DETRITAL ZIRCONS OF SOFT SEDIMENTS WITHIN THE WERNECKE BRECCIA REVEAL A LOCAL SOURCE OF 1.68 – 1.78 GA SEDIMENT IN NORTHWESTERN LAURENTIA

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 from metasomatic titanite. The host rock is the Wernecke Supergroup, a sedimentary succession deposited on the Laurentian margin between 1.66-1.60 Ga. The breccia zones are dominated by Wernecke Supergroup lithologies, but also contain a minority of clasts that were not sourced from the surrounding country rock. These clasts are locally several hundreds of metres in size and consist of plutonic, volcanic and sedimentary rock. The igneous clasts are inferred to be sourced from Bonnetia, a non-Laurentian volcanic arc terrane that was thrust upon the Wernecke Supergroup prior to 1.60 Ga

The exotic sedimentary clasts comprise red interbedded mud and sandstone, a red soft sediment breccia, and green mudstone .The red lithologies occur as clasts and irregular patches up to 20 m across that grade into “normal” Wernecke Breccia. Clast boundaries and internal layering of the soft-sediment breccia are commonly curved or contorted. Together, these features suggest that the red sedimentary units underwent soft-sediment deformation during Wernecke brecciation. The green mudstone is locally concentrated in between breccia clasts. It appears to have been completely unconsolidated at the time of its incorporation into the breccia zones and was probably sourced from the Earth’s surface. We speculate that the Wernecke breccia zones breached the surface and allowed partially to non-lithified sediments to pour into fall-back depressions as semi-consolidated sediment and liquefied mud.

Detrital zircons from the soft-sediment breccia were dated by U-Pb using LA-ICPMS and SHRIMP. The age profile shows a large peak at 1.78-1.68 Ga with lower abundances of older ages. The 1.78-1.68 Ga grains are subangular in contrast to more rounded grains of similar age from the Wernecke Supergroup, and are likely to have been more locally sourced. Proximal Laurentian sources with 1.78-1.68 Ga ages are scarce. A possible source is Bonnetia, which contains rocks of 1.71 Ga and possibly a broader range of ages. Alternatively, sources of this age are abundant in the Gawler craton of eastern Australia, which may have been close to the Laurentian margin around the time of the Wernecke brecciation event.