Geochemistry and Petrogenesis of Nepheline Syenites: Kasungu-Chipala, Ilomba, and Ulindi Nepheline Syenite Intrusions, North Nyasa Alkaline Province, Malawi

The North Nyasa Alkaline Province of Malawi consists of seven Late Precambrian intrusions emplaced along a north–south trend roughly parallel to that of the current rift valley. The intrusions are predominantly nepheline syenite, but minor pyroxenite is found at Ilomba and alkali syenite and granite are associated with nepheline syenite at Mphompha. The chemistry and mineralogy of four representative nepheline syenite intrusions (Kasungu, Chipala, Ilomba, and Ulindi) have been investigated. Pyroxenes vary in composition from diopside (pyroxenite) through aegirine-augite to aegirine. Al-rich micas (Ilomba and Ulindi) may reflect primary magmatic compositions. Rocks from three intrusions plot near the minimum in the 1 kbar Nepheline–Albite–Orthoclase–Kalsilite phase diagram, indicating that they represent evolved liquids. Rocks from the fourth intrusion (Ilomba) are apparently cumulates. NbTa, Zr/Hf, and Th/U ratios vary from those typical of ocean-island basalt (OIB) to very high ratios which may reflect a hydrothermal overprint. Rare earth element patterns are steep, slightly to moderately concave upwards, and typically lack Eu anomalies. The Ulindi and Ilomba nepheline syenites were derived by nepheline, pyroxene, and titanite fractionation of a basanite–nephelinite magma, whereas the Kasungu and Chipala nepheline syenites were fractionated from an alkali basalt magma. These magmas originated from a subcontinental OIB-like source.