Metamorphism of the Palaeoproterozoic Magondi mobile belt north of Karoi, Zimbabwe

The Palaeoproterozoic Magondi mobile belt flanks the Zimbabwe Archaean Craton to the northwest. The belt is composed of metamorphosed sedimentary, volcanic and volcaniclastic rocks associated with quartzofeldspathic gneisses intruded by granitoids, some charnockitic, in the high-grade part of the belt. The belt is metamorphosed from low-grade greenschist-facies in the south and middle to upper amphibolite-facies in the north. Granulite-facies rocks are developed in the extreme north and northwestern part of the belt. Garnet-biotite geothermometry in metapelites indicates that temperatures increase from 590-600°C in the mid-amphibolite-facies through 640-690°C in the upper amphibolite-facies terrain and up to 730°C in the granulite-facies areas. In the granulite-facies terrains, garnet-biotite temperatures are similar to temperatures calculated using garnet-cordierite, garnet-clinopyroxene and, to some extent, two-feldspar geothermometers. Pressures calculated with the GASP barometers are 6 ± 1 kbar for both upper amphibolite- and granulite-facies, suggesting that the granulite-amphibolite-facies transition is primarily isobaric. The calculated pressures for granulites do not support models which invoke the formation of granulites by continent-continent collision. Instead the P-T data suggest that the Magondi mobile belt granulites were formed in a region of high heat flow, with heat possibly being supplied by deep-seated plutons

Pan-African structures and metamorphism in the Makuti group, north-west Zimbabwe

The Makuti Group is composed of arkosic gneisses, amphibolites, marbles, calc-silicate rocks and minor pelitic schists and forms part of the Pan-African supracrustal sequence within the Zambezi belt in north-west Zimbabwe. The group was affected by two generations of folds (Dz1 and DZ2). The DZ1 folds are isoclinal and plunge gently to the NW and SE. Dz2 folds are upright and refold Dz1 folds on an almost coincident axis giving rise to a type III fold interference pattern. Metamorphism within the Makuti Group is syn-DZ1 and pre-DZ2. Temperatures calculated from garnet-biotite (grt-bt) geothermometers increase to the south from 538±49°C in upper greenschist/low amphibolite facies, to 595±46°C for mid-amphibolite facies and 718±30°C for the upper amphibolite grade. Upper amphibolite grt-bt temperatures are supported by amphibole-plagioclase and garnet-hornblende (grt-hbl) temperatures. Compositional zoning is reflected by high anorthite content in plagioclase rims (An24–30) relative to the cores (An10–16) and by high Xgro+sps cores and low Xpyp+alm cores relative to the rims in garnets. Temperatures calculated using garnet and plagioclase rim compositions are 50°C higher than core temperatures. Thus growth zoning in plagioclase and garnet preserve a portion of the prograde P-T path. The metamorphic and structural data concur with the southward thrusting of the Zambezi belt

Geochemistry of amphibolites and quartzofeldspathic gneisses in the Pan-African Zambezi belt, northwest Zimbabwe: Evidence for bimodal magmatism in a continental rift setting

The Zambezi belt separates the Congo and Kalahari cratons in southern Africa and is a key part of the regional Pan-African orogenic framework related to amalgamation of Gondwana in the Neoproterozoic-early Palaeozoic. Several thick, probably correlative, supracrustal sequences are preserved in the belt in Zimbabwe and Zambia. The Makuti Group, a major assemblage of supracrustal rocks within the belt in northwestern Zimbabwe, consists dominantly of amphibolite-facies quartzofeldspathic gneisses of supracrustal origin interlayered with horizons of marble, calc-silicate rock, quartzite, and pelitic schist. Numerous thick, concordant amphibolites derived from mafic sills and/or lava flows are intercalated within the supracrustal sequence. Major- and immobile trace-element geochemistry indicates dominantly tholeiitic affinities for the amphibolites, with some samples showing transitional to alkaline affinities. High-field-strength trace-element contents and LREE-enriched patterns are consistent with a within-plate setting for the mafic rocks. Major- and trace-element data show the quartzofeldspathic gneisses to be dominantly of igneous origin. Their protoliths are inferred to be mainly peralkaline rhyolites and trachytes. High Zr contents (up to 1500 ppm) are a diagnostic signature for these rocks. The bimodal nature of the magmatism and the abundance of peralkaline felsic rocks point to a continental rift zone as the setting for the Makuti Group. Other examples of pre-orogenic, mafic or bimodal magmatic rocks are found in the Zambezi belt elsewhere along strike in Zambia and Zimbabwe. All these rocks are inferred to represent widespread, rift-related magmatism associated with initiation of the depositional basin within which the Neoproterozoic sequences of the Zambezi belt accumulated

Tectonic evolution of the Zambezi orogenic belt: geochronological, structural, and petrological constraints from northern Zimbabwe

In southern Africa, the Zambezi belt forms the eastern part of a transcontinental orogenic system that connects with the East African orogen and records interactions between the Congo and Kalahari cratons during collisional assembly of the Gondwana supercontinent at the end of the Neoproterozoic. We report the results of reconnaissance studies in the eastern part of the Zambezi belt in northern Zimbabwe, where thick-skinned thrusting has inverted a crustal column comprising a Neoproterozoic supracrustal sequence tectonically overlain by rocks exhumed from the lower crust. An extensive felsic gneiss with A-type geochemical signatures that is inferred to represent a metarhyolitic unit within the supracrustal sequence has yielded a U–Pb zircon crystallization age of ca. 795 Ma, which helps to constrain the timing of supracrustal deposition in this part of the Zambezi belt. The dominant ductile structures in the area record south-vergent thrusting, during which the supracrustal rocks underwent prograde, amphibolite-facies metamorphism as they were overridden by a crystalline thrust stack partly preserving high-pressure granulite-facies assemblages. Complex U–Pb zircon geochronological results for a layered, metagabbroic to meta-anorthositic intrusive complex in the upper part of the thrust stack are interpreted to indicate a minimum igneous crystallization age of ca. 1830 Ma. Polydeformed orthogneisses in the lower part of the thrust stack were derived from granitoid protoliths with U–Pb zircon crystallization ages of ca. 1050 and 870 Ma. The younger granites are inferred to be parts of an A-type magmatic province that can be recognized throughout the Zambezi belt. U–Pb zircon and titanite geochronological results from the layered complex and the underlying orthogneisses are interpreted to record metamorphism associated with thrust emplacement at ca. 550–530 Ma. This is consistent with isotopic age data from adjacent areas, indicating that a major orogenic event affected the Zambezi belt in this time frame, during assembly of central Gondwana

Tectonic evolution of the Zambezi orogenic belt: geochronological, structural, and petrological constraints from northern Zimbabwe

In southern Africa, the Zambezi belt forms the eastern part of a transcontinental orogenic system that connects with the East African orogen and records interactions between the Congo and Kalahari cratons during collisional assembly of the Gondwana supercontinent at the end of the Neoproterozoic. We report the results of reconnaissance studies in the eastern part of the Zambezi belt in northern Zimbabwe, where thick-skinned thrusting has inverted a crustal column comprising a Neoproterozoic supracrustal sequence tectonically overlain by rocks exhumed from the lower crust. An extensive felsic gneiss with A-type geochemical signatures that is inferred to represent a metarhyolitic unit within the supracrustal sequence has yielded a U–Pb zircon crystallization age of ca. 795 Ma, which helps to constrain the timing of supracrustal deposition in this part of the Zambezi belt. The dominant ductile structures in the area record south-vergent thrusting, during which the supracrustal rocks underwent prograde, amphibolite-facies metamorphism as they were overridden by a crystalline thrust stack partly preserving high-pressure granulite-facies assemblages. Complex U–Pb zircon geochronological results for a layered, metagabbroic to meta-anorthositic intrusive complex in the upper part of the thrust stack are interpreted to indicate a minimum igneous crystallization age of ca. 1830 Ma. Polydeformed orthogneisses in the lower part of the thrust stack were derived from granitoid protoliths with U–Pb zircon crystallization ages of ca. 1050 and 870 Ma. The younger granites are inferred to be parts of an A-type magmatic province that can be recognized throughout the Zambezi belt. U–Pb zircon and titanite geochronological results from the layered complex and the underlying orthogneisses are interpreted to record metamorphism associated with thrust emplacement at ca. 550–530 Ma. This is consistent with isotopic age data from adjacent areas, indicating that a major orogenic event affected the Zambezi belt in this time frame, during assembly of central Gondwana

Mesoproterozoic intraplate magmatism in the Kalahari Craton : a review

The Kalahari Craton was initially stabilized following cessation of Palaeoproterozoic orogenesis in southern Africa at ca. 1.8 Ga. Subsequent Mesoproterozoic intraplate magmatism at ca. 1.4–1.35 Ga formed a series of alkaline and carbonatitic complexes in the southern part of the craton. Original volcanic structures are partly preserved in some of the complexes, and a variety of intrusive rocks (e.g., quartz syenite, nepheline syenite, pyroxenite, ijolite, carbonatite) are present. The Premier kimberlite cluster was emplaced in the same region at ca. 1.2 Ga, but available geochronology indicates that the main alkaline magmatism preceded 1.2–1.0 Ga orogenesis in the Namaqua–Natal–Maud Belt along the southern craton margin. Another, more extensive intraplate magmatic event at ca. 1.1 Ga formed the Umkondo Igneous Province, which is recognized over an area of 2.0 · 106 km2 on the Kalahari Craton, including a detached fragment now located in Antarctica. Much of the province comprises high-level mafic intrusions, but erosional remnants of basalt lava piles and bimodal basalt/rhyolite assemblages are also present. Most of the mafic rocks are continental tholeiites, but trace-element geochemistry reveals distinct subgroups that cannot be related by crustal-level assimilation/fractional crystallization processes or by partial melting of a uniform mantle source. Geochronological and palaeomagnetic data indicate that enormous volumes of tholeiitic magma were emplaced within the province in a narrow time frame at ca. 1112–1106 Ma, which is inferred to record uprise of a mantle plume behind the Namaqua–Natal–Maud Belt

Mesoproterozoic intraplate magmatism in the Kalahari Craton: A review

The Kalahari Craton was initially stabilized following cessation of Palaeoproterozoic orogenesis in southern Africa at ca. 1.8 Ga. Subsequent Mesoproterozoic intraplate magmatism at ca. 1.4–1.35 Ga formed a series of alkaline and carbonatitic complexes in the southern part of the craton. Original volcanic structures are partly preserved in some of the complexes, and a variety of intrusive rocks (e.g., quartz syenite, nepheline syenite, pyroxenite, ijolite, carbonatite) are present. The Premier kimberlite cluster was emplaced in the same region at ca. 1.2 Ga, but available geochronology indicates that the main alkaline magmatism preceded 1.2–1.0 Ga orogenesis in the Namaqua–Natal–Maud Belt along the southern craton margin. Another, more extensive intraplate magmatic event at ca. 1.1 Ga formed the Umkondo Igneous Province, which is recognized over an area of ∼2.0 × 106 km2 on the Kalahari Craton, including a detached fragment now located in Antarctica. Much of the province comprises high-level mafic intrusions, but erosional remnants of basalt lava piles and bimodal basalt/rhyolite assemblages are also present. Most of the mafic rocks are continental tholeiites, but trace-element geochemistry reveals distinct subgroups that cannot be related by crustal-level assimilation/fractional crystallization processes or by partial melting of a uniform mantle source. Geochronological and palaeomagnetic data indicate that enormous volumes of tholeiitic magma were emplaced within the province in a narrow time frame at ca. 1112–1106 Ma, which is inferred to record uprise of a mantle plume behind the Namaqua–Natal–Maud Belt

The precambrian mafic magmatic record, including large igneous provinces of the kalahari craton and its constituents : A paleogeographic review

The study of Precambrian dyke swarms, sill provinces and large igneous provinces on the Kalahari craton in southern Africa has expanded greatly since the pioneering work initiated almost four decades ago. The main contributors to this progress have been a large number of precise U–Pb crystallization ages of mafic rocks, published in a number of recent papers. This information is compiled here into a series of maps that provide a nearly 3 billion year intraplate magmatic record of the Kalahari craton and its earlier constituents, the proto-Kalahari, Kaapvaal and Zimbabwe cratons. We also review their possible paleogeographic relations to other cratons or supercontinents. This review provides a more accessible overview of individual magmatic events, and mostly includes precise U–Pb ages of mafic dykes and sills, some of which can be linked to stratigraphically well-constrained volcanic rocks. The extrusion ages of these volcanic units are also starting to be refined by, among others, in situ dating of baddeleyite. Some mafic dyke swarms, previously characterized entirely on similarity in dyke trends within a swarm, are found to be temporally composite and sometimes consist of up to three different generations. Other mafic dyke swarms, with different trends, can now be linked to protracted volcanic events like the stratigraphically well preserved Mesoarchean Nsuze Group (Pongola Supergroup) and Neoarchean Ventersdorp Supergroup. Following upon these Archean events, shorter-lived Proterozoic large igneous provinces also intrude the Transvaal Supergroup, Olifantshoek Supergroup and Umkondo Group, and include the world’s largest layered intrusion, the Bushveld Complex. Longer-lived late Paleoproterozoic magmatic events are also preserved as mafic intrusions and lava units within the Waterberg and Soutpansberg groups as well as the granitic basement. Many gaps in our knowledge of the Precambrian mafic record of the Kalahari craton remain, but further multi-disciplinary studies combining the latest advances in U–Pb geochronology and both paleomagnetism and geochemistry will help solve the Precambrian paleogeographic puzzle