Pan-African Orogeny in Northern Nigeria

PATRICIA McCURRY Department of Earth Sciences, The Open University, Walton, Bletcbley, Bucks, England

ABSTRACT reached the status of an orogeny in northwest- An area of approximately 12,000 sq km of ern Nigeria. Precambrian to lower Paleozoic rocks around Zaria in northern Nigeria has recently been INTRODUCTION mapped for the first time. Results from this sur- Nigeria lies in the mobile zone separating the vey have been combined with limited data from older West African and Gabon cratons (Clif- adjacent areas to determine the nature of the ford, 1970). This mobile belt was extensively Pan-African event in this part of Nigeria. affected by the widespread Pan-African ther- A crystalline basement complex of gneisses, mo-tectonic event (450 to 680 m.y. ago), migmatites (Dahomeyan), and relicts of an an- which had little influence on the stabilized era- cient metasedimentary sequence (Birrimian) tonic areas (Fig. 1). French geologists have es- underlies most of the region. Infolded into this tablished the complex nature of the event in the basement, as north-south-trending synclinoria, Ahaggar region (Caby, 1970; Fabre, 1969); is a younger metasedimentary sequence (Ka- but farther south in West Africa, its effects are tangan) forming the remnants of an earlier, rather uncertain (Fitches, 1970; Grant, 1970; more extensive supracrustal cover. A suite of McCurry, 1971). This is largely due to our lim- syntectonic to late tectonic granites and ited knowledge of the pre-Mesozoic rocks in granodiorites (late Precambrian to lower these countries. For instance, in Nigeria, which Paleozoic) has intruded both the basement and occupies a key position between the two cra- its cover. tons, geological mapping has been mainly con- The structure of the area is dominated by centrated on the Cretaceous and younger north-south trends typical over much of Ni- sediments of the southern oil fields, and on the geria. There is evidence to suggest that the tin fields and Younger Granite complexes in whole area was affected by two successive central Nigeria. As a result, very little is known phases of intense deformation resulting in tight of the older metamorphic rocks which underlie isoclinal folding about (1) east-northeast-west- the greater part of the country. southwest, and (2) north-south axes. The crys- An area of approximately 12,000 sq km of talline basement was reactivated and the Precambrian to lower Paleozoic rocks around supracrustal cover regionally metamorphosed. Zaria in northern Nigeria—the Geological Sur- Two phases of progressive metamorphism ac- vey of Nigeria degree sheet 21—was recently companied the deformations, which were sepa- mapped for the first time (McCurry, 1970). rated and followed by static metamorphic Reconnaissance techniques of photogeological phases. Pressure-temperature conditions of interpretation and selected field traverses were metamorphism appear to have remained fairly used to compile four geological maps on the constant throughout, giving rise to a low pres- scale of 1:100,000, and the major geological sure facies series similar to the Buchan type. features from these maps have been transferred Extensive migmadzation of the crystalline onto the reduced scale map (Fig. 3). Results basement is believed to have accompanied the from this survey, combined with data from first deformation, followed by further differen- previously mapped areas lying to the northwest tiation during the second phase to produce (Nigeria Geological Survey, 1966) and the homogeneous gneisses and intrusive granites. southwest (Truswell and Cope, 1963), and Jointing, fracturing, and faulting followed the reconnaissance traverses into adjacent un- orogeny, and a major northeast-southwest- mapped areas, have enabled the compilation of trending transcurrent fault system crosses the Figure 2 to be made, giving a broader picture area. It is obvious from the structural and meta- of the geology of northwestern Nigeria. The morphic evidence that the Pan-African event writer intends to build on this framework by

Geological Society of America Bulletin, v. 82, p. 3251-3262, 9 figs., November 1971

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Fig. 3). The latter are generally richly siliceous or calcareous rocks and have sharp contacts with the crystalline host rocks. They are called the older metasediments, to distinguish them from the much younger supracrustal sediments described below. The crystalline complex in Nigeria is continuous with ancient basement rocks in Dahomey and the Camerouns (Mitch- ell-Thome, 1964), while Grant (1969, 1970) has inferred that rocks of Birrimian age (about 2,000 m.y.) and older are present in both the Dahomeyan and Nigerian basements. This complex is therefore considered to be of Dahomeyan to Birrimian age. 2. A supracrustal cover of low- to medium- grade metasediments occupies north-south- trending belts which are now known to be more extensive than previously shown (compare Fig. 2 with Nigeria Geological Survey Map, 1964). The majority of these metasedimentary belts are characterized by micaceous schists, phyllites, quartzites, and concordant amphibolites (unit 4a on Fig. 3). Similar rocks in the Kusheriki area to the southwest (Fig. 2) have been assigned to the Kushaka Schist Formation by Truswell and Cope (1963), to distinguish them from a lithologically different metasedimentary belt characterized by a mudstone con- Figure 1. Part of West Africa showing the distribution of cratonic areas which have remained stable for at glomerate layer (the Birnin Gwari Schist least 2,000 m.y. (horizontal lines) and mobile belt areas Formation of Truswell and Cope, 1963). In the which have been affected by the Pan-African thermo- northwest (unit 4b on Fig. 3) is a group of tectonic event of 450 to 680 m.y. ago (diagonal lines). hornfelsic schists with intercalated calc-silicate The dotted line marks the approximate boundary of Ni- geria (after Caby, 1970). rocks, which continues northward between Gusau and Dutsen Ma (Fig. 2). further mapping; but in the meantime, this pa- Concordant lenses of mafic rocks occur lo- per will place on record some of the data col- cally in the metasediments and are represented lected during the recent survey. The aim of this by anthophyllite-cordierite schists west of study is (1) to describe three major geologic Zaria, actinolite-chlorite schists southeast of subdivisions of the rocks shown on Figure 2 Gusau, and talc-carbonate and chlorite schists in and to place these in a chronological sequence the Kusheriki area. Serpentinites have been applicable to all the metamorphic rocks of Ni- found northwest of Gusau (J. B. Wright, 1970, geria and adjacent countries; (2) to bring evi- personal commun.). dence for at least two successive phases of deformation; and (3) to correlate these with progressive and static metamorphic phases, showing that the Pan-African event attained the Figure ,2. Geological map of part of northwestern status of a full scale orogeny in this part of Nigeria showing the extent of metasedimentary belts, Nigeria. Older Granite bodies, and rocks of the underlying crystalline basement. The cross-section shows the envisaged GEOLOGY relation between the metasedimentary belts and the crystalline complex. Key—open stipple: sediments, Geologically the area can be divided into Cretaceous and younger; dashes: Older Granite bodies; three units (Figs. 2 and 3). fine stipple: younger metasediments; coarse stipple: 1. Underlying most of it is a crystalline com- older metasediments of possible Birrimian age; blank: plex of quartzo-feldspathic biotite and horn- gneisses and migmatites; heavy line (F on cross-section): transcurrent fault zone; square: limit of Figure 3. blende gneisses, migmatites, and high-grade (Modified from Geological map of Nigeria, Nigeria Geol. metasedimentary relicts (units 1, 2, and 3 on Survey, 1964.)

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Figure 3- Major structures and geological features of Geological boundaries—solid line: observed; dashed the Nigeria Geological Survey degree sheet 21. Key—1: line: approximate or inferred; dot-dip sign-dot: grada- migmatites; 2: gneisses; 3: older metasediments; 4a: tional. Structure—thick lines: faults; medium lines: fold younger metasediments, belonging to the Kushaka axial plane traces; "{ : syncline; ^: anticline; £$: over- schist formation of Truswell and Cope (1963); 4b: turned syncline; ^: overturned anticline; narrow lines: younger metasediments, hornfelsic schists; 5a: biotite generalized trends (mainly fractures) taken from aerial granites; 5b: hornblende granites; 5c: granodiorites; photographs on scale of 1:48,000. black lenses: quartzites; double lines: main tarred roads.

Despite different lithologies in the metasedi- Contacts between the younger metasedimentary belts, no structural or stratigraphic evi- ments and the crystalline rocks seem to be dence has been found for assigning them to everywhere gradational, marked by the appear- different depositional sequences. Age dates are ance of increasing amounts of microcline, not yet available for these rocks, which are be- which is absent in the metasediments proper, lieved to be Katangan sediments, deposited be- toward the crystalline rocks. The relation en- tween 800 and 1,000 m.y. ago, and folded visaged between the crystalline rocks and and metamorphosed during the Pan-African metasedimentary belts is shown in the sche- orogeny. matic cross-section to Figure 2. The metasedi-

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ments are considered to occupy narrow TABLE 1. GENERALIZED CHRONOLOGY FOR synclinoria which evolved by crustal compres- METAMORPHIC ROCKS OF NIGERIA sion and downsagging, contemporaneous with gravity controlled upswellings of the mobile 467 to 618 m.y. Syntectonic to late tectonic intrusive granites and Older granitic crystalline complex into intervening granodiorites, accompa- Granites anticlinoria (Bennett, 1970). Such basement nied by contact meta- mobility would have obliterated any basal un- morphism in parts of the conformity. metasediments 435 to 850 m.y. Pan-African thermo-tec tonic 3. Intrusive into both the crystalline complex event—2 successive phases and the belts of younger metasediments is a of deformation, metamor- suite of richly potassic syntectonic to late tec- phism, extensive migmati- tonic granites and granodiorites—the Older zation, and reactivation of the crystalline complex Granites of Nigeria—ranging in age from 467 and older metasediments to 618 m.y. (Grant, 1969). These granites resulting in the formation (units 5a, b, and c on Fig. 3) are believed to be of a suite of orogenic the products of widespread mobilization and granites (the Older reactivation of the crystalline complex during Granites) 800 to 1,000 m.y. Geosynclinal deposition Younger the Pan-African orogeny. They have effected Katangan in mobile belt meta- contact metamorphism where they intrude sediments parts of the metasedimentary belts. The rela- 1,900 ±250 m.y. Eburnian cycle—folding tions between these granites and the other units and metamorphism of Bir- rimian sediments, and are described in detail in McCurry and Wright reactivajion of the (1971). Dahomeyan basement to A generalized chronology of events for the produce a suite of orogenic Nigerian metamorphic rocks, based on availa- granites (the Eburnian Granites) ble age determinations and correlations with ~2,500 m.y. Geosynclinal deposition Older other parts of West Africa (Mitchell-Thome, Birrimian meta- 1964; Grant, 1969, 1970), is given in Table 1. sediments 2,800 ±200 m.y.? Liberian cycle?—Forma- STRUCTURE tion of banded gneiss- quartzite complex near It has not been possible to make more than Ibadan (Grant, 1970) a brief preliminary analysis of the structure, and >2,800 m.y. Crystalline basement descriptions of fold geometry are incomplete, Dahomeyan mainly because of poor exposure over most of the region. Figure 3 shows that, apart from a Dates/rom Grant 1969, 1970 major northeast-southwest-trending transcurrent fault system which crosses the area, the compositional banding. Foliation planes trend dominant structures have the typical north- essentially north-south and are steeply westerly south trends common over much of Nigeria. dipping, so that the poles to these planes plot in The crystalline complex and its supracrustal a well-defined cluster when plotted on equal cover have been folded together to produce area projections (Fig. 4, c, d, e, and f). mainly overturned, tight isoclinal, noncylindri- An earlier east-northeast-west-southwest fo- cal major folds, with approximately north-south liation has been defined at some localities, and axial trends. Curved axial planes are moder- these foliation planes form a minor peak at ap- ately to steeply westerly dipping, and undulat- proximately 70° when all foliation trends are ing fold axes plunge shallowly north and south. plotted on a rose diagram (Fig. 8a). To avoid Lithological boundaries, fold axial planes, and rigidity in the event of other deformational foliation trends are effectively parallel. phases being recorded during subsequent mapping, the earlier and later foliations will be re- Planar Structure ferred to as the FE and FL foliations, relating to A tectonic foliation which forms a well- FE and FL deformational phases respectively. It developed schistosity in the metasediments is is obvious that the FL deformation has largely defined by parallel alignment of platy, pris- obliterated or transformed the FE foliation. At matic, and tabular minerals such as mica, am- four localities, it was possible to measure both phibole, and microcline megacrysts, and by the FE and FL foliation planes (Fig. 4a), and at

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Figure 4. Equal-area projections (x = fold axes, o = subarea among the younger metasediments west of lineations, numbers in parentheses indicate numbers of Zaria. Fold axes and lineations lie along a great circle S-poles plotted, and figures in boxes are contour inter- plane, while the S-poles fall near the pole to this plane. vals as percentage of points per 1 percent area), (a) @- (d) S-poles within the metasedimentary belts, (e) S-poles diagram of foliation planes at four localities where two for the granites and gneisses, (f) S-poles for the migma- foliations were measured; a and b in metasedimentary tites. (g) Lineations and fold axes for the metasedimen- belts, c and d in migmatites. (b) Linear structures along tary belts. S represents the S-pole maximum from d. (h) a single quartzite ridge in the metasedimentary belt west Lineations and fold axes for the gneisses and granites, (i) of Zaria. Great circle plane shows average foliation along Lineations and fold axes for the migmatites. this ridge, (c) Linear structures and S-poles for a small

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one of these localities, the FE foliation was clearly seen to be folded into small chevron folds while the FL schistosity has developed parallel to the axial planes of these folds (Fig. 5a). Similar relations between FE and FL are seen in thin sections of some schists (Figs. 6 and 9a). Linear Structure The most common kind of linear structure appears as corrugations on the rock surface. At one locality within migmatites, such a lineation was seen to be a result of the intersection of two foliation surfaces. A penetrative mineral lineation, formed principally of amphibole and feld- spar, is often found among the gneissic rocks where it frequently obscures foliation. Boudin- age, pinch and swell, and rodded structures occur in all stages of formation, usually developed in vein quartz parallel to fold axes. Stretching in a north-south direction along the foliation has given rise to elongate folds (Fig. 5b) and has drawn out the pebbles in the mudstone con- glomerate (Truswell and Cope, 1963, PI. 2B). Lineations and minor fold axes related to the FE deformation have been rotated by the de- Figure 5. Examples of minor fold structures: (a) Folded F E schistosity and the development of a superim- velopment of later folds. Variations in plunge posed Fi. axial plane schistosity (both shown on stereo- and direction occur on the scale of a single gram); (b) Fi: fold has been rotated and stretched during outcrop. Such variations along a single north- the Fi. deformation; (c) FE fold. The fold axis is at a high south-trending quartzite ridge approximately 1 angle to the trend of foliation, but lies within the foliation plane (stereogram); (d) FI. folds and stereogram, km in length are systematically arranged along showing foliation plane and fold axis; (e) Fi. fold and a great circle coinciding with the average folia- stereogram showing foliation plane, fold axis, and axial tion plane (Fig. 4b). plane cleavage; (f) two stages of granitization in migma- A similar pattern emerges from a small sub- tites: parallel to compositional banding, and parallel to area within the metasediments west of Zaria F i. axial plane. (Fig. 4c). The linear structures scatter about a north-south-trending, westerly dipping plane, disharmonic, ptygmatic, and convolute folding while the S-poles plot close to the pole to this (Turner and Weiss, 1963, p. 114-116). This is plane. When lineations are plotted over the presumably due to the formation of differential whole of the metasedimentary belts, the pattern stresses arising from multilayering of adjacent becomes slightly more random (Fig. 4g). This competent and less competent bands in partially may be partly due to the noncylindrical nature molten rocks, so that buckling and differential of the folding, and to variable amounts of com- movement resulted (Ramsay, 1967; Holland pressive strain accompanying the folding in and Lambert, 1969). different subareas (Ramsay, 1967). Linear Minor folds are divided into two groups ac- structures and fold axes associated with the FL cording to their orientation: (1) FE folds whose folding have generally north-south trends and fold axial planes are parallel to the regional shallow plunge values (Fig. 4, h and i). foliation with general north-south trends and westerly dips, but whose fold axes vary in direc- Minor Folding tion within these planes, as described in the Minor fold structures reflect the major struc- previous section (Fig. 5c; Fig. 6, a and b). (2) tures, being generally tight isoclinal, slightly FL folds whose fold axial planes are parallel to asymmetrical, and similar (Fig. 6, a, b, and c), foliation but whose fold axes also trend north- although more open folds are also found (Fig. south and plunge shallowly (Fig. 5, d and e; 5e). The migmatites display more complex Fig. 6c).

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Early folds have only been definitely iden- is in the order of 10 km, caused a swing in tified among the metasediments. Widespread structural trends in the metasediments by drag mobility of the crystalline complex would on either side of the fault. The fault is not a doubtless have destroyed or transformed most single feature, but a zone marked by a series of of the earlier structure in these rocks. disconnected prominent ridges of brecciated Interference patterns resulting from the and silicified country rocks, and recrystallized superposition of the FL folding on the FE struc- quartz. Its trend varies with the rocks it crosses, tures cannot be positively identified in this area. from about 40° in the crystalline rocks, swing- The peculiar dome structure within the crystal- ing to almost north-south in the metasedimen- line complex north of Funtua (Fig. 3) could be tary belt. Toward the southwest of Malumfashi interpreted as refolding of an east-west axial the fault splits into two features, and movement plane about a north-south axis. Such an inter- has been dissipated along each so that the pretation would be consistent with the fold ax- amount of movement on the southern branch is ial plane traces to the west of this feature. only about 5 km. Beyond this, it cannot be However, it could equally well be the result of traced with any certainty until it reappears in differential stresses during anticlinal folding the Kusheriki area (Truswell and Cope, 1963). about a north-south axis. Joints and fractures are common in all rock More definitive examples occur among the groups as generally vertical to subvertical major structures mapped in the Kusheriki area planes which show up well on aerial photo- (Truswell and Cope, 1963) where sillimanite graphs (Fig. 3). When the trends of these fea- quartzite bands among migmatites show intri- tures are plotted on a rose diagram (Fig. 8b), cate outcrop patterns (Fig. 7a) and axial plane four peaks can be recognized. Many northeast- traces swing markedly from east-west to north- and southeast-trending fractures show subhori- south trends (Fig. 7b). zontal slickensides and dextral (northeast set) or sinistral (southeast set) displacements. This Jointing, Fracturing, and Faulting suggests that they are shear fractures, and they A major transcurrent fault has dextrally dis- are thus labelled S i and 82. The S i set is paral- placed the central metasedimentary belt south lel to the major transcurrent faulting. The of Malumfashi (Fig. 3). The movement, which north-south joint set parallels the FL fold axes and is therefore designated the be joint set, and the peak at 100° is assumed to define the ac joint set. The over-all pattern is one of a single

Figure 7. (a) Outcrop patterns of sillimanite quartzites among migmatites, showing the effects of refolding, (b) Structures in younger metasediments (stipple) and crystalline complex (blank) showing prominent swings of fold axial plane traces from east-west to north-south trends (adapted from Truswell and Cope, 1963).

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There is some indication of two periods of syntectonic metamorphism, separated and followed by periods of static metamorphism. There is no evidence to suggest that the pressure-temperature conditions of metamorphism changed significantly during these episodes, which are labelled Ml to M4, and described as follows: (Ml) Syntectonic metamorphism associated with the FE deformation: a schistosity, defined mainly by biotite, developed parallel to the east-northeast-west-southwest fold axial planes. (M2) Static phase: biotite, garnet, and cordierite porphyroblasts grew in some rocks. (M3) Syntectonic metamorphism accompanying the FL deformation: earlier porphyroblasts were rotated (Fig. 9, b and c) and the Figure 8. Rose diagrams of (a) foliation trends, and earlier schistosity was folded in some rocks, (b) joint and fracture trends. with development of a new axial plane schistosity (Fig. 6, d and e; Fig. 9a). In most of the rocks, the earlier foliation has been completely reori- episode of fracturing, such as might be ex- ented into the north-south direction. Continued pected to accompany uplift, cooling, and con- growth of garnet porphyroblasts from the static traction of folded and metamorphosed rocks. M2 into the M3 phase produced S-trails of inclusions in some specimens (Fig. 9b). Cordier- METAMORPHISM ite porphyroblasts that grew during the M2 A bimodal distribution of metamorphic fa- phase remained stable through the M3 episode des exists between the greenschist to middle (Fig. 9d). The alteration of garnet porphyro- amphibolite facies of the metasediments on the blasts to plagioclase in some amphibolites one hand, and the upper amphibolite facies of among the metasediments is accompanied by the crystalline rocks on the other. Evidence of the renewed growth of hornblende across the mobility and plastic deformation in the latter margins of the altered porphyroblasts. It is not indicates that metamorphic conditions in these yet clear whether this is a retrogressive effect, rocks were similar to those described by Hol- and work is in progress to determine the nature land and Lambert (1969) in their regime 4. and history of this alteration. (M4) Late tec- Because of the scarcity of pelitic lithologies and tonic static phase: large xenoblastic poikilo- hence diagnostic index minerals, division into blasts of cordierite up to 1 cm in diameter occur metamorphic zones is difficult (see Table 2). Al- in the hornfelsic schists adjacent to a late tec- mandine garnet is a common constituent in all tonic granodiorite (unit 5c, north of Funtua, the rocks. Sillimanite has been found at one Fig. 3). Their formation is assumed to be due locality among the crystalline rocks and kyanite to a local raising of the thermal gradient accom- has been described from two localities believed panying emplacement of the granite. Random to lie within the crystalline complex in the Ku- growth of actinolite laths across the foliation sheriki area to the southwest (Truswell and planes in actinolite-chlorite-magnetite schists Cope, 1963; Russ, 1957). Cordierite is not un- southeast of Gusau (Fig. 2) may also be due to common among the metasediments, and al- contact effects from an adjacent granite body. though not found in this area, staurolite has been recorded from a number of metasedimen- Migmatization tary belts elsewhere in Nigeria (Truswell and Two stages of quartzo-felspathic injection Cope, 1963; Jones and Hockey, 1964; Russ, have been recognized among the migmatites, 1957). The metasedimentary assemblages seem parallel to lithological boundaries and parallel to indicate a low-pressure facies series similar to to fold axial planes (Fig. 5f)- Approximately the Pyrenean or Buchan type (Hietanen, 1967; east-west-trending lobate boundaries of the Miyashiro, 1961). Sacchi (1968) records a migmatite complex (Fig. 3) indicate that the close affinity with the Buchan type in the region first major episode of migmatization occurred around Bena about 200 km west of Zaria. during the FE deformation and that this belt

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TABLE 2. MINERAL ASSEMBLAGES IN METAMORPHIC ROCKS OF FIGURE 3

Rock Type Mineral Assemblages

phyllite sericitic mica + quartz mica schists quartz + muscovite ± green biotite ± almandine feldspathic schist green biotite + quartz ± oligoclase Anie anthophyllite-cordierite schist anthophyllite + cordierite quartzite quartz ± muscovite ± almandine hornfelsic schist quartz + brown biotite ± muscovite ± sillimanite ± almandine ± cordierite

.g g .§ calc-silicate quartz + andesine/labradorite + diopside + blue-green hornblende ± epidote ± almandine ~S~la " fj, g 2 amphibolite green hornblende + oligoclase/andesine + quartz ± almandine ± epidote ± diopside

gneiss and migmatite oligoclase ± microcline + quartz + red-brown biotite ± muscovite ± hornblende ± almandine ± epidote (± sillimanite) calcareous schist brown biotite ± tremolite ± plagioclase An3o_36 ± quartz feldspathic quartzite quartz + plagioclase ± muscovite ± sillimanite

was then folded along north-south axes by the FL deformation, which was also accompanied by a second (less intensive) axial plane granitization. If, as is suggested by McCurry and Wright (1971), the granites are the final result of extensive migmatization, it is likely that migmatization and granitization during the FE deformational episode formed a prelude to granitization during the FL deformational phase, when extreme compositional differentiation produced potassic granites on the one hand, and more homogeneous, comparatively potash- poor gneisses on the other. CONCLUSIONS 1. Complexities in the minor structure imply that at least two major episodes of deformation affected both the reactivated crystalline complex and the younger metasedimentary cover. During the earlier FE episode east-northeast- Figure 9. Microphotographs. (a) Chevron folds and axial plane schistosity defined by sillimanite (s) and bio- west-southwest trending structures predomi- tite (b). (b) Rotated garnet porphyroblast (g) showing nated, with foliation planes and fold axial S-trails of inclusions. The F, Foliation denned by biotite planes dipping moderately to north and south. laths (b) has been deflected by the porphyroblast, and This compares well with early structures iden- the strain shadow infilled with granulated quartz (q). (c) Biotite porphyroblasts (b) have been rotated during the tified in the Ahaggar area (Caby, 1970). Be- F, deformation so that they tend toward augen shapes cause foliation is approximately parallel to parallel to the axial plane foliation, (d) Cordierite por- lithological boundaries, it is concluded that phyroblast (c) affected by the F, deformation. Composi- folding during this episode was of the tight iso- tional banding (not believed to be primary) is marked by alternate quartz (q) and biotite (b), and is almost perpen- clinal type. Later FL folding, also tight isoclinal, dicular to the axial plane foliation, defined by the align- has produced the approximately north-south- ment of the biotite laths. trending structures common over much of Ni-

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geria. Early lineations and fold axes have been Fitches, W. R., 1970, "Pan-African orogeny" in the rotated through as much as 70° so that they coastal region of Ghana: Nature, v. 226, p. 744- have variable distributions within the parallel 746. Grant, N. K., 1969, The late Precambrian to early to subparallel FL fold axial planes. Paleozoic Pan-African orogeny in Ghana, Togo, 2. It is possible to correlate the deformational Dahomey and Nigeria: Geol. Soc. America phases with four phases of metamorphism. Bull., v. 80, p. 45-56. Pressure-temperature conditions appear to 1970, Geochronology of Precambrian base- have remained fairly constant throughout the ment rocks from Ibadan, southwestern Nigeria: orogeny, and the rocks show no evidence of Earth and Planetary Sci. Letters, v. 10, p. 29-38. intradeformational cooling or fracturing. The Hietanen, A., 1967, On the facies series in various present fracture system can be related to post- types of metamorphism: Jour. Geology, v. 75, p. tectonic cooling. 187-214. Holland,;. G., and Lambert, R. St.J., 1969, Struc- 3. It is obvious that these deformations and tural regimes and metamorphic facies: Tectono- their accompanying metamorphic phases were physics, v. 7, p. 197-217. successive episodes during a period of intense Jones, H. A., and Hockey, R. D., 1964, The geology orogenesis in late Precambrian to lower Paleo- of south-western Nigeria: Nigeria Geol. Survey zoic times. This corresponds with the Pan-Afri- Bull. 31, 87 p. can event elsewhere in Africa, which clearly McCurry, P., 1970, The geology of degree sheet 21 achieved the status of an orogeny in northwest- (Zaria) [M. Sc. thesis]: Zaria, Nigeria, Ahmadu ern Nigeria. It has been suggested that this Bello Univ. orogeny was perhaps the result of collision be- 1971, Plate tectonics and the Pan-African tween continental blocks on a "Gabon plate" orogeny in Nigeria: Nature, v. 229, p. 154-155. McCurry, P., and Wright, J. B., 1971, On place and and a "West African plate" at the end of time in erogenic granite plutonism: Geol. Soc. Proterozoic times (McCurry, 1971). America Bull., v. 82, p. 1713-1716. ACKNOWLEDGMENTS Mitchell-Thome, R. C, 1964, The Precambrian of West Africa: Geol. Rundschau, v. 54, p. 1088- The research forming the basis of this paper 1143. was undertaken during 1968-1970 as part of an Miyashiro, A., 1961, Evolution of metamorphic M.Sc. thesis study at Ahmadu Bello University, belts: Jour. Petrology, v. 2, p. 277-311. northern Nigeria. I am indebted toj. B. Wright Nigeria Geological Survey, 1964, Geological map of for supervising the project and for critically Nigeria, scale 1: 2,000,000. reading the manuscript. 1966, Geological map sheet 8, Gusau, scale 1: 250,000. RamsayJ. G., 1967, Folding and fracturing of rocks: New York, McGraw-Hill Book Co., 568 p. REFERENCES CITED Russ, W., 1957, The geology of parts of Niger, Zaria, and Sokoto provinces: Nigeria Geol. Sur- Bennett, J. D., 1970, Craton-mobile belt relations vey Bull. 27, 42 p. with particular reference to the Mosetse-Mat- Sacchi, R., 1968, The geology of the region around sitama area, north-eastern Botswana: Geol. Bena in northern Nigeria: Mem. Instituti di Mag., v. 2, p. 113-123. Geologia e Mineralogia dell'Universita di Caby, R., 1970, La chame Pharusienne dans le nord- Padova, Italy, v. 24, 47 p. ouest de 1'Ahaggar (Sahara central, Algerie); sa Truswell, J. F., and Cope, R. N., 1963, The geology place dans 1'orogenese du Precambrien supe- of parts of Niger and Zaria provinces, northern rieur en Afrique [These Ph.D.]: Montpellier, Nigeria: Nigeria Geol. Survey Bull. 29, 52 p. Montpellier Univ., 335 p. Turner, F. J., and Weiss, L. E., 1963, Structural Clifford, T. N., 1970, The structural framework of analysis of metamorphic tectonites: New York, Africa, in Clifford, T. N., and Cass, I. G., eds., McGraw-Hill Book Co., 545 p. African magmatism and tectonics: Edinburgh, Oliver and Boyd, p. 1-26. Fabre, J., 1969, Remarques sur la structure du Sahara Occidental et Central: Bull, de la Soc. d'Histoire Narurelle de PAfrique du nord, v. 60, p. 43-73. MANUSCRIPT RECEIVED BY THE SOCIETY JUNE 10, 1971

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