Low-pressure metamorphism of the mafic volcanic rocks of the Rossland group, southeastern British Columbia

Mafic volcanic rocks of the Rossland Group have been metamorphosed in the subgreenschist to lower amphibolite facies. Subgreenschist-facies regional metamorphic rocks are subdivided into prehnite-pumpellyite zone and prehnite-epidote zone. Fluid inclusions in two subgreenschist-facies veins yielded mean homogenization temperatures of 139 and 151 degrees C. Assuming a reasonable maximum temperature limit of 275 degrees C for the subgreenschist facies, the fluid-inclusion isochores indicate a pressure <250 MPa for regional metamorphism in the subgreenschist facies. This is consistent with the widespread occurrence of prehnite-chlorite-bearing assemblages. Metamorphic grade increases sharply northward approaching the large plutons of the Nelson suite. The contact aureoles of the Nelson batholith and the related Bonnington pluton encompass most of the region, producing an extensive region underlain by rocks within the hornblende-oligoclase zone. Intrusion of the Nelson plutonic suite overlapped with the development of the Hall Creek syncline and Silver King shear zone. The pattern of isograds across the Rossland Group indicates superimposed contact and regional metamorphism rather than progressively deeper structural levels northward.