Geochronometry of the Eagle Plutonic Complex and the Coquihalla area, southwestern British Columbia

Abstract

New U–Pb, K–Ar, and Rb–Sr dates from the Eagle Plutonic Complex and adjacent map units place timing constraints on intrusive and deformational events along the southwestern margin of the Intermontane Belt. U–Pb zircon minimum dates for Eagle tonalite and gneiss (148 ± 6, 156 ± 4, and 157 ± 4 Ma) document previously unrecognized Middle to Late Jurassic magmatism and syn-intrusive deformation along the eastern margin of the Eagle Plutonic Complex and the southwestern margin of the Intermontane terrane. Widespread mid-Cretaceous (Albian–Cenomanian) resetting of K–Ar and Rb–Sr isotopic systematics in Jurassic and older rocks is coeval and cogenetic with emplacement of plutons of the Fallslake Plutonic Suite (110.5 ± 2 Ma, U–Pb) which crosscut Jurassic plutons and structures but were themselves ductilely deformed along the Pasayten fault during sinistral, east-side-up, reverse displacement. K–Ar and Rb–Sr cooling dates for the Fallslake Suite of ca. 100 Ma, including dates from mylonites along the Pasayten fault, suggest that uplift, cooling, and unroofing of the Eagle Plutonic Complex occurred in mid-Cretaceous time along the Pasayten fault. Regional geologic evidence suggests that this thermal and unroofing event affected much of the southwest margin of the Intermontane Belt. Initial 87Sr/86Sr ratios and U–Pb geochronometry for the Fallslake Plutonic Suite suggest that it was derived, in part, from preexisting and relatively nonradiogenic Paleozoic to Mesozoic crust. K–Ar dating of several stocks demonstrates widespread Early Eocene plutonism in the Coquihalla area, and dating of the Needle Peak pluton indicates plutonism continued into Middle Eocene time.