Zimbabwe Craton Research Articles

Neoarchaean tectonic evolution of the Zimbabwe Craton

Abstract An overview is presented of the field relations, age data and geochemical characteristics of the Neoarchaean granites and greenstones of the Zimbabwe Craton, southern Africa. A major tectono-magmatic event at c. 2.7 Ga produced two distinct greenstone successions. One succession is reminiscent of rift- or back-arc environments and is associated with an old continental fragment. A second succession is indicative of arc magmatism and is associated with juvenile crust. Both were affected by a major accretionary event that, in an apparent sense, swept across the craton between 2.68 and 2.60 Ga. During this 80 Ma time period, concomitant late volcanism, regional deformation, the development of syntectonic sedimentary successions in foreland-type basins, and late syntectonic plutonism took place in selected shear-zone-bounded tectonic domains over limited periods of time (<10–20 Ma). Deformation led to isostatically stable, 30–40 km thick continental crust, without significant exhumation of high-pressure rocks, suggesting that lithospheric shortening was accommodated independently in a rheologically strong upper and weak lower crust. Deformation was followed by pan-cratonic crustal melting and strike-slip shear motions, and led to stabilization of the crust at 2575 Ma, heralded by the emplacement of the Great Dyke.

Configuration of Columbia, a Mesoproterozoic Supercontinent

A supercontinent, here named Columbia, may have contained nearly all of the earth's continental blocks at some time between 1.9 Ga and 1.5 Ga. At that time, eastern India, Australia, and attached parts of Antarctica were apparently sutured to western North America, and the eastern margin of North America, southern margin of Baltica/North China, and western margin of the Amazon shield formed a continuous zone of continental outbuilding. Fragmentation began at ∼1.6 Ga, when rifting occurred in North China. Rifting continued until about 1.4 Ga in most of Columbia, and a similar age of rifting north of the Zimbabwe craton of southern Africa suggests that an entire continental block stretching from Australia to South Africa separated from Columbia at this time. Further separation of North America from South America/Africa and rotation of the different blocks ultimately resulted in their reattachment during the Grenville orogeny to form the supercontinent Rodinia.

Precise U–Pb mineral ages, Rb–Sr and Sm–Nd systematics for the Great Dyke, Zimbabwe—constraints on late Archean events in the Zimbabwe craton and Limpopo belt

U–Pb dating of zircon and rutile from bronzitites of the P1 pyroxenite layer of the Great Dyke precisely constrains the crystallization age of this part of the intrusion to 2575.4±0.7 Ma. Whole rock Rb–Sr and Sm–Nd data of various rock types sampled along the entire length of the Great Dyke record inhomogeneous initial isotope ratios, and also later (<1.5 Ga) disturbances of the Rb–Sr and Sm–Nd isotope systems. The 2575.4±0.7 Ma emplacement age of the Great Dyke is ≈120 Ma older than assumed until recently, and calls for new interpretations of the crustal development of the Zimbabwe craton. Close temporal links between the intrusion of the Great Dyke and the emplacement of late Archean granitoids (Chilimanzi and Razi suites) of the Zimbabwe craton are indicated by the new precise age data. The Chilimanzi and Razi suites of granitoids form at least two sub-suites, which can be described as pre- and post-Great Dyke in age. The Great Dyke age now falls within the range of ages for tectonic events in the Limpopo belt including granitoid magmatism, metamorphism, and thrusting of the Northern Marginal Zone over the Zimbabwe craton. A west-to-east diachroneity in both thrusting and crustal stabilization is suggested by the observations that the Great Dyke cuts across the thrust in the west, but syn-tectonic granitoids that are younger than the Great Dyke are deformed by the thrusting in the east. Intrusion of the Great Dyke cannot be linked to collision of the Zimbabwe and Kaapvaal cratons. The overlap in ages of intrusion of the Great Dyke and late Archean events in the Zimbabwe craton shows that Archean crust was cratonized shortly after large-scale melting and granite intrusion.

Tectospheric structure beneath southern Africa

P‐wave and S‐wave delay times from the broadband data of the southern Africa seismic experiment have been inverted to obtain three‐dimensional images of velocity perturbations in the mantle beneath southern Africa. High velocity mantle roots appear to extend to depths of at least 250 km, and locally to depths of 300 km beneath the Kaapvaal and Zimbabwe cratons. Thick roots are confined to the Archean cratons, with no evidence for similar structures beneath the adjacent Proterozoic mobile belts. The Kaapvaal craton was modified ca. 2.05 Ga by the Bushveld magmatic event, which affected a broad swath of cratonic mantle beneath and to the west of the exposed Bushveld Complex. The mantle beneath the extended Bushveld province is characterized by seismic velocities lower than those observed in regions of undisturbed cratonic mantle. The mantle beneath the Limpopo Belt, an Archean collisional zone sandwiched between the Kaapvaal and Zimbabwe cratons, exhibits a cratonic signature.

Crustal structure beneath southern Africa and its implications for the formation and evolution of the Kaapvaal and Zimbabwe cratons

The formation of Archean crust appears to involve processes unique to early earth history. Initial results from receiver function analysis of crustal structure beneath 81 broadband stations deployed across southern Africa reveal significant differences in the nature of the crust and the crust‐mantle boundary between Archean and post‐Archean geologic terranes. With the notable exception of the collisional Limpopo belt, where the crust is thick and the Moho complex, the crust beneath undisturbed Archean craton is typically thin (∼35–40 km), unlayered, and characterized by a strong velocity contrast across a relatively sharp Moho. This crustal structure contrasts markedly with that beneath post‐Archean terranes and beneath Archean regions affected by large‐scale Proterozoic events (the Bushveld complex and the Okwa/Magondi belts), where the crust tends to be relatively thick (∼45–50 km) and the Moho is complex.

Mantle deformation beneath southern Africa

Seismic anisotropy from the southern African mantle has been inferred from shear‐wave splitting measured at 79 sites of the Southern African Seismic Experiment. These data provide the most dramatic support to date that Archean mantle deformation is preserved as fossil mantle anisotropy. Fast polarization directions systematically follow the trend of Archean structures and splitting delay times exhibit geologic control. The most anisotropic regions are Late‐Archean in age (Zimbabwe craton, Limpopo belt, western Kaapvaal craton), with delay times reduced dramatically in off‐craton regions to the southwest and Early‐Archean regions to the southeast. While thin lithosphere can account for weak off‐craton splitting, small or vertically incoherent anisotropy is a more likely explanation for the Early‐Archean region. We speculate that this difference in on‐craton anisotropic structure is the result of two different continent‐forming processes operating.

Tabular intrusion and folding of the late Archaean Murehwa granite, Zimbabwe, during regional shortening

The Murehwa granite belongs to the late Archaean Chilimanzi suite of granites that occupy more than 50% of the Zimbabwe craton. The granite has compositional layers on metre to tens of metre scales with generally shallow dips, and dip directions towards the centre of the granite near its margins. The layering is systematically folded about sub-horizontal east–west axes. Microcline phenocrysts are generally strongly aligned parallel to the layering, but locally crosscut the layering because they are axial planar to the folds. Crystals are not significantly deformed: fabrics are thus magmatic. The granite was intruded in a tabular sheet that may have been only about 6 km thick and fed by dykes, contrary to previous concepts of diapirism/ballooning for some Archaean granites of the Zimbabwe craton. The systematic orientation of magmatic folds and axial planar fabrics demonstrate that the granite was intruded during regional north-south shortening. This shortening is consistent with the orientation of cratonic-scale fractures that were intruded by the Great Dyke within 25 Ma of the intrusion of the Murehwa granite. The granite may have intruded in a relatively short interval of less than 1–26 Ma, within the longer duration of the entire Chilimanzi granite event.

Orogenic gold and geologic time: a global synthesis

AbstractOrogenic gold deposits have formed over more than 3 billion years of Earth’s history, episodically during the MiddleArchean to younger Precambrian, and continuously throughout the Phanerozoic. This class of gold deposit is characteristi-cally associated with deformed and metamorphosed mid-crustal blocks, particularly in spatial association with major crustalstructures. A consistent spatial and temporal association with granitoids of a variety of compositions indicates that melts andfluids were both inherent products of thermal events during orogenesis. Including placer accumulations, which arecommonly intimately associated with this mineral deposit type, recognized production and resources from economicPhanerozoic orogenic-gold deposits are estimated at just over one billion ounces gold. Exclusive of the still-controversialWitwatersrand ores, known Precambrian gold concentrations are about half this amount.The recent increased applicability of global paleo-reconstructions, coupled with improved geochronology from most ofthe world’s major gold camps, allows for an improved understanding of the distribution pattern of orogenic gold in spaceand time. There are few well-preserved blocks of Middle Archean mid-crustal rocks with gold-favorable, high-strain shearzones in generally low-strain belts. The exception is the Kaapvaal craton where a number of orogenic gold deposits arescattered through the Barberton greenstone belt. A few )3.0 Ga crustal fragments also contain smaller gold systems in theUkrainian shield and the Pilbara craton. If the placer model is correct for the Witwatersrand goldfields, then it is possiblethat an exceptional Middle Archean orogenic-gold lode-system existed in the Kaapvaal craton at one time. The latter half ofthe Late Archean ca. 2.8–2.55 Ga was an extremely favorable period for orogenic gold-vein formation, and resulting oresŽ.preserved in mid-crustal rocks contain a high percentage of the world’s gold resource. Preserved major goldfields occur ingreenstone belts of the Yilgarn craton e.g., Kalgoorlie , Superior province e.g., Timmins , Dharwar craton e.g., Kolar ,Ž. Ž . Ž.Zimbabwe craton e.g., Kwekwe , Slave craton e.g., Yellowknife , Sao Francisco craton e.g., Quadrilatero Ferrifero , andŽ. Ž . Ž .Tanzania craton e.g., Bulyanhulu , with smaller deposits exposed in the Wyoming craton and Fennoscandian shield. SomeŽ.workers also suggest that the Witwatersrand ores were formed from hydrothermal fluids in this period.The third global episode of orogenic gold-vein formation occurred at ca. 2.1–1.8 Ga, as supracrustal sedimentary rocksequences became as significant hosts as greenstones for the gold ores. Greenstone–sedimentary rock sequences nowexposed in interior Australia, northwestern Africarnorthern South America, Svecofennia, and the Canadian shield were the

ORIGINS OF CHROMIAN AND ALUMINOUS SPINEL MACROCRYSTS FROM KIMBERLITES IN SOUTHERN AFRICA

A comparison is made of the major- and minor-element compositions of spinel-group minerals that occur as single-crystal macrocrysts in kimberlites from southern Africa (approximately 1500 grains from 20 kimberlites on the Kaapvaal and Zimbabwe cratons) with those from a variety of mantle xenoliths from southern Africa and North America (approximately 150 xenoliths). Extensive compositional overlap between macrocryst and xenolith samples leads to the conclusion that most macrocrysts of spinel-group minerals are likely to be xenocrysts from garnet-bearing peridotite. None are considered to be igneous phenocrysts related to the host kimberlites. Chromian spinel with Cr/(Cr + Al) > 0.88 is derived from garnet-free assemblages that have bulk Cr/(Cr + Al) values too low to form garnet, or from assemblages in which aluminum is partitioned into a different phase, such as metasomatic phlogopite. Such Cr-rich macrocrysts are more common in Group-I kimberlites than in Group-II kimberlites. Moderate degrees of metasomatism produced spinel-group minerals enriched in Fe, Ti and Cr in many Group-I suites, but chromite xenocrysts from highly metasomatized assemblages ( e.g. , zircon-bearing) are uncommon. Unusually Fe- and Ni-rich macrocrysts unique to the Lace mine are from Fe-rich orthopyroxene-bearing peridotites or pyroxenites that may be the product of impact-induced mantle melting associated with the Vredefort structure. Regional variations in chemical characteristics of spinel-group minerals suggest that the mantle is more “fertile” [ i.e. , lower Cr/(Cr + Al) values] near craton margins ( e.g., Liqhobong on the southeastern margins of the Kaapvaal Craton). Though situated on the Kaapvaal Craton, the Jwaneng suite of “spinel” has chemical characteristics similar to Orapa and Letlhakane suites, which suggests closer affinity to the Zimbabwe cratonic mantle than the Kaapvaal mantle.

SHRIMP baddeleyite and zircon ages for an Umkondo dolerite sill, Nyanga Mountains, Eastern Zimbabwe

Extensive mafic sills known as the Umkondo dolerites intrude sedimentary strata of the Umkondo Group, which overlie the eastern margin of the Archaean Zimbabwe craton. Zircons and composite baddeleyite-zircon crystals were recovered from a sample of coarse-grained felsic gabbro collected near the upper contact of a thick sill in the Nyanga area. Ion microprobe analyses of baddeleyite yield a mean 207Pb/2°Pb age of 1098 ± 10 Ma. Analyses of single zircons and zircon overgrowths on baddeleyite are concordant to strongly discordant, and their dispersion suggests that they have• experienced more than one episode of radiogenic Pb loss. The four least discordant zircon analyses indicate a mean 207Pb/2°Pb age of 1103 ± 32 Ma. The best estimate of the age of crystallisation of the sill, based on data for baddeleyite and the four most concordant zircons, is 1099 ± 9 Ma. Zircon overgrowths exhibit concentric euhedral zoning, and high and variable U and Th contents and Th/U ratios. These characteristics are similar to those of single zircons from the same sample, and are typical of late-crystallising zircon in dolerite intrusions, indicating that the overgrowths formed at the same time as the single zircons, during initial cooling of the sill. A re-interpretation of discordant U-Pb data obtained previously for zircons from a similar dolerite sill in the Chimanimani area, taking into account the likelihood of recent Pb loss, indicates an age of 1107 ± 6 Ma. Combining isotopic ages from both samples yields an estimate of 1105 ± 5 Ma for the Umkondo magmatic event in eastern Zimbabwe. The sill dated in this study has yielded characteristic Umkondo palaeomagnetic directions, therefore the Umkondo palaeopole is dated precisely at 1105 Ma, and can be used with increased confidence in defining the late Mesoproterozoic position of the Kalahari Craton.

The Mushandike granite: further evidence for 3.4 Ga magmatism in the Zimbabwe craton

The conflict between independently published ages for the Mushandike Granite, Zimbabwe (2.92 ± 0.17 Ga and 3.45 ± 0.13 Ga) has been resolved in favour of the older age by SHRIMP U–Pb analyses of zircon. Two samples yield indistinguishable estimates of 3374 ± 7 and 3368 ± 11 Ma for the crystallization age of the magma. Together with published data from elsewhere in southern Zimbabwe, the results imply a widespread magmatic event at about 3.35 Ga. A single zircon core giving 3.46 Ga, together with the granite's previously measured Nd model age, suggests that the Mushandike magma could have incorporated remobilized basement similar to the c. 3.5 Ga Tokwe gneisses which crop out 30 km to the west. The published Rb–Sr and Pb–Pb datasets show evidence of late Archaean disturbance of Sr and Pb isotope systematics. In the absence of exposed contacts between the Mushandike granite and the neighbouring Mushandike stromatolitic limestone, the new U–Pb emplacement age suggests that the limestone is unconformable on the granite.

3-D surface wave group velocity distribution in central-southern Africa

Group velocities estimated from fundamental mode Love and Rayleigh waves are used in a tomography process in central-southern Africa. The waves were generated by eighteen earthquakes, which occurred along the East African Rift and recorded at BOSA, LBTB and SLR seismic stations in southern Africa. The group velocities from Love and Rayleigh waves were isolated using the Multiple Filter Technique (MFT) at the period range of 10 to 50 seconds. The tomography method developed by Ditmar and Yanovskaya (1987) and Yanovskaya and Ditmar (1990), was applied to calculate the lateral distribution of surface wave group velocities in central-southern Africa. The results of the tomographic inversion were plotted as distribution maps. In addition to the maps, I also produced two velocity cross-sections across the area of study. The velocity distribution maps show the regional tectonic units, though with poor resolution. The azimuthal bias of the surface wave paths is reflected in the distribution of the group velocities. The Moho depth appears to correlate with velocities at a period of about 30 s. A low velocity feature observed beneath the Zimbabwe craton implies a thickening upper asthenosphere and lithospheric thinning beneath the Zimbabwe craton. Also estimated was a shear wave velocity model beneath the Zimbabwe craton.

2.8–3.0 Ga plutonism and deformation in the SE Amazonian craton: the Archaean granitoids of Marajoara (Carajás Mineral Province, Brazil)

The granitoids of Marajoara in the Rio Maria terrain (Carajás Mineral Province, Brazil) consist of: (i) a broad unit of 2.96 Ga syntectonic tonalites (Arco Verde Tonalites) displaying a trondhjemitic differentiation trend; (ii) 2.93 Ga syntectonic monzogranites (Guarantã); and (iii) 2.87 Ga post-tectonic monzogranites (Mata Surrão) and granodiorites (Rio Maria), displaying a calc-alkaline differentiation trend. Deformation of the Arco Verde tonalites is heterogeneous with low strain domains (well preserved magmatic banding and textures) and orthogneissic domains displaying an E–W trending mainly subvertical foliation, associated with horizontal lineations, upright folds and subvertical shear zones. Microstructures and phase assemblages suggest that deformation occurred within a large temperature range (i.e. during magma emplacement and cooling), from high-T conditions (synmagmatic shear zones and subsolidus ductile deformation with intense quartz and feldspar recrystallization; Pl+Qtz+Hbl+Bt assemblages) to medium- and low-T conditions (ductile to brittle deformation with weakly recrystallized quartz and undulose extinction in feldspars; Qtz+Pl+Bt+Mu or Chl+Ep+Ab+Qtz assemblages). These data, finite strain analysis and structures reported from the surrounding greenstone belts suggest that deformation did not result from a post-emplacement prograde tectono-metamorphic event as considered previously, but that the Marajoara granitoids are synkinernatic intrusions which were deformed together with the supracrustal rocks during a regional NS horizontal shortening. Although the Rio Maria terrain presents similarities with Archaean domains controlled by diapiric processes (lithologies dominated by thick greenstone sequences and TTG plutons, and forming a dome-and-keel structure), its structural evolution is controlled dominantly by a transpressional event which shaped the granite–greenstone terrains. The Rio Maria area, probably as many Archaean ‘grey gneisses’ domains, represents an intermediate case between terrains controlled by Raleigh–Taylor instabilities in a thermally softened crust with insignificant external forces related to plate convergence (e.g. east Pilbara craton) and those controlled by thrust tectonics related to convergence of rigid plates (e.g. Superior Province). The closest analog to the Rio Maria terrain seems to be the Chilimanzi area in the Zimbabwe craton.

The 1998 edition of the National Geological Map of Botswana

A new National Geological Map of Botswana incorporates data acquired from a variety of sources; the map is produced as a 1:1 million hardcopy as well as in digital format. The new map shows the pre-Kalahari Group geology. The oldest rocks are exposed in eastern Botswana where three Archaean terranes are recognised: the western parts of the Kaapvaal and Zimbabwe Cratons and the western part of the Limpopo Mobile Belt. All three terranes are lithologically similar but differ in their structural styles and in the timing of major thermal events. The oldest (pre-3.0 Gal high-grade metamorphic rocks are found in the Kaapvaal Craton, and the youngest in the Limpopo Mobile Belt, which appears to record Palæoproterozoic ductile shearing. Proterozoic orogenic belts, mostly concealed beneath Karoo rocks, define the western limits of the Archaean terranes and pprogressively young westwards away from the Archaean rocks. The Palwoproterozoic Magondi and Kheis Belts are well-defined by regional magnetic maps, but both are very poorly exposed in Botswana. The Kheis Belt trends due north from South Africa into central Botswana to define the western edge of the Kaapvaal Craton. The western part of the Magondi Belt, as well as all of a Mesoproterozoic (Kibaran) belt and rift are overprinted by the Neoproterozoic Damara Belt; all have pronounced northeasterly trends. During the Palæoproterozoic, there was also significant intraplate magmatism, sedimentation and deformation within the Archæan terranes. Some of the magmatism (in southeastern Botswana) was contemporaneous with, and lithologically similar to, the Bushveld Igneous Complex of South Africa. The main feature of the Mesoproterozoic geology of Botswana is a northeast trending rift that extends right across the northwest of the country and which is partly infilled with ca 1 106 Ma volcanic rocks. Neoproterozoic sedimentary rocks overlie the volcanics within the rift. The various rocks are exposed along the Ghanzi Ridge and to the northeast in the Chobe District. New detailed airborne magnetic surveys in northwest Botswana (Ngamiland) show the detailed geology of the northeast trending inland branch of the Damara Belt and exactly define its northwestern and southeastern boundaries. The southeastern part of the Damara Belt comprises the Mesoproterozoic volcanics of the Kgwebe Formation and the Neoproterozoic Ghanzi Group sedimentary strata. The full extent of the volcanics, and of the three formations recognised in the Ghanzi Group, is shown on the new map. Deformation of these rocks increases to the northwest where they are bounded by the tectono-stratigraphical Roibok Group. To the northwest of the Roibok Group are poorly dated granitoid rocks separated into several units that are locally overlain by carbonate-dominated sequences. A cover sequence of metasedimentary rocks with northnorthwest trending folds lies northwest of the Damara Belt. These sediments may overlie the southernmost part of the Congo Craton in the extreme northwest of Botswana. Neoproterozoic/ Lower Palæozoic sediments of the Nama Group partly infill a foreland basin to the south of the Damara belt in western Botswana. Karoo strata deposited within the Kalahari Basin underlie central Botswana. The distribution of the four major sedimentary groups, as well as of the capping basalts, is shown. The total thickness of the sediments is < 2000 m and the basalts are up to 1000 m in thickness. The sediments comprise a lower sequence (Dwyka and Ecca Groups) related to regional sagging and an upper sequence (Beaufort and Lebung Groups) that succeeded regional uplift that created intra-Karoo unconformities. Karoo sedimentation commenced towards the end of the Carboniferous Period and the basalts were extruded at about 180 Ma before Present. Wherever there have been detailed studies undertaken on the Karoo rocks, they show intense faulting that may or may not mimic structures in the pre-Karoo bedrock. The faulting appears to be post-sedimentation. No evidence was found for growth faults producing abnormal thicknesses of Karoo sediments. It is always possible to correlate the internal stratigraphy, at least at the formational level across the faults. Abnormal thicknesses of the basalts are preserved on the downthrow sides of the major faults. A major dyke swarm coeval with the extrusive basalts trends east-southeast right across north-central Botswana to cut across older structural trends. Over 200 kimberlites are shown on the new map. The kimberlites are distributed throughout Botswana in a number of separate clusters. Most of the kimberlites are of Cretaceous age. Isopachs are shown of the Kalahari Group, which is generally < 180 m in total thickness.

The origin of fluids and nature of fluid-rock interaction in mid-crustal auriferous mylonites of the Renco mine, southern Zimbabwe

Geothermometric constraints on auriferous shear zones of the Renco mine in the Northern Marginal Zone of the late-Archaean, granulite-facies Limpopo Belt in southern Zimbabwe indicate that deformation and associated mineralization occurred at temperatures of at least 600 °C up to more likely 700 °C. Mid- to upper-amphibolite facies conditions during mineralization correspond to the regional-scale retrogression of granulite facies wall rocks during the late-Archaean thrusting of high-grade metamorphic rocks of the Northern Marginal Zone onto low- to medium-grade granite-greenstone terrains of the Zimbabwe craton. Mineral assemblages indicate that the ore fluid was moderately oxidized with log fO2 values between 10−17 and 10−18 bars with high H2S activities of 0.25–0.75. Elements enriched in the shear zones include Au, S, Fe, Cu, Mo, Bi, Te, Ni, Co, and H2O, Au and Cu being the most enriched. Geochemically, Au correlates with Cu but not with S, which, together with the fact that gold is only rarely intergrown or in direct contact with sulfides, possibly indicates a transport of gold as a chloride complex. The siting of gold along fractures or within implosion breccias suggests that gold was precipitated due to fluid immiscibility induced by catastrophic fluid pressure drops during seismic slip events. Fluid inclusions are predominantly CO2 (±CH4 ± N2)-rich, but petrographic work indicates that fluid inclusions have undergone extensive post-entrapment modifications due to the pervasive recrystallization of mineral textures in the high-temperature shear zones. The mineralized shear zones are enriched in 18O compared to wall-rock enderbites, which is interpreted to represent an influx of externally derived fluids of probably metamorphic origin. Based on temporal and spatial relationships between mineralization, late-Archaean overthrusting of the Northern Marginal Zone onto the Zimbabwe craton, and coeval amphibolite-facies hydration of granulites, we suggest that the Renco mineralization formed in a mid-crustal environment from metamorphic fluids that were generated from dehydration of subcreted greenstone terrains of the Zimbabwe craton.

Two-dimensional numerical modeling of pressure–temperature-time paths for the exhumation of some granulite facies terrains in the Precambrian

Pressure ( P)–temperature ( T) paths accurately record movement of metamorphic rocks within the Earth’s crust. 1D geodynamic modeling of P– T-time paths of regional metamorphism have explained many important features of the P– T evolution of metamorphic rocks. Further progress may be achieved using 2D numerical geodynamic modeling. Different types of P– T paths obtained for some granulites allow their 2D numerical modeling in terms of a theory of gravitational redistribution of material within the Earth’s crust. This modeling was done using the finite differences method for a Newtonian incompressible fluid. The P– T paths obtained geothermobarometrically for metapelites from the Limpopo granulite complex were compared with the P– T-time paths calculated numerically by monitoring the movement of different ‘samples’ in the thermal and gravitational field. The results of this monitoring support a hypothesis of isobaric cooling of granulites during the thermal interaction between rising hot granulites and sinking cool cratonic rocks. Effective viscosities of the rocks were gradually adjusted during the modeling by using the shape of the P– T paths. The calculated average viscosity of 10 19 Pa s for the granulites, supports an idea that the lower crust is a weak layer between upper crust (10 21 Pa s) and the Upper Mantle. The average rate of exhumation of granulites during the gravitational redistribution process is estimated at about 2.5 mm per year. This suggests exhumation of granulites from depths of about 30 km during approximately 10 Myr. The results of numerical modeling successfully explain the presence of two types of P– T paths observed in metapelites of the Limpopo granulite complex emplaced between the >3 Ga Kaapvaal and Zimbabwe cratons at ∼2.65 Ga.

Tectonic evolution of a greenstone sequence in northern Zimbabwe: sequential early stacking and pluton diapirism

Structural‐metamorphic relationships in the 2.71–2.64 Ga Harare‐Shamva greenstone belt in northern Zimbabwe demonstrate that the Archean stratigraphy of the Zimbabwe craton can not be regarded as a coherent and nonrepetitive, autochthonous succession of (ultra)mafic and felsic greenstones. The greenstone belt shows an early set of amphibolite‐grade D1/M1 structures and associated synkinematic metamorphic assemblages that were formed during an episode of layer‐parallel shearing at about 2.64 Ga. This resulted in sequential west directed imbricate stacking and recumbent folding of distinct tectonostratigraphic greenstone sequences and includes the development of a syntectonic sedimentary succession. D2/M2 fabrics, strain patterns, and contact aureoles overprint D1 geometries and are related to the diapiric emplacement of the Chinamora batholith. The structural data indicate that granite‐gneiss terrains can be subdivided into two types: (1) structurally emplaced terrains (the Madziwa gneiss terrain) and (2) diapiric domes (the Chinamora batholith). The history of the Harare‐Shamva greenstone belt indicates that the late Archean (2.7–2.6 Ga) evolution of the Zimbabwe craton must allow for accretion of diverse crustal fragments including oceanic and back arc mafic crust, volcanic arc felsic crust, continental crust, and related sedimentary sequences. The process of accretion was probably diachronous across the craton and may have involved concomitant diapirism, strike‐slip faulting, magma intrusion, and sedimentation.

Structural, mineralogical and geochemical studies of the Paleoproterozoic Omai gold deposit, Guyana

The granitoid- and greenstone-hosted Omai gold deposit is located in the Paleoproterozoic Barama-Mazaruni greenstone belt, Guiana Shield. At regional scale, the emplacement of the deposit was controlled by the Makapa-Kuribrong crustal shear zone. At local scale, the gold-bearing quartz veins are associated mainly with a quartz monzodioritic intrusion (Omai stock, Fennell pit) and subvolcanic quartz-feldspar porphyry and rhyolite dikes, and subordinately, with intermediate volcanic flows and metasedimentary rocks (Wenot pit). Six gold-bearing undeformed subhorizontal and subvertical vein sets can be distinguished in the two ore zones. On the basis of their internal structures and textures, the veins can be classified as crack and seal, laminated, breccia, and open-space-filling veins. The geometrical and textural relationships between the vein sets suggest that they are broadly contemporaneous. The formation of most veins can be summarized by two filling stages and a late fracture-filling stage related to a protracted hydrothermal process. Although the metallic minerals represent less than 1 percent of the vein volume, their mineralogy is complex and consists of various sulfides, together with tungstates, native elements, tellurides, and sulfosalts. The metallic paragenesis is defined by the Au-Ag-Te-W-Bi-Pb-Zn-Cu-Hg-Mo assemblage. The nonopaque gangue includes mainly quart and subordinately carbonates, albite, sericite, chlorite, tourmaline, rutile, and epidote.Wall rocks of gold-bearing veins are affected by synmineralization alteration. The dominant alteration assemblages include carbonates, sericite, silica, chlorite, albite, epidote, argillic minerals, pyrite, and pyrrhotite. There is an obvious association between the alteration assemblages, the vein internal textures, and the chemical nature of the host rocks.Vein-forming scheelite has 87 Sr/ 86 Sr ratios between 0.7019 and 0.7021 and delta 18 O values between 3.8 and 4.3 per mil, which suggest both consistent temperature and isotopic composition of the hydrothermal solutions during its deposition. Oxygen isotopes measured in vein quartz vary between 13.2 and 14 per mil, similar to the delta 18 O values of carbonates (avg 13.8ppm for calcite and 14.4ppm for ankerite). The carbon isotopes of carbonates range between 1.7 and 4.7 per mil. The delta 18 O values of the mineralizing fluids vary between +5.6 and -2.7 per mil and the delta D values between -52 and +18 per mil. The isotopic composition of the hydrothermal fluids plots outside both magmatic and metamorphic water boxes, therefore suggesting a significant component of surface-derived water.The Omai deposit is late orogenic and its emplacement was controlled by the last brittle to brittle-ductile stages of the Trans-Amazonian orogeny. It can be considered as a Paleoproterozoic equivalent of the Archean epizonal orogenic deposits described in the Yilgarn (Wiluna, Racetrack) and Zimbabwe (Shamwa, Commoner) cratons.

U–Pb and 40Ar/ 39Ar geochronological constraints on the tectonic evolution of the easternmost part of the Zambezi orogenic belt, northeast Zimbabwe

The Zambezi belt is a key segment of the network of Neoproterozoic/lower Paleozoic orogenic belts in southern Africa that formed during amalgamation of central Gondwana. We present new geochronological data from the easternmost Zambezi belt in northeast Zimbabwe, near the junction between that belt and the Mozambique belt. Allochthonous high-pressure granulite-facies migmatitic and mylonitic rocks at the highest exposed structural levels in this part of the Zambezi belt are tectonically juxtaposed against amphibolite-facies supracrustal rocks, which are intruded by a regionally extensive, sheet-like composite granitoid batholith having peralkaline affinities. This batholith separates the Zambezi supracrustal rocks from a zone of Archean basement that wraps the northeast margin of the Zimbabwe craton and shows variable degrees of structural and thermal overprinting. U–Pb zircon and titanite and 40Ar/ 39Ar hornblende geochronological data from the allochthonous granulites are interpreted to record high-grade migmatization at ca 870 to 850 Ma, with pervasive amphibolite-facies retrogression at ca 535 Ma during tectonic emplacement into mid-crustal levels. U–Pb zircon and titanite data from the peralkaline batholith indicate that it crystallized at 805.2±11.1 Ma; partial thermal resetting of the U–Pb system occurred in the same time frame as retrogression of the allochthonous granulites. Hornblende from two samples in thermally overprinted Archean basement farther south yields 40Ar/ 39Ar plateau ages of 507.9±2.5 and 491.3±2.1 Ma. Together these new data indicate that tectonic elements in the easternmost Zambezi belt have a protracted history, involving early, lower crustal, granulite-facies metamorphism (ca 870–850 Ma) followed by intrusion of a peralkaline granitic batholith (ca 800 Ma) into supracrustal rocks within the belt. The major tectonostratigraphic units in the easternmost Zambezi belt were juxtaposed under amphibolite-facies conditions at ca 535 Ma, followed by relatively rapid cooling through Ar closure temperatures in hornblende. The 535 Ma event reflects deformation in the Zambezi belt in the same time frame as widespread orogenesis that is recorded in other Pan-African belts in southern Africa and is related to final stages in Gondwana assembly.

U-Pb zircon evidence for an extensive early Archean craton in Zimbabwe: A reassessment of the timing of craton formation, stabilization, and growth

U-Pb single-zircon analyses provide direct evidence for an enlarged early Archean craton forming the core to the present Zimbabwe craton. Virtually identical dates from the south-central Tokwe segment (3455 ± 2 Ma) and Midlands (3456 ± 6 Ma) parts of the craton strongly suggest their synchronous formation, during an event that formed a single early cratonic nucleus which we propose to call the “Sebakwe protocraton.” This is considered to underlie most of the current Zimbabwe craton. Parts of the craton are at least 3565 ± 21 Ma, a rock age reported here that represents the oldest rock dated from Zimbabwe. A ca. 3350 Ma relatively undeformed and unmetamorphosed intrusive granitic phase constrains the timing of the high-grade metamorphism and the stabilization of the protocraton. Comparison with published Re-Os data for the Zimbabwe craton strongly indicates a depleted subcontinental lithospheric mantle underlying the entire Sebakwe protocraton. Subsequent intrusive and volcanic activity from 3.0 to 2.6 Ga represents a second major period of magma genesis and crustal formation within which the predominant rocks of the exposed Zimbabwe craton were generated.