The alkaline intrusives of the Bancroft area lie along the contact between the “Laurentian” granites and the Grenville marble. Alkaline rocks pass in sweeping arcs about two predominantly granitic areas. The principal alkaline intrusives fall into two quite distinct groups: a series of saturated or nearly saturated rocks ranging from syenite through shonkinite to perknite; an undersaturated group including urtite, ijolite, foyaite, and jacupirangite. Three sill-like bands of marble lie within the alkaline complex but are in the main indistinguishable from the Grenville marble immediately south of the complex.
The oval-shaped granite areas are often heavily contaminated with bands and lenses of amphibolite. About these amphibolite impurities the granite develops a marginal syenitic facies, and most of the shonkinites have been formed by the intimate mixing of amphibolite and syenitic liquid. The nepheline rocks show no constant association with syenite, granite, or amphibolite. Typically the coarse urtite or nepheline pegamatite lies within limestone. There seems to have been only a small supply of truly foyaitic magma, and most of the undersaturated rocks are migmatites or hybrids. Where urtite has been injected into amphibolite, ijolites and jacupirangites have been formed; where it penetrates syenite, foyaite results.
The alkaline rocks as a whole are younger than the granites of the area but are themselves cut by acid pegmatites. Some of these late pegmatites are albite-aegirine-quartz rocks closely related to the alkaline suite.
The carbonate rocks of the alkaline complex are intrusive in the sense that they cut and enclose most of the other rocks of the complex, but their intrusive habit is merely the final expression of a long metamorphic history. The region is rich both in primary calcite and in intrusive carbonate rocks, but the primary calcite is not intrusive, and the intrusive carbonate rocks are not primary.
The granite-syenite-amphibolite association is best explained by the limestone-syntexis hypothesis, and nothing in the field relations of the carbonate-urtite group contradicts the theory. None of the carbonate is truly magmatic, a small percentage of it is hydrothermal, but the vast bulk—including that which forms carbonate-intrusives—is surely metamorphic.