Alkaline Rocks and Carbonatites of the World

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Occurrence number: 
Longitude: 10.17, Latitude: 13.17

Adjacent to the border with Niger in northern Nigeria over a level plain of blown sand rise occasional rocky hills of arfvedsonite granite with a few volcanic rocks. The area underlain by these rocks, on the Nigerian side of the border, is of the order of 300 km2 (see also Mounio occurrence - Niger No. 121-00-023). The granites, the most extensive and numerous outcrops of which are clustered around Matsena, are all arfvedsonite-bearing with aegirine and sometimes aenigmatite or astrophyllite. Turner (1974) distinguished six petrographic varieties that he considered probably represented distinct intrusions. Other minerals identified include hedenbergite, a calcic amphibole, probably hornblende, amblygonite, zircon, fluorite, allanite and pyrochlore. About 25 km east of Matsena occur a group of hills of volcanic rocks that are rhyolitic and mainly tuffs. The tuffs are generally very fine-grained but in some welded pumice fragments and pieces of rhyolite are present. A more homogeneous rhyolite contains phenocrysts of arfvedsonite, aegirine and aenigmatite in a matrix of quartz and feldspar with a little astrophyllite and spongy crystals of fluorite. In the northern part of the area are outcrops of a granite porphyry the aligned distribution of some of the outcrops suggesting they may be part of a ring-dyke. They contain quartz and perthite phenocrysts with poikilitic amphibole plates zoned from hornblende cores to arfvedsonite rims and aegirine rimmed by arfvedsonite in a groundmass of feldspar and quartz.

Rb-Sr determinations on a specimen of arfvedsonite granite and separate light and heavy fractions gave an age of 258(5 Ma (Rahaman et al., 1984).
RAHAMAN, M.A., VAN BREEMEN, O., BOWDEN, P. and BENNETT, J.N. 1984. Age migrations of anorogenic ring complexes in northern Nigeria. Journal of Geology, 92: 173-84.TURNER, D.C. 1974. The younger granites of the Matsena area, Bornu Province, northern Nigeria. Records of the Geological Survey of Nigeria, 8: 5-19.
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