The Fish Canyon magma body, San Juan volcanic field, Colorado: Rejuvenation and eruption of an upper-crustal batholith

More than 5000 km³ of nearly compositionally homogeneous crystal-rich dacite (∼68 wt % SiO₂: ∼45% Pl + Kfs + Qtz + Hbl + Bt + Spn + Mag + Ilm + Ap + Zrn + Po) erupted from the Fish Canyon magma body during three phases: (1) the pre-caldera Pagosa Peak Dacite (an unusual poorly fragmented pyroclastic deposit, ∼200 km³); (2) the syn-collapse Fish Canyon Tuff (one of the largest known ignimbrites, ∼5000 km³); (3) the post-collapse Nutras Creek Dacite (a volumetrically minor lava). The late evolution of the Fish Canyon magma is characterized by rejuvenation of a near-solidus upper-crustal intrusive body (mainly crystal mush) of batholithic dimensions. The necessary thermal input was supplied by a shallow intrusion of more mafic magma represented at the surface by sparse andesitic enclaves in late-erupted Fish Canyon Tuff and by the post-caldera Huerto Andesite. The solidified margins of this intrusion are represented by holocrystalline xenoliths with Fish Canyon mineralogy and mineral chemistry and widely dispersed partially remelted polymineralic aggregates, but dehydration melting was not an important mechanism in the rejuvenation of the Fish Canyon magma. Underlying mafic magma may have evolved H₂O–F–S–Cl-rich fluids that fluxed melting in the overlying crystal mush. Manifestations of the late up-temperature magma evolution are: (1) resorbed quartz, as well as feldspars displaying a wide spectrum of textures indicative of both resorption and growth, including Rapakivi textures and reverse growth zoning (An_27–28 to An_32–33) at the margins of many plagioclase phenocrysts; (2) high Sr, Ba, and Eu contents in the high-SiO₂ rhyolite matrix glass, which are inconsistent with extreme fractional crystallization of feldspar; (3) oscillatory and reverse growth zoning toward the margins of many euhedral hornblende phenocrysts (rimward increases from ∼5·5–6 to 7·7–8·5 wt % Al₂O₃). Homogeneity in magma composition at the chamber-wide scale, contrasting with extreme textural and chemical complexities at the centimeter–millimeter scale, is consistent with a dynamic environment, wherein crystals with a variety of growth and resorption histories were juxtaposed shortly before eruption by convective currents.