The Grays River volcanics are part of the Coast Range basalt province and consist of ~3500 m of tholeiitic basalt fl ows and volcani clastic rocks that erupted in the Cascadia forearc from 42 to 37 Ma. Chemical and isotopic data, combined with migration of the location of magmatism through time, indicate that Grays River volcanics magmatism was related to subduction of a plumeinfl uenced spreading ridge that produced a northward-migrating slab window. Involvement of a mantle plume source is indicated by ocean-island basalt (OIB)-like incompatible element enrichments and radiogenic Pb isotopic compositions ( 206 Pb/ 204 Pb > 19.3).These Pb isotope data are distinct from most Cascade arc rocks and from Cascadia sediment, but they overlap with compositions of other Coast Range basalts. A slab window setting accounts for the northward-younging age progression of the Grays River volcanics as well as geochemical traits, including low B/Be, that indicate the Grays River magmas ascended through the mantle wedge and subducting slab without acquiring an arc signature. Differentiation of Grays River magmas was dominated by clinopyroxene fractionation, which resulted in evolved compositions (Mg# = 59-32), low Sc contents, and Sr contents that increase with fractionation. Geochemical differences between the Grays River volcanics and other Cascadia forearc volcanic units that range from ca. 55 Ma (Crescent Basalts) to <3 Ma (Boring Lavas) were mainly caused by transient changes in tectonic setting (i.e., arrival of a mantle plume, ridge subduction) and do not record progressive chemical modifi cation of the mantle wedge.

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Heavy rare earth element (HREE)-depleted trace-element patterns are a relatively common feature of granitoids within mature Cordilleran continental margin arcs (e.g., Sierra Nevada batholith, Coast Mountains batholith, North Cascades, Peninsular Ranges batholith). This depletion is commonly interpreted to indicate the presence of garnet during granitoid melt formation, which requires thick arc crust (>40 km) to achieve the necessary pressure conditions to stabilize garnet in the lower crust. This work focused on understanding the evolution of thickened crust in an ancient continental arc using whole-rock geochemical data, high-precision chemical abrasion-isotope dilution-thermal ionization mass spectrometry (CA-ID-TIMS) U-Pb zircon geochronology, and in situ zircon Hf and O isotopic data from three contemporaneous Cretaceous plutons in the North Cascades of Washington State, USA. New data show that, over time, magmas from three North Cascades plutons became more felsic and more HREE-depleted but lacked a significant change in zircon δ 18 O. The gradual depletion in HREEs through time is interpreted as evidence for a progressively thickening crust in the North Cascades arc that only became thick enough to stabilize garnet between 90 Ma and 87 Ma. This time frame also coincides with the end of a major period of contraction and plutonism in the region, suggesting a link between thick crust and the end of a major magmatic flare-up. The absence of appreciable change in zircon δ 18 O values during this time suggests that thickening may have been the result of crustal shortening within the arc, causing migration of the magma source region to below the garnet stability threshold.

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