Voluminous lava-like precursor to a major ash-flow tuff: low-column pyroclastic eruption of the Pagosa Peak Dacite, San Juan volcanic field, Colorado

The Pagosa Peak Dacite is an unusual pyroclastic deposit that immediately p redated eruption of the enormous Fish Canyon Tuff (similar to 5000 km(3)) f rom the La Garita caldera at 28 Ma. The Pagosa Peak Dacite is thick (to 1 k m), voluminous (>200 km(3)), and has a high aspect ratio (1:50) similar to those of silicic lava flows. It contains a high proportion (40-60%) of juve nile clasts (to 3-4 m) emplaced as viscous magma that was less vesiculated than typical pumice. Accidental lithic fragments are absent above the basal 5-10% of the unit. Thick densely welded proximal deposits Bowed rheomorphi cally due to gravitational spreading, despite the very high viscosity of th e crystal-rich magma, resulting in a macroscopic appearance similar to flow -layered silicic lava. Although it is a separate depositional unit, the Pag osa Peak Dacite is indistinguishable from the overlying Fish Canyon Tuff in bulk-rock chemistry, phenocryst compositions, and Ar-40/Ar-39 age. The unusual characteristics of this deposit are interpreted as consequences of eruption by low-column pyroclastic fountaining and lateral transport as dense, poorly inflated pyroclastic flows. The inferred eruptive style may be in part related to synchronous disruption of the southern margin of the Fish Canyon magma chamber by block faulting. The Pagosa Peak eruptive sourc es are apparently buried in the southern La Garita caldera, where northerly extensions of observed syneruptive faults served as fissure vents. Cumulat ive vent cross-sections were large, leading to relatively low emission velo cities for a given discharge rate. Many successive pyroclastic flows accumu lated sufficiently rapidly to weld densely as a cooling unit up to 1000 m t hick and to retain heat adequately to permit rheomorphic flow. Explosive po tential of the magma may have been reduced by degassing during ascent throu gh fissure conduits, leading to fracture-dominated magma fragmentation at l ow vesicularity. Subsequent collapse of the 75 x 35 km(2) La Garita caldera and eruption of the Fish Canyon Tuff were probably triggered by destabiliz ation of the chamber roof as magma was withdrawn during the Pagosa Peak eru ption. (C) 2000 Elsevier Science B.V. All rights reserved.