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The mafic mineralogy of the peralkaline syenites and granites of the Mulanje complex, Malawi

Published online by Cambridge University Press:  05 July 2018

R. Garth Platt  and
Alan R. Woolley
R. Garth Platt
Affiliation:
Department of Geology, Lakehead University, Thunder Bay, Ontario, Canada
Alan R. Woolley
Affiliation:
Department of Mineralogy, British Museum (Natural History), London, England
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Abstract

Studies of the mafic mineralogy of the Mulanje granite-quartz-syenite-syenite Massif of southern Malawi delineate two mineralogically distinct complexes—the Main complex and the Chambe complex. Each complex is associated with its own trend of pyroxene evolution. The Main complex pyroxenes exhibit initial enrichment in hedenbergite before subsequent enrichment in acmite (i.e. sodic-salite-sodic-hedenbergite-aegirine-hedenbergite-aegirine), whereas the Chambe pyroxenes display constantly increasing acmite content with no significant enrichment in hedenbergite (i.e. sodic-salite-aegirine-augite-aegirine). This phenomenon is also reflected in the more Mg-rich amphiboles and biotites of the Chambe rocks when compared to those of the Main complex.

The general evolutionary trend of the Main complex amphiboles is katophorite → ferroricherite → arfvedsonite which broadly correlates with a change in rock type from syenite to granite. Superimposed on this trend is an essentially similar, yet less extensive trend of the more Mg-rich Chambe amphiboles. The micas of both complexes show a general evolution to more iron-rich compositions with relatively constant Al content. Those of the Main complex, however, display extreme iron enrichment with ultimate formation of essentially pure ferrous annite.

Aenigmatite, astrophyllite, fayalite, chevkinite, yttrofluorite, and unidentified RE minerals are characteristic of the Main complex rocks but totally absent from those of the Chambe complex. Ilmenite constitutes the only iron oxide phase in the Main complex rocks whereas titaniferous magnetite (now unmixed) and ilmenite are both present in the rocks of Chambe.

The differences between the two complexes are explained in terms of oxygen fugacity, silica activity, crystallization interval, and the relative rates of development of peralkalinity. In essence, the Chambe magmas are considered to have crystallized under high fO2 conditions with an earlier development of peralkaline tendencies when compared to those of the Main complex magmas. Moreover, a lower initial silica activity, a smaller alkali to alumina ratio, and a correspondingly smaller crystallization interval could account for the lack of highly evolved granitic magmas in the Chambe complex, whereas such magmas are integral in the evolution of the Main complex.

Keywords

mafic mineralsperalkaline syenitesgranitesMulanje complexMalawi
Type
Mineralogy
Information
Mineralogical Magazine , Volume 50 , Issue 355 , March 1986 , pp. 85 - 99
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1986

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