PRESSURE-TEMPERATURE AND TECTONIC EVOLUTION OF TRIASSIC LAWSONITE - ARAGONITE BLUESCHISTS FROM PINCHI LAKE, BRITISH-COLUMBIA

A blueschist and eclogite terrane is associated with one of the larges t faults in the Canadian Cordilleran Orogen, the Pinchi fault. Bluesch ists (in situ) and retrogressed eclogite blocks occur along the Pinchi fault zone near 54 degrees 30'N and 124 degrees W. Critical blueschis t facies mineral assemblages include lawsonite-glaucophane, jadeite-la wsonite-glaucophane-quartz, and aragonite. White mica Ar-40/Ar-39 spec tra on blueschist and eclogite yield ages in the range 221.8 +/- 1.9 t o 223.5 +/- 1.7 Ma, establishing a direct link between the blueschists and eclogites. Preservation of aragonite sets rigid constraints on th e pressure-temperature-fluid-time conditions of unroofing. K-Ar dates indicate that this is some of the oldest documented metamorphic aragon ite. Comparison with computed petrogenetic grids suggests that-metamor phic temperatures were in the range 200-300 degrees C, with pressures greater than 8-10 kbar (1 kbar = 100 MPa). Unroofing likely occurred d uring collision of the Cache Creek terrane with Quesnellia in the Late Triassic to Middle Jurassic. The fault was initiated as a plate bound ary and was active as late as Eocene time as a strike-slip zone. The P inchi blueschist terrane is similar to others in the North American Co rdillera and highlights a tectonic regime of repeated blueschist metam orphism and rapid unroofing along many parts of the western margin of North America in the early Mesozoic.