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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 εtNd 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.

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