THE CONTRASTING AGE RANGES OF LARGE PLUTONS AND MONOTONOUS INTERMEDIATES

Monotonous intermediates (MIs) are homogenous crystal-rich dacites that can erupt in very large volumes (e.g., Fish Canyon Tuff (FCT), 5,000 km³). Because of the chemical similarity of MIs to large granodiorite plutons some have proposed that the plutons represent MIs that stalled in the crust prior to eruption. Intermediate magma is hypothesized to accumulate incrementally until a large, persistent mush (50+% crystals) is developed. Injection of a mafic magma may rejuvenate the mush resulting in eruption of a crystal-rich ignimbrite. If the mush is not rejuvenated, it will fully crystallize as a granodiorite pluton.

Zircon U‑Pb ages from intrusive suites consistently yield magmatic fluxes of 10^‑3‑10^‑4 km³/a. In contrast, zircon data for MIs yield fluxes that are at least an order of magnitude greater than plutonic rocks (10^‑2 km³/a). Thus, whereas plutons typically preserve a record of zircon crystallization that spans 10⁶-10⁷ a, MIs of comparable volume preserve a crystallization record on the order of 10⁵ a. This difference might indicate that the hypothesized link between the two rock types is incorrect, or could be a reflection of the proposed difference in their T-t histories. One possibility is that the rejuvenation events inferred to have affected MIs prior to their eruptions dissolved zircon during reheating (e.g., FCT, 715 – 760°C). However, simulations based on Watson (1996) suggest that even in the favorable case of reheating over 200 ka in a melt with no dissolved Zr, zircons with radii ~100 μm may survive the event. Events with shorter heating timespans and less Zr undersaturation permit much smaller zircons to survive. These results suggest it is unlikely that zircon evidence for multi-Ma mush growth in MIs would be obliterated – a result that is supported by the occurrence of xenocrystic zircon cores preserved in many MIs. We suggest that the difference in age spans between MIs and plutons is related to different magma accumulation rates. Thus, the information recorded by zircon in MIs is indicative of high flux events that favor large eruptions rather than stalling magma in the crust.