The use of An-content of interstitial plagioclase for testing slurry models for the origin of Bushveld massive chromitites

Abstract

Several recent petrogenetic models propose that layers of stratiform chromitites in the Bushveld Complex are produced by mechanical separation of chromite from co-existing silicate minerals (e.g., olivine, orthopyroxene, plagioclase) within crystal-rich slurries. One feature in common for the slurry models is that they imply that interstitial liquid in stratiform chromitites solidifies underneath a crystal pile ranging from 20 to 100 m thick. This means that the interstitial liquid would not be able to chemically communicate with the resident melt overlying the crystal pile. Equilibrium crystallization of interstitial melt within chromitites will, therefore, produce interstitial plagioclase with an average Ca/Na ratio similar to that of the interstitial liquid itself, i.e., plagioclase must be evolved in composition (∼40–50% An-content). In contrast to this prediction, extensive microprobe data show that interstitial plagioclase in Bushveld chromitites has a rather primitive composition (∼60–80% An-content) that is close to that of cumulus plagioclase (∼70–80% An-content) from adjacent norites and anorthosites that are produced by fractional crystallization. This implies that interstitial plagioclase is not a product of equilibrium crystallization within a deeply buried chromite-rich pile. Modification of interstitial plagioclase composition by various additional processes (fractional crystallization, selective diffusion, reactive melt flow, etc.) are considered but shown to be inconsistent with field, textural or chemical observations. We conclude that either slurry models for the origin of stratiform chromitites are incorrect and should be abandoned or must be substantially modified to address the issue of the high An-content of interstitial plagioclase in stratiform chromitites.