In:
Canadian Journal of Earth Sciences, Canadian Science Publishing, Vol. 40, No. 2 ( 2003-02-01), p. 285-300
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 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 Chaudière nappe, a Cretaceous thermal overprint might have occurred on the basis of a low KAr value at 256 Ma. Furthermore, the KAr dates of 491467 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 KAr dates of 439418 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 KAr dates that, in this case, could be of meaningful geological significance increase to 455445 Ma. Such ages are consistent with Ordovician 40 Ar/ 39 Ar metamorphic ages attributed to the Taconian orogeny in the internal Humber zone.
Type of Medium:
Online Resource
ISSN:
0008-4077
,
1480-3313
Language:
English
Publisher:
Canadian Science Publishing
Publication Date:
2003
detail.hit.zdb_id:
417294-2
detail.hit.zdb_id:
1491201-6
SSG:
16,13
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