Time scales of pluton construction at differing crustal levels: Examples from the Mount Stuart and Tenpeak intrusions, North Cascades, Washington
Deciphering the magmatic history of continental magmatic arcs, in general, and the growth history of individual intrusions, in particular, is key to understanding the complex history of magma generation, segregation, and transport that define the dynamics of crustal growth. We utilize high precision U-Pb geochronology to resolve a detailed magmatic history from two composite intrusions, the 2–4 kbar Mount Stuart batholith and the 7–10 kbar Tenpeak pluton, emplaced in the Cretaceous North Cascades arc. This temporal framework provides a way to evaluate models of pluton growth that explain common features of intrusions such as concentric compositional zoning and internal magmatic contacts. U-Pb zircon crystallization ages were obtained from 12 samples of the Mount Stuart batholith and 8 samples of the Tenpeak intrusion, representing the range of compositional diversity and geographical extent. These dates indicate that the Mount Stuart batholith was constructed over a ∼5.5 m.y. time period that was punctuated by four intervals of high magma flux. The durations of the high-flux periods are short (a few hundred thousand years) relative to the duration of the batholith. The consistent pattern of magmatic fabrics and the lack of distinct contacts in the batholith may be explained by the juxtaposition of melt-rich and mush zones with subtle contacts between mineralogically and texturally similar tonalite and time-transgressive magma fabrics. In contrast, the Tenpeak intrusion was constructed over a ∼2.6 m.y. time period, with magma influx distributed throughout the intrusive history and texturally distinct magma bodies. The Tenpeak intrusion lacks distinct age domains, which suggests that any magma reservoir was smaller in size and potentially more ephemeral. Although the distinct age domains and discrete compositional and textural phases indicate that pluton growth occurred incrementally, neither pluton bears resemblance to a purely end-member incremental growth model whereby a pluton is constructed from hundreds to thousands of discrete magma pulses that have little, if any, interaction. In particular, ages from the youngest domain of the Mount Stuart batholith indicate that a melt-rich magma reservoir of ≥520 km 3 existed over a 170 ± 90 k.y. time span.