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 δ18O. 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 δ18O 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.