Alkaline Rocks and Carbonatites of the World

Setup during HiTech AlkCarb: an online database of alkaline rock and carbonatite occurrences



Occurrence number: 
Longitude: 45.67, Latitude: -17.87

Two discontinuous lines, 2 km apart and extending over 15 km, of nepheline-bearing gneisses lie conformably within Precambrian gneisses and migmatites. The nepheline-bearing rocks are schistose with individual layers generally no more than 2 m thick. In places they are continuous for several kilometres but elsewhere may be only a few tens of metres in length. The marginal migmatites are in places mylonitised. The rocks comprise microcline perthite, albite, abundant aegirine-augite and hastingsitic amphibole, the latter forming prisms up to several centimetres in length in pegmatitic facies. Nepheline is generally interstitial to the feldspar and may be abundant or scarce, while cancrinite, scapolite, fluorapatite, fluorite and magnetite are ubiquitous. Green biotite, monazite, garnet, titanite and calcite occur in places. A rock analysis is given by Welter (1964).Immediately south of the zone of nepheline gneisses is an approximately circular, 3-4 km diameter peralkaline granite intrusion which forms a massif rising to the summit of Makaraingobe. The granite is coarse-grained and comprises quartz, microcline, albite, abundant riebeckite and a little aegirine. Lacroix (1922b) gives a whole rock analysis.

LACROIX, A. 1922b. Minéralogie de Madagascar. 2. Minéralogie appliquée lithologie. A. Challamel, Paris. 694 pp.WELTER, C. 1964. Contribution a la pétrographie et a la genèse du complexe des gneiss a néphéline du Makaraingobe (ouest de Madagascar). Comptes Rendus de la Semaine Géologique 1964, Comié National Malgache de Géologie. Tananarive, 57-62.
Fig. 3_140 The distribution of nepheline-bearing gneisses of Makaraingobe (after Welter, 1964, Plate 12).
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