FROM IGNIMBRITE TO BATHOLITH IN THE SOUTHERN ROCKY MOUNTAINS: CRYSTALLIZATION AGES AND DURATION OF MAGMA ASSEMBLY

Multistage histories of incremental magma accumulation, fractionation, and solidification during construction of large subvolcanic chambers that remained sufficiently liquid to erupt are recorded by numerous Tertiary ignimbrites, source calderas, and granitic intrusions associated with a large gravity low in the Southern Rocky Mountain volcanic field (SRMVF). Age and compositional similarities indicate that intrusions and ignimbrites are closely related, but the extent that the plutons record former magma reservoirs that lost melt to volcanic eruptions remains controversial. Ar/Ar-feldspar and U/Pb-zircon ages for plutons spatially associated with ignimbrite calderas document final crystallization of granitic intrusions at times that are indistinguishable from the tuff to ages several million years younger. Some researchers interpret these results as recording pluton assembly in small increments that crystallized rapidly, leading to temporal disconnects between ignimbrite eruption and intrusion growth. Alternatively, crystallization ages of the granitic rocks are here inferred to record late solidification, after a prolonged history of open-system evolution involving recurrent magmatic recharge, lengthy residence (>>100 ky) as near-solidus crystal mush, and intermittent separation of liquid to supply volcanic eruptions. Evidence for lengthy construction of caldera-related intrusions includes: (1) age-depth correlations—shallow granitoids are commonly indistinguishable from associated ignimbrites, but ages become younger for deeper intrusions; (2) a few subcaldera intrusions yield ages modestly older than the associated ignimbrite, indicating presence of cogenetic “antecrysts”; (3) Precambrian-source zircons are scarce in caldera-related plutons, in contrast to their abundance in smaller peripheral waning-stage intrusions of the SRMVF, implying resorption of inherited crustal zircon during prolonged magma assembly for the ignimbrite flare-up and construction of a subvolcanic batholith; and (4) m.y.-age ranges of zircons within single pluton samples, commonly averaged and interpreted as “intrusion-emplacement age”, alternatively record intermittent crystallization during incremental assembly and final solidification only when the system began to wane.