ARE the Merensky Reef and Massive Chromitites of the Bushveld Complex Formed from Crystal Slurries or Superheated Magmas?

Abstract:

Many recent models attribute the origin of the Merensky Reef and massive chromitites of the Bushveld Complex, South Africa to replenishment of a magma chamber by phenocryst-rich magmas (crystal slurries). In particular, the emplacement of chromite-rich slurries from the staging chamber is currently thought to be responsible for the formation of massive chromitites of the Bushveld Complex. There are, however, first-order observations that are not compatible with this popular idea. One of the key features of the Merensky Reef and almost all layers of massive chromitites is their close association with so-called potholes, the circular to elliptical depressions with gently to steeply inclined sidewalls that are transgressive relative to their footwall rocks. Portions of magmatic stratigraphy are totally absent from the footwall rocks in pothole areas. Here we summarize abundant field evidence from several localities of the Eastern and Western Bushveld Complex that provide strong support to an idea that these portions of footwall rocks were thermally and partly mechanically eroded away by new magma pulses refilling the chamber. To be able to erode the footwall rocks so effectively, the new magmas must have been superheated upon emplacement into the chamber (no phenocrysts in the magmas). Otherwise the phenocrysts will immediately settle to the floor of the chamber to form a blanket protecting footwall rocks from the thermal erosion. The geological observations thus suggest that the origin of the Merensky Reef and massive chromitites must be tackled in the frame of the models that involve the emplacement of superheated, rather than phenocryst-laden magmas. The important lesson to be drawn from this study is that the field observations are still one of the primary tools for the rigorous testing of our hypotheses in modern igneous/ore petrology.