Filiya Area

In the vicinity of Filiya, and up to 40 km to the north and 60 km east, over 300 volcanic plugs have been mapped by Carter et al. (1963). The great majority of these plugs are of alkali olivine basalt but one basanite, eleven trachyte, five trachyphonolite and six of phonolite were identified. The plugs vary geomorphologically from small mounds several metres high to substantial peaks. Tangale peak, for instance, rises to more than 600 m above low sandstone hills; Biliri Hill and Labore peak are other prominent plugs. Wright and McCurry (1970) investigated the structure of Labore peak and another phonolite plug and concluded that they are extrusive in origin, an earlier tholoid-building phase being succeeded by extrusion of a spine. Occasionally dykes and sills are associated with the plugs. Biliri Hill consists of a basanite composed of zoned olivine and augite in a groundmass of the same minerals together with plagioclase, nepheline, which may be relatively abundant and form euhedra, magnetite and rare apatite. The trachytes vary between sodic varieties dominated by oligoclase and varieties with mainly K-feldspar. Aegirine or aegirine-augite is usual and an amphibole occasionally present; analcime is abundant. The trachyphonolites are mineralogically closely similar to the trachytes but with the addition of a little nepheline and biotite. The most substantial plugs, those of Tangale and Labore peaks, together with Jaragwol Hill, are all phonolitic. Nepheline is now a major constituent and sodic feldspars are largely or entirely absent; the pyroxene is aegirine. Wright and McCurry (1970) also identified sodalite and possible aenigmatite. Analyses of phonolite and trachyte are given by Carter et al. (1963).