Alkaline Rocks and Carbonatites of the World

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Occurrence number: 
The North Nyasa Alkaline Province
Longitude: 33.23, Latitude: -9.52

The Ulindi intrusion lies only 6 km east of Ilomba (No. 103-00-002) and is emplaced in Songwe gneisses (No. 103-00-001). It is a small body of 0.8x0.5 km which forms a steep-sided hill. Biotite gives the rock a marked foliation that parallels the regional trend (Ray, 1974). According to Ray (1974), near the summit coarse syenite is cut by a finer grained syenite containing dark, diffuse xenoliths. The coarse-grained syenite is leucocratic and comprises microcline, albite, nepheline, which in places reaches 50%, and blue sodalite, that occurs as separate grains but in the summit area forms irregular and discontinuous veins up to several centimetres in thickness. Biotite and minor muscovite occur in patches and magnetite and ilmenite are accessory; zircon, zoned pyrochlore and bastnasite have been identified (Eby et al., 1998). Analyses of two rocks are given by Eby et al. (1998).

A Rb-Sr whole rock isochron on nepheline syenites from Ulindi and Ilomba (No. 103-00-002) gave 685(62 Ma (Ray, 1974).
EBY, G.N., WOOLLEY, A.R., DIN, V. and PLATT, G. 1998. Geochemistry and petrogenesis of nepheline syenites: Kasungu-Chipala, Ilomba, and Ulindi nepheline syenite intrusions, North Nyasa Alkaline Province, Malawi. Journal of Petrology, 39: 1405-24.RAY, G.E. 1974. The structural and metamorphic geology of northern Malawi. Journal of the Geological Society of London, 130: 427-40.
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