Mapping, petrological studies, and U-Pb (zircon) dating have identified four bimodal plutons herein named Gillis Brook, Leonard MacLeod Brook, Easach Ban, and Barren Brook in the southern part of the Ganderian Aspy terrane in Cape Breton Island. The plutons consist mainly of gabbroic to dioritic rocks and syenogranitic rocks which are closely associated and locally display mingled relationships with one another, consistent with their similar ages of ca. 435-430 Ma. They display alkalic to calc-alkalic chemical signatures and it is not clear if they formed in a subduction-related magmatic arc or within-plate extensional setting. They were emplaced at shallow depth and are co-magmatic with their Silurian volcanic and sedimentary host rocks of the Sarach Brook Formation. They are petrologically distinct from and not related to the younger (Devonian) and entirely granitic Bothan Brook pluton which also intruded the Sarach Brook Formation in the same area. None of these plutons shows evidence for significant economic mineralization potential. In contrast, the Devonian Gillis Mountain pluton in the Avalonian Mira terrane of southern Cape Breton Island hosts porphyry-style copper-molybdenum mineralization. New studies of trace element characteristics of surface and drill core samples from the Gillis Mountain pluton confirm previous work which showed that it is calc-alkalic and emplaced in a subduction zone setting, intruded into Cambrian shale and siltstone. The presence of sulphide minerals and elevated background metal contents are evidence of its economic potential. Comparison of these plutons shows contrasts in field and petrological characteristics. These differences are consistent with the fundamental distinction between the Aspy and Mira terranes in terms of origin, geological evolution, and tectonic setting through to the Devonian.