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Sheeted intrusive complexes represent unequivocal examples of incremental intrusive activity in plutonic systems and thus provide an opportunity to constrain processes associated with incremental magma emplacement. We present field observations, U-Pb zircon data, and whole-rock and mineral chemistry from the sheeted intrusive complex of the ca. 92 Ma Tenpeak pluton located in the North Cascades, Washington. Samples collected from an ~750-m transect of the sheeted complex, including at least 58 individual sheets, provide an opportunity to understand the time scales, magma sources, and magmatic processes responsible for the generation of incrementally emplaced magma bodies. High-precision chemical abrasion-thermal ionization mass spectrometry (CA-TIMS) geochronology show relatively rapid construction of the complex over an interval of 94,000 ± 62,500 yr (95% confidence). With a volume estimate of ~17 km3, this is equivalent to a magma emplacement rate of 1.1–5.4 × 10–4 km3 yr–1. In general, thinner sheets are found closer to the pluton margin and transition to thicker sheets in the interior. Although magmas of the sheeted complex show compositional and mineralogical similarities with the nearby voluminous Schaefer Lake tonalite phase of the Tenpeak pluton, U-Pb zircon ages from individual sheets suggest that formation of the sheeted complex occurred as a waning phase of the Schaefer Lake emplacement. Individual sheets also show relatively simple zircon populations consistent with low overall magma volumes and relatively rapid cooling following emplacement. We suggest that the sheeted complex resulted from localization of magma intrusions along anisotropies in the shear zone. In addition, the thermal boundary formed with the adjacent meta-supracrustal wall rocks likely facilitated rapid cooling of sheets and localized subsequent intrusive events. The formation of the range of sheet compositions requires at least three different parental magmas. The observed range of sheet compositions can be produced by mixing between a single mafic parental magma (SiO2 ~50 wt%) and an array of felsic magma compositions (SiO2 from 59 to 67 wt%). Mineral populations within mafic samples suggest that the felsic parental magmas either had low crystallinity prior to mixing and/or substantial crystallization occurred after mixing. Textural and field evidence, along with mineral chemistry, suggest that final mixing between the mafic parent and felsic array occurred late, almost immediately prior to emplacement.

Formation of a sheeted intrusive complex within the deep-crustal Tenpeak pluton, North Cascades, Washington