THE CHETICAMP AREA, CAPE BRETON ISLAND, NOVA SCOTIA: A GEOLOGICAL ENIGMA PARTLY RESOLVED BY MAPPING, U-PB (ZIRCON) GEOCHRONOLOGY, AND PETROLOGY

New LA-ICP-MS U-Pb (zircon) dating combined with geological mapping and new petrographic and geochemical studies in the Chéticamp area have resulted in a revised geological interpretation of the western part of the Ganderian Aspy terrane of Cape Breton Island. The oldest metamorphic rocks in the area are assigned to the Jumping Brook Metamorphic Suite (JBMS), now confirmed to have an age of <530 Ma. The JBMS includes a lower mafic metavolcanic unit with N-MORB-affinity (Faribault Brook formation), overlain by turbiditic metasedimentary and metatuffaceous rocks. The enigmatic relationship between these rocks and the plutonic rocks of the former “Chéticamp pluton” which appeared to intrude them but yet yielded a ca. 550 Ma U-Pb (zircon) age reported in the 1980s has been at least partly resolved by the recognition that the pluton consists of components of different ages, only two of which (Pembroke Lake monzogranite with an newly acquired age of ca. 570 Ma and Grand Falaise granodiorite with an age of ca. 547 Ma) are older than the JBMS. The Chéticamp River tonalite (ca. 490 Ma) is younger than the JBMS which it intruded. Other units intruded into the JBMS are even younger: MacLean Brook granodiorite and Lavis Brook quartz diorite are both Early Silurian (ca. 443 and 442 Ma, respectively) and French Mountain syenogranite is Devonian (366 Ma). The bimodal gabbroic and granitic Salmon Pool pluton intruded along the boundary between the low-grade and high-grade (amphibolite-facies) rocks in the JBMS, but the relationship between the low- and high-grade rocks remains uncertain, as is whether or not they are all part of a single unit. All of the pre-Devonian plutonic units have petrographic and chemical characteristics consistent with calc-alkaline affinity and emplacement in a volcanic-arc tectonic setting, indicating a long history of subduction-related magmatism in this part of Ganderia. The relationship of this area to the rest of the Aspy and Bras d’Or terranes remains enigmatic, as is the apparent absence of effects of regional deformation and metamorphism in the older plutonic units.