EVOLUTION OF THE WESTERN METAMORPHIC BELT IN THE COAST MOUNTAINS ARC, BELLA BELLA, BRITISH COLUMBIA (LAT 52-53° N): PRELIMINARY RESULTS

Moderate to shallow crustal levels of the Late Cretaceous Coast Mountains arc are documented by new data from the western metamorphic belt near Bella Bella, >250 km along strike from Prince Rupert. The highest grade rocks occur in the northeast next to the Coast shear zone; metamorphic grade drops to greenschist facies 30 km across strike. In the highest grade rocks, early isoclinal folds and axial planar gneissic fabric are refolded into upright NW-SE plunging folds with local axial planar gneissic foliation. Low-grade rocks show upright N-S plunging folds with axial planar spaced cleavage. Sheath folds are common in the most ductile rocks. A dextral-reverse mylonitic shear zone separates the high and low grade parts of the belt, and dextral and SW-directed thrust slip is recorded on small ductile shear zones within the belt. Syn-kinematic metamorphism produced sillimanite-biotite-garnet gneisses, with relic kyanite in the NE. Si-ky gneiss yielded ~6.5 kb and ~600°C (GASP and ga-bi). Zircon U-Pb dating of meta-granite suggests sillimanite grade metamorphism and deformation were ongoing at ~77 Ma and over by ~74 Ma when intruded by a cross-cutting dike. Tonalitic plutons are abundant; most post-date penetrative deformation and metamorphism. Post-tectonic plutons ~20 km west of the sill-ky gneiss are 79-82 Ma with Al-in-hornblende pressures that decrease westward from 6.2 kb to 5.6 kb to 4.2 kb. Overall, the data show similarities to areas to the NW: metamorphic grade and intrusive pressures are highest in the northeast and the roots of the arc were shortened significantly during metamorphism. In contrast, this area preserves shallower crustal levels, younger deformation and metamorphism, and younger, more static plutonism. In addition, dextral shear occurred here and in near-surface rocks east of the Coast shear zone. Our result show that contraction of the arc and inferred crustal thickening continued until 75 Ma and was accompanied by dextral shear.