Polyphase deformation and crustal evolution in the Pipestone Lake area of the Archean Wabigoon Subprovince, Superior Province, Canada

Archean supracrustal rocks in the western Wabigoon Subprovince, Superior Pr ovince, Canadian Shield, have undergone four phases of deformation, D1-D4. D1, confined to the oldest rocks, includes a large, refolded, reclined, iso clinal anticline (F1) with moderately developed axial-planar cleavage (S1). Rocks affected by D1 are overlain by a regional unconformity. D2 includes post-unconformity, steeply plunging, north-northeast-south-southwest-striki ng isoclinal folds (F2) that are approximately coaxial with F1, but have co ntorted the F1 axial trace. D3 is represented by east-west-striking, steepl y plunging folds at various scales (F3) which occur mainly in rocks near th e regional Manitou Stretch - Pipestone Lake Shear Zone, also interpreted to be a D3 structure. D4 is represented by pervasive cleavage (S4), locally p enetrative but mainly spaced, including crenulation cleavage, fractures, sm all faults, and brittle-ductile shears. S4 is parallel to the margin of the Jackfish Lake Pluton. D1-D3 are ascribed to convergence of the Wabigoon Su bprovince with adjacent subprovinces, ending with formation of the Manitou Stretch - Pipestone Lake Shear Zone as a (presently) steep thrust and obliq ue-ramp structure. D4 is the result of either diapiric emplacement of the J ackfish Lake Pluton, or marginal strain intensification due to the rigidity of the older Ash Bay Dome during late north-south compression. Previously available zircon U-Pb geochronology provides a maximum age for D1 of 2728 M a, and an approximate age for D4 of 2698 Ma. The unconformity developed bet ween D1 and D2, 2725-2713 Ma.