Geology and geochronometry of the Cogburn creek-settler creek area, northwest of Harrison lake, B.C.

Metamorphic supracrustal rocks in the Cogburn Creek area belong to the Cogburn Creek Group and the Settler Schist. These are separated by a melange zone, which has been correlated with the Shuksan thrust zone and contains Baird Metadiorite and ultramafic rocks, and intruded by the Spuzzum batholith and minor younger granodiorite. Three phases of folding are recognised in the schist units: f₁ is associated with contact metamorphism that preceded regional metamorphism, f₂ produced pervasive mica foliation and tight folds, and kinks and broad warps are associated with f₃, which was locally pervasive approaching pluton margins. Mineral assemblages indicate increasing metamorphic grade from west to east from garnet to garnet-staurolite, andesine-epidote amphibolite, staurolite-kyanite, fibrolite, and coarse sillimanite zones. Metamorphic conditions vary from 300 to 500 °C in Cogburn Creek Group rocks to 550 to 700 °C at 6 to 8 kbar for pelites in the Settler Schist. Conditions deduced for metamorphism of the ultramafic rocks are consistent with those for enclosing pelitic schists. Geochronometry indicates that the Baird Metadiorite is probably Precambrian and equivalent to the Yellow Aster Complex of the North Cascade Mountains, Washington. The Cogburn Creek Group was dated as Late Paleozoic (296 ± 58 Ma, Rb-Sr WR isochron), and is provisionally correlated with the Bridge River Group. The protolith of the Settler Schist was deposited around 210 ± 27 Ma (Rb-Sr WR isochron), and it contains 2450 ± 230 Ma detrital zircon indicating partial ultimate derivation from Precambrian basement rocks. The Spuzzum batholith was intruded at 95 to 110 Ma, before the culmination of regional metamorphism. Rb-Sr biotite dates from all units and K-Ar Hb isochron dates in the range 66 to 88 Ma are metamorphic cooling dates. The youngest intrusive rocks, granodiorite dated at 32 ± 2 Ma to 42 ± 14 Ma, postdate the regional metamorphic and intrusive event. Movement on the Shuksan Thrust is bracketed as Albian, after regional blueschist metamorphism of the Shuksan Suite in the North Cascade Mountains and before intrusion of Spuzzum batholith and regional metamorphism east of Harrison Lake.Science, Faculty ofEarth, Ocean and Atmospheric Sciences, Department ofGraduat