Peritectic assemblage entrainment as the main compositional driver in the I-type Vredenburg Granite, north-western Pan-African Saldania Belt, South Africa: A whole-rock chemical perspective

Abstract

The late- to post-collisional Cape Granite Suite (CGS), located in the ~560–520 Ma Pan-African Saldania Belt of south-western South Africa, comprises S-, I-, and A-type granites, mafic intrusive rocks, and pyroclastic rocks. The CGS formed during the closing of the Adamastor Ocean during the Neoproterozoic to Early-Cambrian, related to Gondwana construction. The S-type granites of the suite have received much attention concerning their petrogenesis and sources. The I-type granites, however, remain largely little studied. Whole rock geochemical and isotopic data are used to assess the petrogenesis and sources of the I-type Vredenburg Granite in the north-western Saldania Belt.The Vredenburg Granite is weakly peraluminous, ferroan, and calc-alkalic to alkali-calcic. Associated with the granite are weakly peraluminous, magnesian, and calc-alkalic magmatic enclaves (ME), of which two major varieties are present, a granodioritic variety (ME1) and a biotite- and plagioclase-rich variety (ME2). The former has a similar composition to the host granite, whereas the latter is more mafic and dioritic in nature.The εNd(t) values, Sm–Nd model ages and geochemical characteristics of the enclaves and host granite reflect melting of heterogeneous crustal sources of Mesoproterozoic-age. Biotite-, and subordinate amounts of amphibole-bearing rocks, dominantly metagreywackes and psammites, were the likely sources; minor amounts of basalts likely also played a role. These rock types are typical of a continental arc setting. Previously, the granites were interpreted to have formed by fractionation. However, such processes are incompatible with the full chemical variation displayed by the granites and enclaves. They exhibit positive linear trends between whole-rock major and trace elements vs. maficity (molar Fe + Mg), which can be best explained by entrainment of a peritectic assemblage comprising plagioclase, garnet, an Fe-Ti-oxide (ilmenite or Ti-magnetite) and, potentially, orthopyroxene. An amphibolitic source component may have contributed clinopyroxene and ilmenite, the former, along with plagioclase, contributing to the Ca content of the magma. These, and the co-entrainment of such accessory minerals as zircon, apatite and allanite, can largely explain the major-, minor- and trace-element variations found in the granites and enclaves. Fractional crystallisation may have acted as a minor compositional driver in the granites and enclaves, and then only at high SiO2 content.