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Pb zircon and monazite dating
Precambrian Research, 56 (1992) 89-96
89
Elsevier Science Publishers B.V., Amsterdam
Reassessment of Proterozoic granitoid ages in Ghana on the basis of U / P b zircon and monazite dating W. Hirdes a, D.W. Davis b and B.N. Eisenlohr~ aBundesanstalt 3~r Geowissenschaften und Rohstoffe, Stilleweg 2, D 3000 Hannover, Germany bj. Satterly Geochronology Lab., Royal Ontario Museum, 100 Queen's Park, Toronto, Ont., M5S 2C6 Canada CKeyCentre for Strategic Minerals, University of Western Australia, Nedlands, W.A., Australia (Received March 6, 1991; accepted after revision July 31, 1991 )
ABSTRACT Hirdes, W., Davis, D.W. and Eisenlohr, B.N., 1992. Reassessment of Proterozoic granitoid ages in Ghana on the basis of U/Pb zircon and monazite dating. Precambrian Res., 56: 89-96. The West African Craton in Ghana is of Early Proterozoic age and forms alternating volcanic belts and sedimentary basins made up of rocks of the Birimian Supergroup, as well as younger conglomerates and quartzites of the Tarkwaian Group, and extensive granitoid plutons. No consensus exists on absolute and relative ages of plutons within belts and basins. U-Pb geochronology of four different granitoid complexes indicates that, contrary to long-held views, belt granitoid plutons (Ashanti belt, 2172 + 2 Ma; Sefwi belt, 2179 + 2 Ma) are ~ 60 to 90 Ma older than basin plutons (Kumasi basin, 2116 + 2 Ma; Sunyani basin, 2088 + 1 Ma). Belt granitoids are approximately coeval with volcanic rocks of the Birimian Supergroup, while basin plutons are late-kinematic and postdate Tarkwaian sedimentation. The first high-precision geochronological data for rocks from Ghana, i.e., the eastern part of the Birimian domain on the West African craton, presented here show that magmatism within this region occurred over a time-span of at least 90 Ma, and provide further evidence that the period between 2.1 and 2.2 Ga was one of important magmatism in West Africa.
Introduction
Testing of episodic versus continuous models proposed for the growth of the continental crust requires acquisition of precise geochronology for all major areas of exposed basement. Available data suggest that much of Western Africa may be underlain by crust formed ~ 2.1 Ga ago (Taylor et al., 1988; Abouchami et al., 1990), a time that has generally been considered to be one of relatively slow crustal growth, intervening between two well-documented geochronological peaks at approximately 2.6-2.8 Ga and 1.6-1.9 Ga. It is the purpose of this paper to present the first high-precision geochronological data for rocks from Ghana which constitutes the easternmost portion of Proterozoic rocks of the West African craton (Fig. 1 ). Sev-
eral age determinations on zircons from Proterozoic rocks of the western part of the craton (Senegal, Guinea, Mali, part of Ivory Coast) have been carded out by Calvez as well as Boher (summarized by Milesi et al., 1989), giving an age range between 2115 Ma and 2073 Ma for various "gneisses" and granitoids. Rocks of Early Proterozoic age occupy large parts of the West African craton and crop out extensively in Ghana where they are subdivided into the Birimian Supergroup and the volumetrically subordinate Tarkwaian Group. Both were deformed and metamorphosed to greenschist facies in the course of the Eburnean tectono-thermal event. The Birimian comprises an assemblage of sedimentary/volcaniclastic rocks forming sedimentary basins which separate a series of subparallel, roughly
0301-9268/92/$05.00 © 1992 Elsevier Science Publishers B.V. All fights reserved.
90
W. HIRDES ET AL.
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equally spaced, northeasterly trending volcanic belts of mainly tholeiitic basalts (Fig. 2 ). Rocks of the Tarkwaian Group represent erosional products of the Birimian, are dominated by coarse clastic sediments and are located exclusively within the volcanic belts. Both, belts and basins include extensive outcrops of granitoids. The Birimian supracrustal rocks and, with a minor exception, the granitoids in Ghana represent juvenile crustal additions with Sm/Nd model ages of 2.0 to 2.3 Ga (Taylor et al., 1988 ). Recent field and isotope work suggests that the Birimian volcanic belts and sedimentary basins formed contemporaneously as lateral facies equivalents (Taylor et al., 1988; Leube et al., 1990 ). Traditionally, the Lower Proterozoic in Ghana was interpreted to have undergone two deformational events (Junner, 1940). However, new structural data show that the Birimian Supergroup and the Tarkwaian Group were deformed jointly in a single progressive deformation event (Eisenlohr and Hirdes, in
press). This event involved initially a regional penetrative low-strain phase and subsequently the formation of spatially restricted high-strain zones. Proterozoic granitoids in Ghana
Four types of Proterozoic granitoid suites are present in Ghana; traditionally they have been referred to as Winneba, Cape Coast, Dixcove and Bongo granitoids (Junner, 1940; Kesse, 1985), the latter three having recently been termed "basin", "belt" and "K-rich" granitoids (Mauer, 1990; Leube et al., 1990). Absolute ages of these granitoid types and, in particular, their emplacement age relative to each other and the host rocks are crucial to understanding the evolution of the Proterozoic terrane in Ghana. Basin and belt granitoid rocks are widespread, both other types occur only locally. The Wjnneba granitoid is the only rock suite so far encountered in Ghana which shows evidence for an Archean sialic precursor (Sm/Nd model
91
PROTEROZOIC GRANITOID AGES IN GHANA 3°
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92
w. HIRDES ET AL.
TABLE 1 Characteristics of belt and basin granitoids in Ghana/West Africa Belt granitoids
Basin granitoids
Small to medium-sized plutons, restricted to Birimian volcanic belts
Large batholiths restricted to Birimian sedimentary basins
Contact aureoles of a few tens of metres maximum
Extensive contact-metamorphic aureoles
Seldom foliated (except for local intense shearing), no compositional banding
Foliated, compositional banding ubiquituous and pronounced
Hornblende dominant mafic constituent
Biotite dominant mafic constituent
Metaluminous
Peraluminous
Typically dioritic to granodioritic in composition
Typically granodioritic in composition
Compositional range from granite s.s. to diorite
Compositional range from granite s.s. to tonalite
Na20, CaObe, > Na20, CaObasi°
Rb, K2Obasin > Rb, K2Obelt
Pronounced retrograde mineral alteration
Little mineral alteration
Similar geochemical characteristics as tholeiitic basalts in belts for some elements
No evidence for geochemical similarity to tholeiitic basalts in belts
TABLE 2 Isotopic data for zircon, monazite and rutile from granitoid rocks in Ghana. Fractions are zircon unless specified otherwise Mineral fractions
Sefwibelt-
Weight (mg)
U (ppm)
Pbcom (pg)
Measured 2°7pb/ 2O4pb
Corrected
2°7pbF°6pb Age (Ma)
2OapbFO6pb
2°6pb/238U
207pb/235U
6668
(1) Ab (2)Ab (3)Unab
0.103 0.018 0.149
89 105 116
4 3 20
7147 1970 2789
0.100 0.111 0.085
0.40189 0.39719 0.36931
7.547 7.449 6.912
2179.2_+ 2.9 2177.0_+ 3.6 2173.4-+ 1.8
0.056 0.036 0.060
102 43 191
13 18 18
1485 311 1884
0.118 0.101 0.112
0.40098 0.39232 0.33216
7.497 7.333 6.207
2171.6-+2.5 2171.2+3.6 2170.6_+ 1.8
135 138 164
3 4 50
2088 1906 1678
0.084 0.092 0.070
0.38722 0.38651 0.34551
7.010 7.000 6.198
2115.2-+2.5 2116.0_+3.6 2099.1 + 1.8
2977 3823 225 7
59 14 24 11
813 1759 994 207
2.078 1.585 0.020 0.008
0.38487 0.38148 0.37499 0.36347
6.858 6.799 6.681 6.424
2087.4+2.9 2088.0_+2.9 2087.3+2.5 2073.2-+4.3
A s h a n t i belt - 6 4 2 7
(4) Ab (5) Ab (6) Unab
K u m a s i b a s i n - 6898,4
(7) Ab (8) Ab (9) Unab
0.013 0.016 0.176
Sunyani basin - 8909A
(10) ( 11 ) (12) (13)
Unab, M Ab, M Ab Ab, R
0.005 0.002 0.034 0.102
Ab = abraded; Unab = unabraded; M = monazite; R = rutile; Pbcom = Common Pb, including blank 2tr U / P b errors are 0.6-1.0% for zircon and monazite and 1.6% for futile.
93
PROTEROZOIC GRANITOID AGESIN GHANA
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.38 2040
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Fig. 3. U - P b data for zircon, m o n a z i t e and rutile ( R ) in granitoid rocks from Ghana. The lines represent best-fit regression lines to igneous mineral fractions from each rock. One discordant analysis from the Ashanti belt is off scale.
age of ~ 2.6 Ga, model H-value of 8.4, 87Sr/86Sr initial ratio of 0.7412 ___0.00287 ) (Taylor et al., 1988). The K-rich granitoids possess a clear intrusive relationship with Tarkwaian sediments which overlie Birimian rocks of the Bole-Navrongo volcanic belt in northern Ghana (Fig. 2); their Rb/Sr whole-rock isochron age is 1968___49 Ma (Lenz in Hirdes et al., 1988 ). In this paper, Winneba and K-rich types are not treated in further detail. Principal features of belt and basin granitoids are given in Table 1 which includes data
of Junner (1940), Leube et al. (1990), Mauer (1990) and Eisenlohr and Hirdes (in press). Age relationships between these major Ghanaian granitoid types and their time relationship to rocks of the Birimian Supergroup and to those of the Tarkwaian Group have not been established, due to the absence of intrusive relationships between basin and belt granitoids and the fact that the latter yield poorly constrained Rb/Sr, Pb/Pb and Sm/Nd isotopic ages (Taylor et al., 1988). Also, firm, databased time relationships between the various granitoids and deformation are lacking. Based on their degree of foliation, earlier workers assumed that basin granitoids were intruded during regional deformation, whereas belt granitoids were emplaced after deformation (Junner, 1940; Kesse, 1985). Results of U/Pb zircon, monazite and rutile dating
Igneous crystallization ages of four granitoid plutons were investigated by means of U / P b zircon and/or monazite dating (No. 6427, Ashanti belt; No. 6668, Sefwi belt; No. 6898A, Kumasi basin, No. 8909A, Sunyani basin) (Fig. 2). Unfractured zircon fractions were abraded to remove outside layers of crystals before dissolution in order to reduce lead loss (Krogh, 1982). Sample treatment followed standard techniques (Krogh, 1973) and isotope ratios were determined on a VG354 mass spectrometer. Results of U / P b analyses are presented in Table 2 and Fig. 3. Ages were de-
TABLE 3
Summary of age data and regression parameters Sample
Age (Ma)
Lower intercept (Ma)
Probability of fit (%)
Number of points
Sefwi belt - 6668 Ashanti belt - 6427 Kumasi basin - 6898A Sunyani Basin - 8909A
2178.9+2.3 2171.6+ 1.9 2116.2+2.3 2087.6 + 1.4
131 9 275 -
49 85 68 96
3 3 3 3
94
termined by best-fit regression lines to concordant and discordant data points using the computer program of Davis (1982). Ages are presented in Table 3. All errors are given at the 95% confidence level. Zircon fractions consisted of uniform populations of euhedral, weU-facetted grains, with the exception of sample 8909A. This sample contains zircons of different morphology and colour, and monazite as well as rutile which were also analyzed isotopically for U and Pb. Monazite is typically found as a magmatic mineral in muscovite-bearing granitoid rocks. These rocks often intrude sediments and contain cores of inherited zircons derived from contamination with sediments (Percival and Sullivan, 1988). In sample 8909A, the zircon grains analyzed yielded the same age as grains from both the abraded and unabraded monazite fractions. Rutile in this sample produced a discordant data point with an age of 2073 _+4 Ma, which is a minimum estimate for its growth. The rutile probably is the product of hydrothermal alteration and/or metamorphism as indicated by its depleted T h / U ratio of 0.03 (calculated from the 2°Spb/2°6pb ratio ). Discussion
The above data are in contrast with long-held views; they demonstrate that ( 1 ) the investigated belt ("Dixcove") granitoid plutons from both the Sefwi belt and the Ashanti belt formed approximately contemporaneously at ~ 2175 Ma; (2) the investigated belt ("Dixcove") granitoids are 60 to 90 m.y. older than the investigated basin ("Cape Coast" ) granitoid plutons present in the Sunyani and Kumasi depositories; (3) in contrast to belt-type granitoids, the investigated basin plutons show an age difference of 30 m.y., the younger one being the Sunyani basin granitoid. Ages of the belt plutons coincide closely with
W. HIRDES ET AL.
a 2166 + 66 Ma S m / N d whole-rock isochron age of tholeiitic basalts in Birimian volcanic belts (Taylor et al., 1988), and strongly suggest that belt volcanic rocks and belt granitoids are coeval, comagmatic and parts of the same igneous event. Comagmatism is supported by the intimate association of belt granitoid intrusions and basalts (Hirst, 1946; Eisenlohr and Hirdes, in press). Early pre-deformation emplacement of belt granitoid plutons is also indicated by their absence in Tarkwaian sediments, by their extensive and intensive retrograde alteration and the fact that locally they have been subjected to shearing (Mauer, 1990; Eisenlohr and Hirdes, in press). It is noteworthy that - to the authors' knowledge - the obtained 2178.9 Ma and 2171.6 Ma belt granitoid ages are the oldest precise Proterozoic granitoid ages reported from the West African craton. Such data are strictly incompatible with a recent hypothesis of Milesi et al. (1989). These authors claim that in Ghana and elsewhere in West Africa volcanic and plutonic rocks of the Birimian volcanic belts ("B 2", Milesi et al. ) postdate a major Eburnean tectono-metamorphic phase D~ around 2100 to 2090 Ma, and that such volcanic and plutonic belt rocks are younger than the sedimentary rocks in the basins ("B 1", Milesi et al. ) which are claimed to predate D~ deformation. Basin granitoid intrusions, which were emplaced ~ 60 m.y. later than belt granitoids in the Kumasi basin and ~ 90 m.y. later in the Sunyani basin (Fig. 2 ) show no mineral alteration and exhibit features which suggest that their foliation formed at high temperatures during emplacement, i.e., is not a function of tectonic overprint. The orientation of contactmetamorphic muscovite and biotite grains is discordant with regional foliation in the country rocks indicating that basin granitoid plutons were intruded toward the end of Eburnean deformation. The 2116 and 2088 Ma ages indicated in this study for the basin granitoids thus mark a minimum age for Eburnean defor-
PROTEROZOICGRANITOIDAGESIN GHANA
mation with respect to the Kumasi and Sunyani basins. Recent structural data (Eisenlohr and Hirdes, in press) suggest that the conglomerates, quartzites and arcoses of the Tarkwaian Group were also affected by the Eburnean event and thus the above ages also give a minimum age for these sediments in the area investigated. There are several possible interpretations to account for the 30 m.y. age difference between the two (late-kinematic) basin granitoid plutons of the Kumasi and Sunyani depositories. One possible explanation is compositional difference: the older Kumasi basin sample (2116 + 2 Ma) is a biotite tonalite, whereas the younger Sunyani basin sample (2088_ 1 Ma) is a muscovite bearing granitoid. Such an explanation would be in line with observations of Percival and Sullivan (1988) and Card (1990) in the Superior Province of Canada. These authors describe an age difference of several million years between consanguineous biotite- and muscovite-bearing granitoid intrusions. A second hypothesis would involve a scenario in which contemporaneously formed neighbouring volcanic belts are accreted progressively onto each other. More specifically, in the area studied, the Sefwi belt would have been accreted onto the Ashanti belt prior to accretion of the Bui belt onto the Sefwi belt. Such progressive accretion would involve diachronous crustal shortening and deformation, and could trigger the generation of basin granitoid melts. The two differing age dates of 2116 (Kumasi basin) and 2088 (Sunyani basin) would thus represent "docking ages" for the respective neighbouring volcanic belts (Fig. 2 ). The latter hypothesis would be compatible with structural data recently acquired in Ghana (Eisenlohr and Hirdes, in press): accretion would result initially in regional penetrative low-strain deformation forming S l. Additional strain related to continuing accretion and crustal shortening would be accomodated subsequently in spatially well-definedand restricted high-strain zones producing $2 foliation; these
95
would form preferably at margins of volcanic belts. More extensive precise geochronology - in particular from granitoids of the Cape Coast and Maluwe basins which occur adjacent to the Kumasi and Sunyani depositories (Fig. 2) will be required to evaluate either one of these hypotheses.
References Abouchami, W., Boher, M., Michard, A. and Albarede, F., 1990. A major 2.1 Ga event of mafic magmatism in West Africa: An early stage of crustal accretion. J. Geophys. Res., 95 (B11 ): 17,605-17,629. Card, K.D., 1990. A review of the Superior Province of the Canadian Shield, a product of Archean accretion. Precambrian Res., 48:99-156. Davis, D.W., 1982. Optimum linear regression and error estimation applied to U-Pb data. Can. J. Earth Sci., 19: 2141-2149. Eisenlohr, B.N. and Hirdes, W., in press. Structural development of an early Proterozoic rock suite: the example from the Birimian and Tarkwaian rocks in Southwest Ghana, West Africa. J. Afr. Earth Sci. Hirdes, W., Saager, R. and Leube, A., 1988. New structural, radiometric and mineralogical aspects of the Aubearing Tarkwaian Group of Ghana. Bicentennial Gold 88, Melbourne, Abstracts, 1, pp. 146-148. Hirst, T., 1946. Reports on the Bibiani goldfield. Part 1: The geology of the field. Gold Coast Geol. Surv. Mem., 9: 1-16. Junner, H.R., 1940. Geology of the Gold Coast and Western Togoland (with revised geological map). Gold Coast Geol. Surv. Bull., I 1: 40 pp. Kesse, G.O., 1985. The Mineral and Rock Resources of Ghana. Balkema, Rotterdam, 610 pp. Krogh, T.E., 1973. A low contamination method for hydrothermal decomposition of zircon and extraction of U and Pb for isotopic age determination. Geochim. Cosmochim. Acta, 37: 485-494. Krogh, T.E., 1982. Improved accuracy of U-Pb zircon ages by the creation of more concordant systems using air abrasion technique. Geochim. Cosmochim. Acta, 46: 637-649. Leube, A., Hirdes, W., Mauer, R. and Kesse, G.O., 1990. The early Proterozoic Birimian Supergroup of Ghana and some aspects of its associated gold mineralization. Precambrian Res., 46: 139-165. Mauer, R., 1990. Petrographische und geochemische Untersuchungen tier pr[ikambrischen (Birimian) Granitoide Ghanas. Dissertation, Technische Universit~it Berlin, 202 pp.
96
Milesi, J.P., Feybesse, J.L., Ledru, P., Dommanget, A., Ouedraogo, M.F., Marcoux, E., Prost, A., Vinchon, Ch., Sylvain, J.P., Johan, V., Tegyey, M., Calvez, J.Y. and Lagny, Ph., 1989. West African gold deposits in their Lower Proterozoic lithostructural setting. Chron. Rech. Mini~re, 497: 3-98. Percival, J.A. and Sullivan, R.W., 1988. Age constraints on the evolution of the Quetico belt, Superior Province, Ontario. Radiogenic and isotopic studies, 2. Geol. Surv. Can. Pap., 88-2: 97-107.
W. HIRDES ET AL.
Taylor, P.N., Moorbath, S., Leube, A. and Hirdes, W., 1988. Geochronology and crustal evolution of early Proterozoic granite-greenstone terrains in Ghana/West Africa. Int. Conf. and Workshop on the Geology of Ghana with Special Emphasis on Gold, October 1988, Accra. Ghana Geol. Surv. Dept. and Geol. Soc. Ghana. Abstracts, pp. 43-45.
89
Elsevier Science Publishers B.V., Amsterdam
Reassessment of Proterozoic granitoid ages in Ghana on the basis of U / P b zircon and monazite dating W. Hirdes a, D.W. Davis b and B.N. Eisenlohr~ aBundesanstalt 3~r Geowissenschaften und Rohstoffe, Stilleweg 2, D 3000 Hannover, Germany bj. Satterly Geochronology Lab., Royal Ontario Museum, 100 Queen's Park, Toronto, Ont., M5S 2C6 Canada CKeyCentre for Strategic Minerals, University of Western Australia, Nedlands, W.A., Australia (Received March 6, 1991; accepted after revision July 31, 1991 )
ABSTRACT Hirdes, W., Davis, D.W. and Eisenlohr, B.N., 1992. Reassessment of Proterozoic granitoid ages in Ghana on the basis of U/Pb zircon and monazite dating. Precambrian Res., 56: 89-96. The West African Craton in Ghana is of Early Proterozoic age and forms alternating volcanic belts and sedimentary basins made up of rocks of the Birimian Supergroup, as well as younger conglomerates and quartzites of the Tarkwaian Group, and extensive granitoid plutons. No consensus exists on absolute and relative ages of plutons within belts and basins. U-Pb geochronology of four different granitoid complexes indicates that, contrary to long-held views, belt granitoid plutons (Ashanti belt, 2172 + 2 Ma; Sefwi belt, 2179 + 2 Ma) are ~ 60 to 90 Ma older than basin plutons (Kumasi basin, 2116 + 2 Ma; Sunyani basin, 2088 + 1 Ma). Belt granitoids are approximately coeval with volcanic rocks of the Birimian Supergroup, while basin plutons are late-kinematic and postdate Tarkwaian sedimentation. The first high-precision geochronological data for rocks from Ghana, i.e., the eastern part of the Birimian domain on the West African craton, presented here show that magmatism within this region occurred over a time-span of at least 90 Ma, and provide further evidence that the period between 2.1 and 2.2 Ga was one of important magmatism in West Africa.
Introduction
Testing of episodic versus continuous models proposed for the growth of the continental crust requires acquisition of precise geochronology for all major areas of exposed basement. Available data suggest that much of Western Africa may be underlain by crust formed ~ 2.1 Ga ago (Taylor et al., 1988; Abouchami et al., 1990), a time that has generally been considered to be one of relatively slow crustal growth, intervening between two well-documented geochronological peaks at approximately 2.6-2.8 Ga and 1.6-1.9 Ga. It is the purpose of this paper to present the first high-precision geochronological data for rocks from Ghana which constitutes the easternmost portion of Proterozoic rocks of the West African craton (Fig. 1 ). Sev-
eral age determinations on zircons from Proterozoic rocks of the western part of the craton (Senegal, Guinea, Mali, part of Ivory Coast) have been carded out by Calvez as well as Boher (summarized by Milesi et al., 1989), giving an age range between 2115 Ma and 2073 Ma for various "gneisses" and granitoids. Rocks of Early Proterozoic age occupy large parts of the West African craton and crop out extensively in Ghana where they are subdivided into the Birimian Supergroup and the volumetrically subordinate Tarkwaian Group. Both were deformed and metamorphosed to greenschist facies in the course of the Eburnean tectono-thermal event. The Birimian comprises an assemblage of sedimentary/volcaniclastic rocks forming sedimentary basins which separate a series of subparallel, roughly
0301-9268/92/$05.00 © 1992 Elsevier Science Publishers B.V. All fights reserved.
90
W. HIRDES ET AL.
................ !:i:i::::::.:~:::~ .....................................................i[," ii~i~:-":-::!g'~R'(5:~ ~i!~ii~i~i~ii~iiiii~iii~i~:i:i:i:~:i:i:i~il i!:!:~:;~;!~;..' .:~!~:i~!~!!~!~ ~i~i~ i:i::.,.':!:: ~ i !?~:.~ •/ r .~•.. ) ::?.:.~::: :,.:.:::~:.'.: : ::.-.~ ~ :~
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Fig. 1. D i s t r i b u t i o n o f m a j o r r o c k u n i t s i n W e s t A f r i c a : 1 = A r c h e a n c r a t o n i c n u c l e u s ; 2 = A r c h e a n - P r o t e r o z o i c t r a n s i t i o n zone; 3=Lower Proterozoic supracrustal and intrusive rocks with position of Birimian volcanic belts; 4=Panafrican province; 5 = Upper Proterozoic/Phanerozoic sedimentary rocks.
equally spaced, northeasterly trending volcanic belts of mainly tholeiitic basalts (Fig. 2 ). Rocks of the Tarkwaian Group represent erosional products of the Birimian, are dominated by coarse clastic sediments and are located exclusively within the volcanic belts. Both, belts and basins include extensive outcrops of granitoids. The Birimian supracrustal rocks and, with a minor exception, the granitoids in Ghana represent juvenile crustal additions with Sm/Nd model ages of 2.0 to 2.3 Ga (Taylor et al., 1988 ). Recent field and isotope work suggests that the Birimian volcanic belts and sedimentary basins formed contemporaneously as lateral facies equivalents (Taylor et al., 1988; Leube et al., 1990 ). Traditionally, the Lower Proterozoic in Ghana was interpreted to have undergone two deformational events (Junner, 1940). However, new structural data show that the Birimian Supergroup and the Tarkwaian Group were deformed jointly in a single progressive deformation event (Eisenlohr and Hirdes, in
press). This event involved initially a regional penetrative low-strain phase and subsequently the formation of spatially restricted high-strain zones. Proterozoic granitoids in Ghana
Four types of Proterozoic granitoid suites are present in Ghana; traditionally they have been referred to as Winneba, Cape Coast, Dixcove and Bongo granitoids (Junner, 1940; Kesse, 1985), the latter three having recently been termed "basin", "belt" and "K-rich" granitoids (Mauer, 1990; Leube et al., 1990). Absolute ages of these granitoid types and, in particular, their emplacement age relative to each other and the host rocks are crucial to understanding the evolution of the Proterozoic terrane in Ghana. Basin and belt granitoid rocks are widespread, both other types occur only locally. The Wjnneba granitoid is the only rock suite so far encountered in Ghana which shows evidence for an Archean sialic precursor (Sm/Nd model
91
PROTEROZOIC GRANITOID AGES IN GHANA 3°
2°
i
B
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I
1o
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+ + + + + ++ + + + ++ + + Bolgatang + + + -+-+ + + 4+ + + ~ + + ~~ ~ ' + + + + + + + + + ÷ + 4-+ + _ _ / + ~ ' / - + + + + + + + +
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M O D I F I E D G E O L O G Y OF CENTRAL A N D W E S T E R N G H A N A
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TOGO SERIES
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Fig. 2. Geology of central and western Ghana and sample localities of granitoids under investigation (after Leube et al., 1990).
92
w. HIRDES ET AL.
TABLE 1 Characteristics of belt and basin granitoids in Ghana/West Africa Belt granitoids
Basin granitoids
Small to medium-sized plutons, restricted to Birimian volcanic belts
Large batholiths restricted to Birimian sedimentary basins
Contact aureoles of a few tens of metres maximum
Extensive contact-metamorphic aureoles
Seldom foliated (except for local intense shearing), no compositional banding
Foliated, compositional banding ubiquituous and pronounced
Hornblende dominant mafic constituent
Biotite dominant mafic constituent
Metaluminous
Peraluminous
Typically dioritic to granodioritic in composition
Typically granodioritic in composition
Compositional range from granite s.s. to diorite
Compositional range from granite s.s. to tonalite
Na20, CaObe, > Na20, CaObasi°
Rb, K2Obasin > Rb, K2Obelt
Pronounced retrograde mineral alteration
Little mineral alteration
Similar geochemical characteristics as tholeiitic basalts in belts for some elements
No evidence for geochemical similarity to tholeiitic basalts in belts
TABLE 2 Isotopic data for zircon, monazite and rutile from granitoid rocks in Ghana. Fractions are zircon unless specified otherwise Mineral fractions
Sefwibelt-
Weight (mg)
U (ppm)
Pbcom (pg)
Measured 2°7pb/ 2O4pb
Corrected
2°7pbF°6pb Age (Ma)
2OapbFO6pb
2°6pb/238U
207pb/235U
6668
(1) Ab (2)Ab (3)Unab
0.103 0.018 0.149
89 105 116
4 3 20
7147 1970 2789
0.100 0.111 0.085
0.40189 0.39719 0.36931
7.547 7.449 6.912
2179.2_+ 2.9 2177.0_+ 3.6 2173.4-+ 1.8
0.056 0.036 0.060
102 43 191
13 18 18
1485 311 1884
0.118 0.101 0.112
0.40098 0.39232 0.33216
7.497 7.333 6.207
2171.6-+2.5 2171.2+3.6 2170.6_+ 1.8
135 138 164
3 4 50
2088 1906 1678
0.084 0.092 0.070
0.38722 0.38651 0.34551
7.010 7.000 6.198
2115.2-+2.5 2116.0_+3.6 2099.1 + 1.8
2977 3823 225 7
59 14 24 11
813 1759 994 207
2.078 1.585 0.020 0.008
0.38487 0.38148 0.37499 0.36347
6.858 6.799 6.681 6.424
2087.4+2.9 2088.0_+2.9 2087.3+2.5 2073.2-+4.3
A s h a n t i belt - 6 4 2 7
(4) Ab (5) Ab (6) Unab
K u m a s i b a s i n - 6898,4
(7) Ab (8) Ab (9) Unab
0.013 0.016 0.176
Sunyani basin - 8909A
(10) ( 11 ) (12) (13)
Unab, M Ab, M Ab Ab, R
0.005 0.002 0.034 0.102
Ab = abraded; Unab = unabraded; M = monazite; R = rutile; Pbcom = Common Pb, including blank 2tr U / P b errors are 0.6-1.0% for zircon and monazite and 1.6% for futile.
93
PROTEROZOIC GRANITOID AGESIN GHANA
7
2~oo
ru
.40
.38 2040
/"
.36 / / / / /
bT"/
"// / / /
#
217Y+/-2 Mo
OKUMASI BASIN z,,B./-z Mo
.34 .0
6.5
7.0
7.5
Fig. 3. U - P b data for zircon, m o n a z i t e and rutile ( R ) in granitoid rocks from Ghana. The lines represent best-fit regression lines to igneous mineral fractions from each rock. One discordant analysis from the Ashanti belt is off scale.
age of ~ 2.6 Ga, model H-value of 8.4, 87Sr/86Sr initial ratio of 0.7412 ___0.00287 ) (Taylor et al., 1988). The K-rich granitoids possess a clear intrusive relationship with Tarkwaian sediments which overlie Birimian rocks of the Bole-Navrongo volcanic belt in northern Ghana (Fig. 2); their Rb/Sr whole-rock isochron age is 1968___49 Ma (Lenz in Hirdes et al., 1988 ). In this paper, Winneba and K-rich types are not treated in further detail. Principal features of belt and basin granitoids are given in Table 1 which includes data
of Junner (1940), Leube et al. (1990), Mauer (1990) and Eisenlohr and Hirdes (in press). Age relationships between these major Ghanaian granitoid types and their time relationship to rocks of the Birimian Supergroup and to those of the Tarkwaian Group have not been established, due to the absence of intrusive relationships between basin and belt granitoids and the fact that the latter yield poorly constrained Rb/Sr, Pb/Pb and Sm/Nd isotopic ages (Taylor et al., 1988). Also, firm, databased time relationships between the various granitoids and deformation are lacking. Based on their degree of foliation, earlier workers assumed that basin granitoids were intruded during regional deformation, whereas belt granitoids were emplaced after deformation (Junner, 1940; Kesse, 1985). Results of U/Pb zircon, monazite and rutile dating
Igneous crystallization ages of four granitoid plutons were investigated by means of U / P b zircon and/or monazite dating (No. 6427, Ashanti belt; No. 6668, Sefwi belt; No. 6898A, Kumasi basin, No. 8909A, Sunyani basin) (Fig. 2). Unfractured zircon fractions were abraded to remove outside layers of crystals before dissolution in order to reduce lead loss (Krogh, 1982). Sample treatment followed standard techniques (Krogh, 1973) and isotope ratios were determined on a VG354 mass spectrometer. Results of U / P b analyses are presented in Table 2 and Fig. 3. Ages were de-
TABLE 3
Summary of age data and regression parameters Sample
Age (Ma)
Lower intercept (Ma)
Probability of fit (%)
Number of points
Sefwi belt - 6668 Ashanti belt - 6427 Kumasi basin - 6898A Sunyani Basin - 8909A
2178.9+2.3 2171.6+ 1.9 2116.2+2.3 2087.6 + 1.4
131 9 275 -
49 85 68 96
3 3 3 3
94
termined by best-fit regression lines to concordant and discordant data points using the computer program of Davis (1982). Ages are presented in Table 3. All errors are given at the 95% confidence level. Zircon fractions consisted of uniform populations of euhedral, weU-facetted grains, with the exception of sample 8909A. This sample contains zircons of different morphology and colour, and monazite as well as rutile which were also analyzed isotopically for U and Pb. Monazite is typically found as a magmatic mineral in muscovite-bearing granitoid rocks. These rocks often intrude sediments and contain cores of inherited zircons derived from contamination with sediments (Percival and Sullivan, 1988). In sample 8909A, the zircon grains analyzed yielded the same age as grains from both the abraded and unabraded monazite fractions. Rutile in this sample produced a discordant data point with an age of 2073 _+4 Ma, which is a minimum estimate for its growth. The rutile probably is the product of hydrothermal alteration and/or metamorphism as indicated by its depleted T h / U ratio of 0.03 (calculated from the 2°Spb/2°6pb ratio ). Discussion
The above data are in contrast with long-held views; they demonstrate that ( 1 ) the investigated belt ("Dixcove") granitoid plutons from both the Sefwi belt and the Ashanti belt formed approximately contemporaneously at ~ 2175 Ma; (2) the investigated belt ("Dixcove") granitoids are 60 to 90 m.y. older than the investigated basin ("Cape Coast" ) granitoid plutons present in the Sunyani and Kumasi depositories; (3) in contrast to belt-type granitoids, the investigated basin plutons show an age difference of 30 m.y., the younger one being the Sunyani basin granitoid. Ages of the belt plutons coincide closely with
W. HIRDES ET AL.
a 2166 + 66 Ma S m / N d whole-rock isochron age of tholeiitic basalts in Birimian volcanic belts (Taylor et al., 1988), and strongly suggest that belt volcanic rocks and belt granitoids are coeval, comagmatic and parts of the same igneous event. Comagmatism is supported by the intimate association of belt granitoid intrusions and basalts (Hirst, 1946; Eisenlohr and Hirdes, in press). Early pre-deformation emplacement of belt granitoid plutons is also indicated by their absence in Tarkwaian sediments, by their extensive and intensive retrograde alteration and the fact that locally they have been subjected to shearing (Mauer, 1990; Eisenlohr and Hirdes, in press). It is noteworthy that - to the authors' knowledge - the obtained 2178.9 Ma and 2171.6 Ma belt granitoid ages are the oldest precise Proterozoic granitoid ages reported from the West African craton. Such data are strictly incompatible with a recent hypothesis of Milesi et al. (1989). These authors claim that in Ghana and elsewhere in West Africa volcanic and plutonic rocks of the Birimian volcanic belts ("B 2", Milesi et al. ) postdate a major Eburnean tectono-metamorphic phase D~ around 2100 to 2090 Ma, and that such volcanic and plutonic belt rocks are younger than the sedimentary rocks in the basins ("B 1", Milesi et al. ) which are claimed to predate D~ deformation. Basin granitoid intrusions, which were emplaced ~ 60 m.y. later than belt granitoids in the Kumasi basin and ~ 90 m.y. later in the Sunyani basin (Fig. 2 ) show no mineral alteration and exhibit features which suggest that their foliation formed at high temperatures during emplacement, i.e., is not a function of tectonic overprint. The orientation of contactmetamorphic muscovite and biotite grains is discordant with regional foliation in the country rocks indicating that basin granitoid plutons were intruded toward the end of Eburnean deformation. The 2116 and 2088 Ma ages indicated in this study for the basin granitoids thus mark a minimum age for Eburnean defor-
PROTEROZOICGRANITOIDAGESIN GHANA
mation with respect to the Kumasi and Sunyani basins. Recent structural data (Eisenlohr and Hirdes, in press) suggest that the conglomerates, quartzites and arcoses of the Tarkwaian Group were also affected by the Eburnean event and thus the above ages also give a minimum age for these sediments in the area investigated. There are several possible interpretations to account for the 30 m.y. age difference between the two (late-kinematic) basin granitoid plutons of the Kumasi and Sunyani depositories. One possible explanation is compositional difference: the older Kumasi basin sample (2116 + 2 Ma) is a biotite tonalite, whereas the younger Sunyani basin sample (2088_ 1 Ma) is a muscovite bearing granitoid. Such an explanation would be in line with observations of Percival and Sullivan (1988) and Card (1990) in the Superior Province of Canada. These authors describe an age difference of several million years between consanguineous biotite- and muscovite-bearing granitoid intrusions. A second hypothesis would involve a scenario in which contemporaneously formed neighbouring volcanic belts are accreted progressively onto each other. More specifically, in the area studied, the Sefwi belt would have been accreted onto the Ashanti belt prior to accretion of the Bui belt onto the Sefwi belt. Such progressive accretion would involve diachronous crustal shortening and deformation, and could trigger the generation of basin granitoid melts. The two differing age dates of 2116 (Kumasi basin) and 2088 (Sunyani basin) would thus represent "docking ages" for the respective neighbouring volcanic belts (Fig. 2 ). The latter hypothesis would be compatible with structural data recently acquired in Ghana (Eisenlohr and Hirdes, in press): accretion would result initially in regional penetrative low-strain deformation forming S l. Additional strain related to continuing accretion and crustal shortening would be accomodated subsequently in spatially well-definedand restricted high-strain zones producing $2 foliation; these
95
would form preferably at margins of volcanic belts. More extensive precise geochronology - in particular from granitoids of the Cape Coast and Maluwe basins which occur adjacent to the Kumasi and Sunyani depositories (Fig. 2) will be required to evaluate either one of these hypotheses.
References Abouchami, W., Boher, M., Michard, A. and Albarede, F., 1990. A major 2.1 Ga event of mafic magmatism in West Africa: An early stage of crustal accretion. J. Geophys. Res., 95 (B11 ): 17,605-17,629. Card, K.D., 1990. A review of the Superior Province of the Canadian Shield, a product of Archean accretion. Precambrian Res., 48:99-156. Davis, D.W., 1982. Optimum linear regression and error estimation applied to U-Pb data. Can. J. Earth Sci., 19: 2141-2149. Eisenlohr, B.N. and Hirdes, W., in press. Structural development of an early Proterozoic rock suite: the example from the Birimian and Tarkwaian rocks in Southwest Ghana, West Africa. J. Afr. Earth Sci. Hirdes, W., Saager, R. and Leube, A., 1988. New structural, radiometric and mineralogical aspects of the Aubearing Tarkwaian Group of Ghana. Bicentennial Gold 88, Melbourne, Abstracts, 1, pp. 146-148. Hirst, T., 1946. Reports on the Bibiani goldfield. Part 1: The geology of the field. Gold Coast Geol. Surv. Mem., 9: 1-16. Junner, H.R., 1940. Geology of the Gold Coast and Western Togoland (with revised geological map). Gold Coast Geol. Surv. Bull., I 1: 40 pp. Kesse, G.O., 1985. The Mineral and Rock Resources of Ghana. Balkema, Rotterdam, 610 pp. Krogh, T.E., 1973. A low contamination method for hydrothermal decomposition of zircon and extraction of U and Pb for isotopic age determination. Geochim. Cosmochim. Acta, 37: 485-494. Krogh, T.E., 1982. Improved accuracy of U-Pb zircon ages by the creation of more concordant systems using air abrasion technique. Geochim. Cosmochim. Acta, 46: 637-649. Leube, A., Hirdes, W., Mauer, R. and Kesse, G.O., 1990. The early Proterozoic Birimian Supergroup of Ghana and some aspects of its associated gold mineralization. Precambrian Res., 46: 139-165. Mauer, R., 1990. Petrographische und geochemische Untersuchungen tier pr[ikambrischen (Birimian) Granitoide Ghanas. Dissertation, Technische Universit~it Berlin, 202 pp.
96
Milesi, J.P., Feybesse, J.L., Ledru, P., Dommanget, A., Ouedraogo, M.F., Marcoux, E., Prost, A., Vinchon, Ch., Sylvain, J.P., Johan, V., Tegyey, M., Calvez, J.Y. and Lagny, Ph., 1989. West African gold deposits in their Lower Proterozoic lithostructural setting. Chron. Rech. Mini~re, 497: 3-98. Percival, J.A. and Sullivan, R.W., 1988. Age constraints on the evolution of the Quetico belt, Superior Province, Ontario. Radiogenic and isotopic studies, 2. Geol. Surv. Can. Pap., 88-2: 97-107.
W. HIRDES ET AL.
Taylor, P.N., Moorbath, S., Leube, A. and Hirdes, W., 1988. Geochronology and crustal evolution of early Proterozoic granite-greenstone terrains in Ghana/West Africa. Int. Conf. and Workshop on the Geology of Ghana with Special Emphasis on Gold, October 1988, Accra. Ghana Geol. Surv. Dept. and Geol. Soc. Ghana. Abstracts, pp. 43-45.