TIMING AND P-T CONDITIONS FOR PARTIAL MELTING AND INTRUSION IN THE NASON RIDGE MIGMATITIC GNEISS: NORTH CASCADES, WA

The southern end of the Coast Plutonic Complex - North Cascades Core (CC) - is dominated by two metamorphic culminations: the Swakane Gneiss in the north and the Nason Ridge Migmatitic Gneiss (NRMG) in the south. The NRMG crops out in a NW-SE trending swath of gneiss and migmatite with Chiwaukum Schist to the NE and SW. Metamorphic textures and P-T-t paths indicate that a 4 to 6 km thick thrust? wedge loaded the presently exposed rocks of the Chiwaukum Schist and NRMG prior to ca. 90 Ma. Differentiating between igneous rocks that originated from partial melt and those derived from intrusive magma has been difficult in the NRMG, in part due to the sparse age data with which to compare the timing of intrusion and metamorphism. Oscillatory zoned zircon cores further compound the challenge of relating age data to igneous petrogenesis. Zircon cores with Precambrian U-Pb ages are inferred to have been incorporated into melt from the wall rock. Zircon cores with late Cretaceous U-Pb ages are inferred to have grown from melt. Unzoned or chaotically zoned zircon rims are interpreted as metamorphic overgrows because U-Pb ages of 93.9±1.9 to 84±2.3 are synchronous with 83.1±1.6 to 90.7±1.5 garnet Sm-Nd ages from Chiwaukum Schist samples between the NRMG and the Mt. Stuart batholith. These interpretations concur with petrologic studies, which indicate that a combination of partial melting and intrusion of external magma were important in evolution of the NRMG. Similarity of P-T paths for Chiwaukum and NRMG rocks and the NRMG U-Pb zircon ages are compatible with near synchronous rapid heating event that postdates loading of the Nason terrane. Restriction of high igneous rock modes [10-25%] to the elongate NRMG outcrop and petrologic studies suggest that heating and partial melting were linked to intrusion of external magma.