Formation and emplacement of two contrasting late-Mesoproterozoic magma types in the central Namaqua Metamorphic Complex (South Africa, Namibia): Evidence from geochemistry and geochronology

Petrogenesis of Mesoproterozoic Granites of the Swartoup Hills Region, Kakamas Domain, Namaqua Belt, South Africa

“…The Mesoproterozoic Kakamas Domain (Figure 1) consists of medium-to high-grade metamorphic supracrustal metamorphic rocks which commonly show extensive anatexis. Predominantly granitoid plutons intruded this basement between ∼1,220 and ∼1,180 Ma, and in a second episode at ∼1,100 Ma (Bial et al, 2015a;Bial et al, 2015b). The region of the Swartoup Hills, east of the Lower Fish River-Onseepkans Thrust Zone (LFROTZ; Figure 1B), along which the Kakamas Domain has been thrusted over the Paleoproterozoic Richtersveld Magmatic Arc in the west (Thomas et al, 2016;Macey et al, 2017) is arguably the highest grade metamorphic region within the Kakamas Domain.…”

“…The region of the Swartoup Hills, east of the Lower Fish River-Onseepkans Thrust Zone (LFROTZ; Figure 1B), along which the Kakamas Domain has been thrusted over the Paleoproterozoic Richtersveld Magmatic Arc in the west (Thomas et al, 2016;Macey et al, 2017) is arguably the highest grade metamorphic region within the Kakamas Domain. In this area, the Swartoup and Polisiehoek plutons and their metamorphic host rocks formed and developed in a high-to ultrahigh temperature (HT/UHT) and low-pressure environment reaching up to about 900°C, at pressures not exceeding 500-550 MPa (Bial et al, 2015a;Bial et al, 2015b). Spinel-quartz-and osumilite-bearing parageneses in felsic host rocks indicate the UHT increment of the P-T path.…”

“…The crustal evolution of the Namaqua Metamorphic Province in South Africa and Namibia (Figure 1A) is characterized by a long-standing high-temperature regime in the Mesoproterozoic, particularly between ∼1,230 and ∼1,080 Ma, with further episodes of early (∼1,300 Ma) and late (∼1,000 -1,030 Ma) magmatism in some parts of the belt (Robb et al, 1999;Raith et al, 2003;Clifford et al, 2004;Bial et al, 2015a;Bial et al, 2015b;Abrahams and Macey, 2020;Groenewald and Macey, 2020). Granitic to charnockitic plutonism is predominant and widespread FIGURE 1 | Regional geology of the Namaqua Sector of the Namaqua-Natal Metamorphic Province.…”

“…The position of the map in Figure 2 is highlighted. Maps A and B are modified after Bial et al (2015b) and references therein. SA: Swartmodder antiform.…”

“…The traditional model developed in the 1980s (Joubert, 1986;Stowe, 1986;and reviewed by Miller, 2012) proposes multiple oceanic basins separating small continental fragments in the early Mesoproterozoic, presuming multiple subduction zones and eventual terrane collisions at ∼1,200 Ma in the context of Rodinia assembly. More recently, this model has been challenged (Bial et al, 2015a;Bial et al, 2015b;Macey et al, 2016;Büttner, 2020), particularly on the basis of the lack of convincing indicators for subduction-or collision-related metamorphism, or evidence for the existence of oceanic basins once separating crustal entities in the region at that time. Bial et al (2015a) proposed a long-lived continental back-arc, with regionally and chronologically variable heat and magma transfer into thin lithosphere, as the most likely tectonic setting for the Namaqua Sector in the Mesoproterozoic.…”