STRUCTURAL CHARACTERIZATION AND PRELIMINARY K-AR DATING CONSTRAINTS FOR THE TACONIAN ALLOCHTHONS, QUEBEC APPALACHIANS

In the Northern Appalachians, the Taconian Allochthons consist of siliciclastic rocks, limestones and mafic volcanic rocks that are deformed into a series of imbricated northwest-directed thrust nappes. In the Québec Appalachians, these rock units vary from unmetamorphosed to sub-greenschist metamorphic facies toward the contact with greenschist- to amphibolite-facies metamorphic rocks from the Appalachian internides. Detailed mapping (1:200) of key outcrops in the Taconian Allochthons along two transects in the Québec city and Rivière-du-Loup areas indicates a more complex structural evolution than previously recognized in the Québec Appalachians. Four deformational episodes have been documented along the Chaudière River transect (Québec city area): (i) bedding-parallel thust faults and associated folds result in stratigraphical repetitions; (ii) NW-dipping and SE-verging folds that increase in intensity toward a faulted contact between the allochthons to the NW and the Appalachian internides to the SE; (iii) NE- to SE-trending upright and open folds devoid of axial-planar fabrics and related to dome-and-basin interference pattern, finally; (iv) a series of normal faults marked by brittle fault materials. The timing of the emplacement of the Taconian Allochthons and related low-grade metamorphism is currently mainly constrained by faunal control, which suggests emplacement ages of latest Middle Ordovician to Late Ordovician. In the Québec city area, 14 shales and phyllites of Cambrian to Ordovician age were sampled for K-Ar isotopic dating on their <2 um-size fractions. Among these samples, K-Ar dates of 491-467 Ma were mainly obtained and more likely represent mixed values of detrital material affected by diagenetic or tectonic processes. Toward the SE, the thermal grade of the Taconian Allochthons increases to epizonal conditions and K-Ar dates of 455-445 Ma were obtained. These are consistent with Ordovician metamorphic isotopic ages obtained in the metamorphic rocks of the internides and could be of meaningful significance for the tectonic evolution of the Taconian Allochthons. Ongoing K-Ar analyses of the <0.5, 0.5 to 1, 1 to 1.5 and 1.5 to 2 um-size fractions of fault gouge and other foliated samples will provide more precise isotopic age constraints in the near future.