EMPLACEMENT HISTORY OF THE ECSTALL PLUTON FROM STRUCTURAL KINEMATICS AND METAMORPHISM; PRINCE RUPERT AREA, BRITISH COLUMBIA

The Baja-BC debate is focused on the paleogeography of western North America during the Mesozoic. Interpretations of paleomagnetic data indicate either: (1) much of the Canadian Cordillera was at the latitude of Mexico at 90 Ma or (2) the terranes have been at northern latitudes since Jurassic assembly and discordant paleopoles reflect in situ deformation post-magnetic locking. The Ecstall pluton has been a key part of this debate because paleopoles for the western side of the pluton are highly discordant, decreasingly across the pluton to the east. This has been attributed to folding of the pluton into an antiform during waning stages of emplacement or reheating and generation of a secondary magnetization. Here we examine the models by comparing the geologic structure of the pluton to the predictions of the paleomagnetically-based folding model.

The Ecstall is an epidote-bearing hbl qtz diorite (94-90 Ma) located between the Grenville Channel (GCSZ) and Coast Shear Zones in northwestern British Columbia. The pluton intruded a ductile fold and thrust belt built on the eastern margin of the Insular Terrane. Emplacement was synkinematic with sinistral shearing along the GCSZ at 25 km (0.8 GPa), followed by exhumation. Mapping focused along the northeast margin in previously inaccessible regions. Country rocks contain northeast-verging folds with an average fold axis of 45/147 and axial surfaces concordant with the pluton's contact (110/56 S). Country rock gneiss contains abundant leucosome layers concentrated parallel to the foliation (100/44 S), suggesting syn-deformation migmatization. Asymmetrical tails on garnet porphyroclasts from contact metamorphism incorporated in the deformation fabric indicate top-to-the northeast shear, consistent with fold vergence.

The magmatic foliation defines a basin-like structure, with all foliations moderately dipping and concordant to the pluton boundary. Pegmatite dikes contained within the Ecstall fan radially, normal to the pluton margins, and are consistent with lateral, oblique expansion of the pluton away from the GCSZ. Our data indicates that the pluton was not folded after the last gasps of crystallization as the pegmatites were intruded, and therefore interpret that the paleomagnetic signature results from partial reheating of the pluton from an eastern source.