Nd ISOTOPIC SIGNATURES OF GRANITOID PLUTONS ACROSS THE CRYSTALLINE CORE OF THE NORTH CASCADES, WASHINGTON: IMPLICATIONS FOR THE TECTONIC EVOLUTION OF A MAGMATIC ARC

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 plate margin. 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 and 45 Ma plutons along a transect across the Cascades core, comparing the change in initial εNd signatures across major terrane boundaries. The Tertiary Entiat fault divides the Cascades core into the Wenatchee and Chelan blocks. Predominantly tonalitic and associated subordinate gabbroic magmas intrude both blocks of the core from ca 96 Ma to 88 Ma. Tonalitic plutons that range in age from ca. 76 Ma to 72 Ma and granodioritic plutons that range in age from ca. 48 Ma to 45 Ma are generally restricted to the Chelan block. The initial εNd signatures of all plutons studied range from 1.5 to 6.3; however plutons that intrude the Wenatchee block exhibit a restricted range from 3.1 to 4.4. The initial εNd values indicate that these plutons were derived from or interacted with an older crustal component. Initial εNd values from ca. 46 Ma plutons in the Chelan block range from 2.9 to 1.5, and are significantly lower than the εNd calculated for the older plutons at that time suggesting that the youngest suite of plutons were derived from a predominantly continental crustal source. This interpretation is also supported by the observation that only the youngest suite of plutons contains significantly older (ca. >240 Ma) inherited zircons. This inferred change in degree of involvement of the crustal source region between ca. 73 Ma and ca. 46 Ma can be tied to major tectonic changes occurring within the arc as evidenced by the burial history of the 9-11 kbar metasedimentary Swakane Gneiss. Detrital zircons from the gneiss are as young as 73 Ma and indicate that this unit was buried to ≥40 km depth within the arc after 73 Ma. The burial of the Swakane Gneiss potentially thrust arc crust over older lithosphere, providing a more evolved source region for the ca. 46 Ma plutons.