Unraveling the record of successive high grade events in the Central Zone of the Limpopo Belt using Pb single phase dating of metamorphic minerals (original) (raw)
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Origin and Evolution of Precambrian High- …, 2011
The Northern Marginal Zone (NMZ) of the Limpopo Belt, southern Africa, is a high-grade gneiss belt dominated by magmatic granulites of the charnoenderbite suite, which intruded minor mafi c-ultramafi c and metasedimentary rocks between 2.74 and 2.57 Ga. The intrusive rocks have crustal and mantle components, and occur as elliptical bodies interpreted as diapirs. Peak metamorphism (P ≤800 MPa, T = 800-850 °C) occurred at ca. 2.59 Ga. The highly radiogenic nature of the rocks in the NMZ, supplemented by heat from mantle melts, led to heating and diapirism, culminating in the intrusion of distinctive porphyritic charnockites and granites. Horizontal shortening and steep extrusion of the NMZ, during which crustal thickening was limited by high geothermal gradients, contrast with overthickening and gravitational collapse observed particularly in more recent orogens. The granulites were exhumed by the end of the Archean. The pervasive late Archean shortening over the whole of the NMZ contrasts with limited deformation on the Zimbabwe Craton, possibly owing to the strengthening effect of early crust in the craton. In the southeast of the NMZ, strike-slip kinematic indicators occur within the Transition Zone and the Triangle Shear Zone, where dextral shearing reworked the Archean crust at ca.
Gondwana Research, 2003
The geochemistry of the Leisure Bay Formation, Natal Metamorphic Province suggests that its protoliths were greywackes, pelites and arkoses that were deposited in an oceanic island arc environment. These rocks contain the mineral assemblage biotite + hypersthene + cordierite (with hercynite inclusions) + garnet + quartz + feldspar. Numerous generations of garnet genesis are evident from which a long history of metamorphism can be interpreted MI involved syn-D, high tempcrature/low pressure metamorphism (-4kb and > 8 5 0~) and dehydration melting to produce essentially anhydrous assemblages particularly in the vicinity of, and probably related to the intrusion of the Munster Suite sills. The inclusions of hercynite in cordierite and the garnet + quartz symplectites after hypersthene + plagioclase (-550°C and-5kb) suggests isobaric cooling after MI. This indicates an anticlockwise P-T loop related to the early intrusion of subduction related calc alkaline magmatic rocks. M, involved syn-D, dehydration melting of hydrous assemblages possibly related to the emplacement of many A-type rapakivi charnockite granitoids, which provided heat and loading. The D, tectonism postdated all lithologies in the region, except for synto late-D, granitoid plutons, and is interpreted as a transpressional tectonotherinal reworking of pre-existing (Proterozoic) crust at-1030Ma.
Lithos, 2008
The Bulai pluton represents a calc-alkaline magmatic complex of variable deformed charnockites, enderbites and granites, and contains xenoliths of highly deformed metamorphic country rocks. Petrological investigations show that these xenoliths underwent a high-grade metamorphic overprint at peak P-T conditions of 830-860°C/8-9 kbar followed by a pressure-temperature decrease to 750°C/5-6 kbar. This P-T path is inferred from the application of P-T pseudosections to six rock samples of distinct bulk composition: three metapelitic garnet-biotite-sillimanite-cordierite-plagioclase-(K-feldspar)-quartz gneisses, two charnoenderbitic garnet-orthopyroxene-biotite-K-feldspar-plagioclase-quartz gneisses and an enderbitic orthopyroxene-biotite-plagioclase-quartz gneiss. The petrological data show that the metapelitic and charnoenderbitic gneisses underwent uplift, cooling and deformation before they were intruded by the Bulai Granite. This relationship is supported by geochronological results obtained by in situ LA-ICP-MS age dating. U-Pb analyses of monazite enclosed in garnet of a charnoenderbite gneiss provide evidence for a high-grade structuralmetamorphic-magmatic event at 2644 ± 8 Ma. This age is significantly older than an U-Pb zircon crystallisation age of 2612 ± 7 Ma previously obtained from the surrounding, late-tectonic Bulai Granite. The new dataset indicates that parts of the Limpopo's Central Zone were affected by a Neoarchaean high-grade metamorphic overprint, which was caused by magmatic heat transfer into the lower crust in a 'dynamic regional contact metamorphic milieu', which perhaps took place in a magmatic arc setting.
Tectonophysics, 2003
U -Pb sensitive high resolution ion microprobe (SHRIMP) dating of zircons from charnockitic and garnet -biotite gneisses from the central portion of the Mozambique belt, central Tanzania indicate that the protolith granitoids were emplaced in a late Archaean, ca. 2.7 Ga, magmatic event. These ages are similar to other U -Pb and Pb -Pb ages obtained for other gneisses in this part of the belt. Zircon xenocrysts dated between 2.8 and 3.0 Ga indicate the presence of an older basement. Major and trace element geochemistry of these high-grade gneisses suggests that the granitoid protoliths may have formed in an active continental margin environment. Metamorphic zircon rims and multifaceted metamorphic zircons are dated at ca. 2.6 Ga indicating that these rocks were metamorphosed some 50 -100 my after their emplacement. Pressure and temperature estimates on the charnockitic and garnet -biotite gneisses were obscured by post-peak metamorphic compositional homogenisation; however, these estimates combined with mineral textures suggest that these rocks underwent isobaric cooling to 800 -850 jC at 12 -14 kbar. It is considered likely that the granulite facies mineral assemblage developed during the ca. 2.6 Ga event, but it must be considered that it might instead represent a pervasive Neoproterozoic, Pan African, granulite facies overprint, similar to the ubiquitous eastern granulites further to the east. D
Precambrian Research, 1999
Ion microprobe, evaporation and vapour digestion single zircon emplacement ages for granitoid rocks throughout the Central Zone (CZ) of the Limpopo mobile belt, South Africa define three broad age groups, namely at 3188–3314, 2637–2734 and ca 2500 Ma, respectively. All these granitoids, including the youngest group, display polydeformational features suggesting that much of the tectonic history of the CZ occurred in the late Archaean and early Proterozoic.The oldest group of predominantly tonalitic–trondhjemitic–granodioritic gneisses was found in relatively small domains from the extreme W to the E of the CZ and probably represents remnants of a once more widespread early Archaean terrain. The most abundant granitoid gneisses belong to the 2.6–2.7 Ga age group and were emplaced into already ductilely deformed gneisses of the oldest group. Several of these rocks were derived from intracrustal melting. Rare metamorphic zircons with ages of ∼2560–2575 Ma reflect a high-grade metamorphic event in the CZ that has also been postulated by other workers. A strongly deformed orthogneiss with a protolith emplacement age of ∼2510 Ma proves beyond doubt that at least part of the structural history in the CZ occurred in the early Proterozoic. An anatectic granite in the extreme W of the CZ intruded at ∼2022 Ma and is probably related to the peak of granulite-facies metamorphism previously dated at 2027±6 Ma in the Messina area.The second period of granitoid magmatism is broadly contemporaneous with charnockite and enderbite emplacement and granulite-facies metamorphism in the Northern and Southern Marginal Zones of the Limpopo belt. Broad contemporaneity of extensive granitoid magmatism in all three Zones raises doubts about the exotic nature of the CZ and suggests a revised evolutionary model for the Limpopo belt.
Journal of African Earth Sciences, 2008
Published literature argues that the Limpopo Belt can be subdivided into three zones, each with a distinctive geological character and tectono-metamorphic fingerprint. There are currently two contrasting schools of thought regarding the tectono-metamorphic evolution of the CZ. One camp argues that geochronological, structural and prograde pressure-temperature (P-T) evidence collectively indicate that the CZ underwent tectono-metamorphism at ca. 2.0 Ga which followed a clockwise P-T evolution during a transpressive orogeny that was initiated by the collision of the Kaapvaal and Zimbabwe cratons. Deformation and metamorphism consistent with this scenario are observed in the southern part of the NMZ but are curiously absent from the whole of the SMZ. The opposing view argues that the peak metamorphism associated with the collision of the Kaapvaal and Zimbabwe cratons occurred at ca. 2.6 Ga and the later metamorphic event is an overprint associated with reactivation along Archean shear zones. Post-peakmetamorphic conditions, which at present cannot be convincingly related to either a ca. 2.6 or 2.0 Ga event in the CZ reveal contrasting retrograde paths implying either nearisothermal decompression and isobaric cooling associated with a 'pop-up' style of exhumation or steady decompression-cooling linked to exhumation controlled by erosion. Recent data argue that the prograde evolution of the ca. 2.0 Ga event is characterised by isobaric heating prior to decompression-cooling. Contrasting P-T paths indicate that either different units exist within the CZ that underwent different P-T evolutions or that some P-T work is erroneous due to the application of equilibrium thermobarometry to mineral assemblages that are not in equilibrium. The morphology of the P-T path(s) for the ca. 2.6-2.52 Ga event are also a matter of dispute. Some workers have postulated an anticlockwise P-T evolution during this period whilst others regard this metamorphic event as following a clockwise evolution. Granitoid magmatism is broadly contemporaneous in all three zones at ca. 2.7-2.5 suggesting a possible causal geodynamic link. P-T contrasts between and within the respective zones prevent, at present, the construction of a coherent and inter-related tectonic model that can account for all of the available evidence. Detailed and fully-integrated petrological and geochronological studies are required to produce reliable P-T-t paths that may resolve some of these pertinent issues.
2021
The well exposed granulite facies rocks in the Southern Marginal Zone (SMZ) of the Limpopo Belt (LB), South Africa, represent an exceptional window into the lower crust where partial melting processes can be directly observed and investigated. Previous studies have demonstrated that these rocks experienced a single granulite facies metamorphic event at ca. 2.71 Ga under conditions of ~850 o C and 11 kbar along with partial melting via a sequence of biotite incongruent melting reactions, which produced melt together with peritectic garnet, orthopyroxene, cordierite, sillimanite and plagioclase. In rare cases, peritectic garnets host an abundance of euhedral plagioclase inclusions with variable compositions. Despite their potential that they present in understanding details of the melting processes they have not been a focus of a previous study. This thesis focuses on these inclusions in order to understand the anatectic processes responsible for the generation of the metasedimentary migmatites in the SMZ, and importantly also characterises their retrograde metamorphic history. Specifically, this work integrates geochemical, geochronological and petrological investigations of metapelitic granulites from two localities in the Bandelierkop Formation (the Bandelierkop quarry and the Brakspruit quarry), where Neoarchean high-grade metamorphic textures have been preserved. Electron beam microanalysis show that some of the euhedral plagioclase inclusions have high anorthite content (up to An83) compared to the majority of plagioclase inclusions, as well as the matrix plagioclase, in these rocks. This is interpreted to reflect disequilibrium melting of plagioclase in the source due to slow diffusion in plagioclase. The variable composition of these euhedral plagioclase crystals suggests their entrainment by the melt from different localised domains and the entrapment at the sites where peritectic garnet was growing from the melt. The preservation of these garnets must be due to rapid and efficient melt loss from the residual source, possibly aided by short residency of these granulites at high temperature due to fast exhumation. The peak metamorphic assemblage is partially retrogressed under amphibolite facies conditions. The detailed petrological investigations of the retrogressed metapelitic granulites, combined with LA-ICP-MS dating on rutile suggests that the SMZ of the LB has experienced a previously unrecognised Paleoproterozoic regional amphibolite facies event. This Paleoproterozoic event is 600 Myrs younger than the age of peak metamorphism in the SMZ. Within the Kaapvaal Craton, it relates to the ca. 2.15 Ga U-Pb age from metamorphic monazite and titanite in the Ghaap group, Chuniespoort Group and the Pretoria Group of the Transvaal Supergroup and the ca. 2.1 monazite age from the Central Rand Group of the Witwatersrand, all of which have been interpreted to indicate craton-scale fluid flow associated with thrusting and folding in the Kaapvaal Craton. Stellenbosch University https://scholar.sun.ac.za Firstly, I would like to express my sincere gratitude to my supervisor, Professor Gary Stevens, who gave me this incredible opportunity to undertake this project under his supervision, who always supported me along the way and to whom I am greatly indebted. Thank you for all the time and effort that you spent with me, sharing your knowledge on metamorphic petrology and geology all together, guiding my written work, helping me to improve my scientific writing, providing me with motivational letters, allowing me to work at the Electron Microscopy unit and for providing me with financial support. I am truly grateful for all that you have done for me and I feel very privileged to have had you as a supervisor and mentor.
Precambrian Research, 2011
This paper presents the results of a detailed and fully integrated pressure-temperature-timedeformation (P-T-t-D) study of the country rocks around Venetia Mine in the Central Zone of the Limpopo Belt, South Africa. Detailed structural mapping around Venetia Mine delineates four deformation events (D 1 -D 4 ). Relict S 1 comprises quartzofeldspathic bands in biotite gneiss and amphibolite. S 2 , defined by biotite in biotite schist, gneissic banding in biotite gneiss, long axes of quartzofeldspathic augen, and the long axes of amphibolite lenses/boudins, is axial planar to F 2 . F 3 occurs predominantly at the contacts between biotite gneiss and biotite schist, forming open to closed, upright to inclined, E-W-to ENEtrending, shallowly plunging folds. D 4 , which was constrictional-prolate in nature, refolded S 1 , S 2 , F 2 and F 3 . F 4 folds and an L 4 mineral lineation, defined predominantly by sillimanite in metapelitic schist, plunge moderately NE to NNE and overprint all previous fabrics. Poles to refolded S 2 foliations show a characteristic great circle distribution. In turn, the pole to this great circle coincides with the orientation of L 4 . Such patterns are also found in the Avoca, Bellevue and Ha-Tshansi sheath folds in the Central Zone, albeit that the plunge of the lineations at Venetia are predominantly NE-wards, rather than SWwards. The moderately NE-to NNE-plunging F 4 sheath folds and L 4 lineations accord with the NE-SW trend of sheath folds and associated mineral elongation lineations observed elsewhere in the Central Zone. However, PbSL dating of syntectonic garnets constrain the minimum and maximum age of D 2 and D 4 structures respectively to c. 2037-2040 +/− 22 Ma. The metamorphism developed in metapelitic lithologies is characterized by peak amphibolite facies conditions of ∼6.5 kbar and 680 • C. Pseudosection modelling of growth zoning in garnet provides evidence of a prograde pressure and temperature increase from ∼5 kbar at 600 • C to ∼6-6.5 kbar at 650-680 • C, which is interpreted to be a consequence of tectonic thickening. Collectively, the integrated P-T-t-D data unequivocally demonstrates that the country rocks around Venetia experienced a structural-metamorphic event in the palaeoproterozoic. Coupled with published data we suggest a simple two-fold tectonic model, which involves the Central Zone as separate terrane docking with the Kaapvaal during the Neoarchean and later, during the Palaeoproterozoic this Central Zone-Kaapvaal amalgam collides with the Zimbabwe Craton.
Precambrian Research, 1999
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.
Geological Society of America Memoirs, 2011
Petrological and fl uid-inclusion data of high-grade metapelitic gneisses that occur as enclaves and in the immediate surroundings of the 2.612 Ga old Bulai granitoid intrusive are presented in this chapter. The Bulai intrusive is an important time marker in the tectono-metamorphic evolution of the Central Zone of the Limpopo Complex. The host-rock gneisses show one generation of garnet, cordierite, and sillimanite, whereas the enclave gneisses show two different generations of gar-*Present address: