[1] Zircon U-Pb geochronologic data for plutonic rocks in the Latir volcanic field, New Mexico, demonstrate that the rocks are dominated by plutons that post-date ignimbrite eruption. Only zircon from the ring dike of the Questa caldera yields the same age (25.64 ± 0.08 Ma) as zircon from the caldera-forming Amalia Tuff (25.52 ± 0.06 Ma). The post-caldera Rio Hondo pluton was assembled incrementally over at least 400 ka. The magma accumulation rate for the exposed portion of the Rio Hondo pluton is estimated to be 0.0003 km 3 a −1 , comparable to rates for other plutons, and too slow to support accumulation of large eruptible magma volumes. Extrapolation of the accumulation rate for the Rio Hondo pluton over the history of the Latir volcanic field yields an estimated volume of plutonic rocks comparable to the calculated volume under the field as determined by geophysical studies. We propose that the bulk of the plutonic rocks beneath the volcanic center accumulated during periods of low volcanic effusivity. Furthermore, because the oldest portion of the Rio Hondo pluton is the granitic cap exposed beneath a gently dipping roof contact, the roof granite cannot be a silicic liquid fractionated from the deeper (younger) portions of the pluton. Instead, our data suggest that the compositional heterogeneity of the Rio Hondo pluton is inherited from lower crustal sources. We suggest that if magma fluxes are high enough, zoned ignimbrites can be formed by evolution of the melt compositions generated at the source with little or no shallow crustal differentiation.

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The Mount Givens Granodiorite, a large pluton in the central Sierra Nevada batholith, California, is similar in area to zoned intrusive suites yet is comparatively chemically and texturally homogenous. New zircon U-Pb geochronology indicates that the pluton was constructed over at least 7 Ma from 97.92 ± 0.06 Ma to 90.87 ± 0.05 Ma. Combining the new geochronology with the exposed volume of the pluton yields an estimated magma flux of <0.001 km 3 /a. The geochronologic data are at odds with the previously speculated links between plutons such as the Mount Givens Granodiorite and large-volume homogeneous ignimbrites (often termed monotonous intermediates). Existing data indicate that large plutons accumulate at rates of ≤0.001 km 3 /a, 1-2 orders of magnitude less than fluxes calculated for dated monotonous intermediates. If monotonous intermediates are remobilized, erupted plutons accumulated at rates comparable to dated examples, they should preserve a record of zircon growth of up to 10 Ma. Alternatively, the long history of zircon growth recorded in plutons may be erased during the processes of reheating and remobilization that precede supervolcano eruption. However, zircon dissolution modeling, based on hypothetical temperature-time histories for preeruptive monotonous intermediates, indicates that rejuvenation events would not sufficiently dissolve zircon. We suggest that eruptions of monotonous intermediates occur during high magmatic flux events, leaving little behind in the intrusive rock record, whereas low fluxes favor pluton accumulation.

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