TACONIAN OROGENESIS, SEDIMENTATION AND MAGMATISM IN THE SOUTHERN QUEBEC – NORTHERN VERMONT APPALACHIANS : STRATIGRAPHIC AND DETRITAL MINERAL RECORD OF IAPETAN SUTURING

Cambrian-Ordovician units of the southern Quebec Appalachians oceanic domain include obducted ophiolites and an overlying syncollisional sedimentary basin represented by the Saint-Daniel Mélange and the Magog Group. These terrains were thrust onto the Laurentian continental margin during the Taconian orogeny and are unconformably overlain by Silurian-Devonian successor basins. We present stratigraphic data acquired in the oceanic domain, as well as U-Pb and ⁴⁰Ar/³⁹Ar geochronological data on detrital zircons and muscovites sampled at various stratigraphic levels, from the base of the sedimentary units of the oceanic domain, to the Silurian-Devonian units. This work, combined with a compilation of existing data for the southern Quebec and western New England Appalachians, allows for a better definition of the tectonostratigraphic evolution of the studied units, and to revisit their correlation with rocks of the northern Vermont Rowe-Hawley belt, which represents a collage of Laurentian margin and oceanic units. It is shown that although well-preserved ophiolites and flysch units are lacking in the Rowe-Hawley belt, Cambrian-Ordovician rocks of southern Quebec and northern Vermont have undergone a similar tectonic evolution. The ophiolites were obducted onto the Laurentian margin as a more or less coherent slab of oceanic lithosphere. Both the ophiolites and continental margin rocks were uplifted, eroded and unconformably overlain by Middle to Late Ordovician units of the Saint-Daniel Mélange and Magog Group in a forearc setting. The presence of mafic and felsic volcanic flows that are interstratified within the Saint-Daniel Mélange, can be correlated with an episode of delamination and slab breakoff during the final stages of obduction and exhumation of the orogenic wedge. The Magog Group was deposited during the emplacement and uplift of the Taconian nappes, as the outboard volcanic arc was still active but progressively shutting down. The provenances suggested by the age of the dated detrital minerals can be entirely reconciled with the peri-Laurentian context of the studied rock units and testify of the tectonic evolution and sedimentary recycling of the Taconian orogen, from ophiolite obduction onto the Laurentian margin, to the collapse of the orogenic belt and the formation of successor basins.