Magmatic-petrogenetic & structural relationships of the Peninsula Granite of the Cape Granite Suite (CGS) with the Malmesbury Group, Sea Point contact, Saldania Belt, South Africa
The Sea Point contact, Cape Town, South Africa, exposes the contact between the Neoproterozoic Malmesbury Group metasedimentary rocks of the Pan-African Saldania belt and the intrusive S-type Peninsula Granite of the Neoproterozoic-Paleozoic Cape Granite Suite (CGS). The exposure outcrops over an area of approximately 170 m × 60 m with the northern end of the exposure being characterized by the country rock–microgranite intrusive contact. Heading further south, the outcrop transitions to the main contact zone, which is a predominantly gradational zone marked by sheets of compositionally variable granitic injections (collectively referred to as hybrid granite phases) concordant to the country rock structure, before reaching the main pluton area comprising the voluminous coarse-grained porphyritic granite. Using a combined study incorporating field, structural, geochemical, isotopic and U-Pb geochronological data, the intrusive contact is investigated to determine the construction history of the pluton and delineate possible emplacement mechanisms. The granitic phases, which vary from fine-grained leucocratic, medium-grained porphyritic to coarse-grained porphyritic, are peraluminous, magnesian to ferroan, and alkali-calcic. Based on the linear trends between the whole-rock major and trace element content of the granites vs. maficity (molar Fe + Mg), their initial Sr ratios and εNd(t) values, the granites of the study area are consistent with the currently proposed petrogenetic model for the CGS (e.g. Stevens et al., 2007; Villaros et al., 2009a; Harris & Vogeli, 2010); i.e., they are crustally derived and their chemical variability is controlled primarily by peritectic assemblage entrainment. The fractional crystallization of K-feldspar is identified as the primary mechanism for the local geochemical variability of the granites. The fractionation of K-feldspar as a mechanism of variability was evaluated using binary log-log diagrams of Ba, Sr and Eu and is interpreted to have taken place at levels close to the emplacement site after source entrainment processes. Although there is outcrop evidence, particularly in the main contact zone, to suggest that local assimilation and filter pressing took place, this was not reflected by the whole-rock and isotope geochemistry of the granites. This suggests that these processes are very localized and will need further rigorous testing to ascertain the extent to which they caused variability. Outcrop evidence for assimilation includes gradational country rock-granite contacts and the ductile behaviour of the country rock, whereas the occurrence of K-feldspar megacrysts embedded in the country rock at the main contact zone suggests melt accumulation and escape consistent with the filter pressing mechanism. In the case of the latter, the melt fraction of the granite was easily mobilized and driven out compared to the crystal fraction (K-feldspars) during the emplacement of the granites. Field relationships and the structural interpretation of the Malmesbury Group country rocks and the granites reveal that: (1) the various granites are late syn-tectonic and (2) were emplaced as incrementally assembled, repeated pulses of inclined granitic sheets more or less normal (i.e. at high angles) to the regional NE-SW shortening (D1) of the Malmesbury forearc during the Saldanian orogeny. Given the lack of a controlling shear zone in facilitating granite emplacement in the study area, the pre-existing planar anisotropies (bedding planes and foliations) in the country rock provided preferential pathways for magma emplacement and propagation during deformation. This implies that the tensile strength normal and parallel to the bedding and foliation anisotropy of the country rock was larger than the regional differential stress (σ1 – σ3, with σ1 ≥ σ2 ≥ σ3), allowing for magma emplacement relative to shortening. Sheet propagation is interpreted to have occurred through the balance of the following conditions: (1) density contrasts between host rocks and magmas, (2) the pressure differential along the subvertical fractures/sheets, and (3) the melt pressure equalling the lithostatic pressure to keep the magma pathways open and being sufficiently high such that it exceeds the sum of σ1 and the tensile strength of the rock parallel to σ1. The crystallization ages of the dated granite samples are identical within error and vary between 538.7 ± 3.6 Ma and 542.7 ± 2.9 Ma. They, therefore, cannot prove which granite phase intruded first and which one proceeded and so forth. Field relationships, however, suggests that the microgranites were first to intrude given their fine-grained nature and the localized chilled contacts they show with the country rock. The various coarser-grained and porphyritic phases were next to intrude, with their coarse grain-sizes and lack of chilled margins with the country rock suggesting that the time interval between their successive emplacements was not too long; this prevented the country rock from completely cooling down between each magma batch. Magma stoping and the ductile flow of the host material (owing to highly viscous magma flow) to accommodate granite emplacement are interpreted to be secondary emplacement processes.