HOW GEOLOGICAL MAPPING, PETROLOGICAL STUDIES, AND U-PB (ZIRCON) DATING LED TO RECOGNITION OF SEPARATE NEOPROTEROZOIC, CAMBRIAN, ORDOVICIAN, AND SILURIAN CONVERGENT MARGIN MAGMATISM IN WESTERN CAPE BRETON ISLAND, NOVA SCOTIA, CANADA

The western part of the Cape Breton Highlands of Nova Scotia has been recognized for decades as an area of complex and enigmatic relations among metamorphic and igneous rocks of poorly constrained Neoproterozoic to Devonian ages. Recent mapping resulted in a revised geological map of the area, with units constrained by new U-Pb (zircon) ages. In addition, petrological studies have led to new interpretations of tectonic setting, and clarified the relationship of the rocks west and east of the mylonitic Western Highlands shear zone. The area west of the shear zone (Cheticamp-Jumping Brook belt) includes Neoproterozoic metasedimentary rocks and a granitoid suite formed in a continental margin subduction zone. These rocks are linked to the Bras d'Or terrane of central Cape Breton Island, as well as to poorly preserved units of similar age in the Crippleback Intrusive Suite of the Exploits terrane in Newfoundland. These older rocks are in faulted contact with Cambrian sulphide-bearing back-arc sedimentary and basaltic rocks of the Jumping Brook Metamorphic Suite and intruded by large Ordovician tonalitic and dioritic arc-type plutons, possibly correlative with the Victoria Lake Supergroup and associated plutonic rocks in the Exploits terrane of Newfoundland. Previously unrecognized but widespread Silurian dioritic to granitic rocks formed in a volcanic-arc setting are associated with the Neoproterozoic rocks. The youngest rocks are a widespread cover sequence of ca. 374-372 Ma bimodal volcanic and associated sedimentary rocks and related gabbroic and granitic plutons. The plutons are undeformed and cut the Western Highlands shear zone, thus constraining the timing of juxtaposition of the Cheticamp-Jumping Brook belt west of the shear zone with the Ordovician(?)-Silurian metasedimentary, amphibolitic, and orthogneissic rocks of the Pleasant Bay Complex (PBC) east of the shear zone. The PBC is intruded by abundant Devonian S- and A-type granitoid rocks. Chemical characteristics and ages of amphibolitic and orthogneissic rocks in the PBC indicate that they formed in a mid-Silurian rifted arc. The documentation of arc-related Neoproterozoic, Cambrian, Ordovician, and Silurian rocks strengthens the evidence of correlation of the Aspy terrane with the Ganderian Exploits terrane of Newfoundland and New Brunswick.