Structural analysis of Grenville rocks near Bancroft, Ontario, Canada.
Creators: name: Divi, Sri Ramachandra Rao.
Publisher: name: Université d'Ottawa / University of Ottawa
Year: 1972
Subjects: subject: Geology.

description: Southeast of Bancroft, Ontario, Precambrian metasedimentary and metavolcanic rocks of the amphibolite facies lie tightly folded between a gneiss complex and granitic plutons underlying the Haliburton-Hastings highlands to the north, and the Weslemkoon granitic batholith to the southeast. The metamorphic rocks were subjected to at least three discontinuous phases of deformation, with resultant folds F1, F2 the dominant, and F3. F1 folds, which are only exposed in metasedimentary rocks at a few localities, are small-scale and isoclinal. The preservation within later garnet porphyroblasts of a fine-grained schistosity S1 that is axial-planar to micro F1 folds in relict layering suggests that F1 formed under conditions of relatively low-grade metamorphism. The orientation of S1 before overprinting by later folding and crystal growth is not clear, but in some areas the geometry of low-plunging F2 folds suggests that it was subhorizontal. F2 folds affect both bedding and S1 on all scales, vary in tightness, and are upright to overturned northwest. An axial-surface foliation S2, either a schistosity or gneissic layering, is regionally penetrative and extends into synkinematic or earlier plutons. Some sillimanite crystals are bent by F2 folds, but most lie within S2, and a few are crosscutting; thus high-grade metamorphism preceded, accompanied, and outlasted the folding. Hinge lines of minor F2 folds and axial lineations L 2 and irregular in detail, but commonly plunge at northeast or southwestward along the axial traces of large-scale F2 folds. As demonstrated by rake isogons and moving averages, towards the northern complex of gneiss and plutons L2 progressively changes direction and steepens until, as in the gneiss, it plunges southeast down the dip of S2. To the southeast near the large Weslemkoon pluton, F2 axes and L 2 also plunge steeply down-dip. Flattening of F2 folds, as calculated from systematic changes in thickness of calc-silicate layers in fold profile, varies locally, but in general increases with the axial steepening towards the plutons. Thus it appears that the presence or emplacement of relatively rigid plutons during folding imposed an element of constrictional strain that influenced the orientation and shape of adjacent F2 folds. F3 folds, only locally present, are open, recumbent structures bending S1 and S2 foliations; mineral grains disrupted in the axial regions indicate that folding was virtually post-metamorphic. Narrow mylonite zones occur within the gneiss complex near its southern border. Structures within the mylonites are of similar form and orientation to those associated with F1, F2, and F3. The similarity of the small-scale structures and related fabrics within the gneisses, mylonites, and metasedimentary-metavolcanic group of rocks indicates that the mylonite zones were initiated at an early stage in a common deformational history. K-Ar radiometric ages obtained from minerals forming S2 or L2 fabrics, nepheline from the gneiss complex, biotite and hornblende in the southern metasediments, and biotite from a foliated part of the Weslemkoon pluton, are compatible with a Grenvillian age (about 1000 m.y.) of deformation and metamorphism throughout the area.