The circular, funnel-shaped Abontorok intrusion is 2.6 km in diameter and has a low central hill of quartz syenites and syenite breccias surrounded by an arcuate depression underlain principally by anorthosites, which are obscured by recent sediments in the north and east.
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Most of the occurrences of alkaline rocks in Niger are concentrated in the Air Massif (Fig. 212) a full summary of the geology of which, including the alkaline rocks, is given by Black et al. (1967). The ring-complexes are listed by Moreau (1987) and Husch and Moreau (1982) describe the anorthositic rocks in a number of complexes. Ages of seven complexes are given by Karche and Vachette (1978) and a fuller compilation of age data, including ages for 13 ring complexes, is tabulated in Moreau et al. (1994) who also present a tectonic model for the spatial distribution of the alkaline complexes. A bibliography of the geology of Niger has been compiled by Kinnaird (1982).
This very small intrusion, situated 12 km west of Egalah (No. 121-00-012), is composed of a sodic amphibole-biotite granite identical to that of Egalah.
This is a circular intrusion 15 km in diameter composed of a central biotite granite around which is a coarse-grained amphibole-biotite granite.
Situated between Iskou (No. 121-00-014) and Egalah (No. 121-00-012) Aroyan consists of an amphibole-biotite granite of alkali feldspar, quartz, ferrohastingsitic and arfvedsonitic amphiboles, hedenbergite and biotite. Rhyolite dykes occur in the centre of the intrusion.
Emplaced in Precambrian gneisses and granites this complex has an essentially concentric structure and consists of gabbros and syenites, of which the latter are the more abundant.
The 20 km-diameter, approximately circular Bilet complex forms an imposing 1700 m high mountain around which is a ring-dyke of microsyenite. At the base of the volcanic pile is a conglomerate which is overlain by tuffs intercalated with basaltic flows.
This occurrence is listed by Moreau (1987, Table 1) as a peralkaline microgranite. No other details have been traced.
An oval complex of 20x40 km Bagzane forms a plateau some 700 m high. The volcanic nature of the complex is attested by the presence of a vestigial patch of rhyolites in the central part.
This is a small pluton north of Taghouaji (No. 121-00-019) which is listed by Moreau (1987, Table 1) as a peralkaline granite.
The approximately circular, 16x20 km Taghouaji complex is composed essentially of granites with biotite granites covering 28% and rings of sodic amphibole and aegirine granites extending over 72% of the area.