Zircon growth and recycling during the assembly of large, composite arc plutons

Concordant U–Pb zircon dates have been interpreted traditionally to date the crystallization ages of plutons because until recently analytical uncertainties have generally been large enough to encompass the anticipated duration of pluton growth. Advances in zircon TIMS analysis and geochronological studies by SIMS on zircon from young volcanic rocks, along with evolving views of the evolution of magmatic systems, makes it necessary to revaluate this interpretation. Assignment of unique pluton crystallization ages based on zircon dating is complicated by the propensity of zircon to survive multiple intrusive events that culminate in a large pluton. Distinctions must be made between true inherited zircon, xenocrystic zircon derived from host rocks during magma ascent and/or emplacement, and multi-stage growth of zircon during waxing and waning of magma reservoirs that solidify to form large plutons. These complications are exemplified by recent high-precision U–Pb zircon age dating from the Cretaceous Tuolumne and Mt. Stuart batholiths, which clearly show that the timeframe of pluton assembly was long (> 5 Ma for Mt. Stuart and > 8 Ma for Tuolumne). Zircon crystals from samples from both batholiths exhibit appreciable concordant age dispersion for zircon (several 105 yr up to 2 × 106 yr), and assignment of statistically valid rock solidification ages is not possible from these data. Low Zr in the rocks of each batholith indicates that magmas were initially strongly undersaturated in zircon when emplaced and inherited zircon is rare or absent. Recycling of zircon antecrysts during successive magmatic injections is the primary cause of the modest age dispersion of concordant zircon ages, and is compatible with progressive growth of a large, long-lived, crystal mush body. Because eruption of magma requires low to moderate crystal fraction and high temperatures (relative to stored crystal mushes), zircon crystals are more likely to be eradicated during thermal rejuvenation preceding major eruptions.