Obliquely convergent tectonics and granite emplacement in the Trans-Saharan belt of Eastern Nigeria: a synthesis

Precambrian Research 114 (2002) 199– 219 www.elsevier.com/locate/precamres

Obliquely convergent tectonics and granite emplacement in the Trans-Saharan belt of Eastern Nigeria: a synthesis Eric Ferre´ a,*, Ge´rard Gleizes b, Renaud Caby c a

Department of Geology and Geophysics, Uni6ersity of Wisconsin — Madison, Madison, WI 53706, USA Laboratoire de Pe´trophysique et Tectonique, Uni6ersite´ Paul Sabatier, 38 rue des Trente-Six-Ponts, UMR CNRS 5563, 31400 Toulouse, France c Laboratoire de Tectonophysique, Uni6ersite´ des Sciences et Techniques du Languedoc-Montpellier II, Place E. Bataillon, 34095 Montpellier, Cedex 05, France b

Accepted 18 October 2001

Abstract The Eastern Nigeria terrane belongs to the 3000 km-long Trans-Saharan belt which was formed in the Neoproterozoic, between 750 and 500 Ma by continental collision between the converging West African craton, Congo craton and East Saharan block. The study area consists mostly of gneisses and migmatites that underwent granulite facies metamorphism ( \800 °C, 800 MPa). In contrast with Western Nigeria and Cameroon, no basement-cover relationship has been identified which is in agreement with a monocyclic metamorphic history. Around 640 Ma, the area underwent a nappe tectonics with eastward displacement, emplacement of Bt– Ms granites and granulite facies peak. Later on, around 615 Ma, nappes were towards the north. Numerous Hbl– Bt granite plutons were emplaced around 585 Ma during north–south strike-slip regional deformation. The anisotropy of magnetic susceptibility fabric helps in defining strain localization along narrow north– south to NNE– SSW vertical shear-zones during the exhumation history. Exhumation of the terrane, assessed using U – Pb, Ar– Ar and Rb– Sr methods, was presumably slow with uplift rates around 0.2–0.5 mm/year. Preliminary estimates of contemporaneous horizontal movements suggest that they were one order of magnitude larger. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Granite; Nigeria; Pan African; Tectonics; Neoproterozoic; AMS

1. Introduction Granite magmatism and orogeny are often associated in time and space. This connection occurs in a variety of tectonic settings and is

* Corresponding author. Fax: + 1-608-262-0693. E-mail address: [email protected] (E. Ferre´).

reasonably well understood, particularly in Phanerozoic examples. The causal link between plate movement and Proterozoic or older granite intrusions is, at times, more controversial than for recent examples. Still, older examples are particularly interesting for two reasons, they may point to differences in the deformation of the continental lithosphere and they may show deeper levels of orogenic belts.

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Obliquely convergent orogens are a favorable tectonic setup which leads to continuous and massive granite magma production (e.g. Petford et al., 1994). This setting is favorable for granite magmatism because the continental crust becomes thicker and, therefore hotter, which, together with heat advection from the mantle, promotes crustal melting. The Trans-Saharan Belt in Nigeria is a good laboratory to study granite magmatism and orogeny because it is part of continental-scale oblique collision zone, 4000 km-long and 1000 km wide, and because it has a large number of very well exposed granite intrusions. For this study, we have applied the Anisotropy of Magnetic Susceptibility technique (AMS; Borradaile and Henry, 1997), as a strain analysis tool for three plutons representative of eastern Nigeria: the Solli Hills granite pluton (Ferre´ et al., 1995), the bimodal granite– diorite Toro pluton (De´ le´ ris et al., 1996) and the granite– monzonite Rahama pluton (Ferre´ et al., 1997). This paper presents a new synthesis of our results on the granites and a broader tectono– metamorphic study of their host-rock.

metamorphic assemblages with preserved coesite (Caby, 1994; Jahn et al., 2001), in Togo by kyanite eclogites and in Ghana by high-pressure granulites (Attoh, 1998). The movement of nappes during the initial stage of convergence and crustal thickening was to the west or southwest (Caby, 1989). The late-orogenic tectonics is characterized by North–South to NNE dextral strike-slip deformation (e.g. Djouadi et al., 1997) mostly localized along the continental-scale shear-zones and faults,

2. Neoproterozoic tectonic setting of Nigeria The Neoproterozoic Trans-Saharan belt formed between 750 and 500 Ma by accretion of terranes between the converging Archaean blocks of the West African Craton, the Congo Craton and the East Saharan Block (Fig. 1). The East Saharan Block was probably a craton up until 700 Ma (Black and Lie´ geois, 1993) when it was largely reactivated except in a few areas. The Trans-Saharan belt is characterized by high-grade metamorphism, early thrust-nappe development, numerous granite intrusions and late orogen-parallel tectonics (e.g. Black and Lie´ geois, 1993). The Aı¨r –Hoggar segment of this belt (Fig. 1) is formed by oblique docking of north– south elongated terranes (e.g. Lie´ geois et al., 1994). The initial lithospheric plate convergence was accommodated along a Neoproterozoic east-dipping subduction zone. The main Pan-African suture is marked in Mali by 620 Ma old ultra-high-pressure

Fig. 1. Geological sketch map of the Hoggar – Aı¨r – Nigeria province (modified from Wright, 1985 and Caby, 1989) showing the Neoproterozoic Trans-Saharan belt resulting from terrane amalgamation between the cratons of West Africa and Congo and the East Saharan block. Terrane boundaries so far recognized by Black et al. (1994), Lie´ geois et al. (1994), Ferre´ et al. (1996). The Massenya – Ounianga positive gravimetric anomaly is considered as a possible suture zone (Freeth, 1984; Fairhead and Okereke, 1987).

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such as the Kandi-4°50 fault (e.g. Caby, 1989). Undeformed dacitic volcanics injected along such faults around 500 Ma (K– Ar dating, McCurry and Wright, 1977) mark the end of the orogeny. We suggest that the transpressive tectonics and terrane accretion model proposed by Black et al. (1994) for Hoggar, may also apply to Nigeria. In Nigeria, the existence of two main terranes, Western Nigeria and Eastern Nigeria, has been proposed by Ferre´ et al. (1996) on the basis of: 1. a 500 km-long N– S lineament visible on Landsat images, separating to the west and to the east two domains with distinct tectonic fabrics (Ananaba and Ajakaye, 1987); 2. contrasted metallogenic domains with Au deposits, banded iron formations and greenstone-type deposits to the west, and U deposits to the east (Woakes et al., 1987); 3. Archaean basement with monocyclic cover to the west and Eburnian protolith with no cover to the east (Dickin et al., 1991; Ferre´ et al., 1996); 4. total magnetic field map showing two contrasted domains with distinct anomaly wavelengths on both sides of the assumed boundary (Ajakayie et al., 1991). The Western Nigeria terrane has been the focus of a number of studies showing the remarkable continuity of structures from north to south (see references in Caby, 1989). This terrane displays numerous characteristics of an Archaean basement such as gray gneisses showing TTG (tonalite–trondjhemite– granodiorite) affinities, amphibolites displaying greenstone belt affinities, Archaean U–Pb zircon ages and numerous Nd model ages older than 2.7 Ga (Bruguier et al., 1994; Dada, 1998). By contrast, the Proterozoic cover consists mostly of monocyclic metasediments of greenschist facies metamorphic grade (e.g. Fitches et al., 1985) and amphibolite facies grade in the south (Caby, 1989). Voluminous granitic plutons and widespread migmatization, both dated between 620 and 580 Ma, attests that this region has been substantially affected by tectono –metamorphic events of Late Neoproterozoic age (Tubosun et al., 1984; Dada, 1998). The Eastern Nigeria terrane, by contrast, is characterized by the lack of low-grade schist belts

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and by a Neoproterozoic migmatitic complex intruded by two successive Neoproterozoic granite suites (Wright, 1971): Bt–Ms granite around 605 Ma and Hbl–Bt granite around 580 Ma (Table 1). The Neoproterozoic Hbl–Bt granites display trans-alkaline compositions and similar ages (Ferre´ et al., 1998). The eastern Nigeria terrane is bounded to the east by the N40°-trending Massenya–Ounianga gravimetric anomaly (Fig. 1), interpreted as a hidden suture (Freeth, 1984; Fairhead and Okereke, 1987). Beyond Eastern Nigeria, to the East, the Neoproterozoic units show a low-grade volcano-sedimentary cover (e.g. Ngako et al., 1992) and an Archaean basement (e.g. Ne´ de´ lec et al., 1993).

3. Neoproterozoic migmatites of Eastern Nigeria

3.1. Petrography The Eastern Nigeria terrane is overlain by highgrade, migmatitic metasediments intruded by several voluminous Proterozoic plutonic suites (Figs. 2 and 3). By contrast with Western Nigeria or Cameroon, our new field observations indicate no basement-cover relationship within the Proterozoic units. Most rock types are coarse grained and include banded migmatite (Fig. 4a), paragneiss, anatectic granite, granitic gneiss, kinzigite, charnockite, diorite, amphibolite, quartzite and calc-silicate. Kinzigite and calc-silicate form boudins within gneiss and migmatite. Lenses of almost pure sillimanite and almost pure garnet occur conformably with the layering of migmatites. Igneous charnockites form dikes of a few tens of centimeters in width, cutting through the migmatites and hosting angular xenoliths of mafic granulites. Small gabbroic bodies of a few tens of meters in dimension occur sporadically and are boudinaged within migmatites. All these rock types are intruded by early biotite–muscovite granites forming elongate plutons, which themselves are cut by late biotite–hornblende granites usually forming more rounded intrusions. Some sedimentary features of the protolith, such as bedding (Fig. 4b) or cross-bedding in quartzites, are preserved in

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Table 1 Compilation of radiometric dates from Eastern Nigeria Age/error (Ma)

Isotope method

Material

Rock

Locality

References

530925 535935 530925 535935 510920 546920 510920 51798 5399 8 52598 5609 10 5479 15 52695 5119 10 512910 56095 56595 6059 10 618910 623920 63894 58597 589911 57999 6569 5 6649 17 5829 3 581910 616912 605928 665916

K–Ar K–Ar K–Ar K–Ar K–Ar K–Ar K–Ar Rb–Sr Rb–Sr Rb–Sr Rb–Sr Rb–Sr Rb–Sr Rb–Sr Rb–Sr Ar–Ar Ar–Ar U–Pb 206–207 U–Pb 206–207 U–Pb (U.I.) U–Pb (U.I.) U–Pb (U.I.) U–Pb (U.I.) U–Pb (U.I.) U–Pb (U.I.) U–Pb (U.I.) U–Pb (U.I.) U–Pb evaporation U–Pb evaporation U–Pb evaporation U–Pb evaporation

Bt Hbl Bt Hbl Bt Ms Kfs WR/Bt Bt WR/Bt WR/Bt Ms WR WR Bt Hbl Hbl Zircon Monazite Zircon Zircon Zircon Zircon Zircon Zircon Zircon Zircon Zircon Zircon Zircon Zircon

Charnockite Charnockite Charnockite Charnockite Granulite Pegmatite Bt granite Monzonite Bt granite Monzonite Migmatite Pegmatite Felsic dike Hbl–Bt granite Granite Hbl–Bt granite Hbl–Bt granite Bt–Ms granite Gneiss Metadiorite Monzonite Charnockite Hbl–Bt granite Hbl–Bt granite Granite Granite Hbl–Bt granite Migmatite Anatectic granite Migmatite Schist

Bauchi Bauchi Bauchi Bauchi Dass Jos Rukuba Bauchi Nassarawa Eggon Rahama Toro Wamba Liga Hill Jato Aka Mkar–Gboko Solli Hills Rahama Panyam Badikko Badikko Bauchi Toro Toro Adada Jalingo Mika Zing Toro Toro Obudu Obudu

1 1 1 1 1 2 2 3 4 3 5 6 7 8 4 9 9 10 11 12 13 14 14 15 15 15 15 5 5 16 16

References: 1, Snelling (1964); 2, Tougarinov et al. (1968); 3, Rahaman and van Breemen (unpublished); 4, Umeji and Caen-Vachette (1984); 5, Ferre´ et al. (1996); 6, Ku¨ ster (1995); 7, Okeke and Fitton (1982); 8, Caen-Vachette and Umeji (1983); 9, this study; 10, van Breemen et al. (1977); 11, Dada et al. (1993); 12, Lar (1988); 13, Dada and Respaut (1989); 14, Dada et al. (1989); 15, Tubosun et al. (1984); 16, Ekwueme and Kro¨ ner (1998).

boudins within migmatites, and even better preserved in xenoliths within the thermal aureoles of some plutons.

3.2. Metamorphism Our observations, by contrast with Onyeagocha and Ekwueme (1990), indicate that metamorphism reached grades higher than upper amphibolite facies, with the formation of abundant metatexites and diatexites. All metamorphic rocks are coarse-grained gneisses or anatexites (grain

size commonly, 0.5–1 cm). Orthopyroxene and mesoperthite, commonly observed in leptynites and associated leucosomes, are typical minerals of the granulite facies, suggesting peak temperatures well above 800 °C. Coexisting two pyroxenes and garnet+ orthopyroxene assemblages are only met in Ca –Fe rich metasediments. Aluminous metapelites found as refractory, partly restitic lenses and layers enclosed in biotite– garnet diatexites are always devoid of primary muscovite and contain K-feldspar, frequently mesoperthite, garnet, sillimanite and Ti-rich biotite (4% TiO2,

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XMg =0.60). Cordierite (XMg ]0.60) overgrowing garnet appears in both gneisses and diatexites. Preliminary investigations have been focused on garnet-rich metapelites that best record a part of the metamorphic evolution. Fossilized relict cleavage defined by quartz, minute ilmenite and prismatic rutile interpreted as a medium-temperature prograde feature has been found in garnet cores from a few samples. The preservation of staurolite, kyanite, rutile and Zn-rich spinel (up to 20 wt.% ZnO) inclusions armored towards the external part of such garnet cores is also significant, as well as their chemical zoning (from alm65 pyr24 gro07 spe04 to alm74 pyr18 gro03 spe04; from core to rims). Preliminary temperature estimates (Ferry and Spear, 1978) on one sample have given 600 °C for pressure fixed at 800 MPa for the prograde stage armored in garnet cores, and 750– 830 °C for peak temperature (garnet rims/external biotite). The development of late cordierite in most samples and in leucosomes of aluminous

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pelites indicates that final equilibration was coeval with melting occurring at 400–500 MPa pressure at these temperatures (Spear, 1993). Cordierite– garnet geothermometry (Holdaway and Lee, 1977) gives temperatures 5 600 °C. Late muscovitisation is frequently observed in most aluminous metapelites and diatexites close to leucogranites. Foliated and lineated hornfelses of metapelitic derivation with a fine-grained tectonic fabric were found as xenoliths in the Ribina garnet– biotite leucogranite sheet. These rocks display similar K-feldspar–garnet–sillimanite9 cordierite mineral assemblages and are free of any low-temperature retrogression. Prograde Zn-poor spinel is also present as small inclusions in plagioclase, garnet and cordierite. The temperature of equilibration of one cordierite-free siliceous and graphitic sample is 6859 10 °C for biotite inclusions -garnet cores, and 7809 20 °C for rims (Ferry and Spear, 1978). Dark xenoliths derived from Mg–Fe-rich

Fig. 2. Geological map of the Pambegua – Bauchi area. Foliations compiled from field data, SLAR images and previous maps (Wright, 1971). The Bt – Ms granites form elongate plutons parallel to the regional structures suggesting either a syntectonic emplacement. The Bt – Hbl granites have more rounded shapes casted by country rock structures in conformity envelopes suggesting a late-tectonic emplacement (see text for details). The rectangle corresponds to the area where structural data of Fig. 3a, were collected.

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Fig. 3. (a) Field and AMS data from framed area Fig. 2. Lower hemisphere, equal area. Dash lines represent structural domain boundaries for S and L. (b) Cross-section in the Jos –Bauchi area.

sedimentary protoliths and their associated plagioclase–leucosomes contain metamorphic orthopyroxene. Such mafic inclusions are mimetic from igneous enclaves of noritic composition enriched in euhedral apatite.

3.3. Structures Landsat and SLAR images at 1:250 000 scale were used to delineate granite intrusions, trace foliation trends and locate shear zones. Mineral or stretching lineations and kinematic criteria were observed directly in the field. This allowed the recognition of four major tectonic phases, D1 –D4, on the basis of their geometrical and kinematic features (Figs. 2 and 3 Tables 1 and 2):

D1 — early Pan African nappe tectonics marked by biotite–muscovite sheet-like granites with flat-lying foliations, E–W lineations that were formed during eastward displacements along synmetamorphic low-angle thrusts identified in the metasediments. These intrusions are concordant and contemporaneous with fayalite-bearing monzonite sheet-like intrusions in the Bauchi area. This phase was contemporaneous with metamorphic thermal peak in the stability field of sillimanite since this mineral defines the early regional lineations. Domains that clearly exhibit D1 structures are surrounded by diatexitic migmatites; D2 — these structures are best recognized in other biotite–muscovite granite sheets and in

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some migmatites in which it formed the secondary low-angle, west-dipping foliation bearing N –S lineations. Top-to-north renewed displacements occurred during this second stage, as shown by shear sense indicators.

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D3 — ductile N–S to NNE –SSW dextral wrench tectonics was responsible for the refolding and steepening of the older structures, now dipping to the west. This event also formed kilometer-wide N–S shear zones bearing nearly

Fig. 4. Field photographs. (a) Migmatites –diatexites, 10 km north of Toro pluton which dip to the west; (b) boudins of host-rocks filled in by anatectic granites in the aureole around the Toro pluton, 20 m from contact to the west; (c) dikes of porphyritic granite intruding in medium grained granite, shear zone to the northwest of Rahama pluton; (d) C – S structures in the Solli Hills granites.

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Table 2 Tectono–metamorphic history of Eastern Nigeria Deformation phases

Metamorphic events

Radiometric dates

D4

MT–LT shear zones NNE–SSW strike-slip S4–L4

550 95 Ma

Ar–Ar, Amp; Rb–Sr, Bt–WR; granites–migmatites

D3

HT shear zones N–S oblique-slip S3–L3

585 910 Ma

U–Pb zircon; granites–migmatites

D2

Ductile thrusting shallow dips S2 N–S horizontal L2

615 9 10 Ma

U–Pb zircon; granite–diatexites, charnockites

D1

Ductile thrusting flat-lying S1 E–W horizontal L1

upper greenschist lower amphibolite Bt–Chl–Ms in leucosomes granite pegmatite dikes LP granulite–amphibolite Bt–Grt in leucosomes Hbl–Bt granites MP–LP granulite Opx, mesoperthite, Sill Amp in leucosomes Bt–Grt granites, charnockites HP granulite, Ky+Rut–Ilm–Qtz, Opx in leucosomes

640 9 20 Ma

U–Pb zircon, fayalite-bearing monzonite

horizontal lineations. D3 structures are best marked in the biotite– hornblende granites and migmatites. The shear zones display microstructures such as highly polygonized grainboundaries, grain triple-points, recovered internal structures and minute brown biotite recrystallised in C-planes. Both D2 and D3 took place at high temperatures and were possibly connected with near isothermal decompression to 5 400–500 MPa. D4 is characterized by N– S to NNE – SSW subvertical ductile fine-grained mylonites with dextral strike-slip kinematics (e.g. Turner, 1986). Quartz microstructures, such as undulose extinction and the minute green biotite constrain deformation temperature to 5350 °C (Ferre´ et al., 1995). Similar mylonites have been described elsewhere in the Trans-Saharan Belt (see references in Caby, 1989).

3.4. Geochronology The most significant radiometric ages published on Eastern Nigeria are listed in Table 1. The protoliths age is mainly Lower Proterozoic as shown by the 2.5 Ga upper intercepts of Dada et al. (1993) and by the 2.0 Ga Nd model ages of Dada (1989), Dickin et al. (1991), Ferre´ et al. (1996).

The early nappe tectonics D1 is coeval with the intrusion of fayalite-bearing monzonites in the Bauchi area (Fig. 2). These particular rocks are more abundant to the East of Bauchi and were emplaced during regional deformation under granulite facies conditions. One of these monzonite intrusions, in Bauchi, yielded a U–Pb zircon age of 63893 Ma (Dada and Respaut, 1989). The D1 Bt –Ms granites have not yet been dated. The D2 event is tentatively dated between 623 and 605 Ma by various U–Pb concordant ages, on zircons from a metadiorite, on a monazite from a granite-gneiss (Dada et al. 1993) and on zircons from Bt–Ms granite sheets (van Breemen et al., 1977). The best constrained phase, D3, is dated between 598 and 577 Ma by the U–Pb zircon data for the intrusion of the Hbl–Bt granites (Dada et al., 1989; Ferre´ et al., 1996, 1998). The D4 mylonites cut through host rock that has recorded cooling through the 350 °C isotherm, which is the blocking temperature for Rb –Sr in biotite (Cliff, 1985), around 560 Ma (Ferre´ et al., 1996). These mylonites, displaying quartz microstructures indicative of deformation between 350 and 250 °C, should have formed after 560 Ma. The same mylonites cut through pegmatites dated by Ku¨ ster (1995) which constrains the D4 phase to younger than 550 Ma.

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4. Late Pan African Hbl – Bt granites 4.1. Rock types The Hbl–Bt granites of Eastern Nigeria display similar petrographic features (Wright, 1971) and U – Pb zircon ages approximately, 580 Ma (Dada et al., 1989). The three studied plutons, Toro, Rahama and Solli Hills, have a post-collisional trans-alkaline character (Ferre´ et al., 1998). These plutons are made of three groups of rocks: (1) porphyritic Bt9 Hbl granites, (2) porphyritic Bt– Hbl –Cpx 9Opx 9Fa monzonites and (3) aphyric Bt – Hbl –Cpx – Opx diorites. Magmatic accessory minerals include magnetite, zircon, apatite, titanite and allanite. A petrographic zonation, either concentric or lateral, occurs in each pluton (Fig. 5a, b and c). The pressure of granite emplacement is constrained around 540 MPa (Ferre´ et al., 1997). The Toro pluton is a composite intrusion made of a granite body showing a normal zonation (more felsic towards the center) and of a body of diorite showing a reverse zonation. The northern margin of the pluton shows a Bt–Hbl granodiorite rim (Fig. 5a). The diorite intrusion in the granite pluton is shown by the discordance of its magmatic structures and by a magmatic interaction aureole (Fig. 5a), mostly made of aphyric granite, where the cooling granites were reheated. The Rahama pluton shows a reverse zonation with a monzonite core and a few diorite bodies (Fig. 5b). The Cpx-out line at Rahama marks the limit beyond which petrographic evidence for a former Cpx or Cpx relicts are no longer observed. The Solli Hills pluton, entirely granitic, displays a lateral zonation with no Hbl along the western margin (Fig. 5c). Numerous dikes of porphyritic Bt granite, parallel to the foliation in the country rocks, occur along the western margin of the pluton. An aureole of leucocratic Bt9 Ms 9Grt foliated granite forms a zone of 10– 100 m in thickness at the periphery of Toro and Rahama and along the west side of Solli Hills. This granite is mingled with boudinaged diatexites and migmatitic country rocks (Fig. 4b) and was formed by in-situ melting of country rocks (Ferre´ et al., 1998).

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4.2. Geochemical and isotopic data Major, trace and REE data on the Pan African plutonic rocks of Eastern Nigeria are given in Dada et al. (1995) and Ferre´ et al. (1998). The three studied plutons display similar compositions. The Hbl–Bt granites and Bt–Hbl –Cpx 9 Opx 9 Fa monzonites display SiO2 contents ranging from 63 to 70%, metaluminous and ferriferous compositions (A/CNK : 0.93), a trans-alkaline character and high K2O contents. In contrast with shoshonitic suites, the Hbl–Bt granitic rocks are ferriferous. The diorites show SiO2 contents from 52 to 62%, non alkaline and metaluminous compositions (A/CNK : 0.76) and high-K calc-alkaline affinities. Initial Sr ratios for the granites and monzonites range from 0.707 to 0.711 and oNd(580 Ma) between − 10 and −15 with depleted mantle Nd model ages around 2.090.1 Ga. Pb evaporation zircon ages (Ferre´ et al., 1998) are in good agreement with U-Pb conventional method ages on zircons (Dada et al., 1989) and constrain emplacement around 5809 10 Ma. Ar–Ar ages on amphiboles record the cooling of the plutons at a temperature of 550 °C, taken as the closure temperature by Dahl (1996), at about 560 Ma.

4.3. Microstructures Microscopic observations were systematically made on thin sections from cores at each AMS sampling station. Positions of samples are given in Ferre´ et al. (1995, 1997) and De´ le´ ris et al. (1996). Three main types of deformation microstructures were distinguished: (1) magmatic, (2) near-solidus, i.e. high temperature incipient solid state and (3) high to medium temperature solid state. Definitions for 1 and 2 are from Paterson et al. (1989) and for 3 in Bouchez et al. (1992). In the Toro pluton, magmatic microstructures occur mainly in the diorite and the aphyric granite (Fig. 6a). The porphyritic granite shows near solidus microstructures, similar to those of Bouchez et al. (1992), except along N–S corridors where solid-state deformation is more prominent (Fig. 6a).

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Fig. 5. Petrographic (a, b and c) and magnetic susceptibility (d, e and f) maps of three Pan African granites. Note the magnetic susceptibility zonation patterns in granites, normal at Toro, reverse at Rahama and parallel-lateral at Solli Hills. Data for Toro from De´ le´ ris et al. (1996), for Rahama from Ferre´ et al. (1997) and for Solli Hills from Ferre´ et al. (1995).

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In the Rahama pluton, magmatic microstructures are restricted to the mafic stocks and monzonites (Fig. 6b). Near-solidus microstructures are ubiquitous in the granites of the main pluton and of the eastern satellite. Medium temperature, solid state deformation microstructures are observed in

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the SW part of the pluton and in the NW satellite. In the Solli Hills pluton, magmatic microstructures are the most common (Fig. 6c). Near-solidus to solid state microstructures are restricted to late magmatic shear zones of a few tens of meters in width.

Fig. 6. Magnetic structure maps for the three studied plutons from anisotropy of magnetic susceptibility (from Ferre´ et al., 1995, 1997; De´ le´ ris et al., 1996). Shaded areas display solid-state deformation microstructures. Offsets of the pluton outlines along thick gray dash lines. Shear sense is normal with black arrow on top.

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The three plutons are cut by dextral NE– SWstriking high-to medium-temperature solid state shear zones. Dextral sense of shear is established on asymmetrically pinched K-feldspars and C– S structures. The stability of biotite within the foliation and C-planes indicates deformation occurred at T\350 °C in these shear zones. At Toro, only one shear zone occurs along the NW margin whereas in Rahama, the main body is bounded by two shear zones characterized by numerous anastomosed dikes of Hbl– Bt granites (Fig. 4c). These shear zones are about 1 km wide. By contrast, in Solli Hills, medium-temperature shear zones are seldom, only a few tens of centimeters wide and located within the pluton (Fig. 4d). The NE– SW orientation of the shear zones and their dextral kinematics are similar to those observed in the country rocks.

4.4. Magnetic susceptibility data The magnetic fabric study is based on 544 stations corresponding to 2479 core samples from the plutons and country rocks. The structural study of the granites with the AMS method is correlated with field and microstructural observations. The vast majority of samples display a ferromagnetic behavior which is essentially controlled by non-altered magmatic magnetite. The anisotropy of magnetic susceptibility in ferromagnetic granites originates from the shape anisotropy of magnetite grains and not from the anisotropic distribution of the magnetite grains (Gre´ goire et al., 1998). This magnetic fabric is related to the magmatic fabric, i.e. the magnetic foliation is parallel to the flow plane (in a continuum between magmatic and solid-state deformation) and the magnetic lineation corresponds to the stretching direction of the magma (e.g. SaintBlanquat and Tikoff, 1997). The parallelism between magnetic and magmatic structures has been verified in the field for the three studied plutons. Magnetic fabrics, however, often record the last strain increment, of either late- or post-plutonic origin, and thus the interpretation of plutonic rock fabric relies on microstructural observations to characterize the rheological and thermal state

of deformation. The magnetic structures of the Toro, Rahama and Solli Hills plutons that display similarities and differences are summarized below. In paramagnetic granites, the bulk rock magnetic susceptibility increases with the Fe content and can be used as a magmatic differentiation index (Gleizes et al., 1993). In ferromagnetic granites, the bulk rock magnetic susceptibility shows a correlation with petrographic types (Fig. 5) where the high susceptibilities correspond to the more mafic rocks. The maps of magnetic susceptibility reveal zoning patterns that were not directly visible in the petrography. For example, at Toro, the low susceptibility Bt–Hbl monzogranite displays a normal zonation (i.e. more felsic towards the center) cut by the high susceptibility Opx–Cpx diorite body.

4.4.1. Foliations In spite of contrasted outline shapes and foliation patterns (Fig. 6), the three studied plutons exhibit foliations trending N–S to NNE –SSW and steeply dipping to the west. However, differences appear between the plutons: (i) in the Toro granite, two domains are distinguished (Fig. 5a and d Fig. 6a), those where magnetic foliations strike parallel to the border and generally dip moderately and the others corresponding to N– S corridors (shaded in Fig. 6a) in which foliations strike N–S with steep dips. In the diorite body foliations tend to dip shallowly towards the center. (ii) at Rahama (Fig. 5b and e Fig. 6b), foliations strike N–S on average. On the western side of the main pluton, foliations dip to the east. By contrast, on the eastern side, foliations dip to the west. Near the margins, planar structures tend to follow the shape of pluton and to dip towards the center. The NW satellite body shows NE–SW striking and NW dipping foliations, in conformity with regional structures. (iii) the Solli Hills pluton (Fig. 5c and f Fig. 6c) exhibits a simple pattern of foliations with NNE strikes and westerly dips. To the north of the pluton foliations become parallel to the contact and to country rocks in a narrow tail. In the southern end, foliations are truncated by a Jurassic granite. Each of these three plutons is surrounded by an aureole of country rock, of a few tens to hundreds of meters, in which foliation

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the Toro granite (Fig. 6d Fig. 7e and f), the same two domains that were defined for the magnetic foliations exhibit contrasted linear features. In the bulk of the granitic part, magnetic lineations trend E–W and plunge to the west. Along the SW and the SE margins, lineations are down-the-dip. In the N–S corridors, lineations trend N– S and are sub-horizontal. In the diorite body, lineations are less organized but tend to plunge shallowly towards the center suggesting a central feeding zone. Lineations in the Rahama Complex generally trend N–S and are sub-horizontal (Fig. 6e Fig. 7g). Few NE–SW corridors contain sub-horizontal lineations plunging towards the NE. At less than 2 km from the pluton, lineations in the country rocks are parallel to those in the granite. Finally, the Solli Hills pluton (Fig. 6f Fig. 7h) displays remarkably consistent linear features in which lineations trend NNE–SSW and are subhorizontal.

5. Collision, crustal thickening and high grade metamorphism

Fig. 7. Anisotropy of magnetic susceptibility stereonets for the granites, lower hemisphere, equal area. Squares show the peak of density and structural consistency between the three plutons.

trajectories become parallel to the limit of the pluton (conformity envelope). In spite of differences, the structural similarity between the three plutons is consistent with their emplacement during the same tectonic event.

4.4.2. Lineations The Rahama and Solli Hills plutons show a trend of N–S to NE– SW sub-horizontal lineations which seems to be a regional feature. In

The Proterozoic metamorphic history of Eastern Nigeria was monocyclic as indicated by: (i) the fact that no basement-cover relationship was identified, by contrast with Western Nigeria (e.g. Grant, 1978), (ii) the Nd model ages between 2.1 and 1.8 Ga (Dada et al., 1993; Ferre´ et al., 1996), (iii) the preserved sedimentary features (such as cross-bedding in quartzites) in boudins and hornfels around plutons and finally (iv) the prograde metamorphic history recorded in garnet cores. All metamorphic rocks seem to represent anatectic metasediments of age younger than Birimian. The single event responsible for high-grade metamorphism and partial melting is recorded by MP –HT prograde granulitic assemblages preserved in the cores of large garnet porphyroblasts in metapelites (: 8009 100 MPa, \ 8009 50 °C). Such assemblages are thought to represent the peak of metamorphism but obviously these P-T conditions should be regarded as minimum estimates for the metamorphic peak. These MP –HT assemblages are contemporaneous with the development of regional flat-lying foliations

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(D1), interpreted as collision-related nappe tectonics (Fig. 8). The fayalite-bearing monzonite of Bauchi forms a 5 km diameter zoned pluton concordant with the flat-lying foliation metamorphic granulites (Dada and Respaut, 1989). The emplacement of this monzonite is constrained by a U –Pb age at 6389 3 Ma which, considering the syntectonic character of the intrusion, is interpreted as the age of granulite facies metamorphism. Kyanite– sillimanite MP metapelites occur in numerous other localities throughout eastern Nigeria (e.g. Ekwueme and Onyeagocha, 1985). In granulites, the retrograde history is better documented than the prograde part because early reaction microstructures are erased and only the post T-peak history is preserved. Different leucosomes (Opx –mesoperthite-, hornblende-, cordierite- or biotite9 garnet-bearing) record melting history during the decompression orogenic P– T path. Emplacement of the Hbl– Bt granites is coeval with partial melting in the country rocks and formation of Bt9Grt leucosomes at pressures of : 5409 40 MPa and temperatures : 700 925 °C (Ferre´ et al., 1997). Such conditions, assuming an upper-middle crustal density of 2750

kg/m3, correspond to a depth of 209 1 km, hence an average crustal geotherm of 359 3 °C/km. Such a geotherm is high compared with non-orogenic continental zones which suggests that crustal isotherms had been deflected upward at about 580 Ma, i.e. after the main collision. The high-pressure granulitic conditions, with a pressure of 800 MPa and assuming an average crustal density of 2825 kg/m3, attest to a depth of about 30 km. The average crustal geotherm during the medium pressure event was 279 4 °C/km. Such a geotherm is normal compared with non-orogenic continental zones which suggests that crustal isotherms had been slightly deflected downward during collision at about 640 Ma (Fig. 9). At the moment, these medium pressure assemblages are at the surface of a 359 2 km-thick continental crust (Fairhead and Okereke, 1987) and probably the Neoproterozoic crust continues at depth. The lowermost continental crust, even with a mafic composition, would probably have melted at temperatures around 1000 °C. Assuming that the geotherm calculated for the upper 35 km of the crust prevailed at greater crustal depths, the

Fig. 8. Block diagram showing the large-scale structures resulting from post-collisional transpressive tectonics and granite emplacement in collision-inherited flats-and-ramps.

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Fig. 9. Simplified model of the clockwise tectonothermal evolution of an orogen, based on Eastern Nigeria example, showing a slight thickening followed by pervasive melting of the lower crust and slow exhumation along strike-slip faults.

remaining 200 °C between the observed assemblages (800 °C) and the base of the crust (1000 °C) would be approximately covered in about 6 km thus making the overall thickness of the crust about 41 km. The difference in thickness between the Neoproterozoic crust and today’s crust would be accounted for by both Jurassic and Quaternary mafic magma underplating. In the study area the grade of metamorphism increases gradually from west to east. This change is shown by: (i) less metasedimentary relicts, (ii) more granulite facies assemblages and (iii) larger garnet porphyroblasts in metapelites.

6. Granites and deformation of the Eastern Nigeria crust Two granite suites successively intruded Eastern Nigeria during the Trans-Saharan orogeny: S-type

Bt – Ms granites at : 605 Ma (2.5× 105 km3) and ferro-potassic trans-alkaline Hbl–Bt granites at : 580 Ma (105 km3). Geochemical and isotopic data show that the first suite formed from a Lower to Mid-Proterozoic aluminous crustal protolith (e.g. Dickin et al., 1991) while the second suite derived from a Lower to Mid-Proterozoic hornblende granodiorite protolith (Ferre´ et al., 1998).

6.1. Thermal e6olution and exhumation of the crust of Eastern Nigeria Granites significantly contribute to heat transfer and transient heat heterogeneity in the continental crust. This affects regional deformation because rocks surrounding granites are warmed up by the granite intrusion during emplacement and become easier to deform. Emplacement occurs over a time period less than 1 My (e.g.

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Petford et al., 1994) which is one order of magnitude shorter than orogenic time scales. Granite magmas, during their relatively fast crystallization, display a wide range of rheological behaviors from Newtonian to solid. In early stages, granite plutons behave like a non-localizing medium homogeneously recording regional deformation. As cooling proceeds, internal deformation becomes localized along discrete shear zones (Bouchez, 2000). Hence, granites record a regional deformation snapshot during emplacement. The larger the pluton, the longer the cooling lasts, hence larger plutons record a longer period of regional deformation than smaller ones. This is supported by a younger Ar– Ar amphibole age (Ferre´ et al., 1998) for the Rahama pluton (260 km2) than for the Solli Hills pluton (180 km2). Cooling also depends on the exhumation rate that may vary regionally. Pluton emplacement is constrained by a high closure temperature isotopic system/mineral such as U–Pb/zircon (850 °C; Watson and Harrison, 1984). The Hbl–Bt granites were emplaced in a short time interval, 589910 for Toro (Dada et al., 1989); 57791.6 Ma for Rahama (Ferre´ et al., 1998); and 598911 Ma for Solli Hills (Ferre´ et al., 1998). All these ages are contemporaneous within error margins. Ar –Ar/amphibole and Rb– Sr/WR – biotite isotopic systems/minerals, with lower blocking temperatures (550 °C and 450 °C, respectively; Dahl, 1996; Cliff, 1985), provide further constrains on the cooling history. The estimated cooling rates vary from 99 5 °C/My for Solli Hills to 1795 °C/My for Rahama. Assuming a geotherm of 35 °C/km, the estimated cooling features correspond to exhumation rates of : 0.25 –0.40 mm/year for the 20 My, following emplacement which is relatively slow in comparison with rates for metamorphic core complexes (e.g. Vanderhaeghe et al., 1999). Therefore, post-thickening gravitational collapse may not account for the observed exhumation in two scenarios: (1) if thickening rate keeps up or is faster than collapse rate in a late-orogenic transpressional tectonics; or (2) if crustal melts were continuously extracted from their source region; hence crustal thickness would be con-

stantly balanced by upward magma transport and continuous erosion. A slow exhumation could be explained by low erosion rates which might result from planetary-scale glaciation (Snowball Earth hypothesis).

6.2. Structural e6olution of the granite plutons The Rahama and the Solli Hills plutons exhibit similar structures (Figs. 6 and 7) and microstructures. Microstructures in the Rahama and the Solli Hills plutons, which are the two larger, are dominantly of magmatic type except along narrow shear zones where near-solidus and high-T solid-state microstructures are observed. In the Toro granite, microstructures are generally of near-solidus type and of high-T solid-state type along shear zones. Thus, the bulk of deformation at Toro has occurred at slightly lower temperatures than in the other two plutons, which is in agreement with a shorter cooling history. Magmatic structures are remarkably homogeneous at the scale of the pluton which shows that granitic magmas behaved as homogeneous and non-localizing media. Internal structures are similar to those in the migmatitic and contemporaneous host-rocks (Fig. 3). Foliations strike N–S to NNE –SSW with dips generally to the west. Lineations in Rahama and Solli Hills are sub-horizontal and trend to the north or NNE. This is consistent with a substantial N–S strikeslip component of regional deformation. The syn- to late-magmatic dextral sense of shear documented both in the plutons and in the hostrocks suggest emplacement occurred in a N– S dextral strike-slip regime. The Toro pluton displays emplacement-related magmatic structures which are oblique to the regional structures. This obliquity to regional structures might be comparable to that observed elsewhere in fast cooled intrusions (Saint-Blanquat et al., 2001). Several models of emplacement have been proposed for the three plutons (e.g. Ferre´ et al., 1995). The models are compatible with the depth of granite emplacement of about 20 km. At such depths, the transcurrent regime results in the buckling of one side of the wall rocks which in

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turn initiates a site for magma collection (e.g. Hutton, 1988). This mechanism is documented by the rectilinear shape of the Solli Hills pluton and by kinematic criteria around the Toro pluton. At Toro, the N –S elongate diorite body was emplaced before full solidification of the granites and the dioritic magma heat input partially remelted the granites. The three plutons were emplaced during regional deformation of their country rocks. All emplacement models emphasize the active role played by shear zones during emplacement in draining magmas that had formed at different levels of the lower crust. At Toro and Rahama, the pluton outlines display offsets along N– S zones (Fig. 6) which may be the result of dextral shear during emplacement before full-crystallization. Granites in the offset zones display magmatic microstructures while subsolidus microstructures are remarkably absent. Assuming that pluton emplacement occurred within 0.5 to 1 Myr, a time span close to those currently accepted (Clemens and Mawer, 1992), with an offset of : 2.5 km at Rahama and : 1 km at Toro, this would require strike-slip rate of about 1 –5 mm/annum. Such figures are within the range of known velocities along strike-slip faults.

6.3. Relationship between shear zones and plutons The spatial association between granites and regional strike-slip shear zones has led to the view that major crustal shear zones play an essential role in magma extraction, ascent and emplacement (e.g. D’Lemos et al., 1992). On the contrary, others consider that incompletely solidified plutons represent rheological heterogeneities in the continental crust which would lead to strain localization and shear zone nucleation (e.g. Neves et al., 1996). Finally, others hold the view that there is no spatial relationship between regional shear zones and plutons (Paterson and Schmidt, 1999). Eastern Nigeria is a good example of association between plutons and shear zones (Figs. 2 and 8). At Rahama, the northwest and southeast sides of the intrusion are flanked by high-temperature ductile shear zones. Diorites and norites form enclaves in the Hbl-Bt granites along these shear

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zones. The enclaves have large aspect ratios ranging from 4 to 10, prolate shapes and magmatic microstructures. This indicates that mafic magmas were mingled with granitic magma and were emplaced along the shear zones. At Solli Hills, the pluton is NNE –SSW elongated and flanked on the NW by a ductile shear zone. The large volume of leucosome in the host rock on the pluton NW flank and zonation patterns in the intrusion suggest magmatic feeding along a NNE–SSW discontinuity. The outline of the Solli Hills pluton and its internal geometry is compatible with an emplacement in a shear zone termination by failure of the inclined shear zone footwall. Mylonitic shear zones (D4) cut through the plutons and show no preferred geometrical orientation with the pluton shape or position (Fig. 2).

7. Trans-Saharan geodynamics The tectonic evolution of Eastern Nigeria, reconstructed from our data and described as four steps (Fig. 8 Table 2), most likely resulted from a continuum from orogen-normal thrusting to orogen-parallel slip. A kinematic switch occurred between an early E–W thrusting (D1) and a later N–S thrusting (D2). This deformation is contemporaneous with the production of voluminous Bt –Ms granites. The steep NNE–SSW ductile shear zones (D3) reworked the flat-lying penetrative domains in the migmatites (D1 and D2). D2 and D3 are interpreted as one single event D2 – 3 with slip along flats and lateral ramps. Strain and temperature of deformation are similar in the two domains. The N–S and NNE –SSW mylonitic zones (D4) cut through the steep ductile shear zones. Two sets of brittle NW–SE sinistral and NE –SW dextral faults cut the mylonites. Progressive strain localization can be explained by deformation occurring at decreasing temperature during orogenic exhumation. The Eastern Gondwana plate velocity is difficult to estimate for the lack of reliable markers but estimated slip rates in Hoggar yielded moderate shear strains of 1.2 (Djouadi et al., 1997) corresponding to a strike-slip of 10 km in less than 1 My, hence an hypothetical plate veloc-

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ity of 10 mm/year at 523 Ma (Paquette et al., 1998). Estimates based on pluton outline offset at Rahama and Toro (this study) yielded slip rates of 2 –5 mm/annum at 580 Ma. These figures should be regarded as first order approximation and point to moderate to slow plate velocities. The trans-alkaline nature of the syn-D3 Hbl – Bt granites suggests either a thinner lithosphere or a delaminated mantle (e.g. Black et al., 1993). The post-collisional setting along with the lack of direct evidence for lithospheric mantle delamination favors the hypothesis of a thinner lithosphere at about 580 Ma. Lithospheric thinning would be compatible with lithospheric mantle shear-heating (Kincaid and Silver, 1996; Schott et al., 2000) and shear-heating in continental strike-slip shear zones (Leloup et al., 1999). Lithospheric thinning is also consistent with the estimated 35 °C/km geotherm. Vertical variations of lithospheric strength may also have resulted in heat production along specific horizons. Heat may have had three origins during the Pan African orogeny: (1) crustal thickening and corresponding increase in radiogenic heat production at the initial stage of the orogeny; (2) viscous shear heating; (3) lithospheric thinning towards the end of the orogen leading to asthenospheric heat input. The incubation period between the onset of the continental collision and the production of S-type collision-related granites is about 40 My in the Himalayan orogen (e.g. Thompson et al., 1997). In the study area collision started before the MP granulite-facies event. To thicken the continental crust to about 45 km, approximately 30 My of convergence are required. Thus collision probably began around 670 Ma. The S-type granites were emplaced around 610 Ma. This would make the incubation period to be 60 Ma. Like in the Himalayan orogen, the LP– HT tectono– metamorphic event has obliterated evidence of the earlier history. By contrast with other orogenic belts (e.g. Hollister, 1993), the presence of large amounts of granitic melts did not promote rapid uplift. Considering the available evidence, partial melting occurred initially under medium-pressure granulite facies conditions and continued at decreasing pressure. This is in agreement with a

thickening-related early metamorphism followed by a thinning event contemporaneous with strikeslip tectonics. Evidence for a post-collisional gravitational collapse of the orogen is lacking. Orogen-normal extensional structures are also lacking probably because the wrench tectonics obliterated it.

8. Summary The Eastern half of Nigeria forms a separate Neoproterozoic terrane exhibiting significant lithological differences with the neighboring provinces of Western Nigeria and Cameroon. No basement-cover relationship was identified in the gneisses and migmatites of Eastern Nigeria. Nd model ages and Pb upper intercept ages suggests that the migmatites derived from 2.0 to 1.8 Ga metasediments. The Neoproterozoic tectono– metamorphic evolution of Eastern Nigeria was most likely monocyclic and reached granulite facies conditions (\800 °C, \ 800 MPa). The Neoproterozoic evolution of Eastern Nigeria can be described as a continuum of four events: D1 (ca. 640 Ma)—early nappe with eastward displacement, preserved in Bt–Ms granites and granulite facies metamorphism (peak T\ 800 °C, P\ 800 MPa); D2 (ca. 615 Ma)—nappe tectonics but with northwards transport and associated with migmatite sheets; D3 (ca. 585 Ma)—high-temperature N– S to NNE –SSW dextral wrench tectonics during regional decompression and coeval with geotherm peak (:35 °C/km), massive intrusion of Hbl– Bt granitic rocks and moderate lithospheric thinning; D4 (younger than 550 Ma)— low-temperature N–S to NNE –SSW subvertical strike-slip tectonics forming continental scale shear-zones. The emplacement of numerous granites, analyzed in detail with the AMS method, occurs when the geotherm is the highest and during strike-slip deformation. This plutonic event is well defined around 585 Ma and marks a change in tectonic setting from collisional to strike-slip. Pre-

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existing crustal anisotropies probably guide the ascent and final emplacement of magma bodies. Structures within the plutons reflect mostly magmatic flow but also record strain localization in solid state during the exhumation. Exhumation was relatively slow with uplift rates probably less than 0.4 mm/year after granite emplacement. Such slow exhumation is possibly linked to significantly slower erosion rates that in turn might result from planetary-scale glaciation.

Acknowledgements This research was supported by the Foreign Affairs Ministry (France), through the French Embassy in Lagos, in the framework of the Applied Geological Mapping Project (1990– 1995). Elf Petroleum Nigeria Ltd provided logistic help in the field. The staff of the Department of Geology and Mining, University of Jos provided help in the field, preparing and analysing samples. Pierre Lespinasse did most of the AMS measurements. This manuscript benefited from comments by Christian Teyssier and David Foster. C. Archanjo and J. Myers are gratefully acknowledged for their positive reviews. This is a contribution of Center National de la Recherche Scientifique (UMR 5563) and University of Toulouse who provided scientific and technical support.

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