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Pressure – temperature and tectonic evolution of Triassic lawsonite – aragonite blueschists from Pinchi Lake, British Columbia

1996; NRC Research Press; Volume: 33; Issue: 5 Linguagem: Inglês

10.1139/e96-061

1480-3313

Edward D. Ghent, Philippe Erdmer, Douglas A. Archibald, Mavis Z. Stout,

High-pressure geophysics and materials

A blueschist and eclogite terrane is associated with one of the largest faults in the Canadian Cordilleran Orogen, the Pinchi fault. Blueschists (in situ) and retrogressed eclogite blocks occur along the Pinchi fault zone near 54°30'N and 124°W. Critical blueschist facies mineral assemblages include lawsonite–glaucophane, jadeite–lawsonite–glaucophane–quartz, and aragonite. White mica 40 Ar/ 39 Ar spectra on blueschist and eclogite yield ages in the range 221.8 ± 1.9 to 223.5 ± 1.7 Ma, establishing a direct link between the blueschists and eclogites. Preservation of aragonite sets rigid constraints on the pressure–temperature–fluid–time conditions of unroofing. K–Ar dates indicate that this is some of the oldest documented metamorphic aragonite. Comparison with computed petrogenetic grids suggests that metamorphic temperatures were in the range 200–300 °C, with pressures greater than 8–10 kbar (1 kbar = 100 MPa). Unroofing likely occurred during collision of the Cache Creek terrane with Quesnellia in the Late Triassic to Middle Jurassic. The fault was initiated as a plate boundary and was active as late as Eocene time as a strike-slip zone. The Pinchi blueschist terrane is similar to others in the North American Cordillera and highlights a tectonic regime of repeated blueschist metamorphism and rapid unroofing along many parts of the western margin of North America in the early Mesozoic.