40AR 39AR MINERAL DATES RELATED TO DEVONIAN EXTENSION IN THE SOUTHWESTERN SCANDINAVIAN CALEDONIDES

Middle Devonian sedimentary basins exposed in west-central Norway formed during post-Caledonian extension. They developed along west-dipping, low-angle, normal faults which effected ductile shearing of pre-Caledonian basement units of the Western Gneiss Region (WGR). Kinematic indicators suggest that the west-vergent ductile deformation overprinted older, east-vergent Caledonian fabric elements. Ductile extensional deformation led to development of a 2-km-thick shear zone beneath the Devonian basins. Five amphibole and nine muscovite concentrates from rocks within the shear zone and from unaffected gneisses of the Western Gneiss Region have been analyzed with 40Ar/39Ar incremental-release dating techniques in an attempt to calibrate the timing of extensional events. Well-defined plateau ages are recorded by muscovite concentrates within the shear zone. Amphiboles display disturbed plateau ages clearly related to extraneous argon. Outside the shear zone, muscovite records Caledonian ages. Results suggest that a late tectonometamorphic event affected the rocks within the shear zone. This event, which not exceed ca. 500-degrees-C, completely rejuvenated muscovite argon systems and perturbed argon distribution within amphiboles. This metamorphism is correlated with the extensional deformation which effected exhumation of the Caledonian high-pressure rocks and controlled formation of Devonian basins.