Infiltration metasomatism in layered intrusions revisited: a reinterpretation of compositional reversals at the base of cyclic units

Summary Irvine (1980) proposed an elegant mechanism termed “infiltration metasomatism” to explain upward displacement of discontinuities in mineral compositional trends relative to contacts between cyclic units in the Muskox intrusion. It was concluded that the offsets of Mg/(Mg + Fe) discontinuities in olivine and chromite are a secondary postmagmatic feature that resulted from reaction between the cumulus minerals and intercumulus liquid that was frontally displaced upwards from the underlying crystal pile as a result of compaction. We reinterpret this feature in the Muskox and other layered intrusions as basal reversals that arise from a temperature gradient-driven flux of low melting point components from the hot magma parental to cyclic units towards a relatively cold cumulate floor. In this interpretation basal reversals are a primary magmatic feature that does not involve intercumulus liquid migration.