Evolution of the Cordilleran orogen (southwestern Alberta, Canada) inferred from detrital mineral geochronology, geochemistry, and Nd isotopes in the foreland basin

摘要

The provenance of sedimentary strata that accumulate in foreland basins record the growth and denudation of the adjacent orogen. We use U-Pb geochronology of detrital zircon and monazite, Sm-Nd isotope geochemistry, trac- and rare earth element geochemistry, and petrographic data from synorogenic clastic sedimentary rocks in the Cordilleran foreland basin of southwestern Alberta to provide new perspectives on the evolution of the orogen. Foreland basin clastic rocks comprise three major pulses of sediment delivery: (1) upper Fernie Formation–Kootenay Group (154–142 Ma), (2) Blairmore Group (115–103 Ma), and (3) Milk River Group–Porcupine Hills Formation (78–58 Ma). Nd isotope data are dominated by _Nd values of –7 to –12, interpreted to represent a well-mixed provenance from Devonian through Triassic strata and subordinate contributions from thrust-imbricated pr-Devonian strata of the Cordilleran miogeocline. Significant deviations to less negative (more juvenile) values between –5 and +1 represent periods when the foreland was flooded by juvenile detritus from oceanic arc sources such as Quesnel terrane and from syndepositional continental magmatic arcs of mid- and Late Cretaceous ages.