Academic literature on the topic 'Granulite – Namibia'

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.

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.

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.

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.

Thesis (PhD (Earth Sciences))--University of Stellenbosch, 2009.
Fluid-present partial melting has generally been regarded a poor candidate for effecting crustal differentiation. In this study I report on anatectic metasediments from the Pan-African Damara Belt in Namibia that have undergone fluid-present biotite melting at a relatively low temperature, yet appear to have lost a significant volume of melt. In situ anatectic features have been identified on the basis of the existence of new generations of cordierite and/or garnet produced as the solid products of incongruent anatexis within or adjacent to leucosomes, that most commonly occur as lens shaped pods at a high angle to the lineation and formed during extension in a direction parallel to the long axis of the orogeny. Within these sites biotite underwent incongruent melting via the reaction Bt + Qtz + Pl + H2O = Melt + Grt + Crd. Cordierite nucleated on preexisting crystals within the bounding gneiss; garnet nucleated within the fracture sites (leucosomes) and typically occurs as individual, large (50 to 120 mm in diameter) poikiloblastic crystals. Thermobarometry applied to the anatectic assemblage yields low-temperature, granulitefacies peak conditions of 750 °C, 0.5 GPa. This temperature is approximately 100 °C lower than the accepted conditions for the onset of fluid-absent biotite melting. This, coupled to the focussing of anatexis on extensional fractures, suggests that anatexis occurred through waterpresent biotite incongruent melting. In order to better understand this process, both fluid-absent and water present partial melting experiments were conducted within the temperature interval 700 to 900 °C at 0.7 GPa. In the fluid-absent experiments, biotite incongruent melting started between 800 and 850 °C to produce melt coexisting with peritectic garnet and cordierite. In contrast, in water-saturated experiments, biotite melted via the reaction Bt + Pl + Q + H2O = Grt + Crd + Melt, between 700 and 750 °C, to produce melt, cordierite and garnet in the proportions 73:24:3.

The western Namaqua Metamorphic Complex (NMC) is a Mesoproterozoic mediumto low-pressure, high temperature metamorphic belt that is characterised by a complex, polyphase Mesoproterozoic metamorphic history. Using an integrated approach, the P-T-t evolution of a portion of the marginal zone between two major tectonic domains in the western NMC: the Richtersveld and Gordonia Subprovices is investigated, with the aim of resolving a P-T-t path for the peak to retrograde evolution of a major regional thrust, that will in turn help to elucidate its geodynamic significance. The Kum Kum Klippe represents a structural outlier of the Gordonia Subprovince, located at the frontal zone of the Grunau Terrane, where the Grunau Terrane (hangingwall), Pella Terrane (footwall) and the bounding Lower Fish River Thrust Zone (LFRTZ) are all well exposed. The Pella Terrane in this region shows signi ficant lithological heterogeneity and is constituted by felsic orthogneisses, metama fic rocks and leucogranite intrusives, while the Grunau Terrane is dominated by pelitic granulites. The LFRTZ along the southwestern margin of the Kum Kum Klippe is a ~30-40 m wide tabular zone primarily localised in the Pella Terrane. Phase equilibria modelling of pelitic granulites from the hangingwall constrains peak metamorphic P-T conditions of ~5.2-5.9 kbar and ~790-815°C which are correlated to a monazite growth event at c. 1262-1184 Ma. Sheared pelitic granulites constrain the P-T conditions for early stage shearing in the LFRTZ to ~2.8-5.0 kbar and ~640-785°C which likely occurred shortly after peak metamorphism. Pella amphibolite samples from the footwall and LFRTZ constrain the conditions of tectonic juxtaposition to ~2.7 kbar and ~542°C which must have occurred subsequent to cooling from suprasolidus conditions and is thus constrained at <1184 Ma. The constrained peak to retrograde P-T-t path is characterised by a short segment of substantial decompression subsequent to peak metamorphic conditions at c. 1262-1184 Ma. This is followed by a period dominated by cooling with only a small component of concomitant decompression, which together with the early decompressive segment likely correspond to the period of retrograde shearing, which ultimately led to tectonic juxtaposition in the mid-amphibolite facies at c. <1184 Ma. The P-T path presented suggests that the LFRTZ does not represent a terrane bounadry that juxtasposes crustal entities which converged and collided as the result of Wilson cycle subduction to collision tectonics. Contrastingly the metamorphism at highly elevated geotherms and largely cooling dominated retrograde trajectory determined in this study are more compatible with a continental backarc setting, which has recently been proposed as an alternative model. Thus the P-T constraints presented in this study contradict the collisional model and terrane concept that has long been widely accepted for the western NMC.

The high-grade metamorphic Epupa Complex (EC) of north-western Namibia constitutes the south-western margin of the Archean to Proterozoic Congo Craton. The north-eastern portion of the EC has been geochemically and petrologically investigated in order to reconstruct its tectono-metamorphic evolution. Two distinct metamorphic units have been recognized, which are separated by ductile shear zones: (1) Upper amphibolite facies rocks (Orue Unit) and (2) ultrahigh-temperature (UHT) granulite facies rocks (Epembe Unit). The rocks of the EC are transsected by a large anorthosite massif, the Kunene Intrusive Complex (KIC). The Orue Unit and the Epembe Unit were affected by two distinct Mesoproterozoic metamorphic events, as is evident from differences in their metamorphic grade, in the P-T paths and in the age of peak-metamorphism: (1) The Orue Unit consists of a Palaeoproterozoic volcano-sedimentary sequence, which was intruded by large masses of I-type granitoids and by rare mafic dykes. During the Mesoproterozoic (1390-1318 Ma) the Orue Unit rocks underwent upper amphibolite facies metamorphism. The volcano-sedimentary sequence is constituted by interlayered basaltic amphibolites and rhyolitic felsic gneisses, with intercalations of migmatitic metagreywackes, migmatitic metapelites, metaarkoses and calc-silicate rocks. The Orue Unit was subdivided into three parts, which record similar heating-cooling paths but represent individual crustal levels: Heating led to the partial replacement of amphibole, biotite and muscovite through dehydration melting reactions. The peak-metamorphic P-T conditions of c. 700°C, 6.5 +/- 1.0 kbar (south-eastern part), c. 820°C, 8 +/- 0.5 kbar (south-western part) and c. 800°C, 6.0 +/- 1.0 kbar (northern part) correlate well with the mineral assemblage in the metapelites, i.e. Grt-Bt-Sil gneisses and schist in the south-eastern and south-western region and (Grt-)Crd-Bt gneisses in the northern part. Peak-metamorphism was followed by retrograde cooling to middle amphibolite facies conditions. Contact metamorphism, related with the intrusion of the anorthosites, is restricted to the direct contact to the KIC and recorded by massive metapelitic Grt-Sil-Crd felses, formed under upper amphibolite facies conditions (c. 750°C, c. 6.5 kbar). (2) The Epembe Unit consists of a Palaeoproterozoic volcano-sedimentary succession, which was intruded by small bodies of S-type granitoids and by andesitic dykes. All these rocks underwent UHT granulite facies metamorphism during the early Mesoproterozoic (1520-1447 Ma). The volcano-sedimentary succession is dominated by interlayered basaltic two-pyroxene granulites and rhyolitic felsic granulites. Migmatitic metapelites and metagreywackes are intercalated in the metavolcanites. Sapphirine-bearing MgAl-rich gneisses occur as restitic schlieren in the migmatitic metagreywackes. Reconstructed anti-clockwise P-T paths are subdivided into several distinct stages: During prograde near-isobaric heating to UHT conditions at c. 7 kbar biotite- or hornblende-bearing mineral assemblages were almost completely replaced by anhydrous mineral assemblages through various dehydration melting reactions. A subsequent pressure increase of 2-3 kbar led to the formation of the peak-metamorphic mineral assemblages Grt-Opx and (Grt-)Opx-Cpx in the orthogneisses and Grt-Opx, Grt-Sil and (Grt-)(Spr-)Opx-Sil-Qtz in the paragneisses. UHT-Metamorphism is proved by conventional geothermobarometry (970 +/- 70°C; 9.5 +/- 2.5 kbar), by the very high Al content of peak-metamorphic orthopyroxene (up to 11.9 wt.% Al2O3) in many paragneisses and by Opx-Sil-Qtz assemblages in the MgAl-rich gneisses. Post-peak decompression is recorded by several corona and symplectite textures, formed at the expense of the peak-metamorphic phases: Initial UHT decompression of about ca. 2 kbar to 940 +/- 60°C at 8 +/- 2 kbar is mainly evident from the formation of sapphirine-bearing symplectites in the Opx-Sil gneisses. Subsequent high-temperature decompression to 6 +/- 2 kbar at 800 +/- 60°C resulted in the formation of Crd-Opx-Spl, Crd-Opx and Spl-Crd symplectites. Subsequent near-isobaric cooling to upper amphibolite conditions of 660 +/- 30°C at 5 +/- 1.5 kbar led to the re-growth of biotite, hornblende, sillimanite and garnet. During continued decompression orthopyroxene and cordierite were formed at the expense of biotite in several paragneisses. In a geodynamic model UHT metamorphism of the Epembe Unit is correlated with the formation of a large magma chamber at the mantle-crust boundary, which forms the source for the anorthosites of the KIC. In contrast, amphibolite facies metamorphism of the Orue Unit is ascribed to a regional contact metamorphic event, caused by the emplacement of the anorthositic crystal mushes in the middle crust
Epupa-Komplex (EK) Nordwest-Namibias bildet den südwestlichen Rand des archaischen bis proterozoischen Kongo-Kratons. Der nordöstliche Teil des EK wurde geochemisch und petrologisch untersucht, um seine tektono-metamorphe Entwicklung zu rekonstruieren. Hierbei wurden zwei unterschiedliche metamorphe Einheiten erkannt, die durch duktile Scherzonen getrennt sind: (1) Gesteine der oberen Amphibolitfazies (Orue-Einheit) und (2) Ultrahochtemperatur (UHT)-granulitfazielle Gesteine (Epembe-Einheit). Die Gesteine des EK werden von einem gewaltigen Anorthosit-Massiv, dem Kunene-Intrusiv-Komplex (KIK), durchschlagen. Unterschiede im Metamorphosegrad, in den P-T Pfaden und den Metamorphose-Altern belegen, dass die Orue-Einheit und die Epembe-Einheit von zwei unterschiedlichen mesoproterozoischen Metamorphosen erfasst wurden: (1) Die Orue-Einheit setzt sich aus einer paläoproterozoischen vulkano-sedimentären Abfolge zusammen, die von I-Typ Granitoiden und Basaltgängen intrudiert wurde. Während des Mesoproterozoikums (1390-1318 Ma) wurde die Orue-Einheit unter Bedingungen der oberen Amphibolitfazies metamorph überprägt. Die vulkano-sedimentäre Abfolge wird von einer Wechsellagerung von basaltischen Amphiboliten und rhyolitischen felsischen Gneisen aufgebaut, in die migmatitische Metagrauwacken, migmatitische Metapelite, Metaarkosen und Kalksilikate eingeschaltet sind. Die Orue-Einheit wurde in drei Regionen untergliedert, die ähnliche Aufheizungs-Abkühlungs-Pfade aufweisen, aber unterschiedliche Krustenbereiche repräsentieren: Aufheizung führte zur partiellen Verdrängung von Amphibol, Biotit und Muskovit durch Dehydratations-Schmelz-Reaktionen. Die höchstgradigen P-T Bedingungen von ca. 700°C, 6.5 +/- 1.0 kbar (südöstlicher Teil), ca. 820°C, 8 +/- 0.5 kbar (südwestlicher Teil) und ca. 800°C, 6.0 +/- 1.0 kbar (nördlicher Teil) stimmen mit den jeweiligen Mineralparagenesen der Metapelite überein (Grt-Bt-Sil-Gneise und –Schiefer im südöstlichen und –westlichen Teil und (Grt-)Crd-Bt-Gneise im nördlichen Teil). Abkühlung erfolgte unter Bedingungen der mittleren Amphibolitfazies. Kontaktmetamorphose, verbunden mit der Intrusion der Anorthosite, ist auf den direkten Kontaktbereich zum KIK beschränkt und durch undeformierte metapelitische Grt-Sil-Crd Felse überliefert, die unter Bedingungen der oberen Amphibolitfazies (ca. 750°C, ca. 6.5 kbar) gebildet wurden. (2) Die Epembe-Einheit besteht aus einer paläoproterozoischen vulkano-sedimentären Abfolge, die von kleinvolumigen S-Typ Granitoiden und Andesitgängen intrudiert wurde. Die Gesteine wurden im frühen Mesoproterozoikum (1520-1447 Ma) von einer UHT-granulitfaziellen Metamorphose erfasst. Die vulkano-sedimentäre Abfolge wird durch wechsellagernde basaltische Zwei-Pyroxen Granulite und rhyolitische felsische Granulite dominiert. Migmatitische Metapelite und Metagrauwacken sind in die Metavulkanite eingeschaltet. Sapphirin-führende MgAl-reiche Gneise treten als restititische Schlieren in den migmatitischen Metagrauwacken auf. Die rekonstruierten P-T Pfade verlaufen entgegen des Uhrzeigersinnes und sind in mehrere Stufen gegliedert: Während annähernd isobarer Aufheizung zu UHT-Bedingungen bei ca. 7 kbar wurden Biotit- und Hornblende-führende Mineralparagenesen weitgehend oder vollständig im Zuge von Dehydratations-Schmelzreaktionen verdrängt. Ein anschließender Druck-Anstieg um 2-3 kbar führte zur Bildung der höchstgradigen Mineralparagenesen Grt-Opx und (Grt-)Opx-Cpx in den Orthogneisen und Grt-Opx, Grt-Sil und (Grt-)(Spr-)Opx-Sil-Qtz in den Paragneisen. UHT-Metamorphose ist durch konventionelle Geothermobarometrie (970 +/- 70°C; 9.5 +/- 2.5 kbar), den sehr hohen Al-Gehalt von höchstgradigem Orthopyroxen (bis zu 11.9 Gew.% Al2O3) in zahlreichen Paragneisen und die Paragenese Opx-Sil-Qtz in den MgAl-reichen Gneisen belegt. Anschließende Dekompression ist durch zahlreiche Korona- und Symplektit-Gefüge um die höchstgradigen Minerale überliefert. Initiale UHT-Dekompression um ca. 2 kbar (940 +/- 60°C; 8 +/- 2 kbar) ist hauptsächlich durch Sapphirin-führende Symplektite in den MgAl-reichen Gneisen belegt. Anhaltende Dekompression unter granulitfaziellen Bedingungen (800 +/- 60°C; 6 +/- 2 kbar) führte zur Bildung von Crd-Opx-Spl, Crd-Opx und Spl-Crd Symplektiten. Anschließende annähernd isobare Abkühlung zu Bedingungen der oberen Amphibolitfazies (660 +/- 30°C; 5 +/- 1.5 kbar) führte zum Wiederwachstum von Biotit, Hornblende, Sillimanit und Granat. Während anhaltender Dekompression wurde in den Paragneisen Orthopyroxen und Cordierit auf Kosten von Biotit gebildet. In einem geodynamischen Model wird die UHT-Metamorphose wird mit der Bildung einer Magmenkammer an der Kruste-Mantel-Grenze in Zusammenhang gebracht, welche zugleich die Magmenquelle für die Anorthosite des KIK darstellt. Die amphibolitfazielle Metamorphose der Orue-Einheit wird dagegen mit einer regionalen Kontaktmetamorphose während der Platznahme der anorthositischen Magmen in Verbindung gebracht