Geology of the Matthew Creek metamorphic zone, southeast British Columbia: A window into Middle Proterozoic metamorphism in the Purcell Basin

Southwest of Kimberley, southeastern British Columbia, the Matthew Creek metamorphic zone occupies the core of a structural dome in Mesoproterozoic rocks of the Lower Aldridge formation (lower Purcell Supergroup). It comprises (1) a core zone of ductilely deformed sillimanite-grade metapelites, thin foliated mafic sills, and sheared quartz-plagioclase-tourmaline pegmatites; and (2) a thin transition zone of ductilely deformed metasediments which marks a textural and metamorphic transition between the core zone and overlying regionally extensive, brittlely deformed, biotite-grade semipelitic Lower Aldridge formation metasediments and thick Moyie sills. The core zone and transition zone in combination cover an area of 30 km(2). The deepest exposed rocks in the core zone have a strong foliation and lineation (D-1 deformation) formed during late M-1 metamorphism at conditions of 580-650 degrees C and 3.5 + 0.5 kbar. The timing of this metamorphic-structural episode is constrained to the interval 1352-1341 Ma based on near-concordant U-Pb ages from monazite in pelitic schist near the mouth of Matthew Creek. Later, weaker metamorphic and deformation episodes variably overprinted the rocks of the Matthew Creek metamorphic zone. The juxtaposition of low-grade, weakly deformed rocks above high-grade, strongly deformed rocks across a zone of ductile deformation is interpreted to be due to a subhorizontal shear zone.