Chronology of nappe assembly in the Pan-African Dahomeyide orogen, West Africa: evidence from 40Ar39Ar mineral ages

Pregmdrjap Hesenrrn

ELSEVIER

Precambrian Research 82 (1997) 153-171

Chronology of nappe assembly in the Pan-African Dahomeyide orogen, West Africa: evidence from 4°Ar/39Armineral ages Kodjopa Attoh a.,, R.D. Dallmeyer b, Pascal Affaton c a Dept of Geological Sciences, Cornell University, lthaca, NY14853, USA b Dept of Geology, University of Georgia, Athens, GA 30602, USA c Dkpartement de Gkologie, Universitk de Niarney, BP 338, Niamey, Niger Received 9 October 1995; accepted 18 June 1996

Abstract

The Pan-African Dahomeyide orogen represents the southwestern segments of the eastern tectonic zone along which the West African Craton was incorporated into Gondwana. Orogenic contraction produced nappe complexes comprised of passive margin sediments and accreted exotic magmatic rocks in Ghana and Togo. External nappes include the Atacora, composed of lower amphibolite facies quartzite and mica schist which were structurally imbricated with mylonitic gneiss derived from c. 2.0 Ga foreland basement. The suture zone is represented by high-pressure granulite facies garnet hornblende gneiss. We have analyzed muscovite and homblende concentrates from external and suture-zone nappes in incremental heating 4°Ar/39Ar experiments. Muscovites which define the penetrative foliation in the Atacora nappes and a shear foliation in the basement parautochthon record 4°Ar/agmr plateau ages of 579.4_ 0.8 Ma and 578.1 __+0.5 Ma respectively. In contrast, c. 600 km to the north, muscovite in quartz schist near the base of the Atacora nappes displays a spectrum which gives a minimum age of 608.1 + 1.2 Ma, whereas, muscovite in the overthrust Kara nappes yields a plateau age of 633.8 +0.5 Ma. The c. 30 m.y. difference between the Atacora nappes in the northern Dahomeyides and the younger dates of the southern external nappes indicate out of sequence thrust imbrication, however, forward-imbricating thrusting is also suggested by the older age of Kara nappes relative to Atacora nappes in the northern segment. The youngest ages are inferred to date orogen parallel nappe transport at c. 575 Ma. Hornblende from the structurally lower sections of the suture-zone nappes record a 36Ar/4°Ar versus 39Ar/4°Ar isotope correlation age of 587 +4.3 Ma. Hornblende from a tonalitic vein in the host garnet-mafic gneiss yields an isotope correlation age of 581.9 + 2.4 Ma. These ages are interpreted to date the exhumation of the suturezone nappes and thus provide a minimum age of crustal thickening in the Dahomeyide orogen. The age data are consistent with coeval forward- and hindward- propagating thrusting and nappe imbrication, and constrain the amalgamation in northwest Gondwana to have occurred in the latest Proterozoic rather than Cambrian. © 1997 Elsevier Science B.V.

Keywords: 4°Ar/39Ar geochronology; Gondwana assembly; Pan-African

* Corresponding author. E-mail: [email protected] 0301-9268/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved.

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1. Introduction It has been proposed that Gondwana was assembled from various continental fragments derived from the breakup of the Neoprotemozoic supercontinent Rodinia during the Pan-African orogenic events (Hoffman, 1991; Dalziel, 1991). During assembly, the West African Craton (WAC), which is underlain by Archean and Paleoproterozoic continental crust, was incorporated into northwest Gondwana. Because Grenvillian orogenic rocks are not exposed in WAC it has been difficult to determine its location with respect to any of the other cratonic blocks; consequently, the preGondwana locations of WAC have not been well constrained in current reconstructions (e.g. Powell et al., 1993; Hoffman, 1991). In this sense WAC appears to be a vagrant craton during part of the Neoproterozoic. However, suturing of WAC to form northwest Gondwana culminated in a major orogeny along its eastern margin, and produced a sequence of orogenic rocks that can be traced for > 2000 km from the Sahara to the Gulf of Guinea (Caby, 1987; Trompette, 1994). The northern part of this Pan-African orogen is exposed in the TransSahaman belt where a complex history of magmatism, deformation and metamorphism involving polycyclic gneisses has been documented (Boullier, 1991). The southwestern segments of this Pan-African belt comprise the Dahomeyide orogen where deeplevel crustal rocks are tectonically juxtaposed with the foreland fold and thrust belt in c. 100 km wide transect. This section contains a tectonic record which allows the resolution of the current hypotheses regarding Gondwana assembly, and evaluation of the kinematic linkage between orogenic processes at deep and shallow crustal levels. To constrain the chronology of tectonic events 4°Ar/39Ar ages have been determined for hornblende and muscovite. The results are presented herein, and document the chronology of deformation and metamorphism related to the assembly of Gondwana in the Dahomeyide orogen. Recent studies have shown that the regions affected by the Pan-African orogeny extend between the WAC and Congo craton in the Benin-Nigerian shield, and that orogeny involved reworking of older

continental crust (Castaing et al., 1993; Ajibade and Wright, 1989; Caby, 1989; Affaton et al., 1991). The present study was carried out in the Dahomeyide orogen exposed in Togo and southeastern Ghana which represents the southwestern part of the Benin-Nigerian shield.

2. The Dahomeyide nappe complexes In the study area, the Dahomeyide orogen consists of three tectonic zones: (i) external zone (foreland) underlain by Atacora and basement nappes, (ii) suture zone, and (iii) extensive internal zone comprising the Benin Plains and Accra Plains gneisses (Fig. 1) that underlie much of the Benin-Nigerian shield. The geology of these regions has been reported by Affaton (1990) and Sylvain et al. (1986) in Togo, and in Ghana by Attoh (1990) based on Ghana Geological Survey reports (Kesse, 1985; p. 68-74) 2.1. External nappes

The external zone of the Dahomeyide orogen is underlain by nappe complexes derived from the basement and the sedimentary rocks exposed along the deformed edge of the West African craton. The principal lithotectonic units comprising the external zone are (from west to east): the Buem unit, Kande schists, Atacora quartzites and gneisses representing the parautochthonous Erbunean basement (Fig. 1). The Buem unit consists of a succession of massive arkoses and a thick section of shales and mudstones interbedded with cherts and limestones and alkali-volcanic rocks. Buem strata are predominantly E-dipping and folded about a N-S steep dipping axial planes (Jones, 1990); however, westward thrusting of the Buem strata has been inferred from structures on the western boundary which indicate that the Buem unit is the westernmost unit of the external zone (Affaton, 1990; Castaing et al., 1993). A distinct tectonic and metamorphic front separates the N - S structures and sub-greenschist facies Buem mocks from the NE-SW structures of the Atacora nappes. Along this zone serpentinite bodies are exposed in pervasively sheared graphitic

K Attoh et al. / Precambrian Research 82 (1997) 153-171

155

northern Togo is shown in Fig. 2. Here, it occupies a belt c. 40-100 km wide and comprises the Buem unit, Kande, Atacora, Kara and Kabye nappes (Caby, 1987; Affaton, 1990). The distribution of external zone structural units in the southern Dahomeyides are shown in Fig. 3. The Atacora nappes are exposed immediately east of the Kande schists. They display penetrative structures in quartzites and quartz-mica schists including prominent lineations typified by fold rods. Recv.mbent isoclines with hinges rotated NW and SSW are interpreted to indicate nappe transport directions. Within the Atacora nappes, metamorphic grade and shear strain systematically increase eastwards and are associated with northwesterly thrusting and later southerly nappe transport. The highest, metamorphic intensity attained in the external zone was upper greenschist-lower amphibolite facies (evident in chloritoid bearing quartz schists, Table 1). The c. 2.0 Ga basement is composed of mylonite gneisses derived from biotite granodiorite. The eastern edge of the WAC is marked by an extensive ductile shear zone within which the granitoids have been transformed into mylonitic gneisses (Ho gneiss) and locally more highly sheared varieties which comprise ductile imbricate structures exposed in the Atacora eastern outliers. This shear zone is characterized by a shallow SE-dipping foliation that indicates NW-verging thrust propagation. In this paper the Ho gneiss (Fig. 3) is correlated with the Kara gneiss (Fig. 2). Fig. 1. The principal tectonicelementsof Dahomeyideorogen in (modifiedafter Castainget al., 1993and referencestherein). The lines of section indicate the approximate transects in the northern segment (Togo) and southern segment (Ghana). Kandi fault (KF) and other major transcurrent faults are shown. The inset map showsthe locationof the Dahomeyides with respect to WestAfricanCraton (WAC).

and chloritic schists (Attoh, 1990) which comprise the Kande phyllonitic nappes (Sylvain et al., 1986; Castaing et al., 1993). The Atacora nappes are largely composed of quartzites and quartz schists which are overthrust by and imbricated with the c. 2.0 Ga (Eburnean) Kara gneiss. A schematic cross-section of the Dahomeyide external zone in

2.2. Suture-zone nappes

The Kabye gneiss exposed in northern Togo (Fig. 2) and the Shai Hills gneiss in Ghana (Fig. 3) comprise the suture-zone nappes. This zone coincides with a prominent gravity anomaly that can be traced along the entire eastern margin of the WAC (Bayer and Lesquer, 1978; El-Hadj Tidjani et al., 1994). The suture-zone nappes are composed of garnet-bearing mafic-ultramafic rocks which can be traced continuously for c. 1000km. Extensive shearing affected the mafic gneiss where it was thrust over the external nappes along a crustal-scale ductile shear zone. Above this shear zone the granulite facies Shai Hills gneiss was partially retrogressed to garnet amphibolite.

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BUEM

,, I ! N

KANDE

ATACORA N APPES (~)~.

KARA GNEISS ,

/

KABYE NAPPES ,

,o.

R~NI~ P J A J ~ ~=,~=~

. . . . . . . . . . . . . . . . . . .

I

Fig. 2. Interpretive section of the Dahomeyides in northern Togo (modified after Caby, 1987) showing the inferred nappe stacks and the locations of the samples dated. Ornamentations as in Figs. 1 and 3.

Fig. 3. Tectonic map of the Dahomeyide orogen in southeastern Ghana and adjoining part of Togo (sources: Attoh, 1990; Sylvain et al., 1986). Locations of samples dated in this study are shown by stars.

K. Attoh et al. / Precambrian Research 82 (1997) 153 171

157

Table 1 Description of Samples Analyzed Sample number

Tectonicunit

Description: Mineral assemblagea

1 2 3 4 5

Atacora nappes Kara nappes Atacora nappes Atacora nappes Basement nappes (Ho gneiss) Suture zone nappes (Shai Hills gneiss) Suture zone nappes (Shai Hills gneiss) Suture zone nappes (Shai Hills gneiss) Accra Plains allochthon (Dzodze gneiss)

Quartz schist: Qz + Ms + Cld Protomylonite: Pl + Qz + Ms Quartz schist: Qz + Ms + Cld Micaceous quartzite: Qz+Ms+Cld Mylonitic gneiss: Qz + P1+ Ms + Bio

6 7 8 9

Garnet amphibolite: Hbl + Grt + Czo Vein in mafic gneiss: P1 + Hbl + Sc(scapolite)+ Grt + Di Garnet hornblende gneiss: Hbl + Grt +Di+PI+Sc +I1+ Ru Diorite gneiss: Qz + P1+ Hbl + Bio+ Grt

aMineral abbrevations after Bucher and Frey (1994).

Typical Shai Hills gneiss is a garnet-hornblende gneiss previously described by Knorring and Kennedy (1958), Burke (1959) and Attoh (1990). It is a distinct, variably deformed rock, locally displaying a shear-induced layering which is characterized by a streaky appearance due to discontinuous development of alternating garnet-rich and hornblende-rich bands. Partial melting during high temperature-pressure metamorphism produced tonalitic veins which occur both as foliation parallel and discordant veins. The older veins are folded and foliated, and attain a maximum thickness of 0.5m. The suture-zone gneiss represents the boundary between the autochthonous WAC and inferred allochthonous rocks, exposed to the east, in the Accra Plains allochthon. Kinematic indicators and microstructures in the suture-zone nappes and the external zone indicate an early NW-thrusting followed by southerly nappe transport in the southern Dahomeyides (Attoh, 1990; Castaing et al., 1993). These displacements produced the dominant N E - S W folding and superposed folds with ESE axial surfaces (Fig. 3). 2.3. Aecra plains allochthon

Granitoid gneisses exposed east of the suture zone comprise the Accra Plains allochthon (Fig. 3) and have been subdivided into two units: (i) biotite

migrnatite and (ii) the Dzodze gneiss (to the east). The migmatite was intruded by synkinematic porphyritic-granitic gneiss with aplitic veins and was thrust over the suture-zone nappes. The prominent regional structure of the migmatitic gneiss is a shallow (20o-30 °) SE-dipping foliation apparently developed during NW-thrusting. A subvertical dextral shear zone marks the eastern boundary of the biotite-migrnatitic gneiss. The Dzodze gneiss, exposed east of the shear zone, is distinguished from the migmatite by the occurrence of amoeboid garnet in hornblende-rich layers, and by an overall dioritic composition. The Dzodze gneiss is interpreted as an orthogneiss derived from a dioritic protolith that was tectonically emplaced along a crustal-scale dextral shear. Fig. 4 is a schematic cross-section of the southern Dahomeyides in Ghana showing the relation among the external and suture-zone nappes and internal-zone nappes of the Accra Plains allochthon. 2.4. Previous geochronology

Agyei et al. (1987) reported a R b - S r wholerock isochron and K-Ar mineral ages for gneisses exposed in southeastern Ghana. They obtained 615-590 Ma ages from K-Ar analyses of hornblende from suture-zone gneiss and 611_+16 Ma for the Dzodze gneiss. Biotites from the migmatite

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K. Attoh et aL /Precambrian Research 82 (1997) 153-171

BASEMENT NAPPES

®

ATACORA NAPPES

KANOE(~

SUTURE ZONE

NAPPES

(~

® ~, '::iii!!!iiiii!!!iiiii!!!iii! "'L~--
..................

1 km

I--

10 km

Fig. 4. Schematiccross-sectionof the southern Dahomeyideorogen in Ghana showing the principal nappes and relative structural levels of the samplesanalyzed projectedonto the line of section. Ornamentations as in Fig. 3. and Dzodze gneiss record younger K-Ar ages of 542-506 Ma. Agyei et al. (1987) also presented a 2176 + 44 Ma whole-rock Rb-Sr isochron for basement (Ho gneiss) which provided the first Eburnean (c. 2.0 Ga) age in the Dahomeyide orogen of Ghana. This permitted correlation of the Ho gneiss with the Kara gneiss in Togo which was previously dated to be Eburnean (2064_+90 Ma) (Vachette et al., 1979). Results of U-Pb analysis of zircon separates from Dahomeyide gneisses exposed in Ghana were reported by Attoh et al. (1991). A zircon fraction from the Shai Hills gneiss gave a concordant age of 610 _+2.0 Ma and is interpreted to date the highpressure granulite-facies metamorphism. U-Pb analyses of zircon fractions from the granitoid gneisses yielded strongly discordant ages which suggest Pb loss at c. 560 Ma following crystallization at c. 650 Ma. Attempts to determine the depositional age of the Buem and Atacora units include Rb-Sr analysis of glauconite and K-Ar analysis of illite (Clauer et al., 1982). The data suggest diagenesis may have occurred between 650 Ma and 550 Ma in the Buem and correlative cratonic sediments. Jones (1990) reported a c. 500 Ma whole-rock K-Ar age of Buem volcanic rocks which he interpreted to be significantly younger than their depositional age.

3. Analytical methods The techniques used during 4°Ar/39Ar analysis of mineral concentrates from the Dahomeyide

orogen generally followed those described by Dallmeyer and Gil-Ibarguchi (1990). Optically pure (>99%) mineral concentrates were wrapped in aluminum foil packets, encapsulated in sealed quartz vials, and irradiated in the TRIGA reactor at the US Geological Survey in Denver. Variations in the flux of neutrons along the length of the irradiation apparatus were monitored with several mineral standards, including MMhb-1 (Samson and Alexander, 1987). The samples were incrementally heated until fusion in a double-vacuum, resistance heated furnace. Measured isotopic ratios were corrected for total system blanks and the effects of mass discrimination. Interfering isotopes produced during irradiation were corrected using factors reported by Dalrymple et al. (1981). Apparent 4°Ar/39Ar ages were calculated from corrected isotopic ratios using the decay constants and isotopic abundance ratios listed by Steiger and J~ger (1977). Intralaboratory uncertainties have been calculated by statistical propagation of uncertainties associated with measurement of each isotopic ratio (at two standard deviations of the mean) through the age equation. Interlaboratory uncertainties are ca. + 1.25-1.5% of the quoted age. Total-gas ages have been computed for each sample by appropriate weighting of the age and percentage 39Ar released within each temperature increment. A 'plateau' is considered to be defined if the ages recorded by two or more contiguous gas fractions (with similar apparent K/Ca ratios) each representing >4% of the total 39Ar evolved (and together

K. Attoh et al. / Preeambrian Research 82 (1997) 153-171

constituting > 50% of the total quantity of 39Ar evolved) are mutually similar within a + 1% intralaboratory uncertainty. Analyses of the MMhb-1 monitor indicate that apparent K/Ca ratios may be calculated through the relationships 0.518 ( - t - 0 . 0 0 0 5 ) × (39Ar/37Ar)e . . . . cted. Plateau portions of the hornblende analyses have been plotted on 36Ar/4°Ar versus 39Ar/4°Ar isotope correlation diagrams. Regression techniques followed methods described by York (1969) and a mean square value of the weighted deviated (MSWD) has been used to evaluate the isotopic correlations. Muscovite concentrates have been prepared from schist and quartzite collected at three locations within the Atacora nappes (1, 3 and 4) and from mylonitic gneiss collected at two locations within the basement parautochthon (2 and 5). Hornblende concentrates were prepared from three samples (6, 7 and 8) collected within the suturezone nappes (Shai Hills gneiss) and from the Dzodze gneiss (9) representing the Accra Plains allochthon. Sample locations are indicated in Figs. 2-4, (coordinates are provided in the Appendix), and mineral assemblages are listed in Table l.

4. Results 4.1. Muscovite

4°Ar/39Ar analytical data for muscovite samples are listed in Table 2 and portrayed as incrementalrelease apparent age spectra in Figs. 5 and 6. The muscovite concentrate from sample 1 is characterized by high apparent K/Ca ratios but the apparent ages between 545°C and 900°C only range between 601.8_+0.7 Ma and 608.1 + 1.2 Ma with oldest age corresponding to the highest temperature. Sample 2 displays a more internally discordant apparent age spectrum. Although intermediate temperatures do not rigorously define a plateau, the apparent ages only range between 630 Ma and 640 Ma. Muscovite concentrates from samples 3, 4 and 5 display only slightly discordant apparent age spectra which yield well-defined younger plateau ages that range between 579-+0.4Ma (sample 5) and 575.2+0.5Ma

159

(sample 4). Apparent K/Ca ratios are very large with considerable associated uncertainties and, consequently, are not shown with the age spectra. The ratios display minor and non-systematic intrasample variation suggesting that experimental evolution of gas occurred from compositionally uniform intracrystalline 'sites'. The plateau ages in Fig. 6 are considered geologically significant and are interpreted to date the last cooling through temperatures required for intracrystalline argon retention within muscovite. Although not fully calibrated experimentally, the preliminary data of Robbins (1972) used in the diffusion equations of Dodson (1973) suggest that temperatures of c. 375 ___25°C may be appropriate for muscovite. 4.2. Hornblende

4°Ar/39Ar analytical data for the four hornblende concentrates are listed in Table 3. The four hornblende concentrates display variable spectral discordance. The relatively small volume increments evolved at low experimental temperatures display considerable variation in apparent ages. These are matched by fluctuations in apparent K/Ca ratios (Fig. 7) which suggest that experimental evolution of argon may have occurred from several, compositionally distinct domains near grain margins. Most intermediate- and high-temperature gas fractions display little intrasample variation in apparent K/Ca ratios, suggesting that experimental evolution of gas occurred from compositionally uniform intracrystalline sites. The intermediate-temperature increments of samples 7 and 8 (Fig. 7) combine to define plateaux of 587.1_+1.3Ma (7) and 573.8__+0.9Ma (8). 36Ar/4°Ar versus 39Ar/4°Ar isotope correlations of the plateau data on Fig. 7A and B are well defined (MSWD <2.0) (Table 4). Inverse ordinate intercepts in the correlations are significantly larger than the 4°Ar/36Ar ratio in the present-day atmosphere suggesting considerable intracrystalline contamination with extraneous ('excess') argon components. Using the inverse abscissa intercept (4°Ar/39Ar ratio) in the age equation yields intermediate- and high-temperature plateau isotope correlation ages of 581.9+2.4 Ma (7) and 566.2_+ 1.9 Ma (8) shown on Fig. 7 Because calcu-

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Table 2 4°Ar/39Ar analytical data for incremental-heating experiments on muscovite concentrates from external nappes in the Dahomeyide Orogen, Ghana-Togo Release temp (°C)

(4°mr/a9Ar)a

(36Ar/39Ar)a

(37Ar/39Ar)b

39Ar % of total

%4°Ar non-atmos,c

36Arc,%

Apparent age (Ma) d

0.094 0.010 0.007 0.020 0.017 0.007 0.016 0.014 0,017 0,022 0,018 0.030 0.121 0.019

1.69 8.87 7.36 8.38 11.64 12.87 11.67 8.56 6.52 6.51 9.67 4.31 1.96 100.00 96.36

89.79 98.99 99.66 99.68 99.67 99.59 99.58 99.73 99.51 99.42 99.47 99.47 98.19 99.34

0.19 0.20 0.44 1.35 1.11 0.39 0.80 1.16 0.75 0.79 0.73 1.22 1.33 0.79

572.6 +__1.5 601.8+0.7 604.2-+0.5 604.6+__0.3 606.1 _+0.5 606.1 _+0.4 608.4_+0.4 610.8_+0.5 611.8_+1.3 611.0+0.8 608.8+0.5 608.1 + 1.2 624.2+0.5 606.9-+0.6 607.2 +0.6

0.045 0.012 0.023 0.013 0.032 0.014 0.014 0.032 0.028 0.013 0.039 0.024 0.021

1.37 6.38 7.07 7.21 11.55 13.69 18.75 7.56 12.08 9.48 3.56 1.30 100.00 77.91

86.04 97.64 99.11 99.29 99.51 99.21 99.21 99.31 99.22 98.80 97.59 89.07 98.74

0.06 0.10 0.52 0.37 1.32 0.36 0.36 0.93 0.72 0.21 0.30 0.04 0.53

562.2+_0.7 654.6+0.5 640.3+0.4 635.1 +0.4 633.3 +_0.5 631.9+__0.5 632.3 +0.5 630.9+0.4 636.1 +0.5 648.2+0.9 666.4+0.7 672.6 _ 1.6 637.2+0.5 633.8 + 0.5

0.013 0.021 0.028 0.015 0.028 0.009 0.028 0.007 0.015 0.009 0.020 0.036 0.138 0.018

4.09 14.22 9.94 7.80 7.55 9.52 9.34 8.90 9.73 10.45 5.46 2.77 0.21 100.00 95.70

92.71 98.74 99.65 99.90 99.40 99.83 99.48 99.50 99.32 99.54 99.53 98.59 76.59 99.10

0.04 0.36 1.80 3.66 1.03 1.22 1.18 0.31 0.50 0.45 0.94 0.55 0.11 1.01

512.8+__0.8 582.1 +0.5 583.7 + 0.3 582.0+_0.3 575.0_+0.5 576.6+0.4 575.4_+0.6 575.1 _+0.6 575.1 +_0.4 576.2-+0.6 576.8 -+0.6 579.4-+0.8 519.3_+3.6 575.3_+0.5 578.1 _+0.5

Sample 1 (mica schist, Atacora nappes) J=0.010485 485 545 580 610 645 680 715 750 785 825 860 900 Fusion Total Total without 485°C, and fusion

39.68 38.15 38.07 38.09 38.21 38.24 38.41 38.53 38.70 38.67 38.49 38.44 40.15 38.40

0.01372 0.00129 0.00041 0.00040 0.00042 0.00051 0.00053 0.00034 0.00063 0.00075 0.00068 0.00067 0.00247 0.00085

Sample 2 (protomylonite, Kara nappes): J=0.010482 480 540 575 610 645 680 715 750 785 820 860 Fusion Total Total without 480-540°C, 820°C --fusion

40.55 42.75 41.02 40.55 40.33 40.35 40.37 40.23 40.66 41.75 43.69 48.40 40.95

0.01914 0.00340 0.00122 0.00095 0.00066 0.00107 0.00106 0.00093 0.00106 0.00168 0.00355 0.01789 0.00177

Sample 3 (mylonitic quartzite, Atacora nappes): J=0.010283 480 540 570 600 630 665 700 735 770 805 840 875 Fusion Total Total without 480°C, and fusion

34.49 37.52 37.29 37.08 36.73 36.70 36.74 36.71 36.77 36.78 36.82 37.37 42.37 36.87

0.00850 0.00158 0.00043 0.00011 0.00073 0.00020 0.00064 0.00060 0.00083 0.00056 0.00057 0.00178 0.03358 0.00110

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161

Table 2 (continued) Release temp (°C)

(40Ar/a9Ar)a

(36Ar/a9Ar)a

(37Ar/a9Ar)b

39Ar % of total

%4°Ar non-atmos, c

36Arca%

Apparent age (Ma) d

0.008 0.008 0.010 0.021 0.004 0.005 0.003 0.004 0.004 0.003 0.006 0.005 0.012 0.015 0,006

0.70 4.88 3.66 3.76 6.00 6.94 11.37 11.92 11.18 13.07 12.21 7.40 5.20 1.69 100.00 85.31

82.61 97.80 99.62 99.47 99.44 99.02 99.18 99.48 99.15 99.15 99.43 99.58 99.39 98.54 99.12

0.01 0.07 0.60 0.87 0.14 0.ll 0.09 0.16 0.09 0.09 0.25 0.29 0.45 0.23 0.21

481.0+1.2 585.7+0.5 586.1 +0.5 580.5+0.6 578.1 +0.2 574.3+0.3 574,9_+0.4 576,0+0.7 574,0 + 0.6 573.8+0.7 575.5+0.3 578.3_+0.2 576.0+0.3 576.5+0.8 575.8+0.5 575.4 _+0.4

0.059 0.040 0.022 0.029 0.010 0.016 0.017 0.013 0,013 0.010 0.029 0.055 0.484 0.018

0.67 3.18 3.33 5.46 8.85 14.72 13.35 14.52 15.82 13.28 4.79 1.77 0.26 100.00 90.79

83.17 94.66 98.77 97.35 97.22 98.23 98.29 98.11 98.25 99.23 99.32 98.68 85.73 98.05

0.07 0.16 0.38 0.24 0.08 0.20 0.22 0.15 0.16 0.30 0.95 0.90 0.61 0.25

559.7+ 1.8 585.3+ 1.0 584.7+0.3 580.4+0,7 579.9___0,3 579.1 +0.5 579.2___0.3 579.5+0.4 577.4+0.4 578.1 +0.6 580.4+0.4 587.8 + 1.3 603.8+3.1 579.4+0.5 579.0 + 0.4

Sample 4 (mica schist, Atacora nappes)." J=0.010295 470 530 560 590 620 650 680 710 740 770 800 830 860 Fusion Total Total without 470-590°C~ and fusion

35.93 38.09 37.43 37.07 36.90 36.77 36.76 36.73 36.70 36.69 36.71 36.86 36.76 37.12 36.85

0.02113 0.00281 0.00046 0.00065 0.00068 0.00120 0.00100 0.00063 0.00104 0.00104 0.00069 0.00050 0.00075 0.00185 0.00108

Sample 5 (mylonitic gneiss, Ho gneiss)." J =0.010425 480 540 575 610 645 680 710 740 770 800 830 865 Fusion Total Total without 480-575°C, 865°C - - fusion

41.95 38.84 37.18 37.40 37.41 36.96 36.95 37.04 36.83 36.51 36.65 37.44 44.46 37.07

0.02388 0.00701 0.00153 0.00335 0.00351 0.00220 0.00212 0.00235 0.00216 0.00094 0.00083 0.00167 0.02158 0.00247

~Measured. bCorrected for post-irradiation decay of 37Ar (35.1 day i/2-1ife). 36 [40 Artot.-(Arat~os.) (295.5)]/4°Artot.. dCalculated using correction factors of Dalrymple et al. (1981 ); two sigma, intralaboratory errors.

lation of isotope correlation ages does not require assumption of a present-day 4°Ar/a6Ar ratio these ages are considered more reliable than those directly calculated from the analytical data. The two isotope correlation ages are considered geologically significant and are interpreted to date the last cooling through temperatures required for intracrystalline retention of argon in constituent hornblende grains. Harrison (1981 ) suggested that

temperatures of c. 500+25°C are appropriate for argon retention within most hornblende compositions for cooling rates likely to be encountered in most geologic settings. The intermediate- and hightemperature increments evolved from sample 6 (Fig. 7) show slightly more intrasample variations in apparent age compared to samples 7 and 8. Although a plateau is not rigorously defined, the intermediate- and high-temperature increments

162

K. Attoh et al. / Precambrian Research 82 (1997) 153 171

650

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650

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(i)

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600

t

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Plateau age = 578,1 + 0.5 Ma

I

~====,=~

550

550

Total - gas age = 575.3 +_0.5 Ma

Total - gas age = 606.9 + 0.6 Ma I

I

[

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700t~ '

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500

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Plateau age = 579.4 + 0.4 Ma

Plateau age = 633.8 + 0.5 Ma ,

I

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Total 550

I

0

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10 20

-

gas age

= 637.2 + 0.5

Total - gas age = 579.4 + 0.5 Ma

Ma

I

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30

40

50

60

70

80

I

90 1OO

Cumulative Percentage 39 Ar Released Fig. 5. 4°Ar/39Ar apparent age spectra of muscovite concentrates from external nappes in northern Dahomeyides (in Togo) based on Table 2. Analytical uncertainties (two
yield a well-defined isotope correlation corresponding to an age of 587.1_+4.3 Ma. It is noteworthy that samples 6 and 7 record ages that are distinctly older than sample 8 (Fig, 7(C)), because the three samples were collected within the same tectonic unit. To explore the possibility of compositional dependence of the argon retention temperature we obtained electron microprobe analyses of representative hornblende grains in thin sections prepared from the samples dated. The hornblende compositional parameters listed in Table 5 suggest a correlation between Fe/Mg and Ca/Mg ratios and

I

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Total - gas age = 575.8 + 0.5 Ma

590 0

I

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10

20

30

40

50

Cumulative

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60 70 80 90 39 Percentage Ar Released

100

Fig. 6. 4°Ar/39Ar apparent age spectra of muscovite concentrates from external nappes in southern segment of the Dahomeyides in Ghana from Table 2. Analytical uncertainties and experimental temperatures as in Fig. 5. Plateau age for increments shown is listed on each spectrum. See Table 1 for sample descriptions, Figs. 3 and 4 for sample locations and Table 2 for total gas ages.

K. Attoh et al. / Precambrian Research 82 (1997) 153-171

163

Table 3 4°Ar/39Ar analytical data for incremental-heating experiments on hornblende concentrates from suture zone and internal nappes in the Dahomeyide Orogen, Ghana-Togo Release temp (°C)

(4°Ar/39Ar)a

(36Ar/39Ar)a

(37Ar/39Ar)b

39Ar % of total

%4°Ar non- atmos, c

36Arca %

Apparent age (Ma) d

0.23417 0.07732 0.06843 0.02858 0.02291 0.01719 0.02650 0.02035 0.01393 0.01254 0.01460 0.01741 0.01655 0.01314 0.01363 0.01310 0.02457

14.843 51.954 72.449 45.998 52.698 57.024 64.676 51.626 39.695 39.669 41.714 44.946 45.179 43.656 42.387 37.202 46.296

2.09 2.09 2.13 2.30 4.35 2.88 6.89 15.12 11.79 4.80 7.36 10.18 9.18 8.60 6.17 4.05 100.00

26.44 62.72 71.43 89.37 94.18 98.82 93.59 95.54 97.80 98.74 97.68 96.30 96.91 99.07 98.46 97.81 93.66

1.72 18.28 28.80 43.77 62.57 90.25 66.39 68.99 77.51 86.05 77.70 70.20 74.23 90.36 84.57 77.24 69.90

389.6+20.2 497.3±5.8 566.3± 10.1 609.9 ± 10.9 626.6+5.6 638.1 ± 5.0 597.3±4.3 613.3±2.4 626.5 ±3.5 619.1 +5.1 619.4±4.0 610.6±3.3 606.l ±3.1 617.4+_3.4 609.7±3.7 601.7±6.9 603.5 ± 4.4

0.14418 0.01634 0.01028 0.00297 0.00297 0.00278 0.00267 0.00222 0.00177 0.00275 0.00178 0.00231 0.00180 0.00299

5.924 7.624 5.807 5.732 5.814 5.811 5.762 5.808 5.889 5.929 5.826 5.903 6.207 5.837

0.34 0.24 1.90 10.69 5.49 8.90 12.22 22.11 10.31 5.02 12.97 6.64 3.19 100.00 96.24

50.09 90.42 93.76 98.91 98.93 99.06 99,37 99,49 99.86 99,12 99,83 99.45 99.91 99.12

1.12 12.69 15.36 52.46 53.32 56.80 66.22 71.17 90.75 58.63 88.88 69.61 94.80 69.21

637.8 ± 12.0 607.1 ± 15.2 592.3±8.8 588.8+2.6 585.9+2.7 584.5 +- 1.3 586.6+0.8 586.9±0.8 588.3 ± 1.4 587.1 +2.7 590.7+-0.6 590.6 ± 1.2 599.7_+1.4 588.4_+1.5 587.8 ± 1.3

0.29468 0.09684 0.06739 0.06151 0.02202 0.00235 0.00231 0.00201 0.00180 0.00163 0.00161 0.00147 0.00192 0.00534 0.00294

16.141 7.361 6.793 7.934 4.912 4.357 4.612 4.680 4.717 4.740 4.673 4.797 6.498 11.384 5.062

0.14 0.12 0,06 0.12 1.18 9.80 17.68 16.32 11.18 8.52 13.92 10.69 6.36 3.90 100.00 88.10

31.69 63.31 65.85 70.17 86.57 99.08 99.15 99.40 99.58 99.71 99.72 99.86 99.87 98.22 99.10

1.49 2.07 2.74 3.51 6.07 50.43 54.24 63.23 71.21 78.94 78.87 89.01 92.15 57.98 68.23

608.7+24.2 713.2+20.1 573.7__+32.2 624.8+21.4 600.6+2.6 576.7 ___1.1 571.9_+0.5 571.9 ± 1.1 573.0+1.6 572.7+0.8 573.4+ 1.1 576.8 + 1.0 578.9+ 1.1 570.4 ± 1.2 574.3 ± 1.0 573.5 ± 0.9

Sample 6 (Shai Hills Gneiss)." J =0.009772 700 800 850 870 890 905 920 935 950 995 1010 1025 1040 1055 1070 Fusion Total

92.45 50.10 50.42 44.73 43.63 42.36 41.17 41.96 42.36 41.38 41.79 41.59 40.97 41.00 40.68 40.45 43.05

Sample 7 (Shai Hills Gneiss). J=0.009991 700 800 850 880 900 915 930 950 970 990 1020 1050 Fusion Total Total without 700-800°C, and fusion

84.43 44.07 41.34 38.92 38.68 38.52 38.57 38.54 38.51 38.70 38.70 38.84 39.35 38.88

Sample 8 (Shai Hills Gneiss): J=0.009982 600 700 750 800 850 880 900 920 940 960 980 1010 1040 Fusion Total Total without 600-850°C, 1040°C - - fusion

125.59 76.39 56.72 58.81 45.58 37.99 37.59 37.49 37.51 37.43 37.49 37.69 37.80 37.66 37.91

164

K. Attoh et al. /Precambrian Research 82 (1997) 153-171

Table 3 (continued) Release temp (°C)

(4°mr/39Ar)"

(36Ar/39Ar) a

(37Ar/39Ar) b

39Ar % of total

%4°Ar non- atmos, c

36Arca %

Apparent age (Ma) a

5.905 2.301 3.834 4.494 4.633 4.619 4.538 4.430 4.310 4.244 4.195 4.205 4.264 4.307 4.409

1.02 1.53 5.02 11.83 16.71 17.64 8.19 6.78 6.45 8.38 6.31 5.72 2.33 2.10 100.00

98.23 98.46 97.87 99.56 99.62 99.63 99.93 99.84 99.95 99.81 99.73 99.67 99.79 99.55 99.58

1.70 3.13 15.19 58.02 65.95 69.09 93.99 85.26 95.72 81.39 71.02 66.64 76.47 60.08 68.70

5042. + 40.2 2780. +6.5 1056. ± 2.7 841.6 + 1.1 758.7+ 1.2 690.4 __+1.4 660.6 _ 1.7 636.6+ 1.5 631.3 +0.9 649.9+0.9 764.7+_0.9 770.5 __+1.7 755.2 + 3.1 756.0 _ 3.2 814.7+_ 1.4

Sample 9 (Dzodze Gneiss). J=0.010033 700 800 850 870 890 905 920 940 960 980 1010 1040 1080 Fusion Total

1554.10 371.09 80.89 60.25 52.17 46.52 43.99 42.14 41.68 43.21 52.64 53.16 51.81 51.99 71.14

0.09468 0.01997 0.00687 0.00211 0.00191 0.00182 0.00131 0.00141 0.00122 0.00142 0.00161 0.00172 0.00152 0.00195 0.00319

aMeasured. bCorrected for post-irradiation decay of 37Ar (35.1 day l/2-1ife). e[4°hrtot.-(36Aratmos.)(295.5)]/4°hrtot.. dCalculated using correction factors of Dalrymple et al. (1981 ); two sigma, intralaboratory errors.

isotope correlation and apparent ages. The data indicate that Mg-rich hornblende may have a somewhat higher argon retention temperature. However, hornblendes analyzed show no evidence of significant compositional zoning; for example, Fe/Mg values for both grain center and edge domains of sample 7 lie between 0.53 and 0.58 (0.55 in Table 5 is the average of five analyses). On the other hand, Onstott and Peacock (1987) interpreted hornblende apparent age variation in granulites to reflect composition zoning and contamination by minor alteration minerals. They also suggested that Fe-rich hornblende recorded younger 4°Ar/39Ar age. This contrasts with the results of the recent study which suggested that the composition of metamorphic calcic-hornblende is not an important factor in terms of argon closure temperature (Cosca and O'Nions, 1994). The hornblende concentrate prepared from sample 9 (not figured) from the Dzodze gneiss (Accra Plains allochthon) was characterized by an internally discordant, saddle-shaped apparent age spectrum (Table 3) which corresponds to a totalgas age of 814.7+1.4Ma. No reliable isotope correlations are defined by any combinations of

the incremental data. Many workers have demonstrated that these types of discordant 4°Ar/39Ar apparent age spectra result from experimental evolution of gas from mineral phases characterized by complex intracrystalline extraneous argon contamination (e.g. McDougall and Harrison, 1988). Therefore, the c. 815 Ma total-gas age recorded by sample 9 is interpreted as having no apparent geologic significance.

5. Implications for nappe assembly and exhumation 5.1. Chronology of deformation in the external nappes The4°Ar/39Ar muscovite plateau ages provide important constraints for the chronology of deformation and metamorphic events recorded in the external nappes, and suggest new interpretations of the geology. The dated samples were collected along two transects, one located in northern Togo and the other c. 600 km to the south in southeastern Ghana (Fig. 1). These transects cross N-S and NE-SW segments of the Atacora and parautoch-

K. Attoh et al. / Precambrian Research 82 (1997) 153 171 "¢

0.200 m

,<

650 600 O~ <

550 Q. 12. ,<

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Total -gas age =603.5 _+4.4 Ma

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400

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39 A r R e l e a s e d

Fig. 7. 4°Ar/39Ar apparent age spectra and apparent K/Ca of hornblende concentrates from the Shai Hills gneiss based on data from Table 3. Analytical uncertainties and experimental temperatures as in Fig. 5. Isotope correlation age for increments shown is listed on each spectrum. See Table 1 for sample descriptions, and Figs. 3 and 4 for sample locations.

165

thonous basement nappes. The microstructures of sample 1, from near the base of the Atacora nappes (Fig. 2), suggest it experienced at least two episodes of deformation. However, the muscovite analyzed apparently grew during the peak metamorphism associated with formation of first metamorphic schistosity (S1). The muscovite occurs in subparallel foliae c. 0.03 mm thick that define prominent S1 cleavage which is weakly crenulated. Because there appears to have been little or no recrystallization associated with crenulation, the mineral assemblage Qz + Mus + Cld indicates peak metamorphic temperatures of c. 450:C (Bucher and Frey, 1994; pp. 206-208). The stepwise increase in age with release temperature in the spectrum of sample 1 (Fig. 5(A)) provides evidence of some post-crystallization diffusional argon loss, so the age of 608 Ma (Table2) is interpreted to represent a minimum age of metamophism and to closely date emplacement of Atacora nappes. Sample 2, which was collected along the northern transect in the Kara gneiss, represents a relatively higher structural level than sample 1. Muscovite in the Kara gneiss is medium grained (c. 0.06 mm), and apparently grew during the formation of a c s fabric near the brittle-ductile transition (evident in the quartz ribbons and healed hair-line fractures in the feldspar; Fig. 8(A)). The fabric indicates that dynamic recrystallization and cleavage formation occurred at c. 400°C (Suppe, 1985; pp. 145-146). The 634+_00.5 Ma muscovite plateau (sample 2, Fig. 5(B)) likely represents a maximum age of recrystallization in the Kara gneiss as it may represent a mixing age between older and neoblastic age components. However, the age spectrum is dominated by a neoblastic muscovite age component. This would indicate that ductile shearing and thrusting of Kara gneiss may have occurred betbre the Atacora nappes were emplaced and suggests foreland-propagating thrusting. The 672.6+1.6 Ma total fusion age (Table 2) of this sample is significantly older than the plateau age and this is considered as evidence of excess argon (Cosca and O'Nions, 1994). Therefore, the age of deformation of the Kara gneiss, comprising the parautochthonous basement in the northern Dahomeyide transect, may be older

166

K. Attoh et al. /Precambrian Research 82 (1997) 153-171

Table 4 36Ar/4°Arversus 39Ar/4°Arisotope correlationsfrom incremental-heatingexperimentson hornblende concentratesfrom suture zone nappes in the DahomeyideOrogen, Ghana-Togo Sample

Isotopecorrelation age (Ma)a

4°Ar/36Ar interceptb

MSWD

Increment included

% total 39Ar

Calculated4°Ar/39Ar plateau age (Ma)b

6 7 8

587.1 +4.3 581.9+ 2.4 566.2+ 1.9

547.3+ 16,9 936.5 + 49,1 873.3± 32.7

2.03 1.59 1.22

920-fusion 850-1050 880-1010

73.40 96.24 88.10

NA 587.8+ 1.3 573.5+ 0.9

Calculated using the inverse abscissa intercept (4°Ar/39Ar ratio) in the age equation. "Inverse ordinate intercept. ~q'able 3 Table 5 Correlation of hornblende compositionwith age Sample ID

Fe/Mg Ca/Mg Age Ma Total Gas

6 7 8

0.43 0.55 0.64

0.63 0.66 0.84

Isotope Cot

603.5+04.5 587,1___+4.3 587.8+01.3 581.9_____2.4 574.3+1.0 566.2+__1.9

but close to the age of Atacora nappes. The 4°Ar/39Ar ages lead to the conclusion that nappe stacking in the northern Dahomeyides occurred between 634 Ma and 608 Ma. Muscovite ages were determined from three structural levels in the external zone of the southern Dahomeyides (Fig. 4). Muscovite from mylonitic quartzite (sample 3) is very fine-grained ( < 0 . 0 2 m m ) , defines a subhorizontal foliation (Fig. 8(B)) and prominent lineation parallel to the fold-hinge that indicate NW-thrusting. Sample 4 represents a higher level in the Atacora nappe. Muscovite separated is coarse-grained (> 0.2 mm), typical of lower amphibolite-facies micaceous quartzite, and defines a strong lineation on a gentle NE-plunging fold-hinge. Sample 5 is from the Ho augen gneiss which was thrust over and imbricated with the Atacora nappes. This sample represents the highest level of the external nappes dated. The mylonitic gneiss locally displays a variable c-s fabric in which fine muscovite fish (>0.03 mm) indicate top to the northwest (Fig. 8(C)). The oldest muscovite age of 579.0_+0.4Ma (Fig. 6(B), in Atacora nappes in the southern segment), is recorded within the highest structural

level and is interpreted to date shear-induced recrystallization. This age is indistinguishable from the 578.1_+0.5 Ma (Fig. 6(A)) age of the lower part of the Atacora nappes represented by mylonitic quartzite. If these ages date cleavage formation related to nappe emplacement they indicate synchronous NW-thrusting of basement and Atacora units in the southern external zone. On the other hand, the 575_+0.5 Ma age (interpreted to date the SSW-trending linear structures), is slightly younger than the NW-thrusting (by c. 3-4 m.y.) indicating that orogen parallel nappe transport may have been a late-tectonic phase in the Dahomeyides. These ages provide a rigorous constraint on the minimum age of the orogeny in the southern Dahomeyide foreland because the muscovites apparently recrystallized close to their argon retention temperature of c. 350-400°C. The age data suggest that folding and crustal-scale ductile shearing along the eastern margin of the WAC was kinematically linked to foreland nappe assembly. Nappe emplacement by thrusting occurred earlier in the northern Dahomeyldes than in the south. 5.2. D a t i n g uplift in the suture zone

The hornblendes dated represent the lower structural levels of the suture-zone nappes and occur in different mineral assemblages (Table 1). Sample 6 is a garnet amphibolite from a klippen of Shai Hills gneiss (Fig. 3); it represents the lowest structural level (Fig. 4) and displays a prominent SE-dipping foliation developed parallel to an alternating amphibolite and garnet granulite layer-

K Attoh et al. / Precambrian Research 82 (1997) 153 171

167

ing. Because the garnet amphibolite recrystallized at c. 700°C, the age must represent cooling following nappe formation. The isotope correlation age of 587.1 _+4.3 Ma is the oldest of the three suture-zone samples and is interpreted to date initial phase of exhumation of the suture zone. Sample 7 is a foliated, concordant vein composed of 60% plagioclase, 20% hornblende and 15% garnet from the mylonitized lower level of suture-zone gneiss. The hornblende occurs as porphyroclasts with asymmetric tails which consist of neoblastic fine grained hornblende that likely formed during localized SSW-thrusting of the Shai Hill gneiss along a ramp (Fig. 3). Mineral compositions and phase equilibria considerations of the host gneiss indicate the tonalitic vein formed by partial melting during high temperature-pressure metamorphism at 800-900°C and c. 13 kbar (Attoh, 1990; Attoh, 1994). Consequently, the 581.9_+2.4Ma isotope correlation age records post-metamorphic unroofing. Therefore, the c. 582Ma age represents a minimum date for unroofing that likely accompanied nappe transport in the southern Dahomeyides. The highest structural level dated in the suture-zone nappes is represented by sample 8 (Fig. 4) which is a typical Shai Hills gneiss with characteristic high-pressure granulite-facies mineral assemblage (Table 1). The analyzed hornblende is finer grained than that in the vein. However, sample 8, on the whole, is the least sheared of the three samples analyzed. Its youngest isotope correlation age of 566 Ma may be interpreted as evidence for later exhumation by thrusting in the higher structural level of the suture-zone nappes.

6. Discussion: implications for Pan-African orogeny

Fig. 8. Representative microstructures of muscovite-bearing samples dated from the external zone: (A) Kara gneiss (sample 2) showing quartz ribbon in lower part and healed hairline fracture in K-feldspar (Ms=muscovite, Qz=quartz, K f = K feldspar); (B) mylonitic quartzite (Atacora nappes, sample 3) with fine muscovite (<0.02 ram) in S1 foliation; (C) c s fabric of dynamically recrystallised muscovite in mylonite gneiss (Ho gneiss).

The chronology of tectonic events in the PanAfrican orogens of West Africa has implications for current hypotheses regarding Neoproterozoic global tectonic reorganization. In addition, chronologic information can lead to better understanding of the connection between mountain building at deep and shallow crustal levels. Recent geological studies in the Dahomeyides (Attoh, 1990;

168

K. A~toh et al. / Precambrian Research 82 (1997) 153-171

Castaing et al., 1993) have provided sufficient structural and petrological information to evaluate the geochronologic data, and, as such, the 4°Ar/39Ar results permit dating of distinct tectonic events during the assembly of the external and suture-zone nappes. Because muscovite closure temperatures for argon diffusion is close to the inferred peak metamorphic temperatures (c. 450°C) in the external zone, the muscovite ages likely closely date thrust imbrication and metamorphism in the external zone. The ages reported are also considered especially significant because the argon retention temperature of muscovite, as with other minerals, is expected to be grain-size dependent (McDougall and Harrison, 1988). However, because the grain size of the samples dated was relatively coarse this additional complication may not be significant. For example, the fine-grained muscovite in mylonitic quartzite gave an older age than the coarse-grained mica schist (Fig. 6) whereas in the northern segment (Fig. 5), where the grain size difference between the two samples dated is not as great, the finer grained muscovite gave the younger age. Taken together it is unlikely that the sample grain-size differences could account for the age differences and so the muscovite ages are close to the date of synkinematic recrystallization in the external nappes. Muscovite ages within the Atacora nappes in the southern Dahomeyides are significantly younger than those from the northern segment. This difference is consistent with the relative ages of the principal geological structures (Fig. 3); the older NS structures which occur in the northen segment and the Buem are clearly truncated by NE-SW thrust faults in the southern Dahomeyide in the region south of Lat. 7 ° N. The 4°Ar/39Ar results establish two distinct episodes of deformation in the external Dahomeyides which were separated by c. 25m.y. These include: (i) an early E W shortening in the north which also produced folding of the Buem unit; and (ii) WNW-thrusting in the south. This sequence of thrusting events indicate hindward imbricating thrusting (out-ofsequence), whereas, the data are consistent with forward propagating thrusting in northern segments of the external zone. In the NNE-striking Atacora and basement nappes of the southern

segment, the ages are more consistent with synchronous WNW-thrusting and ductile imbrication of quartz schists and basement gneisses. Because the hornblende argon closure temperature (c. 500°C) is, on the other hand, much lower than the estimated metamorphic temperatures for the suture-zone nappes, the hornblende ages date cooling during post-metamorphic unroofing in the suture zone. The ages are therefore consistent with a 610_ 2 Ma date for peak metamorphic recrystallization of the suture-zone rocks (Attoh et al., 1991). This would indicate exhumation from 900°C to 500°C in the suture zone ocurred over c. 22 25 m.y. The hornblende ages compared with muscovite ages in the southern Dahomeyide indicate minimum regional cooling rates of c. 7-10°C/m.y. between 587 Ma and 578 Ma and a slightly faster rate of 15-20°C/m.y. in the 610-585 Ma interval. Corona mineral textures in gabbroic rocks intruded into the suture-zone gneiss suggest that this earlier period of unroofing was episodic (Attoh, 1994), consisting of relatively fast decompression followed by slow isobaric cooling; thus it appears that unroofing slowed down with time. Hornblende ages for the lower sections of the suture-zone nappes are consistently older than the muscovite ages of the external nappes in the southern transect. The available age data suggest that prograde metamorphic recrystaUization occurred in the suture zone before muscovite recrystallization in the external nappes. The 4°Ar/39Ar ages indicate that unroofing, following crustal thickening in the suture zone, preceded deformation in the external zone of the southern segment. It suggests cratonward-thrust propagation such as has been proposed and documented in many Phanerozoic orogens (e.g. Boyer and Elliot, 1982). Furthermore, the ages suggest hindward-thrust imbrication indicating that orogeny in the Dahomeyides involved out-of-sequence thrusting (Fig. 9). Such kinematics of fold-thrust belt deformations have been deduced for other areas such as the Pyrenees (Burbank et al., 1992). The sequence of nappe stacking reconstructed for the Dahomeyide orogen based on the geochronology therefore indicates a protracted record of deformation and unroofing, which are likely related to

K. Attoh et al. / Precambrian Research 82 (1997) 153 171

579 Ma 607 Ma ~-~ ~E_~587Ma

.

169

575AMa >

10 km

I

I

Fig. 9. Inferred sequenceand age of thrusting in the Dahomeyideorogen: ( 1), Westward thrusting of Atacora nappes and Buem unit in the northern segment; (2), exhumationof suture zone nappes; (3A) and (3B), WNW directed thrusting and imbrication involving Atacora and basement nappes in southern external zone; (4), regional dextral shear. Ornamentations as in Fig. 3. continued convergence following collision of WAC with exotic terranes to the east. Current hypotheses for the assembly of Gondwana predict that earliest deformations involving crustal shortening occurred after c. 600 Ma (Hoffman, 1991 ) and that final amalgamation may have occurred as late as the Middle Cambrian (Powell et al., 1993). However, adequate geochronological data to test the hypothesis have not been available. In northwest Gondwana the hypothesis requires that the closure of the Pharuside ocean (between WAC and poorly defined continental fragments) to form the TransSaharan and Dahomeyide belts occurred after c. 600 Ma. The 4°Ar/39Ar mineral ages impose a significant constraint on the minimun date for the oceanic closure which is represented by the age of significant uplift in the suture zone at c. 587 Ma. The record of imbrication of the eastern edge of WAC was clearly diachronous, beginning in the north, with E - W convergence followed by N W - S E shortening which resulted in rotations of fold hinges and produced highly strained rocks. If the minimum age of the earliest deformations in the external zone was c. 608 Ma, corresponding to the thrusting of the Atacora nappes, a minimum age for Gondwana assembly would require early dispersal of WAC by rifting on the eastern margin of Rodinia. The ages presented herein clearly preclude Middle Cambrian tectonic assembly in northwest Gondwana if the Cambrian period began at c. 5 4 4 M a (Bowring et al., 1993). However, these ages are consistent with latest Neoproterozoic consolidation which may have preceded final assembly in eastern Gondwana. In the

Mauritanides, on the western side of the WAC, Dallmeyer and Villeneuve (1987) reported muscovite 4°Ar/a9Ar ages of 5 9 0 - 5 7 0 M a and 550-540 Ma. They interpreted the 590-570 Ma ages to date a Pan-African I event which may be correlated with later phases of Gondwana assembly. Thus there are accumulated data to support early incorporation of WAC into an early northwest Gondwana continent. If correct, the dispersal of the WAC may be correlated with early rifting on the periphery of Rodinia where the WAC is commonly located in many reconstructions.

Acknowledgment

This project was supported by grants from American Chemical Society ( P R F 24709-B 8) and American Philosophical Society to K.A. Field assistance in Ghana by Ben Fiebor is acknowledged. The manuscript has benefited from comments by the journal reviewers, especially those of C. Friend.

Appendix

Sample ID Locality Name

Lat.

1 (RT 76) 2 (RT 75) 3 (AC71)

9 ° 49.6'N 1" 4.7'E 9° 41.4'N 1° 8.3'E 5° 38.2'N 0 ° 16.1'W

4 (TG32)

Baga, Togo Pya, Togo AchimotaQuarry Ghana Kabakaba Hills Ho, Ghana

Long.

6° 36.2'N 0 ° 26.7'E

170 5 (NV9) 6 (HZ25) 7 (SH 42) 8 ( A D 18) 9 ( D Z 28)

K. Attoh et al. / Precambrian Research 82 (1997) 153 171 Nyive, G h a n a Hodzo Aviefe Hills Ghana M a m p o n g Quarry Shai Hills, G h a n a Adaklu Ahloefe Ghana Kagakofe Quarry Dzodze, G h a n a

6 ° 44.8'N 6 ° 42.0'N

0 ° 3.3'E 0°36.6'E

5 ° 51'N

0 ° 3.1'E

6 ° 29.6'N

0 ° 29.6'E

6 ° 19.0'N

l ° 0'E

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