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Garth Platt - One of the best experts on this subject based on the ideXlab platform.
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Geochemistry and Petrogenesis of Nepheline Syenites: Kasungu–Chipala, Ilomba, and Ulindi Nepheline Syenite Intrusions, North Nyasa Alkaline Province, Malawi
Journal of Petrology, 1998Co-Authors: G. Nelson Eby, Alan R. Woolley, Vic Din, Garth PlattAbstract:The North Nyasa Alkaline Province of Malawi consists of seven INTRODUCTION Late Precambrian intrusions emplaced along a north–south trend The North Nyasa Alkaline Province (NNAP, Bloomfield, roughly parallel to that of the current rift valley. The intrusions are 1970) of central and northern Malawi consists of seven predominantly Nepheline Syenite, but minor pyroxenite is found at intrusions (Kasungu, Chipala, Chikangawa, Mphompha, Ilomba and alkali Syenite and granite are associated with Nepheline Telelele Hill, Ilomba, and Ulindi) that lie along a north– Syenite at Mphompha. The chemistry and mineralogy of four south trend roughly parallel to the current rift valley representative Nepheline Syenite intrusions (Kasungu, Chipala, Il(Fig. 1). The dominant lithology is Nepheline Syenite, omba, and Ulindi) have been investigated. Pyroxenes vary in but alkali Syenite and granite occur at Mphompha and composition from diopside (pyroxenite) through aegirine-augite to pyroxenites outcrop adjacent to, and within, the Ilomba aegirine. Al-rich micas (Ilomba and Ulindi) may reflect primary intrusion. Intrusions of gabbro, ijolite, carbonatite, nemagmatic compositions. Rocks from three intrusions plot near the pheline Syenite and Syenite in adjoining Tanzania and minimum in the 1 kbar Nepheline–Albite–Orthoclase–Kalsilite Zambia have also been included in the NNAP (Bloomphase diagram, indicating that they represent evolved liquids. Rocks field, 1970). from the fourth intrusion (Ilomba) are apparently cumulates. Nb/ All the published radiometric ages for the NNAP Ta, Zr/Hf, and Th/U ratios vary from those typical of oceanintrusions of northern and central Malawi, plus possible island basalt (OIB) to very high ratios which may reflect a additional members of the province in southern Malawi hydrothermal overprint. Rare earth element patterns are steep, slightly (Tambani), Mozambique, Tanzania, and Zambia are to moderately concave upwards, and typically lack Eu anomalies. listed in Table 1. Except for Ngualla, which may be The Ulindi and Ilomba Nepheline Syenites were derived by Nepheline, considerably older, the radiometric ages suggest that pyroxene, and titanite fractionation of a basanite–nephelinite magma, there was widespread emplacement of alkaline intrusions whereas the Kasungu and Chipala Nepheline Syenites were fracin this part of Africa in late Precambrian time. Rb–Sr tionated from an alkali basalt magma. These magmas originated whole-rock isochron and Pb–a zircon ages obtained for from a subcontinental OIB-like source. Ilomba (685 ± 62 Ma and 655 Ma) and Chikangawa
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geochemistry and petrogenesis of Nepheline Syenites kasungu chipala ilomba and ulindi Nepheline Syenite intrusions north nyasa alkaline province malawi
Journal of Petrology, 1998Co-Authors: Nelson G Eby, Alan R. Woolley, Vic Din, Garth PlattAbstract:The North Nyasa Alkaline Province of Malawi consists of seven INTRODUCTION Late Precambrian intrusions emplaced along a north–south trend The North Nyasa Alkaline Province (NNAP, Bloomfield, roughly parallel to that of the current rift valley. The intrusions are 1970) of central and northern Malawi consists of seven predominantly Nepheline Syenite, but minor pyroxenite is found at intrusions (Kasungu, Chipala, Chikangawa, Mphompha, Ilomba and alkali Syenite and granite are associated with Nepheline Telelele Hill, Ilomba, and Ulindi) that lie along a north– Syenite at Mphompha. The chemistry and mineralogy of four south trend roughly parallel to the current rift valley representative Nepheline Syenite intrusions (Kasungu, Chipala, Il(Fig. 1). The dominant lithology is Nepheline Syenite, omba, and Ulindi) have been investigated. Pyroxenes vary in but alkali Syenite and granite occur at Mphompha and composition from diopside (pyroxenite) through aegirine-augite to pyroxenites outcrop adjacent to, and within, the Ilomba aegirine. Al-rich micas (Ilomba and Ulindi) may reflect primary intrusion. Intrusions of gabbro, ijolite, carbonatite, nemagmatic compositions. Rocks from three intrusions plot near the pheline Syenite and Syenite in adjoining Tanzania and minimum in the 1 kbar Nepheline–Albite–Orthoclase–Kalsilite Zambia have also been included in the NNAP (Bloomphase diagram, indicating that they represent evolved liquids. Rocks field, 1970). from the fourth intrusion (Ilomba) are apparently cumulates. Nb/ All the published radiometric ages for the NNAP Ta, Zr/Hf, and Th/U ratios vary from those typical of oceanintrusions of northern and central Malawi, plus possible island basalt (OIB) to very high ratios which may reflect a additional members of the province in southern Malawi hydrothermal overprint. Rare earth element patterns are steep, slightly (Tambani), Mozambique, Tanzania, and Zambia are to moderately concave upwards, and typically lack Eu anomalies. listed in Table 1. Except for Ngualla, which may be The Ulindi and Ilomba Nepheline Syenites were derived by Nepheline, considerably older, the radiometric ages suggest that pyroxene, and titanite fractionation of a basanite–nephelinite magma, there was widespread emplacement of alkaline intrusions whereas the Kasungu and Chipala Nepheline Syenites were fracin this part of Africa in late Precambrian time. Rb–Sr tionated from an alkali basalt magma. These magmas originated whole-rock isochron and Pb–a zircon ages obtained for from a subcontinental OIB-like source. Ilomba (685 ± 62 Ma and 655 Ma) and Chikangawa
S. H. Jazayeri - One of the best experts on this subject based on the ideXlab platform.
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Kinetic model for isothermal sintering of porcelain stoneware body in presence of Nepheline Syenite
Thermochimica Acta, 2010Co-Authors: Amin Salem, S. H. Jazayeri, E Rastelli, G TimelliniAbstract:Abstract The effect of Nepheline Syenite on porosity–soaking time relationship of porcelain stoneware bodies has been studied using the experimentally measured total porosity. A new kinetic model was applied to quantify porosity as function of temperature and soaking time. In the first part of work, the effect of Nepheline Syenite was evaluated on the kinetic parameters and the porosity variations were found to be related to surface tension/viscosity ratio of melted phase which influences the minimum porosity of ceramic body. The kinetic analysis of data indicated that the optimum soaking time to achieve minimum porosity in ceramic body is controlled by amount of Nepheline Syenite and milling time. The validity of presented theory to predict the experimental results was substantiated by computing total porosity at other temperatures and soaking times. The proposed model can be used to describe the sintering process of ceramic bodies that are sintered by diffusing melted phase.
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dilatometeric study of shrinkage during sintering process for porcelain stoneware body in presence of Nepheline Syenite
Journal of Materials Processing Technology, 2009Co-Authors: Amin Salem, S. H. Jazayeri, E Rastelli, G TimelliniAbstract:Abstract The shrinkage of porcelain stoneware body containing different values of Nepheline Syenite was investigated. Dilatomeric tests were carried out on suitable specimens to obtain the expansion-shrinkage behavior of ceramic bodies. The differences found in shrinkage data were evaluated by kinetic model and determination of activation energy. The results showed that the activation energy increases with Na2O + K2O/quartz ratio and reaches to constant value when 10.0 wt.% Nepheline Syenite is added to starting composition. The maximum value of shrinkage also approximately remains constant with increasing fluxing agent. It was found that the fluxing effect of Nepheline Syenite was quite evident with the observed enhanced rate of densification accompanied by the typical compensation effect.
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THE USE OF Nepheline-Syenite IN A BODY MIX FOR PORCELAIN STONEWARE TILES
Ceramics International, 2005Co-Authors: Leonardo Esposito, A. Salem, Antonella Tucci, Alessandro F. Gualtieri, S. H. JazayeriAbstract:In the study, the possibility to use Nepheline-Syenite, as fluxing agent, in a body mix used for porcelain stoneware tile, was determined. Starting from the reference mix composition, different amounts, 5.0, 10.0 and 15.6 wt.%, of sodium feldspar were replaced with the same amounts of Nepheline-Syenite. Different sintering cycles were used to understand the role of the Nepheline-Syenite in the development of the final microstructure. The presence of Nepheline-Syenite strongly favours the sintering behaviour, by reducing the sintering time necessary to reach the water absorption values, requested by the International Standard. Furthermore, the microstructure results more homogeneous and the mechanical characteristics are increased. The better mechanical performances can also be attributed to the different mineralogical compositions, in particular with lower amount of glassy phase, higher flexural strength values are obtained.
Vic Din - One of the best experts on this subject based on the ideXlab platform.
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Geochemistry and Petrogenesis of Nepheline Syenites: Kasungu–Chipala, Ilomba, and Ulindi Nepheline Syenite Intrusions, North Nyasa Alkaline Province, Malawi
Journal of Petrology, 1998Co-Authors: G. Nelson Eby, Alan R. Woolley, Vic Din, Garth PlattAbstract:The North Nyasa Alkaline Province of Malawi consists of seven INTRODUCTION Late Precambrian intrusions emplaced along a north–south trend The North Nyasa Alkaline Province (NNAP, Bloomfield, roughly parallel to that of the current rift valley. The intrusions are 1970) of central and northern Malawi consists of seven predominantly Nepheline Syenite, but minor pyroxenite is found at intrusions (Kasungu, Chipala, Chikangawa, Mphompha, Ilomba and alkali Syenite and granite are associated with Nepheline Telelele Hill, Ilomba, and Ulindi) that lie along a north– Syenite at Mphompha. The chemistry and mineralogy of four south trend roughly parallel to the current rift valley representative Nepheline Syenite intrusions (Kasungu, Chipala, Il(Fig. 1). The dominant lithology is Nepheline Syenite, omba, and Ulindi) have been investigated. Pyroxenes vary in but alkali Syenite and granite occur at Mphompha and composition from diopside (pyroxenite) through aegirine-augite to pyroxenites outcrop adjacent to, and within, the Ilomba aegirine. Al-rich micas (Ilomba and Ulindi) may reflect primary intrusion. Intrusions of gabbro, ijolite, carbonatite, nemagmatic compositions. Rocks from three intrusions plot near the pheline Syenite and Syenite in adjoining Tanzania and minimum in the 1 kbar Nepheline–Albite–Orthoclase–Kalsilite Zambia have also been included in the NNAP (Bloomphase diagram, indicating that they represent evolved liquids. Rocks field, 1970). from the fourth intrusion (Ilomba) are apparently cumulates. Nb/ All the published radiometric ages for the NNAP Ta, Zr/Hf, and Th/U ratios vary from those typical of oceanintrusions of northern and central Malawi, plus possible island basalt (OIB) to very high ratios which may reflect a additional members of the province in southern Malawi hydrothermal overprint. Rare earth element patterns are steep, slightly (Tambani), Mozambique, Tanzania, and Zambia are to moderately concave upwards, and typically lack Eu anomalies. listed in Table 1. Except for Ngualla, which may be The Ulindi and Ilomba Nepheline Syenites were derived by Nepheline, considerably older, the radiometric ages suggest that pyroxene, and titanite fractionation of a basanite–nephelinite magma, there was widespread emplacement of alkaline intrusions whereas the Kasungu and Chipala Nepheline Syenites were fracin this part of Africa in late Precambrian time. Rb–Sr tionated from an alkali basalt magma. These magmas originated whole-rock isochron and Pb–a zircon ages obtained for from a subcontinental OIB-like source. Ilomba (685 ± 62 Ma and 655 Ma) and Chikangawa
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geochemistry and petrogenesis of Nepheline Syenites kasungu chipala ilomba and ulindi Nepheline Syenite intrusions north nyasa alkaline province malawi
Journal of Petrology, 1998Co-Authors: Nelson G Eby, Alan R. Woolley, Vic Din, Garth PlattAbstract:The North Nyasa Alkaline Province of Malawi consists of seven INTRODUCTION Late Precambrian intrusions emplaced along a north–south trend The North Nyasa Alkaline Province (NNAP, Bloomfield, roughly parallel to that of the current rift valley. The intrusions are 1970) of central and northern Malawi consists of seven predominantly Nepheline Syenite, but minor pyroxenite is found at intrusions (Kasungu, Chipala, Chikangawa, Mphompha, Ilomba and alkali Syenite and granite are associated with Nepheline Telelele Hill, Ilomba, and Ulindi) that lie along a north– Syenite at Mphompha. The chemistry and mineralogy of four south trend roughly parallel to the current rift valley representative Nepheline Syenite intrusions (Kasungu, Chipala, Il(Fig. 1). The dominant lithology is Nepheline Syenite, omba, and Ulindi) have been investigated. Pyroxenes vary in but alkali Syenite and granite occur at Mphompha and composition from diopside (pyroxenite) through aegirine-augite to pyroxenites outcrop adjacent to, and within, the Ilomba aegirine. Al-rich micas (Ilomba and Ulindi) may reflect primary intrusion. Intrusions of gabbro, ijolite, carbonatite, nemagmatic compositions. Rocks from three intrusions plot near the pheline Syenite and Syenite in adjoining Tanzania and minimum in the 1 kbar Nepheline–Albite–Orthoclase–Kalsilite Zambia have also been included in the NNAP (Bloomphase diagram, indicating that they represent evolved liquids. Rocks field, 1970). from the fourth intrusion (Ilomba) are apparently cumulates. Nb/ All the published radiometric ages for the NNAP Ta, Zr/Hf, and Th/U ratios vary from those typical of oceanintrusions of northern and central Malawi, plus possible island basalt (OIB) to very high ratios which may reflect a additional members of the province in southern Malawi hydrothermal overprint. Rare earth element patterns are steep, slightly (Tambani), Mozambique, Tanzania, and Zambia are to moderately concave upwards, and typically lack Eu anomalies. listed in Table 1. Except for Ngualla, which may be The Ulindi and Ilomba Nepheline Syenites were derived by Nepheline, considerably older, the radiometric ages suggest that pyroxene, and titanite fractionation of a basanite–nephelinite magma, there was widespread emplacement of alkaline intrusions whereas the Kasungu and Chipala Nepheline Syenites were fracin this part of Africa in late Precambrian time. Rb–Sr tionated from an alkali basalt magma. These magmas originated whole-rock isochron and Pb–a zircon ages obtained for from a subcontinental OIB-like source. Ilomba (685 ± 62 Ma and 655 Ma) and Chikangawa
Alan R. Woolley - One of the best experts on this subject based on the ideXlab platform.
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Geochemistry and Petrogenesis of Nepheline Syenites: Kasungu–Chipala, Ilomba, and Ulindi Nepheline Syenite Intrusions, North Nyasa Alkaline Province, Malawi
Journal of Petrology, 1998Co-Authors: G. Nelson Eby, Alan R. Woolley, Vic Din, Garth PlattAbstract:The North Nyasa Alkaline Province of Malawi consists of seven INTRODUCTION Late Precambrian intrusions emplaced along a north–south trend The North Nyasa Alkaline Province (NNAP, Bloomfield, roughly parallel to that of the current rift valley. The intrusions are 1970) of central and northern Malawi consists of seven predominantly Nepheline Syenite, but minor pyroxenite is found at intrusions (Kasungu, Chipala, Chikangawa, Mphompha, Ilomba and alkali Syenite and granite are associated with Nepheline Telelele Hill, Ilomba, and Ulindi) that lie along a north– Syenite at Mphompha. The chemistry and mineralogy of four south trend roughly parallel to the current rift valley representative Nepheline Syenite intrusions (Kasungu, Chipala, Il(Fig. 1). The dominant lithology is Nepheline Syenite, omba, and Ulindi) have been investigated. Pyroxenes vary in but alkali Syenite and granite occur at Mphompha and composition from diopside (pyroxenite) through aegirine-augite to pyroxenites outcrop adjacent to, and within, the Ilomba aegirine. Al-rich micas (Ilomba and Ulindi) may reflect primary intrusion. Intrusions of gabbro, ijolite, carbonatite, nemagmatic compositions. Rocks from three intrusions plot near the pheline Syenite and Syenite in adjoining Tanzania and minimum in the 1 kbar Nepheline–Albite–Orthoclase–Kalsilite Zambia have also been included in the NNAP (Bloomphase diagram, indicating that they represent evolved liquids. Rocks field, 1970). from the fourth intrusion (Ilomba) are apparently cumulates. Nb/ All the published radiometric ages for the NNAP Ta, Zr/Hf, and Th/U ratios vary from those typical of oceanintrusions of northern and central Malawi, plus possible island basalt (OIB) to very high ratios which may reflect a additional members of the province in southern Malawi hydrothermal overprint. Rare earth element patterns are steep, slightly (Tambani), Mozambique, Tanzania, and Zambia are to moderately concave upwards, and typically lack Eu anomalies. listed in Table 1. Except for Ngualla, which may be The Ulindi and Ilomba Nepheline Syenites were derived by Nepheline, considerably older, the radiometric ages suggest that pyroxene, and titanite fractionation of a basanite–nephelinite magma, there was widespread emplacement of alkaline intrusions whereas the Kasungu and Chipala Nepheline Syenites were fracin this part of Africa in late Precambrian time. Rb–Sr tionated from an alkali basalt magma. These magmas originated whole-rock isochron and Pb–a zircon ages obtained for from a subcontinental OIB-like source. Ilomba (685 ± 62 Ma and 655 Ma) and Chikangawa
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geochemistry and petrogenesis of Nepheline Syenites kasungu chipala ilomba and ulindi Nepheline Syenite intrusions north nyasa alkaline province malawi
Journal of Petrology, 1998Co-Authors: Nelson G Eby, Alan R. Woolley, Vic Din, Garth PlattAbstract:The North Nyasa Alkaline Province of Malawi consists of seven INTRODUCTION Late Precambrian intrusions emplaced along a north–south trend The North Nyasa Alkaline Province (NNAP, Bloomfield, roughly parallel to that of the current rift valley. The intrusions are 1970) of central and northern Malawi consists of seven predominantly Nepheline Syenite, but minor pyroxenite is found at intrusions (Kasungu, Chipala, Chikangawa, Mphompha, Ilomba and alkali Syenite and granite are associated with Nepheline Telelele Hill, Ilomba, and Ulindi) that lie along a north– Syenite at Mphompha. The chemistry and mineralogy of four south trend roughly parallel to the current rift valley representative Nepheline Syenite intrusions (Kasungu, Chipala, Il(Fig. 1). The dominant lithology is Nepheline Syenite, omba, and Ulindi) have been investigated. Pyroxenes vary in but alkali Syenite and granite occur at Mphompha and composition from diopside (pyroxenite) through aegirine-augite to pyroxenites outcrop adjacent to, and within, the Ilomba aegirine. Al-rich micas (Ilomba and Ulindi) may reflect primary intrusion. Intrusions of gabbro, ijolite, carbonatite, nemagmatic compositions. Rocks from three intrusions plot near the pheline Syenite and Syenite in adjoining Tanzania and minimum in the 1 kbar Nepheline–Albite–Orthoclase–Kalsilite Zambia have also been included in the NNAP (Bloomphase diagram, indicating that they represent evolved liquids. Rocks field, 1970). from the fourth intrusion (Ilomba) are apparently cumulates. Nb/ All the published radiometric ages for the NNAP Ta, Zr/Hf, and Th/U ratios vary from those typical of oceanintrusions of northern and central Malawi, plus possible island basalt (OIB) to very high ratios which may reflect a additional members of the province in southern Malawi hydrothermal overprint. Rare earth element patterns are steep, slightly (Tambani), Mozambique, Tanzania, and Zambia are to moderately concave upwards, and typically lack Eu anomalies. listed in Table 1. Except for Ngualla, which may be The Ulindi and Ilomba Nepheline Syenites were derived by Nepheline, considerably older, the radiometric ages suggest that pyroxene, and titanite fractionation of a basanite–nephelinite magma, there was widespread emplacement of alkaline intrusions whereas the Kasungu and Chipala Nepheline Syenites were fracin this part of Africa in late Precambrian time. Rb–Sr tionated from an alkali basalt magma. These magmas originated whole-rock isochron and Pb–a zircon ages obtained for from a subcontinental OIB-like source. Ilomba (685 ± 62 Ma and 655 Ma) and Chikangawa
Tom Andersen - One of the best experts on this subject based on the ideXlab platform.
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Contrasting trends of agpaitic crystallization in Nepheline Syenite in the Pilanesberg Alkaline Complex, South Africa
Lithos, 2018Co-Authors: Tom Andersen, Marlina Elburg, Muriel ErambertAbstract:Abstract Different types of peralkaline Nepheline Syenite in the Pilanesberg Alkaline Complex show contrasting Zr- and Ti-bearing mineral assemblages: In some members of the green foyaite suite, eudialyte is the dominant Zr mineral, as is common in many other agpaitic Nepheline Syenites. Eudialyte has formed together with aegirine ± arfvedsonite, alkali feldspar(s), sodalite and Nepheline. Aenigmatite is the main Ti-bearing mineral, but some samples have normandite (a Ti-Zr disilicate mineral) as a late magmatic mineral. Other varieties of green foyaite do not have eudialyte, but carry lamprophyllite (a Ti-Sr disilicate mineral) as a prominent, early crystallizing mineral. In contrast, pockets of trapped melt in white foyaite crystallized a series of titanium minerals starting with clearly miaskitic assemblages (with titanite and ilmenite), through increasingly agpaitic assemblages with astrophyllite, aenigmatite and lorenzenite, with hilairite forming at the end, as the only mineral with essential Zr. In both green and white foyaite, aegirine has percent- to sub-percent amounts of ZrO2. Chemographic analysis of melt-mineral equilibria suggests that the main driving force behind the evolution of mineral assemblages in both white and green foyaite is increasing peralkalinity of the melt. In the white foyaite, this took place at high aH2O throughout, whereas the green foyaite started its evolution at much lower aH2O. The different Zr and Ti mineral assemblages observed in the latter rock type can be explained by differences in water activity during crystallization of the magma, reflecting different pre-emplacement fractionation and degassing histories of batches of genetically related Nepheline Syenite magma.
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the miaskitic to agpaitic transition in peralkaline Nepheline Syenite white foyaite from the pilanesberg complex south africa
Chemical Geology, 2017Co-Authors: Tom Andersen, Marlina Elburg, Muriel ErambertAbstract:Abstract The Mesoproterozoic Pilanesberg Complex, South Africa, is built up by several distinct, ring-shaped intrusions of Syenite and peralkaline Nepheline Syenite. A mildly peralkaline ((Na + K) / Al = 1.04–1.09), medium-to coarse grained Nepheline Syenite makes up the outermost ring in the southwestern part of the complex (“Matooster type white foyaite”). In this rock, mafic silicate minerals (amphibole, biotite, aegirine) and Ti-bearing minerals (ilmenite, astrophyllite, aenigmatite, lorenzenite, bafertisite, jinshajiangite) are interstitial to feldspar and Nepheline, and define a series of mineral assemblages reflecting a change from a miaskitic crystallization regime (with Na-Ca amphibole, titanite and ilmenite) to increasingly agpaitic conditions (with arfvedsonite, aegirine, astrophyllite, aenigmatite, lorenzenite). The main driving force behind the evolution was an increase in peralkalinity of the trapped liquid, mainly by adcumulus growth of alkali feldspar and Nepheline, which in the later stages of evolution was combined with increases in oxygen fugacity and water activity. Unlike in most other agpaitic rock complexes, Zr remained compatible in aegirine (and to some extent in amphibole) almost to the end of the process, when a hydrous zirconium silicate mineral (hilairite) crystallized as the only mineral in the rock having essential zirconium. The presence of minerals such as hilairite, bafertisite, jinshajiangite and a Na-REE-Sr rich apatite group mineral (fluorcaphite ?) in the latest assemblages suggests that the last remaining interstitial melt or fluid approached a hyperagpaitic composition. The isolated melt pockets in the Pilanesberg white foyaite follow a pattern of evolution that can be seen as a miniature analogue of the fractional crystallization processes controlling magma evolution in large, alkaline igneous rock complexes.
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petrology of Nepheline Syenite pegmatites in the oslo rift norway zirconium silicate mineral assemblages as indicators of alkalinity and volatile fugacity in mildly agpaitic magma
Journal of Petrology, 2010Co-Authors: Tom Andersen, A. O. Larsen, Muriel Erambert, Rune S SelbekkAbstract:Agpaitic Nepheline Syenites contain complex zirconium silicate minerals as primary magmatic phases; agpaitic liquidus mineral assemblages are controlled by compositional parameters such as alkalinity and concentrations of water and halogens in the magma. The Larvik Plutonic Complex in the late Palaeozoic Oslo Rift, Norway consists of hypersolvus monzonite (larvikite) and different varieties of Nepheline Syenite. The Nepheline Syenite members have miaskitic mineralogy (i.e. zircon stable), except for a suite of late, mildly agpaitic pegmatites, which contain zirconium silicate minerals of the wohlerite, rosenbuschite and eudialyte groups, in some cases together with zircon. These minerals form part of the liquidus mineral assemblage of the Nepheline Syenite magma together with fluorite and the rock-forming minerals alkali feldspar, aegirine(–augite), alkali amphibole, biotite, Nepheline and sodalite. Low-variance mineral assemblages with two or three zirconium silicate minerals, fluorite and the major rock-forming minerals can be modelled in terms of phase equilibria in a multi-component system involving the sodium disilicate component of the melt and the volatile components water, fluorine and chlorine. A semiquantitative isothermal and isobaric petrogenetic grid in log activity space constructed from the observed mineral assemblages and the compositions of the coexisting minerals indicate that there are at least three trends of evolution leading from a miaskitic to an agpaitic crystallization regime in peralkaline Nepheline Syenites: (1) the increasing alkalis trend, typically leading to eudialyte-dominated liquidus assemblages (and possibly further to hyperagpaitic residual liquids); (2) the increasing water trend, leading to catapleiite crystallization; (3) the increasing fluorine trend, giving transitionally agpaitic liquidus assemblages with wohlerite, lavenite and hiortdahlite, with or without zircon, but eventually with magmatic fluorite; this type of evolution is applicable to weakly agpaitic systems such as the Oslo Rift pegmatites. The evolution of such magmas will most probably terminate at liquidus boundaries also involving eudialyte and/or rosenbuschite, and they are unlikely to evolve to more strongly agpaitic compositions.
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Crystallization and metasomatism of Nepheline Syenite xenoliths in quartz-bearing intrusive rocks in the Permian Oslo rift, SE Norway
Norsk Geologisk Tidsskrift, 1993Co-Authors: Tom Andersen, Henning SørensenAbstract:N of the Larvik pluton in the Vestfold Graben of the late Paleozoic Qslo rift of SE Norway. The Nepheline Syenite has a metaluminous major element composition, and its primary igneous mineralogy is: alkali feldspar + Nepheline + clinopyroxene + titanite + magnetite + apatite ± amphibole. The mafic silicate minerals have lower (Na + K)/AI than comparable minerals in other felsic intrusions in the Oslo Rift. Gamet (grossular-andradi te), analcime, sodalite, thomsonite and gonnardite occur as interstitial minerals in the )east altered parts of the Nepheline Syenite. The xenoliths were metasomatized as a result of interaction between Nepheline Syenite and younger silica-saturated to oversaturated magrnas and their associated fluids. Early, pervasive metasomatism led to breakdown of Nepheline, replacement of pyroxene by biotite ± garnet and crystallization of quartz. Recr ystallization took place at solidus-near temperatures (7 00-725°C), and was controlled by an increase in silica activity and oxygen fugaci ty. Titanite + magnetite were replaced by rutile + quartz + hematite + calcite at a late stage of the metasomatic history, at oxygen fuga cities above the HM buffer, and T < 450°C. The xenoliths indicate the former presence of larger bodies of Nepheline Syenite in an area where no such rocks were known previously. These intrusions were alm ost completely disgested by later intrusions.