Search

Mafic layered intrusions show abundant evidence for the formation through the progressive inwards growth of a mushy solidification layer, with a central body of nearly crystal-free resident melt. The thickness and permeability of the mushy layer play a vital role in operation of several petrogenetic processes but remain poorly constrained in layered intrusions. To address this issue, we have re-examined the cumulate sequence in the Upper and Critical Zones of the Bushveld Complex. Although some observations (e.g., soft-sediment deformation) are indicative of cumulate rocks being mushy and having experienced interstitial melt percolation (e.g., metasomatic anorthosites), other field and textural evidence suggest the contrary. Some of the clearest evidence comes from sections through chromitite and magnetitite layers as well as through the Merensky Reef, layers that were deposited on the eroded rocks of the temporary chamber floor. Below the erosional surface, the layered rocks (1) were tilted and truncated without losing their internal coherence, (2) have knife-sharp contacts with the overlying rocks, (3) show minimal evidence of penetration by downward-percolating melt, (4) reveal sharp truncation of individual crystals, particularly beheading of pyroxene oikocrysts and intersecting of polysynthetic twinning in plagioclase, and (5) show resumed growth on the truncated crystals into the overlying rocks. These observations indicate that the cumulates of the inward-growing chamber floor were almost entirely solid at the time of the erosion and deposition of these layers. To reconcile these two sets of contrasting observations, we propose that only a few metres of the uppermost portion of the cumulate pile of the Upper and Critical Zones of Bushveld magma chamber were mushy, with other underlying rocks being almost totally solidified. The mushy layer tends to be thin and locally even non-existent (e.g., in magnetitite layers) due to high efficiency of primary adcumulus growth at the crystal-liquid interface of the temporary chamber floor. The solid chamber floor results from thermochemical erosion of the entire mushy layer by magmas replenishing the evolving chamber. The study implies that such petrogenetic processes as the hydrodynamic sorting of crystals and reactive porous flow may only operate within this very thin mushy portion of the cumulate pile and cannot therefore be responsible for any large-scale petrological features (e.g., platinum reefs, massive chromitites, stratiform anorthosite layers) of the Bushveld Complex. • One set of field and textural observations indicates that Bushveld's cumulate pile was mushy and plastically deformable. • In contrast, another set of field and textural evidence suggests that the cumulate pile was almost entirely solid. • Only a few uppermost meters of the cumulate pile were mushy, with other underlying rocks almost completely solidified. • The solid chamber floor is due to thermochemical erosion of the mushy layer by magmas replenishing the evolving chamber. • The hydrodynamic sorting of crystals and reactive porous flow operate within a thin mushy portion of the cumulate pile. [ABSTRACT FROM AUTHOR]

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. • Slurry models imply chromitite interstitial liquid solidifies at depth. • Crystallization of interstitial melt should produce plagioclase with An40-50 content. • Interstitial plagioclase in Bushveld chromitites shows up to 70-80% An content. • Interstitial plagioclase did not form in a deeply buried chromitite-rich pile. • Slurry models must be modified to explain the plagioclase composition. [ABSTRACT FROM AUTHOR]