Heterogeneous amphibolite facies deformation of a granulite facies layered protolith: Matches Island shear system, Parry Sound domain, Grenville Province, Ontario, Canada

Abstract

Amphibolite facies transposition of granulite facies gneiss resulted in the formation of a nappe-bounding km-scale shear zone at the margin of the Parry Sound domain, Grenville Province. The Matches Island shear system illustrates the earlier stages of transposition in which heterogenous retrogression of the well layered (mafic–felsic) granulite facies gneiss along pegmatites orthogonal to layering controlled the location of shear zones and ensured that unretrogressed granulite persisted as strong elements. Shear zones curve anticlockwise from their original orientation accompanied by rigid rotation of wall rock layering which remains orthogonal to the shear zones. This relationship is modified so that wall rock layering form sigmoidal mafic ‘fish’ where shear zone linkage occurs via: (i) merging of established parallel shear zones by wall rock ‘collapse’; (ii) merging of an established shear zone with a new shear zone formed within preexisting wall rock or; (iii) linking of two established zones by an oblique new shear zone. All of these wall rock – shear zone relations are displayed in the maximally transposed nappe-bounding shear zone but contrast with those at the boundary of other nappes where uniform amphibolite facies protolith is transposed with a buckle-and-shear style.