Precambrian Crustal Development in Western Nigeria: Indications from the Iwo Region

FRED H. HUBBARD Department of Geology, The University, Dundee DDI 4HN, Scotland

ABSTRACT drafted (Hubbard, 1963, in Geological Survey Nigeria Bulletin no. 38, unpub.). The rocks of the Iwo region of southwestern Nigeria comprise a The early work of de Swardt (1953) in the northeast quadrant of migmatite-gneiss-granite complex and a metasupracrustal se- the area and the mapping of I. K. Ogbukago (unpub. data) in the quence. Though dominantly of amphibolite facies grade, pyroxene northwest quadrant are acknowledged. These quadrants were granulite and charnockite remnants are found in the migmatite- briefly resurveyed by me and integrated with my mapping of the gneiss-granite complex. Old supracrustal remnants, best identified southern quadrants. Whereas the original descriptions of some in resistant quartzite horizons, are also represented, and a variety rock units by de Swardt and Ogbukago are drawn on in the follow- of granitic plutonic rocks form well-marked circumscribed associa- ing account, the interpretations presented are my own and are not tions. The main metasupracrustal belt is associated with a major necessarily subscribed to by other workers in the area or by the dislocation zone and has two principal lithologies — dominant Geological Survey of Nigeria. amphibolite and pelite west of the fault zone and quartzite and The 1:250,000 sheet 60 (Iwo), which comprises an area of about quartzofeldspathic gneiss to the east. Oligoclase-granite gneiss and 12,000 sq km east of Ibadan, is centrally placed in the Basement pegmatite are associated with the schist. All the rocks of the area Complex of southwestern Nigeria (Fig. 1). Despite the presence of have been affected to varying extents by late reworking, known gold deposits, little has been published on the geology other metasomatism, and granitic activity. than de Swardt's bulletin (1953). Accounts of aspects of the char- On the basis of the indicated field relations and the limited nockitic rocks of the area and their associations with the nonchar- radiometric age data available, a development sequence for the re- nockitic granite have, however, been published (Hubbard, 1965, gion is proposed. Archean crustal rocks are considered to have 1966, 1968). been involved in an Eburnian (1,950 ± 250 m.y.) reactivation of Proterozoic mobile belt type. In the Kibaran period (1,200 ± 200 GENERAL FEATURES OF THE GEOLOGY m.y.), activity of greenstone belt—granite type brought about the formation of the Ife supracrustal—granite gneiss association, with The generalized map of Figure 2 is modified and simplified after extensive basement reactivation limited to the proximity of the 1:250,000 sheet 60 (Iwo) published by the Geological Survey of downwarping supracrustal sequence. Late, widespread reworking Nigeria. and granitic plutonism is related to the Pan-African (Older Granite) Schistose rocks dominate in a north-northeast—trending belt in event (600 ± 150 m.y.). the east of the region and in a strip along the western boundary. The possible relevance of the proposed scheme to the interpreta- Between these zones and in the southeast, the country is underlain tion of the development of the eastern parts of the West African platform is discussed. Key words: areal geology, West Africa, Nigeria, metamorphic petrology, igneous petrology, geochronology, Pre- cambrian.

INTRODUCTION

The recent advances in knowledge of the processes of the Pre- cambrian (Anhaeusser and others, 1969; Salop and Scheinmann, 1969; Windley and Bridgewater, 1971; Windley, 1973) and the availability of new radiometric age data (Grant, 1971) allow a modified interpretation of the nature and development of the basement complex of western Nigeria to be attempted. In the late 1950s and early 1960s a semireconnaissance, field-mapping program was carried out by officers of the Geologi- cal Survey of Nigeria in a belt across western Nigeria from the Dahomey border to the Niger River. I was employed in the mapping of the Iwo area, 1:250,000 sheet 60, and a map of the region was published in 1964. Circumstances, however, dictated that the explanatory bulletin was never printed. The Iwo area is located immediately east of the Ibadan area described by Jones and Hockey (1964). In this paper, the outlines of the geology of the Iwo area are presented, and an interpretation is proposed that takes into account Figure 1. Location map showing position of Iwo region within Base- the new age data. The interpretative views discussed here differ ment Complex of Nigeria. Cretaceous and younger sedimentary cover is in- from those held at the time the map was prepared and the bulletin dicated by stippled pattern.

Geological Society of America Bulletin, v. 86, p. 548-554, 4 figs., April 1975, Doc. no. 50414.

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Migmatite-Gneiss-Granite Complex Metasupracrustal Schists & Granite-Gneiss

Figure 2. Generalized geologic map of Iwo region modified and simplified after 1:250,000 sheet 60 (Iwo) of Geological Survey of Nigeria. 1, Migmatite-gneiss complex; 2, charnockite; 3, early granitic phases of Older Granite Cycle; 4, younger granitic phases of Older Granite Cycle; 5, amphibolite, amphibole schist, and pelitic schist; 6, quartzite, quartz-schist, and quartzofeldspathic gneiss; 7, granite-gneiss-dominated regions; p, pegmatite; T, tonalité. Thick, broken lines indicate major faults.

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by a complex sequence of gneiss and migmatite that is broken by late metasomatism, the central enderbitic mass shows progressive discrete, normally elongate, granitic complexes. All the rocks of the marginal degradation to mafic-spotted granodiorite and por- region have, to a greater or lesser degree, been modified by the ef- phyroblastic granite. fects of a late reworking that was accompanied by intense Biotite-quartz-diorite (tonalite) bodies are common to most as- K-metasomatism, and interpretations of the early history are con- sociations. Their marginal relations are normally obscured by sequently hampered. Some lithologies, however, have to a great ex- metasomatism, but it is thought they represent diapiric emplace- tent been able to resist the late metasomatism (for example, quartz- ments of homogenized and mobilized basement. Xenolithic evi- ite and amphibolite), and in many other instances, it is possible to dence from the southeastern Ile-Oluji complex shows emplacement isolate the metasomatic effects and gain some picture of the nature into a high-grade supracrustal sequence. No time relation with the of the rocks before their transformation. In the following account charnockite can be deduced from outcrop evidence, but the spatial of the regional geology, the subdivision into a "migmatite-gneiss- association of tonalite and charnockite in the northwest and lack of granite complex" and "metasupracrustal schist" is adopted solely any evidence of diaphthoresis in the tonalite's petrography suggest for descriptive convenience. Each of these gross lithological units that the tonalite emplacement stage postdates the period of granu- contains rock formations of different ages, and no implication of lite facies conditions. As with the charnockite, in zones of strong stratigraphic significance is intended. metasomatism the quartz-diorite is converted through a granodioritic stage to porphyroblastic granite. Migmatite-Gneiss-Granite Complex Although generally demonstrable of in situ metasomatic deriva- tion from old plutonic and gneissic rocks, the porphyroblastic gran- In the central and southern parts of this unit and in the whole of ite and granodiorite often have an at least partial intrusive relation the eastern zone, intense metasomatism, with contemporaneous at- with their gneiss-migmatite envelope showing that mobilization tainment of a high degree of plasticity, has in large part determined was locally attained during the transformation. Restricted migma- the nature of the rocks. Widespread wild plastic folding and shear tite aureoles are commonly developed at the intrusive interfaces. flow is characteristic in these areas. An essentially granodioritic A suite of younger, magmatic, often two-mica granites intrudes compositional mode prevails, and locally massive, granitic, all other granitic rocks and shows a marked restriction of massive homogenized patches have developed. Basic-to-ultrabasic pods and development to within the boundaries of the complexes. Within schlieren are generally all that survive to attest to an originally such suites, a time sequence of emplacement can be determined more inhomogeneous progenitor sequence. Late crosscutting showing that the culminating phases were aplitic and pegmatitic. leucogranitic and pegmatitic veins and dikes are widespread. Minor intrusive rocks attributable to each phase of these late grani- In the northern part of the western belt, metasomatism has lo- tic intrusions are of widespread development throughout the re- cally been less severe, and both homogenization and mobility gion. have been more restricted. Greater insight into the nature of The north-south linear aggregation of the circumscribed rocks, the old complex is thus afforded. Polydeformational biotite- particularly in the west, is very marked and would tend to suggest amphibole—banded gneiss, containing evidence of an early that all the units relate to the one plutonic episode localized along and now deformed basic and ultrabasic intrusive phase, is the most some structural lineament. The charnockite, however, crystallized widespread rock type. Discontinuous, strongly deformed quartzite in a granulite facies environment and, as far as can be determined, and quartz-sillimanite schist bands indicate a supracrustal origin was intruded into nonmigmatized high-grade metasupracrustal for at least part of the complex. Remnant zones of pyroxene granu- rocks. The evidence suggests that the tonalite was emplaced lite and charnockite, now in large part diaphthoritized and mig- into an amphibolite facies environment before the main meta- matized, testify to an early high-grade metamorphism. This is also somatic/migmatitic event and postdating the charnockite- indicated in the xenoliths of the charnockitic intrusions (now in- granulite stage. The extent of the time period separating these in- corporated in the granitic complexes), which are khondalitic trusive events is unknown. All the other units represent subsequent garnet-sillimanite schist. It is not clear to what extent the banded stages of the plutonic cycle, perhaps heralded by the emplacement gneiss is diaphthoritic. Agmatitic zones on a rather coarse scale are of the tonalite, which involved an intense metasomatic phase and locally recognized, particularly in regions of high amphibolite con- affected transformations and local remobilizations of the charnock- tent, but the extent to which this feature may perhaps be developed ite, tonalite, and parts of their envelopes. This cycle culminated in on a grander scale is inestimable in such a poorly outcropping ter- the magmatic granite, pegmatite, and aplite. This is the Older rane. Granite Cycle of Nigeria. With the exception of the minor granulite facies remnants men- If such a development sequence applies, it suggests that the tioned above, the metamorphic grade is uniformly in the amphibo- localizing influences of the charnockitic intrusion stage remained as lite facies. an effective control in the later plutonic activity. It is of interest, Because the detail of the circumscribed granitic associations is however, that the zone of greatest intensity of metasomatic activity very complex, only general features of the associations will be out- does not conform with this meridional trend but is concentrated in lined. Several components, although not all found in any one as- a transverse, broad, northeast-trending belt. sociation, characterize these complexes — enderbitic and granodioritic charnockite, biotite-quartz-diorite (tonalite) and Metasupracrustal Schist granodiorite, porphyroblastic granodiorite and granite, fine- to medium-grained, often two-mica granite, aplite, and pegmatite. The main development of this unit is in a broad Charnockite remnants are recognized only in the north and north-northeast—trending belt in the west of the region, but rocks center of the large elongate Ikire-Iwo complex in the west; how- of essentially similar lithologies also crop out at the western limit of ever, it is thought that some of the mafic-spotted granodiorite of the sheet area. A striking subdivision of the lithologies of the east- other complexes may be, at least in part, diaphthoritized charnock- ern Ile-Ife belt by the intense Ife fault system is evident (Fig. 2). The ite (especially that in the northern part of the southeastern Ile-Oluji western flank is dominantly composed of amphibolite, amphibole- complex). The petrography points to a magmatic crystallization schist, and pelitic schist with associated granite gneiss and pegma- origin in a granulite facies environment (Hubbard, 1968), and this tite, whereas east of the fault zone the rocks are mainly quartzite, intrusive phase is probably time related to the granulite facies quartz-schist, and quartzofeldspathic gneiss, including minor metamorphic stage indicated in the migmatite-gneiss complex. Al- iron-rich schist and quartzite. The possible relationship of the two though the northern charnockite has, to a great extent, escaped the facies is indicated in the northeast where a quartzitic sequence over-

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the eastern quartzitic zones has no associated pegmatite.) Delimita- tion is imprecise, but on the map (Fig. 2), regions dominated by granite-gneiss and those of extensive massive pegmatite are outlined. It is difficult to define the western limit of the schist belt. Fresh outcrop is extremely rare but information from weathered material from incised paths and streams, together with data from damsite investigation drill holes (which never reached fresh rock), suggests pegmatized schist, west of the granite-gneiss/pegmatite belt, in the areas outlined on the map. However, the body of massive pegmatite at the southwest extremity of the granite-gneiss/pegmatite belt, that is, associated with the western fringe of pegmatized schist, is of the distinctive, tourmaline- and beryl-bearing, biotite-muscovite- microcline-pegmatite type elsewhere seen directly linked with the late stages of the Older Granite Cycle activity.

Ife Fault Zone

This major fault system, which is generally north-northeast- or northeast-trending and is associated with the schist belt, extends both north and south from the Iwo sheet area. Its full extent is not known but a photo-geological interpretation of the area south of the sheet boundary indicated that the fault continues southward to pass under the Cretaceous sediment cover, and its striking align- Figure 3. Some general features of geology of eastern parts of West Afri- can craton in Ghana, Togo, Dahomey, and Nigeria. Voltaian-Buem-Togo ment with the Okitipupa Ridge may be more than merely fortui- system is shown by stippled pattern, Cretaceous and younger sedimentary tous. cover by broken-line pattern. Fine lines within Basement Complex are There is evidence to suggest that the clearly demonstrated post- generalized structural trend lines in schist belts; strong broken lines indicate metasomatism fault activity is only a young manifestation of re- traces of major fault systems; dash-dot line indicates limit of occurrence of peated movements in a long-established major zone of dislocation. schist belts within Basement Complex. Compiled and modified from Grant The general concordance of its trend with the major faulting as- (1969) and Burke (in Oyawoye, 1972). sociated with schist belts in northern Nigeria (Truswell and Cope, 1963), the eastern limit of schist-belt occurrence in the Nigerian lies amphibole-schist apparently identical to that of the west. A Basement Complex (see Burke's generalized structural map in thrusted interface may, however, be involved. Oyawoye, 1972), and the Ghana-Togo-Dahomey orogenic belt In general the rocks show an amphibolite fades grade of (Grant, 1969) suggest that the Ife fault system is one of a series of metamorphism, but locally, greenschist fades assemblages are pre- fundamental lineaments that has existed as least as long as from the served. Tight isodinal or slightly overturned folding on generally period of schist-belt development (Fig. 3). north-northeast—trending axes, with extensive strike-slip faulting, is the characteristic structural pattern throughout, but it is best dis- DISCUSSION played in the quartzitic zones where strike ridges stand out in bold relief. As Oyawoye (1972) emphasized in his recent review, interpreta- A supracrustal volcanic and sedimentary origin is indicated for tions of the evolution of the Basement Complex of Nigeria are the rocks of the unit, although irrefutable depositional structures likely to be generalized and "liable to individual prejudice." This is have not been identified. East of Ile-Ife, adjacent to the Ife fault, a a natural consequence of the paucity of detailed data. Comparisons metaultrabasic-to-basic intrusive complex crops out but is too ex- with models erected from investigations of more completely extensively deformed and transformed to allow its detailed nature to posed Precambrian platform regions of the world may help in- be identified. terpretation. Of great importance, however, is the availability of Associated with the schist belts are conformable, often margi- radiometric age data and, although more dates are becoming avail- nally diffuse bodies of foliated granite (or granite-gneiss), which able, in large part due to the efforts of Grant (1971), they are still characteristically include significant proportions of partially di- sparse and selective for the Nigerian complex. The great prepon- gested pelitic or amphibolitic material. These granitic bodies, derance of known ages falls within the Pan-African range (600 ± where only weakly affected by later metasomatism, are seen to be 150 m.y.) but, as Grant (1971) suggested, this may in large part be trondhjemitic — the oligoclase-granite-gneiss complex of de due to bias of sampling and method. The whole-rock Rb-Sr ages Swardt (1953). published (one from a northern Nigerian migmatite, another from In the west, the granite-gneiss is found in association with zones a granite-gneiss from Ibadan just west of the Iwo area, and a third of schist so intensely pegmatized that in some areas only occa- from the Ile-Ife granite-gneiss formation) all give much older ages, sional vague remnants of schistose texture attest to the largely re- although K-Ar mineral ages from the same rocks, where available, placive character of the pegmatite. Particularly in the more massive gave Pan-African ages (Table 1). development, there is clear evidence of an early oligoclase- King and de Swardt (1949) and de Swardt (1953), discussing pegmatite being transformed by a later K-rich metasomatism. A rocks from western Nigeria, and Truswell and Cope (1963), for genetic association of the granite-gneiss and the pegmatite is likely, northern Nigerian rocks, suggested on the basis of field evidence both relating to an early oligoclase granitic stage predating the late that there might exist granitic rocks older than those of the Older widespread K-metasomatic event. They may represent loci of dif- Granite Cycle. Grant and others (1972) came to a similar conclu- fering intensity of granitic activity, although it may be that sion on isotopic grounds. De Swardt (1953) further suggested that amphibolite-dominated areas- tended to give rise to granite-gneiss the quartzitic rocks of the Ilesha region (northeast part of the Iwo whereas the pelitic zones were prone to more extensive transforma- area) might represent a younger supracrustal sequence. Hubbard tion to pegmatite. (It is noteworthy that the granite-gneiss flanking (1963, unpub. bulletin; 1968) considered that the granulite and

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TABLE 1. RADIOMETRIC AGES IN THE RANGE 1,950 ± 250 M.Y. FROM THE NIGERIAN BASEMENT COMPLEX lated to each of these periods are found from the western Nigerian Basement Complex. Locality Sample Age (m.y.) Age (m.y.) The oldest whole-rock date known from the western Nigerian (Rb-Sr method) (K-Ar method) region is the Eburnian age from the Ibadan granite-gneiss, which Ibadan Granite-gneiss* 2,190 ± 30 Grant and others, was emplaced, in part by replacement, in a basement characterized 1969 (isochron age) by banded gneiss and extensive refolded quartzite (Grant, 1970). Ibadan Hornblende 500 ± 20+ Grant and others, from granite- 1969 Hockey (Jones and Hockey, 1964) ascribed the Ibadan granite- gneiss gneiss to the Older Granite Series, a belief that seems no longer ten- Ibadan Biotite 480 ± 20+ Grant and others, from granite- 1969 able in the light of the subsequently determined radiometric ages. gneiss There is, rather, evidence of an Eburnian granitic event affecting Muslim Migmatite 2,220 ± 30§ Hurley, 1966 even older rocks. The supracrustal remnants associated with the cemetery, (JF1392)* Kaduna granite-gneiss may represent an early depositional phase related to Muslim Biotite 480 ± 20+ Snelling, 1964 the Eburnian event or may be relict from some Archean stage. The cemetery, from Kaduna migmatite Pan-African (Older Granite Cycle) is evidenced here only in minor Ile-Ife Granite-gneiss* 1,150 ± 140# Grant and others, granitic and pegmatitic veining, mineral recrystallization, and (isochron age) 1972 basalt dike intrusion (Grant, 1970). There is no isotopic evidence Note: Modified from Grant, of Kibaran activity. * Whole rock. 1 1 The Ibadan sequence continues into the Iwo sheet area as the t "K = 5.305 x 10" " yr" . s § "Rb • 1.475 x 10"11 yr"1 'Rb/' Sr = 1.933, "Sr/'Sr » 0.7693; initial western strip of schist and granitic gneiss (Fig. 4). The basement of B7Sr/86Sr ratio = 0.705. I "Rb = 1.45 x 10"" yr"1; initial »'Sr/,sSr ratio - 0.705. this sequence is the migmatite-gneiss complex. It seems possible that the banded gneiss may, at least in part, represent Archean rocks reworked in the Eburnian period with the massive charnock- charnockite of the Iwo area were remnants of an old basement on ite intrusions resisting deformation in their inner parts. Such an in- which a supracrustal sequence, in part represented by the amphibo- terpretation is, of course, speculative but may serve to indicate loci lite and schist, was deposited and that the two were subsequently of interest for further age determinative investigation. If the indi- reworked together in the Older Granite Cycle activity. The now cated effects of the Pan-African activity are subtracted, as far as is available radiometric age data tend to confirm the contention that possible, the features of the region correspond very closely to those the Nigerian Basement Complex includes old rock sequences that predictable for Archean regions reworked in a Proterozoic mobile have been partially reworked and restructured in successive belt of the mobile zone type as defined by Windley (1973), that is, younger events of which the Older Granite Cycle, or Pan-African those Proterozoic mobile belts consisting mostly of medium- to event, is the youngest major episode. With reference to Africa as a high-grade reactivated basement with some infolded supracrustal whole, T. N. Clifford (1975; here quoting from Windley, 1973) rocks. It is proposed that the Eburnian event in Nigeria was such a recognized three Proterozoic events — the Eburnian and Huabian Proterozoic event reworking a portion of the Archean West African (1,850 ± 250 m.y.), the Kibaran (1,100 ± 200 m.y.), and the craton. Damaran-Katangan or Pan-African (550 ± 100 m.y.). Dates re- Before Grant and others' (1972) determination of a

Figure 4. Compilation map of principal geologic features of Iwo area and adjacent regions. 1, Migmatite-gneiss complex (in part reworked Archean); 2, Eburnian metasupracrustal sequence and associated rocks (Ibadan metasupracrustal belt); 3, Kibaran metasupracrustal sequence and associated rocks (Ife metasupracrustal belt); 4, charnockite (Archean?); 5, Pan-African granitic complexes; broken lines indicate major faults; X, Rb/Sr age 2,190 ± 30 m.y.; O, Rb/Sr age 1,250 ± 90 m.y. Compiled from 1:250,000 sheets 59 (Ibadan) and 60 (Iwo) of Geological Survey of Nigeria and Figure 1 of Cooray (1972).

1,150-m.y. Kibaran age for the Ife granite-gneiss, it perhaps transformed to granite and localTy remobilized. Extensive de- seemed most reasonable to relate the main schist belt of the Iwo velopments of the late granitic intrusions are restricted to the west- sheet to the Ibadan schist sequence. The better preservation of, for ern Ikere-Iwo linear complex, although relatively minor dikes and example, the quartzite/quartz-schist sequence of the former com- veins are widespread. The possibility that the early pared with the isolated disjointed quartzitic regions farther west, tonalitic/granodioritic stage might relate to the Kibaran, or even to however, led de Swardt (1953) and Hubbard (1963, unpub. bulle- the Eburnian, granitic activity cannot be overlooked. It would be of tin) to speculate that they might be part of a younger supracrustal great interest to obtain whole-rock age data from these rocks, par- sequence. However, if the new Kibaran granite-gneiss age is correct ticularly from the northwestern tonalite where later events have and if the quartzitic and amphibolitic/pelitic sequences are both had a less-extreme effect. part of the same supracrustal event, the depositional period must The petrogenetic and geochronologic model developed may be be pre- or early Kibaran. Grant's age does not, of course, necessar- summarized by reference to the compilation map of Figure 4, ily preclude the correlation with the pre- or early Eburnian schist of which relates the principal geologic features of the Iwo area to the Ibadan area. those of the areas adjoining to both east and west. Three features The occurrence of dates indicating intense Kibaran activity at Ife immediately stand out in this complex belt of some 22,000 sq km: and their apparent absence less than 50 km to the west at Ibadan (1) the occurrence of two distinct metasupracrustal belts, both in- requires explanation. It must be presumed that the Ife oligoclase- folded in the basement complex but relating to distinct Proterozoic granite-gneiss sequence is a product of a granitic event distinct events; (2) the linearity, on submeridional trends, of the massive from that which produced the Ibadan granite-gneiss. It is, however, plutonic rock complexes; and (3) the marked spatial link of the difficult to define (based on present knowledge of the geology) a Paleozoic granite plutonism with the massive charnockite de- possible limit of Kibaran overprint on the Eburnian. The lithologies velopments. and structural features of the Ife schist belt show marked The extensively reworked, older, metasupracrustal sequence, similarities to those described for Archean greenstone belts centered on Ibadan is most clearly distinguished in the quartzite (Anhaeusser and others, 1969). The lithological comparison is and quartz-schist remnants. The geologic and geochronologic evi- perhaps most apposite in the amphibolite/pelite areas where the dence from the associated granitic rocks demands an early Ebur- only anomaly is the comparatively high metamorphic grade. Hub- nian or older age for the supracrustal activity — and thus an even bard (1963, unpub. bulletin) suggested that these rocks might rep- older age for the basement and provenance sequence. It may be resent a reworked greenstone belt, drawing particular attention to reasonably argued, therefore, that the Archean West African craton the common economic mineral associations (gold and nickeliferous extends this far east, and that its rocks are now represented, in pyrrhotite). The comparability of the northern Nigerian schist belts strongly reworked form, in the gneiss-migmatite basement to the with greenstone belts was noted by Wright and McCurry (1970). A supracrustal belts. feature highlighted by Anhaeusser's synthesis is the common screen The eastern belt, in the Ile-Ife region, is better preserved and can of low K/Na ratio, diapiric granite with associated chemically simi- be seen to have contained important basic volcanic and pure lar pegmatite, flanking the metavolcano-sedimentary synclinal arenaceous sedimentary facies. Both proved effective resistors to keels. These are interpreted as developing in gravitative response to later extensive metasomatic transformation and, in addition, af- the downwarping of the denser supracrustal sequences. The forded protection to more susceptible included pelitic horizons. oligoclase-granite-gneiss and pegmatite stage rocks of the Iwo re- The associated oligoclase-granitic rocks are believed to have de- gion are similar to these diapiric rocks of the greenstone belts in veloped subcontemporaneously with the supracrustal belts, and the chemistry and location, and the possibility that the Iwo schist Kibaran age obtained from the former indicates a petrogenetic belt-granitic gneiss association may represent a Kibaran green- episode distinct from and younger than that at Ibadan. The absence stone belt type association, reworked in the Pan-African event, of any evidence of Kibaran ages from the Ibadan rocks, despite must be considered. Pettijohn (1970), from observations of the late their relative proximity, points to spatially restricted recycling, and Paleozoic and Mesozoic associations in the Sierras of California, greenstone belt/granite type activity related to crustal suturing (the showed that such a mechanism of diapiric granite intrusion in re- Ife fault zone) would satisfy the imposed conditions. sponse to downwarping of denser supracrustal belts with a high Consideration of the massive plutonic associations east of the Ife basic volcanic component is not confined to the Archean. This supracrustal belt strongly reinforces the impression gained from the model for the development of the Ife rocks may be of significance in Iwo region that an important link exists between the charnockitic explaining the apparent localization of Kibaran ages. At the level of intrusive rocks and the massive plutonic rocks of the Pan-African the crust now exposed, extensive reworking, with consequent reset- (Older Granite) Paleozoic event. The only radiometric age from ting of the isotopic clocks, may perhaps have been confined to the these charnockitic rocks known to me is a K/Ar mineral age of 490 marginal zones responding to the downwarping of the supracrustal ± 16 m.y. from biotite in the charnockitic mass northwest of Iwo belts. (Snelling, 1965), that is, an age commensurate with the late Pan- The linear circumscribed granitic complexes of the Iwo sheet are African. Geologic considerations, however, deny this as being an probably in large part attributable to the late Pan-African (Older original emplacement age for the charnockitic rocks. The relation- Granite Cycle) event. It seems clear, however, that a number of de- ships are poorly displayed, but the available evidence, particularly velopmental stages were involved: an early tonalitic/granodioritic that from the region northwest of Iwo, strongly favors the existence stage, an intense K-Si metasomatic stage, and finally, a late, cross- of the charnockitic intrusive rocks within the complex that fur- cutting, magmatic granite stage with associated pegmatite and ap- nished the basement for the Ibadan supracrustal belt, and an Ar- lite. The early tonalitic/granodioritic rocks are in part represented chean age is thus indicated. Much of the coarse, porphyroblastic by discrete intrusive rocks and in part by transformations of older granodiorite and granite of both the Iwo and Akure areas can be rocks, including charnockitic intrusions. The emplacement of the shown to have formed by the transformation of pre-existing mas- syenitic mass north of Iwo town may belong to this stage. The sive rocks. In many cases the latter were demonstrably the char- submeridional linear form of the complexes was apparently nockitic rocks. This is particularly well displayed in the Akure- stabilized at this time. The subsequent metasomatism was very Ado-Ekiti region where the transformations are less complete. This widespread and affected almost all the rocks of the region. Its ef- may provide an explanation of the spatial association found, with fects, however, were particularly concentrated along a broad, the older massive charnockitic complexes providing the loci for the southwest-trending zone, that is, oblique to the circumscribed later, in situ, Pan-African, massive granitic developments. complex linearity. The early plutonic rocks falling in this zone were In the Iwo area, however, the complete picture of the Cambrian

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plutonic episode can be seen to be much more complex. Apart from REFERENCES CITED the widespread metasomatism that is a feature of the whole region, there also occur within the large Iwo-Ikire complex both early to- Anhaeusser, C. R., Mason, R., Viljoen, M. J., and Viljoen, R. P., 1969, A nalitic and syenitic diapirs and late magmatic, granitic, pegmatitic, reappraisal of some aspects of Precambrian shield geology: Geol. Soc. and aplitic intrusions. These phases effectively link the centers of America Bull., v. 80, p. 2175-2220. charnockite/porphyroblastic granodiorite development into the cir- Bridgewater, D., Watson, J., and Windley, B. F., 1973, The Archaean craton of the North Atlantic region: Royal Soc. London Philos. Trans., cumscribed complexes described, and they point to the activity in ser. A, v. 273, p. 493-512. this region of some further, markedly submeridional linear, localiz- Clifford, T. N., 1975, The evolution of the crust of Africa, in Conference on ing influence (thermal or tectonic) during the development of the the Precambrian, Morocco, 1970 (in press). Pan-African cycle of plutonism. Cooray, P. G., 1972, Note on the charnockites of the Akure-Ado Ekiti area, The basement complex of the Iwo region is thus interpreted as western Nigeria, in Dessauvagie, T.F. J., and Whiteman, A. J., eds., Af- comprising Archean crustal remnants, reworked in a Proterozoic rican geology: Univ. Ibadan, p. 45-54. mobile belt of Eburnian age, a Kibaran greenstone belt/granite type de Swardt, A.M.J., 1953, The geology of the country around Ilesha: association, and circumscribed granitic complexes largely related to Nigeria Geol. Survey Bull., no. 23, 54 p. Grant, N. K., 1969, The late Precambrian to early Palaeozoic Pan-African the plutonic activity of the Pan-African (Older Granite Cycle). The orogeny in Ghana, Togo, Dahomey, and Nigeria: Geol. Soc. America widespread transformations of older rocks in the Pan-African, par- Bull., v. 80, p. 45-56. ticularly in the zones of intense metasomatism, have, in large part, 1970, Geochronology of Precambrian basement rocks from Ibadan, controlled the present nature of the bulk of the rocks, but it is still southwestern Nigeria: Earth and Planetary Sci. Letters, v. 10, p. possible to recognize and interpret older sequences and events. 26-38. Attempts to extrapolate the suggested Iwo pattern to the wider 1971, A compilation of radiometric ages from Nigeria: Nigerian Jour. context of the whole Nigerian Basement Complex and, even more Mining and Geology, v. 6, p. 37-54. so, to the West African Platform must be tentative. Grant (1969) Grant, N. K., Rex, D. C., Burke, K. C., and Freeth, S. J., 1969, The geologi- showed that a complete sedimentary and otogenic cycle was as- cal sequence and geochronology of old basement rocks from Ibadan, Nigeria: [France] Clermont-Ferrand Univ. Annales Fac. Sci. 41, sociated with the Pan-African event in Ghana, Togo, and Dahomey Géologie et Minérologie, fasc. 19, p. 84. (the Voltaian, Buem, and Togo Formations), but in Nigeria this Grant, N. K.,. Hickman, M., Burkholder, F. R., and Powell, J. L., 1972, event is seen only as crustal reworking and granitic plutonism. The Kibaran metamorphic belt in Pan-African domain of West Africa?: early suggestion of Grant (1969) that the Nigerian schist belts may Nature (Phys. Sci.) v. 238, p. 90-91. be infolded remnants of a more extensive Pan-African supracrustal Hubbard, F. H., 1965, Antiperthite and mantled feldspar texture in cover seems negated by his later age data. The extent of the Kiba- charnockite (enderbite) from S.W. Nigeria: Am. Mineralogist, v. 50, ran activity is not known, but if all the well-preserved Nigerian p. 2040-2051. schist belts are found to have similar ages, interesting inferences 1966, Myremekite in charnockite from south-west Nigeria: Am. may be drawn. These schist belts lie within a rather well-defined, Mineralogist, v. 51, p. 762-773. north-northeast—trending broad zone that parallels the Pan-African 1968, The association charnockite—Older Granite in south-west geosynclinal belt to the west (Fig. 3). Present knowledge indicates Nigeria: Nigerian Jour. Mining and Geology, v. 3, p. 25-32. an association with major faulting of the same trend. It may be Hurley, P. M., 1966, Continental drift investigations: Massachusetts Inst. Technology, 14th Ann. Rept. (for 1966). that, subsequent to crustal stabilization after the Eburnian mobile Jones, H. A., and Hockey, R. D., 1964, The geology of part of southwest- belt activity, a north-northeast—trending tensional zone developed ern Nigeria: Nigeria Geol. Survey Bull., no. 31, 97 p. in this part of the West African Platform with local rifting giving King, B. C., and Swardt, A.M.J., de, 1949, The geology of the Osi area, rise to activity similar to the Archean greenstone belt/granite type Ilorin Province: Nigeria Geol. Survey Bull., no. 20, 92 p. association to produce the schist belts. This tensional crustal re- Oyawoye, M. O., 1972, The basement complex of Nigeria, in Dessauvagie, gime may have culminated in localization of rupture in the site of T.F.J., and Whiteman, A. J., eds., African geology: Univ. Ibadan, p. the Voltaian-Buem-Togo depositional zone. The reworking and 66-102. plutonism of the Nigerian area in the Pan-African would then be Pettijohn, F. J., 1970, The Canadian Shield — A status report, 1970: attributable to crustal activity associated with the orogenic stage in Canada Geol. Survey Paper 70-40, p. 239-265. Salop, L. I., and Scheinmann, Yu. M., 1969, Tectonic history and structures the adjacent geosynclinal region to the west. of platforms and shields: Tectonophysics, v. 7, p. 565-597. The general model which emerges for the development of the Snelling, N. J., 1964, Overseas Geological Survey annual report for 1963: West African Platform is one characterized by repeated Proterozoic London, Directorate Overseas Geol. Surveys, p. 113. reworkings of, and supracrustal incorporations in, a widespread 1965, Overseas Geological Survey annual report for 1964: London, and already complex Archean craton. Such a model has many Directorate Overseas Geol. Surveys, p. 101. points of similarity with that deriving from the recent considera- Tougarinov, A. I., Knorre, K. G., Shanin, L. L., and Prokofieva, L. N., 1968, The geochronology of some Precambrian rocks of southern tions of the much better known North Atlantic region (Wynne- West Africa: Canadian Jour. Earth Sci., v. 5, p. 639-642. Edwards and Hasan, 1970; Bridgewater and others, 1973; Wind- Truswell, J. F., and Cope, R. N., 1963, The geology of parts of Niger and ley, 1973). In West Africa, the apparent general decrease in age of Zaria Provinces, northern Nigeria: Nigeria Geol. Survey Bull., no. 29, formations eastward from the cratonic Archean zone of Sierra 52 p. Leone, suggested by any general survey of radiometric age distribu- Windley, B. F., 1973, Crustal development in the Precambrian: Royal Soc. tion (Tougarinov and others, 1968), indicates, perhaps, the vector London, Philos. Trans., ser. A, v. 273, p. 321-341. of platform stabilization of the original large West African Archean Windley, B. F., and Bridgewater, D., 1971, The evolution of Archaean low- craton rather than any process of continental accretion on a smaller and high-grade terrains: Geol. Soc. Australia Spec. Pub., v. 3, p. nucleus. 33-45. Wright, J. B., and McCurry, P., 1970, A reappraisal of some aspects of Pre- cambrian shield geology: Discussion: Geol. Soc. America Bull., v. 81, p. 3491-3492. ACKNOWLEDGMENTS Wynne-Edwards, H. R., and Hasan, Z-U., 1970, Intersecting orogenic belts across the North Atlantic: Am. Jour. Sci., v. 268, p. 289-308. My thanks are due to Norman K. Grant. Without his repeated promptings, it is doubtful if my Nigerian file would ever have been MANUSCRIPT RECEIVED BY THE SOCIETY JANUARY 16, 1974 reopened. REVISED MANUSCRIPT RECEIVED MAY 22, 1974

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