PRELIMINARY P-T-TIME MODELS FOR REGIONAL METAMORPHISM IN THE MOUNT WADDINGTON AREA OF THE SOUTHERN COAST MOUNTAINS BATHOLITH, BRITISH COLUMBIA CANADA

The Coast Mountains batholith (CMB) is an ideal place to study high-flux events (HFE) because multiple HFE have been identified and metamorphic rocks record deformation and metamorphism above a well-documented Late Cretaceous subduction zone. Driving mechanisms behind HFE are debated; however, one theory suggests that metamorphism in the deeper parts of fold and thrust belts contributes to, if not causes, HFE through metamorphic dehydration and/or partial melting. In this study of the CMB, we present P-T-t paths for metamorphism in order to evaluate the role of metamorphism in HFE.

Preliminary P-T paths for 8 samples from Bute Inlet, the Jewakwa Mountain area, the Mount Waddington area, and the Waddington fold-thrust belt were constructed using THERIAK-DOMINO and garnet compositional isopleths. Observed assemblages include garnet, sillimanite, biotite, quartz, plagioclase, ilmenite, ±staurolite, and ±muscovite, ±clinoamphibole, ±chlorite, ±clinozoisite. Garnet Mn and Fe# zoning for all samples indicate preservation of prograde growth with minimal diffusional resetting. P-T path results indicate that garnet growth began at 4-5 kbar at 550-650°C and continued to peak metamorphic conditions of 5-7 kbar at 600-750°C, with 5 samples reaching peak conditions at ~7 kbar and 3 samples at 5-6 kbar. The predicted peak P-T conditions are similar to previously published thermobarometric estimates in the Waddington fold-thrust belt (5-7 kbar at 675°C) and in the Mount Raleigh area (5.5-6.5 kbar at 675°C). Sillimanite is the only aluminum silicate found in our samples, compatible with similar medium pressure metamorphism throughout the region. The near isothermal pressure increases predicted for the P-T paths are compatible with thrusting and crustal thickening during garnet growth.

Our new peak P-T estimates are similar for all samples, indicating mid-crustal exposures along an ~80 km transect from Bute and Knight inlets eastward to the Waddington fold-thrust belt. Although metamorphic conditions are similar throughout this part of the CMB, two, likely separate, metamorphic events have thus far been identified with garnet Sm-Nd geochronology: 73-71 Ma and 99-90 Ma. Future work will construct new P-T paths, refine existing paths, and evaluate the relationship between metamorphism and the timing and nature of HFE.