Detrital zircon U-Pb geochronology of the Magog Group, southern Quebec – stratigraphic and tectonic implications for the Quebec Appalachians

Abstract

In the Quebec Appalachians, the Laurentian continental margin (Humber Zone) and adjacent oceanic domain (Dunnage Zone) were amalgamated during the Ordovician Taconian orogeny. The Dunnage Zone includes ophiolites and overlying synorogenic deposits of both the Saint-Daniel Mélange and Magog Group. The latter consists of a ∼3 km-thick pile of sandstone, felsic volcaniclastic rocks and graphitic slate at the base (Frontière, Etchemin and Beauceville formations) overlain by a ∼7 km-thick turbiditic flysch sequence, constituting the Saint-Victor Formation. The maximum upper age limit of the Magog Group was considered to be Darriwilian based on graptolite fauna. This was proven consistent with a 462⁺⁵/₋₄ Ma (U-Pb ID-TIMS) from a felsic tuff of the Beauceville Formation, but contradicts a detrital zircon U-Pb age of 424 ± 6 Ma recently measured in the Saint-Victor Formation. A new detrital zircon U-Pb geochronology study (HR-LA-ICP-MS and ID-TIMS), focused on the Saint-Victor Formation, yields young detrital populations that suggest that the Saint-Victor Formation is not exclusively Ordovician and extends into the Silurian, as indicated by a maximum age of sedimentation around 430 Ma. Detrital zircon U-Pb geochronology associated with fossil age constraints and stratigraphic correlations in adjacent areas attest that the Saint-Victor Formation should be considered as an upper Magog Group sequence that is separated from lower units (the Frontière-Etchemin-Beauceville formations) by an unconformity corresponding to a sedimentary hiatus of ≤ 10 m.y. Regional tectonic considerations imply that the Magog Group evolved from a syn-Taconian forearc basin in Middle-Late Ordovician time to a syn-Salinic peri-continental basin in early Silurian time. Several NW and SE erosional sources are invoked for the sedimentation of the Magog Group, evolving from the erosion of both the southern Quebec ophiolites and adjacent sedimentary rocks of the Laurentia margin to the NW, and volcanic arc rocks of the Ascot Complex, Shelbourne Falls and Bronson Hill massifs to the SE. Potential sources for ca. 430 Ma zircons found towards the top of the Saint-Victor sequence are the Silurian Frontenac Formation and the East Inlet granitic pluton, both located in the vicinity of the Quebec-Maine border.