Plagioclase disequilibrium induced during fluid‐absent biotite‐breakdown melting in metapelites

Abstract

AbstractIn the Bandelierkop Quarry (South Marginal Zone of the Limpopo belt), two generations of leucosome contain peritectic garnet produced by incongruent melting of biotite‐bearing assemblages, either in the presence or absence of sillimanite, respectively. This study focuses on mineral inclusions in large garnet crystals from both leucosome generations, which, in rare cases, host an abundance of euhedral plagioclase inclusions with variable compositions. Plagioclase in the residuum adjacent to the leucosomes ranges from An32 to An39, while the euhedral plagioclase inclusions are considerably more calcic with compositions that range from An50 to An83. These grains are also significantly zoned, with inclusions displaying both Na‐ and Ca‐enriched rims. Garnet zoning directly adjacent to the inclusions demonstrates that the Ca‐rich nature of the inclusions is not a consequence of Ca‐uptake from garnet. The euhedral plagioclase inclusions are interpreted to reflect disequilibrium during anatexis due to slow diffusion in plagioclase. Thus, the kinetic pathway followed by the incongruent melting reaction appears to be one where only the outer portions of the reactant plagioclase grains were involved, via dissolution, not diffusion, and with the precipitation of new Ca‐rich peritectic plagioclase as a consequence. This is predictable, as the coupled substitutions involved in plagioclase solid solution result in very slow intracrystalline diffusion of the major elements, while plagioclase dissolution in granitic melt is faster by several orders of magnitude. Phase equilibrium modelling demonstrates that the Ca‐content of the peritectic plagioclase is inversely proportional to the amount of plagioclase that participates in the fluid‐absent incongruent melting reaction in biotite‐bearing metapelites, with the specific compositions of the peritectic grains, reflecting local variations in effective bulk composition as the melting reaction progressed. The variation in composition and zoning patterns within plagioclase inclusions from individual garnet grains is interpreted to reflect melt segregation and flow simultaneously with garnet growth, allowing individual garnet grains to trap euhedral peritectic plagioclase grains that had formed within a range of effective bulk compositions. The preservation of this unusual garnet, which was most likely due to rapid and efficient melt loss from the residual source, as well as rapid cooling, provides a rare insight into the details of the anatectic process.