Geological and geochronological relationships of the Thompson nickel belt, Manitoba: Discussion
The conclusion advanced by Cranstone and Turek (1976) that the migmatitic gneisses of the Wabowden subprovince represent early Precambrian Pikwitonei basement rocks remobilized during an early stage of the Kenoran orogeny, or during an orogeny earlier than the Kenoran, is in part based upon the assumption that the gneissic fabric of the rocks from the western part of the Wabowden subprovince is coeval with the transposition fabric of the retrograded Pikwitonei granulites of the eastern part of the subprovince. Such an assumption may not be valid. The gneisses of the western part of the Wabowden subprovince between Setting Lake and Paint Lake include quartzofeldspathic gneisses, biotite-, pyroxene-, and garnet-bearing amphibolites, diopside K-feldspar calc-silicate rocks and marbles, coarse-grained quartzites containing plagioclase, biotite, and garnet with zonally arranged felted inclusions of sillimanite, and pegmatite with coarse pyroxenes mantled by amphibole. Although the gneisses contain abundant small scale folds, the texture of the rocks in hand specimen is generally granoblastic, with phyllosilicate minerals tending to form random orientation patterns. It is evident therefore that the gneisses are deformed rocks that have undergone high temperature post-deformation thermal annealing. However, in the Paint ' Lake region it is notable that quartz grains in quartzites containing randomly oriented flakes of biotite invariably have sutured boundaries (triple point boundaries are only preserved in plagioclase aggregates), contain abundant deformation larnellae, and exhibit a moderate degree of preferred crystallographic orientation. Furthermore, when traced eastwards, the gneisses take on a pronounced schistosity, and biotite as well as quartz exhibits a strong degree of preferred crystallographic orientation. If these features can be taken to indicate that the schistosity of the Paint Lake gneisses is a transposition feature developed after rather than prior to the high temperature annealing metamorphism of these rocks, the possibility arises that the Setting Lake Paint Lake gneisses are little modified homologues of the gneisses of the Pikwitonei province, and the Paint Lake schistosity a deformation feature coeval with the transposition fabric of the remobilized Pikwitonei gneisses of the eastern margin of the Wabowden subprovince. In this case two conclusions may be drawn. Firstly, the older 'lit par lit' gneissosity and granoblastic fabric of the gneisses from the western part of the Wabowden subprovince may be primary Kenoran features developed in Archean rocks (or preKenoran features developed in pre-Archean rocks if the age of 2820 + 135 Ma determined by Turek for the gneisses is considered to be a minimum age) rather than transpositional structures formed by remobilization of Pikwitonei basement during an early stage of the Kenoran orogeny. Secondly, the transposition fabric of the eastern margin of the Wabowden subprovince and of the Paint Lake region may represent a Hudsonian structural feature developed contemporaneous with the primary foliation found in amphibolitic metagabbro bodies2 within the Wabowden gneisses near Soab Mine