Early Paleozoic post-breakup magmatism along the Cordilleran margin of western North America: new geochronological and geochemical results from the Kechika Group, Yukon, Canada

Abstract

Lower Paleozoic igneous rocks were emplaced along the Cordilleran margin of western North America following lithospheric breakup. Several competing rift models have been proposed to explain the significance of this magmatism to Cordilleran passive margin evolution. New field and laboratory studies of the Kechika group, south-central Yukon, were conducted to test these rift models and constrain the timing and tectonic setting of postbreakup magmatism. The Kechika group contains vent-proximal and sediment-sill facies that developed outboard of continental margin shelf and trough basins. Zircon U-Pb dates indicate that Kechika group rocks crystallized during the late Cambrian (488-483 Ma) and Early Ordovician (473 Ma). Whole-rock trace element and Nd-Hf isotope results are consistent with the low-degree partial melting of a lithospheric mantle source during margin-scale extension. Post-breakup rocks emplaced along the magma-poor North Atlantic margins, including the Orphan Knoll and Galicia Bank crustal blocks, are likely modern analogues for the Kechika group.