Petrogenesis of Mesoarchean high-K rocks in the Kaapvaal Craton: Investigating the Boesmanskop Alkaline Complex and Salisbury Kop batholith

The Boesmanskop Alkaline Complex (BAC) and Salisbury Kop batholith (SB) are two contrasting units of the large ca. 3105 Ma Granodiorite-Monzogranite-Syenogranite (GMS) suite, situated on the eastern margin of the Kaapvaal Craton. The BAC consists of small intrusions at the southern end of the Barberton greenstone belt, recording shear deformation and containing the most potassic and REE-rich rocks of the GMS suite. In contrast, the SB is a homogeneous batholith at the north of the belt with a more sodic character. This study examines the BAC and SB through field observations, petrography, mineral chemistry, whole-rock compositions, and U-Pb zircon geochronology, alongside Rb-Sr and Sm-Nd isotopic data and phase equilibrium modeling for the BAC rocks. The BAC includes coarse-grained syenite, quartz monzonite and leucogranite. Some syenites display porphyritic textures that indicate K-feldspar phenocryst accumulation, with clinopyroxene and Ca-amphibole as intercumulus phases. Field observations indicate that the BAC magmas were emplaced at shallow levels along active shear zones. New U-Pb zircon dating of a quartz monzonite yielded a crystallization age of 3118 ± 9 Ma and inherited cores of ca. 3220 Ma. Zircon, apatite, and titanite compositions reflect high-temperature crystallization from REE-rich felsic magmas, with Ti-in-zircon thermometry recording temperatures up to 915 °C. Additionally, the high K2O+Na2O, low MgO and CaO contents of all the rocks, the presence of inherited zircon cores, and crustal isotopic signatures of the cumulus rocks suggest that they derive from magmas produced by low-degrees of fluid-absent anatexis of the lower crust (including recycled sediments). Phase equilibrium modeling conducted at pressure of 1.5 kbar and temperature range from 800 to 950 °C, consistent with intrusion at shallow depths, demonstrated that a subset of high-alkali content GMS granitic compositions is capable of crystallizing K-feldspar as the first tectosilicate. These experiments support the hypothesis that the syenites and quartz monzonites originated from hot granitic melts that underwent significant crystal accumulation of alkali feldspar. These textural and geochemical features and zircon U-Pb ages advocate that the BAC quartz monzonites and syenites formed earlier than the surrounding granitic bodies, representing the onset of the K-high magmatism responsible for transferring incompatible and heat-producing elements to shallow levels, thereby contributing to the stabilization of the Kaapvaal Craton. Conversely, the leucogranites and granodiorites of the SB lack cumulus textures. U-Pb zircon dating of granodiorites yields a crystallization age of 3094 ± 10 Ma and the titanite dating yields a within error identical age of 3100 ± 11 Ma. The idiomorphic biotite as the only mafic phase suggests that these rocks originated from more evolved and colder magma than the syenites and quartz monzonites. Taken together, the BAC and SB represent two key magmatic complexes that record high-K magmatism associated with the partial melting of the lower crust. The features reported highlight the importance of crustal reworking and lower crustal melting in stabilizing the early cratons, such as the Kaapvaal Craton. The BAC represents the initiation of the magmatic event.

Grantor dc:publisher

Stellenbosch : Stellenbosch University

Year dc:date.issued

2025

Author dc:creator

• Leandro, Marcel Vinicius Santos

Advisors dc:contributor.advisor

• Stevens, Gary
• Kisters, Alex
• Moyen, Jean-Francois

Repository record dc:identifier.uri

https://scholar.sun.ac.za/handle/10019.1/132616

OAI identifier oai:identifier

oai:scholar.sun.ac.za:10019.1/132616