STRUCTURAL AND METAMORPHIC HISTORY OF THE CENTRAL CAPE BRETON HIGHLANDS, NOVA SCOTIA, CANADA: IMPLICATIONS FOR THE SALINIAN AND ACADIAN OROGENIES

The central Cape Breton Highlands of Nova Scotia have a complex early Paleozoic history related to convergence between the Ganderian Aspy and Bras d’Or terranes along the Eastern Highlands shear zone (EHSZ). Low-grade mainly metasedimentary rocks of the Calumruadh Brook Formation (CBF) in the core of the central highlands are in the hanging wall of the EHSZ. Interlayered metarhyolite yielded a U-Pb zircon age of 428 Ma, confirming the early to mid-Silurian age of the CBF and its correlation with the dominantly volcanic Sarach Brook Formation (SBF) and associated plutons (ca. 440-425 Ma; U-Pb zircon) in the south-central highlands. These formations (regionally assigned to the Money Point Group) are subduction-related volcanic-sedimentary successions related to Ganderian accretion to Laurentia in the Salinian orogeny. They are in tectonic contact along mainly brittle faults with high-grade metasedimentary rocks of the Ordovician Middle River Formation (MRF). The MRF was intruded by protoliths of the ca. 442 Ma Belle Cote Road tonalitic orthogneiss and ca. 430 Ma Taylors Barren granitic orthogneiss prior to regional deformation and metamorphism that formed staurolite-kyanite assemblages in in the MRF. This event occurred at 420-410 Ma during the first phase of compressional deformation along the EHSZ, coinciding with the Acadian orogeny. Continuing prograde metamorphism caused local migmatization at ~395 Ma, synchronous with highest-grade metamorphism in the western Aspy terrane interpreted as tectonic burial during terrane convergence. The EHSZ then evolved into a transpressional system, with lateral transport of Bras d’Or terrane to the south along the collision zone and deformation of basal units of the CBF and eastern part of the SBF at chlorite-grade conditions. The MRMS was exhumed concurrently along the Coinneach Brook shear zone, with retrograde breakdown of high-grade assemblages. The Park Spur pluton was emplaced at 375 Ma during exhumation along an east-west shear zone, with overall kinematics of south- and east-directed imbrication of the Park Spur pluton, CBF, SBF, and MRF. Subsequent emplacement of the ca. 363 Ma Margaree and related plutons marked the beginning of extension in the central highlands.