The structural geometry and geological history of Archean rocks at Kenora, north-western Ontario—a proposed type area for the Kenoran Orogeny

Abstract

The Kenora area, at the south margin of the English River Subprovince, consists of polydeformed gneisses intruded by granitoid plutons, some of which are also deformed. The gneisses are mainly tonalites of igneous origin, but contain remnants of an earlier, dominantly mafic, supracrustal sequence and severely deformed pegmatites.Within the gneissosity are intrafolial F1 folds subsequently refolded by tight-to-open F2 folds. Major F2 folds can be delineated from the geometry and distribution of minor F2 structures. During deformation the area was subjected to upper amphibolite facies metamorphism (650–750 °C, 4–7 kbar (400–700 MPa)) with K-feldspar megacrysts and garnet developed in the deeper levels. Major F2 folds are refolded in a rim synform (F3), which developed synchronously with the emplacement of the Dalles granodiorite, an event that also created a plunge culmination uplifting part of the deeper K-feldspar megacryst – garnet structural level. Cumulative apparent dextral displacement of 6 km in two periods of movement, related partly to the emplacement of the Dalles granodiorite, is suggested for a major fault 1.5 km north of, and parallel to, the English River – Wabigoon Subprovince interface. Open, upright F4 folds caused buckling of dioritic dikes and pegmatites. Later deformation involved minor faulting and joint development.Rb–Sr isotopic studies indicate that this geological history took place within about 150 Ma with most activity over by 2600 Ma. It is proposed that this should be selected as the type area for the Kenoran Orogeny.