Description:
<jats:title>Abstract</jats:title><jats:p>The mineralogy, including the accessory phases låvenite, rosenbuschite, and catapleiite, and consequent petrogenetic implications have been investigated for a group of four overlapping nepheline syenite complexes (Chikala, Chaone, Mongolowe, and Chinduzi) and for spatially associated silica-saturated and over-saturated perthosites, from the northern part of the Chilwa Alkaline Province, Malawi.</jats:p><jats:p>The complexes are thought to have formed by injection into high-level chambers of magma pulses genetically related to a common source magma at depth. Evidence for the source magma is preserved in salitic cores observed in the pyroxenes and a trend to more hedenbergite-rich compositions is believed to have formed by evolution of this magma. Subsequent trends of acmite enrichment followed magma injection into the higher-level chambers; the actual pyroxene trend associated with each individual complex is a function of the evolution attained by the source magma, oxidation potential, and perhaps even alkali activity. On the basis of such a two-stage model, the pyroxene data suggest emplacement of the Chaone and Mongolowe magmas somewhat earlier than that of Chikala, with the Chinduzi magma migrating even later.</jats:p><jats:p>Amphiboles and biotites are believed to have formed after high-level injection of the magmas. Their compositions broadly reflect the nature of the crystallizing pyroxenes in that magnesian hastingsitic hornblendes and more Mg-rich biotites are associated with more Mg-rich sodic pyroxenes, whereas katophorites and annite-rich micas are generally associated with sodic pyroxenes somewhat richer in hedenbergite. Sub-solidus crystallization in some of the complexes is represented by aegirine and magnesio-arfvedsonite. Nepheline compositions indicate broadly similar crystallization temperatures within the complexes, namely 950 to 750°C. Oxygen fugacities for these magmas obtained from biotite/annite compositions vary from 10<jats:sup>−19</jats:sup> to 10<jats:sup>−14</jats:sup> bars for this temperature range. Mineralogical data, particularly from pyroxenes and amphiboles, strongly suggest that the perthosites, spatially associated with the nepheline syenite complexes, are genetically unrelated.</jats:p>