ASSEMBLY OF LARGE CORDILLERAN MAGMA CHAMBERS: THE IGNIMBRITE-CALDERA RECORD

Recent inferences that Mesozoic Cordilleran plutons grew incrementally during >10⁶-year intervals, without presence of voluminous eruptible magma, minimize the close associations with large ignimbrite calderas. Alternatively, multi-stage histories of magma accumulation, fractionation, and solidification in upper parts of many large plutons must have included episodes when they were sufficiently liquid to erupt ignimbrites with volumes of 1-5x10³ km³ and accommodate caldera collapse. Spacing of Tertiary calderas at distances of 10s to 100s of km is comparable to pluton spacing for similar durations. Individual calderas, to 75 km across with 2-5 km subsidence, are direct evidence for shallow magma bodies comparable to the largest granitic plutons. Downwind ash in eastern Cordilleran sediments records large-scale explosive volcanism concurrent with batholith growth. Nested calderas that erupted compositionally diverse tuffs document deep composite subsidence and rapid evolution in subvolcanic magmas. Mineral fabrics and gradients suggest flowage of pluton interiors before complete solidification, and some plutons contain ring dikes or other textural evidence for roof subsidence. Geophysical data shows that low-density upper-crustal rocks, inferred to be plutons, are 10 km or more thick beneath many calderas. Most ignimbrite compositions are more evolved than associated plutons, requiring that subcaldera chambers retained voluminous residua from fractionation. Initial incremental pluton growth in the upper crust is likely recorded by modest eruptions from central volcanoes; preparation for caldera-scale ignimbrite eruption must involve recurrent magma input and homogenization high in the chamber. Some eroded calderas expose shallow granites of similar age and composition to tuffs, recording sustained postcaldera magmatism. Plutons thus provide a composite record of prolonged magmatic evolution, while volcanism offers snapshots of conditions at early stages; some evidence cited for incremental pluton assembly may record late events during/after volcanism. Lack of geophysical evidence for voluminous magma beneath young calderas suggests that near-solidus plutons can be rejuvenated rapidly by mantle melts, causing large explosive eruptions with only brief precursors.