Abstract

The Beemerville carbonatite–alkalic rock complex of Late Ordovician age consists of two stocklike bodies of nepheline syenite and dikes or sills of phonolite, tinguaite, lamprophyre micromalignite, lamprophyre micromelteigite, and carbonatite. The complex also includes several lamprophyric diatremes with xenoliths of sedimentary rock and gneiss and autoliths of carbonatite, potassic syenite, and lamprophyre micromelteigite. The largest diatreme also contains a small pluglike body of nepheline syenite; fenite after graywacke occurs adjacent to one of the diatremes. Intense hydrothermal alteration is particularly evident in phonolite, lamprophyre, diatreme autoliths, and diatreme matrix rock.

Field and bulk chemical evidence suggests that parental magma was of either highly carbonated melteigitic or slightly carbonated malignitic composition. A petrogenetic model based on each of these possibilities is explored. The model that begins with highly carbonated melteigite magma involves immiscibility relations between melteigite and carbonatite magmas as well as fractional crystallization processes. The model that begins with slightly carbonated malignite magma includes, in addition, immiscibility relations between carbonated melteigite and syenite magmas.

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