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The Peralkaline Nepheline Syenites of the Junguni Intrusion, Chilwa Province, Malawi

Published online by Cambridge University Press:  05 July 2018

Alan R. Woolley  and
R. Garth Platt
Alan R. Woolley
Affiliation:
Department of Mineralogy, British Museum (Natural History), Cromwell Road, London SW7 5BD, U.K.
R. Garth Platt
Affiliation:
Department of Geology, Lakehead University, Thunder Bay, Ontario, Canada
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Abstract

The mineralogy of the highly peralkaline Junguni nepheline syenite intrusion of the Chilwa alkaline province has been investigated. The rocks comprise alkali feldspar, very abundant nepheline, locally exceptionally abundant sodalite, sodic pyroxenes, scarce biotite, rare amphibole, and an extensive range of accessory minerals. Electron microprobe analyses indicate that the pyroxenes define an evolutionary trend from salite through aegirine-augite to aegirine, which is unusual in its broadness and ill-definition. This is explained by a series of overlapping trends produced by fluctuating Fe3+/Fe2+ ratios caused by variations in alkali content of the magma, probably produced by periodic alkali loss by surface de-gassing. Much of the sodalite occurs as a primary liquidus phase, but the paragenesis of certain ramifying sodalite veins is more problematical. It is possible that such ramifying masses are the product of coupled migration of alkalis and volatiles by diffusion in a gravitation field under pressure gradients generated by eruptive events, or by the formation of immiscible Na- and Cl-rich liquids. Both early calcic and late mangan-fluor eckermannitic/arfvedsonitic amphiboles occur. The micas vary from biotites with Mg: Fe ratios < 0·5 to almost pure annites; they are fluor-micas and characterized by high Mn contents. Analyses of niobian rutile, mangan ilmenite, ferroan pyrophanite, manganese-rich eucolite, låvenite, mangan-titan låtvenite, rosenbuschite, wöhlerite, pyrochlore, eudialyte and what is believed to be only the second occurrence of kupletskite are given. Many of these minerals are rich in Zr, Nb, Na and Mn and thus typical of the assemblages found in extreme agpaitic complexes such as Ilimaussaq and Lovozero.

Keywords

nepheline syenitesChilwa provinceMalawirutileilmenitepyrophaniteeucolitelåveniterosenbuschitepyrochlorewöhleriteeudialytekupletskite
Type
Petrology and Geochemistry
Information
Mineralogical Magazine , Volume 52 , Issue 367 , September 1988 , pp. 425 - 433
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1988

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References

Barker, D.S. (1976) Phase relations in the system NaAlSiO4-SiO2NaCI-H2O at 400°-800°C and 1 kilobar, and petrologic implications. J. Geol. 84, 97–106.CrossRefGoogle Scholar
Binsted, N. (1981) The system Ab-Ne-NaC1H2O. In Progress in Experimental Petrology (Ford, C.E., ed.). NERC Publ. Ser. D. No. 18, 34-6.Google Scholar
Bloomfield, K. (1965) The geology of the Zomba area. Bull. Geol. Surv. Malawi 16, 193 pp.Google Scholar
Deer, W.A., Howie, R.A., and Zussman, J. (1962) Rockforming minerals, Vol. 3, Sheet silicates. Longmans, London. 270 pp.Google Scholar
Dixey, F., Smith, W.C. and Bisset, C.B. (1937) The Chilwa Series of southern Nyasaland; a group of alkaline and other intrusive and extrusive rocks and associated limestones. Bull. Geol. Surv. Nyasaland 5, 85 pp.Google Scholar
Ferguson, J. (1964) Geology of the Ilimaussaq alkaline intrusion, South Greenland: description of map and structure. Medd. om Gronland, 173(3), 1-82.Google Scholar
Fudali, R.F. (1965) Oxygen fugacities of basaltic and andesitic magmas. Geochim. Cosmochim. Acta, 29, 106-375.Google Scholar
Hamilton, D.L. (1961) Nephelines as crystallization temperature indicators. J. Geol. 69, 321-9.Google Scholar
Kogarko, L.N. (1987) Alkaline rocks of the eastern part of the Baltic Shield (Kola Peninsula). In Alkaline Igneous Rocks, (Fitton, J.G. and Upton, B.G.J. eds) Geol. Soc. London, Spec. Publ. 30, 531-44.Google Scholar
Kogarko, L.N. Ryabchikov, I.D., and Sorensen, H. (1974) Liquid fractionation. In The Alkaline Rocks (Sorensen, H., ed.) John Wiley, London. 488-500.Google Scholar
Kogarko, L.N. Ryabchikov, I.D., and Sorensen, H. Burnham, C., and Shettle, D. (1977) Water regime in alkalic magmas. Geochem. Int. 14(3), 1-8.Google Scholar
Koster van Groos, A.F. (1966) The effect ofNaF, NaCl, and Na2CO3 on the phase relationships in selected joins of the system Na2OCaO-Al2Oa-SiO2H2O at elevated temperatures and pressures. PhD. Thesis, Leiden.Google Scholar
Larsen, L.M. (1976) Clinopyroxenes and coexisting mafic minerals from the alkaline Ilimaussaq intrusion, South Greenland. J. Petrol. 17, 258-90.Google Scholar
Leake, B.E. (1978) Nomenclature of amphiboles. Mineral. Mag. 42, 533-63.CrossRefGoogle Scholar
Mitchell, R.H. and Platt, R.G. (1978) Mafic mineralogy of ferroaugite syenite from the Coldwell alkaline complex, Ontario, Canada. J. Petrol. 19, 627-51.Google Scholar
Mitchell, R.H. and Platt, R.G. (1982) Mineralogy and petrology of nepheline syenites from the Coldwell alkaline complex, Ontario, Canada. Ibid. 23, 186-214.Google Scholar
Nash, W.P. and Wilkinson, J.F.G. (1970) Shonkin Sag laccolith, Montana. I. Mafic minerals and estimates of temperature, pressure, oxygen fugacity and silica activity. Contrib. Mineral. Petrol. 25, 241-69.Google Scholar
Paul, A. and Douglas, R.W. (1965) Ferrousferric equilibrium in binary alkali silicate glasses. Phys. Chem. Glasses, 6, 207-11.Google Scholar
Platt, R.G. and Woolley, A.R. (1986) The mafic mineralogy of the peralkaline syenites and granites of the Mulanje complex, Malawi. Mineral. Mag. 50, 85-99.CrossRefGoogle Scholar
Semenov, E.I. (1956) Kupletskite: a new mineral of the astrophyllite group. Dokl. Akad. Nauk. SSSR 108, 93-36.Google Scholar
Stephenson, D. and Upton, B.G.J. (1982) Ferromagnesian silicates in a differentiated alkaline complex: Kungnat Fjeld, South Greenland. Mineral. Mag. 46, 283-300.Google Scholar
Tilley, C.E. (1954) Nepheline-alkali feldspar paragenesis. Am. J. Sci. 252, 65-75.Google Scholar
Wellman, T. (1968) Stability of sodalite in the system NaA1SiO4NaC1H2O. Am. Geoph. Union Trans, 49, 34-23.Google Scholar
Woolley, A.R. and Jones, G.C. (1987) The petrochemistry of the northern part of the Chilwa alkaline province, Malawi. In Alkaline Igneous Rocks, (Fitton, J.G. and Upton, B.G.J., eds) Geol. Soc. London, Spec. Publ. 30, 335-55.Google Scholar
Woolley, A.R. and Jones, G.C. and Platt, R.G. (1986) The mineralogy of nepheline syenite complexes from the northern part of the Chilwa Province, Malawi. Mineral. Mag. 50, 597-610.Google Scholar