Relationships Between Metamorphism, Deformation, And Plutonism As Constrained By Garnet Geochronology From The Central Coast Mountains, British Columbia

Abstract

Metamorphism, deformation, and plutonism are the processes that are responsible for the generation of the continental crust and the tectonic evolution of plate margins. Understanding the interactions of these processes is hampered by our ability to directly date rock forming minerals. Lu-Hf geochronology of garnet is a powerful technique that provides the ability to date the growth of garnet that can be directly tied to metamorphism and deformation. This study utilized Lu-Hf geochronology in combination with structural and metamorphic analysis of two metamorphic belts in coastal British Columbia. This work showed that direct dating of garnet contradicts prior results derived from U-Pb dating of cross-cutting relationships. The new results require revision of the tectonic history of the Canadian Cordillera and have important implications for the entire North American Cordillera. Profound crustal thickening across thrust faults and transpressive shear zones characterizes the structure of the Prince Rupert area. Newly obtained Lu-Hf garnet ages date metamorphism, deformation, and the development of a regionally inverted metamorphic sequence. Prior work had previously inferred that the emplacement of the 90 Ma Ecstall pluton was synchronous with regional metamorphism and deformation. The garnet ages show that the Ecstall pluton was emplaced 10 to 15 m.y. after metamorphism and deformation. This result demonstrates that discordant paleomagnetic poles for the Ecstall pluton cannot be the result of deformation of the pluton and instead are consistent with the pluton being displaced from more southerly latitudes. To the east, within the central gneiss complex (CGC), Lu-Hf ages date the inception of granulite facies metamorphism and document strain partitioning. Crosscutting relationships observed in the field require deformation to have progressed temporally between adjacent domains; however, the garnet ages require that deformation was synchronous between domains requiring strain partitioning during dextral transpression. This contrasts with theoretical models for transpression that predict distributed deformation within a single oblique slip shear zone. In both studies, the Lu-Hf system resolves the age of garnet growth despite metamorphic conditions in the upper-amphibolite- to granulite-facies. This indicates that the Lu-Hf system in garnet is an extremely robust tool for deciphering metamorphic and deformation ages despite complex thermal histories.