Abstract

East of Bancroft, Ontario, metasedimentary and metavolcanic rocks lie tightly folded between a syntectonic granitic batholith in the southeast and a complex of feldspathic gneisses and plutons to the north. At least three generations of folds are well displayed in marble and calc-silicate layers. F1 folds are isoclines with few hinges exposed and an axial-surface foliation that closely parallels stratification. F2 folds are dominant on all scales. These folds bend the stratiform foliation, vary in tightness, and are upright to overturned northwest toward the gneiss complex. During and after F2 deformation, minerals of the amphibolite facies crystallized. F3 folds, only locally present, are open and recumbent; folding was virtually postmetamorphic.

Axes of minor F2 folds and parallel mineral lineations commonly plunge at low angles along the axial traces; but toward the gneiss complex and near the granitic batholith, the axes progressively increase in rake until, as in the gneiss, folds are reclined and plunge approximately down the dip of axial surfaces. Variations in thickness of folded layers indicate that folds have undergone flattening and that this strain progressively increased with increase in rake.

The evidence supports the concept that during medium-grade metamorphism, constriction near plutons or feldspathic massifs led to areal changes in shape and orientation of the dominant folds.

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