SENSE OF SHEAR ON LOW-ANGLE NEOPROTEROZOIC DUCTILE SHEAR ZONE, CAPE BRETON, NOVA SCOTIA

Late Neoproterozoic ductile shear zones that juxtapose low-grade over high-grade assemblages are characteristic features of parts of the peri-Gondwanan terranes of the Canadian Appalachians. One such ductile shear zone, in the Creignish Hills of Cape Breton Island, Nova Scotia, brings low-grade platformal metasedimentary rocks of the George River Suite into contact with underlying high-grade rocks of the Bras d’Or Gneiss. The low-grade assemblage includes quartzite, marble, schist and phyllite with interlayered felsic volcanogenic units and mafic flows, whereas the high-grade unit comprises low-pressure, high-temperature gneisses and migmatites, including pelitic paragneisses of likely volcanogenic origin. The contact between the two assemblages is defined by a broad mylonite zone that envelopes the high-grade rocks in the form of a westerly plunging antiform. The structural dome is truncated to the east against Carboniferous strata by high-angle faulting. Kinematic indicators within the mylonites, including asymmetric porphyroclasts, S-C fabrics and folded mylonitic foliation, suggest a broadly top-to-the-east (dextral) sense of shear, while the presence of gneissic granitoid sheets that are broadly concordant but locally cross-cut and are folded about the mylonitic foliation, suggest that mylonitization was accompanied by partial melting and syntectonic intrusion. Monazite from the gneisses and zircon from the granitoid sheets have yielded near-identical U-Pb ages of ca. 550 Ma. The Creignish Hills forms one of a number of peri-Gondwanan basement blocks that project through the Carboniferous cover of central Cape Breton. The juxtaposition of low-grade over high-grade assemblages in many of these blocks suggests that the folded ductile shear zone exposed in the Creignish Hills is part of a regional low-angle structure that repeatedly intersects the present erosion surface. Deformation along this low-angle tectonic boundary was accompanied by low-pressure, high-temperature metamorphism and placed low-grade rocks over high-grade rocks in a manner consistent with extensional detachment. If so, the Creignish Hills ductile shear zone suggests broadly eastward detachment at ca. 550 Ma, perhaps in response to collapse of the peri-Gondwanan magmatic arc or to core complex development related to ridge-trench collision.