Alkaline Rocks and Carbonatites of the World

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Nuanetsi-Save Nephelinites (Sabi)

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Occurrence number: 
181-00-010
Country: 
Zimbabwe
Location: 
Longitude: 31.7, Latitude: -21.17

At the base of the Karoo basalts which extend for 200 km along the Sabi Monocline nephelinite lavas and intrusions are found at a number of localities. They have not been mapped in detail but there are three main areas of outcrop (Bristow, 1984a), namely to either side of the Lundi River, adjacent to the Chiredzi River and west of the Save River. The nephelinites form a thin series of lavas that overlie Karoo sandstones and are themselves overlain by picrite basalts of the Letaba Formation. There are associated nephelinite dykes and sills. These nephelinites are closely similar to the Mashikiri nephelinites of South Africa (South Africa, No. 151-00-002) with which they are coeval. The lavas generally give rise to flat country but in the west the summit of the hill of Bendezi close to the Lundi River is composed of nephelinite flows 6-7 m thick (Swift et al., 1953) that show a good deal of variation (Cox, 1972). The most abundant type contains stellate glomeroporphyritic clusters of clinopyroxene, phenocrysts of nepheline and large mica flakes; phenocrysts of olivine and an opaque phase are more rare. The groundmass is composed essentially of nepheline and clinopyroxene with some amphibole and biotite. The pyroxene phenocrysts are often rimmed by aegirine-augite with groundmass pyroxene also sometimes aegirine-augite; perovskite, cancrinite and analcime may also occur (Swift et al., 1953). There are a number of nephelinite dykes, which are petrographically similar to the lavas, and sills including a teschenite and an ijolite. According to Swift et al. (1953) theralite and basanite also occur. Analyses of nephelinite are in Swift et al. (1953), Bristow (1984a) and Duncan et al. (1984) and of intrusive rocks in the first reference.

Age: 
K-Ar dating of three nephelinites from near Bendezi Hill gave an age of 178(5 Ma (Fitch and Miller, 1984). Rb-Sr isotope data are given and discussed in petrogenetic terms by Bristow et al. (1984).
References: 
BRISTOW, J.W. 1984a. Nephelinites of the North Lebombo and south-east Zimbabwe. In A.J. Erlank (ed), Petrogenesis of the volcanic rocks of the Karoo Province. Special Publication of the Geological Society of South Africa, 13: 87-104. BRISTOW, J.W. 1984b. Picritic rocks of the North Lebombo and south-east Zimbabwe. In A.J. Erlank (ed) Petrogenesis of the volcanic rocks of the Karoo Province. Special Publication of the Geological Society of South Africa, 13: 105-23.BRISTOW, J.W., ALLSOPP, H.L., ERLANK, A.J., MARSH, J.S. and ARMSTRONG, R.A. 1984. Strontium isotope characterization of Karoo volcanic rocks. In A.J. Erlank (ed) Petrogenesis of the volcanic rocks of the Karoo Province. Special Publication of the Geological Society of South Africa, 13: 295-329.COX, K.G. 1972. Karroo lavas and associated igneous rocks of southern Africa. Bulletin Volcanologique, 35: 867-86.DUNCAN, A.R., ERLANK, A.J. and BETTON, P.J. 1984. Appendix 1. Analytical techniques and database descriptions. In A.J. Erlank (ed), Petrogenesis of the volcanic rocks of the Karoo Province. Special Publication of the Geological Society of South Africa, 13: 389-95.FITCH, F.J. and MILLER, J.A. 1984. Dating Karoo igneous rocks by the conventional K-Ar and 40Ar/39Ar age spectrum methods. In A.J. Erlank (ed), Petrogenesis of the volcanic rocks of the Karoo Province, Special Publication of the Geological Society of South Africa, 13: 247-66.SWIFT, W.H., WHITE, W.C., WILES, J.W. and WORST, B.G. 1953. The geology of the Lower Sabi coalfield. Bulletin, Geological Survey of Southern Rhodesia, 40: 1-99.
Location: 
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