Abstract

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 Chaudière River transect, fourteen shales and phyllites of Cambrian to Ordovician age were sampled for determination of the K–Ar values of their < 2 μm-size fractions. To interpret the K–Ar 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 K–Ar dates are mixed or partly reset values. However, it can be stated that in the Quebec Promontory nappe and part of the Chaudière nappe, a Cretaceous thermal overprint might have occurred on the basis of a low K–Ar value at 256 Ma. Furthermore, the K–Ar dates of 491–167 Ma obtained in the Chaudière nappe to the southeast, most probably represent mixed values of detrital material affected by diagenetic or tectonic processes. It might also be stated that the base of the Sainte-Hénédine nappe appears to be characterized by southeast-dipping thrust sheets with K–Ar dates of 439–418 Ma, which may represent either a late-stage emplacement of the nappe or a yet unrecognized structural effect of hinterland-directed deformation that characterizes the internal Humber zone. In the southeastern part of the Sainte-Hénédine nappe, the thermal grade increases to epizonal conditions and the K–Ar dates that, in this case, could be of meaningful geological significance increase to 455–445 Ma. Such ages are consistent with Ordovician 40Ar/39 Ar metamorphic ages attributed to the Taconian orogeny in the internal Humber zone.

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