New U–Pb geochronology from Timiskaming-type assemblages in the Shebandowan and Vermilion greenstone belts, Wawa subprovince, Superior Craton: Implications for the Neoarchean development of the southwestern Superior Province

Abstract

Timiskaming-type assemblages in the Wawa–Abitibi terrane of the Superior Craton and other Archean cratons worldwide are important because of their spatial and temporal association with world-class orogenic gold mineralization, especially in the Kirkland Lake area of Ontario. These mainly locally derived sedimentary assemblages are deposited in transpressional, pull-apart basins that developed during the final stages of terrane accretion, and are commonly associated with calc-alkalic to alkalic volcanic and plutonic rocks. The Shebandowan and Vermilion greenstone belts (SGB, VGB) in the Wawa subprovince have significantly less economic base and precious metal mineralization than the greenstone belts of the Abitibi subprovince and the timing of deformation and magmatism in the SGB and VGB is less well constrained. This study presents new U–Pb geochronology from the SGB and VGB that demonstrating that the sedimentary and volcanic rocks of the Lake Vermilion Formation in the VGB are coeval with similar rocks in the SGB and their interpreted depositional environment are consistent with these units being Timiskaming-type assemblages. Analyses of magmatic zircons in a dacitic tuff breccia from the Gafvert Lake sequence, the volcanic member of the Lake Vermilion Formation, provided a high precision U–Pb age of 2689.7±0.8Ma using thermal ionization mass spectrometry. Detrital zircon geochronology using LA-ICP-MS confirms that the source of the detritus in the Lake Vermilion Formation was locally derived from the Gafvert Lake metavolcanic lithofacies. A similar temporal pattern was observed in many of the Timiskaming-type sedimentary rocks that were sampled in the SGB. The locally derived, alluvial–fluvial to shallow marine depositional environment in long, narrow basins with rapid lateral facies changes described in the SGB and VGB are consistent with deposition in structurally controlled, pull-apart basins. However, one sample from the SGB had detrital zircons with ages that were more akin to the age range found to the north in the Wabigoon subprovince, which is suggestive of trans-terrane transport of detritus in a foreland-type basin. Geochemically, the Timiskaming-type volcanic rocks in the VGB and SGB are less enriched in incompatible elements and are comparatively less alkalic than the volcanic rocks in the Timiskaming assemblage of Kirkland Lake. This may indicate that the structures that formed the basins within the SGB and VGB in the Wawa were not as deeply penetrating as those in the Abitibi, which may explain the paucity of economic gold mineralization associated with the former. The Timiskaming-type deposits of the northern Wawa–Abitibi terrane are 20Ma older than the Timiskaming assemblage in the Abitibi subprovince, and co-depositional with the flysch-like deposits of the Porcupine assemblage in the southern part of that terrane.