Metamorphism, deformation, and plutonism in the Mount Raleigh Pendant, Coast Mountains, British Columbia

Vancouver and about 25 km from the eastern margin of the Coast Mountains. Lower Cretaceous volcanicla stic rocks, greywacke and conglomerate in the Mount Raleigh pendant were deposited on a basement of quartz diorite and folded granitoid rocks of the Central Gneiss Complex. Upper Triassic(?) and Lower Jurass ic( ?) amphibolite, f eldspathic schist and marble may have been thrust over the Lower Cretaceous strata. The pendant strata were folded into a gently-plunging open syncline prior to emplacement of most of the plutons that bound the pendant. The metamorphic grade of pelitic rocks increases from andalusite+garnet in the northeast part of the pendant to sillimanite+garnet+cordierite to the southwest. Temperatures during metamorphism reached about 700°C at pressures of about 6.5 kb. Metamorphism resulted from regional thermal gradients and is independent of plutonism at the present level of exposure. Peak metamorphism probably occurred during uplift and was accompanied by the diapiric emplacement of synmetamorphic plutons that deformed and modified the earlier structure of the pendant. Five K-Ar dates indicate this period of metamorphism and deformation was concluded by 70 Ma ago. Posttectonic plutons were emplaced during the Early Tertiary; accompanying contact metamorphism reached about 800°C. Most pendants in the southern Coast Mountains may be graben-like structures. The high lithostatic pressures reached during metamorphism of the Mount Raleigh pendant may be due to emplacement of plutonic diapirs above the pendant. Most plutons in the map-area were intruded by forcible injection and wedging apart of the strata with accompanying radial distension and def or ma ti on of the wall rocks. Most Early Tertiary plutons in the southern Coast Mountains may have been emplaced at relatively shallow levels.