Late Proterozoic Schist Belts and Plutonism in NW Nigeria
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J. geol. Soc. London, Vol. 142, 1985, pp. 319-337, 7 figs, 1 table. Printed in Northern Ireland Late Proterozoic schist belts and plutonism in NW Nigeria W. R. Fitches, A. C. Ajibade", I. G. Egbuniwe?, R. W. HoltS & J. B. WrightS Geology Department, University College of Wales, Aberystwyth,Dyfed, UK; 'Geology Department, MinnaUniversity, Minna, Nigeria; ?.Geology Depart- ment, Federal University of Technology, PMB 0248, Bauchi, Nigeria; $Depart- ment of Earth Sciences, TheOpen University,Walton Hall, Milton Keynes, Bucks., UK. SUMMARY: The deformed, low-grade, metasedimentary-volcanic schist belts of NW Nigeria, and the voluminous granitoidplutons which invaded them,are expressions of late Proterozoic-early Phanerozoic activity in the terrain separatingthe W African and Congo cratons. Recent interpretationsof Nigerian geotectonic evolution have invoked two generations of schist belt, one aproduct of Kibaran (c. 1lOOMa) ensialic processes, theother due to Pan-African (70W50 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 adominantly 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 weredeformed congruently and simply before c. 750 Ma, following thedevelopment of flat-lying, possibly thrust-related structures in the Maru and Birnin Gwari Belts. Subsequent plutonism, beginning c. 750Ma ago, has the calcalkaline, I-type characteristics of subduction zones but younger plutons have mildly peralkaline compositions due to thickening crust. Nigeria lies in the late Proterozoic-early Phanerozoic the Pan-African orogenic cycle by McCurry (1973, terrain which separatesthe W African and Congo 1976, 1978), Wright & McCurry (1970), Grant (1969) cratons (Fig. 1). Thisterrain in Nigeria is conven- and Burke & Dewey (1972), among others. More tionally divided into three units. The basement unit recent interpretations, however, have pointed to the comprises gneisses and migmatites, with entrained possibility of more than one generation of schist belt. supracrustal relics, yielding Archaean (c. 2700 Ma) Grant (1978) used structuraland lithological criteria and early Proterozoic (c. 2000Ma) isotopic ages (e.g. together with isotopicarguments to distinguish an Grant 1970; Grant et al. 1972; Oversby 1975). Late earlier, Kibaran (c. 11OOMa) Kushaka Schist Belt Proterozoic-early Phanerozoicevents are most from the Pan-African (700-600 Ma) Birnin Gwari obviously expressed by the abundant granitoid plutons Schist Belt in NW Nigeria. This view was supported by of the Older Granitesuite. The few dated plutons have Turner (1983) who, using similar criteria, assigned all ages in the range 75&600 Ma (e.g. van Breemen et al. Nigerian schist belts toone or other cycle. This 1977; Ogezi 1977). The thirdunit comprises the hypothesis appears to explain the c. 11OOMa isotopic 300 km wide zone of NNE-trending schist belts age obtained by Ogezi (1977) from phyllites of the composed of low-grade, deformed, supracrustal Maru Belt (Holt et al. 1978) and the Kibaran isotopic assemblages. These rocks are cut by OlderGranite overprinting of basement gneisses in several regions plutons and are generally considered to rest on the (Grant et al. 1972). It also corroboratesHubbard's basement as a late Proterozois cover. (1975) interpretation of schist belt relations in south- One of themajor problems of Nigerian geology central Nigeria. concerns the geotectonic significance of the schist belts On the basis of this hypothesis, Holt et al. (1978) and a part of that problem stems from the difficulties and Turner (1983) suggested that the Kibaran schist of establishing theiractual and relative ages. They belts were the result of the opening and closing of an were assigned collectively to the subsidence stage of essentially ensialic basin which was aprecursor to more extensive subduction-related Pan-African subsi- dence. It is widely held that these younger events, that *Chemicaland isotopic dataquoted here from Ajibade is the subsidence of the Pan-African sedimentary- (1980) Egbuniwe (1982) and Holt (1982) are available on volcanic basin or basins, the deformationand low- request from W.R.F. The full set of analytical data will be grademetamorphism, andthe emplacement of the stored in the UK-IGBA datafile,from which it can be Older Granites, are due to a cycle of spreading and retrieved via the NationalGeochemical Databank of the collision during late Precambrian times. An eastward- British Geological Survey. dipping subductionzone at the margin of the W Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/142/2/319/4888824/gsjgs.142.2.0319.pdf by guest on 23 September 2021 320 W. R. Fitches et al. ----\ ----\ - 19 2,0 LP km m Msozoic 8 Tertiary mPon-African Pluton m Zungeru Hylonites SCHIST BELTS m Anka Birnin Gwari Haru m Kushaka m Malwnfashi IIIII] Wonaka m Ushama Zuru m Volcanics m Anka Pogmatito canplox 0pp,Gneisses L FIG. 1. Geological Map of NW Nigeria. Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/142/2/319/4888824/gsjgs.142.2.0319.pdf by guest on 23 September 2021 Late Proterozoicbelts, schist NW Nigeria 32 1 African craton has been inferred from various lines of The age of this basement is unknown. Ogezi (1977) evidence such as the presence of the thrust front and obtained a three point Rb/Sr whole rock errorchron adjacent belt of high density, high pressure rocks at or indicating an age of 1158 * 30 Ma with an initial near the surface which extends NNE from the 87Sr/86Srratio of 0.7117 from which he inferreda Ghanaian coast through Benin (Grant 1969; Shackle- Kibaranmetamorphic overprint on rocks which ton 1971; Trompette 1979) and beyond into Mali and already had a long crustal history. the Sahara (e.g. Bertrand & Caby 1978; Black et al. The basement rocks adjacent to the Birnin Gwari 1979). Ogezi (1977), Holt et al. (1979) and Turner were intensely deformed for the first time during the (1983) suggested that the younger Nigerian schist belts early stages of cover deformation.They were de- are due to back-arc extension related to that subduc- scribed originally asmetasediments by Russ (1957), tion zone. Truswell & Cope (1963) and Ajibade (1976) and as a Interpretations of the Nigerian sector, notably those bimodal acid-basic volcanic assemblage with minor concerning the actual and relative ages of the schist sedimentary intercalations by Bafor (1982) and Turner beltsand their geotectonicsettings, are severely (1983). However, Ajibade et al. (1979), elaborating on hindered by lack of data and are based extensively on the suggestions of Grant (1978), interpreted them as a broad regional information together with isotopic data cataclastic-mylonitic suite derived from the basement which, although valuable, are still very sparse. Very and termed them the Zungeru Mylonites. In the least little new,detailed information has been published deformed areas it can be demonstrated thatthe recently on specific aspects of the schist belts whereby original rocks were porphyritic granodiorites lacking a models can be tested or firmly established. tectonicfabric, containing acid plutonicxenoliths, Here we present a synopsis of unpublished work, zoned feldspar megacrysts and a variety of minor sheet now completed, on the Birnin Gwari, Maru and Anka intrusions including basic dykes. Quartz mylonites and belts (Fig. 1) carried out by Ajibade (1980), Egbuniwe shearedstaurolite schists previously assigned to a (1982) andHolt (1982), respectively. This synopsis separate Ushama Schist Belt (Ajibade, 1976), and provides new data on: sedimentationand tectonics; whose original relations to thegranodiorites have been chemical aspects of volcanic rocks and minor intru- masked by deformation and extremely poor exposure, sions intimately associated with the metasedimentary are probably basement metasedimentary relics. rocks; chemical characteristics of the Pan-African A noteworthy feature of the basement adjacent to the plutons cutting the schist belts; isotopic aspects of the Birnin Gwari Belt is its unusual composition compared metasediments,minor intrusions and major plutons. with the amphibolite facies tonalitic-granodioritic Most of this paper is concerned with the presentation gneisses with metasedimentary relics found elsewhere and interpretation of the new data independently of in northern Nigeria. This is the first record in the age and regional context. The questions of the ages of basement of Nigeria of an undeformed plutonic body events and geotectonic settings of the schist belts are older than the schist belt cover. Whether ornot it is an considered in later sections. integral part of the early Precambrian basement or a later, possibly early Pan-African pluton is unknown. The basement Here we give only a brief outline of the basement Schist belt sedimentation and insofar as it concerns the evolution of the three schist volcanism belts. It is exposed on the eastern margin of the Maru Belt, where it is represented by the Gusau Migmatites, The Maru Belt and on either side of the Birnin Gwari Belt, where it has been extensively modified by later deformation. No lithostratigraphic succession for this belt has The Gusau Migmatites are dominantlybanded