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‐eastern Alaska, USA, constrain the timing of thermal events in polyphase metamorphic rocks of the western metamorphic belt and provide new data on the spatial extent of Cretaceous regional metamorphism. Bulk garnet–whole‐rock Sm–Nd ages for a sillimanite‐zone amphibolite (Taku Inlet) and a biotite‐zone metapelite (Tracy Arm) are 77±17 Ma and 59±12 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 Ma) ages were obtained from a sample collected 200 m west of a Palaeocene Coast plutonic–metamorphic complex sill‐like pluton that separates medium‐grade metamorphic rocks from high‐grade metamorphic rocks and voluminous 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 metamorphism of rocks to the east. Similar ages for the younger plutons, the youngest garnets and the rim of a multistage garnet ( c. 59 Ma) indicate a later episode of contact metamorphic garnet growth. Documentation of pre‐71 Ma garnet‐zone metamorphism along the western edge of the Coast plutonic–metamorphic complex confirms that Albian to Late Cretaceous metamorphism associated with crustal thickening affected this part of the orogen. The similarity of garnet Sm–Nd ages to independent age estimates for metamorphic events confirms that this technique provides useful estimates for the timing of Late Cretaceous to Tertiary thermal events. The c. 20 Myr difference between garnet core and rim ages suggests that the Sm–Nd isotope systematics of a single garnet grain can be used for distinguishing between multiple metamorphic events.