An investigation of caldera-forming magma chambers using the timing of ignimbrite eruptions and pluton emplacement at the Mt. Aetna caldera complex

Abstract

The temporal and chemical relationships of volcanic and plutonic rocks exposed at the Mt. Aetna caldera complex, central Colorado, provide insight into the emplacement history of pre-, syn-, and postcaldera plutons and the origin of caldera-related silicic magmas. 40Ar/39Ar sanidine ages indicate the rhyolitic Wall Mountain Tuff erupted at 37.3Ma. LA-ICP-MS U/Pb zircon ages of the compositionally zoned Mt. Princeton batholith, which has been interpreted by several previous studies to be the nonerupted, less-fractionated residuum of the Wall Mountain Tuff magma chamber, indicate that it was emplaced between 35.9 and 35.2Ma during postcaldera magmatism. Nested within the Mt. Princeton batholith is the Mt. Aetna caldera. The dacitic Badger Creek Tuff erupted at 34.3Ma during Mt. Aetna caldera collapse. The Badger Creek Tuff is deposited on some of the Mt. Princeton intrusive units, demonstrating that the Wall Mountain Tuff caldera was obliterated during exhumation and erosion prior to 34.3Ma. Shortly after the Mt. Aetna caldera collapse, the intracaldera ignimbrite and caldera ring faults were intruded by magma that is compositionally and temporally similar to the Badger Creek Tuff, suggesting that these intrusions represent a nonerupted, geochemically equivalent portion of the magma chamber. Emplacement of postcaldera luecogranites at ~31Ma caused localized thermal resetting of biotite and K-feldspar in older, adjacent intrusions. Most samples contain antecrystic zircon. Antecrystic zircon populations range from 38.8 to 33.6Ma, indicating that open system magmatic processes operated throughout the history of the caldera complex. The temporal, chemical, and spatial relationships of plutonic and volcanic rocks at the Mt. Aetna caldera complex indicate that the majority of the exposed plutons were emplaced during pre- and/or postcaldera magmatism. None of the exposed plutons represent the less-fractionated, mafic residuum of either caldera-forming magma ignimbrite. This suggests that either the mafic roots of each caldera-forming magma chamber are not exposed or upper crustal in situ fractionation did not generate the caldera-forming silicic magmas.