoned rhyolitic eruptions from the Valles caldera. Here we analyze 25 samples of a minor component of compositionally and texturally variable silicic dacite pumice (~ 67 to 72% SiO2) that is widely distributed through the unit. The dacite has a phenocryst assemblage dominated by feldspar and hornblende and is presumed to be a recharge magma. Quenching of dacite against cooler rhyolite, melting of rhyolitic crystal mush, and mixing between dacite and rhyolite contributed to textural complexity. The dacite can be broken into three petrographic pumice types resulting from different degrees of dacite–rhyolite interaction. The earliest stage in the history of the dacite discernable from mineral chemistry, thermobarometry and hygrometry is mid-crustal storage at temperatures close to 900 °C and water content ~ 5 wt%. Plagioclase zoning suggests that the dacite was subject to more mafic recharge at this stage. The dacite was then injected into rhyolitic crystal mush at temperatures between 700 and 800 °C and pressures ~ 0.3 GPa. Consequences of mixing with mushy rhyolite include the following: (1) cooling and partial crystallization of dacite; (2) growth of large, dendritic feldspars with ternary compositions; (3) ingestion of and melting of feldspar and quartz from the rhyolitic mush; (4) enrichment in fluorine due to melting of biotite in the mush; (5) enrichment in light REE contents in some samples due to melting of chevkinite-rich domain(s) in the mush; (6) second boiling of quenched dacite rendering it buoyant and distributing dacite ‘enclaves’ through the zoned rhyolite magma column. The dacite was likely injected into the rhyolite over a protracted period and eventually triggered the Tshirege eruption. Keywords Calderas · Silicic magma systems · Magmatic recharge · Supereruptions · Ignimbrites
Introduction The origins, growth, and eruption of large silicic magma reservoirs and associated caldera systems are problems of abiding interest for understanding crustal evolution and long-term catastrophic volcanic hazards. Communicated by Mark S Ghiorso. Electronic supplementary material The online version of this article (https://doi.org/10.1007/s00410-020-01725-w) contains supplementary material, which is available to authorized users. * Joseph R. Boro [email protected] 1
School of the Environment, Washington State University, Pullman, WA 99163, USA
School of Earth and Environmental Sciences, University of Michigan, Ann Arbor, MI 48109, USA
2
Large silicic ignimbrites may be crystal-rich, crystalpoor, compositionally homogeneous, strongly zoned,
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