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

Powell, W.G., and Ghent, E.D.

1996, Canadian Journal of Earth Sciences, v. 33, pp. 1402-1409

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 151C. Assuming a reasonable maximum temperature limit of 275C for the subgreenschist facies, the fluid inclusion isochores indicate a pressure of <250MPa 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 Groupindicates superimposed contact and regional metamorphism rather than progressively deeper structural levels northward.