The Abitibi-Opatica transition, Superior Province, Quebec, Canada: structural analysis, 40Ar/39Ar thermochronology and implications for Archean tectonics
Résumé
We present a structural and metamorphic study of the Abitibi greenstone belt (AGB) and Opatica Plutonic Belt (OPB) of the Archean Superior Province. The AGB-OPB contact is considered as an archetype example of Archean subduction, based on a LITHOPROBE seismic profile showing a North-dipping lithospheric-scale reflector interpreted as the vestige of subduction. Our mapping indicates that the AGB overlies the OPB, and that the AGB-OPB contact does not show evidence of significant shear deformation, as expected for a major upper plate-lower plate boundary. There is no metamorphic break at the AGB-OPB transition but rather a progressive increase of metamorphic grade toward the OPB. Therefore, we think that the OPB simply exposes the deepest part of a composite AGB-OPB sequence. 40 Ar/ 39 Ar ages suggest that the AGB and OPB rocks were exhumed from amphibolite-facies conditions at 2678 Ma and ~ 2668 Ma, respectively, and then that both terranes share the same succession of thermal disturbances at c. 2650 Ma and 2600 Ma, the latter corresponding to shearing along the Nottaway River Shear Zone. We suggest that progressive cooling of both assemblages was accompanied by strain localisation along strike-slip shear zones and occurred when lateral flow of the lower crust became predominant over the convective overturn and vertical crustal material transfer. Comparison with adjacent areas suggests that regional metamorphism has been coeval over a large region, which is consistent with pervasive deformation and slow cooling as expected for vertical tectonic models and diapiric magmagenesis and ascent during the Archean.
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