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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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