Magma through-flow and embedding of magmatic crystals in metasedimentary country rocks adjacent to the Peninsula Granite, Cape Granite Suite (South Africa)

Abstract

The Sea Point contact in Cape Town, South Africa showcases the interaction between the Malmesbury Group metasedimentary rocks and the intrusive S-type Peninsula Granite of the Cape Granite Suite. The Malmesbury Group, comprising weakly metamorphosed mudstones and siltstones, has been intruded by various granitic injections of varying composition and mineralogical make-up. These granitic injections, displaying diffuse contacts with the country rock and termed “hybrid” granites, indicate varying degrees of assimilation of the country rocks. The injections occurred parallel to the country rock's fabric and anisotropies, particularly along the well-developed S₁ foliation. The granites exhibit compositional variations and contain K-feldspar megacrysts in a variable grain-sized groundmass. Some of these megacrysts are found within the Malmesbury Group, with or without quartzofeldspathic material envelopes. Most researchers agree that large K-feldspar megacrysts in granites develop through near-solidus growth, resulting from prolonged exposure to renewed magma input. However, the origin of K-feldspar and other phenocrysts in the adjacent country rocks is debated, with proposed mechanisms including entrapment due to magma through-flow and in situ growth due to K-metasomatism from granitic magma-derived fluids. This study compares the textures and chemistry of K-feldspar megacrysts in various granite types at the Sea Point contact with those in the Malmesbury Group country rock to determine the origin of the latter. The megacrysts in all lithologies show notable textural and compositional similarities, exhibiting poorly defined compositional zoning and consistent variations in K₂O, Na₂O, and BaO, along with rare earth element concentrations. These attributes suggest that the megacrysts formed through renewed magma injection and prolonged growth involving multiple magma pulses. The study proposes that filter pressing, where melt fractions separate from a crystal mush during crystallisation, occurred during active deformation so that filter pressing was strain-induced. During magma intrusion in a syn- to post-tectonic compressive regime, the melt and associated crystals moved into openings, with the melt fraction more easily mobilised and driven out compared to the crystal fraction during transpression. This left K-feldspar megacrysts embedded or mantled by the country rock. These isolated crystals, with or without associated crystallised magmatic material, mark former magma flow pathways through the country rock, contributing to understanding magma emplacement mechanisms and the formation of the continental crust.