TECTONIC ACTIVITY ALONG THE WESTERN MARGIN OF LAURENTIA RECORDED IN PALEO- TO MESOPROTEROZOIC STRATA IN YUKON, CANADA

Recent field and analytical studies in the Proterozoic inliers of northern Yukon have led to revised interpretations of Paleo-to-Mesoproterozoic successions and the environments in which they were formed. These findings have replaced the depiction of northwestern Laurentia as a passive environment with a model of active tectonism involving terrane obduction, continental collision, brecciation, rifting and drifting. In turn, the role that Laurentia played in the evolution of supercontinent Columbia has been appreciably modified.

The Wernecke Supergroup is the oldest known sedimentary succession exposed in the Northern Canadian Cordillera and was deposited sometime after ~1660 Ma (50 m.y. later than previously proposed). The Wernecke Supergroup was deformed and metamorphosed during the ~1.60 Ga Racklan orogeny, and overthrust by Bonnetia, an oceanic terrane originating outboard of Laurentia. Geochemical analyses from fragments of Bonnetia, including mafic to intermediate plutonic and volcanic rocks, suggest Bonnetia was a volcanic arc affected by intra-plate magmatism, and possibly continental crust.

Following obduction, Bonnetia was overlain by sandstone, mudstone and soft-sediment breccia, and together with the Wernecke Supergroup, these units were cut by the 1599 Ma Wernecke Breccia, causing fragments of all lithologies to founder into the breccia zones and become juxtaposed with the Wernecke Supergroup. Subsequent erosion removed the arc terrane and the upper part of the Wernecke Supergroup prior to Mesoproterozoic sedimentation of the PR1 and Pinguicula basins.

Detrital zircon geochronology and geochemical analyses suggest a prevalent Laurentian source for Wernecke Supergroup sediments with possible inputs from other landmasses. In contrast, detrital zircons from the soft-sediment breccia above Bonnetia display a prominent U-Pb age peak at 1.68-1.78 Ga and reflect derivation from non-Laurentian sources, possibly Bonnetia and Australia. The PR1 basin contains a near-unimodal population of detrital zircons ca. 1500 Ma with a proposed source in the Mt. Isa inlier of Australia, pinning northeastern Australia to northwestern Laurentia in the Mesoproterozoic. The detrital zircon signature from the Pinguicula basin also suggests a non-Laurentian source such as Australia.