STRATIGRAPHIC AND STRUCTURAL RELATIONSHIPS BETWEEN ORDOVICIAN ARC-FOREARC ROCKS AND THE GASPE BELT, STOKE MOUNTAINS AREA, SOUTHERN QUEBEC APPALACHIANS

In the Canadian Appalachians, the Humber and Dunnage zones are remnants of the Laurentian continental margin and fringing oceanic domain that amalgamated during the Ordovician Taconian orogeny. In southern Quebec, the Dunnage zone consists of the Southern Quebec ophiolites, the Saint-Daniel Mélange, the Magog Group forearc sequence, and the Ascot Complex volcanic arc. In the Stoke Mountains, the Magog Group overlies the Ascot Complex and is mainly made up of the St-Victor Formation, which consists of turbiditic and volcanic sandstones with three distinctive horizons of conglomerate. The lowermost conglomerate contains felsic volcanic clasts (including a significant proportion of hematite-rich chert and exhalite), whereas the middle horizon is characterized by abundant boulders of felsic intrusive rocks and associated with turbiditic feldspar-rich sandstones. The uppermost conglomeratic horizon is compositionnally similar to the latter but is associated with massive beds of lithic sandstone and overlies a distinctive sequence of finely-bedded mudstone-siltstone sequence. The contact between the Magog Group and the Ascot Complex, previously described as a major fault, has been observed on the field and is interpreted as a nonconformity. The Ascot Complex is made up of interlayered, mafic and felsic metavolcanic rocks and crosscutting synvolcanic granite. Both the Magog Group and the Ascot Complex are unconformably overlain by the Upper Silurian Lac Aylmer Formation of the Gaspé Belt, which consists of interbedded dolomite, dolomitic sandstone and conglomerate, limestone and shale. The base of the Gaspé Belt is marked by a boulder-rich conglomerate and massive dolomite, whereas its upper part is characterized by discontinuous horizon of slumped calcareous siltstone and limestone conglomerate. To the SE, the Ascot Complex is bounded by the La Guadeloupe fault, a major NW-verging Acadian reverse fault that marks the contact with the Silurian-Devonian rocks of the Connecticut Valley-Gaspé trough to the SE. An ongoing geochemical characterization of volcanic and plutonic rock cobbles and boulders collected in various conglomerate horizons will be used to investigate the nature of the sedimentary source(s) and compare with the different volcanic rock units currently cropping out in the Ascot Complex.