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Crust-forming ages and proterozoic crustal evoluton in Nigeria: a reappraisal of current interpretations
Premmbriun Reseur(h ELSEVIER
Precambrian Research 87 (1998) 65 74
Crust-forming ages and proterozoic crustal evoluton in Nigeria: a reappraisal of current interpretations S.S. Dada Geology Programme, Abubakar Tafawa Balewa UniversiO', Bauchi, Nigeria Received 18 June 1996; accepted 22 September 1997
Abstract
Radiometric ages in the Nigerian basement cluster around 3.5, 3.1 3.0, 2.7 2.5, 2.1 1.8 and 0.6 Ga. However, not all these ages correspond to times of crust formation. There are significant discrepancies between U-Pb and Rb-Sr ages and Nd and Sr model ages with a definite trend for the orthogneisses and granitoids analysed. Results show good agreement between model ages (T TM, T st) and emplacement ages for early Archaean and imply that these model ages date the time of crust-mantle differentiation. The Archaean/Early Proterozoic boundary was most probably a major crust-forming period in SW Nigeria as indicated by U-Pb zircon ages that are in agreement with Nd model ages in that segment of the Nigerian basement. On the other hand, Late Proterozoic and younger rocks, which include migmatized gneisses, Pan African granitoids and Jurassic Younger Granites, have model ages in excess of their crystallization ages. Such model ages that lie between Pan African (or Jurassic) and well-known older geologic ages do not correspond to any specific crust-forming event. Rather they are mean crust-residence ages reflecting contributions from juvenile material and remelted older Archaean crustal components. The overall scenario is consistent with widespread Archaean crust and its involvement in the genesis of Late Proterozoic and younger crust in the Nigerian basement. This is in agreement with an intra-cratonic setting far removed from the suture to the west. © 1998 Elsevier Science B.V.
Keywords: Anorogenic magmatism; Crust-formation; Model ages; Nigeria; Pan African
1. Introduction
It is generally believed that the greater part o f crustal g r o w t h t o o k place by Late Archaean. The identification o f juvenile p o s t - A r c h a e a n crust as opposed to reworked pre-existing material is often difficult in m a n y terrains (Allegre and Ben O t h m a n , 1980; Taylor and M c L e n n a n , 1981; Allegre and Rousseau, 1984). A l t h o u g h the Nigerian basement (Fig. 1) has for long been 0301-9268/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved. PH S0301-9268 (97)00054-5
regarded as p o l y m e t a m o r p h i c , unequivocal geochronological and structural evidence in the postA r c h a e a n is available only for the Pan-African. For the latter, a continental collision plate tectonic model (Burke and Dewey, 1972; Black et al., 1979; Caby, 1989) has been accepted to be at the origin o f the reactivation o f the D a h o m e y a n shield east o f the G h a n a - T o g o - B e n i n suture zone. In this study, available data from Sm Nd, R b Sr and U P b isotopic systematics are used to identify
66
X S . Datkl
Precamhrian Res'earuh ,~¢7 f 199~U 65 74
(a)
(c) 5°
5°E
lOOE
Oft"
.
" i
i
H , i,">:,,
:','~ ,: ~;~.i.!:~: i
i
" i
o Ii
I
16o Kin. I
i
I
NIGERIA
Fig. 1 (a) Location of the Nigerian shield in the major structural units of West Africa and the Brazilian belt l'ollowing the preMesozoic reconstruction of Caby (I 989). (b) Location of Nigeria in the generalized map of Africa. (c) Simplified geological map of Nigeria [alter Oyawoye ( 1972 )]. I, Cretaceous and Younger sediments: 2, Jurassic Younger granites: 3, older granites: 4, undifferentiated metasediments; 5, quartzite and quartz schist: 6, Precambrian basement (undifferentiated): 7, phanerozoic sediments (generalized); 8, cratons (2.0 Ga or older): 9, Upper Proterozoic o[" Western Hoggar: 10. reworked Archaean and Proterozoic cover: II. major shear zone: 12. Late Proterozoic suture: 13, major thrust. Egbe Kabba Okene "Proterozoic" region.
major periods of new crustal formation and in particular, to distinguish between juvenile crust and reworked pre-existing crust.
2. Geological Setting and Geochronology About half of the Nigerian crystalline crust is buried beneath Cretaceous and younger sediments: while the other half outcrops largely in the northcentral, southwestern and in three smaller regions from north to south along the country's eastern boundary with Cameroun (Fig. 1 ). Recent U Pb isotopic work in North Central Nigeria (Dada0 1989; Bruguier et al., 1991, 1994;
Ekwueme and Kroner, 1993) has confirmed earlier indications (Ogezi, 1988) of Early to MidArchaean (3.5-3.0 Ga) crust in the Nigerian basement. Late Archaean (2.4 2.5 Ga) U-Pb ages have been reported from gneisses in SW Nigeria (Oversby, 1975: Pidgeon et al., 1976: Rahaman, 1988) and in NE Nigeria (Dada et al., 1993a). Early Peroterozoic ages (2.1 1.8 Ga) have been obtained on gneisses in SW Nigeria (Grant, 1970; Rahaman et al., 1983; Rahaman, 1988; Annor, 1995). Late Proterozoic ( ~ 6 0 0 M a ) U Pb ages have been reported from both migmatitic gneisses and granitoids in practically all the shield areas in Nigeria; Tubosun et al. (1984), Rahaman et al. (1991) in the SW; Van Breemen et al. (1977),
S.S. Dada / Precambrian Research 87 (1998) 65 74
Dada and Respaut (1989), Dada et al. (1989) and Ferre et al. (1996) in North Central and Ekwueme (1990) in the SE. On the other hand, lower intercept U Pb ages from Archaean zircons cluster ca 600 Ma, arguing for the main tectonometamorphic imprints in Pan African times.
3. Crust formation and Sm-Nd isotopic systematics Ever since the formation of primitive continental crustal masses in the Earth's earliest history, large proportions of them have been subsequently tectonized and recyled. Conversely, newly formed crust separated directly from the mantle has been added in many regions at specific times through geologic history. The considerable progress which isotope geochemistry has witnessed since the early 1970s has helped to resolve the fundamental problem of distinguishing between the reworked crust and juvenile additions (Armstrong, 1981 ; DePaolo, 1983; Patchett and Chauvel, 1984; Allegre and Rousseau, 1984; Patchett and Arndt, 1986). It is in resolving the issues involved herein that the Sm Nd isotopic system has found both geochronologic and petrogenetic applications. Sm and Nd are parent daughter rare-earth elements ( R E E ) that do not fractionate in the geological environments except when phases with significantly different distribution coefficients for Sm and Nd participate in melting, crystallization or metamorphic processes. One such process is mantle melting when heavy rare-earth element ( H R E E ) rich phases such as garnet are present. In such cases, the lighter, more hygromagmatophilic Nd leaves the heavier Sm in the residue and gets incorporated into the melt which leaves the mantle with newly forming crust. The residual mantle is thus gradually depleted with time in Nd and enriched in Sm. Except for certain granulite facies rocks, garnet is not commonly involved in intra-crustal differentiation, metamorphism, crustal melting and weathering. Hence, Sm Nd fractionation is practically absent in crustal processes (McCulloch and Wasserburg, 1978). It is significant to note that this is in marked contrast to the commonly used, older dating methods such as U Pb and R b - S r systems in which both mantle
67
melting and intra-crustal fractionation processes produce melts that are rich in the parent elements U and Rb. The S m - N d isotopic systematics is thus able to 'see back' to the time in the past when continental crustal material separated from the mantle. Following the interpretation of McCulloch and Wasserburg (1978), it is only such mantle separation ages that can be meaningfully referred to as crust formation ages dating the times of addition of juvenile crust from the mantle. Calculations of model ages from this system therefore assumes constancy of the S m - N d ratios subsequent to mantle extraction except in very few cases of highly silicic (> 75% SiO2) magmas where light rare-earth element (LREE)-rich phases (sphene, monazite, etc.) are abundant (Miller and Mittlefehdt, 1982). All Sm Nd data presented in Table 1 used the following parameters: • Present-day 143Nd/144Nd = 0.512638 and 147Sm/a44Nd = 0.1966 (McCulloch and Wasserburg, 1978). • Depleted mantle=S43Nd/144Nd=0.51315 and 147Sm/144Nd=0.2137 (Jahn et al., 1988). • All other expressions and calculations follow the techniques of McCulloch and Wasserburg (1978).
4. The Nigerian Basement and Crustal Growth The Nigerian basement is better known as part of the Pan-African mobile belt of Kennedy (1964) as all crystalline shield areas have been reworked ca 600 Ma. Apart from the Jurassic Ring Complexes, most crystalline rocks of the Nigerian basement bear Pan-African tectonic and mineralogical imprints. Pre-Pan African ages in Nigeria are therefore protolith ages. 4.1. The archaean
The orthogneisses of Kaduna area (Dada, 1989; Dada et al., 1993b) have Nd model ages that overlap U - P b zircon crystallization ages ( Table 1 ), showing that the rocks formed from material that separated from the mantle during a crust-forming
68
S.S. Da&l Pre¢'anlhrkm Research 87 (1998) 65 74
Table 1 Geologic { U Pb, Rb Sr) and model (Nd, Srl ages for rocks of the Nigerian Basement and the Jurassic ring complexes
Ts~
($T-t
3.57
3,49Ga
10Ma
3.54
3.18 Ga
440
3.51
2.73 Ga
760
_., 6
2.36 Ga
[_ithology
Geological age
T...,d (Ga)
Kadunagranodioritegneiss
3 . 4 6 G a ( U Pb) 3 . 4 6 G a ( U Pb) 3.1 Ga ( U Pb, Rb St)
Kaduna carl 3 gneiss
cnd (t)
Tnd Reference
Kadtinu late gneiss Ibadan Aplile ()do ogun gneiss lie-fie grey gneiss lie-fie granilc gneiss lgbetti augen gneiss Egbe gneiss kabba-okemle gneiss
2.75 Ga {Rb Sr) 2 . 7 5 G a ( P b Pb) 2.5 Ga (U Pb) 2.3 Ga439 Ma ( U Pb) 1.85Ga 5 5 0 M a I U Pb) 1.9 Ga {Rb Sr)
Tildcn fulani migmatite Badiko granite gneiss Okemae granodiorile Gn
2.5 Ga500 Ma ( U Pbl 2 . 5 G a 5 0 0 M a ( U Pb) 2.1 (]u {U Pbl
2.51 1.80 2.10 2.10
- 1.3 680 Ma 14.6 740Ma 2.78 Ga
1300 Ma 1600Ma
Sarkin pawa nyntectonic migmatite I kad-abuji rd I Badiko s3nlectonic diorile lkerre massive charnockiie &kure gneissic dmrnockite Akure porplD ritic granite Idanre gneissic charnocki/e Idanre massive charnockite ldam'e porphyritic grunile Toro Biot-Hbde granite Toro charnockitic diorite Bauchi quartz lh,,alite monzonite (bauchiic)
635 Ma ( k T Pb)
1.51)
-3.4
865 Ma
623 Ma ( U 620Ma(U 634Ma(U 621 Ma (t; 580Ma(U 593 M a ( U 587Ma(U 61)7 Ma L 638 Ma L 638 Ma 15
1.91)
Toro migmcltile Toro anaicclic granite
581 Ma 616 Ma 751 Ma 170Ma Rb Sr WR) 170 Ma 170 Ma
Toro Ring Ring Ring
migmatite graniie complex 369 complex 473 complex 412
Pb) Pb) Pb) Pb) Pbl Pb) Pbl Pb) Pb) Pbi
710 Ma
12.3 780 Ma
Bruguier et al. (1994) Ekwueme and Kroner (1993) Dada (1989). Bruguier el al. ( 19941 Dada and Ralmman (in press) Oversby ( 1975 ) Pidgeon et al. 11976) Rahaman (1988) Rahaman (1988) Rahaman ( 19881 Dada and Rahaman (in press)
Duda et al, 11993a,bl Armor ( 1995 }: Dada and Rahaman (in press) Dada (unpublished data)
1297 Ma Tubosunet a1.(1984)
2.10 2.50 1.70 1.75 1.88 2.71 1.15 1.46 1.92
e v e n t a b o u t , o r n o t l o n g b e f o r e 3.5 G a ((ST O ) . U Pb zircon and whole-rock Rb Sr data from adjacent rocks of the same area indicate subsequent isotopic disturbance and rehomogenization. This would suggest two metamorphic events at 3.1 Y 0 a n d 2.75 G a , b u t t h e l o w e r i n t e r c e p t a g e o f 612 M a ( B r u g u i e r et al., 1994) d o e s n o t f a v o u r t h i s i n t e r p r e t a t i o n . T h e h i g h i n i t i a l STSrff°Sr r a t i o s of 0.70749+0.00046 and 0.70768+0.00042, respectively, indicate reworking of pre-existing components. H o w e v e r , t h e T na a g e s f o r t h e s e r o c k s r e m a i n m u c h o l d e r a n d a v e r a g e 3.5 G a . A
--12.6 670Ma - 15.6 1.07Ma 3.9 1.80 Ga 1.86 14 12.4 1.3 3.2 5.6
1493 1915 1062
Dadaet a1.(1989) Dadaet a1.(19891 Dada and Respaut ( 1989 )
1280 1380 1960 880 1290 1750
Ferre et al. (19961 Ferre el al. ( 1996} Ferre el ul. ( 19967 Van Breemen et al. (1975) Dickin et al. ( 1991 Dickin et al. ( 1991 I
s i m i l a r m e t a m o r p h i c a g e o f 2.75 G a w a s p r o p o s e d f o r l b a d a n a p l i t e s in S W N i g e r i a ( O v e r s b y , 1975). The only reasonable interpretation for such rocks is t o c o n s i d e r t h e t w o a g e b r a c k e t s (3.1 3.0 a n d 2.75 G a ) as p u r e l y m e t a m o r p h i c a g e s d u r i n g w h i c h there was no vertical addition of new crust from the mantle. 4.2.
Archaean/proterozoic
bounck,'v
U P b z i r c o n c r y s t a l l i z a t i o n a g e s o f 2.4 2.3 G a oil I l e - l f e g r e y g n e i s s e s f r o m S W N i g e r i a ( P i d g e o n
69
S.S. Dada : Precambrian Research 87 (1998) 65 74
et al., 1976: Rahaman, 1988) and NE Nigeria (Dada et al., 1993a) are in agreement with T Na ages of 2.56-2.51 Ga (Table 1) on orthogneisses from Egbe and Kabba Okene areas (Dada and Rahaman, in press), end values at 2.5 Ga for these samples plot largely above the Chondritic Universal Reservoir (CHUR). They constitute a supporting evidence for juvenile crustal addition at the Archaean/Proterozoic boundary with little or no time lag between crustal formation and rock crystallization (6 T-O).
involving an extended protolith history. It is expected that isotopic data from the better defined Proterozoic metasedimentary cover within the Schist Belt to the west (McCurry, 1976; Caby, 1989) will aid in resolving some of the incertitudes. Such old model ages (T>>t) have also been obtained on the basement and Mesozoic Ring Complexes of the Jos Plateau (Dickin et al., 1991 ). These ages are strongly biased towards an Early Proterozoic (Eburnean) event known in many parts of North and West Africa (Lancelot et al., 1983; Bertrand et al., 1986: Abouchami et al., 1990: Boher et al., 1991) and NE Brazil (Caby, 1989) from basic to intermediate assemblages. However, there is till date no unequivocal evidence for regarding the NE Nigerian basement as belonging to the 2.1 Ga terrane. Neither U - P b zircon ages nor other dates suggest this. Available data restrict the 2.1 Ga event in the Nigerian basement to the SW with T Nd and U - P b ages in agreement only for the Okene granodiorite gneiss (Table 1 ). This spatial restriction has led to the fundamental question on the true nature and significance of the 2.1 Ga ages in Nigeria. Unequivocal Palaeoproterozoic tectonic landmarks that can be widely
4.3. The Nigerian proterozoic The T Nd ages calculated with respect to the depleted mantle (2.1-1.5 Ga) from both migmatite gneisses and granitoids are in general older than the U Pb zircon crystallization ages (Table 1 ). In Fig. 2, where initial Nd values are plotted against U Pb ages, the Pan African granitoids have negative values of - 1 5 to - 4 and therefore plot not only below the depleted mantle ( D M ) evolution trend but also below the C H U R . This would imply a contamination by the pre-existing Archaean crust or magma derivation via a multistage process
.10
~
1.0 T
D
1.5 ~
2.1 T antl¢
2.S T
EVOlutiontV AP
0
-10 . . . .
LU -20
-30
0.5
1.0
1.5
2.0
2.5
3.0
Time (Ga) Fig. 2. end diagram showing possible mixing between proposed Early Proterozoic (2), Late Proterozoic (3) and Mesozoic (4)juvenile crusts adn the Archaean (_> 2.5 Ga) felsic component ( I ) of the Nigerian basement complex. Depleted mantle evolution trend assumes a linear growth from a D M source with present-day e~,d - + 1 0 (Jahn et al., 1988). AP and P represent end fields for TNa=2.56 2.51 k G a and 2.1 1.5 Ga ages, respectively.
70
S, S. Da&t, Precamhrian Re,search 87 / 1998) 65 74
applied within the Schist Belt are yet to be identified. The same sequence of deformation and metamorphic events that affected the Archaean gneisses have been recognized in the 2.1--1.8 Ga gneisses and the metasediments of the Schist Belt, not only in Nigeria but also in the equivalent predrift Borborema Province (Fig. 1) of NE Brazil (Caby, 1989; Caby et al., 1990). Available data appear equivocal with regard to the 2.1 G a ages. They are either related to an accretionary event or are purely metamorphic. If the latter, they should then be interpreted as shield-stabilization to Late Archaean/Early Proterozoic crust (Fig. 2). This will be much like the anorogenic 1.9 1.8 G a magmatism in SW Nigeria ( R a h a m a n et al., 1983; Ajibade et al., 1987: R a h a m a n , 1988) and the Iforas region of Mali (Caby and AndreopoulosRenaud, 1983). In the absence of further 2.1 Ga U - P b age data, the best interpretation that can be given to all such model ages that lie between the well established Archaean and Pan African U - P b zircon crystallization ages, including those of Ferre et al. (1996), is that they are mean crustal residence ages (Arndt and Goldstein, 1987) that represent average ages of mixtures of Pan African and remelted Archaean crust ( D a d a et al. (1995). Annor (1995) obtained a precise U Pb zircon age of 2.1 G a on the K a b b a - O k e n n e granodiorite gneiss. In Fig. 2, the eNa values calculated to zircon crystalization age of 2.1 Ga, for a presumed Proterozoic Egbe K a b b a Okene region, have negative values that plot below the C H U R . The samples have model ages of 2.56 2.51 G a ( D a d a and Rahaman, in press) and therefore suggest that the crust-forming process in the region began earlier than the measured zircon crystallization age of 2.1 G a or that some Archaean crustal component was incorporated as the new('?) Lower Proterozoic crust was formed. The Late Archaean model ages suggest that the K a b b a Okenne and indeed the SW Nigerian basement is of Archaean association, regardless the strong Pan African fabric on the terrain.
4.4. The Nigerian Mesozoic' Ring Complexes The application of multiple radiogenic tracers (Sr, Nd, Pb) by Dickin et al. ( 1991 ) on the Jurassic
Ring Complexes shows the great potential of the Sm--Nd isotopic system to 'see back' into the past and identify old crustal protoliths. Nd model ages ( 1.92 1.15), calculated from Dickin et al. ( 1991 ) as shown on Table 1 indicate large a T in excess of the crystallization age of 170 Ma (Van Breemen et al., 1975). Any juvenile material at 170 Ma must therefore have been essentially contaminated by older components during m a g m a generation and/or ascent. Because the Jurassic Complexes would have contributions from at least two different sources (Archaean and Pan-African, Fig. 2), its proportion of juvenile material must be insignificant.
5. Implications for crustal growth in Nigeria T Nd and U - P b data from Northern Nigeria confirm Early Archaean (3.5 Ga) crustal addition and subsequent disturbances and rehomogenizations at 3.1-.3.0 and 2.75 G a have been recorded in the north and southwest as constrained by U Pb zircon ages (Dada, 1989; Bruguier et al., 1994) and whole-rock Rb Sr ages (Oversby, 1975; Dada and Rahaman, in press). The agreement between Nd model ages and U--Pb ages is also in favour of crust-formation during Late Archaean times (2.5 Ga). Bruguier et al. (1994) suggested that the 3.1 3.0 G a (3050+23 Ma) U - P b zircon age approximates the age of emplacement and crystallization of the granodioritic m a g m a from a crust of extended pre-history (3.56Ga). This is in agreement with Nd model ages that put the extraction age of the crust from the mantle in North Central Nigeria at 3.55 Ga. This interpretation is in agreement with the views of Dada et al. (1993b) in which both Rb Sr and U Pb data put the emplacement age of early gneiss at 3.1 3.0 Ga, with an extended protolith history of 400 Ma. back to 3.5 Ga. The suggestion of a metamorphic event at the well known regional Liberian Orogeny (2.75 Ga) in West Africa has not been corroborated by U Pb zircon data within the Nigerian basement. Only partial rehomogenization or disturbance of the isotopic system can be inferred. The present view of the nature of the Nigerian
71
S.S. Dada / Precambrian Research 87 (1998) 65 74
Proterozoic crustal growth needs to be rethought, particularly because unequivocal evidence of subduction induced magmatism appears to be lacking. Extensive Sm Nd isotopic work on a global scale has shown that: (1) the 2 . 2 - 1 . 8 G a age range represents the average 'crustal residence age' of the upper crust (Allegre et al., 1983; Goldstein et al., 1984), which may not be related to any crustforming event. (2) Nd model ages regularly decrease with the geologic ages and after 2 Ga, the curve flattens and tends to an asymptotic value at ca 1.8 Ga (Allegre and Rousseau, 1984). Therefore, great caution should be exercized in the interpretation of the isotopic data particularly the Nd model ages from these post-Archaean rocks with a polymetamorphic history. Underplating in areas close to Archaean shields can be envisaged for the Mid-Proterozoic which was a quiscent period (lacking magmatism) that followed the 1.9 1.8Ga anorogenic magmatism (Rahaman et al., 1983; Ajibade et al., 1987; Rahaman, 1988). The identification of the Pan-African Ghana Togo Benin suture zone (Burke and Dewey, 1972; Black et al., 1979; Caby, 1989) to the west emphasizes the mid-plate nature of the Nigerian shield 250-800 km eastwards, depending on the sample location. Field evidence has for a very long time (Oyawoye, 1972; McCurry, 1976; Rahaman, 1976; Grant, 1978; Fitches et al., 1985; Ajibade et al., 1987) recognized the Nigerian basement as an assemblage of contrasted terranes with a well developed metasedimentary cover to the west and a largely vestigial crystalline terrain to the east. It is a continuum between the Hoggar to the north (McCurry, 1976) and the Borborema Province to the south [Caby (1989); Caby et al. (1990); see Fig. l]. Recent structural studies (Black et al., 1994; Ferre et al., 1996) have strengthened the view on the contrasted nature of the Nigerian basement. While some vestiges of molasse assemblages have been recognized in the Anka Belt of N W Nigeria (Ogezi, 1988), their periods of initial creation and subsequent accretion to the Proterozoic continental mass are the more urgent questions that need the attention of multiple
isotopic tracer techniques and absolute chronology (Sm Nd, U Pb). Upper mantle convective disturbances resulted in partial melting of the underplated Archaean protoliths during the continental collision in Neoproterozoic times as part of the large-scale Pan-African/Braziliano tectonothermal event. The widespread precise U - P b zircon ages on granitoids of this age; their characteristic deformational and metamorphic patterns (Rahaman, 1976; McCurry, 1976: Ajibade, 1976; Fitches et al., 1985; Annor and Freeth, 1985; Ekwueme, 1987) in Nigeria, N W Africa and N E Brazil (Bertrand et al., 1986; Leigeois et al., 1987; Caby, 1989) emphasize the orogenic nature of the 600 Ma event. Fig. 3 summarizes the relationship between available geologic and Nd model ages, emphasizing at least a twostage accretionary model for Proterozoic and younger rocks with consistent older Nd model ages that argue in favour of significant involvement of Archaean felsic crust in their genesis. The large 6t values correspond to significant time intervals between mantle separation ages and metamorphic emplacement (geologic ages) at latter times. The presence of Archaean crustal segments in the Nigeria basement as in Central Hoggar is compatible with an intracratonic setting. The relict Archaean ages strongly support the continental collision model of Caby et al. (1981) and Caby (1989) that the region and the West African and Sao Luis Cratons must have been part of the same crustal province which was rifted apart, reunited and in part reworked in the Late Proterozoic (Dada et al., 1995; Dada and Rahaman, in press)
3.0z.o-
°
"I
1.0-
0.0
I
0.0
1.0 2!0 Geotogic
Age
3!0 (6a)
4-0
Fig. 3. Comparison of geologic and Nd model ages (see Table 1 for data sources).
72
S.S. Dada
PrecanThrian Research 87 (1998) 65 74
in contrast to the largely juvenile Birrimian rocks (Abouchami et al., 1990: Boher et al., 1991 ). It is therefore clear that the post Archaean ages obtained on rocks in the Nigerian basement cannot be interpreted as representing periods of purely juvenile additions. Such rocks give mean crustal residence (model N d ) ages which neither coincide with U - P b zircon ages nor well established structural or other line of evidence of an orogenic event.
Acknowledgment The author wishes to thank M. Boher and R. Caby for their constructive reviews of the manuscript; M. A. Rahaman and R. Caby for their interest and encouragement in crustal evolution studies o f the Pan African/Brazilian belt. Many thanks to L. Ochibe for drafting the figures.
References Abouchami, W., Boher, M., Michard, A.. Albarede, F.. 1990. A major 2.1 Ga old event of marie magmatism in West Africa: an early stage of crustal accretion. J. Geophys. Res. 95, 17605 17629. Ajibade, A . C . . 1976. Provisional classification and correlation of the Schist belts in NW Nigeria. In: Kogbe, C.A. (Ed.), Geology of Nigeria. Elizabethan Pubh. Lagos, pp. 85 90. Ajibade. A.C., Woakes, M.. R a h a m a n . M.A., 1987. Proterozoic crustal development in the Pan-African regime of Nigeria. In: Kroner, A. (Ed.), Proterozoic Lithospheric Evolution. Geophysical Union Geodynamics Series, vol. 17. pp. 259 271. Allegre, C.J.. Ben Othman, D.. 1980. Nd Sr isotopic relationship in granitoid rocks and continental crust development: a chemical approach to orogenesis. Nature 286, 335 342. Allegre, C.J., Hart, S.R., Minster, J.F.. 1983. Chemical structure and evolution of the mantal and continents determined by inversion of Nd and Sr isotopic data, II. Numerical experiments and discussion. Earth Planet. Sci. Lett. 66. 191 213. Allegre. C.J., Rousseau, D.. 1984. The growth of the continent through geological time studied by Nd isotope analysis of shales. Earth Planet. Sci. Lett. 67, 19 34. Annor, A.E., 1995. U Pb zircon age lk~r Kabba Okene granodiorite gneiss: implication for Nigeria's basement chronology. African Geoscience Review 2 (1), 101 105. Annor. A.E., Ereeth, S.J., 1985. Thermotectonic evolution of the basement complex around Okene, Nigeria; with special reference to deformation mechanism. Precambrian Res. 28. 269 281.
Armstrong, R.L.. 1981. Radiogenic isotopes: the case for crustal recycling on a near steady-state no-continental-growth. Earth Philos. Trans. R. Soc. London Ser. A 301. 443 472. Arndt, N.T.. Goldstein, S.L.. 1987. Use and abuse of crustformation ages. Geology 15, 893 895. Bertrand. J.H., Michard. A.. Boullier, A.M., Dautel, D., 1986. Structure and U:Pb geochronolgy of Central l-toggar (Algeria): a reappraisal of its Pan-African evolution. Tectonics 5 (7), 955 972. Black, R., Caby, R., Moussine-Pouchkine, A., Bayer. R.. Bertrand, J.M., Boullier. A.M., Fabre, J., Lesquer, A., 1979. Evidence for late Precambrian plate tectonics in West Africa. Nature 278, 223 227. Black, R., Latouche, L., Liegeois, J.P.. Caby. R.. Bertrand, J.M., 1994. Pan African displace terranes in the Tuareg shield (Central Sahara). Geology 22, 641 644. Boher, M., Abouchami, W., Michard, A., Albarede, F.. Arndt, N.T.. 1991. Crustal growth in West Africa at 2.1 Ga. J. Geophys. 97, 345 369. Burke. K.C.. Dewey, J.F-., 1972. Orogeny in Africa. In: Dessauvagie, T.EJ.. Whiteman, A.J. (Eds.). African Geology. University Press. lbadan, pp. 583 608. Bruguier, O., Dada. S.S.. Lancelot. J.R., 1991. Time constraints for early crustal formation in Northern Nigeria by singel and multiple crystal U Pb zircon dating. In: Terra 7111 Meeting of European Union Geosciences, Strasbourg. Terra Abstracts 3. 506 Bruguier, O., Dada, S.. Lancelot. J.R., 1994. Early Archaean component ( > 3 . 5 G a ) within a 3.05Ga orthogneiss from northern Nigeria: U Pb zircon evidence. Earth Planet Sci. Lett. 125, 89 103. Caby, R., 1989. Precambrian terranes of Benin Nigeria and northeast Brazil and the Late Proterozoic south Athmtic fit. Geol. Soc. Am. Special paper 230, 145 158. Caby, R., Andreopoulos-Renaud, U., 1983. Age/t 1800 Ma du magmatisme sub-alcaline associe aux metasediments monocycliques dans la Chaine Pan-Africane du Sahara Central. J. Aft. Earth Sci. 1 (3/4), 193 197. Caby, R.. Bertrand. J.M., Black, R., 1981. Pan-African ocean closure and continental collision in the Hoggar Iforas segment, central Sahara. In: Kroner, A. (Ed.). Precambrian Plate Tectonics. Elsevier, Amsterdam, pp. 407 434. Caby, R., Sial, A.N., Arthaud. M., Vauchez, A., 1990. Crustal evolution and the Brasiliano Orogeny in Northeast Brazil. In: Dallmeyer, R.D.. Lecorche, J.P. (Eds.). West African orogens and Circum-Atlantic Correlatives, Springer-Verlag, pp. 353 376. Dada, S.S., 1989. Evolution de la crofite continentale au Nord Nigdria: apport de la geocbimie, de la g~ochimie, de la geochronologie U Pb et des traceurs isotopiques Sr, Nd et Pb. These de l'Universite de Montpellier I1, Montpellier, France. Dada, S., Respaut, J.P., 1989. La monzonite a fayalite de Bauchi (bauchite): nouveau temoin d'un magmatisme syntectonique pan-africain au nord du Nigeria. C.R. Acad. Sci. Paris 309(11).887 892. Dada, S.S.. Rahaman, M.A., in press. Archacau Lower Proterozoic crustal evolution in Nigeria. Aft. Geosci. Re,,'.
S.S. Dada /Precambrian Research 87 (1998) 65 74
Dada. S.S., Lancelot, J.R., Briqueu, L., 1989. Age and origin of the annular charnockitic complex at Toro, Northern Nigeria: U Pb and Sr evidence. J. Aft. Earth Sci. 9 (2), 227 234. Dada. S.S., Tubosun. I.A., Lancelot, J.R., Lar, A.U., 1993a. Late Archaean U Pb age for the reactivated basement of Northeastern Nigeria. J. Aft. Earth Sci. 16, 405 412. Dada, S.S.. Birck, J.L., Lancelot, J.R., Rahaman, M.A., 1993. Archaean Migmatite complex of North Central Nigeria: its geochemistry, petrogenesis and crustal evolution. 16th Int. Coll. African Geology. Mbabane, Swaziland, Geological Survey and Mines, vol. 1, pp. 97 102. Dada, S.S.. Briqueu. L., Lancelot. J.R., Harms, U., Matheis, G., 1995. Charnockitic and monzonitic Pan-African series from North-Central Nigeria: trace element and Nd. Sr. Pb isotope constrains on their petrogenesis. Chem. Geol. 124. 233 252. DePaolo. D.J., 1983. The mean life of continents. Estimates of continental recycling rates from Nd and Hf isotopic data and implications for mantle structure. Geophys. Res. Lett. 10, 705 708. Dickin, A.P., Halliday, A.N., Bowden, P., 1991. A Pb, Sr, Nd isotope study of the basement and Mesozoic ring complexes of the ,los Plateau. Nigeria. Chem. Geol. ( Isotope Geoscience Section) 94.23 32. Ekwueme, B.N., 1987. Structural orientations and Precambrian deformational episodes of Uwet area, Oban Massif, SE Nigeria, Precamb. Res. 34. 269 289. Ekwueme, B.N., 1990. Rb Sr ages and petrologic features of Precambrian Rocks from the Oban Massif, SE Nigeria. Precamb. Res. 47, 271 286. Ekwueme, B.N., Kroner, A., 1993. Preliminary zircon evaporation ages from migmatitic gneisses in Kaduna, N. Nigerai: evidence for Early Archaean IPre-Leonianl event in the Nigeria basement complex. NMGS 29th Annual Conference Abstr., Nigerian Mining and Geosciences Society, p. 61. Ferre, E., Deleris, J.. Bouchez, J.L., Lar, A.U., Peucat, J.J., 1996. The Pan African reactivation of Eburnean and Archaean provinces in Nigeria: structural and isotopic data. J. Geol, Soc. London 153. 719 728. Fitches, W.R., Ajibade. A.C., Egbuniwe. I.G., Holt, R.W., Wright, J.B., 1985. Late Proterozoic Schist Belts and plutonism in NW Nigeria. J. Geol. Soc. London 142, 319 337. Goldstein, S.L., O'Nions, R.K.. Hamilton, P.J., 1984. A Sm Nd isotopic study of atmospheric dust and partculates from major river systems. Earth Planet Sci. Lett. 10, 26 38. Grant, N.K., 1970. Geochronology of Precambrian basement rocks from lbadan. SW Nigeria. Earth Planet Sci. Lett. 10, 26 38. Grant, N.K.. 1978. Structural distinction between a metasedimentary cover and an underlying basement in the 600 Ma old Pan African domain of NW Nigeria. Bull. Geol. Soc. America 89. 50 58. Jahn, B.M., Auvrey, B., Shen, Q.H., Liu, D.Y.. Zhang, Z.Q., Dong. Y.J., Ye, X.J., Zhang, Q.Z.. Cornichet, J., Mace, J., 1988. Archaean crustal evolution in China: the Taishan corn-
73
plex, and evidence for juvenile crustal addition from longterm depleted mantle. Precamb. Res. 38, 381--403. Kennedy, W.G., 1964. The structural differentiation of African in the Pan-African (+500 Ma) tectonic episode. Res. Inst. African Geol., Leeds 8th Ann. Rpt Sci. Results 48. Lancelot, J.R., Boullier, A.M., Maluski, H., Ducrot. J., 1983. Deformation and related radiochronology in a late PanAfrican mylonitic shear zone, Adrar des iforas (Mall). Contrib. Mineral. Petrol. 82, 312 326. Leigeois. J.P., Bertrand, J.M., Black, R., 1987. The subduction and collision related Pan-African composite batholith of the Adrar des lforas (Mali): a review. Geol. J. 22 (Thematic issue). 185 211. McCulloch, M.T., Wasserburg, G.J., 1978. Sm-Nd and Rb Sr chronology of continental crust formation. Science 200, 1003 1011. McCurry. P., 1976. A general review of the geology of the Precambrian to Lower Paleozoic rocks of Northern Nigeria. In: Kogbe. C.A. (Ed.), Geology of Nigeria, 2nd ed. Elizabethan Publ., Lagos, pp. 13 37. Miller, C.F., Mittlefehdt, D.W.. 1982. Light rare earth element depletion in felsic magmas. Geology 10. 129 133. Ogezi, A.E.O., 1988. Origin and evolution of the basement complex of NW Nigeria in the light of new geochemical and geochronological data. h : Oluyude et al. (Eds.), The Precambrian Geology of Nigeria. Geological Survey of Nigeria Publication, Kaduna, Nigeria, pp. 301 312. Oyawoye, M.O., 1972. The basement complex of Nigeria. In: Dessauvagie, T.F.J., Whiteman, A.J. (Eds.), African Geology. lbadan University Press, lbadan, Nigeria, pp. 66 102. Oversby, V.M., 1975. Lead isotopic study of aplites from the Precambrian basement rocks near lbadan, southwestern Nigeria. Earth Planet Sci. Lett. 27, 177-180. Patchett. P.J., Chauvel, C., 1984. The mean life of continents is presently unconstrained by Nd and Nf isotopes. Geophys. Res. Lett. 11, 151 153. Patchett. P.J., Arndt, N.T., 1986. Nd isotopes and tectonics of 1.9 1.7Ga crustal genesis. Earth Planet Sci. Lett. 78, 329 338. Pidgeon. R.T.. van Breemen, O., Oyawoye. M.O., 1976. Pan African and earlier events in the basement complex of Nigeria. 25th IGC, Sydney. Abstr. 3, p. 667. Rahaman, M.A., 1976. Review of the basement geology of Southwestern Nigeria. In: Kogbe, C.A. (Ed.), Geology of Nigeria, 2nd ed. Elizabethan Publ., Lagos, pp. 36 56. Rahaman, M.A.. 1988. Recent advances in the study of the basement complex of Nigeria. 111: Precambrian Geology of Nigeria. Geological Survey of Nigeria Publication, Kaduna, Nigeria, pp. I I 43. Rahaman, M.A., Emofurieta, W.O., Vachette. M.C.. 1983. The potassic granites of Igbetti area: further evidence of the polycyclic evolution of the Pan-African belt in SW Nigeria. Precamb. Res. 22, 75 92. Rahaman, M.A.. Tubosun, I.A., Lancelot, J.R., 1991. U Pb geochronology of potassic syenites from SW Nigeria and the
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Precamhrian Research 87 (1998) 65 74
timing of deformational events during the Pan-African orogeny. J. Aft. Earth Sci. 13, 387 395. Taylor, S.R., McLennan, S.M., 1981. The composition and evolution of the continental crust: rare-earth element evidence from sedimentary rocks. Phil. Trans. R. Sot., London A 103. 381 399. Tubosun, 1.A., Lancelot, J.R.. R a h a m a n . M.A., Ocam O.O., 1984. U - P b Pan-African ages of two chamockite-granitc
associations from SW Nigeria. Contrib. Mineral Petrol. 88, 188 195. Van Breemen. O., Hutchinson, J.. Bowden, P.. 1975. Age and origin of the Nigerian Mesozoic granites Rb Sr isotope study. Contrib. Mineral Petrol. 51), 157 172. Van Breemem O., Pidgeon, R.T., Bowdem P., 1977. Age and isotopic studies of some Pan Afi'ican granites from North Central Nigeria. Precamb. Res. 4. 307 319.
Precambrian Research 87 (1998) 65 74
Crust-forming ages and proterozoic crustal evoluton in Nigeria: a reappraisal of current interpretations S.S. Dada Geology Programme, Abubakar Tafawa Balewa UniversiO', Bauchi, Nigeria Received 18 June 1996; accepted 22 September 1997
Abstract
Radiometric ages in the Nigerian basement cluster around 3.5, 3.1 3.0, 2.7 2.5, 2.1 1.8 and 0.6 Ga. However, not all these ages correspond to times of crust formation. There are significant discrepancies between U-Pb and Rb-Sr ages and Nd and Sr model ages with a definite trend for the orthogneisses and granitoids analysed. Results show good agreement between model ages (T TM, T st) and emplacement ages for early Archaean and imply that these model ages date the time of crust-mantle differentiation. The Archaean/Early Proterozoic boundary was most probably a major crust-forming period in SW Nigeria as indicated by U-Pb zircon ages that are in agreement with Nd model ages in that segment of the Nigerian basement. On the other hand, Late Proterozoic and younger rocks, which include migmatized gneisses, Pan African granitoids and Jurassic Younger Granites, have model ages in excess of their crystallization ages. Such model ages that lie between Pan African (or Jurassic) and well-known older geologic ages do not correspond to any specific crust-forming event. Rather they are mean crust-residence ages reflecting contributions from juvenile material and remelted older Archaean crustal components. The overall scenario is consistent with widespread Archaean crust and its involvement in the genesis of Late Proterozoic and younger crust in the Nigerian basement. This is in agreement with an intra-cratonic setting far removed from the suture to the west. © 1998 Elsevier Science B.V.
Keywords: Anorogenic magmatism; Crust-formation; Model ages; Nigeria; Pan African
1. Introduction
It is generally believed that the greater part o f crustal g r o w t h t o o k place by Late Archaean. The identification o f juvenile p o s t - A r c h a e a n crust as opposed to reworked pre-existing material is often difficult in m a n y terrains (Allegre and Ben O t h m a n , 1980; Taylor and M c L e n n a n , 1981; Allegre and Rousseau, 1984). A l t h o u g h the Nigerian basement (Fig. 1) has for long been 0301-9268/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved. PH S0301-9268 (97)00054-5
regarded as p o l y m e t a m o r p h i c , unequivocal geochronological and structural evidence in the postA r c h a e a n is available only for the Pan-African. For the latter, a continental collision plate tectonic model (Burke and Dewey, 1972; Black et al., 1979; Caby, 1989) has been accepted to be at the origin o f the reactivation o f the D a h o m e y a n shield east o f the G h a n a - T o g o - B e n i n suture zone. In this study, available data from Sm Nd, R b Sr and U P b isotopic systematics are used to identify
66
X S . Datkl
Precamhrian Res'earuh ,~¢7 f 199~U 65 74
(a)
(c) 5°
5°E
lOOE
Oft"
.
" i
i
H , i,">:,,
:','~ ,: ~;~.i.!:~: i
i
" i
o Ii
I
16o Kin. I
i
I
NIGERIA
Fig. 1 (a) Location of the Nigerian shield in the major structural units of West Africa and the Brazilian belt l'ollowing the preMesozoic reconstruction of Caby (I 989). (b) Location of Nigeria in the generalized map of Africa. (c) Simplified geological map of Nigeria [alter Oyawoye ( 1972 )]. I, Cretaceous and Younger sediments: 2, Jurassic Younger granites: 3, older granites: 4, undifferentiated metasediments; 5, quartzite and quartz schist: 6, Precambrian basement (undifferentiated): 7, phanerozoic sediments (generalized); 8, cratons (2.0 Ga or older): 9, Upper Proterozoic o[" Western Hoggar: 10. reworked Archaean and Proterozoic cover: II. major shear zone: 12. Late Proterozoic suture: 13, major thrust. Egbe Kabba Okene "Proterozoic" region.
major periods of new crustal formation and in particular, to distinguish between juvenile crust and reworked pre-existing crust.
2. Geological Setting and Geochronology About half of the Nigerian crystalline crust is buried beneath Cretaceous and younger sediments: while the other half outcrops largely in the northcentral, southwestern and in three smaller regions from north to south along the country's eastern boundary with Cameroun (Fig. 1 ). Recent U Pb isotopic work in North Central Nigeria (Dada0 1989; Bruguier et al., 1991, 1994;
Ekwueme and Kroner, 1993) has confirmed earlier indications (Ogezi, 1988) of Early to MidArchaean (3.5-3.0 Ga) crust in the Nigerian basement. Late Archaean (2.4 2.5 Ga) U-Pb ages have been reported from gneisses in SW Nigeria (Oversby, 1975: Pidgeon et al., 1976: Rahaman, 1988) and in NE Nigeria (Dada et al., 1993a). Early Peroterozoic ages (2.1 1.8 Ga) have been obtained on gneisses in SW Nigeria (Grant, 1970; Rahaman et al., 1983; Rahaman, 1988; Annor, 1995). Late Proterozoic ( ~ 6 0 0 M a ) U Pb ages have been reported from both migmatitic gneisses and granitoids in practically all the shield areas in Nigeria; Tubosun et al. (1984), Rahaman et al. (1991) in the SW; Van Breemen et al. (1977),
S.S. Dada / Precambrian Research 87 (1998) 65 74
Dada and Respaut (1989), Dada et al. (1989) and Ferre et al. (1996) in North Central and Ekwueme (1990) in the SE. On the other hand, lower intercept U Pb ages from Archaean zircons cluster ca 600 Ma, arguing for the main tectonometamorphic imprints in Pan African times.
3. Crust formation and Sm-Nd isotopic systematics Ever since the formation of primitive continental crustal masses in the Earth's earliest history, large proportions of them have been subsequently tectonized and recyled. Conversely, newly formed crust separated directly from the mantle has been added in many regions at specific times through geologic history. The considerable progress which isotope geochemistry has witnessed since the early 1970s has helped to resolve the fundamental problem of distinguishing between the reworked crust and juvenile additions (Armstrong, 1981 ; DePaolo, 1983; Patchett and Chauvel, 1984; Allegre and Rousseau, 1984; Patchett and Arndt, 1986). It is in resolving the issues involved herein that the Sm Nd isotopic system has found both geochronologic and petrogenetic applications. Sm and Nd are parent daughter rare-earth elements ( R E E ) that do not fractionate in the geological environments except when phases with significantly different distribution coefficients for Sm and Nd participate in melting, crystallization or metamorphic processes. One such process is mantle melting when heavy rare-earth element ( H R E E ) rich phases such as garnet are present. In such cases, the lighter, more hygromagmatophilic Nd leaves the heavier Sm in the residue and gets incorporated into the melt which leaves the mantle with newly forming crust. The residual mantle is thus gradually depleted with time in Nd and enriched in Sm. Except for certain granulite facies rocks, garnet is not commonly involved in intra-crustal differentiation, metamorphism, crustal melting and weathering. Hence, Sm Nd fractionation is practically absent in crustal processes (McCulloch and Wasserburg, 1978). It is significant to note that this is in marked contrast to the commonly used, older dating methods such as U Pb and R b - S r systems in which both mantle
67
melting and intra-crustal fractionation processes produce melts that are rich in the parent elements U and Rb. The S m - N d isotopic systematics is thus able to 'see back' to the time in the past when continental crustal material separated from the mantle. Following the interpretation of McCulloch and Wasserburg (1978), it is only such mantle separation ages that can be meaningfully referred to as crust formation ages dating the times of addition of juvenile crust from the mantle. Calculations of model ages from this system therefore assumes constancy of the S m - N d ratios subsequent to mantle extraction except in very few cases of highly silicic (> 75% SiO2) magmas where light rare-earth element (LREE)-rich phases (sphene, monazite, etc.) are abundant (Miller and Mittlefehdt, 1982). All Sm Nd data presented in Table 1 used the following parameters: • Present-day 143Nd/144Nd = 0.512638 and 147Sm/a44Nd = 0.1966 (McCulloch and Wasserburg, 1978). • Depleted mantle=S43Nd/144Nd=0.51315 and 147Sm/144Nd=0.2137 (Jahn et al., 1988). • All other expressions and calculations follow the techniques of McCulloch and Wasserburg (1978).
4. The Nigerian Basement and Crustal Growth The Nigerian basement is better known as part of the Pan-African mobile belt of Kennedy (1964) as all crystalline shield areas have been reworked ca 600 Ma. Apart from the Jurassic Ring Complexes, most crystalline rocks of the Nigerian basement bear Pan-African tectonic and mineralogical imprints. Pre-Pan African ages in Nigeria are therefore protolith ages. 4.1. The archaean
The orthogneisses of Kaduna area (Dada, 1989; Dada et al., 1993b) have Nd model ages that overlap U - P b zircon crystallization ages ( Table 1 ), showing that the rocks formed from material that separated from the mantle during a crust-forming
68
S.S. Da&l Pre¢'anlhrkm Research 87 (1998) 65 74
Table 1 Geologic { U Pb, Rb Sr) and model (Nd, Srl ages for rocks of the Nigerian Basement and the Jurassic ring complexes
Ts~
($T-t
3.57
3,49Ga
10Ma
3.54
3.18 Ga
440
3.51
2.73 Ga
760
_., 6
2.36 Ga
[_ithology
Geological age
T...,d (Ga)
Kadunagranodioritegneiss
3 . 4 6 G a ( U Pb) 3 . 4 6 G a ( U Pb) 3.1 Ga ( U Pb, Rb St)
Kaduna carl 3 gneiss
cnd (t)
Tnd Reference
Kadtinu late gneiss Ibadan Aplile ()do ogun gneiss lie-fie grey gneiss lie-fie granilc gneiss lgbetti augen gneiss Egbe gneiss kabba-okemle gneiss
2.75 Ga {Rb Sr) 2 . 7 5 G a ( P b Pb) 2.5 Ga (U Pb) 2.3 Ga439 Ma ( U Pb) 1.85Ga 5 5 0 M a I U Pb) 1.9 Ga {Rb Sr)
Tildcn fulani migmatite Badiko granite gneiss Okemae granodiorile Gn
2.5 Ga500 Ma ( U Pbl 2 . 5 G a 5 0 0 M a ( U Pb) 2.1 (]u {U Pbl
2.51 1.80 2.10 2.10
- 1.3 680 Ma 14.6 740Ma 2.78 Ga
1300 Ma 1600Ma
Sarkin pawa nyntectonic migmatite I kad-abuji rd I Badiko s3nlectonic diorile lkerre massive charnockiie &kure gneissic dmrnockite Akure porplD ritic granite Idanre gneissic charnocki/e Idanre massive charnockite ldam'e porphyritic grunile Toro Biot-Hbde granite Toro charnockitic diorite Bauchi quartz lh,,alite monzonite (bauchiic)
635 Ma ( k T Pb)
1.51)
-3.4
865 Ma
623 Ma ( U 620Ma(U 634Ma(U 621 Ma (t; 580Ma(U 593 M a ( U 587Ma(U 61)7 Ma L 638 Ma L 638 Ma 15
1.91)
Toro migmcltile Toro anaicclic granite
581 Ma 616 Ma 751 Ma 170Ma Rb Sr WR) 170 Ma 170 Ma
Toro Ring Ring Ring
migmatite graniie complex 369 complex 473 complex 412
Pb) Pb) Pb) Pb) Pbl Pb) Pbl Pb) Pb) Pbi
710 Ma
12.3 780 Ma
Bruguier et al. (1994) Ekwueme and Kroner (1993) Dada (1989). Bruguier el al. ( 19941 Dada and Ralmman (in press) Oversby ( 1975 ) Pidgeon et al. 11976) Rahaman (1988) Rahaman (1988) Rahaman ( 19881 Dada and Rahaman (in press)
Duda et al, 11993a,bl Armor ( 1995 }: Dada and Rahaman (in press) Dada (unpublished data)
1297 Ma Tubosunet a1.(1984)
2.10 2.50 1.70 1.75 1.88 2.71 1.15 1.46 1.92
e v e n t a b o u t , o r n o t l o n g b e f o r e 3.5 G a ((ST O ) . U Pb zircon and whole-rock Rb Sr data from adjacent rocks of the same area indicate subsequent isotopic disturbance and rehomogenization. This would suggest two metamorphic events at 3.1 Y 0 a n d 2.75 G a , b u t t h e l o w e r i n t e r c e p t a g e o f 612 M a ( B r u g u i e r et al., 1994) d o e s n o t f a v o u r t h i s i n t e r p r e t a t i o n . T h e h i g h i n i t i a l STSrff°Sr r a t i o s of 0.70749+0.00046 and 0.70768+0.00042, respectively, indicate reworking of pre-existing components. H o w e v e r , t h e T na a g e s f o r t h e s e r o c k s r e m a i n m u c h o l d e r a n d a v e r a g e 3.5 G a . A
--12.6 670Ma - 15.6 1.07Ma 3.9 1.80 Ga 1.86 14 12.4 1.3 3.2 5.6
1493 1915 1062
Dadaet a1.(1989) Dadaet a1.(19891 Dada and Respaut ( 1989 )
1280 1380 1960 880 1290 1750
Ferre et al. (19961 Ferre el al. ( 1996} Ferre el ul. ( 19967 Van Breemen et al. (1975) Dickin et al. ( 1991 Dickin et al. ( 1991 I
s i m i l a r m e t a m o r p h i c a g e o f 2.75 G a w a s p r o p o s e d f o r l b a d a n a p l i t e s in S W N i g e r i a ( O v e r s b y , 1975). The only reasonable interpretation for such rocks is t o c o n s i d e r t h e t w o a g e b r a c k e t s (3.1 3.0 a n d 2.75 G a ) as p u r e l y m e t a m o r p h i c a g e s d u r i n g w h i c h there was no vertical addition of new crust from the mantle. 4.2.
Archaean/proterozoic
bounck,'v
U P b z i r c o n c r y s t a l l i z a t i o n a g e s o f 2.4 2.3 G a oil I l e - l f e g r e y g n e i s s e s f r o m S W N i g e r i a ( P i d g e o n
69
S.S. Dada : Precambrian Research 87 (1998) 65 74
et al., 1976: Rahaman, 1988) and NE Nigeria (Dada et al., 1993a) are in agreement with T Na ages of 2.56-2.51 Ga (Table 1) on orthogneisses from Egbe and Kabba Okene areas (Dada and Rahaman, in press), end values at 2.5 Ga for these samples plot largely above the Chondritic Universal Reservoir (CHUR). They constitute a supporting evidence for juvenile crustal addition at the Archaean/Proterozoic boundary with little or no time lag between crustal formation and rock crystallization (6 T-O).
involving an extended protolith history. It is expected that isotopic data from the better defined Proterozoic metasedimentary cover within the Schist Belt to the west (McCurry, 1976; Caby, 1989) will aid in resolving some of the incertitudes. Such old model ages (T>>t) have also been obtained on the basement and Mesozoic Ring Complexes of the Jos Plateau (Dickin et al., 1991 ). These ages are strongly biased towards an Early Proterozoic (Eburnean) event known in many parts of North and West Africa (Lancelot et al., 1983; Bertrand et al., 1986: Abouchami et al., 1990: Boher et al., 1991) and NE Brazil (Caby, 1989) from basic to intermediate assemblages. However, there is till date no unequivocal evidence for regarding the NE Nigerian basement as belonging to the 2.1 Ga terrane. Neither U - P b zircon ages nor other dates suggest this. Available data restrict the 2.1 Ga event in the Nigerian basement to the SW with T Nd and U - P b ages in agreement only for the Okene granodiorite gneiss (Table 1 ). This spatial restriction has led to the fundamental question on the true nature and significance of the 2.1 Ga ages in Nigeria. Unequivocal Palaeoproterozoic tectonic landmarks that can be widely
4.3. The Nigerian proterozoic The T Nd ages calculated with respect to the depleted mantle (2.1-1.5 Ga) from both migmatite gneisses and granitoids are in general older than the U Pb zircon crystallization ages (Table 1 ). In Fig. 2, where initial Nd values are plotted against U Pb ages, the Pan African granitoids have negative values of - 1 5 to - 4 and therefore plot not only below the depleted mantle ( D M ) evolution trend but also below the C H U R . This would imply a contamination by the pre-existing Archaean crust or magma derivation via a multistage process
.10
~
1.0 T
D
1.5 ~
2.1 T antl¢
2.S T
EVOlutiontV AP
0
-10 . . . .
LU -20
-30
0.5
1.0
1.5
2.0
2.5
3.0
Time (Ga) Fig. 2. end diagram showing possible mixing between proposed Early Proterozoic (2), Late Proterozoic (3) and Mesozoic (4)juvenile crusts adn the Archaean (_> 2.5 Ga) felsic component ( I ) of the Nigerian basement complex. Depleted mantle evolution trend assumes a linear growth from a D M source with present-day e~,d - + 1 0 (Jahn et al., 1988). AP and P represent end fields for TNa=2.56 2.51 k G a and 2.1 1.5 Ga ages, respectively.
70
S, S. Da&t, Precamhrian Re,search 87 / 1998) 65 74
applied within the Schist Belt are yet to be identified. The same sequence of deformation and metamorphic events that affected the Archaean gneisses have been recognized in the 2.1--1.8 Ga gneisses and the metasediments of the Schist Belt, not only in Nigeria but also in the equivalent predrift Borborema Province (Fig. 1) of NE Brazil (Caby, 1989; Caby et al., 1990). Available data appear equivocal with regard to the 2.1 G a ages. They are either related to an accretionary event or are purely metamorphic. If the latter, they should then be interpreted as shield-stabilization to Late Archaean/Early Proterozoic crust (Fig. 2). This will be much like the anorogenic 1.9 1.8 G a magmatism in SW Nigeria ( R a h a m a n et al., 1983; Ajibade et al., 1987: R a h a m a n , 1988) and the Iforas region of Mali (Caby and AndreopoulosRenaud, 1983). In the absence of further 2.1 Ga U - P b age data, the best interpretation that can be given to all such model ages that lie between the well established Archaean and Pan African U - P b zircon crystallization ages, including those of Ferre et al. (1996), is that they are mean crustal residence ages (Arndt and Goldstein, 1987) that represent average ages of mixtures of Pan African and remelted Archaean crust ( D a d a et al. (1995). Annor (1995) obtained a precise U Pb zircon age of 2.1 G a on the K a b b a - O k e n n e granodiorite gneiss. In Fig. 2, the eNa values calculated to zircon crystalization age of 2.1 Ga, for a presumed Proterozoic Egbe K a b b a Okene region, have negative values that plot below the C H U R . The samples have model ages of 2.56 2.51 G a ( D a d a and Rahaman, in press) and therefore suggest that the crust-forming process in the region began earlier than the measured zircon crystallization age of 2.1 G a or that some Archaean crustal component was incorporated as the new('?) Lower Proterozoic crust was formed. The Late Archaean model ages suggest that the K a b b a Okenne and indeed the SW Nigerian basement is of Archaean association, regardless the strong Pan African fabric on the terrain.
4.4. The Nigerian Mesozoic' Ring Complexes The application of multiple radiogenic tracers (Sr, Nd, Pb) by Dickin et al. ( 1991 ) on the Jurassic
Ring Complexes shows the great potential of the Sm--Nd isotopic system to 'see back' into the past and identify old crustal protoliths. Nd model ages ( 1.92 1.15), calculated from Dickin et al. ( 1991 ) as shown on Table 1 indicate large a T in excess of the crystallization age of 170 Ma (Van Breemen et al., 1975). Any juvenile material at 170 Ma must therefore have been essentially contaminated by older components during m a g m a generation and/or ascent. Because the Jurassic Complexes would have contributions from at least two different sources (Archaean and Pan-African, Fig. 2), its proportion of juvenile material must be insignificant.
5. Implications for crustal growth in Nigeria T Nd and U - P b data from Northern Nigeria confirm Early Archaean (3.5 Ga) crustal addition and subsequent disturbances and rehomogenizations at 3.1-.3.0 and 2.75 G a have been recorded in the north and southwest as constrained by U Pb zircon ages (Dada, 1989; Bruguier et al., 1994) and whole-rock Rb Sr ages (Oversby, 1975; Dada and Rahaman, in press). The agreement between Nd model ages and U--Pb ages is also in favour of crust-formation during Late Archaean times (2.5 Ga). Bruguier et al. (1994) suggested that the 3.1 3.0 G a (3050+23 Ma) U - P b zircon age approximates the age of emplacement and crystallization of the granodioritic m a g m a from a crust of extended pre-history (3.56Ga). This is in agreement with Nd model ages that put the extraction age of the crust from the mantle in North Central Nigeria at 3.55 Ga. This interpretation is in agreement with the views of Dada et al. (1993b) in which both Rb Sr and U Pb data put the emplacement age of early gneiss at 3.1 3.0 Ga, with an extended protolith history of 400 Ma. back to 3.5 Ga. The suggestion of a metamorphic event at the well known regional Liberian Orogeny (2.75 Ga) in West Africa has not been corroborated by U Pb zircon data within the Nigerian basement. Only partial rehomogenization or disturbance of the isotopic system can be inferred. The present view of the nature of the Nigerian
71
S.S. Dada / Precambrian Research 87 (1998) 65 74
Proterozoic crustal growth needs to be rethought, particularly because unequivocal evidence of subduction induced magmatism appears to be lacking. Extensive Sm Nd isotopic work on a global scale has shown that: (1) the 2 . 2 - 1 . 8 G a age range represents the average 'crustal residence age' of the upper crust (Allegre et al., 1983; Goldstein et al., 1984), which may not be related to any crustforming event. (2) Nd model ages regularly decrease with the geologic ages and after 2 Ga, the curve flattens and tends to an asymptotic value at ca 1.8 Ga (Allegre and Rousseau, 1984). Therefore, great caution should be exercized in the interpretation of the isotopic data particularly the Nd model ages from these post-Archaean rocks with a polymetamorphic history. Underplating in areas close to Archaean shields can be envisaged for the Mid-Proterozoic which was a quiscent period (lacking magmatism) that followed the 1.9 1.8Ga anorogenic magmatism (Rahaman et al., 1983; Ajibade et al., 1987; Rahaman, 1988). The identification of the Pan-African Ghana Togo Benin suture zone (Burke and Dewey, 1972; Black et al., 1979; Caby, 1989) to the west emphasizes the mid-plate nature of the Nigerian shield 250-800 km eastwards, depending on the sample location. Field evidence has for a very long time (Oyawoye, 1972; McCurry, 1976; Rahaman, 1976; Grant, 1978; Fitches et al., 1985; Ajibade et al., 1987) recognized the Nigerian basement as an assemblage of contrasted terranes with a well developed metasedimentary cover to the west and a largely vestigial crystalline terrain to the east. It is a continuum between the Hoggar to the north (McCurry, 1976) and the Borborema Province to the south [Caby (1989); Caby et al. (1990); see Fig. l]. Recent structural studies (Black et al., 1994; Ferre et al., 1996) have strengthened the view on the contrasted nature of the Nigerian basement. While some vestiges of molasse assemblages have been recognized in the Anka Belt of N W Nigeria (Ogezi, 1988), their periods of initial creation and subsequent accretion to the Proterozoic continental mass are the more urgent questions that need the attention of multiple
isotopic tracer techniques and absolute chronology (Sm Nd, U Pb). Upper mantle convective disturbances resulted in partial melting of the underplated Archaean protoliths during the continental collision in Neoproterozoic times as part of the large-scale Pan-African/Braziliano tectonothermal event. The widespread precise U - P b zircon ages on granitoids of this age; their characteristic deformational and metamorphic patterns (Rahaman, 1976; McCurry, 1976: Ajibade, 1976; Fitches et al., 1985; Annor and Freeth, 1985; Ekwueme, 1987) in Nigeria, N W Africa and N E Brazil (Bertrand et al., 1986; Leigeois et al., 1987; Caby, 1989) emphasize the orogenic nature of the 600 Ma event. Fig. 3 summarizes the relationship between available geologic and Nd model ages, emphasizing at least a twostage accretionary model for Proterozoic and younger rocks with consistent older Nd model ages that argue in favour of significant involvement of Archaean felsic crust in their genesis. The large 6t values correspond to significant time intervals between mantle separation ages and metamorphic emplacement (geologic ages) at latter times. The presence of Archaean crustal segments in the Nigeria basement as in Central Hoggar is compatible with an intracratonic setting. The relict Archaean ages strongly support the continental collision model of Caby et al. (1981) and Caby (1989) that the region and the West African and Sao Luis Cratons must have been part of the same crustal province which was rifted apart, reunited and in part reworked in the Late Proterozoic (Dada et al., 1995; Dada and Rahaman, in press)
3.0z.o-
°
"I
1.0-
0.0
I
0.0
1.0 2!0 Geotogic
Age
3!0 (6a)
4-0
Fig. 3. Comparison of geologic and Nd model ages (see Table 1 for data sources).
72
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PrecanThrian Research 87 (1998) 65 74
in contrast to the largely juvenile Birrimian rocks (Abouchami et al., 1990: Boher et al., 1991 ). It is therefore clear that the post Archaean ages obtained on rocks in the Nigerian basement cannot be interpreted as representing periods of purely juvenile additions. Such rocks give mean crustal residence (model N d ) ages which neither coincide with U - P b zircon ages nor well established structural or other line of evidence of an orogenic event.
Acknowledgment The author wishes to thank M. Boher and R. Caby for their constructive reviews of the manuscript; M. A. Rahaman and R. Caby for their interest and encouragement in crustal evolution studies o f the Pan African/Brazilian belt. Many thanks to L. Ochibe for drafting the figures.
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