Recrystallization and the origin of layering in the Bushveld Complex

Abstract

Three rock types are distinguished in this study, (a) the layered rocks, (b) the Reef pegmatites which are parallel to the layering and form prominent marker horizons and (c) the iron rich ultramafic (IRUM) pegmatites which are transgressive to the layered rocks. Reaction textures are observed in the layered rocks adjacent to Reef pegmatites and are compared with similar reaction textures from alteration zones adjacent to and caused by the iron-rich ultramafic (IRUM) pegmatites. These include, (a) clinopyroxene-coronas around orthopyroxene, (b) hornblende lamellae along (100) of orthopyroxene, (c) orthopyroxene altered to olivine, serpentine, talc and/or chlorite. In the alteration zones adjacent to IRUM pegmatites the textures were generated by interaction between the pegmatite forming fluid and the wallrocks. A similar origin is proposed for reaction textures in the layered rocks. In the latter case they were formed by the fluid responsible for the formation of Reef pegmatites. In the layered rocks the reaction textures were modified to variable degrees during and after the emplacement of Reef pegmatites. These modifications include in leuconorites, (a) clinopyroxene-coronas overgrown by feldspar, (b) orthopyroxene overgrown by feldspar and hornblende lamellae transforming to biotite and in olivine-pyroxenites (c) olivine-serpentine textures overgrown by a second generation of orthopyroxene. The modifications suggest the subsolidus dissolution of orthopyroxene and precipitation of feldspar in leuconorites and the reverse process in pyroxenites. The textures suggest considerable transfer of matter in the solid rock due to a fluid gradient. Orthopyroxene recrystallized in areas with a high a H2 O , plagioclase in regions of low a H2 O . This process is believed to be responsible for the stratification of the rock into pyroxene and plagioclase-rich layers. Spherical mineral inclusions are relict textures in the three main minerals, chromite, orthopyroxene and plagioclase. A number of similarities between the different inclusions suggest a similar origin. The high content of hydrous minerals in all three suggests their formation during the emplacement of Reef pegmatites. These hydrous minerals were then trapped in the recrystallizing plagioclase, orthopyroxene and chromite. Also spherical mineral inclusions witness the late crystallization of pyroxene in melanorite/pyroxenite and the crystallization of plagioclase in leuconorite/anorthosites after the emplacement of Reef pegmatites.