CORRELATION OF METAMORPHIC EVENTS AND PLUTON EMPLACEMENT IN THE NORTH CASCADES, WA

Garnet Sm-Nd and zircon U-Pb ages provide a direct means of evaluating the timing, duration, and distribution of metamorphic events. In the North Cascades, garnet ages hint that the earliest metamorphism occurred 143-107 Ma west of the Entiat Fault in the Wenatchee block and may be of similar age east of the Entiat Fault in the Chelan block. Garnet and zircon ages date growth of widespread metamorphic mineral assemblages at 92-86 Ma in both fault blocks. In the Wenatchee block where metamorphic conditions reached mid- to upper-amphibolite facies, initial metamorphism predated plutonism, continued during widespread emplacement of tonalite to granodiorite plutons including the 96-91 Ma Mount Stuart batholith and 92-91 Ma orthogneiss at Labyrinth Mountain, and then peaked at 88-86 Ma after intrusion of the Mount Stuart. The last event was locally, but not everywhere associated with emplacement of numerous thin intrusive sheets of the Nason Ridge Migmatitic Gneiss. In the Chelan block where metamorphic conditions reached and/or exceeded upper amphibolite facies, initial 90.1±1.8 Ma (recorded in zircon rims and sector-zoned zircon cores) and later 80.9±1.9 Ma (recorded in narrow zircon rims) metamorphism have no close spatial association with plutons; however, 90 Ma metamorphism was synchronous with ca. 90 Ma emplacement of the Seven-Fingered Jack suite ca. 20 km to the north. These two events have not been characterized in the Chelan block, but the oldest correlates with peak metamorphism in the Wenatchee block. The youngest ‘peak' event in the Chelan block caused growth of zircon rims (72.4±2.3 Ma) and garnet (71.3±2.8 Ma) at 750ºC and 11-13 kbar synchronous with 73-71 Ma emplacement of the adjacent Entiat Pluton above the Dinkelman Decollement. Garnet and zircon ages indicate synchronicity of pluton emplacement and metamorphism throughout large areas of the crust between 92 and 72 Ma. We conclude that the thermal effects of pluton emplacement ranged from local heating and contact metamorphism to more widespread heating beyond local thermal aureoles, and that the Swakane Gneiss likely represents the lowermost crust of the Cascades during the Late Cretaceous.