Field relations, structure, and geochemistry of the Fisset Brook Formation in the Lake Ainslie - Gillanders Mountain area, central Cape Breton Island, Nova Scotia

Abstract

Detailed mapping shows that the Fisset Brook Formation in the Lake Ainslie - Gillanders Mountain area consists of a lowermost sedimentary unit overlain by basaltic and rhyolitic units. The sedimentary unit is mainly arkosic pebble conglomerate and siltstone, and unconformably overlies or is in faulted contact with older metamorphic and plutonic rocks. The basaltic unit consists mainly of subaerial flows, locally interlayered and intermixed with red-brown siltstone. The overlying rhyolitic unit consists mainly of eutaxitic to spherulitic flows or welded tuffs, with less abundant lapilli tuff. The chemical compositions of the basalt and rhyolite in both areas have been modified by alteration, but discrimination diagrams using relatively immobile elements indicate that the basalts and gabbros are continental, within-plate tholeiites. The rhyolites also have features indicative of origin in a within-plate setting, but are depleted in Y, Zr, and rare-earth elements compared to A-type granites. A rhyolite sample yielded a U-Pb (zircon) age of 373 ± 4 Ma, thus indicating that the Fisset Brook Formation in the Lake Ainslie - Gillanders Mountain area is Middle to earliest Late Devonian in age.