FIELD RELATIONS, PETROLOGY, AND TECTONIC SETTING OF MAFIC ROCKS IN THE NORTHWESTERN ASPY TERRANE, CAPE BRETON ISLAND, NOVA SCOTIA, CANADA

Geological mapping, U-Pb (zircon) dating, and petrographic and geochemical studies in the Chéticamp area of the northwestern Cape Breton Highlands have demonstrated that mafic rocks there include five different components: (1) variably metamorphosed and deformed dioritic (and tonalitic) rocks of the Ordovician Georges Brook pluton and inferred Ordovician South Branch Corney Brook pluton emplaced in a continental margin subduction zone; (2) gabbroic (to dioritic rocks) of the bimodal ca. 372 Ma Salmon Pool pluton; (3) metamorphosed basaltic rocks of the mid- to Late Cambrian Faribault Brook Formation that likely formed in a back-arc setting; (4) metagabbroic rocks intruded into the Faribault Brook Formation; and (5) amphibolitic rocks of mid-Silurian age intruded into pelitic, psammitic, and orthogneissic rocks of the Pleasant Bay Complex. Chemical similarity of the metagabbroic rocks in the Faribault Brook Formation to the higher-grade amphibolite in the Pleasant Bay Complex suggests that these mafic rocks may be related and formed in a mid-Silurian rifted arc. The mafic rocks lack the anomalous depletion in the light rare-earth elements (REE), Nb, Hf, and Ta that characterizes mafic metavolcanic rocks in the Faribault Brook Formation. The original contact between the Pleasant Bay Complex and Faribault Brook Formation is obscured by the Western Highlands shear zone and the ca. 373 Ma. Salmon Pool pluton. Gabbroic and dioritic rocks in the Salmon Pool pluton have similar chemical characteristics to the basaltic rocks of the Fisset Brook Formation, and both plutonic and volcanic rocks were emplaced in a post-tectonic extensional setting. The documentation of arc-related Cambrian and Ordovician rocks strengthens the evidence of correlation of the Aspy terrane with the Exploits terrane of Newfoundland and New Brunswick.