METAMORPHIC EVIDENCE FOR TILT OF THE SPUZZUM PLUTON - DIMINISHED BASIS FOR THE BAJA BRITISH-COLUMBIA CONCEPT

To address the question of tilt versus translation as the mechanism responsible for discordance between paleomagnetic directions of Cretaceous plutons in the British Columbia Coast Plutonic Complex and the North American reference direction, metamorphic pressures around the margin of the Spuzzum pluton have been determined. Pressures are derived from microprobe analyses and evaluation of exchange equilibria in the assemblage garnet-biotite-plagioclase-aluminum silicate-quartz. Samples studied come from eight localities in the contact aureole around the pluton and encompass the area of a previous paleomagnetic study. The analyzed samples are coarse grained and exhibit textural features indicative of equilibrium crystallization following emplacement of the Spuzzum pluton. Results of this study indicate a complicated tilt history for the Spuzzum pluton, with tilt first to the northeast then to the southwest. The southwest tilt can fully explain the discordance of the paleomagnetic direction in the Spuzzum but does not preclude translation. However, in view of the tilt history of this pluton, the paleomagnetic data derived from it should not be used in an analysis of north-south translation.