Marlina Elburg | University of Johannesburg, South Africa (original) (raw)

Papers by Marlina Elburg

We analysed 585 My old antitaxial fibrous calcite veins that formed at about 4-6 km depth in dark... more We analysed 585 My old antitaxial fibrous calcite veins that formed at about 4-6 km depth in dark carbonaceous shales, found near Arkaroola, northern Flinders Ranges, South Australia [1]. The veins contain micron-sized biomorph structures, such as coccus-and globular structures, ...

2004 Denver Annual …, Jan 1, 2004

Extremophile microbes are currently known to inhabit the surface and upper kilometres of the Eart... more Extremophile microbes are currently known to inhabit the surface and upper kilometres of the Earth's crust. Although deep microbial life may date back to the beginning of life on Earth, fossil evidence for deep life remains virtually non-existent. A main problem is the difficulty to find and recognise micron-sized fossil remains in the vast volume of rock.

We analysed 585 My old antitaxial fibrous calcite veins that formed at about 4-6 km depth in dark carbonaceous shales, found near Arkaroola, northern Flinders Ranges, South Australia [1]. The veins contain micron-sized biomorph structures, such as coccus- and globular structures, indications of cell division (wall bands, meridian constriction) and thin (0.1 µm) fibres with knots. The combined morphological evidence indicates that these constitute fossil microbes. These microbes must have lived during vein growth and, therefore, possibly played a role in the formation of the veins. The microbes bear some resemblance to the modern-day hyperthermal Archaea, such as Pyrodictium, which thrive at temperatures of about 80-120°C and have been found in hot springs and oil wells. Living by sulphur-metabolism, these Archaea typically alter carbonate solubility, leading to calcite precipitation, as well as minor pyrite as a by-product. Minor sulphides were found inside the veins. XRF and LA-ICPMS analyses of the veins and their immediate wall rock showed mobilisation of Ca, Fe, Mg, Na, Mn, P, S, Sr, Y, Sc, Pb, Zn, U, Cu, and Mo, some of which may indicate microbial activity.

The fossil microbes from Arkaroola indicate that life had penetrated deeper rocks at least at the end of the Proterozoic. If there is a link between the distinct antitaxial fibrous texture of the veins and microbial activity, this type of veins may help in the search for more, and possibly older evidence for deep microbial life in the geological record.

Australian Journal of Earth Sciences, Feb 17, 2016

ABSTRACT The Neoproterozoic to Cambrian Selwyn Block in Central Victoria forms the mainly unexpos... more ABSTRACT The Neoproterozoic to Cambrian Selwyn Block in Central Victoria forms the mainly unexposed basement to the Paleozoic metasediments, granitic rocks and felsic volcanic complexes of the Melbourne Zone of the Lachlan Orogen. The Late Devonian felsic rocks are largely products of partial melting of the Selwyn Block, and their chemistry implies that their sources were most probably arc-related andesite, dacite, volcaniclastic greywackes and some pelites. When plotted against the median longitudes of the plutons and volcanic complexes, the average values for 87Sr/86Srt and ϵNdt (at 370 Ma) reveal broad trends interpreted to reflect possible compositional and/or age structure in the Selwyn Block. Assuming that the trends are real, from W to E, I-type sources are progressively less crustally evolved, probably younging eastward. The S-type sources show no trend in ϵNdt, suggesting that there was efficient sediment mixing. The 87Sr/86Srt values, however, become more evolved eastward (opposite in sense to the apparent variation in the I-type sources). This is interpreted as the original Selwyn Block sediments having been more pelitic eastward, perhaps suggesting a deepening of the basin in this direction, as well as structurally upward in the succession. The opposite senses of variation highlights the spatial separation of the S- and I-type sources and suggest that the granitic magmas here are unlikely to represent any sort of mixing continuum.

Transactions of The Royal Society of South Africa, Aug 2, 2016

Using chemical evidence, particularly the variations between titanium and iron + magnesium, we de... more Using chemical evidence, particularly the variations between titanium and iron + magnesium, we demonstrate that the petrogenetic processes that operated within Central Victorian granitic host magmas and their igneous microgranular enclave suites are dissimilar. Chemical variations within the granitic series result from a variety of what might be called ‘orderly’ processes, resulting in linear or curvilinear trends in chemical variation diagrams. Those that affected the enclave suites commonly resulted in scattered, chaotic variations. Even in cases where an enclave suite displays a more orderly chemical variation, it is demonstrable that the hypothesis of simple mixing between an enclave magma and a host granitic magma, to bring about the overall chemical variations, cannot be supported. The enclave magmas had vastly smaller volumes compared with their host granitic magmas. Thus, they have commonly undergone hybridisation through mixing with deep crustal melts and both chemical and mechanical interactions with wall rocks and their enclosing granitic magmas. As a result of this complex and chaotic set of processes, it remains extremely difficult to unravel the precise mechanisms that produced a given suite of enclave magmas. Due to the similarities between the studied granites and their enclaves with occurrences worldwide, we suggest that our findings are likely to have general applicability.

EGU General Assembly Conference Abstracts, Apr 1, 2017

South African Journal of Geology, Mar 1, 2023

An integrated approach embracing field studies, petrographic and geochemical investigations toget... more An integrated approach embracing field studies, petrographic and geochemical investigations together with zircon U-Pb-Hf data was used to investigate the petrogenesis of potassic granite suites along the southern margin of the Zimbabwe Craton. Zircon U-Pb geochronology identifies age relationships, revealing coeval magmatism of the ca. 2 635 ± 5 to 2 625 ± 3 Ma Chilimanzi Suite, and the ca. 2 627 ± 7 Ma Razi Suite. Both suites represent syn- to late-tectonic, high-K, calc-alkaline, and metaluminous to weakly peraluminous granites and granodiorites with I-type affinity. The granite suites contain xenocrystic zircons, with the Chikwanda Pluton of the Chilimanzi Suite yielding a grain of up to 3 206 Ma old. Both granite suites exhibit eHf values of between -5.6 ± 1.3 and -7.3 ± 1.6 and TDM model ages of ca. 3.4 to 3.5 Ga which suggests a similar crustal source. The unradiogenic zircon Hf isotopic compositions are consistent with formation of the granite suites through partial melting of pre-existing crustal protoliths, including Palaeoarchaean tonalite-trondhjemite-granodiorites (TTGs) of the Zimbabwe proto-craton. Partial melting of lower crust gave rise to granitic melts that became emplaced over a relatively short time interval from 2 635 to 2 625 Ma and heralded the stabilisation of the Zimbabwe Craton.In addition to virtually identical ages, the Razi and Chilimanzi suites have similar geochemistry. Small geochemical differences between the Chilimanzi and the Razi suites are attributed to the crustal level at which they are preserved, the modal mineralogy and the extent to which the melts are evolved. The Razi Suite melts were generated from lower crust partial melting of thickened charnockite-enderbite source rocks rich in heat producing elements. The partial melting occurred under fluid-absent conditions and magmas were emplaced at lower to mid crustal levels. The Chilimanzi Suite magmas were similarly derived by the partial melting of TTG lower crust and were emplaced at upper crustal levels. Accordingly, the Chilimanzi Suite exhibits more evolved magmatic fractionation indices indicated by high Rb/Sr, as well as low K/Rb ratios relative to the Razi Suite. Both suites reveal varying degrees of enrichment in incompatible elements including Rb, Th, and U, as well simultaneous depletions in Ba, Sr, and Hf which underscores the role of fractional crystallisation in the evolution of the granitic magmas.

Sedimentology, Apr 23, 2022

The age and properties of detrital zircon in a sediment reflect properties of the rocks in which ... more The age and properties of detrital zircon in a sediment reflect properties of the rocks in which the zircon crystallized, and not necessarily the immediate precursor of the host sediment. Where clastic sediments are recycled, these properties are preserved so that U-Pb ages and Lu-Hf data on zircon grains will no longer give information on the routing of detritus. Because the crustal evolution and sedimentary recycling history of Southern Africa is well-established, Holocene sediments in the region provide a good test example for detrital zircon geochronology applied to sands or sandstones with a complex recycling history. Data from 16 samples of unconsolidated sand, including dunes from the southern part of the Kalahari Basin, beach sands at the Atlantic coast, and isolated inland dune occurrences, yield ranges of early Mesozoic to late Archaean ages. Uranium-lead and lutetium-hafnium isotope data provide no evidence of direct derivation of zircon from protosources in crystalline bedrock. The complex distribution patterns can be decomposed into seven 'provenance components' that have previously been encountered in Palaeoproterozoic to Jurassic sedimentary rocks in the region. These components occur in non-random combinations, in proportions that suggest mixing of recycled material from different sedimentary precursors. Taking pre-Cenozoic geology, Cretaceous to Pleistocene drainage history of southern Africa and Holocene wind and current patterns into account, it is possible to work out a consistent model for transport of detritus from sedimentary precursors into the basin, followed by aeolian remobilization within the basin itself. The detrital zircon distributions can thus be interpreted from prior knowledge of the basin filling history, regional geology, geomorphology, drainage evolution and wind pattern. This amounts to a line of reasoning from 'sink back to (intermediate) 'source', which is the opposite of the '(proto)source-to-sink' paradigm commonly invoked in detrital zircon studies.

Precambrian Research, Nov 1, 2020

Journal of Metamorphic Geology, Sep 1, 2001

One of the currently popular theories on magma ascent is that it mainly occurs by propagating hyd... more One of the currently popular theories on magma ascent is that it mainly occurs by propagating hydrofractures (dykes) and that magma viscosity is the primary rate-controlling factor. This theory is based on mathematical models for single hydrofractures under idealised conditions. We simulated magma ascent with air ascending through gelatine and observed that the air ascended in batches, following paths made by their predecessors. Multiple batches accumulate at obstacles along the path. Although magma viscosity may control ascent rate during movement, obstacles ultimately control the size and average ascent velocity of ascending batches. We propose that step-wise movement of magma batches is the mechanism of primary accumulation and ascent from the partially molten source rock of a magma to its ®rst emplacement site and therefore the main ascent mechanism for granitic magmas.`Classical' dyking is the mechanism for secondary ascent from a magma chamber.

Eos, Transactions American Geophysical Union, 2005

Truly fibrous calcite veins occur mainly in carbonaceous shales and are characterised by high len... more Truly fibrous calcite veins occur mainly in carbonaceous shales and are characterised by high length:width ratios of their fibres (>10). Previous studies on their Sr isotopic geochemistry (Elburg et al., 2002: Geol. Soc. London Spec. Publ. 200, 103-118; Hilgers and Sindern, 2005: Geofluids, in press) have shown that some of the material could be derived from the local wall rock. These studies also showed that the veins were always enriched in Sr compared to the calcite in the host rocks. Aragonite can contain significantly more Sr than calcite, while it also tends to have a fibrous crystal habit. It is therefore possible that the fibrous habit of these veins, which now consist of calcite, are a reflection of their initial aragonitic mineralogy, rather than of any special tectonic regime during their formation. This idea was investigated by analysing the major and trace element geochemistry of selected fibrous and non-fibrous calcite veins from Arkaroola (northern Flinders Ranges, Australia). The fibrous vein analysed for major elements contains less than 1% MgCO3, whereas calcite in the host rock, with which it is in Sr isotopic equilibrium, contains 18% MgCO3. Calcite can contain significant Mg, whereas the aragonitic structure cannot accomodate this ion, so this result is consistent with the idea of an original aragonitic mineralogy of the veins. The fibrous veins show an enrichment in the middle rare earth elements (REE) compared to the calcite in the host rock and blocky veins. In a Post-Archean Average Shale normalised diagram, Eu is more strongly enriched compared to its neighbouring elements in the fibrous veins, but not in the host calcite, blocky veins, or in the silicate fraction of the host rock, suggesting more reducing conditions during fibrous vein formation. This data cannot be used as direct evidence for the fibrous veins' aragonitic mineralogy. It does, however, show that significant differences exist between calcite in host rocks, blocky and fibrous calcite veins, and this data should be incorporated in any model explaining the origin of fibrous veins.

The Pan-African Damara Belt in southern Africa is a trench-trench-trench triple junctions orogen ... more The Pan-African Damara Belt in southern Africa is a trench-trench-trench triple junctions orogen that formed at 590-470 Ma during the Gondwana Supercontinent assembly. The Damara Belt records up to granulite facies HTLP metamorphism in the core, upper plate of the orogen. However, the cause of this metamorphism is not well understood. To tackle this problem, we focus on the ENE-WSW-trending Namibfontein-Vergenoeg (NV) migmatitic domes. We use P-T-t-d data to investigate the temporal relationships of deformation fabrics, metamorphism and melting.The NV domes formed through the superposition of four folding events. We use LA-(Q/MC)-ICP-MS U-Pb dating of monazite from structurally controlled granitoids and leucosomes to define the relative timing of the deformation phases. These include 1) an early phase of E-W shortening forming upright F1 folds and steep N-S-striking S1 deformation fabrics. D1 was active between ~559 and 530 Ma. 2) N-S shortening followed, forming dome-scale F2 antic...

Contributions to Mineralogy and Petrology

The Hokuroku region of north-eastern Japan is endowed with important volcanic-hosted massive sulp... more The Hokuroku region of north-eastern Japan is endowed with important volcanic-hosted massive sulphide Zn–Pb–Cu deposits, which are considered the archetype of Kuroko (black ore) deposits worldwide. The bimodal, felsic-dominated volcanic succession that hosts the ore was deposited in a continental rift formed during continental extension in the final stages of the Miocene back-arc opening that led to the formation of the Japan Sea. In this study, we define some of the fundamental intensive parameters of this volcanism (temperature, pressure of crystallisation, fluid saturation, fO2) based on rock textures, and analyses of whole-rock samples, minerals and melt inclusions. Based on the melt inclusion analyses, we assess the behaviour of metals during magma evolution and degassing, and evaluate the possible implications for ore deposition. Plagioclase-melt geothermometry in felsic tuff and lava samples collected from both the units underlying and overlying the Kuroko indicates temperatu...

Earth-Science Reviews, 2022

Precambrian Research, 2021

Abstract The Sveconorwegian orogen represents a branch of Grenville-age (~1250–950 Ma) orogenic b... more Abstract The Sveconorwegian orogen represents a branch of Grenville-age (~1250–950 Ma) orogenic belts that formed during the construction of the supercontinent Rodinia. This study traces the Sveconorwegian records from its type-area in the Baltic Shield of South Norway into basement windows underneath Caledonian nappes, by combining zircon U–Pb geochronology and Hf–O isotopes. Samples along a N-S trending transect reveal multiple magmatic episodes during Gothian (ca. 1650 Ma), Telemarkian (ca. 1500 Ma) and Sveconorwegian (1050–1020 Ma vs. 980–930 Ma) orogenesis as well as Sveconorwegian migmatization (1050–950 Ma). Newly documented 1050–1020 Ma magmatism and migmatization extend the Sirdal Magmatic Belt to a 300 km-long, NNW-SSE trending crustal domain, with the northern boundary corresponding to the gradual transition from Telemarkian to Gothian crust. These Precambrian crustal heterogeneities largely controlled the development of Caledonian shear zones. The ca. 1050–1040 Ma granitic and mafic magmas show similar isotopic signatures with slightly negative or positive eHf(t) and moderate δ18O values (6–7‰), which indicates that crustal reworking was more dominant than juvenile inputs during their genesis. The generation of leucosomes and leucogranites at ca. 1030–1020 Ma, which have a more evolved Hf isotopic composition, probably reflects an even higher degree of remelting of older crust. The Hf–O isotopic patterns show that Sveconorwegian magmas differ from typical arc magmas by lower involvement of sedimentary components and juvenile material. This makes the 1050–930 Ma magmatism incompatible with a long-term subduction setting. The ca. 1650–1500 Ma samples, in contrast, generally have juvenile Hf isotopic compositions associated with varying δ18O values of 4.5–9‰, consistent with subduction-accretion processes involving significant sedimentary recycling. This accretionary margin was most likely transformed into the Sveconorwegian orogen through collisional interactions of Baltica, Laurentia and Amazonia in the context of Rodinia amalgamation.

Journal of African Earth Sciences, 2022

Journal of Petrology, 2021

Gneiss domes cored by migmatites and granites represent the principal role of anatectic melts dur... more Gneiss domes cored by migmatites and granites represent the principal role of anatectic melts during the exhumation of high-grade metamorphic complexes. This study explores the exhumation history of a metapelitic granulite within the Ha-Tshanzi structure from the Central Zone of the Neoarchean–Paleoproterozoic Limpopo high-grade Complex, South Africa. Quartzofeldspathic garnet-bearing coarse-grained leucosomes in the rock alternate with attenuated shear bands consisting of biotite, cordierite, sillimanite and quartz that prominently modified the earlier garnet porphyroblasts. Cores of garnet porphyroblasts contain various polyphase inclusions that are interpreted as crystallized inclusions of melt. The phase equilibria modeling and regular zoning of garnet with respect to major (Mg, Fe, Ca) and some trace (P, Cr, Sc) elements reveals that a garnet + biotite + plagioclase + quartz ± sillimanite assemblage in the rock coexisted with the melt during the sub-isothermal (810–830 °C) exhu...

We analysed 585 My old antitaxial fibrous calcite veins that formed at about 4-6 km depth in dark... more We analysed 585 My old antitaxial fibrous calcite veins that formed at about 4-6 km depth in dark carbonaceous shales, found near Arkaroola, northern Flinders Ranges, South Australia [1]. The veins contain micron-sized biomorph structures, such as coccus-and globular structures, ...

2004 Denver Annual …, Jan 1, 2004

Extremophile microbes are currently known to inhabit the surface and upper kilometres of the Eart... more Extremophile microbes are currently known to inhabit the surface and upper kilometres of the Earth's crust. Although deep microbial life may date back to the beginning of life on Earth, fossil evidence for deep life remains virtually non-existent. A main problem is the difficulty to find and recognise micron-sized fossil remains in the vast volume of rock.

We analysed 585 My old antitaxial fibrous calcite veins that formed at about 4-6 km depth in dark carbonaceous shales, found near Arkaroola, northern Flinders Ranges, South Australia [1]. The veins contain micron-sized biomorph structures, such as coccus- and globular structures, indications of cell division (wall bands, meridian constriction) and thin (0.1 µm) fibres with knots. The combined morphological evidence indicates that these constitute fossil microbes. These microbes must have lived during vein growth and, therefore, possibly played a role in the formation of the veins. The microbes bear some resemblance to the modern-day hyperthermal Archaea, such as Pyrodictium, which thrive at temperatures of about 80-120°C and have been found in hot springs and oil wells. Living by sulphur-metabolism, these Archaea typically alter carbonate solubility, leading to calcite precipitation, as well as minor pyrite as a by-product. Minor sulphides were found inside the veins. XRF and LA-ICPMS analyses of the veins and their immediate wall rock showed mobilisation of Ca, Fe, Mg, Na, Mn, P, S, Sr, Y, Sc, Pb, Zn, U, Cu, and Mo, some of which may indicate microbial activity.

The fossil microbes from Arkaroola indicate that life had penetrated deeper rocks at least at the end of the Proterozoic. If there is a link between the distinct antitaxial fibrous texture of the veins and microbial activity, this type of veins may help in the search for more, and possibly older evidence for deep microbial life in the geological record.

Australian Journal of Earth Sciences, Feb 17, 2016

ABSTRACT The Neoproterozoic to Cambrian Selwyn Block in Central Victoria forms the mainly unexpos... more ABSTRACT The Neoproterozoic to Cambrian Selwyn Block in Central Victoria forms the mainly unexposed basement to the Paleozoic metasediments, granitic rocks and felsic volcanic complexes of the Melbourne Zone of the Lachlan Orogen. The Late Devonian felsic rocks are largely products of partial melting of the Selwyn Block, and their chemistry implies that their sources were most probably arc-related andesite, dacite, volcaniclastic greywackes and some pelites. When plotted against the median longitudes of the plutons and volcanic complexes, the average values for 87Sr/86Srt and ϵNdt (at 370 Ma) reveal broad trends interpreted to reflect possible compositional and/or age structure in the Selwyn Block. Assuming that the trends are real, from W to E, I-type sources are progressively less crustally evolved, probably younging eastward. The S-type sources show no trend in ϵNdt, suggesting that there was efficient sediment mixing. The 87Sr/86Srt values, however, become more evolved eastward (opposite in sense to the apparent variation in the I-type sources). This is interpreted as the original Selwyn Block sediments having been more pelitic eastward, perhaps suggesting a deepening of the basin in this direction, as well as structurally upward in the succession. The opposite senses of variation highlights the spatial separation of the S- and I-type sources and suggest that the granitic magmas here are unlikely to represent any sort of mixing continuum.

Transactions of The Royal Society of South Africa, Aug 2, 2016

Using chemical evidence, particularly the variations between titanium and iron + magnesium, we de... more Using chemical evidence, particularly the variations between titanium and iron + magnesium, we demonstrate that the petrogenetic processes that operated within Central Victorian granitic host magmas and their igneous microgranular enclave suites are dissimilar. Chemical variations within the granitic series result from a variety of what might be called ‘orderly’ processes, resulting in linear or curvilinear trends in chemical variation diagrams. Those that affected the enclave suites commonly resulted in scattered, chaotic variations. Even in cases where an enclave suite displays a more orderly chemical variation, it is demonstrable that the hypothesis of simple mixing between an enclave magma and a host granitic magma, to bring about the overall chemical variations, cannot be supported. The enclave magmas had vastly smaller volumes compared with their host granitic magmas. Thus, they have commonly undergone hybridisation through mixing with deep crustal melts and both chemical and mechanical interactions with wall rocks and their enclosing granitic magmas. As a result of this complex and chaotic set of processes, it remains extremely difficult to unravel the precise mechanisms that produced a given suite of enclave magmas. Due to the similarities between the studied granites and their enclaves with occurrences worldwide, we suggest that our findings are likely to have general applicability.

EGU General Assembly Conference Abstracts, Apr 1, 2017

South African Journal of Geology, Mar 1, 2023

An integrated approach embracing field studies, petrographic and geochemical investigations toget... more An integrated approach embracing field studies, petrographic and geochemical investigations together with zircon U-Pb-Hf data was used to investigate the petrogenesis of potassic granite suites along the southern margin of the Zimbabwe Craton. Zircon U-Pb geochronology identifies age relationships, revealing coeval magmatism of the ca. 2 635 ± 5 to 2 625 ± 3 Ma Chilimanzi Suite, and the ca. 2 627 ± 7 Ma Razi Suite. Both suites represent syn- to late-tectonic, high-K, calc-alkaline, and metaluminous to weakly peraluminous granites and granodiorites with I-type affinity. The granite suites contain xenocrystic zircons, with the Chikwanda Pluton of the Chilimanzi Suite yielding a grain of up to 3 206 Ma old. Both granite suites exhibit eHf values of between -5.6 ± 1.3 and -7.3 ± 1.6 and TDM model ages of ca. 3.4 to 3.5 Ga which suggests a similar crustal source. The unradiogenic zircon Hf isotopic compositions are consistent with formation of the granite suites through partial melting of pre-existing crustal protoliths, including Palaeoarchaean tonalite-trondhjemite-granodiorites (TTGs) of the Zimbabwe proto-craton. Partial melting of lower crust gave rise to granitic melts that became emplaced over a relatively short time interval from 2 635 to 2 625 Ma and heralded the stabilisation of the Zimbabwe Craton.In addition to virtually identical ages, the Razi and Chilimanzi suites have similar geochemistry. Small geochemical differences between the Chilimanzi and the Razi suites are attributed to the crustal level at which they are preserved, the modal mineralogy and the extent to which the melts are evolved. The Razi Suite melts were generated from lower crust partial melting of thickened charnockite-enderbite source rocks rich in heat producing elements. The partial melting occurred under fluid-absent conditions and magmas were emplaced at lower to mid crustal levels. The Chilimanzi Suite magmas were similarly derived by the partial melting of TTG lower crust and were emplaced at upper crustal levels. Accordingly, the Chilimanzi Suite exhibits more evolved magmatic fractionation indices indicated by high Rb/Sr, as well as low K/Rb ratios relative to the Razi Suite. Both suites reveal varying degrees of enrichment in incompatible elements including Rb, Th, and U, as well simultaneous depletions in Ba, Sr, and Hf which underscores the role of fractional crystallisation in the evolution of the granitic magmas.

Sedimentology, Apr 23, 2022

The age and properties of detrital zircon in a sediment reflect properties of the rocks in which ... more The age and properties of detrital zircon in a sediment reflect properties of the rocks in which the zircon crystallized, and not necessarily the immediate precursor of the host sediment. Where clastic sediments are recycled, these properties are preserved so that U-Pb ages and Lu-Hf data on zircon grains will no longer give information on the routing of detritus. Because the crustal evolution and sedimentary recycling history of Southern Africa is well-established, Holocene sediments in the region provide a good test example for detrital zircon geochronology applied to sands or sandstones with a complex recycling history. Data from 16 samples of unconsolidated sand, including dunes from the southern part of the Kalahari Basin, beach sands at the Atlantic coast, and isolated inland dune occurrences, yield ranges of early Mesozoic to late Archaean ages. Uranium-lead and lutetium-hafnium isotope data provide no evidence of direct derivation of zircon from protosources in crystalline bedrock. The complex distribution patterns can be decomposed into seven 'provenance components' that have previously been encountered in Palaeoproterozoic to Jurassic sedimentary rocks in the region. These components occur in non-random combinations, in proportions that suggest mixing of recycled material from different sedimentary precursors. Taking pre-Cenozoic geology, Cretaceous to Pleistocene drainage history of southern Africa and Holocene wind and current patterns into account, it is possible to work out a consistent model for transport of detritus from sedimentary precursors into the basin, followed by aeolian remobilization within the basin itself. The detrital zircon distributions can thus be interpreted from prior knowledge of the basin filling history, regional geology, geomorphology, drainage evolution and wind pattern. This amounts to a line of reasoning from 'sink back to (intermediate) 'source', which is the opposite of the '(proto)source-to-sink' paradigm commonly invoked in detrital zircon studies.

Precambrian Research, Nov 1, 2020

Journal of Metamorphic Geology, Sep 1, 2001

One of the currently popular theories on magma ascent is that it mainly occurs by propagating hyd... more One of the currently popular theories on magma ascent is that it mainly occurs by propagating hydrofractures (dykes) and that magma viscosity is the primary rate-controlling factor. This theory is based on mathematical models for single hydrofractures under idealised conditions. We simulated magma ascent with air ascending through gelatine and observed that the air ascended in batches, following paths made by their predecessors. Multiple batches accumulate at obstacles along the path. Although magma viscosity may control ascent rate during movement, obstacles ultimately control the size and average ascent velocity of ascending batches. We propose that step-wise movement of magma batches is the mechanism of primary accumulation and ascent from the partially molten source rock of a magma to its ®rst emplacement site and therefore the main ascent mechanism for granitic magmas.`Classical' dyking is the mechanism for secondary ascent from a magma chamber.

Eos, Transactions American Geophysical Union, 2005

Truly fibrous calcite veins occur mainly in carbonaceous shales and are characterised by high len... more Truly fibrous calcite veins occur mainly in carbonaceous shales and are characterised by high length:width ratios of their fibres (>10). Previous studies on their Sr isotopic geochemistry (Elburg et al., 2002: Geol. Soc. London Spec. Publ. 200, 103-118; Hilgers and Sindern, 2005: Geofluids, in press) have shown that some of the material could be derived from the local wall rock. These studies also showed that the veins were always enriched in Sr compared to the calcite in the host rocks. Aragonite can contain significantly more Sr than calcite, while it also tends to have a fibrous crystal habit. It is therefore possible that the fibrous habit of these veins, which now consist of calcite, are a reflection of their initial aragonitic mineralogy, rather than of any special tectonic regime during their formation. This idea was investigated by analysing the major and trace element geochemistry of selected fibrous and non-fibrous calcite veins from Arkaroola (northern Flinders Ranges, Australia). The fibrous vein analysed for major elements contains less than 1% MgCO3, whereas calcite in the host rock, with which it is in Sr isotopic equilibrium, contains 18% MgCO3. Calcite can contain significant Mg, whereas the aragonitic structure cannot accomodate this ion, so this result is consistent with the idea of an original aragonitic mineralogy of the veins. The fibrous veins show an enrichment in the middle rare earth elements (REE) compared to the calcite in the host rock and blocky veins. In a Post-Archean Average Shale normalised diagram, Eu is more strongly enriched compared to its neighbouring elements in the fibrous veins, but not in the host calcite, blocky veins, or in the silicate fraction of the host rock, suggesting more reducing conditions during fibrous vein formation. This data cannot be used as direct evidence for the fibrous veins' aragonitic mineralogy. It does, however, show that significant differences exist between calcite in host rocks, blocky and fibrous calcite veins, and this data should be incorporated in any model explaining the origin of fibrous veins.

The Pan-African Damara Belt in southern Africa is a trench-trench-trench triple junctions orogen ... more The Pan-African Damara Belt in southern Africa is a trench-trench-trench triple junctions orogen that formed at 590-470 Ma during the Gondwana Supercontinent assembly. The Damara Belt records up to granulite facies HTLP metamorphism in the core, upper plate of the orogen. However, the cause of this metamorphism is not well understood. To tackle this problem, we focus on the ENE-WSW-trending Namibfontein-Vergenoeg (NV) migmatitic domes. We use P-T-t-d data to investigate the temporal relationships of deformation fabrics, metamorphism and melting.The NV domes formed through the superposition of four folding events. We use LA-(Q/MC)-ICP-MS U-Pb dating of monazite from structurally controlled granitoids and leucosomes to define the relative timing of the deformation phases. These include 1) an early phase of E-W shortening forming upright F1 folds and steep N-S-striking S1 deformation fabrics. D1 was active between ~559 and 530 Ma. 2) N-S shortening followed, forming dome-scale F2 antic...

Contributions to Mineralogy and Petrology

The Hokuroku region of north-eastern Japan is endowed with important volcanic-hosted massive sulp... more The Hokuroku region of north-eastern Japan is endowed with important volcanic-hosted massive sulphide Zn–Pb–Cu deposits, which are considered the archetype of Kuroko (black ore) deposits worldwide. The bimodal, felsic-dominated volcanic succession that hosts the ore was deposited in a continental rift formed during continental extension in the final stages of the Miocene back-arc opening that led to the formation of the Japan Sea. In this study, we define some of the fundamental intensive parameters of this volcanism (temperature, pressure of crystallisation, fluid saturation, fO2) based on rock textures, and analyses of whole-rock samples, minerals and melt inclusions. Based on the melt inclusion analyses, we assess the behaviour of metals during magma evolution and degassing, and evaluate the possible implications for ore deposition. Plagioclase-melt geothermometry in felsic tuff and lava samples collected from both the units underlying and overlying the Kuroko indicates temperatu...

Earth-Science Reviews, 2022

Precambrian Research, 2021

Abstract The Sveconorwegian orogen represents a branch of Grenville-age (~1250–950 Ma) orogenic b... more Abstract The Sveconorwegian orogen represents a branch of Grenville-age (~1250–950 Ma) orogenic belts that formed during the construction of the supercontinent Rodinia. This study traces the Sveconorwegian records from its type-area in the Baltic Shield of South Norway into basement windows underneath Caledonian nappes, by combining zircon U–Pb geochronology and Hf–O isotopes. Samples along a N-S trending transect reveal multiple magmatic episodes during Gothian (ca. 1650 Ma), Telemarkian (ca. 1500 Ma) and Sveconorwegian (1050–1020 Ma vs. 980–930 Ma) orogenesis as well as Sveconorwegian migmatization (1050–950 Ma). Newly documented 1050–1020 Ma magmatism and migmatization extend the Sirdal Magmatic Belt to a 300 km-long, NNW-SSE trending crustal domain, with the northern boundary corresponding to the gradual transition from Telemarkian to Gothian crust. These Precambrian crustal heterogeneities largely controlled the development of Caledonian shear zones. The ca. 1050–1040 Ma granitic and mafic magmas show similar isotopic signatures with slightly negative or positive eHf(t) and moderate δ18O values (6–7‰), which indicates that crustal reworking was more dominant than juvenile inputs during their genesis. The generation of leucosomes and leucogranites at ca. 1030–1020 Ma, which have a more evolved Hf isotopic composition, probably reflects an even higher degree of remelting of older crust. The Hf–O isotopic patterns show that Sveconorwegian magmas differ from typical arc magmas by lower involvement of sedimentary components and juvenile material. This makes the 1050–930 Ma magmatism incompatible with a long-term subduction setting. The ca. 1650–1500 Ma samples, in contrast, generally have juvenile Hf isotopic compositions associated with varying δ18O values of 4.5–9‰, consistent with subduction-accretion processes involving significant sedimentary recycling. This accretionary margin was most likely transformed into the Sveconorwegian orogen through collisional interactions of Baltica, Laurentia and Amazonia in the context of Rodinia amalgamation.

Journal of African Earth Sciences, 2022

Journal of Petrology, 2021

Gneiss domes cored by migmatites and granites represent the principal role of anatectic melts dur... more Gneiss domes cored by migmatites and granites represent the principal role of anatectic melts during the exhumation of high-grade metamorphic complexes. This study explores the exhumation history of a metapelitic granulite within the Ha-Tshanzi structure from the Central Zone of the Neoarchean–Paleoproterozoic Limpopo high-grade Complex, South Africa. Quartzofeldspathic garnet-bearing coarse-grained leucosomes in the rock alternate with attenuated shear bands consisting of biotite, cordierite, sillimanite and quartz that prominently modified the earlier garnet porphyroblasts. Cores of garnet porphyroblasts contain various polyphase inclusions that are interpreted as crystallized inclusions of melt. The phase equilibria modeling and regular zoning of garnet with respect to major (Mg, Fe, Ca) and some trace (P, Cr, Sc) elements reveals that a garnet + biotite + plagioclase + quartz ± sillimanite assemblage in the rock coexisted with the melt during the sub-isothermal (810–830 °C) exhu...

Die ältesten Obsidianfunde Sachsens wurden in den zwanziger Jahren des 20. Jahrhunderts aus Schmö... more Die ältesten Obsidianfunde Sachsens wurden in den zwanziger Jahren des 20. Jahrhunderts aus Schmölen bei Wurzen, östlich von Leipzig gemeldet 6 . Zuerst ist nur die Rede von einer Klinge und einem "Abfallstück", die aus der Sandgrube des dortigen Rittergutes stammen sollten. Später tauchen, angeblich vom gleichen Fundplatz, einige aus Obsidian gefertigte flachretuschierte Pfeilspitzen sowie ein Kern aus dem gleichen Material auf. Trotz intensiver Bemühungen seitens des Landesmuseums, die Fundumstände zu klären, gelang dies damals nicht. Zwar sind die Funde nicht mehr auffindbar, doch lassen einige Abbildungen der Objekte im Archiv des Landesamtes