Field relationships and petrology of the late Devonian Fisset Brook formation in the Cheticamp area, western Cape Breton Island, Nova Scotia

The Fisset Brook Formation in the Cheticamp area forms two elongate belts, a western belt which includes the type section of the formation in Fisset B rook and a separate eastern belt. In the western belt, the Fisset Brook For mation consists of (1) a lower, mainly sedimentary unit dominated by a basa l conglomerate that overlies Proterozoic to Silurian metamorphic and igneou s rocks, (2) a dominant middle part that consists of basaltic flows interla yered with minor red elastic sedimentary rocks, and (3) an upper part that consists mainly of rhyolite flows and tuffs. In the eastern belt, the basal sedimentary unit appears to be absent due to faulting, and the upper felsi c unit is thicker than in the western belt. Mafic dykes in overlying sedime ntary strata of the Creignish Formation (Horton Group) are petrochemically similar to basalt in the Fisset Brook Formation, and may represent the wani ng stages of igneous activity in the area, or a separate minor Late Devonia n to Early Carboniferous igneous event. Petrological studies of an extensive suite of samples support the results o f earlier studies showing that the mafic volcanic recks are continental, wi thin-plate tholeiites. The rhyolites are similar to within-plate felsic roc ks, but lack elevated values of elements such as Zr and Nb that characteriz e A-type granites. They are probably the extrusive equivalents of voluminou s granites of similar age in the Cape Breton Highlands, suggesting that the present exposures of the Fisset Brook Formation may be remnants of more ex tensive sequences. The stratigraphy, lithology, age, and petrochemical char acteristics of the Fisset Brook Formation in the Cheticamp area are very si milar to those in the Gillanders Mountain-Lake Ainslie area.