DETRITAL ZIRCON U-PB GEOCHRONOLOGY OF THE UPPER MAGOG GROUP, SOUTHERN QUÉBEC APPALACHIANS: SILURIAN, NOT ORDOVICIAN!

In the Quebec Appalachians, the Laurentian continental margin (Humber zone) and adjacent oceanic domain of the 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 ~3 km pile of sandstone, felsic volcaniclastic rocks, graphitic slate and sandstone at the base (Frontière, Etchemin and Beauceville formations) overlain by a ~7 km-thick of turbiditic flysch sequence, constituting the St-Victor Formation at the top. The maximum age limit for the Magog Group is currently considered to be Caradocian based on graptolite fauna. This has been proven consistent with a 462 +5/-4 Ma (U-Pb ID-TIMS) from a felsic tuff of the Beauceville Formation, but in contradiction with a detrital zircon U-Pb age of 424 ± 6 Ma recently measured in the St-Victor Formation. A detrital zircon U-Pb geochronology study (LA-HR-ICPMS), focused on the St-Victor Formation, has been therefore initiated in order to better constrain the age of the Magog Group. Results were treated according to a Bayesian mixture modeling to highlight different age populations. A feldspar-rich sandstone, directly overlying the Ascot Complex (ca. 460 Ma) and belonging to the base of the St-Victor Formation, yielded ages as young as 431 ± 3 Ma (Wenlockian). Higher in the stratigraphy, a quartz-feldspars sandstone sample contains zircons as young as 419 ±2 Ma (Pridolian). Finally, another sandstone sample from the stratigraphic top of the analyzed sequence yielded a bimodal age distribution, showing prominent populations clustering around ca. 950 Ma and ca. 435 Ma. These preliminary results suggest a time gap as high as 30 m.y. between the St-Victor Formation and underlying rocks of the lower Magog Group and the Ascot Complex. Combined with current mapping in southern Quebec, this implies the probable occurrence of a major unconformity at the base of the St-Victor, and suggest that a large part of the Magog Group deposits has been the result of post-Taconian sedimentation rather than a typical forearc sequence as commonly reported in literature.