ABSTRACT Growth of continental crust requires addition of juvenile material from the mantle and/or oceanic lithosphere. Large-scale addition is most likely to occur by accretion of oceanic arcs and continental arc magmatism. Therefore, evaluation of fluxes of new crustal material into continents requires testing for crustal recycling by melting of older crust and reincorporation of continental sediments into continental arcs. This work uses isotopic data and pressure-temperature-time ( P-T-t ) paths to evaluate the juvenile sedimentary contribution to crustal growth versus evolved sedimentary rocks recycled from preexisting continental crust in the Cretaceous Cascades magmatic arc. Neodymium isotope ratios for 91–75 Ma Swakane Gneiss metasedimentary rocks are compatible with a significant proportion of recycled crustal material, with ε t Nd values ranging from 2.1 to -5.3 and a preponderance of values less than 0.7. Clockwise metamorphic P-T-t paths consist of four segments: (1) initial loading and heating to garnet growth conditions, (2) near-isothermal loading during early garnet growth, (3) near-isobaric heating to 650–710 °C and 8–11 kbar, and (4) decompression and cooling. The transition from isothermal loading to isobaric heating is interpreted to reflect thrust loading and associated subsequent heating. Similarities in the P-T-t paths for all samples require a similar tectonic mechanism to produce the high- P before high- T metamorphic history. We conclude that high-grade rocks of the Swakane Gneiss preserve petrologic evidence for synorogenic deposition, metamorphism that resulted from heating caused by overthrusting, and subsequent exhumation. These results demonstrate the importance of thrust loading in magmatic arcs as a mechanism for growth and recycling of continental crust.

ABSTRACT The incorporation of metasedimentary rocks into the mid- to deep crust of continental magmatic arcs has significant mechanical and geochemical consequences for arc systems. The Late Cretaceous–Eocene North Cascades arc is one of the few continental magmatic arcs in the world that exposes a large amount of exhumed deep-crustal metasedimentary rocks. Here, we investigate a range of processes that may have been important in transferring sediment into the arc by combining field mapping with bulk-rock Nd analyses, U-Pb and Hf-isotopic study of detrital zircons, and U-Pb dating of zircon and monazite to determine the timing of metamorphism and melt crystallization from metasedimentary samples collected in two deep-crustal domains of the North Cascades (the Skagit Gneiss and Swakane Gneiss). We also use these data to examine provenance links between the metasedimentary rocks and potential sediment sources in the accretionary wedge (western mélange belt), the forearc (Nooksack Formation), and the present-day backarc (Methow terrane) to the North Cascades arc. Jurassic strata of the Methow terrane and the Nooksack Formation have unimodal detrital zircon age peaks and near-depleted mantle ε Ηfi values, whereas zircons from the middle Cretaceous strata of the Methow terrane have a bimodal age distribution and less radiogenic ε Ηfi values. In comparison, the accretionary western mélange belt (WMB) has Jurassic to Upper Cretaceous sandstones characterized by multiple Mesozoic age peaks, and the Upper Cretaceous sandstones also reveal distinct Proterozoic zircon populations and unradiogenic Late Cretaceous zircons. The Skagit metasedimentary rocks yield zircon-age signatures that fall into two groups: (1) a wide range of zircon dates from Proterozoic to latest Cretaceous and (2) a more limited range of Late Triassic to latest Cretaceous grains with no Proterozoic zircons. Both groups reveal a mix of ε Ηfi values. The Swakane metasedimentary rocks have similar detrital zircon age signatures to Group 1 Skagit metasediments. For Swakane rocks, >100 Ma zircons have radiogenic ε Ηfi values, whereas younger zircons plot between near-depleted mantle to unradiogenic values. Overall, the data are most consistent with some metasedimentary rocks of the Swakane and Skagit Gneisses being sourced from either the forearc or the accretionary wedge. This sedimentary material was buried to mid-crustal depths by ca. 75–65 Ma, coeval with major magmatism within the North Cascades arc. Moreover, the distinct combination of unradiogenic Late Cretaceous detrital zircons and ca. 1.4–1.3 and 1.8–1.6 Ga Proterozoic peaks is documented in many of the forearc and accretionary-wedge units exposed along western North America. The Proterozoic peaks likely reflect zircon derived from southwestern Laurentian crust, equivalent to the latitude of the present-day Mojave Desert. Therefore, the detrital-zircon results from both the Swakane and Skagit Gneisses, as well as parts of the accretionary wedge, support at least moderate translation of sedimentary material along the margin of western North America during the Late Cretaceous.

The crystalline core of the North Cascades arc records the Cretaceous to Paleogene history of magmatism, deformation, and crustal growth along a segment of the North American Cordillera. The Nd isotopic compositions of granitoid plutons that intrude the Cascades core are a product of their source regions, and they provide probes of the crustal architecture. We present Sm-Nd isotopic data from 96 Ma to 45 Ma plutons and meta-igneous and metasedimentary terranes across the Cascades core. Sm-Nd data from all metamorphic terranes, excluding the much younger ca. 73 Ma Swakane terrane, yield mid-Cretaceous ε Nd values that range from +8.5 to −1.9 and indicate minor involvement of an enriched crustal component. Amphibolites from the Napeequa complex and Chiwaukum Schist yield near-depleted-mantle ε Nd values in the mid-Cretaceous, and ε Nd values from meta-clastic rocks from these terranes (+3.4 to −1.9) have an isotopic character that is intermediate between arc-derived and continental-shelf (miogeocline) sediments, reflecting a mixture of these two sources. Initial ε Nd values of the Swakane Gneiss range from +0.6 to −5.4 and reflect a significant input from the miogeocline. The initial ε Nd values of the Late Cretaceous to Paleogene plutons studied range from +1.5 to +6.3, consistent with geochemical studies that indicate the plutons were generated by mixing of mantle-derived melt and melt derived by anatexis of the underlying terranes. Initial ε Nd values of plutons from the NE part of the Cascades core generally decrease over time, suggesting a greater contribution of melt from evolved crustal sources, which may reflect a change in the physical parameters of melting. The metamorphic terranes of the North Cascades show a close affinity to the Late Triassic to Early Cretaceous arc terranes of the southern Coast Belt. The similarity in isotopic character supports the assumption that the North Cascades terranes formed in a position outboard of the North American craton but in close enough proximity to derive sediments from the miogeocline. Variations in Nd signature are also observed between the northern and southern Coast plutonic complex, and they indicate changes in the sources of crustal melting along the length of the Cretaceous arc.

Abstract The crystalline core of the North Cascades (Cascades core) consists largely of oceanic and arc terranes that were metamorphosed to amphibolite facies and intruded by 96-45 Ma arc plutons. A crustal section recording paleodepths of ~5-40 km is preserved in the southern part of the core and facilitates evaluation of processes at different levels of the arc. After terrane juxtaposition, plutons were intruded during major arc-normal crustal shortening dominated by early recumbent folds and subsequent upright folds. Structural patterns emphasize the heterogeneous vertical partitioning of deformation and complex rheological stratification of arc crust at all scales. Metamorphic and geochronologic data indicate rapid burial of plutons and host rocks during the middle Cretaceous shortening. A subsequent major, cryptic event in the evolution of the Cascades core was the rapid underthrusting of Cretaceous sedimentary protoliths of the Swakane Gneiss to depths of >40 km between 73 and 68 Ma. The emplacement of the gneiss, which lacks arc magmas, may have removed the roots of many of the arc plutons. Plutonic rocks record both focused and unfocused, largely tonalitic magmatism that resulted in large plutons and abundant narrow sheets, respectively. Individual large-volume plutons were constructed over intervals of up to 5.5 m.y. Magmas probably ascended as visco-elastic diapirs, and emplacement was aided primarily by vertical material transfer, including ductile flow and stoping, and possibly regional folding. The magmatic, metamorphic, and structural processes recorded in the Cascades core exemplify the dynamic evolution of arcs and the large vertical and lateral displacements during arc construction.