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Journal articles on the topic 'Granulite – Namibia'

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Author: Grafiati

Published: 4 June 2021

Last updated: 16 February 2022

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1

Franz, Leander, Rolf L. Romer, and D. Pieter Dingeldey. "Diachronous Pan-African granulite-facies metamorphism (650 Ma and 550 Ma) in the Kaoko belt, NW Namibia." European Journal of Mineralogy 11, no. 1 (February 11, 1999): 167–80. http://dx.doi.org/10.1127/ejm/11/1/0167.

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2

Diener, Johann F. A., Åke Fagereng, and Sukey A. J. Thomas. "Mid-crustal shear zone development under retrograde conditions: pressure–temperature–fluid constraints from the Kuckaus Mylonite Zone, Namibia." Solid Earth 7, no. 5 (September 16, 2016): 1331–47. http://dx.doi.org/10.5194/se-7-1331-2016.

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Abstract. The Kuckaus Mylonite Zone (KMZ) forms part of the larger Marshall Rocks–Pofadder shear zone system, a 550 km-long, crustal-scale strike-slip shear zone system that is localized in high-grade granitoid gneisses and migmatites of the Namaqua Metamorphic Complex. Shearing along the KMZ occurred ca. 40 Ma after peak granulite-facies metamorphism during a discrete tectonic event and affected the granulites that had remained at depth since peak metamorphism. Isolated lenses of metamafic rocks within the shear zone allow the P–T–fluid conditions under which shearing occurred to be quantified. These lenses consist of an unsheared core that preserves relict granulite-facies textures and is mantled by a schistose collar and mylonitic envelope that formed during shearing. All three metamafic textural varieties contain the same amphibolite-facies mineral assemblage, from which calculated pseudosections constrain the P–T conditions of deformation at 2.7–4.2 kbar and 450–480 °C, indicating that deformation occurred at mid-crustal depths through predominantly viscous flow. Calculated T–MH2O diagrams show that the mineral assemblages were fluid saturated and that lithologies within the KMZ must have been rehydrated from an external source and retrogressed during shearing. Given that the KMZ is localized in strongly dehydrated granulites, the fluid must have been derived from an external source, with fluid flow allowed by local dilation and increased permeability within the shear zone. The absence of pervasive hydrothermal fractures or precipitates indicates that, even though the KMZ was fluid bearing, the fluid/rock ratio and fluid pressure remained low. In addition, the fluid could not have contributed to shear zone initiation, as an existing zone of enhanced permeability is required for fluid infiltration. We propose that, following initiation, fluid infiltration caused a positive feedback that allowed weakening and continued strain localization. Therefore, the main contribution of the fluid was to produce retrograde mineral phases and facilitate grain-size reduction. Features such as tectonic tremor, which are observed on active faults under similar conditions as described here, may not require high fluid pressure, but could be explained by reaction weakening under hydrostatic fluid pressure conditions.

3

Masberg, H. P., E. Hoffer, and S. Hoernes. "Microfabrics indicating granulite-facies metamorphism in the low-pressure central Damara Orogen, Namibia." Precambrian Research 55, no. 1-4 (March 1992): 243–57. http://dx.doi.org/10.1016/0301-9268(92)90026-k.

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4

Diener, Johann F. A., Richard W. White, Klemens Link, Tanya S. Dreyer, and Adam Moodley. "Clockwise, low- metamorphism of the Aus granulite terrain, southern Namibia, during the Mesoproterozoic Namaqua Orogeny." Precambrian Research 224 (January 2013): 629–52. http://dx.doi.org/10.1016/j.precamres.2012.11.009.

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5

Groenewald, C. A., and P. H. Macey. "Lithostratigraphy of the Mesoproterozoic Yas-Schuitdrift Batholith, South Africa and Namibia." South African Journal of Geology 123, no. 3 (September 1, 2020): 431–40. http://dx.doi.org/10.25131/sajg.123.0029.

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Abstract The granitic and leucogranitic Yas and Schuitdrift Gneisses occur together as a large ovoid pre-tectonic batholith that crosses the Orange River border between South Africa and Namibia. They occur in the central parts of the Kakamas Domain in the Namaqua Sector of the Namaqua-Natal Metamorphic Province where they intrude, and are deformed together with, slightly older (~1.21 Ga) orthogneisses and granulite-facies metapelitic gneisses. The Yas Gneiss occurs mainly on the outer perimeter and northern parts of the batholith and comprises equigranular leucogranite gneiss and biotite granite augen orthogneiss, whereas the Schuitdrift biotite-hornblende augen gneiss is located at the centre and southern parts of the batholith. The batholith is strongly deformed with penetrative Namaqua-aged gneissic fabrics defined by grain-flattening of quartz and feldspar in the equigranular leucogneisses and aligned K-feldspar megacrysts in the augen gneisses. The gneissic fabric is refolded during a large-scale folding event that results in the dome-shape of the batholith and controls the present outcrop pattern of its various components. Flexure along the margins of the batholith refoliated the gneisses into a zone of mylonitic rocks. The Yas and Schuitdrift Gneisses have similar geochemistry and classify as alkali granites and alkali leucogranites. They are felsic (mean SiO2: 74.5 wt%) and potassic (mean K2O: 5.8 wt%) but have low MgO, CaO and Na2O, reflecting their low mafic mineral and plagioclase contents. The Schuitdrift Gneiss yielded U-Pb zircon ages of 1 191 ± 7 and 1 187 ± 6 Ma.

6

Ward, Robert, Gary Stevens, and Alex Kisters. "Fluid and deformation induced partial melting and melt volumes in low-temperature granulite-facies metasediments, Damara Belt, Namibia." Lithos 105, no. 3-4 (October 2008): 253–71. http://dx.doi.org/10.1016/j.lithos.2008.04.001.

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7

Gray, Tim, Judith Kinnaird, Justin Laberge, and Alejandro Caballero. "Uraniferous Leucogranites in the Rössing Area, Namibia: New Insights from Geologic Mapping and Airborne Hyperspectral Imagery." Economic Geology 116, no. 6 (September 1, 2021): 1409–34. http://dx.doi.org/10.5382/econgeo.4828.

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Abstract This study combines historical exploration data with new mapping, underpinned by airborne hyperspectral imagery, to provide a detailed camp-scale geologic view of the Rössing uranium mine area in the Damara orogen, Namibia. The Neoproterozoic Damaran metasedimentary host rocks to uranium deposits of the Rössing area structurally overlie Paleoproterozoic basement rock. Both units were subjected to polyphase deformation and upper amphibolite to lower granulite facies metamorphism during Pan-African orogenesis. The sequence was voluminously intruded by leucogranites, where younger phases may contain ore-grade uranium as magmatic uraninite and traces of betafite, together with secondary uranium minerals. Early, postdepositional modifications to the Damaran sequence included partial dolomitization of marble units and development of evaporite dissolution and diapiric breccias. Major pre-D3 extensional structures developed in conjunction with recumbent, isoclinal folding and acted to focus the intrusion of early, mostly barren leucogranites generated primarily through anatexis of Damaran metasediments. Syn-D4 leucogranites overprint complex interference fold geometries that resulted from D3 deformation. D4 leucogranites were emplaced under predominantly ductile, transtensional conditions, into NNE-trending zones oriented highly oblique to all preexisting structures. These steeply dipping zones provided the prerequisite conditions for partial melt material to be derived from uraniferous basement lithologies. The concentration of magmatic uranium was promoted where leucogranite melt material interacted with carbonates and sulfide-bearing Damaran metasedimentary units. In the Rössing area these horizons occur at the Khan-Rössing Formation contact zone for the SJ, SK, SH, Z20, and Husab deposits and within and above the Arandis Formation for the Z19 deposit leucogranites.

8

Stenvall, C. A., A. Fagereng, J. F. A. Diener, C. Harris, and P. E. Janney. "Sources and Effects of Fluids in Continental Retrograde Shear Zones: Insights from the Kuckaus Mylonite Zone, Namibia." Geofluids 2020 (August 1, 2020): 1–21. http://dx.doi.org/10.1155/2020/3023268.

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Midcrustal rocks in retrograde metamorphic settings are typically H2O-undersaturated and fluid-absent and have low permeability. Exhumed continental retrograde faults, nonetheless, show evidence for the operation of fluid-mediated weakening mechanisms during deformation at midcrustal conditions. To explore the origin and effects of fluids in retrograde faults, we study the Kuckaus Mylonite Zone (KMZ), an exhumed crustal-scale, strike-slip shear zone in the southern Namibian Namaqua Metamorphic Complex. The KMZ deformed quartzofeldspathic migmatised gneisses at midcrustal retrograde conditions (450-480°C, 270-420 MPa) in the Mesoproterozoic, 40 Ma after granulite facies peak metamorphism at 825°C and 550 MPa. The mylonites contain fully hydrated retrograde mineral assemblages, predominantly adjacent to anastomosing high-strain zones, providing evidence of local H2O saturation and fluid presence during deformation. Whole rock and quartz vein δ18O values suggest that at least some of the fluids were meteoric in origin. The rocks across the shear zone retain the effect of different protoliths, implying little effect of fluid-rock interaction on whole rock major element chemistry. Together with a general scarcity of quartz veins, this suggests that fluid/rock ratios remained low in the KMZ. However, even small amounts of H2O allowed reaction weakening and diffusion-precipitation, followed by growth and alignment of phyllosilicates. In the ultramylonites, a fine grain size in the presence of fluids allowed for grain size sensitive creep. We conclude that the influx of even small volumes of fluids into retrograde shear zones can induce drastic weakening by facilitating grain size sensitive creep and retrograde reactions. In retrograde settings, these reactions consume fluids, and therefore elevated fluid pressures will only be possible after considerable weakening has already occurred. Our findings imply that the range of seismic styles recently documented at active retrograde transform faults may not require high fluid pressures but could also arise from other local weakening mechanisms.

9

Jung, S., A. Kröner, and S. Kröner. "A ∼700 Ma Sm–Nd garnet–whole rock age from the granulite facies Central Kaoko Zone (Namibia): Evidence for a cryptic high-grade polymetamorphic history?" Lithos 97, no. 3-4 (September 2007): 247–70. http://dx.doi.org/10.1016/j.lithos.2006.12.013.

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10

Jung, Stefan, Soenke Brandt, Rebecca Bast, Erik E. Scherer, and Jasper Berndt. "Metamorphic petrology of a high-T /low-P granulite terrane (Damara belt, Namibia) - Constraints from pseudosection modelling and high-precision Lu-Hf garnet-whole rock dating." Journal of Metamorphic Geology 37, no. 1 (October 2, 2018): 41–69. http://dx.doi.org/10.1111/jmg.12448.

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11

Doggart, S., P. H. Macey, and D. Frei. "Lithostratigraphy of the Mesoproterozoic Twakputs Gneiss." South African Journal of Geology 124, no. 3 (September 1, 2021): 783–94. http://dx.doi.org/10.25131/sajg.124.0041.

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Abstract The Twakputs Gneiss is a garnetiferous, K-feldspar megacrystic, biotite granite-granodiorite orthogneiss. It represents a major unit in the Kakamas Domain of the Mesoproterozoic Namaqua-Natal Metamorphic Province extending about 250 km between Riemvasmaak in South Africa and Grünau in southern Namibia. The Twakputs Gneiss occurs as foliation-parallel, sheet-like bodies tightly infolded together with granulite-facies paragneisses into which it intrudes along with a variety of other pre-tectonic granite and leucogranite orthogneisses. These rocks were subsequently intruded by late-tectonic garnet-leucogranites, granites and charnockites. The Twakputs Gneiss is a distinctive unit characterised by large ovoid to elongate megacrysts of twinned perthitic K-feldspar, set in a coarse-grained matrix of garnet, biotite, quartz and feldspar. It contains a penetrative foliation defined by the alignment of K-feldspars and streaks of biotite that developed during the main phase D2 of the Namaqua Orogeny (~1.2 to 1.1 Ga). The foliation and an accompanying elongation lineation are more intensely developed along lithological contacts, especially at the margins of the mega-scale F3 domes and basins that refold the regional fabrics. U-Pb zircon dating of the Twakputs Gneiss has yielded concordia ages of between ~1192 and 1208 Ma. Whole-rock geochemistry shows consistent major, trace and REE elemental trends, and thus reflect chemical variability from a single fractionating magma. The Twakputs Gneiss has a granitic to granodiorite composition and is strongly peraluminous. The geochemistry and the ubiquitous presence of garnet and pelitic xenoliths indicate an S-type granite protolith. The Twakputs Gneiss is the most voluminous and widespread member of the Eendoorn Suite which comprises seven textural variants of garnetiferous, K-feldspar-megacrystic granitoid orthogneiss of the same age.

12

Bial, Julia, Steffen Büttner, and Peter Appel. "Timing and conditions of regional metamorphism and crustal shearing in the granulite facies basement of south Namibia: Implications for the crustal evolution of the Namaqualand metamorphic basement in the Mesoproterozoic." Journal of African Earth Sciences 123 (November 2016): 145–76. http://dx.doi.org/10.1016/j.jafrearsci.2016.07.011.

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13

FRYBERGER, STEVEN G., PATRICK HESP, and KATHLEEN HASTINGS. "Aeolian granule ripple deposits, Namibia." Sedimentology 39, no. 2 (April 1992): 319–31. http://dx.doi.org/10.1111/j.1365-3091.1992.tb01041.x.

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14

Upton, Steve J., and Paul S. Freed. "Description of the oocysts of Isospora pachydactyli sp.nov. (Apicomplexa: Eimeriidae) from Bibron's gecko, Pachydactylus bibronii (Reptilia: Gekkonidae)." Canadian Journal of Zoology 66, no. 3 (March 1, 1988): 597–98. http://dx.doi.org/10.1139/z88-088.

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Isospora pachydactyli sp.nov. (Apicomplexa: Eimeriidae) is described from the feces of Pachydactylus bibronii bibronii from Namibia (South-West Africa). Oocysts sporulate endogenously and are spherical or subspherical, 27.4 × 27.1 (25.2–29.6 × 25.2–29.6) μm, with a bilayered wall ca. 1.5 μm thick; shape index (length/width) 1.0 (1.0–1.1). Micropyle, polar granule, and oocyst residuum are absent. Sporocysts are ovoid, 15.4 × 10.6 (14.4–16.8 × 9.6–11.2) μm, with Stieda and substieda bodies. The posterior end of the sporocyst is drawn out slightly to resemble a second Stieda body. Sporocyst residuum present, consisting either of a compact granular mass or of scattered granules. Sporozoites vermiform, with spherical anterior and posterior refractile bodies.

15

Seth, Barbara, Richard A. Armstrong, Sönke Brandt, Igor M. Villa, and Jan D. Kramers. "Mesoproterozoic U–Pb and Pb–Pb ages of granulites in NW Namibia: reconstructing a complete orogenic cycle." Precambrian Research 126, no. 1-2 (September 2003): 147–68. http://dx.doi.org/10.1016/s0301-9268(03)00193-1.

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16

BRANDT, S. "Ultrahigh-Temperature Metamorphism and Multistage Evolution of Garnet-Orthopyroxene Granulites from the Proterozoic Epupa Complex, NW Namibia." Journal of Petrology 44, no. 6 (June 1, 2003): 1121–44. http://dx.doi.org/10.1093/petrology/44.6.1121.

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17

Brandt, Sönke, Thomas M. Will, and Reiner Klemd. "Magmatic loading in the proterozoic Epupa Complex, NW Namibia, as evidenced by ultrahigh-temperature sapphirine-bearing orthopyroxene–sillimanite–quartz granulites." Precambrian Research 153, no. 3-4 (March 2007): 143–78. http://dx.doi.org/10.1016/j.precamres.2006.11.016.

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18

Le Heron, D. P., M. E. Busfield, and C. Kettler. "Ice-rafted dropstones in “postglacial” Cryogenian cap carbonates." Geology, October 21, 2020. http://dx.doi.org/10.1130/g48208.1.

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Dropstones of ice-rafted origin are typically cited as key cold-climate evidence in Cryogenian strata and, according to conventional wisdom, should not occur in postglacial, warm-water carbonates. In Namibia, the Chuos Formation (early Cryogenian) contains abundant dropstone-bearing intervals and striated clasts. It is capped by the Rasthof Formation, composed of laminites in its lower portion and microbial carbonates above. These laminites are locally found to contain pebble- and granule-sized lonestones in abundance. At the Omutirapo outcrop, meter-thick floatstone beds occur at the flanks of a Chuos paleovalley and are readily interpreted as mass-flow deposits. At Rasthof Farm, however, the clasts warp, deflect, and penetrate hundreds of carbonate laminations at both the outcrop and thin-section scale. We propose that these are dropstones, and we infer an ice-rafting mechanism. Evidence for vestigial glaciation concomitant with cap carbonate deposition thus merits a reappraisal of the depositional conditions of cap carbonates and their paleoclimatic significance.

19

Lohmeier, S., B. G. Lottermoser, T. Schirmer, and D. Gallhofer. "Copper slag as a potential source of critical elements - A case study from Tsumeb, Namibia." Journal of the Southern African Institute of Mining and Metallurgy 121, no. 3 (2021). http://dx.doi.org/10.17159/2411-9717/1383/2021.

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SYNOPSIS At a time of resource consumption, it is important to study the chemical composition of mining and metallurgical wastes to prevent the dissipative loss of metals and metalloids from the mining value chain. In particular, the recovery of critical elements from wastes is an option to increase the resources of such materials that are economically significant and have an overall supply risk. In this paper we report on the chemical composition, in particular the critical element content, of granulated slag originating from historical smelting activities in the Tsumeb area, Namibia. Laboratory-based inductively coupled plasma-mass spectrometry (ICP-MS) and X-ray fluorescence (XRF) analyses as well as portable X-ray fluorescence (pXRF) demonstrate that the slags are on average enriched in base metals (Cu 0.7 wt%, Pb 2.7 wt%, Zn 4.7 wt%), trace metals and metalloids (Cd approx. 50 mg/kg, Mo approx. 910 mg/kg) as well as critical elements (As approx. 6300 mg/kg, Bi approx. 3 mg/kg, Co approx. 200 mg/kg, Ga approx. 100 mg/kg, In approx. 9 mg/kg, Sb approx. 470 mg/kg). While metals and metalloids such as As, Mo and Pb can be determined reliably using pXRF instruments, the technique has inherent limitations in evaluating the contents of certain critical elements (Ga, Sb). However, there are positive correlations between the As, Mo, and Pb contents determined by pXRF and the Ga and Sb contents obtained through ICP-MS and XRF. Thus, quantitative pXRF analysis for As, Mo, and Pb allows calculation of Ga and Sb abundances in the slags. This work demonstrates that pXRF analysers are a valuable tool to screen smelting slags for their chemical composition and to predict the likely contents of critical elements. Keywords: base metal slag, portable XRF, critical elements, secondary resource.