Tectonic stacking of metamorphic zones in Cheticamp River area, Cape Breton Highlands, Nova Scotia

Abstract

Metabasic and quartzo-feldspathic schists and gneisses of the Cheticamp River area can be subdivided into three north–south-trending metamorphic belts. The westernmost belt, the low-grade belt, has been metamorphosed to transitional greenschist–amphibolite facies with development of one penetrative foliation. The central medium-grade belt has been metamorphosed to amphibolite facies with development of two penetrative foliations. The eastern high-grade belt has been metamorphosed to amphibolite facies accompanied by development of strong gneissic segregation and nonanatectic migmatite leucosomes during formation of two penetrative foliations.Deformation has been inhomogeneous in each of these belts. Most rocks in each belt show evidence of post-tectonic porphyroblast growth. However, deformation continued after the metamorphic peak in localized zones. Very high strain occurred in many of these zones during and after the metamorphic peak, so that early formed fold axes were rotated towards the stretching direction. Locally, dynamically recrystallized mylonite has formed. The three belts were juxtaposed under waning metamorphic conditions by relative movement on these high-strain zones. The schist and gneiss complex thus consists of stacked metamorphic zones with the lowest grade rocks lying at the bottom of the stack. Stacking occurred by east to west thrusting of tight macroscopic ductile folds whose lower limbs have been sheared off.There are many lithologic similarities between the three metamorphic belts. The rocks may have been derived from the same protolith and have since been variably deformed and metamorphosed before later juxtaposition. There is no evidence for involvement of an older "basement complex" in the stacking tectonics in the studied area.The schist and gneiss complex has been intruded by post-tectonic plutonic rocks, and locally affected by post-metamorphic brittle deformation.