JURASSIC STRATIGRAPHY AND TECTONIC EVOLUTION OF THE WHITEHORSE TROUGH, CENTRAL YUKON: NEW INSIGHTS FROM LASER ABLATION SPLIT STREAM (LASS) DETRITAL ZIRCON U-PB GEOCHRONOLOGY AND HF ISOTOPE GEOCHEMISTRY

The Whitehorse trough records the rapid exhumation and erosion of the Intermontane terranes (Stikinia, Quesnellia, Yukon-Tanana, Cache Creek) during the early growth of the northern Canadian Cordillera. The tectonic evolution of the Whitehorse trough in central Yukon is largely preserved by the Early to Middle Jurassic Laberge Group, a ~3000-m thick succession of synorogenic clastic strata that unconformably overlies shallow-water, arc-marginal rocks of the upper Lewes River Group. New detrital zircon U-Pb and Hf isotope studies have been conducted with the novel laser ablation split steam (LASS) technique to test and further constrain the maximum depositional age, provenance, and tectonic significance of Laberge Group strata.

Detrital zircon samples from the Tanglefoot (marginal-marine to tidal strata with intervening mass-flow conglomerate) and Richthofen (turbiditic strata and mass-flow conglomerate) formations yielded maximum depositional ages that range from Late Triassic to Early Jurassic. Maximum depositional ages of basal strata support a westerly transgression across the axis of the trough after the initial uplift of the western flank. Dominant Late Triassic to Early Jurassic age peaks are consistent with the Laberge Group strata being sourced from 216-178 Ma plutons that intrude the Intermontane terranes and surround the Whitehorse trough. Minor late Paleozoic age peaks similarly indicate provenance from arc successions of Stikinia, Quesnellia and Yukon-Tanana. Detrital zircon Hf isotopic compositions range from juvenile to evolved, and furthermore indicate a progressive change from Late Triassic mantle-derived to Early Jurassic crustally-contaminated magmatism. Future detrital U-Pb and Hf isotope studies will test a westerly transgression of the trough and provide insight into the relationships between the timing of exhumation, sedimentation, and terrane accretion in central Yukon, leading to better constraints on the tectonic evolution of the northern Intermontane terranes.