GEOCHRONOLOGY OF THE SOUTHERN QUÉBEC DUNNAGE ZONE – NEW U-PB ZIRCON DATA, SYNTHESIS AND IMPLICATIONS FOR THE PALEOGEOGRAPHY AND TECTONIC SETTING OF THE TACONIAN OROGENY

Ordovician oceanic rocks of the peri-Laurentian Dunnage Zone crop out for >300 km in the southern Québec Appalachians and consist of ophiolites (Thetford-Mines, TMO; Asbestos, AO; Lac-Brompton, LBO; and Mont-Orford, MOO), the Ascot Complex volcanics, and overlying sedimentary rocks of the St-Daniel mélange and Magog Group. The TMO, AO and LBO represent a single piece of c. 480 Ma oceanic lithosphere whereas the MOO has yielded U-Pb zircon ages of c. 504 Ma, and a significant proportion of Laurentian inherited grains. East of the ophiolite belt, the Ascot Complex is the sole occurrence of an Ordovician peri-Laurentian volcanic arc of the Quebec Appalachians. It consists of bimodal volcanic rocks stratigraphically overlain by black phyllites of the St-Daniel mélange. Both the Ascot and the St-Daniel are unconformably overlain by the Magog Group. Even though Laurentian inheritance is found in all these rocks, the nature of the basement over which ophiolites were obducted, the Ascot complex was accreted, and overlying sediments have been emplaced has recently been questioned. Therefore, we conducted a new structural and U-Pb geochronology study of rocks of the Dunnage zone. Preliminary high-resolution CA-ID-TIMS U-Pb zircon ages of ca. 470 Ma from the Stokes and Eustis domains of the Ascot Complex suggest that it belongs to a single volcanic edifice that was established on, or contaminated by, Laurentian continental rocks. This age is consistent with the age of the overlying St-Daniel mélange, bracketed by detrital ⁴⁰Ar/³⁹Ar muscovite ages of 467-460 Ma, and by ca. 462 Ma magmatic and detrital zircon ages found at the base of the Magog Group.

Regional field relations indicate that the southern Québec ophiolites, the Ascot Complex, and the St-Daniel mélange were accreted to Laurentia from c. 475 to c. 460 Ma, before the deposition of the Magog Group. None of the Québec Dunnage zone units described above (including correlative rocks of the Gaspé Peninsula) show significant Gondwanan inheritance and\or provenance. This raises important questions regarding lithological correlations with New England and extension of the peri-Gondwana Moretown terrane into the Quebec Appalachians. Rather than an orogenic-scale terrane, we suggest that the Moretown terrane does not extend over the entire length of the Appalachian orogen.