USING THE INTRUSIVE RECORD TO UNDERSTAND CALDERA MAGMATISM: THE GEOCHRONOLOGY AND GEOCHEMISTRY OF THE MT. AETNA CALDERA COMPLEX, CENTRAL COLORADO

The combination of Ar-Ar and U-Pb geochronology, Sr, Nd, and Pb whole-rock isotope and elemental geochemistry, and in situ Hf isotope zircon geochemistry sheds light on the timing and origin of caldera magmatism at the Mt. Aetna caldera complex. The Mt. Aetna caldera complex consists of the Mt. Princeton batholith, the nested Mt. Aetna caldera, and associated postcaldera intrusions. Preliminary results indicate that magmatism occurred between 37.3 and 30.8 Ma. The plutons represent pre- and postcaldera magmatism and small-volume remnants of the caldera-forming magma chamber. Plutons and ignimbrites display varying whole-rock elemental and isotopic geochemistry, reflecting long-term variations in the source region.

The Mt. Princeton batholith was previously interpreted to be the intrusive remnant of the 37.3 Ma Wall Mt. Tuff caldera based on paleovalley geometry, whole-rock geochemistry, and a single Pb-Th date. New U-Pb dates indicate that the majority of the Mt. Princeton units were incrementally emplaced between 36 and 35 Ma and are thus too young to be the source of the Wall Mt. Tuff. One small-volume border phase of the Mt. Princeton batholith yielded a LA-ICP-MS U-Pb zircon age of 36.4 ± 0.7 Ma and is the only unit that can be potentially linked to the Wall Mt. Tuff. Radiogenic isotopes indicate a more significant crustal component in Wall Mt. Tuff compared to the Mt. Princeton units. Ar-Ar thermochronology indicates that the thermal history of the batholith was influenced by variable proximity to country rock and younger intrusions.

The Mt. Aetna caldera formed during the 34.3 Ma eruption of Badger Creek Tuff. One pluton in contact with intracaldera ignimbrite has a similar radiogenic isotopic signature to Badger Creek Tuff and Mt. Princeton units. U-Pb zircon ages of the this pluton are between 34.8 and 34.5 Ma suggesting that the pluton represents either residual magma that was remobilized during resurgence or a fully crystallized pluton that became exposed by the caldera-forming event .

Several high-silica, peraluminous plutons were emplaced between 32 and 30 Ma and caused widespread reheating of previously emplaced plutons. The isotopic and trace-element data from these plutons, compared to the older plutons, indicate an increasing asthenospheric component apparently reflecting Rio Grande rift extension.