Relationships between foliation development, porphyroblast growth and large-scale folding in a metaturbidite suite, Snow Lake, Canada

Complex relationships exist between cleavage development, metamorphism and large-scale folding in the well-bedded, polydeformed, staurolite-grade metaturbidite of the Burntwood Suite, internal Paleoproterozoic Trans-Hudson Orogen at Snow Lake, Manitoba, Canada. It is demonstrated: (a) that cleavage in anisotropic pelitic rock develops whenever microfolding is possible and that, commonly, initiation of a cleavage, which is pervasive on the scale of a fold, predates folding; (b) how a new axial planar fabric can develop on one fold limb of a symmetrical fold and not on the other; and (c) how two cleavages of different generations can be present in adjacent beds. It is further shown that porphyroblasts rotate with respect to geographical coordinates during folding. Finally, dissolution of cleavage septa is suggested here as an alternative mechanism for the generation of schistosity. The Burntwood Suite is exposed on the dismembered limb of a macroscopic, isoclinal F2 structure and preserves a domainal cleavage (S2), which locally grades into a schistosity. S2 developed from crenulation of a generally bedding-parallel S1 cleavage that is axial planar to F1 isoclinal folds formed at 1.84 Ga. Porphyroblast growth coincided with crenulation of S1 early during F2 folding at 1.815-1.8 Ga. Early stages of S2 development are recorded by inclusion trails (S1) in the porphyroblasts. During F2 flexural-flow folding, variations in magnitude of bedding-parallel shear in lithologies of different competency resulted in a strong S2 refraction and thus heterogeneous strains between beds. Independent of shear magnitude and resulting S0S2 angle, Si and S2 remained sub-orthogonal everywhere, and thus porphyroblasts and the enveloping S2 rotated by equal amounts with respect to S0. As the different magnitudes of porphyroblast rotation in different beds could not be exactly balanced by the counteracting rotation of the fold limbs (same magnitude for all beds) during fold tightening, most porphyroblasts also rotated with respect to geographical coordinates. S2 was crenulated prior to F3 large-scale folding, where favourably oriented. F3 crenulations were tightened on the eastern F3 limb and unfolded by sinistral layer-parallel shear on the western limb, where F2 and F3 layer-parallel shears were of opposite and the same sense, respectively. As a result, the initial developmental stages of an S3 are developed only on the eastern F3 limb, and there only in incompetent layers, whereas S2 is preserved in the competent layers. On the western limb, S2 is preserved and appears axial planar to the F3 structure. The S2 domainal fabric was locally transformed into a schistosity by dissolution of the septa during widespread fluid activity, which endured until syn- or post-F3.