RELATIONSHIPS BETWEEN FOLIATION DEVELOPMENT, PORPHYROBLAST GROWTH ANDLARGE-SCALE FOLDING IN A METATURBIDITE SUITE, SNOW LAKE, CANADA

Complex relationships exist between cleavage development, metamorphism and large-scare 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 micr ofolding is possible and that, commonly, initiation of a cleavage, whi ch is pervasive on the scale of a fold, predates folding; (b) how a ne w axial planar fabric can develop on one fold limb of a symmetrical fo ld and not on the other; and (c) how two cleavages of different genera tions can be present in adjacent beds. It is further shown that porphy roblasts rotate with respect to geographical coordinates during foldin g. Finally, dissolution of cleavage septa is suggested here as an alte rnative mechanism for the generation of schistosity. The Burntwood Sui te is exposed on the dismembered limb of a macroscopic, isoclinal F-2 structure and preserves a domainal cleavage (S-2), which locally grade s into a schistosity. S-2 developed from crenulation of a generally be dding-parallel S-1 cleavage that is axial planar to F-1 isoclinal fold s formed at 1.84 Ga. Porphyroblast growth coincided with crenulation o f S-1 early during F-2 folding at 1.815-1.8 Ga. Early stages of S-2 de velopment are recorded by inclusion trails (S-i) in the porphyroblasts . During F-2 flexural-flow folding, variations in magnitude of bedding -parallel shear in lithologies of different competency resulted in a s trong S-2 refraction and thus heterogeneous strains between beds. Inde pendent of shear magnitude and resulting S-0/S-2 angle, S-i and S-2 re mained sub-orthogonal everywhere, and thus porphyroblasts and the enve loping S-2 rotated by equal amounts with respect to S-0. As the differ ent, magnitudes of porphyroblast rotation in different beds could not be exactly balanced by the counteracting rotation of the fold limbs (s ame magnitude for all beds) during ford tightening, most porphyroblast s also rotated with respect to geographical coordinates. S-2 was crenu lated prior to F-3 large-scale folding, where favourably oriented. F-3 crenulations were tightened on the eastern F-3 limb and unfolded by s inistral layer-parallel shear on the western limb, where F-2 and F-3 l ayer-parallel shears were of opposite and the same sense, respectively . As a result, the initial developmental stages of an S-3 are develope d only on the eastern F-3 limb, and there only in incompetent layers, whereas S-2 is preserved in the competent layers. On the western limb, S-2 is preserved and appears axial planar to the F-3 structure. The S -2 domainal fabric was locally transformed into a schistosity by disso lution of the septa during widespread fluid activity, which endured un til syn-or post-F-3. (C) 1998 Elsevier Science Ltd.