The Ntumbaw Complex, NW Cameroon: An atypicalanorogenic ring complex of intermediate composition

Abstract

The Ntumbaw anorogenic ring complex, NW Cameroon, is composed of two intersecting centres intruded into the Precambrian metamorphic basement and partly hidden by basalt flows. It covers an area of about 12 km 2 and, despite lack of radiometric dating, is similar to the other anorogenic complexes of Tertiary age along the Cameroon Line. The rocks range from monzodiorite, monzonite and quartz monzonite for centre 1 to quartz syenite, syenogranite and granophyre for centre 2. The two centres are inversely zoned petrographically with the most acid rocks towards the periphery of both. The chemistry of major, trace and rare-earth elements shows that the rocks of both centres are subalkaline to alkaline, cogenetic, and that their variation was governed by fractional crystallization. More than 90% of this complex is of intermediate composition with silica values between 52 and 65%. Such rocks are often absent from anorogenic complexes, their compositions falling in the Daly Gap.