Age and tectonic setting of granitoid plutons in the Chéticamp belt, western Cape Breton Island, Nova Scotia, Canada

Abstract

Geological mapping in the Chéticamp granitoid belt in combination with petrographic and geochemical studies and U–Pb (zircon) dating by laser ablation inductively coupled plasma mass spectrometry have resulted in major reinterpretation of the geology in the western part of the Ganderian Aspy terrane of Cape Breton Island. Nine new U–Pb (zircon) ages show that the former “Chéticamp pluton” consists of 10 separate plutons of five different ages: late Neoproterozoic (ca. 567 Ma), Cambrian–Ordovician (490–482 Ma), Ordovician–Silurian (442–440 Ma), mid-Silurian (ca. 428 Ma), and late Devonian (366 Ma). The three late Neoproterozoic granodioritic to monzogranitic plutons are older than the adjacent metavolcanic and metasedimentary rocks of the Jumping Brook Metamorphic Suite, whereas the tonalitic to quartz dioritic Cambrian–Ordovician plutons intruded those metamorphic rocks. Petrographic characteristics and approximately 100 whole-rock chemical analyses show that with the exception of the mid-Silurian Grand Falaise alkali-feldspar granite, which has A-type within-plate characteristics, the plutonic units have calc-alkaline affinity and were emplaced in a volcanic-arc tectonic setting. These results are evidence that fragments of a long history of episodic subduction-related magmatism and terrane collision are preserved in this small part of Ganderia. Eight new Sm–Nd isotopic analyses are consistent with the Ganderian affinity of the Chéticamp plutonic belt. The ca. 490–482 Ma plutons are the first direct evidence in Cape Breton Island for the Penobscottian event recognized in the Exploits Subzone of central Newfoundland and in New Brunswick. However, the structural relationship of the Chéticamp plutonic belt to the rest of the Aspy and Bras d’Or terranes remains enigmatic, as is the apparent absence of effects of Devonian deformation and metamorphism in the older plutonic units.