J. geol. Soc. London, Vol. 142, 1985, pp. 319-337, 7 figs, 1 table. Printed in Northern Ireland

Late belts and plutonism in NW

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 intothree units. The basement unit recent interpretations, however, have pointed to the comprises and , 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 times. An eastward- British Geological Survey. dipping subductionzone at the margin of the W

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----\ ----\ - 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.

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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, althoughvaluable, 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 lacking a models can be tested or firmly established. tectonicfabric, containing acid plutonicxenoliths, Here we presenta 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 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 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 been established because of the strong, repeated tonalitic gneisses with minor granitic gneisses, veined deformation and very poor exposure. However, very by granodioritic to granitic neosomes. Some amphibo- rare younging evidence indicates an upward-facing lite sheets were originally basic dykes which cut the succession. gneissic fabrics and were themselves deformed; they offer a means of beginning to decipher the complex Metasediments history of the migmatites. Mineral assemblages in the gneisses andamphibolites stabilized in the middle Of the 40 km long segment of the Maru Belt amphibolite facies and no traces of higher facies relics investigated in detail,about 70% is underlain by have been detected. Comparison of small-scale struc- low-grade metamorphosed fine-grained clastic tures suggests that the gneissic banding, the emplace- sedimentaryrocks. Their high phyllosilicate content ment, metamorphism and deformation of basic dykes and the presence of graphiteand pyrite indicate a and the bulk of the migmatisation preceded the first low-energy, euxinic depositionalenvironment. Typi- deformation of the Maru supracrustal rocks. cally, they comprise layers alternately rich in quartz or

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/142/2/319/4888824/gsjgs.142.2.0319.pdf by guest on 23 September 2021 322 W. R. Fitches et al. phyllosilicates andthereby resemble interlaminated 1973). On some discrimination diagrams they plot in siltstones and of very distal turbidite sequences. the field of overlap between island-arc tholeiites and However, much of the lamination is not bedding but a mid-ocean ridge (e.g. Fig. 2A)but on most product of tectonicpressure solution striping. The diagramsthey are confined almostentirely tothe original sediments were more homogeneous, probably island-arc field (e.g. Fig. 2B,C).On the MORB- weakly or unbedded mudstones or shales. normalized diagram (Fig. 2D) they show depletion of Fine-grained, commonly flaggy , sandstones locally certain elements foundas in island-arc low-K dominatethe central and eastern parts of the belt. tholeiites; Sr, Ba and Nb arenot depleted, in common Forming units up to 10 m in thickness in the argillites, with basaltstransitional between within-plate and they are mostly plane bedded, contain partings mid-ocean ridge types (cf. Searle et al. 1980; Thorpe et andrepresent mature quartz-sandbodies. They are al. 1984). On the assumption that the trace-elements almostdevoid of sedimentary structures exceptfor used for discrimination have remained immobile since rare grading andstarved ripples andare regarded eruption it seems that chemically the Maru basalts are as delta-front or possibly inter-deltaicbarrier beach closer to island-arc tholeiites than to basalts of other deposits. major environments. They bear little resemblance to Banded iron formations (B.I.F.) extend almost the basalts of ophiolite suites. It is noted, however, that entire length of this part of thebelt. Described as their association with regional quiet-water conditions, ferruginous quartzites by Truswell & Cope (1963) and their small volumeand the absence of acid or McCurry (1976), these rocks are closely similar to intermediate volcanic rocks is not typical of island-arc B.I.F. of other regions (e.g.Lepp & Goldich 1964; settings.Consequently, it remains doubtful,on the Govett 1966; Bronner & Chauvel 1979). Typically, strength of trace-element chemistry alone, whether they comprise 0.5-2 m thick intercalations in argillites, these volcanics can definitely beattributed to an or more rarely in fine sandstones, in units up to 60m island-arc origin. in thickness. Banding is on a fine (2mm-3 cm) or In this regard, it is pertinentthat low-grade coarse (5-10cm) scale andthe iron-richlayers, metabasalts are also recorded west of the Maru Belt composed of magnetite and hematizedmagnetite, by Ogezi (1977), between the Maiinchi Batholith and alternate with recrystallized chert. the Anka Pegmatite Complex (Fig. 1) in the Sado and The association of B.I.F. with chemically and Maikwonaga areas, 30 km WNW and 50 km SW of texturally mature sediments is consistent with deriva- Maru, respectively. These basalts are also tholeiites, tion of the B.I.F. components from a nearby mature, locally pillowed and in places associated with phyllites, deeply weathered landsurface as invoked in the but they differ from the Maru basalts in several ways. models of Goldich (1973) andBronner & Chauvel Some of theSadwMaikwonaga basalts again have (1979) among others. However, there are metabasalts trace-elementcharacteristics of island-arc tholeiites in the succession, asdescribed below, and although but the whole suite shows a much broader range of none has been located near the B.I.F. units a volcanic traceelement ratios and on discrimination diagrams exhalative origin (e.g. Stanton 1972; Goodwin 1973) is extends across the fields of island-arc, mid-ocean ridge equally plausible. and within-plate (Fig. 2B,C). Moreover, they are accompanied by gabbroic and ultramafic rocks. Metavolcanic rocks Ogezi (1977) considered that the chemical characteris- tics and rock association can be explained by several Pillow basalts are intercalated with argillites in the models but favoured one in which a slab of mantle was River Sokotonear Maru. Although deformedand tectonically emplaced into a small marginal basin low-grade metamorphosed, pillows with chilled mar- behind an island-arc. It may bespeculated thatthe gins and vesicular textures are clearly preserved and Maru basalts represent local activity on the outlying set in recrystallized interpillowcherts. Amphibolite flank of that arc. However, much of the evidence for sheets up to 3 m in thickness are locally interlayered testing thatinterpretation lay in the zone now with argillites elsewhere and represent sills or flows. occupied by the Maiinchi Batholith whilst the Rb/Sr Of uncertain origin arethe 1-3 m thick layers of age of 825 * 300 Ma obtained by Ogezi from the chlorite-magnetite schist intercalated with argillites in Sado-Maikwonaga assemblage is too inaccuratefor the Fon Kada and Kidauli rivers. In mineralogy and attempting correlation with the Maru basalts. form theyresemble those described as metamorph- osed ultramafic rocks from the Zaria area (McCurry 1978) and Chafe (Ogezi 1977). The Birnin Gwari Belt The volcanic rocks offer a meansof investigating the It has not been possible to erect a lithostratigraphy geotectonic setting of the Maru Belt, chiefly through for this belt because of very poor exposureand using their geochemical characteristics. The metaba- complex tectonics. From varioussedimentary struc- salts have tholeiitic compositions, having Y/Nb ratios tures the succession youngs upward on a regional scale consistently above unity for example (Pearce & Cann and structures are upward-facing.

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(A) 20 Field of Islandarc volconicr Field of Abyssal tholeiites .J\L ---A-----

<5 N

I -r IC 50 I00 500 100 Zr (ppm)

Mean of 22 MaruTholeiiie v 4 -ROCK MORB

\ . ., I0l 100 I000 Sr B0 NO C€ P Zr Hf Sm Ti Y 'I0 SE 7r lnnrnl

FIG. 2. The Maru Basalts plotted on discrimination diagrams. Data points from Egbuniwe (1982). Areas enclosed by dotted line on Fig. 2B and 2C contain data points for basic rocks from Maikwonaga and Sado (Ogezi 1977). Fields A (after Miyashiro 1975) and B (after Pearce & Norry 1979), low-K tholeiites of volcanic arc type; B, ocean floor basalts; B & C (after Pearce 1980), calc-alkali basalts; D (after Searle et al. 1980), ocean island and continental basalts. Trianglesmark transitional tholeiites andopen circles markprobable island arc tholeiiteof volcanics beneath Semail Nappe, Oman (see text).

Sedimentation was predominantly under quiet water basement rocks include quartzite, mica-schist, calc- euxinic conditions,as in theMaru Belt, producing silicate rocks and quartz which is commonly blue and graphitic, pyritic shales now represented by slates and tourmaline-bearing. A magmatic-sedimentary terrain phyllites. contributed clasts of acid andintermediate volcanic Into this background low-energy environment tur- rocks, and arkose. Turner (1983) raised the bidity currents and mass flows carried coarse detritus possibility that the clasts are ice-rafted glacial debris, a froma terrain comprising basementelements and manifestation of thelate Precambrian glaciations. younger plutonic-volcanic components.Metre-thick However,in our view thecrude layering, intercala- graded turbidites contain clasts of acid and tions of volcanogenic sandstoneturbidites andthe intermediate volcanic rocks, volcanic feldspars, quartz presence of graded tuffs in the succession support the and reworked shale fragments. debris-flow origin proposed by Truswell & Cope The Durumi Pebbly Schist, originally described by (1963). Truswell & Cope (1963), forms l-15-m thick intercala- Previously unrecognized among the sedimentary tions in the argillites. It is matrix-supporteda rocks are thin (5-10cm) graded layers of acid tuff, conglomerate, usually unbeddedbut locally crudely representingeither ashfall deposits or, more likely, layeredand size-graded. It contains wide variety of material reworked by sedimentary processes from the clast types up to 2m in length. Clasts derivedfrom same volcanic sourceas the clasts described above.

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/142/2/319/4888824/gsjgs.142.2.0319.pdf by guest on 23 September 2021 324 W. R. Fitches et al. Minor sills or flows of quartz- andalkali fieldspar-phyric any of the argillites in the Anka Belt are part of an and intercalated in the sedimentary older, previously deformed succession. rocks have also been newly recognized.Extensive The successions at Bunkasauand Sauna differ in alteration during deformation makes it unlikely that various details because of very rapid changes in lateral meaningful chemical data can be obtained from these and vertical thickness and facies which are characteris- rocks. tics of each area.In general terms, however,both successions represent coarsening upward assemblages, frompredominantly argillites with minorsandstones nearthe base,through sequences dominated by The Anka Belt sandstones with minorconglomerates, to predomi- The Anka Belt is composed chiefly of very poorly nantly conglomeraticunits; at Bunkasau the upper- exposed, commonly homogeneousquiet-water argil- most part of the succession fines upward asmore lites. It differs from the otherbelts in containing major sandstones come in. coarse clastic units and incorporatinga province of These deposits areinterpreted as products of an acid-intermediate volcanic and minor intrusive rocks. alluvial fan-braided stream environment established The coarse clastic units and the magmatic rocks both on the western argillite basin (Fig. 3). Most, if not all, yield informationconcerning palaeoenvironments, the sediments were deposited above wave-base in an geotectonics and timing of events in NW Nigeria. essentially terrestrial environment. The sandstones dominating the lower parts of both The coarse clastic units successions andthe top of the Bunkesausequence have many characteristics of braided stream deposits These are best exposed in the Bunkasau area (Fig. 1) (e.g. Rust 1978) such as troughcross-bedding, andnear Sauna 50 kmto the NE, from where granule-rich sandstones, clast- and sand-supported generalized stratigraphic successions have been estab- gravel layers and laminated, cross-bedded very fine lished. Their relationships tothe argillites remain sandstones. The intercalated fine clastic rocks are unclearbecause of deformation, lack of younging interpreted as overbankdeposits characterized by indicators in the argillites, and above all the very poor interlaminations of shale, silt and fine sand, showing exposure of the fine clastic rocks. There is evidence, somegrading, local cross-lamination and various however, that the coarse clastic units young away from dewatering and slump structures. and overlie the bulk of the argillite sequence but are Uplift of the fault block led to progradation of intercalated with them in the lower parts of the coarse conglomerates across the alluvial plain. The con- clastic succession. There is no evidence to suggest that glomerates are very poorly sorted, with outsize

Subsiding basin with clastic wedges+I and sheets developed on pelitic assemblages. partly intertongue with,partly overlain by coarse clastics.

FIG.3. Schematic diagram to show relationships between sedimentary rocks and source terrain, Bunkasau area of Anka Belt (after Holt 1982).

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blocks, are clast-supported,have a granule-coarse body but only tectonic contacts were observed in this sand matrix, a sheet-like form and no major internal study and age relations remain unsubstantiated. In our erosionsurfaces. They closely resemblefan-head view the Anka Pegmatite Complex is a likely source of debris-flow deposits that debouch from block margins, clasts but not a demonstrable one. as described by Larsen & Steel (1978), for example. Most of the non-plutonic clasts wereprobably Eventually, degradation and stabilization of the mar- derivedfrom sediments, volcanic rocks and minor gins took place and a fining-upward sequence was intrusions formed at an earlier stage in the history of established as the fan-head retreated. theAnka Beltbasin. However,the low-grade de- Accurate reconstructions of the conglomerate mas- formed metasedimentary clasts are probably older and ses cannot be made but there is an overall westward their possible origin is discussed in a later section. thinning and transition into the braided-stream sand- stones as though the fault block lay in the E. Volcanic rocks and minor intrusions Westward-flowing current directionsobtained from current-bedding and the higher proportion of shales in The coarse clastic rocks at Bunkasaucontain tuff the sandstones in thesame directionsupport this layers and sills or flows, all of acid to intermediate interpretation. compositions. Extensive alteration during deformation The positive block against which these clastic hasdestroyed much of their original character wedges accumulated and which provided the locally although plagioclase, alkali feldspar, and deriveddetritus was largely of plutonic origin with amphibole are locally preserved. Small dykes, much volcanic rocks,low-grade and unmetamorphosed altered and deformed with the host sediments, contain sedimentarycomponents. The sandstones of the relict phenocrysts of plagioclase, quartzand biotite alluvial plain are chiefly arkoses, or where indicating an acid-intermediatecomposition. Plagio- feldspar clasts have broken down to clay matrix. Clasts clase-phyric microtonalite forms small sheet intrusions of polycrystalline quartz andquartz-feldspar inter- and also bigger masses up to 1.5 km across. growths appearto be of plutonic origin. The con- The volcanic rocks and minor intrusions at Bunk- glomeratescontain a wide range of clast types. asau are assigned hereto the volcanic province Contributions from plutonic rocks are chiefly granite, recognized elsewhere in the Anka Belt by McCurry & and vein quartz, with smaller amounts of Wright (1977). They described two main centres, one ,gabbro, micro-granite and acid pegmatite. at Kisemi (Fig. 1) comprising agglomerates, breccias, Volcanic rocks and minor intrusions are represented dacite flows andmicrotonalite minor intrusions, the by clasts of acid to intermediate porphyry. Fragments otherat Maradun (Fig. 1) consisting of rhyolite of sedimentary origin include arkose and feldspathic domes, ashes and agglomerates in a caldera sequence. greywacke, resembling sandstoneselsewhere in the Rocks of these centres contain more than 62% silica, succession, with locally reworked shales. Of consider- have low Na20/K20ratios, high contents of light able importance in establishing the age of the Anka lithophile elements and LREE enrichment. Belt is the presence of fragments of previously Chemical data from the Bunkasau rocks expand the deformedand low-grade metamorphosed quartzite, range of characteristics of this volcanic province. The red, green and brown sandstones, and phyllites. No dykes have rare-earth (Fig. 4A,B) and other chemical representatives of the high-grade basement have been characteristics similar to those described by McCurry positively identified althoughsome composite quartz & Wright (1977). The microtonalites are distinctly grains and rare amphibolite clasts possibly orginated different,however, having low LIL concentrations, from that source. flat REEpatterns and high Na20/K20 ratios. The The various characteristics of the source terrain can dykes and microtonalites lie close to the calc-alkaline be inferred from the composition of the clasts but the trend on the AFM diagram (Fig. 5A) and havearc actual source cannot be identified with certainty. The and syn-collisional characteristics on discrimination plutonic clasts cannot have been derived from nearby diagrams (Fig. SC,E,F). Pan-African plotons, such as the Dan Garamfegranite McCurry & Wright (1977) considered the volcanic and the Maiinchi Batholith, becausealmost all are suite has chemical affinities with high-K, calc-alkaline demonstrably intrusive into the sedimentary rocks of late-tectonic volcanic rocks of intra-continental colli- theAnka Belt, including the coarse clastic units. sion belts. The Bunkasau dykes and the volcanic rocks Moreover, neithergabbroic nor dioritic Pan-African of Kisemi and Maradum are also broadly comparable plutons are known near or in this belt. Farghar (1960) tothe high-K, siliceous volcanic types, defined by suggested derivation from the Anka Pegmatite Com- Ewart (1979), found in parts of the western U.S.A., plex (Fig. 2). This major composite mass is distinctive the Andes and the Mediterranean arcs. The Bunkasau for its pink feldspar, that also characterizes the coarse microtonalites belong to Ewart’s (1979)low K-type clastic units, and its wide range of compositions from foundin the S Sandwich and Caribbeanarcs, for gabbro to acid pegmatite. Fargher considered the example.Although thereare notable differences coarse clastic rocks are unconformable on this plutonic between the Bunkasau microtonalites and other rocks

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1000

332 292 100 m 302A 231 A 293

-.-Q L V C L

\ X B 10

1 1’ Tm YbLu La CO Nd TbSm Eu LuTm Yb La CP NdTb GdSm Eu R.€.€

FIG. 4. Chondrite normalized rare-earth diagrams for Bunkesau dykes and microtonalites (Holt 1982).

of the Anka Belt, the importantpoint is that all of the Main deformation volcanic and minorintrusive assemblages are of The main deformation in all belts was essentially magmatic arc type usually associated with subduction simple, producing upright NNE folds on various scales and collisional . and an axial planar cleavage. Most folds are tight to isoclinal in the fine-grained clastic rocks, more open in Schist belt deformation the coarse clastic types of the Anka Belt; hinges have a mean horizontal plunge although individual folds are Thethree schist belts appearto havea similar commonly markedly non-cylindrical. Major folds with tectonic-thermalhistory. Ineach belt folding took wavelengths of severalkilometres have been recog- place on steep NNE axial planes so that the regional nized in theMaru Belt, forexample, the Kparaba structural trends of the belts are mutually parallel. All Anticline with B.I.F. inits core,and in the Birnin belts weremetamorphosed in the low greenschist Gwari Belt where the Gungu Syncline is responsible facies. However, small-scale structural studies have for thesouthern termination of that schist belt. revealed complex tectonic histories for the Maru and Younging indicators arerare but in all known BirninGwari Belts. There,the NNE structures are instances the folds in all belts are upward-facing. imposed on earlier structuresand were themselves deformedlater. In the following sections theNNE Early deformation structures common to all belts are ascribed tothe ‘main deformation’. Probably this is a regional event In the Anka Belt the folds produced by the main butfor reasons discussed ina later section no deformationhave an axial planar slaty cleavage in convincing proof is available thatthe congruent pelitic and finer psammites. These structures represent structuresare contemporaneous. The structurespre- the first deformation in that belt. In the Maru Belt, ceding the main deformation are described as ‘early however, the main cleavage is commonly aspaced deformation’ structures, arguably Kibaran in the Maru crenulationfabric that deforms and transposes an Belt but of uncertain age in the Birnin Gwari Belt. earlier slaty cleavage. This slaty cleavage is axial ‘Laterstructures’ are thoseimposed onthe NNE planar to very rare early isoclinal folds of bedding that structures of all belts. are best observed in outcrops of the B.I.F. and in thin

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 Proterozoic schist belts, NW Nigeria 327 sections of laminated fine clastic sediments. Too few from major batholiths such as the Maiinchi Batholith, dataare available to establish accurately the initial almost 100km in length, to minorbodies less than orientation of theseearly structures but from rare 1 km across. Characteristically, they have an ovoid to interference patterns with main folds it seems that the elongate form concordant to the regional NNE trends, early axial planes and slaty cleavage were reclined to their emplacement having been controlled by the main recumbentand hinges werehorizontal and nearly deformation structures. The major bodies are usually N-S. Thisearly event also affected the Gusau composite, comprising combinations of tonalite, Migmatites, the basement tothe Maru Belt, tightly monzonite, adamellite, granodiorite and granite. The folding the gneissic bandingand producing a new smaller masses are commonly more homogeneous and flattening fabric; like those in the cover these earlier composed of one, rarely two, of the above rock types. structureswere strongly overprinted by the main Unique among thenorthern Nigerian Pan-African deformation and their original orientation is uncertain. plutons are two quartz -granite bodies, the Early cleavage and folds have also been identified in Kanoma and Sabon Gida plutons of the Maru Belt. the Birnin Gwari Belt. There, the nearly N-S gently All 15intrusions studied are high level plutons plunging clast elongationlineation in theDurumi emplaced by stoping and diapiric processes. Contacts Schist and some of the coarser sandstones is assigned with host rocks are very rarely exposed but where seen to the same deformation because it is slightly oblique are sharp anddiscordant, marginal zones contain rafts, to, and folded by, the main folds. screens and angular xenoliths of the host rock,and The most widespread and obvious effects of this characteristically the plutonshave narrow thermal deformation arefound in the basement adjacent to the aureoles with assemblages of , biotite, Birnin Gwari Belt where the original granodiorites andalusite, sillimanite, cordierite and in various were sheared to form the Zungeru Mylonites in two combinations. There is no evidence of significant belts on either side of, and parallel to, the schist belt partial melting or metasomatism at contacts, there are (Fig. 1). In these major shearzones, described by no marginal migmatites except of brittleagmatite Ajibade et al. (1979), the planar fabric is now steep, type,and thereare no indications that any of the strikes NNE and contains a nearly horizontal NNE- bodies formed in situ. These plutons are products of SSW grain elongation lineation produced by extension magmagenesis at levels substantially below the present during shear.The shear fabrics are considered to exposure level. precede, but to owe their present orientations to, the Some plutons are still being unroofed. Low hills in main deformation. The evidence for this conclusion is parts of the Minna Batholith (Fig. l), for example, are that the shear fabrics are deformed by small-scale folds capped by a thin veneer of Birnin Gwari schists. Small of the main deformation. areas within the Maru Belt contain contact metamor- Much of the original character of the early struc- phic chiastolite orgarnet porphyroblastsprobably tures has been masked by the main deformation but marking the roof aureoles of unexposed plutons lying enough information is preserved to infer that both the just beneath the surface. Maru and Birnin Gwari Belts were initially deformed The plutons are late- topost-tectonic with respect to inhorizontal tectonic regimes. The possibility then the main deformation. The regional cleavage is arises that thrusttectonics, involving nearly N-S truncated by pluton margins, rotated in xenoliths and displacement, preceded the main deformation in both in thermal aureoles it is recrystallized and overgrown belts. by contact metamorphic porphyroblasts. Some plutons have a weak foliation due to flow or magmatectonic processes ratherthan regional deformation. Ogezi Late Deformation (1977) records shear zones of unknown age in parts of Crenulation cleavages and kink-bands are imposed the Maiinchi Batholith. on the early and main structures in all belts. Some sets The only pluton known definitely to precede the appearto be common to all schist belts,and are main deformation is the Gusau Granite (Fig. 1) probably due to a regional deformation. These include that separates the Maru Belt from the Gusau Migma- the steep crenulations and kinks striking ENE which tites of the basement. This pluton, which contains rafts are congruent to major warps of the main structures of the , was emplaced afterthe early such as that deflecting the trace of the Birnin Gwari deformation but was thenfoliated by the main Belt (Fig. 1). Other sets are localized around plutons deformation. and are regarded as products of forceful magmatic Chemically, the plutons individually and collectively emplacement. follow a calc-alkaline trendon an AFM diagram (Fig. 5A & B) and, excepting the quartz , are calc-alkaline to alkaline on an alkalinity index diagram The Pan-African Older Granites (Fig. 5D). They all have I-type characteristics (Chap- Between 30 and 40% of the regions studied are pel & White 1974; Hine et al. 1978; Pitcher 1979) underlain by Older Granite plutons (Fig. l), ranging which include NazO above 3.270,normative diopside

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+

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Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/142/2/319/4888824/gsjgs.142.2.0319.pdf by guest on 23 September 2021 330 W. R. Fitches et al. or normative corundum below l%,whilst the s7Sr/86Sr (1977) obtained three-pointa Rb/Sr whole rock initialratios of the Maiinchi and Dan Garamfe plu- errorchron of 1158 f 30 Maand an initial s7Sr/86Sr tonsare below 0.708 (Fig. 6A).On trace-element ratio (R.I.) of 0.7117 f 0.0004. He regarded this age discriminationdiagrams (Fig. 5C,E,F) all analysed as the time of Kibaran amphibolite facies metamorph- samples,except thequartz syenites, lie in theArc ism imposed on older Proterozoic rocks. We broadly and/or syn-collisional fields of Pearce et al. (1984). concur with this view but consider the age is more Accordingly, the quartz-saturated Pan-African plutons likely to be due to Kibaran low-grade metamorphism closely resemble those generated at very deep crustal on high-grade older rocks; there is no evidence of levels or in the upper mantle during subduction and more than low grade metamorphism at about this time collision processes.This model of OlderGranite in the adjacent Maru Schist belt. generation hasbeen advocated since plate-tectonics The only other basement dataare from the have been invoked to explain this Pan-African terrain Malumfashi Gneisses(Ogezi 1977) for which an (e.g. Burke & Dewey 1972); our chemical data eight-point Rb/Sr whole rock errorchron gave an age provide substantive support. of 615 f 65 Ma and R.I. 0.7107 f 0.0021. In this case The composite quartz syenite-granite plutons of aPan-African overprinton olderProterozoic rocks Kanomaand Sabon Gida are also late Pan-African appears to have been particularly strong. intrusions. The Kanoma plutonhas forced aside the Forthe Pan-Africanplutons Ogezi (1977) found main regional fabrics of the Maru Belt and recrystal- thatthe Maiinchi Batholith (Fig. 1) yielded an lized them in its thermal aureole. Its age is not known 11-point Rb/Sr whole rock isochron of 679 f 65 Ma with precision but a single whole rock sample yielded a andR.I. 0.7074 f 0.0008, whilst theZaria Granite model Rb/Sr age of 662 f 36 Ma and a WAr horn- (Fig. l), by thesame method, gave athree-point blende age, probably too high due to excess argon, of errorchron age of794 f 73Ma and R.I. 0.7044 790 f 40 Ma (Snelling 1964). The Sabon Gida pluton f 0.0053. Ogezi considered thatthe age of the intrudedthe Maiinchi Batholithfrom which Ogezi Maiinchi Batholith probably dates post-emplacement (1977) obtained a 675 f 65 Ma Rb/Sr11-point deformation although the date of intrusion is not much isochron age, which is close to the age of emplace- older; the moderately high R.I. (Fig. 7) is consistent ment. with this interpretation. The age of the Zaria Granite The biotite granites forming the pluton cores are not he regardedasdating emplacement. The Gusau chemically distinguishable fromother Pan-African Granite Gneiss yielded an age of 577 f 46 Ma which is granites in the region and conform to products of arc probably dueto post-emplacement open system be- and syn-collisional magmatism(e.g. Fig. 5E,F).The haviour in view of the R.I. 0.7291 f 0.0030 (Fig. 7) quartz syenite shells, however, have mildly peralkaline and because our observations show that it was chemical characteristics (Fig. 5D)and on the discri- deformedduring the main deformation that in turn minationdiagrams of Pearce et al. (1984) (Fig. preceded other plutons with emplacement ages of 5E,F) they overlap into the mid-plate field. It seems, more than 650 Ma. therefore, that these two late plutons show the early The age from theZaria Granite might seem of stages of transition from subduction-related magmat- doubtful significance in view of the small data base and ism to mid-platemagmatism, probably as the crust the unusually old agefor a Nigerian Pan-African thickened and stabilized towardthe end of the granite. However,the Dan Garamfe Granite in the Pan-African events. A similar transition, taken several Anka Belt (Fig. 1) has now yielded similar results, stages further, has beenrecognized in the Pan-African namely a seven-point Rb/Sr whole rock errorchron age terrain of the Nubian Shield (e.g. Gass 1981) and of740 f 26Ma (MSWD 4.66) andR.I. 0.70361 other collisional belts. f 0.0015 (Fig. 6A). The unaltered, undeformedand high-level intrusive character of this pluton as well as Isotopic data from the region the low R.I. (Fig. 7) supports the interpretation that this age is close to the time of emplacement. Ogezi (1977) provided the most recent comprehensive Five samples of granodioritefrom the Minna suite of age determinationsfrom this part of NW Batholith emplaced in the Birnin Gwari Belt (Fig. 1) Nigeria but his data remainunpublished. His main have also beenanalysed isotopically (Fig. 6B).No results and conclusions are briefly reviewed in this meaningful results were obtained, other than that the section which also includes new data from metasedi- batholith is broadly of Pan-African age, because of ments, minorintrusions and plutonicbodies (Figs 6 few data points, low Rb values, limited Rb/Sr ratio and 7). range and difficulties obtaining precise 87Sr/86Srratios. From the Gusau Migmatites of the basement Ogezi The ages of the three dated plutons contrast with

FIG. 6. Rb-Sr Isotope Data from NW Nigeria. (A, C & D) From Holt (1982); analyses carried out at B.G.S. Isotope Unit. (B, E-G) From Ajibade (1980); analyses carried out at Dept. of Earth Sciences, Leeds University. Decay constant of 1.42 X lop" yrpl used in all analyses.

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0.940. p 0.7050 - 0.900 1 / 1 pP1L 0,8600.820 500 MO reference line 0.7045 0.780 1 ( B) MinnaBatholith . 4248 c* R.i.10.7044+0.0002 0.740 426 *424c R.1.=0.70361~0.00026 (Assuminq 500 MO ape) ,.'+. M.S.W.D.-4.66 0.700 /+L:...... 0.7040 2 42 6 8 10 14 12 16 18 20 22 0.05 0.1 0.15

87Rb/86Sr

/ 0.740. + 3028

0.715 0.730. ,;j

Q\0.710 P

0.705 AGE=762*20 MO (2-rigma) AGE=572*10 MO (2-riqma) R.1.~0.70317+0.00018 R.1.=0.70824t0.00028 M.S.W.O.=I.O9 M.S.W.D.=25.28 0.700 * 0.2 0.4 0.6 0.81.4 1.0 1.2 1.0 2.0 3.0 8'Rb/B6Sr 0.728 1 1 0.73 1 251-8./a' 0.722 4 256-B 256-H /

/* AGE=561*18 MO (566k17Mo) o*7101 / . F).~ Birnin GwariPhvllitar RI.= 0.7097*0.0005 (0.705920.0005) AGE-681236Ma M.S.W.D.=6.8(0.5) 0.704 R.1.=0.7059f0.0008 Data in parentheses calculated n".i,.i-r 3Gl-lrl M.S.W.D.rl.0 n 7n Y

I 87R b/ 'Sr

I G) Maru Phyllitec

AGE c. 1110 MO R.I. c.0.703 0.7 1/ 2 4 2 6 8 10 12 14 16 18

B7Rb/86Sr

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0.712 - ,GM I ++mp 0.712 I L 0.710- v) 2 0.700- MG +eo \ l 0.706 - 1 b m MP ZG I 1 BGCP I .-0 0,704- L BrIOGG 7i 0.702 - 0.700 -

I i 1200 1150 1100 1050 1000 950 900 B50 000 750 700 650 600 550 500 Age (Mal FIG.7. Plot of radiometric age against measured initial 87Sr/ssSrratio for NW Nigerian rocks discussed in text. Two sigma error bars given for each suite. A, depleted mantle; B, bulk earth; C, enriched mantle, all from Thorpe et a/. (1984). BD, Bunkasau Dykes (1); BGP, Birnin Gwari Phyllite (2); bGCP, Birnin Gwari Chloritic Phyllite (2); BM, Bunkasau Microtonalite (1); DGG,Dan Garamfe Granite (1); GG,Gusau Granitic Gneiss (3); GM,Gusau Migrnatite (3); MaG, Malumfashi Gneiss (3); MG, Maiinchi Granodiorite (3); MP, Maru Phyllite (3); ZG, Zaria Granite (3). 1, Holt (1982); 2, Ajibade (1980); 3, Ogezi (1979).

those obtainedfrom north-central Nigeria by van To provideisotopic information on minor intrus- Breeman et al. (1977); the Panyam Granite has a U/Pb tions analytical results are given here from four zircon age of 605 f 10 Ma and the Bauchi Granite a microtonalitesamples and six dyke samples from five-point Rb/Sr whole rock isochron of 609 f 23 Ma. intrusionsemplaced in,and deformedwith, the Charnockiticintrusions from Bauchi andRahama Bunkasau coarse clastic units of the Anka Belt. The yielded Rb/Sr wholerock errorchrons with ages of microtonalites (Fig. 6C) define aRb/Sr whole rock 663 5 164 Ma and 668 f 128 Ma which those authors isochron (MSWD 1.09) indicating an age of 762 f 20 considered to be too old for emplacement ages unless Ma and R.I. 0.7032 f 0.0002. The line is little more supported by independent evidence so farnot pro- thana two-point isochron, however, and interpreta- vided. tions from theseresults must remain tentative. Too few data are available from northern Nigeria to Provisionally, the age is considered to be close to the read anyparticular significance intothe finding of age of emplacement and the low initial "Sris6Sr ratio older ages in the west and younger ages in the east. (Fig. 7) implies only a short crustalresidence time. Future work might investigate the possibilities that The results closely resemblethose from theDan there is a real difference in ages across the province, Garamfe Granite, also emplaced in the coarse clastic that the centres migrated eastward with time or that rocks, raising the possibility thatthe two forms of there are two or more distinct age provinces. magmatism were nearly contemporaneous. Forthe volcanic rocks of the region only two Thedata from the BunkasauDykes (Fig. 6D), previousisotopic studies havebeen documented. 572 f 10 Ma and R.I. 0.7082 f 0.0003, are difficult to Unmetamorphosed at Kisemi yielded a WAr interpret.The poor fit (MSWD c. 25) makes the whole rock age of c. 500 Ma (McCurry 1976). McCur- indicated age of 572 f 10 Ma unreliable beyond point- ry & Wright (1977) considered this age to be the date ing to some Pan-African effects on the dyke. Various of crystallization but a possible alternative is that the interpretationsare that the age is the time of age marks a time during uplift. Ogezi (1977) obtained crystallization, that the pointscatter is due to open an Rb/Sr whole rock age of 825 f 300 Ma from the system behaviour during deformation and alteration or Sad0metabasites, which areperhaps contempor- the presence of more than one generation of dyke. aneous with the Maru tholeiitic metabasites described Ogezi (1977), attemptingto define the age of above. However, the result is of uncertain significance deformation and metamorphism or sedimentation of in view of the large error. the Maru Belt, analysed eightsamples of the Maru

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TABLE1: Summary of isotopic constraints on events in the Anka, Birnin Gwari and Maru Schist Belts of NW Nigeria

Uplif 1, Cooling 500 L50 t Partial Resetting 7 Panyam of Gusau Granite 1 lsotoplc BunkasauI l Granite Gneiss Ages Bauchi Partlal 600 of Granite Resetting Birnin 650 Gwari lith Phyllites 700 c -p hnkasau Micro- tonalltes Garamfe Zarla I Amroxin!ate Minimun Granite Granlte 3 age of Birnln Gwari Belt Mam Deformation I GusauGranite and Sedimentation Gneiss i

and Deposition of Anka Belt

TMaru Phyllite 1100 7 Approximate Minimum- -];t4;t;&phism, Deformation age of -,- Partial 11501 Maru Belt Resetting Sedimentation of Gusau Migmatite 1200 and Volcanism

phyllites, obtaining an RbISr whole rock errorchron 6.8 to OS) butmakes little difference tothe age age of 1068 f 65 Ma and R.I. 0.7043 f 0.0057. He (566 f 17 Ma) nor the R.I. (0.7094 f 0.0005). For the argued that this isotopic age is the time of deformation three chloritic phyllites (Fig. 6F) an errorchron age of and metamorphism, not of sedimentation, diagenesis 681 f 36Ma with R.I. 0.7059 * 0.0008 (MSWD 1.0) or inherited detrital material; by implication the Maru was obtained but in view of the small data base the Belt is producta of Kibaransedimentation and results are of uncertain meaning deformation. Becausethese data have such important implica- tions for regional geotectonic interpretation most of The timing and correlation Ogezi’s original samples have been re-analysed in the of schist belt events same laboratory. The resultsobtained (Fig. 6G) are not significantly different from the earlierones and The constraints on the timing of events in NW Nigeria Ogezi’s results and main interpretations seem to be provided by isotopic age data are summarized in Table vindicated. 1. From these data it is seen that the older granites To pursue the problem of the timing of deformation and minorintrusions give a minimum of age of and metamorphismin the schist belts two suites of 650-750 Ma on sedimentation, volcanism and the main phyllites fromthe BirninGwari Belt havebeen deformationin the schist belts.Isotopic information analysed isotopically. The suite sixof graphitic on events before older granite emplacement is sparse, phyllites (Fig. 6E) gave an Rb/Sr whole rock isochron however,and needs to be used with otherdata to indicating a typical Pan-African age of 561 k 18 Ma obtain a broad sequence of events. with R.I. 0.7097 f 0.0005. Dropping one point that Accepting Ogezi’s (1977) interpretation of the lies off the isochronimproves the line-fit (MSWD c. 1100Ma age for the Maru phyllites as the time of

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/142/2/319/4888824/gsjgs.142.2.0319.pdf by guest on 23 September 2021 334 W. R. Fitches et al. first deformation and metamorphism in the Maru Belt, The Maru Belt is considered by Grant (1978) and it follows thatsedimentation and basaltic volcanism Turner (1983) to link southwards through little-known there is older than that age. Ogezi considered that the ground with the Kushaka Belt(Fig. l),largely because isotopic event is relatedto the upright, mainstruc- both contain B.I.F.and metabasalts. Lithological, tures. He was unaware of the flat-lying earlystruc- structural and isotopiccontinuity have yet to be tures, however, and an alternative view suggested here demonstrated but provisionally this correlation is is that the c. 11OOMa isotopic event is closely related accepted. to the early structures; the main structures areyounger The Birnin Gwari Belt was assigned to Pan-African and were not registered isotopically. Accordingly, it is events by Grant (1978), partly because of lithological likely that the rocks of the Maru Beltaccumulated and contrasts with the neighbouring Kushaka Belt and also were first deformed in response to Kibaran events. because the latter belt is considered to have a longer TheAnka Belt appearsto beyounger than the tectonic history. Turner (1983) supported this inter- Maru Belt. This conclusion is based on the single line pretation and added two lines of supporting evidence: of evidence thatsome Anka Beltconglomerates that a stratigraphic unit of intermediate age separates contain clasts of phyllite which were already cleaved the two belts and that the Birnin Gwari Belt contains before being incorporated in the conglomerate. The latePrecambrian tillites represented by theDurumi only known source of these clasts is the Mary phyllites. Pebbly Schists. The criteria used by other authors (e.g. Grant 1978; In our view, the questions of the age of the Birnin Turner 1983) to separate different ages of schist belts GwariBelt and relations with the Kushaka Belt need using cautiously. For example,the strong remain open. Structural analysis of the Birnin Gwari contrasts in minor lithologies betweentwo belts, in Belthas revealed a tectonic history as complex in this case coarse clastic sequencesand acid- terms of deformationstages as that in the Kushaka intermediate volcanic rocks in theAnka Belt com- Belt (Ajibade et al. 1979, Table 1; Ajibade 1980). In pared with B.I.F. and basalts in the Maru Belt, cannot both belts the upright NNE folds are preceded by be used alone to demonstrate anage difference. Those isoclinal folds and cleavage and are followed by other contrasts could be found in a single complex volcano- structures so thatthe two belts are not readily sedimentarybasin. Similarly, the differentstructures distinguishable on available tectonic information. The of the two belts, flat-lying early structures in the Maru unit separating the two belts is interpreted as myloni- Belt but not in the Anka Belt, do not prove an age tized basement by Ajibade et al. (1979), supporting a difference because various parts of single basins can view held by Grant (1978), rather thana separate begin deformingearlier thanother parts. However, stratigraphic unit, whilst theDurumi Pebbly Schists these lithological and structural differences are are regarded here as mass flow deposits with no meaningful in the case of the Anka and Maru Belts particular time connotations. but only in the light of the evidence of the phyllite clasts. The age of the Birnin Gwari Belt remains unknown, Schist belt evolution in Lithologically, the belt has more in common with the NW Nigeria Anka Belt than the Maru Belt but tectonically it more closely resembles the Maru Belt, containing structures In this section the main conclusions from foregoing earlierthan the regionalupright main structures. sections are summarized and briefly considered in the However, as arguedabove, neither lithological nor context of the region as a whole. The earlier stages of structural differences are reliable criteria for correla- evolutionremain poorly defined because of the tion orseparation. Isotopic datafrom the Birnin absence of detailedinformation from most NW Gwari Belt do not resolve the correlationproblem Nigerian schist belts and consequentuncertainties because it is not known to what event or events the concerning their ages and correlations. The later c. 600Ma isotope ages are related. Various possibili- stages, better constrained by isotopic and other data, ties are that the isotopeage relates to (a) the early are more clearly understood. deformationand metamorphism, in which case the The Maru Belt is considered here to be part of a flat-lying structures in the Birnin Gwari and Maru Kibaran basin, which probably links with the Kushaka Belts are of very differentages; (b)to the main Belt. Together, thesebelts appearto represent a deformation,then allowing the correlation of early major region of quietly accumulating fine clastic structures in both belts and a Kibaran age for the sediments that denote slow steady subsidence uninter- Birnin Gwari Belt; (c) to isotopic overprint by older rupted by major tectonic activity. In these respects, a granite lutons which extensively invade the belt. The model invoking an intracratonicextensional basin initial PSd8% ratios (Fig. 7) are low to intermediate, (Ogezi 1977; Holt et al. 1979; Turner 1983) seems however, perhaps implying acrustal residence time justified. Whether or not the little-known Zuru tooshort for the BirninGwari phyllites to be Quartzites (McCurry 1978) are timeequivalents, as correlated with those of the Maru Belt. Turner (1983) suggested, and thereby represent basin

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margin deposits, remains to be tested. Itis not yet clear from which it is separated by basement gneisses, Older how the Maru basalts and those of the Kushaka Belt Granites and the Zuru Quartzites. can be explained by this model. Chemically, the Maru The three schist belts under particular examination basalts have more in common with island arc tholeiites and probably also the other belts, were deformed by than the within-plate basalts anticipated in an intracra- upright NNE folds and cleavage. It is considered that tonicextensional basin. One explanation, which this deformation was a regional, more or less contem- cannot be evaluated on available data, is thatthe poraneousevent, although confirmation fromthe basalts are related to a subduction zone sited near the other, little-known belts is required. In the Anka Belt, SadwMaikwonaga area tentatively invoked by Ogezi and probably elsewhere, this event took place before (1977). In Ogezi’s model, subduction followed the c. 750 Ma, preceding emplacement of the dated Older opening up of a small basin floored by oceanic crust Granite plutons.This deformation is probably a within the framework of the major Kibaran basin. .subduction-related event, marking the closure of one Another enigma in the Maru Belt is the previously or more marginal basins. unrecognized early deformation. The recumbent iso- Most OlderGranites are post-tectonicintrusions clinal folds andcleavage, considered to be dated by whose chemistry is closely comparable to magmas the c. 1100 Ma isotopic event, appear to represent a generated in subduction and continent-continent colli- Kibaran horizontal tectonic regime. Before the signi- sion zones. They are attributed to thesubduction zone ficance of this event can be evaluated its distribution that emerges at the W African craton margin 2.54-350 and effects elsewhere need to be investigated. km to the W. Plutonism was long-lived, spanning at The Anka Belt is one of the younger Nigerian schist least the period 750-600 Ma, and in the closing stages belts. It has been assigned by Turner (1983) to began a transition from calc-alkaline to mildly peralka- Pan-African events and correlated with the Wonaka line as the crust thickened and stabilized. (Fig. l), Kazaure andKaraukarau Belts (140 km E Major crustal mobility in NW Nigeria had virtually and 120 km SE of Anka, respectively) and the Birnin ceased by 450-500 Ma ago. By then, regional uplift Gwari Belt. There are problems with the age of the and loss of heat fromthe oldergranites led to the Birnin Gwari Belt, as discussed above, and it is collapse of isotherms and the region as a whole cooled important to note that the other belts have not been below the argon-blocking K/Ar isotope systems. investigated in detail so that correlation is necessarily provisional. TheAnka Belt, at least, can be explained asa ACKNOWLEDGEMENTS.Theauthors acknowledge variously product of marginal basin developmentrelated to the collaboration of Drs N. Snelling and F. Derbyshire at the subductionat the edge of the West African craton. British Geological Survey and Dr M. Dodson and D. Rex of The minorintrusions and volcanic rocks have close the Leeds University Isotope Laboratories (A.C.A. & R.H.), financial support of the Nigerian Government (A.C.A. & chemical affinities with subduction-related magmas I.G.E.) and of the British Overseas Development Ministry elsewhere. The Anka Belt is not part of an island arc (W.R.F. & J.B.W.). The authors also thank Drs J. Pearce, immediately adjacent to the subduction zone. The belt N.Harris and A. Tindle for allowing us to use their lies some 250 km E of the W African craton margin granite discrimination diagrams prior to publication.

References

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Received 10 February 1984; revised typescript accepted 31 October 1984.

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