Structural analysis of the Creignish Hills Mylonite Zone, Cape Breton Island, Nova Scotia: implications for Neoproterozoic core complex development along the northern Gondwanan margin?

摘要

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 Metamorphic 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 gneiss and rnigmatite, including pelitic paragneiss of likely volcanogenic origin. The ductile shear zone between the two assemblages (Creignish Hills Mylonite Zone) envelopes the high-grade rocks in the form of a WNW-plunging antiform. The structural dome is truncated to the east against Carboniferous strata by high-angle faulting. Kinematic indicators within the mylonite, including asymmetric porphyroclasts, fractured veins, S-C fabrics, and folded mylonitic foliation, suggest a broadly top-to-the-southeast (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 gneiss and zircon from the granitoid sheets have yielded near-identical U-Pb ages of ca. 550 Ma. Juxtaposition of low-grade over high-grade assemblages in several peri-Gondwanan basement blocks in central Cape Breton Island suggests that the Creignish Hills Mylonite Zone is part of a series of regional low-angle detachments with a core complex geometry. Similar ductile shear zones with easterly components of shear and low-angle pre-Carboniferous orientations also place low-grade over high-grade rocks in southern New Brunswick and the Cobequid Highlands of mainland Nova Scotia. Dated at ca. 565-540 and 605 Ma, respectively, they suggest repeated late Neoproterozoic detachment in the peri-Gondwanan arc(s). Detachment broadly coincides with the termination of arc magmatism and may reflect ridge-trench collision.