EVALUATION OF ANOMALOUSLY HIGH-T FOREARC METAMORPHISM OF THE PACIFIC RIM TERRANE ON VANCOUVER ISLAND, BRITISH COLUMBIA

The Pacific Rim Terrane comprises forearc metasedimentary and metaigneous rocks on southern Vancouver Island, British Columbia. Two units of the Pacific Rim Terrane, the Leech River Complex (LRC) and the Pandora Peak Unit (PPU) are fault bound between Wrangellia and the Crescent Terrane, to the north and south respectively, and thus bear on the process and configuration for terrane accretion during the Cenozoic. The LRC comprises tightly folded and transposed psammitic, tuffaceous, mafic, and pelitic protolith rocks. The PPU consists of banded cherts, argillites, melanges, cataclasites, and metabasalts. The contrasting lithologies and spatial relationship of the LRC and PPU has been problematic in interpreting the accretionary history. Past interpretations of the Pacific Rim Terrane metamorphic history invoke two periods (mid-Cretaceous and Eocene) of isobaric heating associated with suites of syn-deformation felsic intrusion to explain the anomalously high forearc metamorphic temperatures.

New estimates of peak metamorphic temperatures in both the LRC and PPU were determined over a larger area and variety of protoliths using Raman carbonaceous material geothermometry. This method shows the PPU was metamorphosed to temperatures of only between 230 and 360 °C, in stark contrast to the LRC which underwent only one primary heating event producing temperatures up to ~600 °C along the Leech River Fault. Peak temperatures show little correlation with exposures of the volumetrically small Eocene felsic intrusions, suggesting the latter are not a source of metamorphic heat, but are rather interpreted as partial melts of the LRC protolith. The monotonic metamorphic gradient in the LRC from north to south could be the consequence of underplating of the Pacific Rim Terrane at ~50 Ma by the young, hot oceanic crust of the Crescent Terrane that structurally underlies the LRC along the Leech River Fault. Past suggestions of a slab window or migration over a hot spot are also being evaluated.