Source and tectono-metamorphic evolution of mafic and pelitic metasedimentary rocks from the central Quetico metasedimentary belt, Archean Superior Province of Canada.
Résumé
A study of the Jean Lake area of the Quetico metasedimentary belt, Superior Province, Canada, was conducted in order to evaluate the origin, source and evolution of sedimentary rocks, including mafic rocks previously mapped as ultramafics rocks. Bulk chemical compositions of these rocks show a mixing with two end members: quartzo-feldspathic sediments and komatiitic basalts. High CaO and MgO contents of the rocks suggest a proximal source of komatiitic basalts, probably from the southern Wabigoon subprovince. The rocks in the Jean Lake area record a pressure-temperature (P-T) path with three tectono-metamorphic stages. The first stage formed staurolite (500-700°C) under medium P-T (MP-MT) metamorphic conditions shortly after the sedimentation. The second stage yielded the biotite-sillimanite-garnet assemblage under the peak condition of 0.6 0.1 GPa and 700 70°C during transpressional deformation. The third stage, low P- medium T (0.25 0.11 GPa, 540 80°C) metamorphism, was associated with regional south-southeast compression and its timing is constrained by a new U-Th-Pb monazite age of 2667 20 Ma. Combining the regional deformation events, we suggest that the sediments in the Jean Lake area were buried up to MP-MT conditions during the D1 deformation at 2698-2689 Ma. They attained the peak metamorphic condition during the regional transpressive D2-D3 deformation (2689-2671 Ma), and retrograded to LP-MT condition during the south-southeast compression of the regional D4 at 2671-2667 Ma. The tectono-metamorphic history of the study area is consistent with the sedimentation in an accretionary prism followed by its docking to the Wabigoon subprovince to the north and Wawa greenstone belt to the south. The P-T-time path of this Archean accretionary prism is similar to that in modern arc accretion systems, except for a higher temperature gradient, ~ 30°C/km, recorded in the Quetico belt compared to ~10°C/km in modern counterparts. The high temperature gradients in the Quetico belt explain the lack of high-pressure metamorphic rocks, such as blueschist, that are common in modern accretionary prisms.
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