Petrogenesis of the Concord gabbro-syenite complex, Cabarrus County, North Carolina

The Concord intrusive complex consists of a gabbro stock partially encircled by a syenite ring dike. A simple gravity model suggests that the gabbro pluton extends to a depth of about 6 km. and that the steep-walled syenite ring dike dips outwardly at an angle of 78° on the west side, and dips vertically on the east side. Isotopic data indicate that crystallization of the gabbro and syenite were approximately contemporaneous, and the close field association of these bodies suggests a relationship through fractional crystallization.

Three gabbroic rock types, based on the presence of different cumulus minerals, occur. Olivine-plagioclase cumulate (OPC) gabbros contain cumulus olivine and plagioclase, and postcumulus clinopyroxene, orthopyroxene and hornblende. Clinopyroxene-plagioclase cumulate (CPC) gabbros contain cumulus clinopyroxene and plagioclase; olivine-clinopyroxene-plagioclase cumulate (OCPC) gabbros contain cumulus olivine, clinopyroxene and plagioclase; both CPC and OCPC gabbros contain postcumulus orthopyroxene and hornblende. All gabbro types contain biotite, Fe-Ti oxides, sulfides, apatite and zircon. The syenite is a coarse-grained seriate porphyry with phenocrysts of perthitic feldspar and a groundmass of perthite, albite, microcline, clinopyroxene, amphibole, biotite, oxides, sulfides, apatite, zircon and monazite. Mafic mineral compositions in both the gabbro and syenite indicate limited Fe-enrichment trends during crystallization.

Cumulate textures suggest that the fractionation of the Concord complex parental magma was dominated by plagioclase, olivine and clinopyroxene. Petrogenetic modelling of major and trace elements indicates that the Concord syenite could be produced by 95% fractional crystallization of olivine, plagioclase and clinopyroxene in roughly cotectic proportions, with minor sulfide and oxide minerals, from a basaltic parental magma.

The Concord gabbro-syenite trend probably represents near-critical plane differentiation of a tholeiitic or transitional basaltic liquid to a syenitic residual liquid with composition close to that of the alkali feldspar join in the system NaAlSiO₄-KAlSiO₄-SiO₂. This fractionation took place at depth, before intrusion of the syenitic liquid into the fractured region between the gabbro and country rock. Fractionation from basaltic to trachytic liquid is not unusual, and it can be shown that any parental magma with composition near the plane of critical silica undersaturation is likely to exhibit this trend.