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 amphibolite s along the Salmon River suture zone, western Idaho, delineate two per iods of amphibolite grade metamorphism separated by at least 16 millio n years. In one amphibolite, P - T studies indicate a single stage of metamorphism with final equilibration at approximately 600-degrees-C a nd 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 approx imately 525-degrees-C at 119 +/- 2 Ma. In a nearby amphibolite, garnet s with a two-stage growth history consist of inclusion-rich cores surr ounded by discontinuous, inclusion-free overgrowths. Temporal constrai nts for core and overgrowth development were derived from Sm-Nd garnet - whole rock pairs in which the garnet fractions consist of varying p roportions of inclusion-free to inclusion-bearing fragments. Three gar net fractions with apparent ''ages'' of 144, 141, and 136 Ma are thoug ht to represent mixtures between late Jurassic (pre-144 Ma) inherited radiogenic components preserved within garnet cores and early Cretaceo us (approximately 128 Ma) garnet overgrowths. These observations confi rm the resilience of garnet to diffusive exchange of trace elements du ring polymetamorphism at amphibolite facies conditions. Our geochronol ogic results show that metamorphism of arc-derived rocks in western Id aho was episodic and significantly older than in arc rocks along the e astern 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 Fer ry. Baker, Wallowa) during the initial phases of their collision with North American rocks. Peak metamorphism at approximately 128 Ma reflec ts tectonic burial along the leading edge of the Wallowa arc terrane d uring its final penetration and suturing to cratonic North America.