K-Ar dating constraints on the tectonothermal evolution of the external Humber zone, southern Quebec Appalachians
In the external Humber zone, the emplacement episodes of allochthonous nappes and related low-grade metamorphism are mainly based on paleontological ages from the St. Lawrence Lowlands. In fact, Late Ordovician flysches are affected by thrusting, and it is not clear if the nappe imbrication is the result of the Taconian or the Acadian deformation. Along the Chaudiere River transect, fourteen shales and phyllites of Cambrian to Ordovician age were sampled for determination of the KAr values of their < 2 µm-size fractions. To interpret the KAr data, mineral composition and clay mineral crystallinity of the < 2 µm-size fractions and maturation of the organic matter were determined. In addition, the intensity of deformation was characterized by describing the rock texture at macroscopic and microscopic scale. The combined approach used in this study emphasized that nearly all KAr dates are mixed or partly reset values. However, it can be stated that in the Quebec Promontory nappe and part of the Chaudiere ...