PETROLOGY AND GEOCHRONOLOGY OF THE LAST GASP OF THE CENTRAL SAN JUAN CALDERA CLUSTER, COLORADO (Invited Presentation)

Large caldera-forming ignimbrite eruptions and their crustal magma reservoirs pose significant hazards to humans, affect climate on a global scale, and play a large role in continental crust development. One of the most intense examples of ignimbrite volcanism is the Southern Rocky Mountain volcanic field in southern Colorado, USA. The San Juan Mountains are the largest erosional remnant of this the mid-Cenozoic volcanic field. Volcanism in the San Juan Mountains started with basaltic andesite to dacite lavas, known as the Conejos Formation, erupting ~25,000 km³ between 36 and 30 Ma, and transitioned to 22 caldera-forming, dacite to rhyolite ignimbrites erupting ~15,000 km³ between 33 and 23 Ma. The last four ignimbrites of the central San Juan caldera cluster, in order of eruption, are the zoned 150 km³ Rat Creek Tuff, unzoned 250 km³ Cebolla Creek Tuff, zoned 500 km³ Nelson Mountain Tuff, and unzoned 500 km³ Snowshoe Mountain Tuff. These eruptions collectively produced 1400 km³ of ignimbrite in rapid succession (27.08-27.05 Ma). Thermobarometry indicates vertically extensive crustal magma reservoirs with a common deep zone at ~16-17 km and upper zones overlapping at ~4-13 km, which agrees with previous thermobarometry of San Juan magmas. Zircon geochronology combined with zircon chemistry, thermal modelling, and synthetic zircon age modelling suggests long-lived magmatic systems (>0.8 m.y.) formed with relatively low vertical accumulation rates of magma (2-5 x 10^-3 m y^-1). These results highlight the complexity of magmatic systems producing ignimbrite flare-ups and show that high magmatic fluxes are not required to produce large amounts of eruptible magma necessary for the largest silicic eruptions on Earth.