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The evolution of the Thor-Odin gneiss dome and related geochronological studies Duncan, Ian James


The Thor-Odin gneiss dome is one of several structural culminations along the eastern margin of the Shuswap terrain. The gneiss domes have previously been ascribed both to diapiric uprise of granitic gneisses and to interference of late-stage buckle folds. The ages of rocks in the gneiss domes have also been the subject of controversy. Age estimates range from Archean to Mesozoic. This study provides the basis for a detailed model for a new interpretation of the structural evolution of the Thor-Odin gneiss dome. Prior to doming, this area was characterized by the formation of large-scale nappe structures and imbrication of Archean basement rocks with the cover rock sequence. The first period of deformation (Phase One) consisted of large-scale infolding of the cover rock sequence into the basement rocks. The Pingston fold in the core of the dome is a product of this event. The second period of deformation (Phase Two) was marked by the forcing of wedges of basement into the cores of northerly-moving nappes. The third period of deformation (Phase Three) was co-axial with Phase Two and consisted of imbrication and refolding of the upper levels of the stack of Phase Two nappes. Strain analysis based on fold shape, distorted lineation patterns and elliptical strain markers demonstrate that finite strains are highest in the imbricated zone between the Basement Cored Nappe Domain and the Cover Rock Domain. Flattening strains are anomalously low in the Autochthonous Core Gneiss Domain. Analysis of Phase Two fold axes distorted into flattened small circles by Phase Three folds shows that Phase Three strain magnitude √(λ₂/ λ₁) varies from 0.81 to 0.49. Rb-Sr whole rock geochronometry has revealed evidence for three Precambrian events within the basement gneisses. Unmigmatized metasedi-mentary basement gneisses form a six-point errorchron with an age of 2.73±0.20 Ga. Granitic gneisses previously dated at 1.96 Ga by the U-Pb zircon technique, give reset ages for gneiss/vein whole rock suites with ages in the range 750 to 860 Ma. Common lead systematics of stratabound sulfide deposits in the cover rock sequence suggest that the cover rocks are Cambrian in age, in agreement with earlier lithological correlations. The lead isotope ratios suggest the deposits were ultimately derived from an approximately 2.0 Ga basement terrain. The outcropping gneissic basement rocks of the Shuswap are dominantly granitic gneisses which give 2.0 to 2.2 Ga Rb-Sr whole rock dates.; Ductile deformation in the terrain appears to have ended by 150 Ma, as this corresponds to the ages of several post-tectonic intrusions which crosscut Phase Three folds and impose a contact metamorphic overprint on the regional metamorphic pattern. The Nelson Batholith is a post-tectonic batholith which intrudes both the Shuswap terrain and the Kootenay Arc to the east. Two sample suites were collected from the different batho-lithic phases and analyzed by Rb-Sr whole rock methods with the following results: Suite I - Porphyritic monzonite with alkali feldspar megacrysts from the West Arm of Kootenay Lake give a seven-point isochron with an age of 158 ± 16 Ma and an initial ratio of 0.7069 ± 0.0001. Suite I has a Rb-Sr feldspar, date of 153 ± 26 Ma, a K-Ar hornblende date of 143 ± 5 Ma, and a K-Ar biotite date of 60 ± 2 Ma. It seems likely that the K-Ar dates have been disturbed by thermal events subsequent to emplacement of the batholith. Suite II- Medium grained granodiorite from the southern tail of the Nelson Batholith gives a nine-point whole rock isochron with an age of 162 ± 6 with an initial ratio of 0.7065 ± 0.0001. Taken together, these data provide significant new information on the nature and timing of the events that formed the metamorphic core zone of the southern Canadian Cordillera.

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