SPATIAL AND TEMPORAL VARIATIONS IN ND ISOTOPIC SIGNATURES ACROSS THE CRYSTALLINE CORE OF THE NORTH CASCADES, WASHINGTON

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 composition of granitoid plutons that intrude the Cascades core reflect source regions at depth and can provide important information about crustal architecture. We present Sm-Nd isotope data from 96 Ma to 45 Ma plutons and the meta-igneous and meta-sedimentary terranes that they intrude across the Cascades core, comparing the change in initial εNd signatures across major terrane boundaries. Sm-Nd data from metamorphic terranes of the North Cascades, excluding the much younger ca. 73 Ma Swakane terrane, yield εNd values in the mid-Cretaceous that range from +9.4 to -2.7 and indicate little incorporation of an evolved crustal component. Amphibolites from the Napeequa Complex and Chiwaukum Schist yield near-depleted mantle εNd values in the mid-Cretaceous, and εNd values from meta-clastic rocks from these terranes (+3.4 to -1.9) are compatible with the inferred arc-derived basinal and oceanic protolith. Initial εNd values of the Swakane Gneiss range from +0.6 to -5.4 and reflect mixing of craton-derived and arc-derived sources with a significant input from the craton.

The initial εNd signatures of all plutons studied range from +1.5 to +6.3; however initial εNd values are influenced by local variations in the Nd signature of the crustal source. The εNd values of plutons are consistent with geochemical studies that suggest the plutons were generated by mixing of mantle-derived melt and melt derived by anatexis of isotopically juvenile terranes. Initial εNd values of plutons from the NE part of the Cascades core generally decrease over time suggesting melting from more evolved crustal sources. These temporal variations in Nd signature are also observed between the northern and southern Coast Plutonic Complex, and may indicate changes in the sources of crustal melting along the length of the Cretaceous arc.