Emplacement mechanisms and magmatic structures associated with granitic pluton evolution: a case study of the Peninsula pluton, Cape Granite Suite, Saldania Belt, South Africa

Abstract

This study investigates how the spatial distribution, structural and petrographic characteristics, mineral fabrics and whole-rock geochemistry of the Peninsula pluton can be used to constrain the mechanisms responsible for its emplacement. The Peninsula pluton forms part of the S-type magmatism of the Cape Granite Suite within the western Saldania Belt and was emplaced into metasedimentary rocks of the Malmesbury Group. Field mapping and sampling of coastal exposures along the Atlantic Seaboard and False Bay coastline were integrated with petrographic analysis, Shape Preferred Orientation (SPO) measurements of K-feldspar megacrysts, major-element geochemistry and trace-element analysis to investigate the petrogenesis, internal evolution and emplacement of the pluton. Three principal granitic lithologies are recognised: porphyritic monzogranite, aphyric granite and finer-grained leucocratic granite. The porphyritic monzogranite is the primary lithology and contains abundant 2 – 8 cm K feldspar megacrysts whose textures indicate prolonged magmatic growth under supra-solidus conditions. Diffuse contacts and shared mineral assemblages between the lithologies indicate genetically related magma batches, whereas replacement and overgrowth textures record prolonged crystallisation, repeated replenishment and late mineral-melt and mineral-fluid interaction. Whole-rock geochemistry indicates a metasedimentary source and demonstrates that the compositional variability of the pluton is better explained by variable entrainment of peritectic and accessory phases than by fractional crystallisation alone. The porphyritic monzogranite and magmatic layering display higher maficity and REE abundances, consistent with greater entrainment of ferromagnesian and accessory phases, whereas the aphyric granite and finer-grained leucocratic granite are more evolved and contain a greater proportion of felsic melt. Fractional crystallisation and residual-melt evolution subsequently enhanced these compositional differences.