Source and tectono-metamorphic evolution of mafic and pelitic metasedimentary rocks from the central Quetico metasedimentary belt, Archean Superior Province of Canada

A study of the sedimentary rocks in the Jean Lake area of the Quetico metasedimentary belt, Superior Province, Canada, was conducted in order to evaluate the origin, source, and evolution of rocks, including mafic rocks previously mapped as ultramafics rocks. Bulk compositions of these sedimentary rocks show a mixing between two end members: quartzo-feldspathic sedimentary rock and komatiitic basalt. High CaO and MgO contents of the rocks suggest a proximal source of the komatiitic basalt. cks in the study 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 conditions of 0.6±0.1 GPa and 700±70 °C during transpressional deformation. The third stage, low P–medium T (LP–MT; 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 the burial metamorphism of sediments up to MP–MT conditions during the D1 deformation at 2698–2689 Ma. They attained the peak metamorphic conditions during the regional transpressive D2–D3 deformation (2689–2671 Ma), and retrograded to LP–MT condition during the south-southeast compression (regional D4) at 2671–2667 Ma. t time span between the sedimentation and MP–MT metamorphism accompanied by D1 deformation suggest that the Quetico sedimentary rocks formed in an acretionary prism. The studied rocks in the Jean Lake area deposited close to the Wabigoon Subprovince, transported towards the south, and buried up to ∼20 km by rapid underthrusting to amphibolite facies conditions. Subsequent dextral transpressive deformation (D2–D3) during the oblique docking of the Quetico belt to the Wabigoon Subprovince to the north and Wawa greenstone belt from the south resulted in the displacement of the sedimentary rocks to the west from the original depositional site. The P–T–time path of the Quetico sedimentary rocks is similar to that in modern arc accretion prisms, except for a high geothermal gradient of ∼30 °C/km recorded in the former compared to ∼10 °C/km in modern counterparts. The high temperature gradient in the Archean accretionary prism explains the lack of high-pressure metamorphic rocks, such as blueschist, that are common in modern accretionary prisms.