Ainslee Nolan's HBSc thesis abstract
The Wabigoon and Quetico subprovinces are east-west trending belts of metasedimentary and metavolcanic rocks located within the Superior province, the world’s largest preserved Archean craton. The Wabigoon-Quetico subprovince boundary in the Decourcey Lake area is complex and several kilometres wide. The 130m-long roadcut on Hwy 527 near Decourcey Lake is composed of a garnet-biotite to biotite schist with three intrusions, a pegmatitic intrusion, a felsic intrusion and a mafic dyke. There are also several quartz-carbonate veins that have been folded and boudinaged; the folded veins crosscut the boudinaged veins that are parallel to foliation.
In the southern portion of the outcrop, mats of fibrolite in the schist indicate peak metamorphism at amphibolite facies or higher. However, some parts of the outcrop show evidence of overprinting retrograde metamorphism. The amount of retrograde metamorphism increases from south to north in the outcrop. In the southern part of the outcrop there are minimal amounts of chlorite (~1%) and the amount increases to about 30% in the north. The chlorite replaces biotite, the main mica in the southern portion of the roadcut. Chlorite replacing biotite is indicative of retrograde metamorphism.
Other evidence for this retrograde metamorphism is the presence of stable garnet in the southern portion of the outcrop and metastable garnet in the northern portion, indicated by euhedral unfractured crystals in the south and anhedral, fractured and separated grains in the north.
Throughout the schist, quartz with undulose extinction, irregular grain boundaries and subgrains provides evidence for deformation by dislocation creep. In the southern portion of the outcrop, undulose extinction in feldspar indicates deformation at amphibolite facies temperatures or higher. This higher temperature deformation of feldspar is not evident in the northern portion of the outcrop where retrograde metamorphism is more pervasive. The pegmatite and felsic intrusions preserve evidence of deformation at the amphibolite facies or higher, including undulose extinction in feldspar and bent mica grains.
The preservation of evidence for amphibolite facies metamorphism and high temperature deformation in schist in the southern portion of the outcrop as well as in the pegmatite and felsic intrusions suggests that the Decourcey Lake roadcut is composed of metamorphosed Quetico lithologies. Therefore, the Quetico-Wabigoon subprovince boundary lies to the north of this roadcut. The increase in retrograde metamorphism toward the northern portion of the outcrop (in the direction of the subprovince boundary) suggests that this lower temperature metamorphism may be associated with the boundary.