The stratigraphy and geological history of the Fourchu group, South Eastern Cape Breton, Nova Scotia

The Fourchu Group of Southeastern Cape Breton consists of sodic felsic and mafic pyroclastics, ignimbrites and minor rhyolitic flows. The succession between Kennington Cove and Cape Breton has been divided into six informal members. The lower members are massive and were erupted and deposited subaerially. They are poor in accidental lithic fragments. This is a contrast with the thinly bedded lithic-rich upper members which were laid down in shallow water. The mafic tuffs and hyaloclastites were extruded from submarine volcanoes. The volcanic stratigraphic sequence, crystal tuffs ⟶ ignimbrites ⟶ lithic tuffs, reflecting the increasing viscosity and explosive character of the magma, is attributed to the upward migration of volatiles in the felsic magma chamber. The felsic magma is calc-alkalic and related to orogenesis. There have been three separate periods of mafic intrusion. With the exception of the Blackrock Point Gabbro, the mafic intrusives are high alumina rocks transitional between calc-alkalic and tholeiitic suites. The Blackrock Point Gabbro is strongly tholeiitic. The mafic sheets appear to be "feeders" for the extrusive Little Lorraine Member. A phase of tectonic deformation produced close folds and an associated penetrative axial planar foliation. A second foliation is present in the more schistose units. The volcanism associated with the Fourchu Group is attributed to magmatic arc environment developed on continental crust.