POST-OROGENIC OVERLAP SUCCESSION SPANNING THE GAWLER CRATON AND NORTHWESTERN NUNA AT 1.6 GA

The Wernecke Breccia is a set of hydrothermal breccia zones that occur in northern Yukon, Canada. The brecciation has been dated at 1.599 Ga by U-Pb on metasomatic titanite. The host rock is the Wernecke Supergroup, a sedimentary succession deposited on the Laurentian margin between 1.66 and 1.60 Ga. The breccia clasts are dominated by Wernecke Supergroup lithologies, but also include anomalous clasts that were sourced from other units. The anomalous clasts consist of plutonic, volcanic and sedimentary rock that are locally hundreds of metres in size. The igneous clasts were sourced from Bonnetia, a non-Laurentian volcanic arc terrane that was thrust over the Wernecke Supergroup prior to 1.60 Ga.

The anomalous sedimentary clasts comprise red interbedded mudstone and sandstone, red conglomerate and green mudstone. Textures of these sedimentary clasts include undulatory and contorted boundaries and indicate that these units were soft during Wernecke Breccia formation. The anomalous sedimentary clasts were incorporated into the breccia zones as unlithified sediments after the hydrothermal systems breached the surface. Detrital zircon extracted from the sedimentary clasts is largely subangular. An age profile obtained by LA-ICPMS and SHRIMP analysis displays a prominent peak at 1.78 to 1.68 Ga, lacks 2.4 – 2.1 Ga ages, and has a minor peak at 2.5 Ga.

The detrital zircon age profile is unlike those from the Wernecke Supergroup. However, it is remarkably similar to age profiles of lithologically similar sedimentary clasts preserved within the ca. 1.59 Ga Olympic Dam breccias on the Gawler Craton of Australia. Based on this similarity and previous reconstructions that link the Wernecke and Olympic Dam hydrothermal provinces, these clasts infer the former existence of an overlap assemblage that was deposited on both the Gawler Craton and northwestern Nuna. The only known relics of this once-extensive succession are the clasts within the Wernecke and Olympic Dam breccias. Much of the zircon was likely derived from metasediments and intrusions of the Gawler Craton. In order to accommodate subsequent deposition of sediment from northeast Australia onto Laurentia at ca. 1.5 Ga, we propose that Australia moved southwards (current coordinates) along the Laurentia margin from 1.6 – 1.5 Ga.