Isotopic ages from the Oban Massif and southeast Lokoja: implications for the evolution of the Basement complex of Nigeria

Jouraal of African Earth Scieaces, Vol. 12, No. 3, pp.489-503, 1991.

0899-5362/91 $3.00 + 0.00 © 1991 Pergamon Press plc

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Isotopic ages from the Oban Massif and southeast Lokoja: implications for the evolution of the Basement Complex of Nigeria B. N. E ~ * ,

M. CAEN-VAc~n~* and A. C. ~ c / 3 c a ~ * * *

*Deparanent of Geology, University of Calab~, Calabar, Nigeria **Laboratoire de Geochronologie,.Univ~it~ de Clermont IT, 5 rue Kessler, Clermont-Ferrand, France ***Deparunent of Geology, University of Nigeria, Nsukka, Nigeria (Accepted for publication 20th April, 1990) Abstract-The evolution of the basement rocks of the Obanmassifand southeastern Lokoja in southeastern and central Nigeria respectively involved two orogenies: Kibaran and Pan-African. The main phase of the Kibaran deformation and metamorphimi in these two m~as oc,ciared at the same lime ca. 1313 + 37 M.a. to 1315 + 72 M.a. ago. In the Oban massif, this Kibaran event was aeompanied by plutonic activity which emplaced rocks of chamockitic affinity about 1289 + 153 M.a. ago. These ages thus confirm the active participation of the Kibasan orogeny in the evolution of the Basement Complex as well as the occurrence of Kibaran chamockites in Nigeria. The Kibaran rocks were subjected to Pan:African tectoganesis, the main phase of which occurred at the same time in both areas at ca. 676 4- 16 M.a. to 687:1:13 M.a. ago. A post-tectonic Pan-African event which was marked by diaphthoresis occurred in the Oban massif 527 + 16 M.a. ago and is believed to be coeval with a 546:1:24 M.a. event which reworked the chm'nockite, in the area.

INTRODUCTION

B R I E F D E S C R I P T I O N S OF ANALYZED R O C K S

The Oban massif a n d s o u t h e a s t LokoJa belong to the Precambrian b a s e m e n t complex of Nigeria (Figs. 1 a n d 2). Compared with southwest and northwest Nigeria, little is k n o w n about the geolog), of these two areas. However, R a e b u r n (1927) did a r e c o n n a i s s a n c e survey of the tinstone in the Calabar district w h e r e a s R a h m a n (1981) a n d R a h m a n et al. (1981) studied the geology of parts of the Oban massif. More recently Ekwuerne (1985) reported on the petrology, geochemistry and Rb-Sr geochronology of m e t a m o r p h o s e d rocks in the Uwet area w h e r e a s E k w u e m e and Onyeagocha (1985, 1986) worked on the metamorphic isograds and geochemistry of Uwet rocks. E k w u e m e (1987) studied structural orientations and deformational episodes of Uwet area: The regional m e t a m o r p h i s m of southeast LokoJ a was studied by E k w u e m e (1983) while E k w u e m e and Onyeagocha (1983) worked on the plagioclases in the rocks of the area. In t h i s c o n t r i b u t i o n we report r a d i o m e t r i c ages obtained on some rocks of the Oban massif a n d of s o u t h e a s t LokoJa a n d a s s e s s t h e i r implications for the evolution of the b a s e m e n t complex of Nigeria.

Oban M a s s i f The Oban b a s e m e n t m a s s i f h a s hitherto been referred to as undifferentiated b a s e m e n t (see Geological m a p of Nigeria 1974). We have, in this study, p r o d u c e d the first comprehensive geological m a p of the Oban m a s s i f in which the rocks have b e e n differentiated into phyUites, s c h i s t s , g n e i s s e s a n d a m p h i b o l i t e s (Fig. 1). Intrusives into these are c h a m o c k i t e s , dolerites, granites, granodiorite, diorite, syenite, adamellite a n d pegmatite. Only a brief description of those schists, gneisses, amphibolites a n d c h a m o c k i t e s which have been isotopically analyzed is given here. Schists Schists are restricted to the western half of the

Oban region. They are grouped on the basis of texture and mineralogy as follows (see Fig. 1). 1) Quartz-mica schist 2) Garnet-mica schist 3) Gamet-sflllrnanlte schist 4) Kyanite-silllmanite schist All these schists are foliated in the N-S to NE-SW direction t h o u g h foliation t r e n d s of 320 ° were also observed. Dips of the foliation range from 20-90 ° .

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Fig. 1. Geological map of Oban massif, southeastern Nigeria. The q u a r t z - m i c a s c h i s t is fine-gralned w h e r e a s t h e m e d i u m - g r a i n e d varieties also occur. The gneisses g a r n e t - m i c a schist is m e d i u m - g r a l n e d . Coarse- are g r o u p e d as follows (Fig. i): grained s c h i s t s i n c l u d e g a m e t - s f l l i m a n i t e a n d 1) bioUte-hornblende gneiss; kyanite-sillimanite s c h i s t s a n d are mlgmatitic a n d 2) kyanite gneiss: c o n t a i n index m i n e r a l s (Table 1) which indicate 3) granite gneiss, t h a t a high-grade regional m e t a m o r p h i s m of t h e 4) mlgmaUtic gneiss. B a r r o v i a n - t y p e o c c u r r e d in t h e O b a n Massif Gneisses are the d o m i n a n t lithologic u n i t s in (Ekwueme a n d Onyeagocha, 1985}. Whole rock the e a s t e r n half of t h e O b a n Massif. FoliaUon Rb-Sr i s o c h r o n w a s obtained only for the kyanite- a n d lineation are defined by parallel a n d linear sfllimanite schist w h i c h crops out at t h e Kwa Falls a r r a n g e m e n t of p h y l l o s i l i c a t e s 0 q u a r t z a n d (Fig. 1). Single whole rock isotope d a t a have b e e n a m p h i b o l e s (hornblende). T h e b a n d e d variety of obtained, however, for t h e g a r n e t - m i c a a n d garnet- the gneisses p r e d o m i n a t e s in t h e e a s t e r n portion sfllimanite schists. of t h e area (Fig. 1). FoliaUons with dips of 40-60 ° t o w a r d s 90 ° or 2 7 0 - 2 8 0 ° are d o m i n a n t . However, Gneisses this regional foliation t r e n d w a s observed at Calaro The g n e i s s e s in the O b a n Massif are all quartzo- Estate a n d NdebiJJi to be t r a n s e c t e d by a n o t h e r feldspathic. T h e y are generally coarse-grained b u t dipping 50-70 ° t o w a r d s 40-50 ° . The angle b e t w e e n

491

Isotopic ages from the Oban Massif and southeast Lokoja: implications for the evolution of the basement Table I. Modal averages of minerals in the Isotopleally analyzed rocks of the Oban massif and southeast Lokoja, Nigeria.

Quartz Plagioclase K-feldspar Biotite Muscovite Garnet Kyenite Stilimanite Staurolite Cordierite Chlorite Epidote Hornblende Calcite Tourmaline Opaques Sphene Olivine Pygoxene Myrmekite

KSS

SS

GGS

KG

BAPT

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17.00 5.00 1.60 31.80

20.00 5.00

30.00 5.00

7.50 17.50

4.50 24.67

20.00 9.00 2.00

27.00 10.00 5.00

26.80 24.00 9.70 20.00

10.00

4.44

30.00 10.50 14.00 10.00

0.60 7.00

5.00

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2.50

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t h e s e two foliatlons is always 30 °. A third foliation t r e n d i n g ENE-WSW (60-70 °) w a s recognized in migrnaUtic g n e i s s e s at NdebiJJi (Fig. I). Folds in t h e s e gneisses are tight to isoclinal a n d have axes t r e n d i n g O °, 100-140 ° a n d 60-70 °. They have generally steep plunges. B o u d i n a g e a n d pinch a n d swell s t r u c t u r e s are a b u n d a n t at t h e limbs of the folds. O t h e r varieties of folds recognized in t h e rnlgrnatitic gneisses are r e c u m b e n t , c h e v r o n a n d p t y g m a t i c types. Pegmatitic dykes a n d veins dissect t h e s e gneisses. The kyanlte a n d biotiteh o r n b l e n d e g n e i s s e s c o n t a i n h e a l e d fractures. Materials fflllng t h e s e f r a c t u r e s are s c h i s t o s e a n d c o n t a i n retrograde chlorite. In s o m e localities, s u c h as Ik-pai a n d NyaJe, t h e gneisses (granite gneiss, in this case) s h o w evidence of s h e a r a n d granulation. S u c h s h e a r e d rocks u s u a l l y c o n t a i n resistant feldspar and quartz porphyrob l a s t s w r a p p e d by folded a n d k i n k e d phyllosilicates. The g n e i s s e s in t h e O b a n Massif grade into each other. For instance, t h e biotite-hornblende gneiss grades into t h e kyanite g n e i s s w h i c h in t u r n g r a d e s into t h e granite a n d migrnatitic gneissos. Index m i n e r a l s recognized in gneisses Include biotite, garnet, kyanite a n d silllmanite (Table I). These m i n e r a l s indicate t h a t t h e y have b e e n m e t a m o r p h o s e d at t h e amphibolite facies grade. The

HAFT = HomogeneousAmphibolite KSS = Kyanite-SillimaniteSchist CKT = ChamockiticRock

a b u n d a n c e of K - f e l d s p a r (microcline mainly) and migmatites indicates that partial melting o c c u r r e d in t h e area. The average m o d a l c o m p o s i t i o n s of t h e a n a l y z e d g n e i s s e s are p r e s e n t e d in Table 1. A whole-rock Rb-Sr i s o c h r o n w a s obtained for t h e kyanite gneiss at Old Netim (Fig. 1). Single whole-rock isotope data have been obtained for three s a m p l e s of t h e biotite-hornblende a n d migmatitic gneisses. Amiphibolltes Bodies of b a n d e d a n d h o m o g e n e o u s a m p h i bolites are enclosed by t h e g n e i s s e s in t h e O b a n Massif. T h e y o c c u r g e n e r a l l y a s m a s s i v e to scattered boulders. The h o m o g e n e o u s variety is d a r k d u e to t h e p r e p o n d e r a n c e of greenish b r o w n h o r n b l e n d e in t h e rock. T h e b a n d e d variety, which crops o u t in the c h a n n e l s of t h e Ekurl, A c h a n a n d E r o k u t rivers (Fig. I) h a s t h i n (about 0.1 c m thick) felsic b a n d s of quartz a n d feldspar. The a m p h i bolites are m e d i u m to coarse-grained. They s h o w variations in foliation trends. For instance, dips of foliation of the b a n d e d amphibolite, whi'~h crops o u t in t h e E r o k u t River (Fig. I), vary from 25 ° to 60 ° t o w a r d s 40 ° . On t h e o t h e r h a n d , t h e h o m o g e n e o u s amphibolite s h o w s foliation a n d lineation t r e n d s of ENE-WSW (60-70 °) with dips a n d plunges, varying

492

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from 60-80 ° t o w a r d s 240-260 °. Folds in t h e banded amphibolite at E r o k u t River are tight to isocllnal a n d have axes .trending 70°; t h e plunge is n e a r vertical. These isoclinal folds are associated with b o u d m a g e . The a m p h i b o l i t e s are similar mineraloglcally. The d o m i n a n t m i n e r a l s (Table 1) are h o r n b l e n d e (57-64 %) a n d plagioclase of An44 c o m p o s i t i o n (18-25 %). O t h e r m i n e r a l s include biotite (10 %) a n d quartz (8 %) in t h e b a n d e d amphibolite as o p p o s e d to 4 % biotite a n d 5 % quartz in t h e h o m o g e n e o u s variety (Table 1). Garnet (up to 5 %) o c c u r s in t h e b a n d e d a m p h i b o l i t e . In s o m e s a m p l e s t h e y s h o w preferred orientation parallel to t h e foliation defined by elongate quartz a n d h o r n b l e n d e . The plagioclase is a n d e s i n e (An44). Both t h e b a n d e d amphibolite in t h e c h a n n e l s of the E r o k u t River a n d h o m o g e n e o u s amphibolite (at Akor a n d A y i p e k u ) (Fig. I) have b e e n isotopical-

ly analyzed. Charnockites Rocks of c h a m o c k i t i c affinity crop o u t m a i n l y in two areas, namely, O r e m - N t e b a c h o t t a n d OwomA c h a n River (Fig. 1) a n d show a generally N-S o r i e n t a t i o n . T h e y are all c o a r s e - g r a i n e d a n d foliated in d i r e c t i o n s sirnflar to t h o s e of t h e cOuntry rocks (granite a n d migmatitic gneiss) t h a t is N-S to NE-SW (0°-60°). In fresh outcrop the r o c k is b l u i s h - g r e e n b u t / o n w e a t h e r e d e x p o s u r e s it is b r o w n i s h a n d s h o w s a r e s i n u o u s lustre. Minerals, recognized in h a n d s p e c i m e n , are large crystals of feldspars, quartz a n d pyroxenes. In the AkomJlk s t r e a m bed. n e a r Orem. the h o s t rock of t h e charnockitic rocks is a migmatltic gneiss which displays gneissose b a n d i n g a n d r e c u m b e n t folding. I n t h e O r e m area, t h e charnockitic rocks occur as boulders, m o s t of which have b e e n highly weathered. A r o u n d Ntebachott (Fig. I), where t h e largest a m o u n t of charnockitie rocks are exposed, t h e s e rocks (mostly bauchites} occur as massive intrusives w h i c h cover the hills in the viUage. The contact of t h e c h a r n o c k i t e s with t h e h o s t (amphibolites, migmatiUc a n d granitic gneisses) is sharp. These o u t c r o p s are less coarse-grained b u t fresher t h a n c h a m o c k i t i c rocks at Orem. They have a foliation w h i c h is conformable with u n m a p p a b l e pockets of a m p h i b o l i t e s with w h i c h they are closely associated. A small a n d isolated granitic p l u t o n crops out n o r t h w e s t of the c h a m o c k l t i c rocks at Ntebachott (Fig. 1). The relationship between t h e granite a n d charnockiUc rocks in t h e area is n o t very clear. Nevertheless, field occurrence rules o u t the likelihood t h a t these rocks are related in age a n d m o d e of e m p l a c e m e n t . For instance, t h e granite is non-foliated a n d there is no evidence t h a t t h e foliated c h a r n o c k i t e s graded into t h e granite (cf. H u b b a r d , 1968).

T h e a v e r a g e m o d a l c o m p o s i t i o n s of t h e charnockitic rocks (charnockites a n d bauchites} are s h o w n in Table 1. Quartz is c o m m o n a n d s h o w s u n d u l o s e extinction; it is cloudy a n d displays preferred orientation as well as quartzquartz triple Junction. The plagioclase is a n oligoc l a s e - a n d e s i n e of c o m p o s i t i o n An~7.3o. S o m e of t h e plagioclase lamellae are bent, indicating t h a t t h e y have b e e n deformed. K-feldspars in t h e c h a m o c k i t e s are n ~ I n l y m e s o p e r t h i t i c intergrowths consisting of beads, r o d s a n d strings. The Kfeldspar intergrows with quartz giving rise to w i d e s p r e a d m y r m e k i t e s . O r t h o - a n d clinopyroxenes were identified w h i c h in m o s t sections, have a reaction relationship with h o r n b l e n d e . The greenish h o r n b l e n d e s h o w s a n intergrowth relationship with plagioclase a n d r e d d i s h b r o w n biotite. The biotite a p p e a r s to form in s o m e sections from the b r e a k d o w n of h o r n b l e n d e b u t in other sections, biotite a p p e a r s to be a primary mineral in the c h a m o c k i t e . Olivine (fayalite 95.9 %) is a n additional m i n e r a l in t h e b a u c h i t e (cf. Oyawoye, 1961; R a h m a n , 1981) a n d displays a late tectonic fabric a n d occurs as i n c l u s i o n s in Kfeldspar. Field a n d petrographic data do n o t indicate t h a t t h e ~ h a r n o c k i t e s have u n d e r g o n e a n y m e t a m o r p h i s m (either regional or contact) at their p r e s e n t level of e m p l a c e m e n t . There is, however, a b u n d a n t evidence to s h o w t h a t t h e y have u n d e r gone defoliation possibly d u r i n g t h e Pan-African orogeny. This gave rise to the N-S t r e n d i n g foliation, recognized in the charnockitic o u t c r o p s in t h e Oban Massif.

Southeast LokoJa S a m p l e s of the staurolite a n d cordierite-gamet schists from s o u t h e a s t LokoJa (Fig. 2) were isotopically dated. The staurolite-bearing schists are light coloured a n d m e d l u m - g r a l n e d . Dip of t h e foliation is 15 ° t o w a r d s 2 9 0 - 3 0 0 ° . Crystals of staurolite are easily distinguishable a n d t h e y v a r y in length from 1 c m to m o r e t h a n 3 crn. Smal! crystals o f g a m e t p o r p h y r o b l a s t s are recognized in h a n d s p e c i m e n of the staurolite schists. Flakes of muscovite a n d biotite are c o n s p i c u o u s in these scl~ists a n d t h e y define their foliation. The cordierite-gamet schist occupies a narrow b a n d striking approximately N-S in the centre of s o u t h e a s t LokoJa area (Fig. 2). The rock is m e d i u m to coarse-gralned a n d h a s suffered i n t e n s e deformation resulting in folds a n d faults. In m o s t outcrops, t h i n b a n d s of quartz-feldspar a n d mica (mainly biotite} alternate. The dip of t h e foliation varies from 25 ° to 45 ° t o w a r d s 290-310 °, b u t in the s o u t h e r n portion, s o m e foliation dips are in the opposite direction, pointing to a synclinal s t r u c t u r e (Fig. 2).

Isotopic ages from the Oban Massif and southeast Lokoja: implications for the evolution of the basement

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GEOCHRONOLOGY A n a l y t i c a l Method The Rb-Sr d e t e r m i n a t i o n s on whole-rocks of schists, gneisses, a m p h i b o l i t e s a n d c h a m o c k l t e s were carried out at t h e Geochronology Laboratory, University of C l e r m o n t - F e r r a n d , France. E a c h s a m p l e weighing a b o u t 5 kg was c r u s h e d to p a s s t h r o u g h 80 m e s h a n d a q u a r t e r e d part was analyzed. The chemical p r e p a r a t i o n of these s a m p l e s w a s done by E. G a m o t a n d J. Serange of the Geochronology Laboratory, C.N.R.S. et Universit8, C l e r m o n t - F e r r a n d . T h e S~Sr/86Sr r a t i o s were

494

B. N. EKWUEME,M. CAEN-V^c~,a and A. C. ONYEAC~'HA

resulting in a single Rb-Sr whole-rock errorchron w h i c h yields a n age of 1315 _+72 M.a. a n d a n initial 8~Sr/~Sr ratio of 0.70160 + 0.00060. (U) C o r d l e r i t e - g a r n e t sch!_At - Five-data points of the cordierite-garnet schist in s o u t h e a s t LokoJa (Fig. 2) produced a whole-rock isochron of 696 + 19 M.a. with a n initial STSr/mSr ratio of 0.70621 + 0.00021 (Fig. 5). MSWD for this isochron is 1.2. Four-data points of this schist also define a Rb-Sr whole-rock errorchron (Fig. 5) which yields a n age of 1005 _+35 M.a.; initial ratio of 0.70425 + 0.00035 and a n MSWD = 3.5. Since the staurolite a n d cordierite-garnet schists both in s o u t h e a s t Lokoja yield similar Pan-African ages, the nine points which defined these isochron RESULTS ages m a y be grouped together. The result of the Table 2 lists the results of the isotopic analyses. •grouping is a n age of 687 + 13 M.a. a n d a n initial Whole-rock Rb-Sr i s o c h r o n s a n d e r r o r c h r o n s ~Sr/~"Sr ratio of 0.70639 + 0.00015 with an MSWD (Figs. 3, 4, 5, 6 a n d 7) have b e e n obtained for the of 1. analyzed rocks in the Oban Massif and south-east Gneisses Lokoja. K y a n i t e g n e i s s - A six-point whole-rock Rb-Sr isochron (Fig. 3) was obtained for the kyanite Schists (a) Oban Massif: K y a n i t e - s U l i m a n i t e s c h i s t - gneiss at Old Netim in the Oban Massif (Fig. 1). This schist which crops out at the Kwa falls area of The isochron yields a n age of 676 + 26 M.a. and the Oban Massif (Fig. 1) yielded a 5 - point whole- an initial 87Sr/SaSr ratio of 0..70682 + 0.00039. rock Rb-Sr isochron (Fig. 3). The age of the iso- The MSWD is 0.53 which indicates a good alignc h r o n is 527 + 16 M.a. The initial S~Sr/S6Sr ratio is m e n t of the points and consequently a good Rb0.70883 _+ 0.00018. A m e a n square of weighted Sr homogenization of the kyanite gneiss. A sample deviates (MSWD) of 0.7 for this isochron indicates of biotite-hornblende gneiss (B1) plots on the a good alignment of the points. However. sample isochron but the fracture-filling materials (OB7 KW3 of this schist (Table 2) h a s a very high value and A7) in these gneisses plot below the isochron. of 87Rb/~Sr (23.7) which m a k e s it different from OB7 yields a n age of 572 M.a. using a n initial 87Sr/ other samples of the kyanite-siUimanite schist. 8eSr ratio of 0.712. A single sample of migmatitic KW3 is however, the only sample of this schist gneiss (AW 12) at Mbarakpa gave a n a p p a r e n t age without retrograde chlorite in its modal com- of 922 + 20 M.a. with a n initial STSr/~5r ratio of position. An age of 672 + 24 M.a. was calculated for "0.70455. this sample using the classical 8VSr/~SSr ratio of 0.712 w h e r e a s a n age of 680 M.a. was obtained A m p h i b o l i t e s using the initial 87Sr/~SSr ratio of 0.70883. (a) B a n d e d a m p h i b o U t e - The b a n d e d amphi(b) S o u t h e a s t Lokoja: (i) S t a u r o l i t e s c h i s t - bolite along the c h a n n e l s of the Erokut River This schist which occurs in s o u t h e a s t LokoJa (Fig. 1) in the Oban Massif p r o d u c e d a five-point (Fig. 2) yields t h r e e i s o c h r o n s (Fig. 4). Four Rb-Sr isochron (Fig. 3) of 784 + 31 M.a. a n d an samples of the staurolite schists (Go: 1, 2, 17, 24) initial 87Sr/S6Sr ratio of 0.70586 + 0.00033. The yield a Rb-Sr whole-rock isochron (MSWD s 0.4) MSWD is 1.46 which indicates a poor alignment of indicating a n age of 679 _+ 18 M.a. and an initial the points and therefore a n incomplete Rb-Sr 87Sr/S6Sr ratio of 0.7066 + 0.0002. A s e c o n d homogenization of these samples by the sub(4 - point) isochron which yields a n age of 1194 _+ sequent t h e r m a l event. 76 M.a. was defined by samples (Go: 1, 3, 6, 29) of (b) H o m o g e n e o u s a m p h i b o l i t e - Five of the six the staurolite schists. This isochron has a n initial samples of a h o m o g e n e o u s amphibolite a r o u n d 8~Sr/~SSr ratio 0f0.70203 + 0.00064 and a MSWD Ayipeku a n d Akor (Fig. 1) in the Oban Massif fit a of 1.1. A third isochron defined by samples (Go: 4, regression line (Fig. 6) corresponding to a n age of 5, 12, 24) of the staurolite schist gave a n age of 1313 + 37 M.a. (MSWD = 1.4) a n d a n initial 87Sr/ 1304 + 113 M.a. a n d a n initial S~Sr/~Sr ratio of S~Sr ratio of 0.70319 + 0.00009. 0.70238 + 0.00091. The MSWD is 0.04. Both the b a n d e d and h o m o g e n e o u s amphibolites The similarity in the initial 87Sr/~SSr ratios and have initial STSr/86Sr ratios which lie below the the large errors for the ages of the second and third strontium development line (0.708 + 0.002 or isochrons permit the grouping of these seven points 0.003) of Faure a n d Hurley (1963) for the source

normalized to a value of 0.1194. The Rb a n d Sr isotopic a n a l y s e s were m a d e using a n AEI~ MS2S a n d a V.G.-mass 54E spectrometer respectively. The analytical uncertainties are based on s t a n d a r d deviations of 2 % a n d 0 . 0 1 % for replicate 87Rb/~Sr and ~Sr/S6Sr determinations respectively. Uncertainties quoted for the ages and initial 87Sr/~Sr ratios are at the 2a level. Ages have b e e n calculated using a c o m p u t e r a n d the 8~Rb c o n s t a n t of ~ = 1.42 x I0 "H yr"~ (Steiger and Jager, 1977). Regression analyses for alignments of data points have b e e n done following the programme of Wflliamson (1968).

I s o t o p i c a g e s f r o m the O b a n M a s s i f a n d s o u t h e a s t L o k o j a : i m p l i c a t i o n s f o r the e v o l u t i o n o f the b a s e m e n t T a b l e 2. R b - S r I s o t o p i c d a t a o f w h o l e - r o c k d e t e r m i n a t i o n s o n s o m e r o c k o f t h e O b a n m a s s i f a n d s o u t h e a s t LokoJa, Nigeria. I/ OBAN MASSIF Sample n °

Analysis n °

Rb content ppm

a) Kyenite-sillimanite schist: KW1 R10563 208 KW5 R10564 204 KW6 R10565 138 KW2 R10566 140 KW3 R10567 106 KW4 R10568 75 5-point - isochron (KW1, KW5, KW6, KW2, KW4) T = 527 ± 16 M.a. ~Sr/~Sr = 0.70883 ± 0.00018 MSWD = 0.7

Sr content ppm

t'~Rb~Sr

~Sr/~Sr normalised

613 788 170 860 13 155

0.9842 ± 0.0243 0.7491± 0.0181 2.3610 ± 0.0676 0.4714 ± 0.0137 23.70 ± 0 . 8 3 1.4193± 0.0585

0.71601 ± 0.00003 0.71455 ± 0.00005 0.72684 ± 0.00006 0.71238 ± 0.00004 0.93925 ± 0.00030 0.71940 ± 0.00013

b) Gamet-sillimanite schist: AB1 R10572

120

249

1.3916± 0.0446

0.71785

c) Garnet-mica schist: W2 R10573

197

76

7.5975 ± 0.0004

0.79119

d) Gametiferous phyltite: Cal5 R10574

75

322

0.6747 ± 0.0317

0.71365

765 670 678 334 341 294 299 356

0.5917 ± 0.0186 0.7496 ± 0.0211 0.5651± 0.0165 1.34154- 0.0413 1.3807+ 0.0392 1.5032± 0.0424 1.5306± 0.0425 1.3986± 0.0398

0.71255 ± 0.71405 ± 0.71391± 0.71910 ± 0.72045 ± 0.71973 ± 0.72168 ± 0.72011±

82 71 438

7.794±0.3711 5.2310:1:0.03855 1.2336± 0.0324

0.75464 0.71844

360 256 314 338 298 592 75

0.5021 ± 0.0272 I.I158 ± 0.0433 1.24734- 0.0363 1.0611± 0.0342 1.077± 0.0379 0.4239 + 0.0190 8.8593± 0.2073

0.71138 0.71876 0.72003 0.71396 0.71640 0.71072 0.82688

e) Kyanite gneiss: El RI0548 156 E2 R10549 173 E3 R10550 132 A4 R10551 155 A5 R10552 162 A7 R10553 153 A8 R19554 157 A9 R10555 172 6-point - isochron (El, E2, A4, A5, A8. A9) T = 676 ± 26 M.a. ~Sr/U'Sr, = 0.70682 _+0.00039 MSWD = 0.53 f) Hornblende-biotite gneiss: D6 R10569 OB 7 R10570 B1 R10571 g) Banded amphibolite: AW 1 R10556 AW "3 R10557 AW 4 R10558 AW 5 R10559 AW 6 RI0560 AW 7 R10561 AW 12 R10562 5-point isochron (AW l, AW 3, AW T = 784 ± 31 M.a. ~Srl~Sr = 0.70586 ± 0.(10033 MSWD = 1.46

218 128 187

62 99 135 124 104 87 227 4, AW 6, AW 7)

h) Homogeneous amphibolites: /~M 1 I1011 AM 3 11012 NN 1 11013 NN2 11014 NN3 11015 N'N4 11016 5-point isoehron (AM 3, NN 1, 2, 3, 4 ) T=1313±37M.a. Io = 0.70319 ± 0.00009 MSWD = 1.4

2 3 13 4 6 11

I 19 173 123 105 210 90

0.00024 0.00006 0.00007 0.OOO48 0.00004 0.00002 0.00021 0.00003

0.80417

0.0527 0.0501 0.3012 O.I 128 0.0729 0.3121

:I:0.00008

± ± ± ± ± ±

0.00003 0.00008 0.00002 0.00004 0.00003 0.00056

0.70591 0.70413 0.70893 0.70547 0.70445 0.70894

495

B. N. Egwb'E~, M. CAE~-VACt~rI~and A. C. OCCF~C_~CHA

496

Table 2{Cont.}. Rb-Sr Isotoplc data of whole-rock determinations on some rock of the Oban masslf and southeast Lokoja, Nigeria.

I/ O B A N M A S S I F Sample n °

Analysis n °

Rb oontent ppm

Sr content pprn

i) Chamockites: OR 1 10982 100 R 1 10983 92 OR lb 10984 73 OR 2 10985 88 OR 2a 10986 87 OR 2b 10987 92 OR 4 10988 77 OR 5 10989 69 OR 6 10990 72 OR 6a 10991 75 OR 6c 10992 77 (1) 5-point ig~c.hron (OR Ib, 2b, 4, 5, 6¢) T = 1289 ± 153 M.a. I = 0.70138 ± 0.00143 MSWD = 0.9 (2) 6-point isochron (OR 1, OR la, OR lb, OR 2, OR 6, OR 6a) T = 546 + 24M.a. I = 0.70743 ± 0.00026 MSWD = 2. 1 H/LOKOJA AREA a) Staurotite schists: Go I II001 53 Go 2 11002 91 Go 3 11003 65 GO 4 11004 50 Go 5 11005 48 Go 6 11006 64 Go 12 11007 57 GO 17 11008 76 Go 24 11009 47 GO 29 11010 56 4-points isochron (Go 1, 2, 17, 24) T = 679 + 18 M.a.: I = 0.7066 + 0.0OY2: MSWD = 0.4 4-points isochron (Go 1, 3, 6, 29) T = 1194 + 7 6 M.a.: Io = 0.70203 + 0.00064: MSWD = 1.I 3-point isochron (Go 4, 12, 24) T = 1304 + 113 M.a.: Io = 0.70238 ± 0.00091: MSWD = 0.04 7-poim errorchron (Go 1, 3, 4, 6, 12, 24, 29) T = 1315 ± 7 2 M.a.: I, = 0.70160 + 0.00060: MSWD = 10.7 b) Conlieritc-gamet schists FIN 1 10993 EIN 2 10994 EIN 3 10995 EIN 4 10996 EIN 6 10997 F.IN 7 10998 EIN 8 10999 F2N9 11000 5-pointisochron(EIN 2, 3, 4, 8, 9) T = 696 ± 19 M.a. I = 0.70621 ± 0.00021 I~ISWD = 1.2 4-point isochron(EIN 1, 6, 7, 9) T = 1005 ± 35 M.a. Io = 0.70425 ± 0.00035 MSWD = 3.5

90 89 76 57 46 46 45 61

SIRb~Sr

S~Sr~Sr nonnalised

407 411 378 387 375 367 344 304 199 192 341

0.7097 0.6501 0.5609 0.6559 0.6703 0.7231 0.6512 0.6566 1.0444 1.1348 0.6587

0.71288 0.71266 0.71167 0.71265 0.71325 0.71 447 0.71350 0.71368 0.71518 0.71650 0.71371

251 167 245 263 266 399 242 180 290 205

0.6106 1.5704 0.7664 0.5480 0 5227 0.4670 0.6808 1.2193 0.4741 0.5673

0.71240 0.72185 0.71529 0.71259 0.71298 0.71017 0.71514 0.71841 0.71126 0.71150

181 156 342 128 198 204 202 342

1.4428 1.6391 0.6453 1.2944 0.6676 0.6491 0.6475 0.5169

0.72467 0.72258 0.71236 0.71901 0.71382 0.71403 0.71278 0.71143

Isotopic ages from the Oban Massif and southeast Lokoja: implications for the evolution of the basement

497

.7'110 Kw5

|

+l

•TlO~ J

dl

k

•tlO

~

Aw3~

l

13 j~l;~Zllwl l

OAd

El P.,AL8

'lk I(l,ontOo lmoloe • K~,qm~- d l l l N ~

kwl

~m ~+

•

+Jl,

0

I

I-O

H

t

1"6

J

IlooOlenNI Mime

,.Jl

I

I-0

11

|

1-0

07 M / ~ l l r Fig. 3. Rb-Sr whole-rock isochrons for the kyanite-sillimanite schist, kyanite gneiss and banded amphibolite of the Oban massif, S.E. Nigeira.

t

tl +~ S r l O l

$r

72

12

~4 0

~OG6

07Rk/l~iJr "tO

Fig. 4. Rb-Sr whole-rock isochrons for the staurolite schist of southeast Lokoja, central Nigeria. ..

region o f b a s a l t s a n d s u g g e s t t h a t their p r e c u r s o r s were magrnatic derivatives of p r i m a r y basaltic m a g m a or m a y have formed directly by partial melting of the mantle.

Chmmockltic Rocks Five d a t a - p o i n t s of rocks of c h a m o c k l t i c affinity from Orem a n d Ntebachott (Fig. 1) of t h e O b a n Massif define a Rb-Sr i s o c h r o n of 1289 + 153 M.a. (Fig. 7) with a n initial STSr/~Sr ratio of 0.70138 +

498

B.N. EKWX~m,M, CAEN-VACI'~I-II~and A. C. ~ c ,

oc~

87 S t / 86 Sr 72

Tq

7062 7042

I

"7C

I

01 R k / 0 0

Jr

|

t

Fig. 5. Rb-Sr whole-rock isochron and errorchron for the cordierite-garnet schist of southeast LokoJa, Central Nigeria.

87 S r / 8 6

Sr

0.00143 a n d a n MSWD of 0.9. The large error on this age is d u e to the similarity in SVRb/~Sr ratios of t h e analyzed s a m p l e s a n d possibly also b e c a u s e only five points were considered. The other six s a m p l e s of t h e c h a m o c k i t i c rocks fit a regression line (Fig. 7) c o r r e s p o n d i n g to a n age of 546 + 24 M.a. (MSWD = 2. I) a n d a n initial 8VSr/~Sr ratio of 0.70743 + 0.00026.

,=~

710 NN~N 4

DISCUSSION

. y .705

.70319

7000

AM3

t .25

87 R b / l l 6 | r

I .SO

Fig. 6. Rb-Sr whole-rock isochron for the homogeneous amphibollte of the Oban massif, S.E. Nigeria.

Previous geochronological investigations indicated t h a t four t h e r m o t e c t o n i c events affected different p a r t s of t h e b a s e m e n t complex of Nigeria (Odeyemi, 1982): (I}t h e Liberian (2700 + 200 M.a.) recorded by Grant (1970) a n d Oversby (1975) in b a n d e d gneisses from Ibadan; (II) t h e E b u m e a n (2000 + 200 M.a.) d o c u m e m e d in granite gneisses from I b a d a n (Grant, 1970) a n d 1904 + 18 M.a. for the a u g e n gneisses from Igbeti area ( R a h a m a n et aL, 1983); (III) the Kibaran (1100 + 200 M.a.) recorded in granite g n e i s s e s from Ife (Grant et aL, 1972) a n d phyllites in Maru belts (Ogezi, 1977; Fitch es eta/., 1985) a n d (IV) the Pan-African (600 _+ 150 M.a. ) d o c u m e n t e d

Isotopic ages from the Oban Massif and southeast Lokoja: implications for the evolution of the basement

17 Sr/86

499

St

~o

.715 21i

.71(

70 74 3

• 705

70138

70q

!2S

AS.•

.? I

ll? R I D / 0 8 lie

Fig. 7. Rb-Sr whole-rock isochrons for the charnockitic rocks of the Oban massif, S.E. Nigeria. by J a c o b s o n et a t (1963), Grant (1970), Harper have s u g g e s t e d t h a t part of t h e m i g m a t i t e - g n e i s s et al. (1973), V a n B r e e m e n et al. (1977), complex is Liberian. The existence of Kibaran R a h a m a n et al. (1983), Umeji a n d Caen- rocks in Nigeria is still d i s p u t e d (cf. Odeyemi, Vachette (1984) a n d E k w u e m e (1985). 1982). The Kibaran event in West Africa h a s b e e n As p o i n t e d o u t by Fitches et aL (1985}, one of confirmed for the deformation in Hoggar a n d t h e the m a j o r p r o b l e m s of Nigerian geology c o n c e r n s gneisses in e a s t e r n Mall w h i c h gave ages of 910establishing t h e actual a n d relative ages of the 1050 M.a. (Odeyemi, 1982). Snelling(1964), Grant b a s e m e n t rocks a n d in particular the geotectonic et a t (1972), Egbuniwe a n d Fitches (1977) a n d significance of the schist belts. These p r o b l e m s Ogezi (1977) have obtained radiometric ages which s t e m from lack of systematic a n d s u p p o r t i n g radio- indicate t h a t the Nigerian b a s e m e n t m a y have metric data. Until recently ( R a h m a n et al., 1981; p a r t i c i p a t e d in t h e 1 3 0 0 - 9 0 0 M.a. K i b a r a n Ekwueme, 1985; E k w u e m e a n d Onyeagocha, 1985, orogeny. D e s p i t e this, s o m e i n v e s t i g a t o r s of 1986) it w a s t h o u g h t t h a t the schist belts are the b a s e m e n t complex of Nigeria still d o u b t t h e c o n f i n e d to t h e w e s t e r n h a l f of Nigeria (see e x i s t e n c e of K i b a r a n rocks.• F o r i n s t a n c e , Oyawoye, 1972; Olade a n d Elueze, 1979; etc...). R a h a m a n (1976 p. 56) q u e s t i o n e d t h e reliability This p o s t u l a t i o n s t e m m e d from lack of a d e q u a t e a n d significance of a n age of 1150 + 140 M.a investigations of the geology of the O b a n Massif obtained by Grant (1970) for granite g n e i s s e s from w h i c h h a s h i t h e r t o b e e n d e s c r i b e d a s u n - n e a r Ue-lfe (see also C a h e n et a£, 1984). Neverthedifferentiated b a s e m e n t complex. We have now less, m o r e recent interpretations (Grant, 1978; differentiated t h e s e b a s e m e n t rocks a n d s h o w n Turner, 1983; Fitches et al., 1985) have pointed to t h a t schists are a m o n g t h e rocks of the Oban the possibility of two g e n e r a t i o n s of schist belts in Massif which have b e e n d a t e d in this study. Nigeria, one Pan-African a n d the other, Kibaran. In the Nigeriari b a s e m e n t , t h e occurrence o f Radiometric d a t a from rocks of t h e O b a n Massif E b u r n e a n rocks COlder Metasediments) a n d Pan- a n d s o u t h e a s t LokoJa c o n f i r m t h a t only two African rocks [Younger Metasediments] a n d their t h e r m o t e c t 0 n l c events were responsible for t h e associated intruslves have b e e n confirmed (Grant, crustal evolution of t h e s e areas, n a m e l y Pan1970; McCurry, 1976; R a h a m a n e t a i , 1983). The African (600 + 150 M.a.) a n d Kibaran (1100 + 200 Liberian a n d Kibaran events have b e e n difficult to M.a.). It is significant t h a t the Pan-African ages isolate. However, Grant {1970) a n d Oversby (1975) obtained for t h e s c h i s t s in LokoJa {687 + 13 M.a.)

50O

B. N. E ~ ,

M. CAEN-VAcI-~n~and A. C. 0 N Y E A ~

a n d the kyanite gneiss in the Oban Massif (676 + fine clastic s e d i m e n t s t h a t denote slow steady sub26 M.a.) a n d their initial 8VSr/S6Sr ratios are very sidence u n i n t e r r u p t e d by major tectonic activity. similar. Sirnll~rly, the Kibaran ages obtained for These belts therefore belong according to t h e m to the staurolite schists in s o u t h e a s t LokoJa (1315 + intracratonic extensional b a s i n proposed by Ogezi 72 M.a.) a n d the h o m o g e n e o u s amphibolite from (1977) and alluded to by T u r n e r (19831. The lower the Oban Massif ( 1313 + 1937 M.a.) are almost the value of the initial SVSr/~Sr ratios (0.70160 _+ s a m e . Vachette (1979) obtained mixed isochron 0.0060) of the Lokoja schists c o m p a r e d to those of ages of 1367 + 111 M.a. and 709 + 28 M.a. on the Kibaran age reported by Fitches eta/. (1985) in NW Mabambato series in Madagascar. C a h e n et o3_ Nigeria suggests a wider extension of the Lokoja (1984, p. 106) s u g g e s t e d t h a t t h e y o u n g e r , b a s i n w h i c h possibly favoured the ascent of mantle isochron age could be interpreted as indicating a materials. The 1313 + 37 M.a. Kibaran event in the Oban late P r e c a m b r i a n rehomogenization in the area. Vachette (1979) considered t h a t in Madagscar, the Massffwas a c c o m p a n i e d by the intrusion of rocks influence of t h e ' Pan-African granitizatlon a n d of charnockitic affinity 1289 + 153 M.a. ago, m e t a m o r p h i s m altered the real age of the first t h u s confirming the existence of c h a r n o c k i t e s of m e t a m o r p h i s m a n d led to a n incomplete isotopic Kibaran age in Nigeria. Hurley (19661 h a d earlier obtained a n age of 873 + 44 M.a. for c h a m o c k i t e at readjustment. The interpretation given to these ages here is that Bauchi. This age was supported by H u b b a r d ( 1968, they indicate t h a t major tectono-thermal events 1975) who showed t h a t c h a r n o c k i t e s in S.W. occurred in t h e s e areas 676 - 687 M.a. and 1313- Nigeria predate the Pan-African orogeny. Since 1315 M.a. ago. These two events are the m a i n then, other workers, e.g. Umeji a n d Caen-Vachette p h a s e s of the Pan-African (about 680 M.a.) a n d (1984) a n d Van B r e e m e n eta/. ( 1977), have acceptKibaran ( 1315 M.a.) cycles in the Oban Massif a n d ed t h a t the foliated granites a n d charnockitic LokoJa areas. For instance, one of the samples of granites in Nigeria yield ages of b e t w e e n 609 M.a. the kyanite-sillimanite schist a n d one sample of a n d 692 M.a. (Pan-AfricanL Also, it appears a the biotite-hornblende gneiss plot on the 676 + c o n s e n s u s t h a t charnockitic rocks in Nigeria are 26 M.a. isochron (Fig. 3). Similarly, one isochron intrusive rocks formed in a p r e s s u r e - t e m p e r a t u r e defined by the staurolite schists yielded a n age of regime characteristic of the base of the c r u s t and 679 + 18 M.a. (Fig. 4) which is very close to the were l a t e r d e f o r m e d d u r i n g t h e P a n - A f r i c a n 676 + 26 M.a. age of the kyanite gneiss. This orogeny (Oyawoye, 1961; R a h m a n , 1981; Coray, confirms that a m a j o r m e t a m o r p h i c and deform- 1972; Eborall, 1976). A n n o r and Freeth (1984) ational episode took place in these areas at the have suggested t h a t the c h a m o c k i t e s in Okene s a m e t i m e d u r i n g t h e P a n - A f r i c a n orogeny. area of SW Nigeria were formed in the u p p e r crust. The low initial 8VSr/SSSr ratio of t h e 1289 + This event a p p e a r s as important a n d extensive in Nigeria as in the r e s t of Africa. For instance, 153 M.a. isochron (R. 1. = 0.70138 + 0.00143) C a h e n et al. (1984) have reported a n age of indicates that the c h a m o c k i t e s were derived from 675 M.a. for rocks in the AnU-Atlas of Morocco the mantle and not from the crust. Field and and in NE Sudan. Dr. M. A. R a h a m a n (pers. petrographic data show that they were intrusive comm., 1984) h a s also obtained a K/At age of rocks from the mantle and did not form u n d e r locad 676 M.a. from Jalingo schists (north of Oban pyroxene hornfels facies (cf. Oyawoye, 1961) Ol Massif). granulite facies (cf Hubbard, 1968, 1975) metaThe high initial S¢Sr/S~Sr ratios obtained for the morphism. The second isochron age of 546 ± rocks yielding Pan-African (676-687 M.a.) ages 24 M.a. (R.I, = 0.70743 + 0.00026) reflects a indicate that this m e t a m o r p h i s m a n d deformation stabilization and a Rb-Sr rehomogenization oJ affected older materials at least of Kibaran age. these charnockites during the Pan-Africar The m a i n p h a s e of m e t a m o r p h i s m a n d deform- orogeny. Sample OR2a which plots between the ation during the Kibaran orogeny at both the Oban two isochrons (Fig. 7) is a witness of this deMassif a n d s o u t h e a s t LokoJa occurred at the stabilization. It is therefore evident t h a t the same time 1315 M.a. ago. These ages confirm t h a t charnockitic rocks which were emplaced durin~ the Kibaran orogeny actively participated in the the Kibaran orogeny were later subjected to a Pan. evolution of the b a s e m e n t complex r o c k s in African tectono-thermal episode. This later evenl Nigeria. In fact, it is t h o u g h t that s o u t h e a s t Lokoja imposed a foliation c o n c o r d a n t with t h a t of th~ c o u l d be a p r o l o n g a t i o n s o u t h w a r d s of the c o u n t r y rocks on the c h a m o c k i t e s . This 546 _~ K u s h a k a schist belt in NW Nigeria. Both schist 24 M.a. age is similar to the 547 + 37 M.a belts s h a r e similar petrologic, s t r u c t u r a l and (Mkar-Gboko granites) a n d 540 _+86 M.a. pyroxen~ isotopic characteristics. Fitches et al. (1985) noted bearlng granites (? charnockites) obtained by Umej that the Kibaran schist belts in NW Nigeria appear and Caen-Vachette (1984). It is also close to the ag( to represent a maj or region of quietly a c c u m u l a t i n g 527 _+ 16 M.a. a n d 572 M.a. obtained for k y a n i t e

Isotopic ages from the Oban Massif and southeast Lokoja: implications for the evolution of the basement sfllirnanite schIst and fracture-filling materials respectively in the Oban Massif (Ekwueme 1985). The high value of the initial STSr/~Sr ratio (0.70743 + 0.00026) for the 546 + 24 M.a. Isochron Is a confirmation t h a t reactivation of older material (Kibaran charnockltes) occurred during tle PanAfrican time in the Oban Massif. The 784 + 31 M.a. age was first obtained by E k w u e m e (1985) for the b a n d e d amphibolite of Uwet area, It is higher t h a n Pan-African (600 + 150 M.a.) (cf. C a h e n and Snelling, 1966). E k w u e m e (1985) interpreted this result as either dating a Klbaran event or a destabfllzation at the b o u n d a r y of the Pan-African and the Kibaran orogenies. He t h o u g h t t h e n t h a t a single whole-rock age of 922 + 20 M.a. he h a d obtained for a migmatltic gneiss at Mbarakpa (Fig. 1) suggests that, thIs 784 +_31 M.a. age ls Klbaran. Additional data presented here (1313 + 37 M.a. a n d 1289 + 153 M.a.) suggest f u r t h e r t h a t this age is most likely the waning phase of the Kibaran orogeny, Structural orientations of the b a n d e d amphtboltte lend support to this interpretation (cf. Grant, 1378; Mullan, 1979; O n y e a g o c h a a n d E k w u e m e , 1982; E k w u e m e , 1986). An isochron age of 527 + 16 M.a. obtained for the kyanite-stllirnanite schist at the Kwa Falls (Fig. 1) fits into the post-tectonic events (460525 M.a.) of the Pan-African orogeny (see C a h e n a n d Snelling 1966; Harper et o2., 1973). The prep o n d e r a n c e of retrograde chlorite in these schIsts and in fracture-filling materials (age = 572 M.a.) indicates t h a t this waning p h a s e of the PanAfrican orogeny was a c c o m p a n i e d by diaphthoresIs in the Oban Massif. It Is t h o u g h t t h a t thts event Is coeval with the 546 + 24 M.a. event which affected the charnockitic rocks during the PanAfrican orogeny. Harper et a t (1973) observed t h a t post-tectonlc Pan-African events involve epeirogenie uplift a n d cooling reflected by whole-rock K-At ages of 550 to 530 M.a. ThIs 527 + 16 M.a. event m a y be responsible for the e m p l a c e m e n t of pegmatitic granites, dolerites and other intrusive rocks which occur in the O b a n Massif. Dating of t h e s e intrusive rocks is in progress. SUMMARY AND CONCLUSIONS

This s t u d y h a s establIshed the following: (1) Two orogenies n a m e l y Kibaran and Pan-African were responsible for the crustal evolution of the b a s e m e n t rocks in the Oban Massif and s o u t h e a s t LokoJa in e a s t e r n a n d c e n t r a l Nigeria. (2) The m a i n p h a s e of the Pan-African orogeny occurred co, 680 M.a. in both a r e a s a n d could have affected m a n y other parts of Nigeria. (3) The 680 M.a. event affected already older material a n d c o n s e q u e n t l y reworked these

501

rocks. However, it did not obliterate evidence of earlier events (cf. Ekwueme, 1987). (4) A m a j o r and a n Important m e t a m o r p h i c and deformational event occurred in the two areas at co_ 1315 M.a. during the Kibaran orogeny. In the Oban Massif the 1315 M.a. (Kibaran) event was a c c o m p a n i e d by plutonic activity which resulted In the e m p l a c e m e n t of rocks of c h a m o c k i t i c affinity 1289 + 153 M.a. ago. (5) The Pan African ages hitherto obtained for c h a m o c k i t e s in Nigeria possibly reflect the ages of the latest tectonic events a n d not their e m p l a c e m e n t ages. These charnockites, as examplifled by those of the Oban Massif could have been derived from the m a n t l e and not from the crust as hitherto postulated. (6) A late or post-tectonic event occurred a r o u n d 530 M.a. ago in t h e O b a n M a s s i f a n d was m a r k e d by diaphthoresis and also the e m p l a c e m e n t cf some intrusive rocks in the area. In conclusion, the a u t h o r s refer to t h e recent authoritative review of the geochronology a n d evolution of Africa in which C a h e n et 02. (1984) assigned a time interval of ca. 1400-1300 M.a. to t h e Kibaran o r o g e n y t h e r e b y r e s t r i c t i n g the occurrence of Kibaran rocks to Africa s o u t h of the equator (east-central, s o u t h - c e n t r a l a n d central Africa). It Is signfficam t h a t some ages obtained in this study: 1315 + 72 M.a., 1313 + 37 M.a. and 1289 + 153 M.a. are very close to 1310 + 25 M.a. regarded by C a h e n et a t ( 1984, p. 194) as characterizing one of the p e a k s of the Kibaran tectonot h e r m a l event. The other is 1370 + 25 M.a. If the time interval of co- 1400-1300 M.a. is therefore adopted for the Kibaran, the ages obtained in this study: are at present, the only evidence indicating t h a t Kibaran tectono-thermal event affected Africa north of the equator. Professor IL J. Vail (pers. comm., 1987) is of the opinion that the mixed Isochrons resulting in two or more dIstinguIshable regression lines reported in thIs s t u d y suggest overprinting a n d reworking by P a n - A f r i c a n events, possible c r u s t a l contamination, and possible mixed ages of rocks. He went f u r t h e r to suggest t h a t the original event m a y have preceeded 1300 M.a a n d t h u s be part of the Liberian or E b u r n e a n basement. Traces of these two orogenies are yet to be isolated in Lokoja a n d Oban Massif. Rather, it h a s b e e n shown t h a t the Kibaran rocks in these a r e a s were reworked and overprinted by a major Pan-Afrlcan event dated at ca. 680 M.a. This event did not however, obliterate all the evidence of earlier events in these regions (cf. Ekwueme, 1987). ~kn~vled~t~. The authors thank Professor IL J. Vall for his critical review and useful suggestions

5O2

B. N. ExwtmME,M. CAEN-VACt~r~ and A. C. ~

~

Grant, N. IC, Hickman, M. H., Burkholder, F. R. and PoweU, J. L. 1972. Klbaran metamorphic belt in PanAfrican domain of west Africa? Nature (Pfu3. ScL), 238, 90-91. RFAzERENCl~ Harper, C. T., Sherrer, G., McCurry, P. and Wright, J. B. 1973. K-Ar retention ages from the Pan-African of norArmor, A. E. and Freeth, S. J. 1984. On the origin and t h e m Nigeria. Geo/. Soc. Am. BUl/., 84, 919-926. thermotectonic history of hypersthene-bearing rocks Hubbard, F. H. 1968. The association charnockitefrom the Okene area, Nigeria. NigerlanJ. Mb-LGeo/., 21, older granite in S.W. Nigeria. Nigerian J. Miru Geol., 3, 73-77. 25-32. Cahen, L. and Snelllng, N. J. 1966. The geochronology Hubbard, F. H. 1975. Precambrian crustal development ofEquatorialAfrica, North Holland, Amsterdam, 195 p. in western Nigeria: Indications from the two region. Cahen, L., Snelling, N. J., Delhal, J. and Vail, J. I~ 1984. Geo/. Soc. Am. BULL,86, 548-554. The geochronology and evoltition of Africa. Oxford Hurley, P. 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which improved the quality of this paper. E. O. Otukak and U. S. Umanah drafted the figures.

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