The Fish Canyon Magma Body, San Juan Volcanic Field, Colorado: Rejuvenation and Eruption of an Upper-Crustal Batholith

from >5·5–6 to 7·7–8·5 wt % Al2O3). Homogeneity in magma More than 5000 km of nearly compositionally homogeneous crystalcomposition at the chamber-wide scale, contrasting with extreme rich dacite (>68 wt % SiO2: >45% Pl + Kfs + Qtz + textural and chemical complexities at the centimeter–millimeter scale, Hbl + Bt + Spn + Mag + Ilm + Ap + Zrn + Po) is consistent with a dynamic environment, wherein crystals with a erupted from the Fish Canyon magma body during three phases: (1) variety of growth and resorption histories were juxtaposed shortly the pre-caldera Pagosa Peak Dacite (an unusual poorly fragmented before eruption by convective currents. pyroclastic deposit, >200 km); (2) the syn-collapse Fish Canyon Tuff (one of the largest known ignimbrites, >5000 km); (3) the post-collapse Nutras Creek Dacite (a volumetrically minor lava). The late evolution of the Fish Canyon magma is characterized by

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