The Ningi-Burra complex, Nigeria: dissected calderas and migrating magmatic centres

A series of overlapping ring structures each contain varied, pre-caldera volcanic sequences, overlain by massive intra-caldera rhyolitic ignimbrites and enclosed within ring dykes of fayalite-hedenbergite granite-porphyry. Early basalts are hypersthene-normative but may originally have been alkaline or transitional to nepheline-normative. They are linked, through a series of andesine basalts, mugearites and trachyandesites, to aphyric trachytes. Pre-caldera rhyolitic rocks, interbedded with and overlying the basic and intermediate volcanics, are mainly ignimbrites which were perhaps originally peralkaline or near peralkaline, but are often recrystallized to subalkaline compositions. They formed mainly through partial melting with zone refining of crustal rocks, the mantle-derived basic to intermediate magmas providing the heat source. In the early stages of each caldera-forming cycle the ignimbrite eruptions were of small volume and the rocks have varied crystal contents. Later, large high level magma chambers must have developed, from which crystal-rich ignimbrites with volumes in the order of 100 km 3 were erupted, with accompanying caldera collapse. The form of the complex as a whole shows that this cycle occurred repeatedly, but with the location of magma generation migrating progressively westwards, resulting in a chain of rhyolitic cauldrons, into which subvolcanic granites and syenites have been emplaced.