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Spatial and temporal variations in Nd isotopic signatures across the crystalline core of the North Cascades, Washington


Spatial and temporal variations in Nd isotopic signatures across the crystalline core of the North Cascades, Washington



Special Paper - Geological Society of America 438: 499-516



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 epsilon (sub 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 epsilon (sub Nd) values in the mid-Cretaceous, and epsilon (sub 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 epsilon (sub Nd) values of the Swakane Gneiss range from +0.6 to -5.4 and reflect a significant input from the miogeocline. The initial epsilon (sub 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 delta (sub 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.

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