EVALUATING THE LINK BETWEEN IGNIMBRITES AND SPATIO-TEMPORALLY RELATED SHALLOW PLUTONIC SYSTEMS AT THE AETNA CALDERA, CO

Published and new preliminary geochronologic data from the Aetna caldera (source of the Badger Creek Tuff in CO), suggest that the shallow Mount Aetna pluton was not part of the same magma system that yielded the tuff. The data are consistent with formation of tuff magmas at structural levels below coeval shallow magma systems, and rapid eruption of the tuff through the shallow system with limited interaction between the two.

The Aetna pluton is a porphyritic quartz monzonite with alkali feldspar and plagioclase phenocrysts. Although the pluton was mapped as resurgent into the caldera formed during eruption of the Badger Creek Tuff, U-Pb zircon geochronologic data (Mills and Coleman, 2013) demonstrate that portions of the Aetna pluton (35 Ma) predate eruption of the Badger Creek Tuff (34.4 Ma). A fine-grained sample of the pluton, originally interpreted to be a marginal facies of the pluton, has a zircon age indistinguishable from the tuff. However, field relations demonstrate that portions of the porphyritic Aetna pluton intrude the tuff. Preliminary geochronologic data for the pluton where it cuts the tuff yield zircons as old as 35 Ma, but also include zircons as young as 34.2 Ma. We interpret the young end of this age spectrum to be the emplacement age of that part of the Aetna pluton (consistent with the field relations). Together, the field and zircon data suggest incremental assembly of the Aetna pre- and post-eruption of the Badger Creek Tuff. Field work conducted in 2013 revealed that the fine-grained sample, originally interpreted to be a marginal facies, is a dike that clearly cuts the porphyritic Mount Aetna pluton. Two features of the geochronologic data are important to note: 1) the young (34.2 Ma) Aetna sample includes zircons that suggest a genetic relationship with older portions of the pluton (35 Ma antecrysts), but it apparently contains no 34.4 Ma zircons, and 2) none of the dated 34.4 Ma samples (Badger Creek Tuff, “fine-grained Aetna”, and ring dike) contain any 35 Ma zircons. We take these observations to indicate that the system associated with formation and eruption of the tuff was distinct from the incremental system responsible for construction of the Aetna pluton. The data suggest that ignimbrite magmas may form in systems distinct from temporally and spatially associated shallow plutonic rocks.