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Volume 32 Issue 6
Dec.  2021
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Russell Bailie, Aidan Leetz. A Comparison between the ~1.08-1.13 Ga Volcano-Sedimentary Koras Group and Plutonic Keimoes Suite: Insights into the Post-Collisional Tectono-Magmatic Evolution of the Eastern Namaqua Metamorphic Province, South Africa. Journal of Earth Science, 2021, 32(6): 1300-1331. doi: 10.1007/s12583-021-1462-7
Citation: Russell Bailie, Aidan Leetz. A Comparison between the ~1.08-1.13 Ga Volcano-Sedimentary Koras Group and Plutonic Keimoes Suite: Insights into the Post-Collisional Tectono-Magmatic Evolution of the Eastern Namaqua Metamorphic Province, South Africa. Journal of Earth Science, 2021, 32(6): 1300-1331. doi: 10.1007/s12583-021-1462-7

A Comparison between the ~1.08-1.13 Ga Volcano-Sedimentary Koras Group and Plutonic Keimoes Suite: Insights into the Post-Collisional Tectono-Magmatic Evolution of the Eastern Namaqua Metamorphic Province, South Africa

doi: 10.1007/s12583-021-1462-7
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  • Along the eastern margin of the Mesoproterozoic Namaqua metamorphic province (NMP) of southern Africa are a bimodal volcano-sedimentary succession, the ~1.13-1.10 Ga Koras Group, composed of rhyolitic porphyries and basaltic andesites, and the ~1.11-1.07 Ga late-to post-tectonic granitoids of the Keimoes Suite. This review examines existing whole-rock major-and trace-element data, along with isotope chemistry (with some new isotopic data), to investigate the role these two magmatic successions played in terms of post-collisional magmatism of the eastern NMP near the boundary with the Archean Kaapvaal Craton. The Keimoes Suite comprises variably porphyritic biotite monzogranites and granodiorites, with a charnockitic member. They are metaluminous to weakly peraluminous, ferroan, and calc-alkalic. They exhibit large ion lithophile (LIL) element enrichment relative to the high field strength elements (HFSE) with depletions in Ba, Sr, Nb, P, Eu and Ti, and enrichments in Th, U and Pb. Isotopic values (eNd(t): 2.78 to -2.95, but down to -8.58 for one granite, depleted mantle Nd model ages (TDM): 1.62-1.99 Ga, but up to 2.55 Ga; initial 87Sr/86Sr: 0.652 82-0.771 30) suggest derivation from weakly to mildly enriched (and radiogenic) sources of Meso-to Paleoproterozoic age, the former of more juvenile character. The Koras Group is characterized by a bimodal succession of calcic to calc-alkalic, magnesian and tholeiitic basaltic andesites and mostly metaluminous to peralkaline rhyolitic porphyries. Two successions are recognised, an older, lower succession that extruded at ~1.13 Ga, and a younger, upper succession at ~1.10 Ga. The rhyolitic porphyries of both successions show similar LILE/HFSE enrichment and the same element enrichments and depletions as the Keimoes Suite granitoids. The upper succession is consistently more fractionated in terms of both whole-rock major and trace element chemistry, and, isotopically, has a greater enriched source component (eNd(t): -0.69 to -4.26; TDM: 1.64-2.44 Ga), relative to the lower succession (eNd(t): 0.74-5.62;TDM: 1.28-2.12 Ga). Crystal fractionation of plagioclase and K-feldspar appears to have played a role in bringing about compositional variation in many of the granites. These were derived from partial melting of mainly igneous with subordinate sedimentary sources from mostly lower crustal depths, although some granitoids have indications of a possible mantle source component. The lower succession of the Koras Group was derived by partial melting of subduction-influenced enriched mantle giving rise to mafic magmas that fractionated to give rise to the rhyolitic porphyries. The upper succession rhyolites were derived by crustal melting due to the input of mafic magmatism. Crystal fractionation was the main compositional driver for both successions. The Keimoes Suite granitoids and the Koras Group are associated with extensional regimes subsequent to the main deformational episode in the eastern NMP.
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A Comparison between the ~1.08-1.13 Ga Volcano-Sedimentary Koras Group and Plutonic Keimoes Suite: Insights into the Post-Collisional Tectono-Magmatic Evolution of the Eastern Namaqua Metamorphic Province, South Africa

doi: 10.1007/s12583-021-1462-7

Abstract: Along the eastern margin of the Mesoproterozoic Namaqua metamorphic province (NMP) of southern Africa are a bimodal volcano-sedimentary succession, the ~1.13-1.10 Ga Koras Group, composed of rhyolitic porphyries and basaltic andesites, and the ~1.11-1.07 Ga late-to post-tectonic granitoids of the Keimoes Suite. This review examines existing whole-rock major-and trace-element data, along with isotope chemistry (with some new isotopic data), to investigate the role these two magmatic successions played in terms of post-collisional magmatism of the eastern NMP near the boundary with the Archean Kaapvaal Craton. The Keimoes Suite comprises variably porphyritic biotite monzogranites and granodiorites, with a charnockitic member. They are metaluminous to weakly peraluminous, ferroan, and calc-alkalic. They exhibit large ion lithophile (LIL) element enrichment relative to the high field strength elements (HFSE) with depletions in Ba, Sr, Nb, P, Eu and Ti, and enrichments in Th, U and Pb. Isotopic values (eNd(t): 2.78 to -2.95, but down to -8.58 for one granite, depleted mantle Nd model ages (TDM): 1.62-1.99 Ga, but up to 2.55 Ga; initial 87Sr/86Sr: 0.652 82-0.771 30) suggest derivation from weakly to mildly enriched (and radiogenic) sources of Meso-to Paleoproterozoic age, the former of more juvenile character. The Koras Group is characterized by a bimodal succession of calcic to calc-alkalic, magnesian and tholeiitic basaltic andesites and mostly metaluminous to peralkaline rhyolitic porphyries. Two successions are recognised, an older, lower succession that extruded at ~1.13 Ga, and a younger, upper succession at ~1.10 Ga. The rhyolitic porphyries of both successions show similar LILE/HFSE enrichment and the same element enrichments and depletions as the Keimoes Suite granitoids. The upper succession is consistently more fractionated in terms of both whole-rock major and trace element chemistry, and, isotopically, has a greater enriched source component (eNd(t): -0.69 to -4.26; TDM: 1.64-2.44 Ga), relative to the lower succession (eNd(t): 0.74-5.62;TDM: 1.28-2.12 Ga). Crystal fractionation of plagioclase and K-feldspar appears to have played a role in bringing about compositional variation in many of the granites. These were derived from partial melting of mainly igneous with subordinate sedimentary sources from mostly lower crustal depths, although some granitoids have indications of a possible mantle source component. The lower succession of the Koras Group was derived by partial melting of subduction-influenced enriched mantle giving rise to mafic magmas that fractionated to give rise to the rhyolitic porphyries. The upper succession rhyolites were derived by crustal melting due to the input of mafic magmatism. Crystal fractionation was the main compositional driver for both successions. The Keimoes Suite granitoids and the Koras Group are associated with extensional regimes subsequent to the main deformational episode in the eastern NMP.

Russell Bailie, Aidan Leetz. A Comparison between the ~1.08-1.13 Ga Volcano-Sedimentary Koras Group and Plutonic Keimoes Suite: Insights into the Post-Collisional Tectono-Magmatic Evolution of the Eastern Namaqua Metamorphic Province, South Africa. Journal of Earth Science, 2021, 32(6): 1300-1331. doi: 10.1007/s12583-021-1462-7
Citation: Russell Bailie, Aidan Leetz. A Comparison between the ~1.08-1.13 Ga Volcano-Sedimentary Koras Group and Plutonic Keimoes Suite: Insights into the Post-Collisional Tectono-Magmatic Evolution of the Eastern Namaqua Metamorphic Province, South Africa. Journal of Earth Science, 2021, 32(6): 1300-1331. doi: 10.1007/s12583-021-1462-7
  • Volcanic and plutonic rocks are important in understanding the processes that give rise to a substantial amount of the continental crust (e.g., Yan et al., 2018a, b; Barth et al., 2012; Gagnevin et al., 2010; Bachmann et al., 2007). Post-collisional magmatism forms an important component of magmatism associated with continental collisional and accretionary events. The role of structures and deformation in generating post-collisional magmatism is also of significance in understanding the sources and plumbing system for the magmatism (e.g., Seghedi et al., 2019). The ~1.13-1.10 Ga bimodal volcano-sedimentary Koras Group, and the ~1.11-1.08 Ga plutonic Keimoes Suite granitoids represent voluminous post-collisional magmatism in the eastern Namaqua metamorphic province (NMP) of southern Africa subsequent to the major collisional, accretionary and deformational events associated with the 1.2-1.0 Ga Namaquan Orogeny. The Keimoes Suite constitutes weakly foliated to unfoliated, medium- to coarse-grained late- to post-tectonic I-type megacrystic granites and charnockites. These biotite-bearing, ferroan, alkali-calcic granitoids intruded within a collisional within-plate tectonic setting (Bailie et al., 2017, 2011a). The Koras Group was extruded in two stages, initially at ~1.13 Ga (Mothibi, 2016), and later at ~1.10 Ga (Bailie et al., 2012; Pettersson et al., 2007). The Koras Group lithologies originate from A-type parental magmas (Bailie et al., 2012), whilst the Keimoes Suite has essentially I-type characteristics, but the high Fe/Mg ratios, as well as high large ion lithophile element (LILE), rare earth elements (REE), and high field strength element (HFSE) contents are suggestive of A-type granites.

    A number of workers in the region (Bailie et al., 2012; Pettersson et al., 2007; Moen, 1987) have suggested that the two are potentially related, with one being the extrusive equivalent of the other, based on their similar ages and whole-rock chemistry. Their temporal and spatial separation, however, tends to preclude such an association. Together, however, they represent an extensive late- to post-tectonic magmatic event within the eastern NMP at ~1.13-1.08 Ga. This review re-examines the textural, whole-rock chemistry and isotopic characteristics of these rocks in order to provide further constraints on their sources, petrogenesis and causes of this extensive post-collisional magmatism. It also seeks to provide further constraints on the tectono- magmatic evolution of the eastern NMP at this time during the 1.2-1.0 Ga Namaquan Orogeny.

  • The Namaqua-Natal metamorphic province (NNMP) is a medium to high-grade metamorphic belt which borders the Archean Kaapvaal Craton of southern Africa to its southeast, south and west (Cornell et al., 2006). The NNMP is composed of para- and ortho-gneisses formed, deformed and metamorphosed during the 1.2-1.0 Ga Namaquan Orogeny, and is potentially associated with the assembly of the supercontinent Rodinia (Jacobs et al., 2008; Li et al., 2008). The Namaqua metamorphic province (NMP) forms the western portion of the greater NNMP to the west of the Kaapvaal Craton (Fig. 1).

    Figure 1.  Simplified geological map of the Namaqua metamorphic province (NMP) of the Namaqua-Natal metamorphic province (taken from Macey et al., 2011; modified after Thomas et al., 1994; Moen and Toogood, 2007). The inset shows the position of the larger map in Africa. Towns: A. Alexander Bay; B. Bitterfontein; G. Garies; K. Karasburg; L. Loeriesfontein; P. Pofadder; S. Springbok; U. Upington; metamorphic facies indicators: Amp. amphibolite; LGr. lower granulite; UGr. upper granulite; structural features: BoSZ. Boven Rugzeer shear zone; BRSZ. Buffels River shear zone; GT. Groothoek thrust; HRT. Hartbees River thrust; NSZ. Neusspruit shear zone; OT. Onseepkans thrust; PSZ. Pofadder shear zone; TSZ. Trooilapspan shear zone; Vr In. Vanrhynsdorp Inlier.

    The NMP can be divided into five distinct domains, each characterized by its own distinctive lithostratigraphy and structural fabric. The term domain is used in preference to terrane, because the latter has tectonic implications, whereas the former is more generic and non-tectonic or origin-based. These domains are, from west to east (Fig. 1): the Bushmanland Subprovince (BSP), itself composed of a number of smaller domains, the Richtersveld magmatic arc (RMA), the Kakamas Domain (KD), the Areachap Domain (AD), and the Kheis terrane on the western margin of the Kaapvaal Craton (Van Niekerk and Beukes, 2019).

    The Kakamas Domain is dominated by poly-deformed ortho- and para-gneisses and is bound between the Boven Rugzeer shear zone (BoSZ) to the east and the Hartbees River thrust to the west (Fig. 1; Cornell et al., 2006). It comprises ~1.2-1.15 Ga granulite facies supracrustal rocks (Pettersson et al., 2007) and ~1.21-1.15 Ga pre- to syn-tectonic granitic gneisses intruded by the later ~1.11-1.07 Ga late- to post- tectonic Keimoes Suite (Bailie et al., 2017). The domain is considered to be a low-angle imbricate mega-nappe stack bounded by major thrust structures, e.g., Macey et al. (2017).

    The Areachap Domain comprises poly-deformed mafic to intermediate volcano-sedimentary gneisses with a calc-alkaline arc-like affinity termed the Areachap Group of ~1.24-1.29 Ga age (Bailie et al., 2010; Cornell and Pettersson, 2007; Pettersson et al., 2007; Cornell et al., 1990). The Areachap Group was subjected to upper amphibolite facies metamorphism to the north, and lower granulite facies metamorphism to the south. The northern part, in particular, is characterized by a greater dominance of Paleoproterozoic model ages (Cornell and Pettersson, 2007; Pettersson et al., 2007) suggesting an older crustal component was present toward the northern end of the magmatic arc(s) that gave rise to the Areachap Group (Pettersson et al., 2009). This succession separates the western, granitoid-dominated NMP from the arenitic meta-sedimentary and volcanic rocks of the ~1.92-1.3 Ga Kheis terrane (Van Niekerk, 2006). The granitoids of the Keimoes Suite extensively intruded the Areachap Group. The BoSZ represents the western boundary with the Kakamas Domain, with the eastern boundary being the Trooilapspan shear zone (TLSZ) and Brakbosch fault (Cornell et al., 2006; Fig. 1). The exposures of the Areachap Group north of Upington are unconformably overlain by rocks belonging to the Koras Group (Fig. 2; Bailie et al., 2012). The Koras Group is, however, far more widespread in the adjacent Kheis terrane. The western NMP is characterised, in general, by a greater Paleoproterozoic signature in Nd model ages (Eglington, 2006) compared to the more juvenile eastern NMP (Pettersson et al., 2009). Both the Areachap and Kakamas domains were intruded by both ~1.19-1.15 Ga pre- to syn-tectonic granitic gneisses, and later ~1.11-1.08 Ga late- to post-tectonic granites, the latter comprising the Keimoes Suite (Bailie et al., 2017, 2011a), during the Namaquan Orogeny.

    Figure 2.  Geological map of the eastern NMP adjacent to the western margin of the Archean Kaapvaal Craton. The Koras Group and its stratigraphy are indicated along with the various granitoids comprising the loosely defined Keimoes Suite.

    Van Niekerk and Beukes (2019) argued that the former Kaaien terrane and Kheis Subprovince (e.g., Cornell et al., 2006) be combined into the Kheis terrane, and all the rocks therein be grouped into the proposed ~1.92-1.35 Ga Keis Supergroup (Fig. 2). The eastern boundary of the Kheis terrane is defined by the ~1.20 Ga Blackridge thrust (Evans et al., 2001). The Brakbosch- Trooilapspan shear zone acts as the western boundary of the Kheis terrane (Figs. 1, 2), becoming the north-south oriented aeromagnetic signature termed the Kalahari line further north. All the successions east of the Trooilapspan-Brakbosch fault/shear zone form the western margin of the Kaapvaal Craton. The Keis Supergroup represents passive margin sediments and comprises meta- arenaceous, shale and schistose units (Van Niekerk and Beukes, 2019), capped by a bimodal volcanic unit (the Leerkrans Formation of the Wilgenhoutsdrif Group; Fig. 2). All these are in sedimentary contact with each rather than in tectonic contact, as previously advocated (e.g., Moen, 1999). These units are unconformably overlain by the Koras Group. The uppermost Wilgenhoutsdrif Group marked a transition to an active continental plate margin and development of an intra-continental back-arc setting behind the Areachap volcanic arc along the western margin of the Kaapvaal Craton (Bailie et al., 2011b; Pettersson et al., 2007), with easterly-directed subduction (Van Niekerk, 2006).

    The Kheis terrane was subjected to east-west-directed shortening, resulting in alternating NW-SE-oriented anticlines and synclines; shortening is bracketed between 1 200 and 1 150 Ma (Van Niekerk and Beukes, 2019). The Kakamas and Areachap domains were thrust eastwards over the Kheis terrane at ~1.24-1.20 Ga (Eglington, 2006) at the onset of the 1.2-1.0 Ga Namaquan Orogeny with the closure of the Areachap Sea/Ocean and the accretion of the Areachap arc onto the western margin of the Kaapvaal Craton (e.g., Pettersson et al., 2007).

    In the Kakamas and Areachap domains four major deformational events are recognised. The first deformation event, D1, is subdivided into two sub-events, D1a that occurred at ~1 200 Ma, and D1b, at ~1 170-1150 Ma (Mathee, 2017). The main D2 event gave rise to large-scale tight to isoclinal, non-cylindrical sub-vertical F2 folds with NW-trending fold axes, as well as thrusts. This deformation event is constrained to have occurred at ~1 120-1 100 Ma (Bailie et al., 2017; Mathee, 2017). The F1 and F2 folding events were the main regional penetrative fabric- forming events. The third event (D3) occurred at ~1 075-1 080 Ma, and gave rise to open east-west-trending upright F3 folds. Subsequent D4 deformation, constrained to have occurred between 1 024 and 1 018 Ma (Mathee, 2017), is characterized by reactivation and steepening of intra-terrane thrusts by transpression as sub-vertical dextral shear zones with both a lateral and vertical displacement (Pettersson et al., 2007). These include the BoSZ, TLSZ and Brakbosch shear zone, which, while likely formed during D2 fold and thrust tectonics, show evidence for extensive reactivation, as well as vertical and lateral movement (e.g., van Bever Donker, 1991, 1980).

    The peak metamorphic event (M2) occurred at ~1 170- 1 150 Ma, with upper amphibolite to granulite facies conditions in the Kakamas Domain (800-850 ℃ at 4-4.5 kbar) but varying to lower amphibolite facies conditions (550 ℃, 1.5-2.5 kbar) along regions adjacent to the BoSZ in the Areachap Domain (Cornell et al., 2006; Humphreys and van Bever Donker, 1990; Stowe, 1983). This was mostly due to the intrusion of the pre- to syn-tectonic granitic gneisses. M3 metamorphism occurred at amphibolite facies conditions (~650 ℃ and 6.5 kbar; Bachmann et al., 2015) and accompanied D3 deformation. It was a retrograde to isothermal regional contact metamorphic event associated with intrusion of the Keimoes Suite at ~1.11-1.08 Ga. D4 deformation was associated with M4 retrograde metamorphism, concentrated especially along dextral shear zones (Pettersson et al., 2007; Cornell et al., 2006; Geringer et al., 1994; van Bever Donker, 1980).

    The lithologies of the Kheis terrane, apart from the Koras Group, experienced three episodes of deformation. The first phase of deformation (D1) resulted in isoclinal folding, with the second event (D2) producing folds that refolded the F1 folds. The third event, D3, is associated with large scale dextral shear movement. The Keis Supergroup experienced low-grade metamorphism (400-480 ℃, 1 kbar), except along the Brakbosch shear zone where higher pressures up to 4 kbar were experienced. Prograde metamorphism was associated with D1 and D2 deformation. The Koras Group is essentially mildly deformed and has been metamorphosed to greenschist facies (Moen, 1987).

    The 1.11-1.08 Ga Keimoes Suite is a group of mostly medium-grained porphyritic or megacrystic intrusions that have compositions ranging from granodioritic to potassic granite (Bailie et al., 2017; Cornell et al., 2012). The suite is dominantly composed of metaluminous, ferroan, calcic to calc-alkalic biotite monzogranites with generally high K2O contents (Bailie et al., 2017, 2011a). They are locally hornblende- and orthopyroxene- bearing with moderate SiO2 contents and arc-like affinities (LILE-enrichment relative to the HFSE, negative Ta-Nb, Ti and P anomalies). All these suggest an I-type nature. They, however, also have high Fe/Mg ratios, along with high HFSE, LILE and REE contents more indicative of A-type granites. The general trend of the suite is in a NW-SE orientation sub-parallel to the major F2 fabric in the Kakamas and Areachap domains, and is confined to the area between the Neusspruit shear zone in the Kakamas Domain, and the Brakbosch fault (Fig. 2; Bailie et al., 2017). The Keimoes Suite straddles the boundary between the Areachap and Kakamas domains, being mostly restricted to in and around the BoSZ, which may have played a role in its emplacement (Bailie et al., 2017). It is more voluminous and widespread in the Areachap Domain. The granitoids intrude the Keis Supergroup and Areachap Group of the Areachap Domain, and the Korannaland Group of the Kakamas Domain, as well as older gneisses on a regional scale (Moen, 2007; Geringer et al., 1988).

    The Koras Group is an ~7 000 m thick subalkaline bimodal volcanic succession characterised by calcic to calc-alkalic, magnesian and tholeiitic basaltic andesites along with mostly metaluminous rhyolitic porphyries, both of which are sandwiched between siliciclastic sediments (mostly sandstones, conglomerates, mudstones and feldspathic sandstones). It outcrops to the east of the Brakbosch fault in the Kheis terrane and is exposed in three domains with the domains comprising certain formations which are correlatives of each other (Figs. 1, 2). The stratigraphy of the succession is given in Fig. 2. The Koras Group comprises two successions (Fig. 2), with the lower succession extruded at ~1.13 Ga (Mothibi, 2016), and the upper succession occurring later at ~1.10 Ga (Bailie et al., 2012; Pettersson et al., 2007). Previous age determinations of the lower succession (Pettersson et al., 2007; Gutzmer et al., 2000) gave an ~1 170 Ma age. Recent age determinations (Mothibi, 2016), however, give a younger age of ~1 130 Ma, more in keeping with the largely undeformed nature of the group. Meter-scale folds, deformation fabrics, and slicken-sided striations, along with lower greenschist facies mineral assemblages, indicate that the Koras Group is not totally undeformed or unmetamorphosed (Moen, 1987; Sanderson-Damstra, 1982).

    Temporal regional correlatives of the Koras Group and Keimoes Suite include the anorthositic gabbroic Oranjekom Complex intruded at ~1 100 Ma toward the western boundary of the Kakamas Domain (Kruger et al., 2000; Fig. 2). Isotopic data suggests a depleted mantle source for this complex.

  • This review mostly uses previously published data to assess the origins, sources and petrogenesis of the Koras Group and Keimoes Suite. These include the whole-rock data published by Bailie et al.(2017, 2011a) for the Keimoes Suite, and that of Bailie et al. (2012) for the Koras Group. Analytical techniques for the Keimoes Suite are those of Bailie et al.(2017, 2011a), and for the Koras Group that of Bailie et al. (2012). The methods used to determine the isotopic ratios of the Keimoes Suite samples are from Bailie et al. (2017). Six new additional samples of the Koras Group were analysed for Sr-Nd isotopes due to lacking Rb-Sr isotopes (Bailie et al., 2012). All analytical techniques are given in ESM1.

  • The lithologies of both successions are briefly described here, with more details given in Table S1 (ESM3).

  • The volcanic portion of the Koras Group is composed of basaltic andesites, rhyolitic porphyries and dacites. The dacites contain quartz and plagioclase phenocrysts with epidote- and quartz-filled amygdales (0.6-6 mm in size) within a fine-grained groundmass, whilst the basaltic andesites comprise skeletal plagioclase, ophitic augite, quartz, and epidotised and chloritised pyroxene phenocrysts in a fine-grained groundmass of plagioclase and augite (Fig. 3a). The metabasalts of the Rouxville Formation are amygdaloidal and contain quartz- and epidote-filled amygdales (1.4-16 mm) in a microcrystalline groundmass (65%) containing augite and lath-shaped plagioclase. The rhyolitic porphyries are composed of quartz, sanidine, plagioclase and chlorite phenocrysts and microphenocrysts within a fine-grained devitrified groundmass with microcrystalline quartz and plagioclase (45%-65%) (Figs. 3b-3d).

    Figure 3.  Petrographic photomicrographs of some of the bimodal volcanic rocks from the two successions of the Koras Group (a)-(d). (a) Basaltic andesite from the Boom River Formation, lower succession (sample K11); (b) rhyolitic porphyry from the Swartkopsleegte Formation, lower succession (sample K9); (c) rhyolitic porphyry from the Leeuwdraai Formation, upper succession (sample K13); (d) rhyolitic porphyry from the Welgevind Formation, upper succession (sample K36). Petrographic photomicrographs of the various granites of the Keimoes Suite (e)-(l). (e) Keboes granite (sample Mkb3); (f) Keboes granite (sample Mkb3); (g) Kanoneiland granite (sample Mka7); (h) Gemsbokbult granite (sample Mge4); (ⅰ) Klip Koppies granite (sample Mks1); (j) Klipkraal granite (sample Mkl5); (k) Straussburg granite (sample Ms2); (l) Friersdale charnockite (sample Mf7). Mineral abbreviations: Aug. augite; Bt. biotite; Chl. chlorite; Mcl. microcline; Ms. muscovite; Or. orthoclase; Pl. plagioclase; Qtz. quartz; Sa. sanidine; Ser. sericite.

  • The Keimoes Suite comprises the Gemsbokbult, Kanoneiland, Keboes, Klipkraal, Kleinbegin, Klip Koppies and Straussburg granites, along with the Friersdale charnockite. These megacrystic biotite granites are dominated by quartz (25%-40%), K-feldspars: orthoclase (12%-30%), and microcline (5%-10%), and plagioclase (5%-35%), with the dominant mafic mineral being biotite (5%-20%), with minor hornblende (2%-5%) present in some granites. The granites vary from unfoliated to poorly foliated. In some of the granites, namely the Gemsbokbult, Kanoneiland, Keboes and Kleinbegin granites, muscovite (1%-15%) is present, but is more prevalent in areas where shear zones crosscut the granites. Clinopyroxene, ranging in size from 0.2-2.4 mm, occurs in the Friersdale charnockite in limited amounts (1%-5%).

    The Gemsbokbult granite is unfoliated to weakly foliated and mostly non-porphyritic with scattered phenocrysts, and has a medium-grained equigranular texture (Fig. 3h). Minor subhedral staurolite is associated with biotite. The Kanoneiland granite is medium- to coarse-grained, non-porphyritic and weakly foliated, varying in texture from inequigranular to equigranular (Fig. 3g). The Keboes granite is porphyritic and fine- to medium-grained (Fig. 3f). The Klipkraal granite is a porphyritic medium- to coarse-grained biotite granite with large orthoclase and plagioclase phenocrysts (Fig. 3j). The Kleinbegin granite, the dominant granite of the Kleinbegin Subsuite (Bailie et al., 2017, 2011a), has two main varieties, being either porphyritic or non-porphyritic. The porphyritic variety contains orthoclase and plagioclase phenocrysts. The non-porphyritic variety varies from medium- to coarse-grained; some samples are weakly foliated. The Klip Koppies granite is a poorly foliated, dark grey to dark porphyritic granite with large orthoclase and plagioclase phenocrysts (Fig. 3i). The Straussburg granite, which is medium- to coarse-grained and locally porphyritic (Fig. 3k), with quartz commonly blue opaline. Where porphyritic it has large perthitic orthoclase phenocrysts and smaller quartz and plagioclase phenocrysts. The Friersdale charnockite is medium- to coarse-grained and has large plagioclase and orthoclase phenocrysts (Fig. 3l). It is characterised by blue opalescent quartz.

    The majority of the granites are porphyritic and medium- to coarse-grained, with quartz primarily constituting the matrix ranging in size from 0.01 to 0.04 mm, commonly accompanied by plagioclase and/or orthoclase of similar grain size in smaller quantities. The constituent minerals of the groundmass are commonly sub-rounded/subhedral-anhedral. Quartz not only occurs as a constituent of the matrix, but commonly also as large individual grains, with grains as large as 3 mm present. K-feldspar phenocrysts, both orthoclase and microcline, and commonly perthitic (20%-33% of the phenocrysts) are common (with grain sizes up to 7 mm). Plagioclase phenocrysts, reaching sizes up to 5 mm, are also present. Phenocrysts throughout the suite typically exhibit euhedral to subhedral shapes. The non-porphyritic granites are divided into either medium-grained equigranular or inequigranular medium- to coarse-grained. Biotite commonly occurs in large aggregates which can reach up to 2.5 cm in size. Hematite and magnetite are common throughout the suite as accessory minerals (Bailie et al., 2017, 2011a; Cornell et al., 2012; Pettersson, 2008; Moen, 2007).

  • A total of 31 samples of the Keimoes Suite were analysed (Tables 1, 2). The overwhelming majority of the samples are monzogranites, with a few granodiorites (Fig. 4). They range from metaluminous to weakly peraluminous. They are dominantly ferroan, except for the Kleinbegin granite, and calc-alkalic, except for the Kleinbegin granite and Friersdale charnockite, the latter nominally calcic, and plot mainly within the A-type field on A-type discrimination diagrams (Fig. 4).

    Sample Granite SiO2 TiO2 Al2O3 FeO MgO CaO Na2O K2O MnO P2O5 Fe+Mg ASI
    Mka 1 Kanoneiland granite 70.91 0.76 12.85 4.92 0.67 2.24 2.49 4.78 0.12 0.25 0.085 0 0.99
    Mka 3 Kanoneiland granite 70.41 0.71 13.33 4.66 0.62 2.35 2.74 4.82 0.11 0.25 0.080 4 0.97
    Mka 6 Kanoneiland granite 69.16 0.74 13.91 4.97 0.73 2.53 2.84 4.75 0.11 0.26 0.087 3 0.99
    Mka 7 Kanoneiland granite 70.14 0.77 13.26 4.90 0.66 2.39 2.70 4.81 0.11 0.26 0.084 6 0.97
    Mkb 1 Keboes granite 71.27 0.49 13.84 3.44 0.69 2.13 2.97 4.89 0.08 0.18 0.065 1 1.00
    Mkb 2 Keboes granite 70.79 0.50 14.23 3.37 0.70 2.25 3.02 4.87 0.08 0.18 0.064 2 1.01
    Mkb 6 Keboes granite 70.50 0.50 14.43 3.37 0.70 2.29 3.09 4.86 0.08 0.18 0.064 2 1.01
    Ms 1 Straussburg granite 68.58 0.97 13.55 5.04 0.96 3.07 2.81 4.60 0.12 0.30 0.093 9 0.91
    Ms 2 Straussburg granite 69.11 0.96 13.17 4.93 0.94 3.00 2.62 4.87 0.12 0.28 0.091 8 0.90
    Ms 6 Straussburg granite 69.16 0.92 13.31 4.95 0.98 3.05 2.76 4.45 0.12 0.30 0.093 2 0.91
    Ms 7 Straussburg granite 69.02 1.01 13.14 4.99 0.94 2.93 2.65 4.92 0.12 0.28 0.092 8 0.89
    Mf 1 Friersdale charnockite 67.02 1.25 12.96 6.43 1.37 3.89 2.52 3.94 0.15 0.48 0.123 4 0.87
    Mf 4 Friersdale charnockite 66.13 1.28 13.15 6.63 1.47 4.21 2.62 3.85 0.16 0.50 0.128 7 0.85
    Mf 6 Friersdale charnockite 66.09 1.31 13.30 6.71 1.36 4.11 2.65 3.79 0.16 0.51 0.127 3 0.87
    Mf 7 Friersdale charnockite 66.53 1.29 12.89 6.73 1.42 4.00 2.55 3.94 0.16 0.49 0.128 8 0.85
    Mge 1 Gemsbokbult granite 70.57 0.72 13.59 4.12 0.66 1.86 2.51 5.58 0.09 0.29 0.073 7 1.03
    Mge 2 Gemsbokbult granite 69.77 0.83 13.22 5.54 0.61 2.17 2.28 5.14 0.13 0.32 0.092 2 1.03
    Mge 3 Gemsbokbult granite 69.93 0.77 13.13 5.46 0.65 2.23 2.30 5.09 0.12 0.32 0.092 1 1.01
    Mge 5 Gemsbokbult granite 66.58 1.03 12.98 7.79 0.61 3.21 2.14 5.03 0.19 0.43 0.123 5 0.91
    Mkle 1 Kleinbegin granite 71.61 0.48 13.80 3.18 0.81 3.11 3.51 3.33 0.08 0.11 0.064 2 0.93
    Mkle 2 Kleinbegin granite 71.02 0.56 13.80 3.61 0.96 3.45 3.42 2.97 0.09 0.13 0.074 1 0.92
    Mkle 3 Kleinbegin granite 71.30 0.54 13.83 3.46 0.94 3.29 3.54 2.90 0.08 0.12 0.071 6 0.93
    Mks 1 Klip Koppies granite 70.02 0.76 13.11 5.39 0.59 2.14 2.28 5.27 0.13 0.31 0.089 6 1.01
    Mks 2 Klip Koppies granite 70.16 0.75 13.15 5.22 0.56 2.14 2.35 5.23 0.12 0.30 0.086 7 1.01
    Mks 3 Klip Koppies granite 69.81 0.81 13.28 5.37 0.61 2.27 2.39 5.01 0.12 0.32 0.089 9 1.01
    Mks 4 Klip Koppies granite 70.05 0.81 13.21 5.16 0.57 2.23 2.50 5.04 0.12 0.31 0.086 0 1.00
    MkI 2 Klipkraal granite 67.19 0.92 13.33 8.12 0.47 2.58 1.54 5.17 0.21 0.48 0.124 8 1.09
    MkI 4 Klipkraal granite 67.66 0.95 13.15 7.96 0.49 2.52 2.22 4.39 0.18 0.48 0.122 9 1.09
    MkI 5 Klipkraal granite 66.34 1.12 14.03 6.61 0.98 3.23 2.29 4.80 0.15 0.46 0.116 3 0.98
    MkI 6 Klipkraal granite 66.53 1.16 13.89 6.81 1.02 3.24 2.23 4.50 0.15 0.47 0.120 0 1.00
    Mkl 3 Klipkraal granite 66.68 0.97 13.34 8.17 0.50 2.60 2.46 4.60 0.19 0.49 0.126 2 1.01

    Table 1.  Whole-rock major and minor element concentrations (wt.%) for the Keimoes Suite granitoids in the eastern NMP

    Rock type Kanoneiland granite Keboes granite Straussburg granite Friersdale charnockite
    Sample Mka 1 Mka 3 Mka 6 Mka 7 Mkb 1 Mkb 2 Mkb 6 Ms 1 Ms 2 Ms 6 Ms 7 Mf 1 Mf 4 Mf 6 Mf 7
    Large ion lithophile elements (LILEs)
    Cs 14.25 12.64 11.56 11.28 10.97 10.70 12.35 6.25 5.64 5.31 13.19 5.27 5.28 5.87 6.82
    Rb 245.90 234.50 233.15 227.81 255.71 242.89 185.62 195.89 224.48 199.24 278.56 156.57 161.38 159.36 167.78
    Ba 873.99 882.11 823.74 869.53 715.25 714.42 255.33 1 103.89 1 076.45 1 193.40 883.65 1 133.75 1 129.59 1 069.73 1 105.29
    Sr 159.63 148.32 142.92 155.34 130.96 130.74 294.31 187.98 171.47 197.30 126.64 279.40 277.26 264.33 240.02
    Pb 32.62 31.97 34.80 31.04 32.49 30.95 18.02 30.78 32.41 37.09 40.57 29.29 27.33 28.80 27.95
    High field strength elements (HFSEs)
    Th 22.42 21.50 34.69 22.56 34.41 34.47 14.47 25.02 34.12 57.78 53.78 17.12 17.07 20.08 17.45
    U 3.8 2.5 4.0 4.5 7.2 7.6 3.5 4.0 4.4 4.9 6.6 3.7 3.2 4.3 5.9
    Zr 350 298 332 337 229 235 308 413 432 411 334 395 422 387 504
    Hf 10.0 8.4 9.5 9.5 6.6 6.8 7.3 11.2 12.0 11.0 9.8 10.3 11.1 10.3 13.5
    Nb 22.4 22.7 21.2 21.2 17.3 16.4 16.4 20.6 22.3 22.9 19.0 22.2 24.9 24.4 27.6
    Ta 2.1 2.6 1.9 1.6 3.0 1.5 1.7 1.5 2.7 1.6 2.0 1.5 1.8 2.4 2.6
    Y 56.2 73.1 69.6 46.7 41.0 40.9 21.1 48.2 50.3 53.2 50.9 63.1 59.7 59.1 59.9
    Transition elements
    Sc 17.5 16.3 20.1 14.3 12.3 12.1 11.8 16.6 16.1 19.0 11.9 22.8 21.9 21.3 20.9
    Cr 155 67 82 116 131 79 102 143 163 69 182 53 80 162 74
    Ni 9.8 12.4 9.7 10.1 11.5 11.7 10.3 12.9 14.8 18.1 13.9 16.1 15.1 18.8 18.6
    Co 7.5 7.0 8.0 7.1 5.9 6.0 8.2 9.4 9.2 10.9 6.8 13.6 12.8 13.3 15.6
    V 61.1 52.6 60.3 58.2 50.4 47.4 79.1 87.5 90.5 91.6 66.8 115.3 115.0 125.9 117.0
    Zn 97.3 93.2 98.7 90.7 71.7 77.8 129.5 75.6 77.9 90.2 76.7 110.5 99.7 104.8 108.8
    Cu 11.7 21.5 14.6 12.2 14.0 20.6 43.2 14.5 26.3 31.4 13.4 67.7 16.0 100.3 28.9
    Rare earth elements (REEs)
    La 65.4 63.0 98.3 67.7 57.3 56.4 32.6 73.2 67.7 120.0 85.7 85.4 82.2 81.6 75.1
    Ce 139.1 131.1 215.5 142.4 124.7 122.7 57.1 148.5 144.4 249.9 189.4 180.2 174.7 174.2 160.3
    Pr 16.4 15.5 25.2 16.5 15.0 14.4 5.9 16.9 17.1 27.6 20.8 21.3 20.7 20.4 19.4
    Nd 62.9 57.8 95.2 63.3 56.9 56.1 21.1 61.8 64.7 96.3 76.1 82.4 81.0 79.8 74.5
    Sm 12.7 12.4 18.0 12.0 11.0 10.9 4.3 11.3 12.1 15.8 13.7 15.6 15.5 14.5 14.8
    Eu 2.04 2.12 2.05 2.02 1.40 1.28 0.88 2.04 2.05 2.35 1.60 3.11 3.05 2.86 2.81
    Gd 10.83 10.88 14.47 9.46 8.43 8.36 3.41 9.14 9.56 11.92 10.54 13.73 12.62 12.22 12.45
    Tb 1.69 1.92 2.23 1.44 1.30 1.25 0.57 1.42 1.54 1.72 1.55 2.02 1.86 1.86 1.82
    Dy 10.54 12.89 13.61 8.76 7.63 7.46 3.35 8.87 9.58 10.23 8.91 12.16 11.09 11.15 11.23
    Ho 2.06 2.67 2.63 1.72 1.56 1.48 0.71 1.80 1.92 1.95 1.75 2.40 2.36 2.25 2.31
    Er 6.09 7.99 7.59 4.89 4.22 4.52 2.25 5.12 5.56 5.43 5.24 6.59 6.67 6.51 6.19
    Tm 0.83 1.27 1.09 0.73 0.62 0.60 0.34 0.79 0.77 0.92 0.77 0.95 0.88 0.94 0.95
    Yb 6.09 8.62 7.29 4.91 4.25 4.31 2.11 5.10 5.46 5.57 5.56 6.60 6.02 6.21 6.24
    Lu 0.85 1.15 1.02 0.65 0.63 0.59 0.25 0.75 0.80 0.86 0.79 0.96 0.88 0.87 0.86
    Large ion lithophile elements (LILEs)
    Cs 13.86 3.51 3.40 2.45 1.85 1.99 2.57 5.73 6.48 6.05 6.02 2.51 4.57 2.46 2.07 7.22
    Rb 292.22 209.68 193.08 189.15 107 99 106 197 206 200 197 174 204 200 186 220
    Ba 930.60 1 115.39 1 102.02 1 369.03 1 135 982 959 1 043 1 084 1 050 1 061 1 443 1 467 1 198 1 186 1 588
    Sr 133.19 165.49 151.13 167.43 154 157 156 142 143 148 145 151 147 165 171 165
    Pb 39.25 36.89 35.25 39.36 18.9 17.7 16.2 34.2 37.5 35.0 34.4 41.3 40.9 41.7 39.3 38.6
    High field strength elements (HFSEs)
    Th 55.47 28.98 28.91 37.46 16.9 13.1 5.9 27.3 29.1 28.9 28.1 36.0 41.8 54.6 51.1 38.3
    U 6.6 3.0 2.3 3.6 2.0 1.6 1.5 3.2 3.2 3.2 2.7 3.6 3.5 4.5 4.4 4.1
    Zr 342 369 351 510 275 307 308 324 326 340 344 483 419 515 524 535
    Hf 10.1 10.3 9.9 14.1 7.9 8.0 8.4 9.2 9.3 9.5 9.6 13.7 12.2 14.4 14.2 14.6
    Nb 20.1 20.9 21.6 28.8 11.4 13.3 10.9 20.4 19.9 21.3 21.2 24.4 23.6 27.3 28.5 29.6
    Ta 2.0 1.4 2.9 3.5 0.8 3.0 3.0 1.9 1.3 2.2 1.3 2.5 2.9 1.5 3.1 3.8
    Y 54.1 52.7 50.3 77.3 59.6 56.7 44.0 49.7 52.3 51.3 50.4 92.5 92.6 69.2 69.0 99.7
    Transition elements
    Sc 12.2 19.2 19.7 24.5 10.0 12.6 10.8 17.8 18.1 18.1 18.1 23.3 25.3 20.9 21.5 26.7
    Cr 243 91 161 133 98.9 167.2 184.0 113.3 72.5 147.4 93.2 101.8 72.3 50.7 84.1 149.6
    Ni 14.4 10.5 12.0 11.4 15.8 22.9 21.2 11.4 22.9 12.2 9.0 7.1 7.4 11.1 11.5 11.7
    Co 6.9 6.7 7.1 8.2 8.1 9.6 9.0 6.5 6.7 6.8 6.5 7.8 7.9 11.7 11.4 9.1
    V 71.8 47.4 53.0 37.6 61.1 77.4 73.6 46.1 42.4 49.6 45.8 30.0 26.2 72.9 80.0 35.1
    Zn 74.8 102.4 103.5 139.0 45.8 53.3 47.5 97.6 101.5 96.1 100.8 122.6 124.8 113.9 107.1 146.1
    Cu 21.4 14.3 17.2 32.6 37.6 76.5 43.2 17.7 38.8 17.9 13.5 20.2 18.5 25.1 26.7 27.6
    Rare earth elements (REEs)
    La 91.0 78.4 77.1 99.3 40.7 43.8 24.7 73.6 73.9 76.4 73.3 103.8 120.8 119.1 115.4 113.0
    Ce 198.8 168.2 165.1 221.4 96.2 98.1 56.6 157.4 154.4 162.5 156.9 223.3 255.0 270.3 260.9 241.5
    Pr 21.9 19.8 19.4 26.6 12.0 12.1 7.4 18.7 18.5 19.2 18.4 26.6 30.5 33.1 32.0 29.1
    Nd 80.8 77.1 75.6 107.3 46.4 46.2 31.6 71.4 73.0 75.2 71.8 104.6 118.9 129.5 128.5 115.4
    Sm 14.4 15.0 14.7 20.3 9.78 9.05 6.99 14.17 14.49 14.95 14.12 20.62 23.20 23.56 22.72 21.73
    Eu 1.79 2.40 2.23 3.08 1.33 1.48 1.24 2.11 2.20 2.25 2.22 3.07 3.37 2.79 2.71 3.49
    Gd 10.82 12.44 11.49 16.51 9.28 8.70 6.68 11.54 11.89 11.89 11.97 17.96 19.40 16.88 16.34 19.64
    Tb 1.62 1.77 1.64 2.44 1.60 1.47 1.16 1.71 1.65 1.68 1.65 2.74 2.91 2.24 2.26 3.06
    Dy 9.54 10.05 9.68 14.73 10.35 9.20 7.31 9.32 9.96 9.57 9.79 16.71 17.69 13.41 13.25 18.93
    Ho 1.98 1.93 1.94 2.96 2.22 1.94 1.57 1.87 1.97 1.87 1.90 3.49 3.61 2.66 2.67 3.72
    Er 5.56 5.61 5.27 8.53 6.18 5.77 4.59 5.17 5.49 5.43 5.54 10.26 9.94 7.24 7.39 10.81
    Tm 0.80 0.77 0.77 1.18 0.85 0.90 0.66 0.73 0.75 0.73 0.78 1.40 1.46 1.06 0.95 1.66
    Yb 5.72 5.42 5.18 8.05 5.59 5.86 4.61 4.99 5.13 4.87 5.12 9.85 9.32 6.83 6.82 10.57
    Lu 0.88 0.77 0.74 1.16 0.76 0.88 0.71 0.70 0.72 0.70 0.75 1.40 1.32 0.85 0.98 1.55

    Table 2.  Whole-rock trace element concentrations (ppm) for the Keimoes Suite granitoids in the eastern NMP

    Figure 4.  General geochemical characteristics and classification of the eastern Namaqua metamorphic province Keimoes Suite granites. (a) The QAP plot of Le Maitre et al. (1989); 1. alkali-feldspar syenite; 2. monzodiorite, monzogabbro; 3. diorite, gabbro; 4. q-monzodiorite, q-monzogabbro; 5. q-diorite, q-gabbro, q-anorthosite; 6. q-alk-fsp syenite; 7. quartzolite; (b) the total alkalis vs. silica (TAS) plot of Cox et al. (1979); (c) the An-Ab-Or ternary plot (after O'Connor, 1965); (d) FeOT/(FeOT+MgO) vs. SiO2 plot (after Frost et al., 2001); (e) the Na2O+K2O-CaO vs. SiO2 plot (after Frost et al., 2001); (f) the molar Al2O3/(Na2O+K2O) (A/NK) vs. aluminium saturation index (ASI) plot of Frost et al. (2001); (g) the FeOT/MgO vs. Zr+Nb+Ce+Y plot of Whalen et al. (1987); (h) the (Na2O+K2O)/CaO vs. Zr+Nb+Ce+Y plot of Whalen et al. (1987).

    The SiO2 values range between 66.09 wt.% and 71.61 wt.%, with FeOT covering a large range between 3.18 wt.% and 8.17 wt.%. The Friersdale charnockite has the highest TiO2, MgO and CaO, and the lowest SiO2 concentrations, whereas the Kleinbegin granite has the highest SiO2, and lowest TiO2 and FeOT concentrations. The Klip Koppies and Kleinbegin granites have low concentrations of both MgO and CaO, and elevated Na2O contents. The granitoids are enriched in Ti, Fe, Mn, P and K, and depleted in Al and Na relative to average granites and granodiorites. In Harker plots (using element concentrations instead of oxides), for many of the granites, Si shows negative correlations with Fe, Mg, Ti, Ca, P and the aluminium saturation index (ASI) (Al/(Ca-1.67P+Na+K)), and broadly positive correlations for Al, K and Na (ESM2, Fig. S1).

    The oldest granite, the Keboes granite, is the most highly fractionated. By contrast, the youngest member, the Friersdale charnockite, is the least SiO2-rich, and the least fractionated, in terms of REE and trace element contents, and also has the highest HFSE content.

    Principal component analysis (PCA) (ESM2, Fig. S2) shows that each granite is distinctly different, forming its own separate cluster, and therefore indicating that each granite is genetically unrelated to the others, coming from different sources. The more fractionated plutons (Kanoneiland, Keboes, Gemsbokbult, Kleinbegin and Klip Koppies granites) show trends parallel to the major component (SiO2), indicating that this played the major role in their compositional diversity, whereas the compositions of the less fractionated, and more mafic granites (Friersdale charnockite and Klipkraal granite) are dominantly controlled by CaO and FeOT, respectively, showing trends parallel to these factors (ESM2, Fig. S2). The Straussburg granite falls between these two clusters. K2O has controlled some compositional variation in the Klip Koppies, Gemsbokbult and Kanoneiland granites, and Na2O in the Kleinbegin granite.

    Ti, Ca and P (along with A/CNK (molar Al2O3/(CaO+ Na2O+K2O) and ASI show positive, and Si, K, Na, Al and Mg# show negative correlations with maficity (molar Fe+Mg) (Fig. 5) (as described for granitic suites, e.g., Clemens et al., 2011). The granitoids with the highest maficity are the Klipkraal granite and the Friersdale charnockite. They also have the highest Fe, Mg, Ti and P, and lowest Si contents of all the granitoids. By contrast, the Kleinbegin and Keboes granites have the lowest maficities and lowest Fe, Ti, and P, and highest Si and Na contents; the Kleinbegin granite has the lowest K contents of the granitoids.

    Figure 5.  Maficity plots for major and minor elements of the Keimoes Suite granitoids and Koras Group rhyolitic porphyries.

    Relative to the primitive mantle (PM) values of Sun and McDonough (1989), the granites are enriched in the large ion lithophile elements (LILE) (Cs, Rb, Ba) relative to the high field strength elements (HFSE) (the rare earth elements (REE), Y, Zr, Hf, Nb, Ta, Ti) (Fig. 6). Overall, there are noticeable depletions in Ba, Nb, Sr, P, Eu and Ti, with mild depletions in Rb, K and Zr (slight), and with enrichments in Th, U, Pb and the LREE (La, Ce, Pr, Nd, Sm). The enrichments emphasise an upper crustal component to the granites due to the fact that these elements are particularly enriched in that portion of the continental crust (cf. avg. crust-normalized plot, Fig. 6). The Nb-Ta depletion suggests subduction-influenced processes. In the average crust-normalized plot, the HREE (Tb, Tm and Yb, along with Y) are enriched relative to the LREE, but there are similar depletions in Ba, K, Sr, P, Zr (slight), and Ti.

    Figure 6.  (a) Multi-element trace element plot (spider diagram) for the Keimoes Suite (normalised to the primitive mantle values of Sun and McDonough, 1989). (b) REE plot for the Keimoes Suite granites normalised to the primitive mantle values of McDonough and Sun (1995). (c) Multi-element trace element plot (spider diagram) for the Keimoes Suite normalised to the average crustal values of Weaver and Tarney (1984).

    The REE plot (normalised to the PM values of McDonough and Sun, 1995) show enrichment of the LREE relative to the HREE [(La/Yb)PM=3.63-14.61], and negative europium anomalies [(Eu/Eu*)PM=0.39-0.70]. The LREE are moderately fractionated [(La/Sm)PM: 2.22-4.78], whereas the HREE show relatively flat patterns [(Gd/Yb)PM: 1.02-1.99].

  • A total of 30 samples of the Koras Group were analysed, of which 16 are rhyolitic porphyries from both successions (Tables 3, 4). This review will focus predominantly on the rhyolites. These have characteristic rhyolitic compositions (Figs. 7a-7c). The majority of the rhyolite samples are metaluminous, with only a few being peralkaline (predominantly those of the Swartkopsleegte Formation of the lower succession), and are mostly alkali-calcic and magnesian (Figs. 7d-7f).

    Sample Rock type Succession SiO2 TiO2 Al2O3 FeO MgO CaO Na2O K2O MnO P2O5 Fe+Mg ASI
    K1 Boom River basaltic andesites Lower 55.25 1.09 14.80 9.41 5.49 9.27 2.14 1.31 0.15 0.14 0.267 2 0.68
    K2 Boom River basaltic andesites Lower 53.76 1.10 15.42 9.48 5.61 11.40 1.98 0.96 0.16 0.13 0.271 2 0.62
    K11 Boom River basaltic andesites Lower 57.41 1.23 14.75 9.86 4.66 7.21 2.65 1.92 0.16 0.16 0.252 7 0.76
    K14 Boom River basaltic andesites Lower 54.82 1.13 15.11 9.85 5.73 9.16 2.98 0.90 0.18 0.14 0.279 4 0.68
    K15 Boom River basaltic andesites Lower 56.06 1.16 15.14 10.07 6.09 6.23 2.71 2.17 0.18 0.20 0.291 2 0.85
    K16 Boom River basaltic andesites Lower 56.42 1.12 14.68 5.02 1.70 9.53 10.05 0.18 1.18 0.15 0.111 9 0.43
    K17 Boom River basaltic andesites Lower 54.62 1.12 15.24 10.16 5.81 8.48 2.31 1.96 0.17 0.13 0.285 7 0.72
    K25 Boom River basaltic andesites Lower 58.73 0.98 14.12 8.89 5.19 10.08 0.65 1.08 0.15 0.13 0.252 5 0.69
    W28 Boom River basaltic andesites Lower 55.43 1.25 15.07 9.96 5.23 8.99 2.18 1.57 0.16 0.16 0.268 4 0.70
    W29 Boom River basaltic andesites Lower 55.61 1.28 14.85 10.01 5.15 8.90 2.24 1.62 0.16 0.16 0.267 2 0.69
    K3 Leeuwdraai rhyolitic porphyry Upper 71.34 0.70 13.30 2.74 0.76 1.78 3.93 5.19 0.18 0.07 0.057 1 0.87
    K3b Leeuwdraai rhyolitic porphyry Upper 73.18 0.73 13.01 2.00 0.73 1.09 3.28 5.73 0.19 0.04 0.046 1 0.96
    K4 Leeuwdraai rhyolitic porphyry Upper 73.82 0.62 12.20 2.20 0.62 1.29 3.64 5.38 0.17 0.07 0.045 9 0.87
    K8 Swartkopsleegte rhyolitic porphyry Lower 70.94 0.96 13.04 2.44 1.04 1.88 4.59 4.70 0.34 0.07 0.059 7 0.82
    K9 Swartkopsleegte rhyolitic porphyry Lower 71.59 0.91 12.63 1.86 0.53 2.59 5.66 3.80 0.34 0.09 0.039 0 0.70
    K12 Leeuwdraai rhyolitic porphyry Upper 72.16 0.70 13.04 2.28 0.73 1.85 3.51 5.47 0.18 0.07 0.049 8 0.87
    K13 Leeuwdraai rhyolitic porphyry Upper 72.26 0.75 12.70 2.06 0.62 1.86 4.21 5.26 0.20 0.08 0.044 0 0.80
    K19 Swartkopsleegte rhyolitic porphyry Lower 70.95 0.91 12.92 1.77 0.90 2.04 4.80 5.26 0.34 0.10 0.047 1 0.75
    K21 Leeuwdraai rhyolitic porphyry Upper 72.32 0.84 11.98 2.74 0.70 1.66 4.20 5.21 0.27 0.07 0.055 5 0.77
    K22 Leeuwdraai rhyolitic porphyry Upper 72.58 0.70 12.74 2.28 0.49 1.88 3.46 5.62 0.18 0.05 0.043 9 0.84
    K23 Leeuwdraai rhyolitic porphyry Upper 72.04 0.73 13.02 2.33 0.69 1.94 3.58 5.44 0.19 0.05 0.049 5 0.85
    K24 Leeuwdraai rhyolitic porphyry Upper 73.09 0.65 13.19 2.10 0.49 1.51 2.82 5.92 0.19 0.04 0.041 3 0.96
    K33 Rouxville rhyolitic porphyry Upper 72.23 0.77 13.12 2.46 0.67 1.47 3.76 5.27 0.19 0.05 0.050 9 0.90
    K35 Welgevind rhyolitic porphyry Upper 68.64 0.98 13.98 3.01 1.24 2.38 5.20 4.14 0.31 0.12 0.072 7 0.81
    K36 Welgevind rhyolitic porphyry Upper 70.40 0.84 13.91 2.87 1.12 1.65 3.75 5.13 0.26 0.07 0.067 7 0.95
    K37 Welgevind rhyolitic porphyry Upper 71.22 0.81 13.79 2.74 0.87 2.08 3.24 4.96 0.23 0.05 0.059 8 0.96

    Table 3.  Whole-rock major and minor element concentrations (wt.%) for the Koras Group bimodal volcanic rocks in the western Kheis terrane

    Formation Boom River Swartkopsleegte
    Rock type Basaltic andesites Rhyolitic porphyry
    Sample K1 K2 K11 K14 K15 K16 K17 K25 W28 W29 K8 K9 K19
    Large ion lithophile elements (LILE)
    Cs 1.8 1.3 2.6 2.1 1.9 0.9 1.8 1.1 0.8 1.3 2.1 2.5 3.6
    Rb 53.4 41.7 70.8 45.3 70.7 50.2 65.8 44.4 52 63.5 198 203 220
    Ba 354 286 727 320 750 373 452 244 383 415 1 152 1 131 1 214
    Sr 214 166 307 296 352 393 218 210 207 230 186 184 173
    Pb 11 14 7 11 8 24 9 10 10 12 21 12 18
    High field strength elements (HFSE)
    Th 7 6.7 7.8 7.3 6.7 6.3 6.7 6.1 7.2 8 23.2 24.4 22
    U 1.5 1.5 1.7 1.6 1.5 1.3 1.5 0.3 1.8 1.9 5.5 5.4 5.1
    Zr 120 118 123 129 115 84 118 107 127 141 348 356 322
    Hf 4 4 4.2 4.6 4.1 3.1 4.3 3.8 4.4 4.7 11.4 11.4 10.4
    Nb 8.4 8.3 9.8 9.7 8.5 7.8 8.7 7.8 10 10.9 26.9 26.4 26.2
    Ta 0.5 0.6 0.7 0.6 0.6 0.5 0.6 0.5 0.6 0.7 1.7 1.9 1.7
    Y 26.2 26 28.5 27.7 25.9 24.6 25.9 24.3 26.4 28.6 66.5 66.1 62.6
    Transition elements
    Sc 25.5 23.7 26.5 30 30.5 29.4 29.6 24.7 25.8 26.3 15.6 15.5 14.8
    Cr 132 121 51 139 137 131 134 190 91 93 8 6 8
    Ni 61 64 50 61 59 60 61 51 68 65 5 3 3
    Co 45 45 38 42 43 44 41 38 44 46 6 7 6
    V 242 230 238 226 244 221 237 204 237 225 45 40 36
    Zn 95 91 102 106 100 98 101 86 96 108 119 120 121
    Cu 104 106 99 116 107 114 95 62 109 241 81 4 5
    Ga 18.9 19.7 17.6 17.7 17.3 18.7 19.2 19.5 18.8 20.1 21.1 21.2 19.8
    Rare earth elements (REEs)
    La 21 21 28 26 24 24 24 21 23 25 86 89 80
    Ce 48.8 47.4 61.3 55.5 51.7 51.8 53.2 45.8 50.4 56.2 197.4 201.5 185
    Pr 5.4 5.2 6.5 6.1 5.5 5.4 5.6 4.9 5.6 6.3 20.6 21 19.7
    Nd 22.1 21.8 27.6 25.9 24.3 23.7 24.8 21.6 23.4 25.3 86 89.4 82.1
    Sm 4.9 4.9 5.6 5.8 5.3 5 5.1 4.6 5.1 5.7 16 16.7 15.3
    Eu 1.4 1.3 1.5 1.4 1.4 1.4 1.3 1.2 1.3 1.5 2.9 3.1 2.9
    Gd 5 5.1 6.2 5.9 5.7 5.3 5.3 4.6 5.3 5.6 15.1 14.5 13.4
    Tb 0.9 0.9 1.1 1.1 1 1 1 0.9 0.9 1 2.5 2.7 2.4
    Dy 5 5.1 6.3 5.6 5.6 5.3 5.7 4.9 5.1 5.4 14.2 14.4 12.9
    Ho 1 1.1 1.2 1.1 1.1 1 1.1 1 1.1 1.2 2.6 2.7 2.5
    Er 4 4 3.3 3.3 3.1 2.8 3.1 2.8 3.9 4.2 7.5 7.8 6.8
    Tm 0.4 0.5 0.5 0.5 0.5 0.4 0.5 0.4 0.4 0.5 1.1 1.2 1
    Yb 3.7 3.6 3.7 3.6 3.3 3.2 3.5 3.1 3.7 3.9 8 8.5 7.4
    Lu 0.4 0.4 0.4 0.5 0.4 0.3 0.4 1.4 0.4 0.5 1 1.1 0.9
    Large ion lithophile elements (LILEs)
    Cs 1.8 2.3 1.7 2.2 2.6 2.4 0.9 3.1 3.3 0.9 2 2.4 2.3
    Rb 194 241 219 201 206 236 231 219 231 125 194 190 211
    Ba 1 173 1 832 1 857 1 490 1 605 1 541 1 552 1 521 1 682 1 639 2 016 1 895 1 661
    Sr 129 142 131 180 150 226 162 170 148 426 247 232 140
    Pb 34 38 38 43 39 45 37 38 34 19 37 26 39
    High field strength elements (HFSEs)
    Th 29.6 41 37 35.2 35.7 36.4 33.4 33.3 33.1 34.7 35.1 37.1 34.6
    U 3.9 5 4 4.9 4.8 5.2 4.2 4.6 4.3 3.9 4.6 4.8 4.5
    Zr 169 220 184 177 191 202 165 168 186 236 178 184 191
    Hf 6 7.9 7.2 6.5 7.2 7.5 6.2 6.3 7.1 8.3 7.1 6.9 7
    Nb 27.6 31.6 27.2 25.7 27.6 27.7 23.6 25.3 25.9 22.6 20.7 20 29
    Ta 1.8 2.2 2 1.8 2 2 1.7 1.8 2 1.3 1.3 1.3 2
    Y 56.1 73.5 60.9 61.9 60.8 64.2 60.7 62.4 60.2 51.7 52.3 51.5 63.5
    Transition elements
    Sc 8 11.2 9.5 8.8 9.1 9.3 9.3 9.1 8.4 15.6 12.1 11.7 10.3
    Cr 7 9 6 7 11 7 7 7 6 9 5 4 8
    Ni 4 9 8 4 7 4 6 14 7 13 4 14 10
    Co 6 9 6 6 7 6 6 6 5 12 5 5 6
    V 36 40 34 36 38 37 33 35 25 77 38 38 35
    Zn 65 93 68 76 77 63 64 67 67 106 83 83 73
    Cu 9 4 8 8 15 10 8 9 9 17 47 8 4
    Ga 15.9 18.5 16 18.4 16.6 16.8 17.4 17.6 15.5 19 19.5 19 17.7
    Rare earth elements (REEs)
    La 85 127 86 115 114 108 113 112 106 118 138 139 117
    Ce 192.7 255.8 216.3 237.6 236 241.8 236.8 229.1 206.1 261.2 297.7 298.9 232.9
    Pr 19.3 25.5 19.6 22.6 22.4 22.1 22.4 22 20.2 25.3 29 30.1 23.7
    Nd 69.7 100.1 78.5 87.4 86.8 87.1 87.1 86.4 78.8 104.9 117.9 118.2 89.5
    Sm 12.3 16.3 14.1 14.8 14.8 15.1 14.9 14.2 13.7 18.1 18 18.5 15.3
    Eu 2.1 3.1 2.7 2.8 2.8 2.7 2.7 2.8 2.5 3 3.4 3.4 2.7
    Gd 10 14.2 11.8 13 13.4 12.8 12.2 12.5 12.3 13.3 13.8 13.3 13.6
    Tb 1.9 2.6 2.2 2.3 2.5 2.4 2.4 2.3 2.2 2.3 2.3 2.4 2.5
    Dy 10.3 14.7 12.5 12.9 13.4 13.8 12.7 13.2 12.8 11.7 11.7 12.3 13.6
    Ho 2.1 2.8 2.5 2.5 2.6 2.7 2.5 2.5 2.5 2.2 2.1 2.2 2.7
    Er 7.6 8.2 7.3 7.3 7.4 7.7 7.2 7.1 7.2 6 5.8 6.2 7.7
    Tm 0.9 1.3 1.1 1.1 1.1 1.2 1.1 1.1 1.1 0.9 0.9 0.9 1.2
    Yb 7.3 9.1 8.2 8.1 8.3 8.5 7.8 7.9 7.9 6.3 6.5 6.7 8.3
    Lu 0.8 1.1 1 1 1 1 1 1 1 0.8 0.8 0.8 1.1

    Table 4.  Whole-rock trace element concentrations (ppm) for the Koras Group bimodal volcanic rocks in the western Kheis terrane (lower succession)

    Figure 7.  Classification plots for the Koras Group rhyolitic porphyries. (a) The QAP plot of Le Maitre et al. (1989); (b) the total alkalis vs. silica (TAS) plot of Cox et al. (1979); 1. alkali-feldspar trachyte; 2. q-alk-fsp trachyte; 3. alk-fsp rhyolite; (c) the An-Ab-Or ternary volcanic plot (after O'Connor, 1965); (d) FeOT/(FeOT+MgO) vs. SiO2 plot (after Frost et al., 2001); (e) the Na2O+K2O-CaO vs. SiO2 plot (after Frost et al., 2001); (f) the molar Al2O3/(Na2O+K2O) (A/NK) vs. aluminium saturation index (ASI) plot of Frost et al. (2001); (g) the FeOT/MgO vs. Zr+Nb+Ce+Y plot of Whalen et al. (1987); (h) the (K2O+Na2O)/CaO vs. Zr+Nb+Ce+Y plot; (ⅰ) the (K2O+Na2O)/CaO vs. 10 000×Ga/Al plot of Whalen et al. (1987).

    The Koras Group rhyolites have higher FeO, MgO, CaO, Na2O, K2O, TiO2, and MnO, and lower SiO2 and Al2O3 relative to average rhyolite. All the major elements, except for K2O, show negative trends relative to SiO2 (ESM2, Fig. S3). Na2O concentrations, when taken with those of the basaltic andesites, display a two-step trend or pattern, with increasing trends from the basaltic andesites up to intermediate compositions and decreasing trends for the higher SiO2 compositions of the rhyolites. Sr follows a similar two-step trend as Na2O, while Ba follows the K2O trend. The abundances of incompatible elements, such as Rb, Nb, Zr, Th, and U are positively correlated with SiO2 content, whereas the negative trends of compatible elements (Ni, Cr and V) with increasing SiO2 correspond with similar trends of MgO and Fe2O3 with differentiation (Bailie et al., 2012).

    In terms of PCA, the lower succession rhyolitic porphyries show patterns or trends suggesting that Na and Ca are the main compositional variable, with the limited number of samples precluding any in-depth interpretation, whereas for the upper succession, Si and K play important roles (ESM2, Fig. S4). The lower succession forms separate trends on most of the correlation plots (ESM2, Fig. S5), suggesting that it possibly formed along its own separate evolutionary path relative to the upper succession. On maficity plots (Fig. 5), the Koras Group rhyolites show separate trends to those of the Keimoes Suite granitoids. Positive trends against maficity are present for Ti, Ca, and Al, and negative trends for Si and K; Na, P, Mg#, A/CNK and ASI have vertical to no proper trends.

    On PM-normalized multi-element trace-element plots (spider diagrams), there are no apparent distinctions between the lower and upper successions (Fig. 8). Relative to the primitive mantle, the rhyolitic porphyries are enriched in the LIL elements relative to the HFS elements (Fig. 8). There are depletions in Ba, Nb, Ta, K, Zr, Hf (slight), Sr, P and Ti (the latter three strongly, with enrichments in Th, U and Pb. The LREE are enriched relative to the HREE, the latter having a flat pattern. Nb is depleted relative to the other HFSE, but Th is enriched relative to both Nb and Ta, as well as the HREE (Bailie et al., 2012). Average crust-normalized patterns show similar enrichments and depletions to the PM-normalized plot, apart from a HREE enrichment relative to the LREE (Fig. 8).

    Figure 8.  (a) Multi-element trace-element plot (spider diagram) for the rhyolitic porphyries of the Koras Group (normalised to the primitive mantle values of Sun and McDonough, 1989). (b) REE plot for the Koras Group rhyolitic porphyries normalised to the primitive mantle values of McDonough and Sun (1995). (c) Multi-element trace-element plot for the rhyolitic porphyries of the Koras Group normalised to the average crustal values of Weaver and Tarney (1984).

    The PM-normalized REE plot (Fig. 8) shows a slightly negative sloping trend, with a slight enrichment of the LREE relative to the HREE [(La/Yb)PM: 7.10-14.39], moderate fractionation in the LREE [(La/Sm)PM: 3.28-4.95], relatively flat HREE patterns [(Gd/Yb)PM: 1.11-1.71], and slight negative europium anomalies [(Eu/Eu*)PM: 0.57-0.66]. The Swartkopsleegte Formation of the lower succession has a much smaller range in terms of ratios (but also fewer samples), and shows a shallower LREE slope, and overall LREE/HREE enrichment, and flatter overall HREE slope compared to the rhyolites of the upper succession.

  • The Keimoes Suite granitoids have 143Nd/144Nd ratios for time of emplacement which vary between 0.510 78 and 0.511 35 (Table 5). The εNd(t) values, calculated relative to each granite's emplacement age, fall within a range from 2.78 to -2.95, with the Kleinbegin granite samples having the most negative values (-4.87 to -8.58) (Fig. 9). The Kanoneiland granite is the only granite with mostly positive εNd(t) values (0.65 to 2.78), apart from one sample of the Friersdale charnockite, with the vast majority of the εNd(t) values being negative, suggesting weakly to mildly enriched sources, with a strongly enriched crustal source for the Kleinbegin granite. Sm-Nd model ages (TDM) vary from 1.62-1.99 Ga, with the Kleinbegin granite having older model ages of 1.91-2.55 Ga, and a younger model age of 1.40 Ga for Kanoneiland granite sample Kan1. Initial 87Sr/86Sr ratios vary from 0.652 82 to strongly radiogenic values of 0.771 30.

    Sample Rock type Sm (ppm) Nd (ppm) Sm/Nd (143Nd/144Nd)0 2SE 147Sm/144Nd (143Nd/144Nd)t CHUR(t) εNd(t) εNd(0) Age (Ma) TCHUR TDM References
    Koras Group
    Lower succession
    K2 Boom River basaltic andesite 4.7 21.8 0.216 0.512 113 14 0.130 414 0.511 146 0.511 179 -0.65 -10.24 1 130 1.206 1.894 Bailie et al. (2012)
    K6 Boom River basaltic andesite 5.5 26.0 0.212 0.512 082 0.127 960 0.511 133 0.511 179 -0.90 -10.85 1 130 1.232 1.895 Bailie et al. (2012)
    K11 Boom River basaltic andesite 6.0 28.2 0.213 0.512 090 11 0.128 702 0.511 135 0.511 179 -0.85 -10.69 1 130 1.227 1.897 Bailie et al. (2012)
    K11B Boom River basaltic andesite 6.0 28.2 0.213 0.512 048 30 0.128 702 0.511 093 0.511 179 -1.67 -11.51 1 130 1.321 1.972 This study
    K14 Boom River basaltic andesite 5.0 23.3 0.215 0.511 981 12 0.129 807 0.511 018 0.511 179 -3.15 -12.82 1 130 1.494 2.118 Bailie et al. (2012)
    W28 Boom River basaltic andesite 5.1 23.9 0.213 0.512 001 13 0.129 079 0.511 044 0.511 179 -2.65 -12.43 1 130 1.434 2.065 Bailie et al. (2012)
    W29 Boom River basaltic andesite 5.1 23.7 0.215 0.512 032 12 0.130 168 0.511 066 0.511 179 -2.20 -11.82 1 130 1.386 2.035 Bailie et al. (2012)
    K8 Swartkopsleegte rhyolite 16 86 0.186 0.512 301 28 0.112 540 0.511 466 0.511 179 5.62 -6.57 1 130 0.611 1.279 This study
    K8B Swartkopsleegte rhyolite 16.0 86.0 0.186 0.512 301 0.112 540 0.511 466 0.511 179 5.62 -6.57 1 130 0.611 1.279 This study
    K20 Swartkopsleegte rhyolite 14.2 73.4 0.193 0.512 085 0.117 024 0.511 217 0.511 179 0.74 -10.79 1 130 1.058 1.677 Bailie et al. (2012)
    Upper succession
    K12 Leeuwdraai rhyolitic porphyry 13.5 76.8 0.176 0.511 849 9 0.106 330 0.511 081 0.511 218 -2.67 -15.39 1 100 1.329 1.843 Bailie et al. (2012)
    K12B Leeuwdraai rhyolitic porphyry 14.8 87.4 0.169 0.511 828 22 0.102 432 0.511 088 0.511 218 -2.53 -15.80 1 100 1.308 1.808 This study
    K13 Leeuwdraai rhyolitic porphyry 14.1 80.4 0.175 0.511 766 11 0.106 083 0.511 000 0.511 218 -4.26 -17.01 1 100 1.464 1.956 Bailie et al. (2012)
    K21 Leeuwdraai rhyolitic porphyry 13.3 75.7 0.176 0.511 812 11 0.106 277 0.511 045 0.511 218 -3.39 -16.11 1 100 1.390 1.894 Bailie et al. (2012)
    K22 Leeuwdraai rhyolitic porphyry 12.9 73 0.177 0.511 812 10 0.106 893 0.511 040 0.511 218 -3.47 -16.11 1 100 1.400 1.905 Bailie et al. (2012)
    K23 Leeuwdraai rhyolitic porphyry 13.1 75.3 0.174 0.511 410 13 0.105 235 0.510 650 0.511 218 -11.10 -23.95 1 100 2.039 2.436 Bailie et al. (2012)
    K23B Leeuwdraai rhyolitic porphyry 14.2 86.4 0.164 0.511 795 19 0.099 416 0.511 077 0.511 218 -2.75 -16.44 1 100 1.319 1.804 This study
    K31 Rouxville basaltic andesite 5.3 24.5 0.216 0.511 973 0.130 856 0.511 028 0.511 218 -3.71 -12.97 1 100 1.537 2.160 Bailie et al. (2012)
    K37 Welgevind rhyolitic porphyry 17 103.3 0.165 0.511 824 7 0.099 548 0.511 105 0.511 218 -2.20 -15.88 1 100 1.276 1.767 Bailie et al. (2012)
    K37B Welgevind rhyolitic porphyry 18.5 118.2 0.157 0.511 866 21 0.094 676 0.511 182 0.511 218 -0.69 -15.06 1 100 1.153 1.642 This study
    Keimoes Suite
    Mkb 6 Keboes granite 4.3 21.1 0.205 0.511 977 14 0.124 222 0.511 072 0.511 205 -2.60 -12.89 1 110 1.388 1.994 Bailie et al. (2017)
    Mka 1 Kanoneiland granite 12.7 62.9 0.202 0.511 968 4.8 0.122 018 0.511 083 0.511 211 -2.52 -13.08 1 105 1.366 1.962 Bailie et al. (2017)
    Mka 6 Kanoneiland granite 18.0 95.2 0.189 0.512 076 10 0.114 616 0.511 245 0.511 211 0.65 -10.96 1 105 1.043 1.650 Bailie et al. (2017)
    Kan1 Kanoneiland granite 1.7 12.1 0.140 0.511 970 0.084 986 0.511 354 0.511 211 2.78 -13.03 1 105 0.912 1.396 This study
    Gems Gemsbokbult granite 14.0 77.9 0.180 0.511 984 0.108 711 0.511 197 0.511 214 -0.33 -12.76 1 103 1.132 1.690 Bailie et al. (2011a)
    Mkle 1 Kleinbegin granite 9.8 46.4 0.210 0.511 698 7 0.127 310 0.510 778 0.511 217 -8.58 -18.34 1 101 2.058 2.552 Bailie et al. (2017)
    Klein Kleinbegin granite 13.7 88.7 0.154 0.511 643 0.093 429 0.510 968 0.511 217 -4.87 -19.41 1 101 1.466 1.906 Bailie et al. (2011a)
    Koras Group
    Upper succession
    Keimoes Suite
    Kle1 Kleinbegin granite 7.4 37.4 0.198 0.511 700 0.119 686 0.510 835 0.511 217 -7.46 -18.30 1 101 1.851 2.343 Bailie et al. (2017)
    Mks 1 Klip Koppies granite 14.2 71.4 0.198 0.511 945 7 0.120 031 0.511 082 0.511 223 -2.77 -13.52 1 096 1.376 1.957 Bailie et al. (2017)
    Klip K Klip Koppies granite 14.6 74.6 0.196 0.512 022 0.118 385 0.511 170 0.511 223 -1.03 -12.02 1 096 1.198 1.801 Bailie et al. (2011a)
    Klip Klip Koppies granite 9.7 53.1 0.183 0.511 950 0.110 500 0.511 155 0.511 223 -1.33 -13.42 1 096 1.216 1.769 Bailie et al. (2017)
    Mkl3 Klipkraal granite 108.6 576.8 0.188 0.511 970 0.113 891 0.511 140 0.511 205 -1.26 -13.03 1 110 1.228 1.799 Bailie et al. (2017)
    MkI 4 Klipkraal granite 23.2 118.9 0.195 0.511 971 10 0.118 034 0.511 111 0.511 205 -1.84 -13.01 1 110 1.291 1.875 Bailie et al. (2017)
    MkI 5 Klipkraal granite 23.6 129.5 0.182 0.511 923 8 0.110 069 0.511 121 0.511 205 -1.64 -13.94 1 110 1.256 1.801 Bailie et al. (2017)
    Klp Klipkraal granite 20.8 108.6 0.192 0.511 920 0.115 856 0.511 076 0.511 205 -2.52 -14.01 1 110 1.352 1.912 Bailie et al. (2017)
    Strauss Straussburg granite 11.6 58.0 0.200 0.511 970 0.120 980 0.511 105 0.511 232 -2.48 -13.03 1 089 1.343 1.936 Pettersson et al. (2007)
    Str1 Straussburg granite 15.4 98.1 0.157 0.511 760 0.094 959 0.511 081 0.511 232 -2.95 -17.13 1 089 1.314 1.781 Bailie et al. (2017)
    Mf 1 Friersdale charnockite 15.6 82.4 0.189 0.512 033 8.4 0.114 288 0.511 224 0.511 246 -0.43 -11.81 1 078 1.119 1.710 Bailie et al. (2017)
    Mf 4 Friersdale charnockite 15.5 81.0 0.191 0.512 010 8.5 0.115 596 0.511 193 0.511 246 -1.05 -12.24 1 078 1.179 1.768 Bailie et al. (2017)
    Mf 6 Friersdale charnockite 14.5 79.8 0.181 0.512 011 9.5 0.109 627 0.511 236 0.511 246 -0.21 -12.23 1 078 1.097 1.666 Bailie et al. (2017)
    S188 Friersdale charnockite 14.2 79.4 0.179 0.512 030 0.108 181 0.511 265 0.511 246 0.36 -11.86 1 078 1.047 1.616 Bailie et al. (2017)
    S815 Friersdale charnockite 14.4 78.8 0.183 0.512 010 0.110 540 0.511 228 0.511 246 -0.36 -12.25 1 078 1.110 1.682 Bailie et al. (2017)
    S367 Friersdale charnockite 14.3 76.6 0.187 0.512 010 0.112 925 0.511 211 0.511 246 -0.69 -12.25 1 078 1.142 1.722 Bailie et al. (2017)
    Friers 1 Friersdale charnockite 14.9 78.7 0.189 0.512 000 0.114 524 0.511 190 0.511 246 -1.11 -12.45 1 078 1.183 1.765 Pettersson et al. (2007)
    Friers 2 Friersdale charnockite 14.7 77.4 0.190 0.511 980 0.114 884 0.511 167 0.511 246 -1.55 -12.84 1 078 1.225 1.802 Pettersson et al. (2007)
    (143Nd/144)DM, 0, the present day value of the depleted mantle (DM=0.513 15; (147Sm/144Nd)DM, 0, present day value of DM=0.213 6; εNd(t) calculated using (143Nd/144Nd)CHUR today=0.512 638 with 146Nd/144Nd=0.712 90, εNd(0)= ((143Nd/144Nd(0)/0.512 638)-1)×10 000; εNd(t)=((143Nd/144Nd(sample, t)/143Nd/144Nd(CHUR, t))-1)×10 000; t in all equations is the time of emplacement of the granite/volcanics; TDM=1/(λ)×ln(1+(143Nd/144Nd(sample, 0)-143Nd/144Nd(DM, 0)/ (147Sm/144Nd(sample, 0)-147Sm/144Nd(DM, 0))×10-9 (after DePaolo, 1981). TCHUR=1/(λ)×ln(1+(143Nd/144Nd(sample, 0)-143Nd/144Nd(CHUR, 0)/(147Sm/144Nd(sample, 0)-147Sm/144Nd(CHUR, 0))×10-9 (after DePaolo, 1981); (147Sm/144Nd)CHUR today= 0.196 7; ages for the Koras Group lower succession are from Mothibi (2016); for the upper succession from Pettersson et al. (2007) and Bailie et al. (2012); ages for the various granites of the Keimoes Suite are from Bailie et al.(2017, 2011a) and Cornell et al. (2012).

    Table 5.  Whole-rock Sm-Nd isotopic data for the Keimoes Suite granitoids and Koras Group volcanic rocks, eastern NMP

    Figure 9.  Sr-Nd isotopic plots for the Keimoes Suite granitoids and Koras Group volcanics. (a) εNd(t) vs. initial Sr (Sri=(87Sr/86Sr)i, where i is the time of emplacement or volcanism). (b) εNd(t) vs. age. (c) Model age (TDM) vs. age of extrusion or intrusion.

    The lower succession basaltic andesites (Boom River Formation) have 143Nd/144Nd ratios (for the time of volcanism) and εNd(t) values which vary from 0.511 02-0.511 15, and -0.65 to -3.15, respectively. These are higher than those of a basaltic andesite sample of the upper succession (0.511 03 and -3.71, a sample of the Rouxville Formation). Likewise, the lower succession rhyolites have higher initial 143Nd/144Nd ratios (0.511 22-0.511 47) and εNd(t) values (0.74 to 5.62) than those of the upper succession (0.510 65-0.511 18, and -0.69 to -4.26, respectively, as well as a strongly negative value of -11.10: Table 6). The lower succession has a much younger model age range (TDM: 1.28-2.12 Ga) compared to the upper succession (1.64-2.44 Ga). Such εNd(t) values suggest derivation from weakly depleted to mildly enriched sources and a probable mixture between juvenile Mesoproterozoic and older Paleoproterozoic sources. The lower succession is characterized by a greater depleted component than the upper succession, with the latter characterized by a greater crustal signature. There are only a few Rb-Sr isotopic data available, with initial 87Sr/86Sr ratios varying from 0.709 71 to 0.716 96. The upper succession has lower initial Sr ratios, in general, than the lower succession (Table 6).

    Sample Rock type Rb (ppm) Sr (ppm) Rb/Sr (87Sr/86Sr)0 2SE 87Rb/86Sr (87Sr/86Sr)t BSE(t) εSr(t) Age (Ma) References
    Koras Group
    Lower succession
    K11B Boom River basaltic andesite 71 307 0.23 0.724 609 12 0.612 581 0.714 700 0.703 325 161.7 1 130 This study
    K8B Swartkopsleegte rhyolite 198 186 1.06 0.758 735 18 2.827 621 0.712 997 0.703 325 137.5 1 130 This study
    Upper succession
    K12B Leeuwdraai rhyolitic porphyry 201 180 1.12 0.763 652 16 2.966 146 0.716 957 0.703 362 193.3 1 100 This study
    K23B Leeuwdraai rhyolitic porphyry 219 170 1.29 0.765 667 20 3.421 875 0.711 798 0.703 362 119.9 1 100 This study
    K37B Welgevind rhyolitic porphyry 190 232 0.82 0.743 957 16 2.175 377 0.709 711 0.703 362 90.3 1 100 This study
    Keimoes Suite
    Mkb 6 Keboes granite 186 294 0.63 0.797 790 13 1.675 313 0.771 175 0.703 350 964.3 1 110 Bailie et al. (2017)
    Mka 1 Kanoneiland granite 246 160 1.54 0.717 529 12 4.091 831 0.652 818 0.703 356 -718.5 1 105 Bailie et al. (2017)
    Mka 6 Kanoneiland granite 233 143 1.63 0.791 095 14 4.333 226 0.722 566 0.703 356 273.1 1 105 Bailie et al. (2017)
    kan1 Kanoneiland granite 130 533 0.24 0.717 529 12 0.645 751 0.707 317 0.703 356 56.3 1 105 Bailie et al. (2011)
    Mkle 1 Kleinbegin granite 107 154 0.70 0.763 797 14 1.851 874 0.734 617 0.703 361 444.4 1 101 Bailie et al. (2017)
    kle1 Kleinbegin granite 221 179 1.23 0.763 797 14 3.279 504 0.712 122 0.703 361 124.6 1 101 Bailie et al. (2017)
    Klip Klip Koppies granite 209 149 1.40 0.723 870 3.725 881 0.665 430 0.703 367 -539.4 1 096 Bailie et al. (2017)
    Mkl3 Klipkraal granite 1 101 826 1.33 0.772 350 3.540 595 0.716 101 0.703 350 181.3 1 110 Bailie et al. (2017)
    MkI 4 Klipkraal granite 204 147 1.39 0.772 353 14 3.694 668 0.713 656 0.703 350 146.5 1 110 Bailie et al. (2017)
    MkI 5 Klipkraal granite 200 165 1.21 0.764 825 12 3.207 373 0.713 870 0.703 350 149.6 1 110 Bailie et al. (2017)
    Klp Klipkraal granite 175 161 1.09 0.764 825 12 2.886 942 0.718 960 0.703 350 221.9 1 110 Bailie et al. (2011)
    Str1 Straussburg granite 158 228 0.69 0.746 117 11 1.839 839 0.717 445 0.703 375 200.0 1 089 Bailie et al. (2017)
    Mf 1 Friersdale charnockite 157 279 0.56 0.736 655 14 1.488 460 0.713 695 0.703 389 146.5 1 078 Bailie et al. (2017)
    Mf 4 Friersdale charnockite 161 277 0.58 0.734 338 15 1.546 078 0.710 489 0.703 389 100.9 1 078 Bailie et al. (2017)
    Mf 6 Friersdale charnockite 159 264 0.60 0.734 072 15 1.601 357 0.709 370 0.703 389 85.0 1 078 Bailie et al. (2017)
    S188 Friersdale charnockite 165 262 0.63 0.736 655 14 1.669 527 0.710 902 0.703 389 106.8 1 078 Bailie et al. (2011)
    S815 Friersdale charnockite 165 277 0.60 0.734 338 15 1.580 713 0.709 954 0.703 389 93.3 1 078 Bailie et al. (2011)
    S367 Friersdale charnockite 160 271 0.59 0.734 072 15 1.569 246 0.709 866 0.703 389 92.1 1 078 Bailie et al. (2011)
    BA. Basaltic andesite; RP. rhyolitic porphyry.

    Table 6.  Whole-rock Rb-Sr isotopic data for the Keimoes Suite granitoids and Koras Group volcanic rocks, eastern NMP

  • The Keimoes Suite, despite being stratigraphically grouped as a suite, constitutes a number of plutons which, despite being temporally similar (~1.11-1.08 Ga) and spatially constrained to within one broadly defined region (Fig. 2), are genetically unrelated. They were derived from different sources and likely underwent different petrogenetic processes to give rise to the compositional variations within each pluton. They, therefore, do not form one coherent unit linked by a certain petrogenetic process.

    The Keimoes Suite granitoids show a variable degree of derivation from basaltic- or amphibolitic-type sources (Figs. 10f, 11a-11c). These mafic components, in general, appear to comprise more than 50% of the source for the parental melts. Relatively mafic sources for the granites are also evident in their higher compatible element (Cr, Co, Ni, Cu and Sc) concentrations. Certain granites, such as the Friersdale charnockite and Klipkraal granite, were derived from predominantly mafic sources with some potential mantle input. There is, however, a substantial sedimentary component in the sources to some granites. The compositions of the Kanoneiland granite samples, for example, overlap those of calculated psammite-derived melts (Fig. 11b). In general, the Keimoes Suite granitoids have εNd(t) values around zero (-2.95 to 2.83), Meso- to Paleoproterozoic model ages (TDM: 1.38-1.99 Ga) and relatively high initial Sr isotope ratios (0.717-0.798), suggesting derivation from relatively depleted sources with a variable enriched and/or crustal component. This is supported by enrichments in Pb, Th, U, and the LREE relative to the HREE, and the LILE relative to the HFSE, with depletions in Ba, Nb, Ta, Sr, Eu, Ti, Al2O3, V and Sc (Fig. 6; Bailie et al., 2017). The I-type characteristics of the granitoids, with biotite±hornblende±orthopyroxene as the main mafic minerals, their granodioritic compositions, and arc-like affinities (negative Nb-Ta, Ti and P anomalies), suggests derivation from mainly igneous sources, with a minor sedimentary component giving rise to dry parental magmas. The Keimoes Suite granitoids show an increasing maficity, metaluminous character, and general decreasing degree of fractionation with decreasing age, becoming more granodioritic concomitant with an increase in mafic minerals and decrease in K-feldspar content (Bailie et al., 2017). In addition, they become more aluminous and less alkalic with decreasing age. This suggests a greater depleted source component in the parental magmas with time.

    Figure 10.  Petrogenetic diagrams of the Koras Group volcanics and Keimoes Suite granitoids. (a) (La/Sm)N vs. La diagram, where the N subscript represents chondrite normalization, showing the trajectories of partial melting, fractional crystallization, and magma mixing. The inset is a schematic CH/CM vs. CH plot, where H and M are highly and moderately incompatible elements, respectively. (b) Sr versus Lu diagram showing trajectories of partial melting and fractional crystallization (after Peccerillo et al., 2003). Sr and Lu are incompatible and compatible elements, respectively (Rollinson, 1993). (c) Sr vs. Lu diagram, showing that the genesis of K-rich granitoids are controlled dominantly by partial melting (Rollinson, 1993), as also seen for the Keimoes Suite granitoids. (d) LaN vs. FeOT+MgO diagram (after Laurent et al., 2014) with BC representing bulk crust, and UC, upper crust, OIB ocean island basalt, and MORB, mid-ocean ridge basalt. (e) (Hf/Sm)N vs. (Nb/La)N diagram (after La Fléche et al., 1998). The N subscript represents chondrite normalization; E-MORB is enriched MORB and N-MORB, normal MORB. (f) CaO/(FeOT+MgO+TiO2) vs. CaO+FeOT+MgO+TiO2 diagram. The fields represent compositions of experimentally-derived partial melts from amphibolites, greywackes, and mafic and felsic pelites (after Patiño Douce, 1999). (g) ASI (aluminium saturation index; Al/(Ca−1.67P+Na+K) versus SiO2 diagram (after Frost and Frost, 2011).

    Figure 11.  Test of source characteristics to the parental melts of the Koras Group volcanics and Keimoes Suite granitoids. (a) CaO/Na2O vs. Al2O3/TiO2. (b) Rb/Ba vs. Rb/Sr. The mixing curve between basalt- and pelite-derived melts is from Patiño Douce and Harris (1998) and Sylvester (1998). (c) A-B diagram (after Villaseca et al., 1998). The arrows represent the compositions of progressive melt fractions produced by experimental melting of different crustal rocks (Villaseca et al., 1998 and references therein). Highly, moderately, low and highly felsic peraluminous granitoids, are indicated by h-P, m-P, l-P and f-P respectively. (d) CaO/Na2O vs. Al2O3/TiO2 plot. Vectors for fractional crystallization of oligoclase (An20), K-feldspar (Kfs), biotite (Bt) and ilmenite (Ilm), and for illustrating compositional variations with increasing melting temperature and clay components in the source are from Sylvester (1998) and references therein.

    The high Fe/Mg ratios (0.82-3.84, avg. 1.66), and high HFSE, LILE and REE contents of the granitoids, are, however, also suggestive of fractionated A2-types (Eby, 1992) (Figs. 4g-4h, 13c). The granitoids were derived from partial melting of mid- to lower-crustal sources, such as amphibolites, and supracrustal material, in the form of meta-greywackes and mafic pelites, In terms of processes that can give rise to A-type granites, the partial melting of pre-existing crustal rocks, and a combination of both fractionation of mantle-derived basaltic magmas and crustal anatexis of granodioritic or tonalitic sources are likely to be the most relevant.

    Figure 12.  Binary plots testing the effects of fractional crystallisation of various minerals on the compositional variation within the Keimoes Suite granites. (a) Rb vs. Sr; (b) Ba vs. Sr; (c) Sr vs. Eu; (d) Rb vs. Eu.

    Figure 13.  Binary plots testing the effects of fractional crystallisation of various minerals on the compositional variation within the Koras Group volcanic rocks. (a) Rb vs. Sr; (b) Ba vs. Sr; (c) Sr vs. Eu; (d) Rb vs. Eu.

    The Keimoes Suite granites were derived by dehydration melting at pressures varying from 6 to > 10 kbar (Fig. 10g), indicating derivation from the mid- to lower crust. The Kleinbegin granite was likely derived from shallower melting (closer to 6 kbar), whereas the others were derived from deeper levels (predominantly 10 kbar and greater). The Klipkraal granite and Friersdale charnockite were likely derived from deeper depths, with a possible mantle component present.

    The Klipkraal granite and the Friersdale charnockite are the most mafic members of the Keimoes Suite. The charnockitic nature of the Friersdale charnockite may be due to either partial melting of dehydrated lower crust that had suffered prior melt extraction (cf. Geringer et al., 1987 for enclaves in the Straussburg granite compositionally similar to the Friersdale charnockite), or from a CO2-enriched crust with some mantle input (Fig. 10d), although the tectonic setting of the Keimoes Suite, related to transtensional rift zones (Bailie et al., 2017), likely favours the melting of continental crust above an upwelling mafic magma, represented by the mafic magmas of the Koras Group. By contrast, the Kleinbegin and Keboes granites were derived by partial melting of crustal material (Fig. 10d), as also seen in their strongly negative εNd(t) values (Fig. 9).

    Sm-Nd isotope characteristics (low εNd(t), variable model ages, and arc-related characteristics suggest the involvement of both juvenile Mesoproterozoic crust, such as the ~1.3-1.24 Ga Areachap arc (Pettersson et al., 2007), as well as the reworking of older ~1.80-2.2 Ga Paleoproterozoic crust (such as the 1.8-1.82 Ga Gladkop Suite, and the ~2.2 Ga Sperrgebiet and ~1.9 Ga Richtersveld arcs; Fig. 9b; Nke et al., 2020; Macey et al., 2018, 2017; Thomas et al., 2016). Mixing of different aged sources is likely (cf. the hybrid origin of Cornell et al., 2012), as seen in the spread of model ages and εNd(t) values (e.g., the Kanoneiland granite). Some granites (e.g., the Friersdale charnockite and Kanoneiland granite) have a larger juvenile source component, whereas others may have had potential Archean crustal source contributions (those with model ages ≥2.5 Ga). The reworking and contribution of these varying sources to the parental melts is evident by the presence of inherited zircons in some of the granitoids (Bailie et al., 2011a). The relatively low initial Sr ratios [(87Sr/86Sr)i] for most of the granitoids (Fig. 9a) does, however, suggest that the parental magmas were largely derived from relatively unradiogenic sources. The lack of sufficient samples precludes evaluating the role of mixing in influencing compositional variation.

  • The Koras Group basaltic andesites were derived from the partial melting of lithospheric mantle wedge sources affected by fluid-related subduction metasomatism (Fig. 10e; Becker et al., 2000; McCulloch and Gamble, 1991). The basaltic andesites of the lower succession were derived from partial melting of spinel lherzolite within a variably enriched shallow mantle source (Fig. 10d; Fig. S6c). Subduction and mantle metasomatism related to the development of a continental to island arc had ceased some 50-80 million years prior to the time of Koras Group volcanism (e.g., Pettersson et al., 2007). The low incompatible element content of both successions indicates relatively high degrees of partial melting (Bailie et al., 2012). The geochemical and isotopic characteristics of the rhyolitic porphyries of both successions, including enrichment in the LILE relative to the HFSE, LREE/HREE enrichment, enrichment in compatible elements (Cu, Cr, Ni, V, Sc) and certain HFSE (Th, U, Pb), a negative Nb-Ta anomaly, and positive to mostly weakly negative εNd(t) values, suggest derivation from weakly depleted sources.

    The lower succession is more mafic and has a greater depleted component to the parental magmas, whereas the upper succession is more felsic and fractionated and was derived from melts having a greater crustal component, as denoted by much lower (Nb/Th)PM and higher (Th/Yb)PM ratios (Bailie et al., 2012), becoming more felsic higher in the succession. The upper succession rhyolitic porphyries were likely derived by partial melting of Paleoproterozoic-aged crustal material (TDM: 1.64-2.44 Ga) (Table 5). This melting was likely due to the heat caused by rifting and the influx of mantle-derived mafic melts (represented by the basaltic andesites), as opposed to being derived by fractional crystallisation of the latter. Lack of sufficient samples of basaltic andesites of the upper succession makes it difficult to assess any potential links between the two. The higher LREE/HREE, Th/Yb and Th/Ta, and lower Nb/Th, Nb/Y and Nb/Zr ratios of the rhyolites of the Northern Domain compared to those of the correlative Central Domain suggests a greater crustal contribution to the parental magmas of the former compared to the latter.

    The lower succession rhyolites have trace element contents suggestive of derivation from mostly sedimentary-based sources (predominantly > 80%) (Figs. 10f, 10g, 11a, 11b), with psammites or greywackes being the predominant source rock types. The lower succession rhyolites are variable in terms of εNd(t) values and model ages (Table 5), suggesting derivation from variable sources and potential mixing. With Paleo-proterozoic model ages (TDM: 1.89-2.12 Ga) and low negative εNd(t) values (-0.65 to -3.15; Table 6), the basaltic andesites were derived from partial melting of weakly to moderately depleted sources. The SrI of the Koras Group volcanics are relatively low (less than 0.72) suggesting derivation from weakly radiogenic sources.

    The Koras Group rhyolites also show characteristics of A2-type magmas (Figs. 7g-7i). A2-type magmatism occurs shortly (10 to 20 million years, or longer) after periods of compressional tectonics (Eby, 1992). In terms of trace elements for A2-type felsic magmatism, both subcontinental lithosphere and crustal source signatures are present, but never OIB-like signatures (Eby, 1992). This suggests that there was no influence of any kind of plume at ~1 100 Ma in the eastern Namaqua Province, as has been argued (e.g., Cornell et al., 2012).

  • The general geochemical characteristics of the Keimoes Suite granitoids are summarized in Table 7 which includes geochemical and potential source characteristics. The limited number of samples, particularly of the individual granites, makes petrogenetic interpretation difficult and warrants further and more detailed studies. Variable amounts of partial melting can bring about variations in the abundances of moderately compatible elements, such as the LREE and Sr, whereas more strongly compatible elements, such as the HREE, most notably Lu, are more strongly influenced by fractional crystallisation, being taken up, in particular, by garnet (notably the HREE) and by accessory phases such as apatite, zircon or monazite. Plots of Sr against Lu (Figs. 10b, 10c) give rise to curved paths for these elements during variable amounts of partial melting due to their moderately compatible versus more compatible behaviour, with Sr able to go into the melt. This may imply that a phase in the residua would retain Lu, Y and other REE and liberate Sr. This phase is likely to be garnet because plagioclase would have the opposite effect (Chiaradia, 2015). By contrast fractional crystallisation leads to flatter patterns due to the compatible nature of both elements.

    Granite Overall composition Maficity Major- and minor-elements Trace-elements Sm-Nd isotopes TDM (in Ga) Potential petrogenesis and sources
    Klipkraal Granodiorite to monzogranite calc-alkalic, ferroan, weakly peraluminous High (second highest) High: Fe (highest), Ti, P, ASI (highest); Low: Si, Na, high K2O/Na2O; high Fe/Mg High: La, Sm, Eu, Y, Yb, Nb, Zr, Hf, Th, Ba, Pb, Sc Low: Sr Low -ve εNd(t), high initial Sr (> 0.714); TDM: 1.89-1.91 Unfractionated, mafic source, sedimentary component; I-/A-type characteristics; deep melting; inheritance from ~1.27 Ga Areachap arc
    Kleinbegin Granodiorite, magnesian, calcic, metaluminous, rel. Kfs-rich Low (among lowest) High: Si (highest), Ca, Al, Na (highest); Low: alkalis, Ti (lowest), Fe, K (lowest), Mn, P (lowest), ASI, K2O/Na2O High: Ni, Cu; Low: La, Sm, Eu, Yb, Nb, Zr, Hf, Th, Rb, Ba, Pb, U, Sc v. -ve εNd(t), high initial Sr (> 0.712); TDM: 1.91-2.55 Fractionated; old, radiogenic crust (> 2.0 Ga) derivation; significant mafic igneous component; I-type; various sources, mostly > 2.3 Ga; relatively shallow melting
    Keboes Monzogranite, w. pealuminous, calc-alkalic, ferroan, Qtz-rich Lowest High: Si, Al (highest); Intermediate: K; Low: Ti, Fe, Ca, Mn, P, K2O/Na2O High: Rb, U; Low: La, Sm, Eu, Y, Yb, Nb, Zr, Hf, Ba, Co, Sc, (Ni) Low -ve εNd(t), high initial Sr (> 0.771, n=1); TDM: 1.99 Amongst most fractionated; crustal derivation, substantial radiogenic sedimentary source; inherited zircons; feldspathic (Kfs-rich)
    Gemsbokbult Monzogranite, calc-alkalic, ferroan, weakly peraluminous Variable (intermediate-high) High: K (highest), P; Low: Mg, Ca, Na high Fe/Mg Mostly intermediate; Low: V, Co, Sr, U εNd(t) -ve but close to 0; no Sr data; TDM: 1.69 Relatively fractionated; mildly depleted source, sedimentary component; lack of sufficient data for interpretation
    Kanoneiland Monzogranite, calc-alkalic, ferroan, weakly per- to metaluminous Intermediate High: Si; Low: Mg, Ti, Ca, ASI low K2O/Na2O; high Fe/Mg Mostly intermediate; High: Rb; Low: Ba, Sr, Co, Cu, (Ni) +ve εNd(t); variable to high initial Sr; TDM: 1.40-1.96 Relatively fractionated; depleted source; juvenile crustal derivation; substantial sedimentary source (psammite?); Bt-rich; I-type; mix from various sources
    Klip Koppies Monzogranite; ferroan; quartz- rich; calc-alkalic; weakly peraluminous Intermediate High: Si, ASI, Fe/Mg; Low: Mg, Ti, Ca, ASI High: LREE relative to HREE (high (La/Yb)N); high (Eu/Eu*)N Low: V, Co, Cu, Sr, U Low -ve εNd(t), relatively low initial Sr value; TDM: 1.77-1.96 Crustal derivation with minor depleted source contribution; sedimentary component (psammitic); inheritance from Areachap Group arc
    Straussburg Granodiorite-monzogranite; calc- alkalic, ferroan, metaluminous Intermediate High: Mg, Ca; Low: Si, K, ASI (lowest) High: V, Zr, Hf, Th, (Co), (Ni); Low: Y, Yb Low -ve εNd(t), moderate initial Sr (0.717, n=1); TDM: 1.78-1.94 Relatively unfractionated; mildly enriched source (mostly igneous-rich); Hbl-bearing; mix of various-aged sources
    Friersdale charnockite Granodioritic-monzogranitic; calcic-calc-alkalic; ferroan; most metaluminous of Keimoes Suite Highest High: Fe, Mg (highest), Ti (highest), Ca (highest), P (highest), Mn; Low: Si (lowest), Al, K High: Sr, Eu, Zr, Hf, Sc, V, Co, Ni, Cu, Nb; Low: Rb, Th, Pb Low +ve to low -ve εNd(t), low initial Sr (< 0.714); TDM: 1.62-1.80 Relatively unfractionated (least fractionated of Keimoes Suite); depleted to weakly enriched source; substantial juvenile component (largely mafic igneous source); plagioclase fractionation; mixture of various sources; I-type

    Table 7.  General geochemical properties for the Keimoes Suite granitoids and generalized petrogenesis and source characteristics

    In terms of differentiating between the effects of partial melting and fractional crystallisation on compositional variation in the granites, certain granites, such as the Friersdale charnockite and Gemsbokbult granite, show trends that suggest that variable degrees of partial melting have brought about some compositional variation (Figs. 10a, 10b). By contrast, the flat trends shown by some granites on these plots, such as the Klipkraal granite, most notably, and possibly the Kleinbegin granite, suggest that fractional crystallisation played a role in bringing about compositional variation. The variations seen in both spider diagram and REE plots between the granites (Fig. 6) suggest that source heterogeneities brought about the inherent compositional differences between the granites, as also seen in terms of differing maficity and isotopic characteristics.

    The fractional crystallisation of plagioclase and K-feldspar has brought about some compositional variation for most of the granites, particularly with regard to the LILE (Ba, Rb, Sr) and the alkalis (Fig. 12), but mostly as a later superimposed effect on the primary source-related compositional variation. Plagioclase retention or removal from the parental magma by fractional crystallisation would explain the prominent negative Eu and Sr anomalies in the spider diagrams, given that garnet would retain Y and the HREE and release Sr during melting (Fig. 6).

    The lack of a sufficient number of samples per pluton, as well as differing age relationships, precludes the assessment of whether peritectic assemblage entrainment (PAE) may have played a role in the Keimoes Suite granitoids. PAE is a process that has been invoked to explain the compositional variation seen in both S- and I-type granites which cannot be produced simply by fractional crystallisation or partial melting reactions (Clemens and Stevens, 2012; Clemens et al., 2011; Stevens et al., 2007). PAE refers to the process of entrainment of peritectic minerals or mineral assemblages produced during incongruent melting of the original source material. The peritectic assemblage produced and, potentially, entrained into the melt is dependent on the composition of the source material, among other factors. The PAE process can be assessed by correlations between maficity (molar Fe+Mg) and other major and minor elements (given in molar abundances).

    Certain compositional variations within the Keimoes Suite granitoids (as shown by positive linear correlations on maficity plots for such elements as Fe, Mg, Ti, Ca, P and ASI for many of the granites, such as the Friersdale charnockite; Fig. 5), may be due to the entrainment of clinopyroxene+plagioclase (for Fe, Mg and Ca) and a Ti-bearing phase (such as ilmenite) for Ti. Such an assemblage would likely be produced by the incongruent melting of predominantly igneous biotite- ±hornblende-bearing sources (Clemens et al., 2011). The higher the maficity the greater the amount of entrained peritectic assemblage which is subsequently dissolved into the melt. The positive correlations of Fe, Mg and Ti against maficity are due to the entrainment and dissolution of the Fe-, Mg- and Ca-rich peritectic assemblage (dominated by clinopyroxene and plagioclase) into the melt. Dissolution of the pyroxenes and such Ti-bearing peritectic phases as ilmenite or titano-magnetite lead to a strong increase in Ti with increasing maficity. The entrainment of plagioclase influences the Ca content of the magma. Entrainment of such an assemblage can also produce a decrease in K with increasing maficity, as seen for some of the granites (Fig. 5) which would not be produced by the fractionation of Ti-bearing minerals (such as biotite and/or hornblende). Being incompatible, K concentrations would be high in pure melts, but its concentration is diluted by a greater amount of entrainment of K-poor peritectic phases and their dissolution into the magma. The positive correlation of P against maficity (Fig. 5) has been used to suggest the co-entrainment, and subsequent dissolution, of apatite, the major P-bearing mineral, into a granitic parental melt (Clemens et al., 2011). The poorly defined, slightly negative trends for Na, K and Rb against maficity (Fig. 5) suggest, however, that these reflect the fractionation of alkali feldspars (e.g., Villaros et al., 2009). These elements act in an incompatible fashion and are taken up into the magma during melting, being concentrated in the first melts. Fractional crystallisation of the feldspars would give rise to negative trends with increasing maficity. More detailed studies, with a significantly larger number of samples per pluton, would be required to more rigorously assess the potential of PAE as a compositional driver in the granites.

    The youngest granites (the Friersdale charnockite and Straussburg granite) have the highest maficity values and are also the most metaluminous (lowest A/CNK) (Figs. 4, 5). By contrast, the oldest members of the Keimoes Suite (e.g., the Keboes granite), which are largely leucocratic, and also weakly metaluminous and garnet-bearing, have the lowest maficity (Fig. 5). Peraluminous leucogranites with low maficities are considered to represent pure sedimentary crustal melts in high-grade regional metamorphic settings (e.g., Bial et al., 2015; Jung et al., 2012).

  • In terms of compositional variations caused by partial melting as opposed to fractional crystallisation (Figs. 10a-10c), the rhyolites of the upper succession, and basaltic andesites of the lower succession show trends on these plots suggesting that variable degrees of partial melting brought about some of their compositional variation. By contrast, the flat-lying trends of the lower succession rhyolites (Figs. 10a-10c) suggests that fractional crystallisation played a greater role in compositional variation than did variable amounts of partial melting.

    The mafic and felsic lavas of both successions show regular differentiation trends related to progressive fractional crystallisation (Fig. 13), explaining the trend from the basaltic andesites to the rhyolitic porphyries of the lower succession, in particular (Bailie et al., 2012). K-feldspar and plagioclase fractionation (more evident on the Sr plots), with lesser amounts of biotite (±hornblende), such as in the Welgevind Formation, where Fe variation is a larger factor (Fig. 13), likely brought about the compositional variation seen. The rhyolites of the Swartkopsleegte Formation, of the lower succession, are controlled by Na and Ca (likely due to plagioclase), whereas those of the upper succession are controlled by Si and K (likely K-feldspar). A slight negative Zr trough in the multi-element plot (Fig. 8) possibly suggests zircon fractionation. The basaltic andesites were affected by crustal contamination (Bailie et al., 2012; Fig. S6a).

    The rhyolites do not form distinctive trends on any maficity plots (Fig. 5), apart from relatively well-defined negative correlations of Si and K with maficity. This suggests that PAE probably did not play a significant role in determining the compositional variation in the Koras Group, although, again, more samples and more detailed study is required.

  • The western margin of the Kaapvaal Craton developed as a passive margin. Initially sedimentation was fluvial in the form of braided rivers, but with rifting, a shallow open sea developed, on the margin of which shallow marine sediments were deposited (the bulk of the Keis Supergroup; Van Niekerk and Beukes, 2019).

    At ~1.35 Ga subduction was initiated, leading to development of a volcanic arc, represented by the Areachap Group, on the western margin of the Kaapvaal Craton due to eastward- directed subduction (Bailie et al., 2010; Cornell and Pettersson, 2007; Pettersson et al., 2007; Geringer et al., 1986). The Areachap arc developed between ~1.3 and 1.25 Ga, having a greater continental signature toward the north and was more juvenile toward the south. Closure of the Areachap Sea commenced at ~1.22 Ga and likely continued until ~1.15-1.13 Ga giving rise to extensive eastward-directed thrusting (Van Niekerk and Beukes, 2019, and references therein). The sea which had developed due to extensional forces was closed leading to eastward-directed thrusting onto the western margin of the Kaapvaal Craton, and westward-directed thrusting onto the Bushmanland Subprovince along the Hartbees River thrust toward the west with the Areachap and Kakamas domains caught in between. Accretion of the Areachap arc onto the western margin of the Kaapvaal Craton likely led to widespread magmatism giving rise to the pre- to syn-tectonic granitic gneisses of the Areachap and Kakamas Domains (Bailie et al., 2017; Bial et al., 2015) and upper amphibolite facies M2 metamorphism reaching its peak at ~1.16-1.15 Ga (Cornell and Pettersson, 2007; Pettersson et al., 2007).

    The lateral escape of the Areachap and Kakamas domains was brought about by prolonged compression which resulted in a releasing bend in the eastern NMP (Sithole, 2013; van Bever Donker, 1991). Potential slab roll-back led to the opening of a rift along the former back-arc region of the Areachap arc, along and adjacent to the Wilgenhoutsdrif Group (Van Niekerk and Beukes, 2019) and the development of rift-related basins. The rift basins were infilled with coarse, immature, locally derived clastic sediments derived from the rift shoulders (Moen, 1987). These volcanic rocks made use of faults along the Brakbosch-Trooilapspan fault system for their propagation and emplacement.

    The subsequent volcanism occurred due to lithospheric thinning, which was likely caused by continued rifting and collapse after ocean closure (Fig. 14). The thinning caused partial melting of the subduction-influenced mantle giving rise to medium- to high-K tholeiitic mafic magmas which then rose. Some crustal assimilation along with crystal fractionation of the mafic magmas during ascent occurred to give rise to rhyolitic porphyries of the lower volcanic succession at ~1.13 Ga (Bailie et al., 2012) (seen in the high concentrations of the LIL and HFS elements) during the waning of the peak metamorphic conditions and prior to, and at the onset of the main D2 deformation phase. This is evident from the similar trace element characteristics of both basaltic andesites and rhyolitic porphyries, and their εNd(t) values that vary from positive to weakly negative.

    Figure 14.  Geotectonic setting plots for the granites of the Keimoes Suite and rhyolitic porphyries of the Koras Group, eastern NMP. (a) The Y+Nb vs. Rb plot (after Pearce et al., 1984). (b) R1-R2 granite tectonic setting plot of Batchelor and Bowden (1985). (c) The Y-Nb-Ce ternary A-type granite classification plot of Eby (1992). (d)-(f) The granite tectonic discrimination plots of Maniar and Piccoli (1989). WPG. Within-plate granite; VAG. volcanic-arc granite; ORG. ocean ridge granite; syn-COLG. syn-collisional granite; IAG. island arc granitoids; CAG. continental arc granitoids; CCG. continental collision granitoids; POG. post-orogenic granitoids; RRG. rift-related granitoids; CEUG. continental epeirogenic uplift granitoids; OP. ocean plagiogranites; IAG, CAG, CCG, POG are orogenic granitoids; RRG, CEUG, OP are anorogenic granitoids.

    Subsequent melting occurred at ~1.1 Ga. This was likely unrelated to any plume activity (e.g., De Kock et al., 2014), but was likely due to continued and/or renewed rifting (slab break-off?). The voluminous magmatism was accompanied by lower grade M3 metamorphism (at ~1.12-1.08 Ga). Mantle melting gave rise to the basaltic andesites of the upper succession of the Koras Group that intruded the lower to mid-crust bringing about copious melting and the generation of the various Keimoes Suite granites. The emplacement of these mafic magmas at various crustal levels, as now exposed at the Oranjekom Complex in the Kakamas Domain, led to local heating and crustal melting giving rise to the generation of crust-derived felsic magmas, represented by the rhyolitic porphyries of the upper succession of the Koras Group, as denoted by their negative εNd(t) values and strongly enriched trace element concentrations relative to PM. Crustal magma chambers experienced renewed magma input, as evidenced in the resorbed phenocrysts in the rhyolitic porphyries (Figs. 3b, 3d). The Keimoes Suite ended with the Friersdale charnockite, the youngest and most voluminous member, centred along the BoSZ suggesting that the latter formed the locus and conduit of the late- to post-tectonic intrusive magmatism. The post-tectonic Friersdale charnockite made use of pre-existing F1 and F2 hinge zones for its intrusion (Mathee, 2017).

    The granites of the Keimoes Suite, emplaced at depths of > 15 km, and the Areachap Group, which experienced upper amphibolite facies metamorphic conditions, are adjacent to the bimodal volcanics of the Koras Group and mafic volcanics of the Wilgenhoutsdrif Group, both of which were only subjected to greenschist facies metamorphism. Continued south-westward- directed movement of the Kaapvaal Craton into the NMP subsequent to ~1.11-1.08 Ga created flower structures associated with dextral strike-slip movement (Jacobs et al., 1993; van Bever Donker, 1991). This occurred between 1 024 and 1 018 Ma (D4 deformation; Mathee, 2017). Substantial exhumation and vertical displacement (of ~12-20 km) must have occurred to create this juxtaposition, giving rise to a positive pop-up flower structure. This uplift was focussed along prominent thrusts, which were subsequently reactivated as shear zones. Examples of reactivated faults include the regionally significant Blauwboschpan and Brakbosch faults, and Trooilapspan and Neusspruit shear zones, the former two forming the present-day boundaries of the Koras Group to the east and west respectively. The Keimoes Suite was exposed due to uplift in a positive flower structure. By contrast, the Koras Group is preserved within a negative flower structure due to the development of graben-like structures.

    The P-T paths do not suggest pressures above 6-8 kbar (equivalent of 22-30 km depth for a geotherm of ~25 ℃/km) for the Areachap Group (e.g., Bachmann et al., 2015), suggesting thicknesses of no more than 50 km for the western margin of the Kaapvaal Craton. Closure of a narrow ocean, represented by the Areachap Sea, and accretion of this arc onto the thinned western margin of the Kaapvaal Craton would have caused thickening of no more than 50 km, rather than to more than 70 km. The latter would be the case in a Himalayan-style continental collision scenario, which is not envisioned here. Thrust stacking is, therefore, a more likely scenario compared to crustal thickening. The positive flower structure of the Kakamas and Areachap Domains (Fig. 2), with eastward- directed thrusting into the Kheis terrane (Van Niekerk and Beukes, 2019) and westward-directed thrusting along the Hartbees River Thrust of the Kakamas Domain over the BSP, also suggests closure of a narrow ocean.

  • The granites of the Keimoes Suite formed largely by melting of crustal material of mainly mafic igneous sources, with a minor sedimentary component, as seen in their isotopic and whole-rock major- and trace-element data. The spread of εNd(t) values and variable model ages suggest derivation from variable degrees of mixing between predominantly Paleoproterozoic crustal material and juvenile Mesoproterozoic sources. In some granites a suggestion of a mantle source component is present. Mafic mantle-derived melts also acted as a source of advective heat and as potential participants in hybrid magma generation. This is seen in the low positive εNd(t) values near zero for some granites, such as the Friersdale charnockite, but assimilation of variably aged crustal material explains the large range in initial Sr isotope values. Magmatism was due to transtensional rifting and asthenospheric upwelling.

    Following closure of the Areachap Sea, late Namaquan transcurrent movements led to decompression melting in the mantle, giving rise to the intrusion of mafic bodies, preferentially into the lower to mid-crust. These intrusions transferred heat, leading to melting of this crust, likely of a multi-layered nature involving components of mostly Paleoproterozoic-aged crust of the NMP interspersed with Mesoproterozoic arc material and lesser Archean to Paleoproterozoic material of the Kaapvaal Craton. This melting then gave rise to the mixed hybrid magmas of the Keimoes Suite. The granitoids have mostly I-type characteristics, but also exhibit certain geochemical features more typical of A-type granites reflecting derivation from, and emplacement within a post-kinematic rift-related tectonic environment.

    The lower succession of the Koras Group was derived by partial melting of subduction-influenced metasomatized, enriched mantle, with fractional crystallisation giving rise to the rhyolitic porphyries. By contrast, the upper succession rhyolites were derived by partial melting of Paleoproterozoic-aged crustal material. The bimodal volcanism was derived by rifting associated with and subsequent to Areachap Sea closure and arc accretion on the western margin of the Kaapvaal Craton due to continued southwest-directed movement of the latter into the NMP.

    More detailed, in-depth studies of both the Keimoes Suite granitoids and rhyolitic porphyries of the Koras Group are necessary to clearly elucidate their sources and petrogenetic processes influencing their compositional variation. This review highlights the role of post-kinematic transtensional rifting in bringing about lithospheric thinning and partial melting giving rise to mafic magmas that can fractionate to give rise to felsic magmas. Intrusion of mafic magmas into the lower crust can also generate copious amounts of felsic crustal melts in such a post-kinematic environment.

  • Thanks to Dr. Petrus le Roux of the Department of Geological Sciences, University of Cape Town for analysing the new set of Rb-Sr and Sm-Nd isotopes for the Koras Group volcanic rocks. Richard Harrison and Janine Botha of the Department of Earth Sciences, UWC are thanked for assistance with crushing and milling of samples for isotopic analysis. UWC International Conference Senate Research funds, which allowed RB to attend the IXth Hutton Symposium on Granites and Related Rocks in Nanjing, China in 2019, are gratefully acknowledged. Reviews by two anonymous reviewers, and a constructive review by special volume editor, Valdecir de Assis Janasi, are gratefully acknowledged. The final publication is available at Springer via https://doi.org/10.1007/s12583-021-1462-7.

    Electronic Supplementary Materials: Supplementary materials containing analytical techniques (ESM1), figures (ESM2), Table S1 (ESM3) are available in the online version of this article at https://doi.org/10.1007/s12583-021-1462-7.

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