Genetic relationships between migmatites and the Swartoup Pluton in the Swartoup Hills (central Namaqua Belt)
- Authors: Schmeldt, Graeme Alvin
- Date: 2021-10-29
- Subjects: Migmatite South Africa Northern Cape , Intrusions (Geology) South Africa , Metamorphic rocks South Africa Northern Cape , Metamorphism (Geology) South Africa Northern Cape , Onseepkans (South Africa) , Namaqualand (South Africa) , Anatexis , Swartoup , Koenap
- Language: English
- Type: Master's theses , text
- Identifier: http://hdl.handle.net/10962/192162 , vital:45201
- Description: The central Namaqua Metamorphic Complex can be characterised by long-standing high-temperature (up to granulite/amphibolite facies) conditions between _ 1300 and 1100Ma, inevitably resulting in widespread metamorphism and plutonism. Hosted within a NW–SE striking antiformal structure about 40 km east of Onseepkans, Northen Cape, South Africa, in the Swartoup Hills, lies the Swartoup Pluton. The Swartoup Pluton was sampled and described in hand specimen and thin section. The study area was photographed, with all data presented in this study. The various rock types are readily discerned in the field due to their characteristic weathering colours and overall fabrics. The Swartoup granodioritic body is hosted within metasediments of the Bysteek and Koenap Formations, of the Arribees Group. The package was later intruded by another later granitoid, the Polisiehoek Granite-gneiss. The Bysteek Formation, a wall rock to the S-type Swartoup Pluton, reacted at the contact with the igneous body resulting in localised feldspathic granites and granodiorites with prominent, often euhedral, garnet, pryoxene and titanite. The Swartoup Pluton is divided into two subgroups. The first is characterised by higher P2O5 contents, _ 0.3 – 0.4 wt.%, shown with a narrower constraint on its Rb contents, _ 80 – 160 ppm, than the second, with _ 0.14 – 0.4 wt.% P2O5 and 20 – 310 ppm Rb. Meanwhile the Polisiehoek Granite-gneiss shows _ 50 – 420 ppm Rb and _ 0.04 – 0:1 wt% P2O5. As a whole, the Swartoup Pluton is characterised by somewhat elevated CaO concentrations (_ 1.5 – 6.0 wt.%), relative to calculated averages of granites (1.8 wt.% CaO, Le Maitre, 1976) and granodiorites (3.9 wt.% CaO, Le Maitre, 1976). Whilst most of the Swartoup specimens were classified as granodiorites, some orthopyroxene-bearing monzodiorite and orthopyroxenebearing monzonite were locally found and sampled. However, much of the body appears to be granodioritic to granitic in composition. The Polisiehoek Granite-gneiss is characterised by its orange-brown weathering colour in the field, sheared texture, lower P2O5 and higher total alkali content than the Swartoup Pluton. The Polisiehoek Granite-gneiss is a highly fractionated S-type granite, as shown by plots of (a) (Na2O + K2O)/CaO and (b) FeOT/MgO versus Zr + Nb + Ce + Y (Whalen et al., 1987; Zhang et al., 2019) and also of (c) (Al2O3 + CaO)/(FeOT + Na2O + K2O) versus 100 × (MgO + FeOT + TiO2)/SiO2 (after Sylvester, 1989). Classification schemes identify the Polisiehoek Granite-gneiss as either a granite (TAS diagram, after Middlemost, 1994) or alkali granite (R1R2 diagram, after De la Roche et al., 1980). , Thesis (MSc) -- Faculty of Science, Geology, 2021
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- Date Issued: 2021-10-29
Ilmenite megacryst-hosted melt inclusions from the Monastery kimberlite: implications for kimberlite origins
- Authors: Van Huyssteen, Aiden
- Date: 2021-04
- Subjects: To be added
- Language: English
- Type: Masters theses , text
- Identifier: http://hdl.handle.net/10962/178387 , vital:42935
- Description: Polymineralic inclusions encapsulating a daughter assemblage of crystalline phases (including silicates, oxides, and carbonates) and an amorphous glass phase, hosted in ilmenite megacrysts from the Monastery kimberlite, were investigated texturally and geochemically in order to constrain their melt origin, modeof formation, and evolution prior to quenching. The isolated nature of the melt inclusions within the ilmenite megacrysts provides an opportunity to study components of primary kimberlitic magma captured within the SCLM (4.5–6 GPa) that has been isolated from pervasive modifying processes that are common in kimberlites. The common daughter phase assemblage within the melt inclusions comprises serpentine, phlogopite, calcite, spinel, kassite, perovskite, ilmenite, and glass. The glass is Si-Mg-Fe-rich, with low Al2O3 contents. It is also K2O- and TiO2-free, with variably depleted REE. In composition, serpentine forms a crystalline equivalent to the glass. However, these phases are optically distinct. Serpentine represents two modes of formation: (i) discrete euhedral grains set within a glass matrix that represent a primary phase, crystallising directly from the entrapped melts, and (ii) as patches of partially crystallised glass that represent a secondary phase formed by the devitrification of the glass. Spinel and phlogopite form along early kimberlitic evolutionary trends and record the depletion of the melt in TiO2, Al2O3, and K2O, which typically decreases from the core to the rim of the crystals. Volatile and alkali-bearing minerals (calcite, apatite, phlogopite) crystallised within the melt inclusions from the captured alkali-rich carbonated-silicate kimberlite melt. The daughter mineral assemblage initially crystallised as euhedral and subhedral grains with a uniform composition under equilibrium conditions. Subsequent crystallisation formed grains that exhibit magmatic zoning due to their crystallisation in a progressively depleted melt. Lastly, the crystallisation of skeletal oxide grains occurred under disequilibrium conditions, at a stage of magma ascent with rapidly changing variables including temperature, melt viscosity, and diffusivity. Prior to complete crystallisation, the residual Si-Mg-Fe melt of this crystallisation process was quenched to form the observed glass. The phases that constitute the common daughter assemblage show large variations in modal proportions, forming a continuum from silicate-rich to carbonate-rich endmember inclusions, with certain daughter phases absent in some inclusions. This suggests that the melt was heterogenous at the time of capture and comprised immiscible silicic/oxidic and carbonate melts. Phase separation, therefore, may have started prior to capturing of magma batches as inclusions in ilmenite, but further segregation and crystallisation continued after these batches had become isolated from the megacryst matrix as melt inclusions. The immiscibility and co-existence of the silicic/oxidic and carbonate melts is preserved by textural features between calcite and glass, such as rounded globules of calcite grains set within a silicate glass matrix, calcite forming the matrix for euhedral silicate and oxide minerals, and calcite occupying the interior void of skeletal oxide grains set within a silicate glass matrix. Furthermore, spherulitic globular domains of Ca- and Ti-rich glasses set within a matrix of the Si-Mg-Fe glass suggest that the silicic/oxidic melt underwent further segregation into oxide-rich (Ca-Ti) and silicate-rich (Si-Mg-Fe-Al-K-Ti) melts, potentially crystallising the oxide and silicate minerals of the daughter assemblage, respectively. The abundance of incompatible trace elements and the Cr-poor composition of secondary low-Mg ilmenite as a daughter mineral within the melt inclusions (~1400 ppm Nb; <0.1 wt% Cr2O3; <0.1 wt% MgO), in addition to the Cr-poor composition of the other daughter phases within the inclusions (i.e. <0.1 wt% Cr2O3 for phlogopite and spinel), indicate that they crystallised from a similar melt as the Cr-poor, but high Mg-ilmenite megacrysts (~1400 ppm Nb; <0.1 wt% Cr2O3; ~10 wt% MgO). Furthermore, the melt inclusions are randomly distributed and no textural and/or geochemical evidence for melt infiltration of the ilmenite megacrysts was associated with the melt inclusions. These features are consistent with a primary origin for the melt inclusions which implies a cognate relationship between the megacrysts and the captured kimberlite melt. , Thesis (MSc) -- Faculty of Science, Geology, 2021
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- Date Issued: 2021-04
Petrography, metamorphism, deformation and P-T conditions in the western arm of the Lufilian Arc - Zambezi, north-western Zambia
- Authors: Chilekwa, Mwango
- Date: 2020
- Subjects: Petrogenesis -- Zambia -- Zambezi District , Metamorphism (Geology) -- Zambia -- Zambezi District , Petrology -- Zambia -- Zambezi District , Formations (Geology) -- Zambia -- Zambezi District , Rock deformation -- Zambia -- Zambezi District , Lufilian Arc , Neoproterozoic Katangan R.A.T. (Roches Argilo Talqueuse) Subgroup
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: http://hdl.handle.net/10962/161971 , vital:40699
- Description: The Zambezi area in north-western Zambia is underlain by Neoproterozoic Katanga Supergroup and older, Archean to Mesoproterozoic Basement Supergroup rocks. The area lies within the Domes Region, which is a structural domain of the Lufilian Arc. The stratigraphic succession within Zambezi area is dominated by the Grand Conglomerate Formation (GC) and Mwashia Group which are the most extensive units, and the less abundant Lower and Upper Roan Groups of the Katanga Supergroup. They wrap around the domal Basement Supergroup units. The mineral assemblage of the Mwashia and the GC schists commonly contains garnet, anthophyllite and biotite. GC rocks show remnants of primary structures such as clasts and sedimentary features. Anthophyllite, garnet and biotite are the dominant Mg-Fe rich metamorphic minerals. However, these are iron rich for each mineral phase and has been attributed to iron rich protoliths. The earliest recognised deformation episode (D1) formed NE-SW S1 foliations within GC which is consistent with the regional structural trend in the western Lufilian Arc. S1 was later affected by D2 that generated downward facing F2 folds and S2 foliations. The other associated feature to D2 is garnet that grew as the result of pro-grade metamorphism. The D3 deformation fabric is not developed and did not affect much of the structural geometry of the Zambezi area. The peak assemblages of the Basement Supergroup and the Katanga Supergroup formed at mid-amphibolite facies conditions of 590 °C and 630 °C at an average pressure of 4.0 kbar. The Basement Supergroup has undergone retrograde metamorphism to greenschist facies condition indicated by presence of chlorite and also determined by biotite-anorthite isopleth in THERIAK DOMINO. At the eastern part of Zambezi area, the Katanga Supergroup rocks were retrogressed in the upper greenschist facies at about ~470°C and ~4.0 kbar due to isobaric cooling.
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- Date Issued: 2020
Gem-bearing granitic pegmatites in Malawi: their mineralogy, geochemistry, age, and fluid compositional variations
- Authors: Kankuzi, Charles Frienderson
- Date: 2019
- Subjects: Granite , Pegmatites , Geochemistry , Fluid inclusions , Nonferrous metals
- Language: English
- Type: Doctoral theses , text
- Identifier: http://hdl.handle.net/10962/97905 , vital:31505 , DOI https://doi.org/10.21504/10962/97905
- Description: The gem bearing granitic pegmatites from different pegmatite fields across Malawi intrude all important geological entities from the Palaeoproterozoic in the north, the Mesoproterozoic in central Malawi and the Pan-African basement in the south. U/Pb zircon and Rb/Sr mineral isochron ages indicate pegmatite emplacement from the Palaeoproterozoic to Pan-African and Mesozoic time. Most pegmatites are related to the Pan-African cycle; no Mesoproterozoic pegmatites were observed in this study. Within the Pan-African pegmatite groups there are two important subgroups. Some pegmatites show Sr isotopic compositions that indicate mantle components contributing to the parental granites from which the pegmatites evolved. Others show higher Sr initials, indicating crustal granites as primary pegmatite sources or significant crustal contamination. Only for few pegmatites, such as the Palaeoproterozoic and Ordovician gem tourmaline pegmatites in the Chitipa and Dowa Districts, the granitic source is evident from their field context. For all others the granitic origin is interpreted by mineralogical and geochemical evidence. All analysed pegmatites belong to either the Rare Element Class or the Miarolitic Class, but they vary in their degree of fractionation. The more evolved pegmatites are more enriched in incompatible elements such as Be, Li, B, and Ta, which resulted in the formation of gem minerals such as beryl, aquamarine, tourmaline and topaz, which may or may not be associated with tantalite. The Rare Element pegmatites can be further subdivided into the REL-Li subclass, beryl type, beryl-columbite subtype, and in the complex type and elbaite subtype. The Miarolitic pegmatites include Mi-Li subclass and beryl-topaz type. Fluid inclusion studies (heating-cooling stage, Raman spectroscopy) identified a variety of fluid compositions that were present at different times and different places, indicating a variety of fluid sources. They range from aqueous-saline to CO2–rich carbonic fluids (CO2 +C3H8+ N2), or aqueous-carbonic fluids (H2O-CO2-CH4 and H2O-CO2-H2-H2S-CH4). The dominant solutes and species for the pegmatites show genetic variations over time and orogen (Paleo-/Meso-/Neoproterozoic). Uniform homogenisation temperatures and salinities in individual samples indicate that the gem-bearing pegmatites contained homogeneous fluids at the time of their capturing in quartz. Based on fluid inclusion data, the estimated trapping conditions of inclusions in quartz for all studied pegmatites except for one pegmatite suggest low pressures between 0.9 to 2.6 kb at temperatures of 400-600 C. The other pegmatite formed at slightly higher pressures of 2.2 to 3.6 kb. However, the pressure range for all the pegmatites is in agreement with the known liquidus conditions of Rare-Element pegmatite crystallisation. The shallow crustal emplacement level (3.4-9.8 km) and the greater depth (8.3 to 13.6 km) favoured the formation of gemstones. , Thesis (PhD) -- Faculty of Science, Geology, 2019
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- Date Issued: 2019
Sulphide textures and compositions associated with the hydrothermal/magmatic system of the Twangiza gold deposit (South Kivu, DRC)
- Authors: Busane, Emmanuel Aganze
- Date: 2019
- Subjects: Gold mines and mining -- Congo (Democratic Republic) , Geology -- Congo (Democratic Republic) , Hydrothermal alteration -- Congo (Democratic Republic) , Sulphide minerals -- Congo (Democratic Republic) , Gold ores -- Geology -- Congo (Democratic Republic) , Geochemistry -- Congo (Democratic Republic) , Twangiza Mine (Congo (Democratic Republic))
- Language: English
- Type: text , Thesis , Masters , MSc
- Identifier: http://hdl.handle.net/10962/76588 , vital:30610
- Description: Twangiza mine is a gold deposit situated in the eastern Democratic Republic of Congo. The rock types at the Twangiza Mine consist of black shale, including carbonaceous mudstone and thin intercalated layers of siltstone, and feldspar-rich granitoid intrusive sills, referred to as albitite, folded into a major antiformal structure. The gold mineralization at the mine is commonly found associated with sulphides. The sulphide textures and compositions of mineralized and unmineralized samples of black shales, albitite sills and hydrothermal veins in the mine are considered for the understanding of the spatial association of gold with sulphides and gold mineralization history of the mine. The sulphides within the Twangiza mine consist of pyrite, arsenopyrite, pyrrhotite, chalcopyrite and rare cobaltite. The primary pyrite texture occurs in unmineralized black shale and is interpreted to be diagenetic. It consists of fine-grained anhedral pyrite crystals aggregating into spherical nodules and formed in replacement of organic material during the diagenesis process. The secondary pyrite textures resulted from the hydrothermal fluids activity and include (i) aggregates of annealed anhedral crystals into sulphide-rich lenses; (ii) elongated anhedral pyrite in the form of short stringers; (iii) fine-grained subhedral to euhedral pyrite randomly distributed within the rock matrix; (iv) euhedral zoned pyrite crystals occurring within veins; (v) aggregations of fine-grained anhedral pyrite, locally distributed in the matrix; (vi) abundant dissemination of fine-grained subhedral to anhedral pyrite crystals within the vein selvedge in the host rock; (vii) and coarse-grained massive pyrite bodies. The pyrite major elemental composition does not vary significantly in the different textures and sample types. The Fe content ranges from 44.57 to 46.40 wt. %, and the S content ranges from 53.75 to 55.25 wt. %. Pyrite from mineralized black shale and hydrothermal veins contains relatively higher concentrations of As (~ 1 wt. %) than pyrite from other sample types. The arsenopyrite commonly occurs as fine-grained anhedral crystals as inclusions within pyrite, medium-grained crystal intergrowing with pyrite and/or as coarse-grained massive arsenopyrite bodies in the massive sulphide veins. The arsenopyrite composition is uniform in all textural and sample type with Fe content ranging from 33.44 to 35.20 wt. %, S content ranging from 21.13 to 22.55 wt. % and As content ranging from 42.20 to 43.97 wt. %. In mineralized black shale and unmineralized black shale, the arsenopyrite shows, however, minor concentrations of Ni with 0.39 and 0.70 wt. % respectively. The pyrrhotite occurs as fine-grained anhedral patchy crystals randomly distributed within the rock matrix of unmineralized black shale and unmineralized granitoid, and / or as inclusions within pyrite in mineralized granitoid. The pyrrhotite shows a uniform composition in all samples and textural types, though minor concentrations of Ni (2.06 wt. %) content are reported in unmineralized granitoid. Chalcopyrite occurs as fine-grained crystals in inclusions within pyrite; and cobaltite occurs as rare fine-grained anhedral crystals occasionally disseminated in the albitite sill matrix. The chalcopyrite composition does not vary considerably in all sample and textural types, and cobaltite shows minor concentrations of Ni (4.55 wt. %) and Fe (3.45 wt. %). Native gold grains are commonly found associated with the secondary pyrite texture especially within the sulphide-rich lenses and in the massive sulphide veins, and are almost pure with ~97 wt. %. A Na-rich hydrothermal fluid from low-grade metamorphism associated with the E-W compressive tectonic event, which caused formation of the antiform structure which control the mineralization in the deposit area, led to the albitization of the deposit rocks and specially the alteration of the granitic assemblage to form albitite, and the deposition of aggregates of fine-grained anhedral crystals and growth and annealing of pyrite in sulphide-rich lenses. Afterward, the CO2-rich hydrothermal fluids influx circulated through reactivated structures, including quartz veins, and led to the precipitation of dolomite, ankerite, siderite and magnesite. They also led to the precipitation of pyrite of secondary textures as well as arsenopyrite, chalcopyrite and formation of pyrrhotite from the desulphurization of early pyrite. The CO2-rich hydrothermal fluids probably leached gold and other trace elements such as As, Co, etc. from the sedimentary host rocks and deposited them into suitable traps, such as the sulphide-rich lenses and massive sulphide bodies, preferably within the hinge zone of anticline axis constituting a hydrothermal fluid pathway.
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- Date Issued: 2019
Chemical weathering on selected nunataks in western Dronning Maud Land, Antarctica
- Authors: Knox, Jenna Tracy
- Date: 2018
- Subjects: Glacial climates -- Antarctica -- Queen Maud Land , Glaciology -- Antarctica -- Queen Maud Land , Chemical weathering -- Antarctica -- Queen Maud Land , Atmospheric carbon dioxide -- Environmental aspects , Climatic changes -- Antarctica -- Queen Maud Land , Nunataks -- Antarctica -- Queen Maud Land
- Language: English
- Type: text , Thesis , Masters , MSc
- Identifier: http://hdl.handle.net/10962/61658 , vital:28046
- Description: High latitude areas are sensitive to the impacts of climate change, and it is expected that the impact of greenhouse warming will be much higher in the polar regions than in any other climatic zones, with the most highly affected area being that of the Antarctic rim (Barsch, 1993). Weathering and pedogenic processes respond to variations in climate, with models predicting that chemical weathering may increase synchronously with global carbon dioxide levels increase, due to dissolution rates and the erosional impact of hydrological cycles in warming climates (Anderson & Anderson, 2010). As liquid water becomes more available in Antarctica the potential for chemical weathering, due to a less moisture-limited environment and increased temperatures, increases (Convey et al., 2009). Weathering processes are important for soil formation and the production of fine-grained material, with chemical weathering being an active constituent of this. Increased rates of soil formation are likely to occur, with global climate changes resulting in greater chemical weathering occurring in Antarctica. Opportunistic sampling was conducted during the Austral summer of 2016/2017, whereby rock, snow and meltwater samples were taken at various sites within the western portion of Dronning Maud Land of Antarctica. Rock samples were placed in resin, and cut with a diamond saw to create thin sections. Optical microscopy and scanning transmission electron microscopy (STEM) were used to analyse mineral weight percentage with depth. Twelve soil samples were dried and weighed, sieved and statistically represented according to particle size. Inductively coupled plasma mass spectrometry (ICP-MS) determined the geochemical analysis for 10 water and snow samples. Rock hardness was inferred through the use of an Equotip, with rebound values recorded for multiple rock faces and samples. Thermal regimes of rock temperature was further recorded using a FLIR infrared camera, and documented for each rock face over a 24 hour period at 2 hourly intervals. The products of increased chemical weathering were evident from particle size analysis; samples were very poorly sorted in nature, and undergo in situ weathering, whereby products were not removed by erosional processes. Weathering rinds were found to be siliceous and ferric, depending on parent lithology. Ferric ratios increased in wt.% from the substrate rock to the external surface, creating the red, iron rich crusts noted on the hand specimens. The observable chemical weathering was found adjacent to intrusions through Precambrian dolerites. Geochemical analysis revealed thin, carbonaceous features, with impurity-rich layers, characteristic of speleothem formation. Carbonaceous layers did not follow underlying substrate features, rather deposited at the external surface, upon which, further precipitation growth could occur, creating karst features. Extensive gypsum coatings (>2mm) under BSE imagery were identified, with the abundance of gypsum salts (below surface level) and rock coatings indicating active sulphuric acid weathering, in western Dronning Maud Land, Antarctica. Were mechanical processes faster than chemical, weathering rinds and solution features on silicate rocks would be uncommon in the Antarctic, periglacial landscape. However, this is not the case as the existence of these landforms implies that chemical weathering may occur faster than mechanical weathering processes (Pope et al., 1995). In a changing world, one needs to monitor these processes at a micro-scale in order to fully understand how periglacial environments react to global climatic changes, and the subsequent impacts on these sensitive environments.
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- Date Issued: 2018
Petrogenesis of the Bysteek and Koenap Formation Migmatites, Central Namaqualand
- Authors: Moodley, Jason Anthony
- Date: 2013
- Subjects: Petrogenesis -- South Africa -- Namaqualand Migmatite -- South Africa -- Namaqualand Granulite -- South Africa -- Namaqualand Thermodynamics Geology, Stratigraphic -- Proterozoic Geology, Stratigraphic -- Proterozoic -- South Africa -- Namaqualand
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:4914 , http://hdl.handle.net/10962/d1001574
- Description: The Mesoproterozoic rocks of the Bysteek and Koenap Formations of the Arribees Group are exposed within a NW-SE striking antiformal structure comprised of mafic granulites and metapelitic diatexites, and a number of marble and calc-silicate rock layers. The mafic granulites of the Bysteek Formation show a typological variety of anatectic features, including nebulitic, stromatitic mesosomes, melanosomes, quartz syenitic leucocratic vein networks and syenitic pools. Melanosomes consist of hedenbergitic to diopside-rich clinopyroxene (XMg: 0.40), anorthitic plagioclase (An90), with some quartz, minor apatite and titanite. Anatexis was caused by biotite dehydration melting and formed a melt of probably granitic composition. The leucosome composition ranges from either alkali-feldspar-granitic to plagioclase rich or granitic. This variation is interpreted as a result of variable extraction of melt from the source to granitic pools. The diatexites of the Koenap Formation are most likely of metapelitic or meta-greywacke origin. They are texturally variable but always contain high modal contents of alkali feldspar and quartz which generally form magmatic textures. Almandine-rich garnet (XMg: 0.18-0.25), cordierite (XMg: 0.71) form secondary biotite, sillimanite and magnetite during retrograde breakdown. Thermodynamic modelling of mafic granulite compositions suggests peak P-T conditions of ~865 °C and 8.6 kbar. Occasionally, garnet rich in ferric iron (XAdr: 0.55) forms by plagioclase-clinopyroxene breakdown under oxidising conditions at ~6 kilobar and ~ 800 °C. At the same stage amphibole forms in some melanosomes. P-T estimations for the diatexites based on thermodynamic modelling suggest the equilibration of the assemblage garnet, cordierite, alkali feldspar and melt at ~860 °C and 5.5 kbar. Conditions comparable to the peak pressure in the mafic granulites could not be established. However, since the diatexites and the mafic granulites are closely related in the field and no evidence of juxtaposition after the thermal peak exists, the P-T record of the diatexites might be incomplete
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- Date Issued: 2013