An estimated 6 km of basic to silicic volcanic flows and clastic rocks of the Early Jurassic Telkwa Formation is exposed in moderately east-dipping fault blocks along the Zymoetz River, British Columbia. Extensive wholesale zeolitic replacement of porous tuff beds suggests widespread hydrothermal activity. Metamorphic grade increases regionally from laumontite–albite facies to prehnite–pumpellyite facies with increasing stratigraphic depth. Telkwa Formation strata and the imparted metamorphic zonation are cut and tilted by rotational block faulting, and are repeated in each of the upturned blocks. Late Mesozoic to Tertiary plutonism locally thermally overprinted the regional facies, particularly in the western part of the area. Fluid-inclusion isochores, combined with calculated mineral equilibria, suggest that metamorphism took place at fluid pressures of 2 kbar (1 kbar = 100 MPa) or less, consistent with estimates of stratigraphic burial. Metamorphic fluids were H2O rich and low in dissolved salts. Maximum temperatures during regional depth-controlled hydrothermal metamorphism, based upon the widespread presence of laumontite and the lack of wairakite in the middle to upper parts of the Telkwa Formation, probably did not exceed about

250 °C at H2O pressures of 2 kbar. Mineral zones, estimated paleotemperatures, and geothermal gradients are comparable to regional hydrothermal metamorphism in active volcanic settings such as Iceland.

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