Formation and evolution of the chromitites of the Stillwater Complex : a trace element study

Large layered intrusions, such as the Stillwater Complex, contain cyclic units of chromite-rich layers (cm to m thick) having kilometre-scale lateral extension. Chromite cumulates are among the first to form after new primitive melt injections into the magma chamber. Therefore, chromite cumulates could be used to investigate the nature of the parental magma, given the fact that chromite preserves its primary original magmatic composition. The cooling and crystallization history of large layered intrusions is long, complex, and involves multiple injections of hot primitive magma into an evolving and fractionating magma chamber. Our study on Stillwater chromites shows that the early crystallized chromite experiences various post-cumulus processes with the interstitial silicate melt, such as the precipitation of chromite overgrowths on early formed cumulus chromite and/or the reaction - reequilibration of early formed cumulus chromite. These processes have modifed the primary magmatic composition of the chromite making it difficult to identify the parental magma. Moreover, mineralogical evidence for chromite - interstitial melt interactions have probably been obliterated during late post-magmatic textural maturation and recrystallization which tends to homogenize chromite grain size and composition