Mortar Texture and Magma Dynamics in the Kunene Anorthosite Complex, Angola

A characteristic feature of Proterozoic massif-type anorthosites is 'mortar texture'—plagioclase primocrysts surrounded by mantles of finer-grained plagioclase neoblasts, where the expulsion of Fe-Ti oxide inclusions from the dark plagioclase primocrysts during recrystallization gives the rock a distinctive bleached appearance. The apparent lack of comparable recrystallization in mafic interstitial phases in these rocks has led some authors to suggest that these textures were produced by mechanical deformation during flow of a crystal-rich magma, for example during ascent and emplacement. However, these interpretations were largely based on qualitative observations or relied on methods that can be significantly improved on today. Previous research has shown that the formation of massif-type anorthosites requires ascent of plagioclase flotation mushes from Moho levels, so these are necessarily crystal-rich systems, yet many questions remain concerning the nature of these mushes and their mode of emplacement. Therefore, a study of mortar textures has the potential to shed light on some fundamental aspects of anorthosite petrogenesis.

We used electron backscatter diffraction to conduct a detailed, quantitative microstructural study of mortar textures in the Kunene Anorthosite Complex of Angola—the largest Proterozoic massif-type anorthosite complex in the world. Our results show that 1) the fabrics (crystal preferred orientations) in these rocks show a strong inherited magmatic component; however, 2) the interstitial mafic phases in these rocks are also deformed and recrystallized; and 3) although deformation may have started under melt-present conditions, the majority of the recrystallization that defines the mortar texture took place under subsolidus conditions, by dislocation creep. This raises the question of whether the deformation that produces mortar textures in massif-type anorthosites can be due entirely to internal stresses caused by the emplacement of the anorthosite itself, as has previously been suggested, or is a response to external tectonic forces.