SM-ND GARNET DATING OF POLYPHASE METAMORPHISM - NORTHERN COAST MOUNTAINS, SOUTH-EASTERN ALASKA, USA

Sm-Nd ages of garnet from the northern Coast Mountains of south-easter n Alaska, USA, constrain the timing of thermal events in polyphase met amorphic rocks of the western metamorphic belt and provide new data on the spatial extent of Cretaceous regional metamorphism. Bulk garnet-w hole-rock Sm-Nd ages for a sillimanite-zone amphibolite (Taku Inlet) a nd a biotite-zone metapelite (Tracy Arm) are 77 +/- 17 Ma and 59 +/- 1 2 Ma, respectively. Garnet core-whole-rock (80 +/- 9 Ma), core-matrix (84 +/- 9 Ma), rim-whole-rock (59 +/- 4 Ma) and rim-matrix (62 +/- 4 M a) ages were obtained from a sample collected 200 m west of a Palaeoce ne Coast plutonic-metamorphic complex sill-like pluton that separates medium-grade metamorphic rocks from high-grade metamorphic rocks and v oluminous Tertiary plutons in the core of the orogen. The garnet core ages of c. 80 Ma indicate that the regional metamorphic grade reached garnet zone prior to the intrusion of the plutons and high-grade metam orphism of rocks to the east. Similar ages for the younger plutons, th e youngest garnets and the rim of a multistage garnet (c. 59 Ma) indic ate a later episode of contact metamorphic garnet growth. Documentatio n of pre-71 Ma garnet-zone metamorphism along the western edge of the Coast plutonic-metamorphic complex confirms that Albian to Late Cretac eous metamorphism associated with crustal thickening affected this par t of the orogen. The similarity of garnet Sm-Nd ages to independent ag e estimates for metamorphic events confirms that this technique provid es useful estimates for the timing of Late Cretaceous to Tertiary ther mal events. The c. 20 Myr difference between garnet core and rim ages suggests that the Sm-Nd isotope systematics of a single garnet grain c an be used for distinguishing between multiple metamorphic events.