Thermo-mechanical intrusion-wall rock interaction and granite emplacement mechanisms of the Peninsula granite at the Sea Point contact, Cape Town, South Africa

Abstract

The Sea Point contact, Cape Town, South Africa exposes the intrusive contact between the ∼540 Ma S-type Peninsula Granite and the ∼560–555 Ma metasedimentary rocks of the Malmesbury Group of the Pan-African Saldania Belt. The western Saldania Belt was subjected to low-grade greenschist facies metamorphism and deformation during the ∼560–540 Ma Saldanian orogeny. The Peninsula Granite intruded as a series of numerous granite sheets which made use of the pre-existing country rock anisotropy in order to propagate. These are the steeply dipping S₀ bedding due to folding during the Saldanian orogeny, and a steeply dipping axial planar S₂ foliation to the F₂ folds developed during the dominant D₂ deformation. Magma overpressure relative to tensile stresses in the country rock and regional NE-SW-orientated compressional stresses allowed intrusion of variably crystal-laden magma along the anisotropies. The granitic sheets are commonly concentrated in the hinge zones of F₂ folds, where structural traps facilitated magma “trapping.” Filter pressing at the tail of the magma-filled hydrofracture caused closing during magma through-flow resulting in the entrapping of magmatic crystals, most notably K-feldspar megacrysts, in the wall rock as well as xenoliths dislodged during magma infiltration and stoping, and possibly magma flow. Magma stresses have brought about the alignment of K-feldspar megacrysts as well as the long axes of xenoliths parallel to the orientation of granite sheets and wall rock septa in the complex lit-par-lit zone and adjacent to the contact. A degree of assimilation (and possibly partial melting) is evident in the rounded shapes of xenoliths, but, more notably in mixed zones where there are gradational contacts not only between different granite types, reflecting multiple granite pulse intrusion, but also between the granites and wall rock, giving rise to hybrid granites. Xenoliths with remnant S₀ and S₂ were picked up and rotated during magma flow, with some embedded in the wall rock during filter pressing. The timing of granite intrusion relative to deformation is evident from examining contact-metamorphic cordierite porphyroblasts and their inclusions. The variation of inclusion patterns relative to the external foliation indicate that thermal metamorphism and, by implication, magma intrusion, occurred from early D₂- to late-D₃ deformation with continued flattening of the schist matrix leading to an enveloping of the porphyroblasts by the S₂ foliation. The intrusion thus was accompanied by progressive deformation and tightening of the F₂ folding. Some granites, notably the coarse-grained megacrystic granite, crosscut other granite types and intruded late-to post-tectonic. Magma loading led to the development of a shallowly dipping, widely spaced S₃ crenulation. Subsequent magma injections led to the development of the Peninsula pluton.