Abstract
The Swartoup and Polisiehoek plutons in the Swartoup Hills (South Africa) formed during an episode of significant magma emplacement in the Mesoproterozoic Namaqua Sector of the Namaqua Metamorphic Province. They intruded into mid-crustal metasedimentary rocks of the metapelitic Koenap and mafic to carbonate-bearing Bysteek Formations during and shortly after the ∼1,200–1,220 Ma regional metamorphic peak that reached ultrahigh temperatures. Subsequent to pluton emplacement, the crust underwent regional high-temperature deformation during slow near-isobaric cooling. A further episode of pluton emplacement associated with fluid circulation truncated the first-order regional tectonic structures at ∼1,100 Ma. Based on their petrography, the Swartoup pluton is subdivided into leuco-granitoids with biotite as the sole mafic phase, pyroxene granitoids, and garnet-bearing granitoids, which may contain significant biotite. These subgroups display distinctive geochemical variations from one another, and from the Koenap Formation migmatites and the Polisiehoek granites, which are exposed nearby. Incompatible trace element distributions suggest that the Swartoup and Polisiehoek granitoids represent modified A-type granite magma, consistent with derivation from partial melting of quartzo-feldspathic crust. The magmas incorporated significant amounts of juvenile mantle-derived magma (εNd1200 of ∼−5, and LREE-depleted), but do not require older, early to late Paleoproterozoic crust. Particularly close to contacts to the calcic Bysteek Formation, localized contamination of the Swartoup granites by supracrustal carbonates is evident. A relatively pervasive alkali metasomatic effect is manifested strongly in the initial 87Sr/86Sr and LILE profiles of the Polisiehoek granites in particular, as well as in some of the Swartoup pyroxene granitoids, which could be either a symptom of CO2 metasomatism related to the Bysteek Formation carbonates, or to post-magmatic fluid metasomatism, perhaps linked to regional shearing. The comparison of our results with literature data suggests that similar sources, A-type granitic, Meso- to Paleoproterozoic crustal, and enriched mantle, have contributed, in locally differing proportions, to granites in most parts of the Namaqua Sector. Most likely, these plutons were generated during crustal and mantle melting in a long-lived hot continental back-arc environment.
Highlights
During its formation and evolution, granitic magma may be exposed to processes and factors that influence its mineral, chemical, and isotope composition, grain size, texture, and contact relationships with the host rocks into which the pluton is emplaced and where it solidifies
We investigate the field relationships, the compositional and geochemical variations, and Nd and Sr isotope signatures of the Swartoup and Polisiehoek plutons and their host rocks
A total of forty-four representative rock samples were collected from the study area: ten from the Koenap Formation migmatites, three from the Bysteek Formation, twenty-one from the Swartoup Pluton, and nine from the Polisiehoek granite gneisses
Summary
During its formation and evolution, granitic magma may be exposed to processes and factors that influence its mineral, chemical, and isotope composition, grain size, texture, and contact relationships with the host rocks into which the pluton is emplaced and where it solidifies. These parameters include the variably homogenous or heterogeneous source rocks from which the granite magma is generated, the lithospheric level in which the primary magma is formed, and the contribution of different magmas that may mix or mingle during ascent or emplacement (e.g., Vernon et al, 1988; Baxter and Feeley, 2002). Paul Macey, 2020), basaltic lavas in the Bushmanland Group (∼1,130 ± 35 Ma; Cornell et al, 2009), the ∼1,100 Ma Koras Group (Bailie et al, 2012), or the ∼1,030 Ma Koperberg Suite (Clifford et al, 1995; Robb et al, 1999), indicate fertile mantle sources throughout much of the evolution of the lithosphere in the Namaqua Sector
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