U-Pb ages and Hf isotopes of detrital zircons from miogeoclinal strata of western North America

U-Pb ages and Hf isotope signatures have been determined from 32 samples of Neoproterozoic, Paleozoic, and lower Mesozoic miogeoclinal (passive margin) strata from western North America. Samples have been collected along five transects in eastern Alaska, northern British Columbia, southern British Columbia, Utah-Nevada, and northern Mexico. Detrital zircon grains from these samples were analyzed for U-Pb by ID-TIMS in the early 1990's to generate a spatial-temporal reference for the ages of grains that accumulated along the western edge of North America. We have recently re-analyzed these samples by CL-based LA-MC-ICPMS to determine more robust U-Pb age distributions (~200 grains/sample) as well as complementary Hf isotope signatures (~50 grains/sample). U-Pb ages from these samples match well with the ages determined by ID-TIMS, and in most cases resemble the ages of nearby basement rocks. Hf isotope signatures for these detrital grains reveal a fascinating history of crustal genesis and recycling. In southern regions, primary age groups are 1.0-1.2, ~1.4, and 1.6-1.8 Ga, which matches the ages of bedrock terranes in the region. Hf signatures of these grains record generation of juvernile crust at 1.6-1.8 Ga, followed by recycling of this crust during younger magmatism. Central transects contain these same age groups, plus a significant Late Archean contribution (presumably from the Wyoming craton). Hf data suggest formation of juvenile crust during Late Archean time, with significant recycling of this crust during 1.6-2.2 Ga magmatism. Little juvenile crust is recorded between 1.6 and 1.8 Ga, whereas, surprisingly, 1.0-1.2 and ~1.4 Ga zircons are considerably more juvenile than coeval grains to the south. Northern transects record protracted magmatism from 3.6 Ga to 300 Ma. Some juvenile crust of early Paleozoic, Early Proterozoic, and Late Archean age is represented, but most grains have Hf compositions that lie between observed 1.8-2.0 and 2.5-2.8 Ga Hf evolution trajectories. This presumably reflects homogeneous mixing of 1.8-2.0 and 2.5-2.8 Ga crust, rather than recycling of 2.0-2.3 Ga juvenile crust, as the 2.0-2.3 Ga grains present in the same samples are not juvenile.