SM-ND DATING OF MULTIPLE GARNET GROWTH EVENTS IN AN ARC-CONTINENT COLLISION ZONE, NORTHWESTERN UNITED-STATES CORDILLERA

Integrated petrologic and Sm-Nd isotopic studies in garnet amphibolites along the Salmon River suture zone, western Idaho, delineate two periods of amphibolite grade metamorphism separated by at least 16 million years. In one amphibolite, P - T studies indicate a single stage of metamorphism with final equilibration at approximately 600-degrees-C and 8-9 kbar. The Sm-Nd isotopic compositions of plagioclase, apatite, hornblende, and garnet define a precise, 8-point isochron of 128 +/- 3 Ma (MSWD=1.2) interpreted as mineral growth at the metamorphic peak. A Ar-40/Ar-39 age for this hornblende indicates cooling through approximately 525-degrees-C at 119 +/- 2 Ma. In a nearby amphibolite, garnets with a two-stage growth history consist of inclusion-rich cores surrounded by discontinuous, inclusion-free overgrowths. Temporal constraints for core and overgrowth development were derived from Sm-Nd garnet - whole rock pairs in which the garnet fractions consist of varying proportions of inclusion-free to inclusion-bearing fragments. Three garnet fractions with apparent ''ages'' of 144, 141, and 136 Ma are thought to represent mixtures between late Jurassic (pre-144 Ma) inherited radiogenic components preserved within garnet cores and early Cretaceous (approximately 128 Ma) garnet overgrowths. These observations confirm the resilience of garnet to diffusive exchange of trace elements during polymetamorphism at amphibolite facies conditions. Our geochronologic results show that metamorphism of arc-derived rocks in western Idaho was episodic and significantly older than in arc rocks along the eastern margin of the Wrangellian Superterrane in British Columbia and Alaska. The pre-144 Ma event may be an expression of the late Jurassic amalgamation of marginal oceanic arc-related terranes (e.g., Olds Ferry. Baker, Wallowa) during the initial phases of their collision with North American rocks. Peak metamorphism at approximately 128 Ma reflects tectonic burial along the leading edge of the Wallowa arc terrane during its final penetration and suturing to cratonic North America.