REPLENISHMENT AND INTERACTION OF MAGMA DURING THE FORMATION OF LAKE CITY CALDERA, COLORADO, USA

Lake City caldera is the youngest caldera in the San Juan Volcanic Field, Colorado. The 23 Ma caldera exhibits an exceptionally complete exposure of volcanic and intrusive rocks from different structural levels. The rocks show a large degree of chemical diversity attesting to a complex magmatic system beneath the caldera. We use field and geochemical data from zoned ignimbrites and intrusions to constrain the configuration of magma storage and magma interaction during the formation of Lake City caldera. Two geochemically distinct magma batches erupted during caldera formation: high silica rhyolites, trachytes, and less evolved trachytes (Batch A), and dacites and trachy-andesites (Batch B). The ignimbrites of the Lower, Middle and Upper Sunshine Peak Tuff represent the majority of the erupted Batch A magma, showing a temporal progression in bulk composition from high silica rhyolite, through rhyolite, to trachyte. Major element, REE, and isotopic data are consistent with the sequential eruption from a zoned magma chamber formed by fractional crystallization. However, certain aspects of the zonation are inconsistent with this interpretation. Compositional jumps and periods of reverse zonation suggest that replenishment and accelerated periods of magma movement occurred in the magma chamber during caldera formation. After collapse, dacite lavas of Batch B erupted, concurrent with resurgent uplift from the shallow intrusion of residual mingled Batch A and B magma. Our results indicate that magma chamber replenishment, amalgamation, and magma interaction play an integral role in caldera development. There is thus a complex interplay between magma chamber processes providing conditions suitable for caldera formation, and, caldera development affecting magma chamber processes.