Hubert Munyanyiwa

U-Pb zircon age for the Umkondo dolerites, eastern Zimbabwe: 1.1 Ga large igneous province in southern Africa–East Antarctica and possible Rodinia correlations

We report a U-Pb zircon age of 1105 ± 2 Ma for the extensive Umkondo dolerites in eastern Zimbabwe, which are part of a large igneous province that can be traced over much of southern Africa and originally contiguous parts of East Antarctica. Other members of the province include widespread tholeiitic intrusions in Botswana and South Africa, bimodal volcanic rocks in Botswana and Namibia, and dolerites and flood basalts in Dronning Maud Land, Antarctica. Available data indicate that substantial parts of the province were emplaced in a restricted interval at ca. 1.1 Ga and originated from a large-scale mantle thermal anomaly inboard of a coeval continental-margin orogen. Striking similarities in age and tectonic setting between the Umkondo igneous province and widespread 1.1 Ga within-plate magmatism in Laurentia are consistent with reconstructions of the early Neoproterozoic Rodinia supercontinent that place southern Africa and Dronning Maud Land off the southern tip of Laurentia.

Geologic evolution of the neoproterozoic Zambezi orogenic belt in Zambia

The Neoproterozoic Zambezi belt links with the Mozambique belt, Lufilian arc, and the inland branch of the Damara belt within the regional Pan-African tectonic framework of southern Africa. The belt contains a thick supracrustal sequence deposited on older sialic basement and penetratively deformed with it during Neoproterozoic (Pan-African) orogenesis. In Zambia, where the entire width of the orogen is exposed, local bimodal volcanic rocks at the base of the sequence are overlain by psammites and pelites, which in turn are succeeded by extensive carbonate and calc-silicate rocks. Abundant scapolite in metamorphic assemblages within the belt is taken as evidence for the original presence of evaporites. The nature of the rock types and the inferred stratigraphic sequence are consistent with deposition in an intracontinental rift basin invaded by marine waters. Available isotopic age brackets for the timing of supracrusta deposition show that the basin developed between 880 nad 820 Ma. Main-phase deformation in the belt involved both transcurrent shearing and south- to southwest-vergent thrusting and was associated with predominantly amphibolite-facies regional metamorphism. Mineral assemblages throughout much of the belt in Zambia, together with limited thermobarometric data, indicate typical Barrovian-type intermediate P/T conditions during metamorphism. Eclogites and other high-pressure metamorphic assemblages in parts of the belt, however, provide evidence that significant crustal thickening occurred, presumably in relation to thrusting. Reworked basement and syntectonic granite were subjected to extensive mylonitization related to strike-slip and oblique, reverse-slip shearing. The major orogenic event is dated at c. 820 Ma, based on an igneous age for a sheet-like, syntectonic batholith injected into a transcurrent shear zone within the central part of the belt. Pan-African orogenesis along the Zambezi-Lufilian-Damara trend was diachronous and records closure of intracratonic basins in the Zambezi belt and Lufilian arc, with evidence for the involvement of oceanic lithosphere present only in the Damara belt.

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.

GEOCHEMICAL STUDY OF THE UMKONDO DOLERITES AND LAVAS IN THE CHIMANIMANI AND CHIPINGE DISTRICTS (EASTERN ZIMBABWE) AND THEIR REGIONAL IMPLICATIONS

Abstract The Umkondo Group is a supracrustal sequence cropping out in eastern Zimbabwe in the Nyanga, Chimanimani and Chipinge Districts. In these areas the sequence has been divided into a weakly metamorphosed and deformed unit of argillaceous, arenaceous and carbonate rocks (Zimbabwe facies) in the west, and a strongly deformed and medium- to high-grade metamorphosed sequence of mainly quartzites and metapelites (Mozambique facies) in the east. The two sequences were tectonically juxtaposed during the Neoproterozoic Pan-African Mozambique Belt deformation. The Zimbabwe facies sedimentary rocks are intruded by extensive dolerite sills and minor interlayered basalts flows. The mafic rocks are sub-alkaline continental tholeiites. They have low mg numbers associated with low Cr, Cu, Ni and Co, which indicate that the parental magma underwent some differentiation processes en route to the surface. They are LREE enriched with ( La Yb N = 5.0–7.6 , high Ce/Yb (>10) and La Nb (>0.5) values, and exhibit troughs at Nb, Sr, Ti and P on a MORB-normalised, multi-element spider diagram. These chemical characteristics, together with the large areal extent of the Umkondo dolerites and basalts, suggest that the Umkondo mafic igneous suite was once widespread and formed part of a continental flood basalt province. This is supported by the depositional environment (shallow water platform type setting) of the sedimentary sequence into which the mafic rocks were emplaced. The widespread occurrence of the Umkondo igneous event is further supported by the similarity in palaeomagnetic poles of a number of mafic units in southern Africa.

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.

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

Abstract 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 marbles and calc-silicate rocks in the Pan-African Zambezi belt, Zambia

Abstract Amphibolite-facies marbles and calc-silicate rocks make up an important part of the supracrustal sequence in the Pan-African Zambezi belt of Zambia. Major- and trace-element geochemistry shows that dolomite marbles in the sequence were derived from nearly pure dolostones, calcite marbles were derived from impure limestones, calc-silicate schists were derived from calcareous shales, and massive calc-silicate rocks were derived from calcareous sandstones. A pronounced compositional banding in abundant calc-silicate gneisses, now folded and transposed, reflects original interbedding of carbonate-rich and pelite-rich sediments. Trends in major and trace elements in the calcite marbles and calc-silicate rocks are controlled largely by original variations in the amount of silicate phases admixed with CaCO3 during deposition, and these compositional trends have been preserved into the amphibolite facies. Dolomite was present in significant amounts only in the protoliths to the dolomite marbles, and played little or no role in the evolution of the calcite marbles and calc-silicate rocks. Correlation of most major and trace elements with Al2O3 indicates that an alumina-bearing phase made up a major fraction of the material admixed with CaCO3. The use of the Niggli parameter al-alk reveals that the Al2O3 was carried largely by clays and detrital micas. Other components contributed by the phyllosilicates include SiO2, TiO2, FeO, Na2O, K2O, Ba, Rb, Cr, Ni and Zn. Sr was added with the carbonate fraction during deposition, and detrital zircon may have been an important carrier of Zr and Y. Some of the massive calc-silicate rocks are geochemically distinct from the other calc-silicate rocks, suggesting that admixture of other detrital phases, such as quartz and feldspar, played a more important role in governing element contents in these rocks. Evidence of significant alkali metasomatism is shown only by some of the massive calc-silicate rocks, in which Na2O replaced K2O.

Mineral assemblages in calc-silicates and marbles in the Zambezi mobile belt: their implications on mineral-forming reactions during metamorphism

Abstract Abundant calc-silicate rocks and marbles form a major proportion of the Katangan supracrustal sequence in central and southern Zambia. The rocks were penetratively deformed and metamorphosed largely to amphibolite facies conditions during the Late Proterozoic Pan-African events. In S Zambia, mineral assemblages formed during the peak of amphibolite facies metamorphism include dolomite, dolomite + diopside, tremolite + calcite + dolomite, calcite + quartz + tremolite + diopside, rutile + sphene + calcite + quartz, phlogopite + dolomite + calcite, scapolite + plagioclase + quartz, grossularite + epidote + calcite + quartz + diopside. Secondary minerals formed during retrogressive green-schist facies include zoisite, chlorite, muscovite, quartz and calcite. Based on these primary/secondary minerals and on reaction textures, it is possible to formulate the nature of mineral reactions occurring during metamorphism. Comparison of these reactions with the experimentally calibrated reactions and the nature of fluid phase attending metamorphism is presented. This shows that grossular-rich garnet, zoisite, diopside and sphene probably crystallized in H2O - rich phase (XCO2 - 0.0 – 0.4) in the calcsilicate rocks. In the dolomite marbles, diopside locally crystallized in CO2 - rich fluid phase (XCO2 > 0.9) as is rutile in the calcite marbles.