Abstract

SummarySmall trachyte and phonolite bodies are shown as eroded plugs on published geological maps of part of north-eastern Nigeria. Field relationships at two such ‘plugs’ strongly suggest them to be extrusive in origin, formed of an earlier tholoid-building phase, followed by extrusion of a spine (plug dome) of very viscous lava, now forming the central peak at each locality. Petrographic data show that the tholoid and spine-forming lavas are phonolites of contrasted composition. Eruption of salic lavas from scattered centres appears to have occurred intermittently between mid-Tertiary and Pleistocene times.

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