Late Proterozoic schist belts and plutonism in NW Nigeria

The deformed, low-grade, metasedimentary-volcanic schist belts of NW Nigeria, and the voluminous granitoid plutons which invaded them, are expressions of late Proterozoic-early Phanerozoic activity in the terrain separating the W African and Congo cratons. Recent interpretations of Nigerian geotectonic evolution have invoked two generations of schist belt, one a product of Kibaran ( c . 1100 Ma) ensialic processes, the other due to Pan-African (700–450 Ma) marginal basin development. The detailed histories of the Anka Belt (Pan-African), Maru Belt (Kibaran) and Birnin Gwari Belt (unknown age), and the plutons emplaced in them, are documented here on the basis of new field, chemical and isotopic data.* Each belt represents a dominantly quiet-water sedimentary environment but volumetrically minor lithologies reveal important differences between the belts. The volcanic rocks and early minor intrusions have strong affinities with those of destructive plate margins. The schist belts were deformed congruently and simply before c . 750 Ma, following the development of flat-lying, possibly thrust-related structures in the Maru and Birnin Gwari Belts. Subsequent plutonism, beginning c . 750 Ma ago, has the calcalkaline, I-type characteristics of subduction zones but younger plutons have mildly peralkaline compositions due to thickening crust.