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

Abstract

The North Nyasa Alkaline Province of Malawi consists of seven INTRODUCTION Late Precambrian intrusions emplaced along a north–south trend The North Nyasa Alkaline Province (NNAP, Bloomfield, roughly parallel to that of the current rift valley. The intrusions are 1970) of central and northern Malawi consists of seven predominantly nepheline syenite, but minor pyroxenite is found at intrusions (Kasungu, Chipala, Chikangawa, Mphompha, Ilomba and alkali syenite and granite are associated with nepheline Telelele Hill, Ilomba, and Ulindi) that lie along a north– syenite at Mphompha. The chemistry and mineralogy of four south trend roughly parallel to the current rift valley representative nepheline syenite intrusions (Kasungu, Chipala, Il(Fig. 1). The dominant lithology is nepheline syenite, omba, and Ulindi) have been investigated. Pyroxenes vary in but alkali syenite and granite occur at Mphompha and composition from diopside (pyroxenite) through aegirine-augite to pyroxenites outcrop adjacent to, and within, the Ilomba aegirine. Al-rich micas (Ilomba and Ulindi) may reflect primary intrusion. Intrusions of gabbro, ijolite, carbonatite, nemagmatic compositions. Rocks from three intrusions plot near the pheline syenite and syenite in adjoining Tanzania and minimum in the 1 kbar Nepheline–Albite–Orthoclase–Kalsilite Zambia have also been included in the NNAP (Bloomphase diagram, indicating that they represent evolved liquids. Rocks field, 1970). from the fourth intrusion (Ilomba) are apparently cumulates. Nb/ All the published radiometric ages for the NNAP Ta, Zr/Hf, and Th/U ratios vary from those typical of oceanintrusions of northern and central Malawi, plus possible island basalt (OIB) to very high ratios which may reflect a additional members of the province in southern Malawi hydrothermal overprint. Rare earth element patterns are steep, slightly (Tambani), Mozambique, Tanzania, and Zambia are to moderately concave upwards, and typically lack Eu anomalies. listed in Table 1. Except for Ngualla, which may be The Ulindi and Ilomba nepheline syenites were derived by nepheline, considerably older, the radiometric ages suggest that pyroxene, and titanite fractionation of a basanite–nephelinite magma, there was widespread emplacement of alkaline intrusions whereas the Kasungu and Chipala nepheline syenites were fracin this part of Africa in late Precambrian time. Rb–Sr tionated from an alkali basalt magma. These magmas originated whole-rock isochron and Pb–a zircon ages obtained for from a subcontinental OIB-like source. Ilomba (685 ± 62 Ma and 655 Ma) and Chikangawa