HIGH-PRECISION GEOCHRONOLOGY OF IGNIMBRITES AND ASSOCIATED PLUTONS OF TWO CALDERA SYSTEMS IN CENTRAL COLORADO AND NORTHERN NEW MEXICO

We have obtained CA-TIMS U/Pb zircon and Ar/Ar sanidine single-crystal laser-fusion ages of two different ignimbrites and associated plutons to help understand the emplacement and preservation of caldera-forming magma chambers. Quantifying the volume of the pre-eruptive caldera magma chamber is difficult because the crystallized remnants, which are preserved as plutons, are rarely exposed before complete erosion of the ignimbrite. Rio Grande rift-related faulting has exposed plutons associated with the Mt. Aetna caldera complex in central Colorado and the Questa caldera in northern New Mexico.

Results from the Aetna and Questa caldera systems are strikingly different. The 34.3 Ma Badger Cr. and Antero Tuffs (~500 km³) erupted from the Mt. Aetna caldera, which is nested in the Mt. Princeton batholith. The U/Pb ages of the Badger Cr. Tuff are similar or slightly older (<100ka) than the Ar/Ar eruption age. The U/Pb ages of the Mt. Aetna Quartz Monzonite pluton, which is geochemically similar to the tuffs, are 300-500 ka older than the caldera-forming ignimbrites. It is unclear whether the pluton represents a resurgent, remobilized pluton or an earlier, fully crystallized part of the magma chamber exposed in the wall of the caldera. The 25.4 Ma eruption of peralkaline Amalia Tuff (~500 km³) produced the Questa caldera. The U/Pb and Ar/Ar ages of the Amalia Tuff are indistinguishable. In contrast to the Aetna system, U/Pb ages of the resurgent pluton (peralkaline phase of the Virgin Canyon) are indistinguishable from the tuff. The remaining resurgent plutons, which are geochemically different than the Amalia Tuff, are 100 to 500 ka younger than the caldera-forming ignimbrite.

Geochronologic and geochemical data from these ignimbrites and associated plutonic rocks indicate that: (1) only a small portion of the total exposed plutonic material represents non-erupted ignimbrite magma, (2) plutons that are geochemically similar to the associated ignimbrite are either the same age or slightly older, (3) resurgent plutons that are chemically different than the associated ignimbrite are demonstrably younger, and (4) the youngest U/Pb zircon ages for ignimbrites are indistinguishable to the Ar/Ar eruption ages.