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A Pb, Sr and Nd isotope study of the basement and mesozoic ring complexes of the Jos Plateau, Nigeria
Chemical Geology (Isotope GeoscienceSection), 94 ( 199 1) 23-32 Elsevier SciencePublishers B.V., Amsterdam
23
A Pb, Sr and Nd isotope study of the basement and Mesozoic ring complexes of the Jos Plateau, Nigeria A.P. Dickin”@, A.N. Halliday”,@and P. Bowdenb aScottish Universities Research and Reactor Centre, East Kilbride, Glasgow G75 OQU, Scotland, UK bDepartment of Geography and Geology University of St. Andrews, Fife KY16 9ST, Scotland, UK
(ReceivedOctober 23, 1989; revisedand acceptedJune 5, 1991)
ABSTRACT Dickin, A.P., Halliday, A.N. and Bowden, P., 1991I A Pb, Sr and Nd isotope study of the basement and Mesozoic ring complexesof the JosPlateau, Nigeria. Chem. Geol. (Isot. Geosci.Sect.), 94: 23-32. Combined Pb, Sr and Nd isotope determinations on severalNigerian Mesozoic ( w 170 Ma) ring complexes,studied previouslyby van Breemenand co-workers,indicate a multistage petrogeneticprocess.Mantle-derived differentiated magmas assimilatedcrustal basementof averageEarly Proterozoicage.After crystallisation,someplutons were subjectedto a secondstageof crustal contamination by circulating hydrothermal fluids. Crustal compositions were constrained by isotopic analysisof the Proterozoic basement of the Jos Plateau. Sm/Nd analysisof six gneissesyielded an averagecrustalresidenceageof 2 Ga, correspondingto the Burkinian event recognised elsewherein western Africa. However, one sampleyields a model age of 3 Ga, suggestingthe presenceof Archean crustal remnants. Pan-Africangranitoids yield a similar rangeof Nd model agesto the gneisses,suggestingthat they were largely generatedby crustalmelting. The Zaranda anorogeniccomplex hasrelatively radiogenicinitial Nd and Pb isotope compositionsand unradiogenic Sr (-0.5126, + 18.4 and +,0.705, respectively),attributed to a mantle-derived differentiated magma which suffered moderate contamination during ascentthrough the crust.Other ring complexestrend toward lessradiogenicNd and Pb isotope ratios and more radiogenicSr,indicative of an increasingcrustalcontribution. Initial Pb isotope compositionsyield a welldefined Pb/Pb isotope array with a slope ageof w 1.8 Ga which is consistentwith the averageNd crustalresidenceagesof basementgneissesand granitoids. The arfvedsonite albite apogranite from the Ririwai anorogenic complex has isotope ratios resembling Pan-African basement,probably resulting from hydrothermal overprinting with fluids equilibrated in the continental crust. Other Ririwai intrusions and one unit from the ShereHills display evidenceof hydrothermal overprinting of Sr and to someextent Nd isotope compositions,but only the Ririwai apogranite hasbeen significantly overprinted by hydrothermal Pb. The isotopic evidencesupports a model for the Mesozoicanorogenic (“A-type”) granitesof Nigeria in which mantlederived magmas suffered crustal contamination during magmatic differentiation to syenitic compositions, followed by sub-solidushydrothermal alteration in the continental crust.
1. Introduction The Nigerian Mesozoic granites form a series of anorogenicring complexeswhich cut the exposedPrecambrian basement of the Jos PlaPresent addresses:
aDepartment of Geology, McMaster University, Hamilton, Ont. L8S 4M 1, Canada. @Department of Geological Sciences,University of Michigan, Ann Arbor, MI 48109, U.S.A.
0168-9622/91/$03.50
teau in central Nigeria (Fig. 1) . They were regardedby Collins et al. ( 1982) and Bowden et al. ( 1984) as typical examples of A-type granites, a terminology first applied to anorogenic granites of distinctive mineralogy forming small post-tectonic intrusions in New England, U.S.A. (Loiselle and Wones, 1979). Different workers have conflicting views on the origins of A-type granites, and the petrogenesis of the Nigerian anorogenic granites is there-
0 1991 Elsevier SciencePublishers B.V. All rights reserved.
4.1’. DIC‘KIN
I I
IOOkm
ET Al.
I
N
Zaranda
) Amc BASEMENT PLATEAU
OF *
CAMEROON
I
SOOkm
5'
i
IO'
15-E
7’
Fig. 1. A. Location map of Nigeria showing areas of Precambrian basement outcrop (shaded)
9’
and Phanerozoic cover
(white).
B. Map of the eastern half of the Jos Plateau, showing locations of Mesozoic ring complexes enclosing plutonic rocks (black) and lavas (white). Labels indicate localities of sampled ring complexes and Precambrian basement rocks.
fore of generalas well as regional significance. The Nigerian ring complexes are thought to be the plutonic equivalents of chains of volcanoeserupting minor quantities of mildly alkaline basalt lavas, occasionally intercalated with hawaiite, trachyandesite and trachytes, but normally interbedded with peralkaline ignimbritic rhyolites which ultimately dominated the volcanism (Bowden et al., 1984). Bowden and Kinnaird ( 1978, 1984) identified a liquid line of descent from a syenite parent liquid (itself possibly derived by fractionation of a basic magma) to fayalite granite compositions. However, an early peralkaline trend leading to riebeckite granites and a late peraluminous trend leading to biotite granites were attributed to sub-solidus hydrothermal alteration. This alteration limits the reliability of much of the elemental chemistry of the plutons as petrogenetic indicators. In this situation, radiogenic isotope tracers may be more diagnostic. A detailed Rb-Sr study of the Nigerian Mesozoic granites was made by van Breemen et
al. ( 1975). Suites were analysed from six ring complexes in order to determine the agesand initial ratios of different units within each complex. Most of the rocks analysed had high Rb/Sr ratios, resulting in high-precision radiometric ages.However, in some casesthe error on initial Sr isotope ratios was very large, leading to uncertainties in petrogenetic interpretations basedon the data. In order to overcome these problems, further study is made here of the same sample suite analysed by van Breemen et al., using mineral separatesto establish more accurate initial Sr isotope ratios, and analysing selected samplesfor Pb and Nd isotope ratios. New isotopic analysesare also presented for samples of crustal basement from the Jos Plateau, including material studied by van Breemen et al. ( 1977). Most isotopic measurements were made on a VG@ 54E mass spectrometer at the Scottish Universities Research and Reactor Centre, using published techniques (Halliday et al., 1983). Five further Sm/Nd analyses were performed on a VG@ 354 mass spectrom-
BASEMENT
AND
MESOZOIC
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OF THE JOS PLATEAU
eter at McMaster University. All quoted Rb/ Sr agesare recalculated using a decay constant of 1.42.10-r’ a-’ for 87Rb.A decay constant of 6.54. lo-l2 a-’ was used for 14’Sm. 2. The Benin-Nigeria crustal basement The Benin-Nigeria shield forms part of the Pan-African belt of western Africa. Studies on the Tuareg shield to the north (Black, 1985) indicate ocean closure by continental collision in late Pan-African times. The continents involved in collision were essentially the 2-Gaold cratons of the Hoggar-Air domain and the Benin-Nigeria shield, but considerable rejuvenation of old agesoccurred during thrusting and folding. In SW Nigeria the Precambrian basement consists in part of charnockitic rocks of unknown ageand a gneissic complex comprising banded gneisses,migmatites, quartzites, schist, biotite -t amphibole gneisses,amphibolites and marbles metamorphosed to almandine-amphibolite facies. Caen-Vachette and Umeji ( 1987) attempted to datetheseparagneissesby the Rb/Sr method. Nineteen analysesdefined a scatterchron with an upper bound of 2202 + 3 1Ma and a lower bound of N I 700 Ma. Caen-Vachette and Umeji interpreted the 2202-Ma ageas a major metamorphic event in the Eburnean orogeny, renamed “Burkinian” by Lemoine et al. ( 1985). This interpretation is supported by an Rb-Sr ageof 2278 t- 34 Ma on foliated granitesfrom SW Nigeria which cut the paragneissic rocks (Grant, 1970). However, a crustal prehistory for the paragneissic rocks dating back to the Archean is indicated by agesof > 2600 Ma from the basement complex of southern Nigeria (Grant, 1970; Oversby, 1975). Pan-African rejuvenation of the basementproduced biotite Rb-Sr agesaround 500-600 Ma (Caen-Vachette and Umeji, 1987). In order to characterise the composition of
25
the crustal basement of the Jos Plateau, two migmatites (295, 297) and four granitoid orthogneisses ( 298, 671, 6 73, 674) were analysed from Malumfashi, north of Zaria (Fig. 1b, Tables I-III). The samples display a complete scatteron an Rb-Sr isochron diagram and yield depleted-mantle model ages ( tDM; DePaolo, 1981) from 1.3 to 3.0 Ga, suggestinga complex evolutionary history commencing in the Archean. However, the suite yields an average crustal residence age of 2 Ga and suggests that the Early Proterozoic “Burkinian” event was an important component in the crust-forming process. The continental crust of the Jos Plateau is largely composed of Pan-African granites with crystallisation agesranging from 596 to 677 Ma (van Breemen et al., 1977). Where possible, the same rocks with the extremes of Sr isotope composition (highest and lowest Rb/Sr) were analysed for Pb, though only one sample was analysed for Nd from each locality. When Nd isotope compositions are age corrected using the Rb-Sr dates, the initial ‘43Nd/144Ndratios, expressedas E(‘) are -2.3, - 10.5 and - 13.2 for samples of the Panyam (336)) Rahama (438) and Bauchi (331) plutons, respectively. These enriched sourcecompositions suggestthat the Pan-African granites were largely derived by melting of older crustal basement. Depleted-mantle Nd model agesfor these samples are 1.29, 1.92 and 1.96 Ga, respectively, again suggestingthat “Burkinian” basement played a role in the genesisof PanAfrican granites. Support for theseconclusions is provided by the Cameroon Line volcanics from Bambouto (Fig. la), which contain granulite enclaves (Halliday et al., 1988) which sample a segment of crustal basement similar to that forming the Jos Plateau. Two granulite enclaves yield tDM agesof 1.47 and 2.09 Ga, which are within the same range as the Jos Plateau samples. However, the Sr isotope composition of the enclaves is less radiogenic, possibly becausethey representRb-depleted lower crust.
26
A.P. DICKIN
ET AL.
TABLE 1 Rb/Sr data Sample No.
Lithology
Age (MaI
biotite granite
162
5.25
biotite granite
161
biotite granite syenite
151 151
biotite granite arfvedsonite granite apogranite
165.8 168 166
129.2 13.27
arfvedsonite granite riebeckite-biotite fayalite granite
160.6 160.6 161
trachy-rhyolite syenite syenite
186 186 186
&I
( 87Sr/86Sr),
20
0.71902
0.7069
0.0005
+37
16.77
0.74497
0.7066
0.0010
+33
13.99 0.31
0.73614 0.70544
0.7062 0.7048
0.0013 0.0002
+27 +7
1.04143 0.80456
0.7345 0.7220 0.7732
0.0070 0.0120 0.0040
+ 429 +251 +978
15.91 172.0 21.57
0.75181 1.10790 0.75942
0.7155 0.7156 0.7100
0.000 1 0.0010 0.0010
+158 +160 +81
85.55 2.33 0.17
0.93815 0.71105 0.70535
0.7150 0.7049 0.7049
0.0100 0.0001 0.000 I
+152 +9 +9
1.66 0.67 7.74 0.30 0.73 0.95 2.09
0.7217 0.7162 0.7763 0.7108 0.7149 0.7173 0.7276
0.7177 0.7146 0.7576 0.7101 0.7131 0.7150 0.7226
+190 +146 +757 +82 +125 +152 +260
3.35 1.25 4.47 2.43 6.35 2.25
0.8210 0.7226 0.7831 0.7286 0.7863 0.7324
0.8129 0.7196 0.7723 0.7227 0.7710 0.7270
+1542 +217 +965 +261 +947 + 322
Amo: 369WR Jos: 473 WR Pankshin: 451 WR 470 WR Ririwai: 412 WR 417-422 427As3 Shere Hills: 35 9 R*“ 361 R*’ 362 R Zaranda: 387 WR 38.5 WR 790 WR
Pan-African granites: isotope ratios at 170 Ma: 336 32s 331 375 377 436 438
Panyam Bauchi Bauchi Bauchi Bauchi Rahama Rahama
Maiumfashi gneisses: 295 297 298 671 673 674
migmatite migmatite granitic gneiss granitic gneiss granitic gneiss dioritic gneiss
*‘Errors t 1% (2~). **Errors average + 0.000 1 (20). *3Arfvedsonite separate. *4Arfvedsonite separate from same pluton as sample 355 analysed for Pb. *‘Riebeckite separate from same pluton as sample 360 analysed for Pb.
BASEMENT
AND MESOZOIC
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27
OF THE JOS PLATEAU
TABLE II Sm/Nd data Sample No.
Sm
Nd
(ppm)
(wm)
147Sm/144Nd
‘43Nd/‘44Nd
20
‘43Nd/‘44Nd
(170Ma)(.) %d
tDM (Gal
Mesozoic ring complexes: 369 473 451 412 417 360 385
10.69 36.98 68.97 33.21 18.48 14.49 18.79
50.62 163.7 338.2 137.1 82.36 62.62 116.8
0.1276 0.1365 0.1233 0.1464 0.1357 0.1399 0.0973
0.512501 0.512410 0.512361 0.512298 0.512436 0.512405 0.512564
0.000016 0.000030 0.000043 0.000018 0.000025 0.000023 0.000019
0.51237 0.5 1227 0.5 1224 0.51214 0.51229 0.51226 0.51245
-1.3 -3.2 -4.0 -5.6 -2.7 -3.4 +0.9
54.65 36.38 54.85
0.1006 0.1022 0.1091
0.511588 0.512101 0.511726
0.000017 0.000016 0.000019
0.51148 0.51200 0.51161
-18.4 -8.3 - 15.9
1.96 1.29 1.92
25.34 19.40 31.06 36.71 12.70 24.86
0.1248 0.0962 0.1221 0.1056 0.1442 0.1189
0.511314 0.511333 0.511967 0.512130 0.512039 0.5 12076
0.000018 0.000011 0.000011 0.000011 0.000011 0.000011
0.51118 0.51123 0.51183 0.51201 0.51188 0.51195
- 24.3 -23.3 -11.5 -1.9 - 10.6 -9.3
3.00 2.22 1.70 1.29 2.21 1.55
Pan-African granitoids: 331 336 438
9.096 6.148 9.896
Malumfbshi 295 297 298 671 673 674
gneisses:
5.232 3.086 6.269 6.414 3.026 4.892
* c(‘) =parts per ten thousand deviation from bulk Earth evolution line at time of emplacement, 1.
3. Revisedchronology of the Mesozoic ring complexes Rocks from the Amo, Jos, Pankshin, Ririwai, Shere Hills and Zaranda anorogenic complexes were analysed from the sample suite of van Breemen et al. ( 1975). When selecting samples for Pb and Nd isotope determination, those with the lowest Rb/Sr ratio from each intrusive unit (according to van Breemen et al. ) were chosenfor analysis. In the caseof the Amo, Jos, Pankshin and Zaranda plutonites, such samples had sufficiently low Rb/Sr ratio for the calculation of accurate initial 87Sr/86Sr ratios using the isochron agesof van Breemen et al. (Table I). Hence in these casesthe initial Pb, Nd, and Sr ratios are based on the same sample. In the Ririwai and Shere Hills complexes,whole-rock Rb/Sr ratios are too high to usethis procedure,and lower-Rb/Sr separated mineral phaseswere analysedin order to more
accurately constrain initial 87Sr/86Srratios. The Zaranda trachy-rhyolite initial ratio cannot be accurately determined in this manner becausethe rock is too fine-grained to allow mineral separation. For the Shere Hills complex an alkali amphibole separatewas analysedfrom eachof the three intrusive phases.Amphibole and wholerock samples from the arfvedsonite granite and riebeckite-biotite granites are colinear, yielding an ageof 160.62 1.2Ma and initial Sr ratio (I.R. ) of 0.715562 0.00009 (20). However, the amphibole separatefrom the arfvedsonite granite with remnant fayalite (362 R) yields a distinctly lower initial ratio of 0.7100 at 161 Ma (Table I). Re-analysis of the least radiogenic wholerock samples of the Ririwai biotite and arfvedsonite granites yielded isochrons with agesof 165.8_+1.6 and 168+ 6 Ma, respectively, in better agreement than in previous work (van
28
A.P. DICKIN
ET AL.
TABLE III Pb isotope data Sample “‘Pb Jzo4Pb No.
207Pb/204Pb
2osPb/‘04Pb
15.591
39.300
5.2
31
15.567 15.572
30.273 39.227
10.8 12.9
15.548 15.556
39.138 38.417
15.582 15.599 15.618 15.645 15.537 15.61 I
39.438 39.314 39.666 39.982 38.769 38.685
9.8 12.3 19.7 8.3 78 198
15.562 15.559 15.555
39.156 38.929 38.574
7.8 8.8 6.9
15.616 15.670 15.665 15.631 15.648
38.566 39.116 39.274 39.404 39.497
15.665 15.568 15.618 15.594 15.584 15.569 15.578
15.510 15.375 15.830 15.645 15.789 15.710
U (mm)
Th (mm)
Pb (wm)
(206Pb/204Pb),
( 204Pb/204Pb),
(20*Pb/204Pb),
14.2
17.93
15.56
38.13
48 53
24 33
17.63 17.55
15.56 15.54
38.13 38.45
36 4*
17.5 7.8
17.56 18.00
15.52 15.55
38.12 38.17
43 40 42 17.3 274 465
17.85 17.81 17.88 18.06 17.65 17.84
15.56 15.57 15.58 15.60 15.51 15.61
38.11 38.41 38.28 38.46 38.27 38.69
32 40 7.8
21 49 74
17.61 17.72 17.68
15.53 15.54 15.55
38.36 38.50 38.52
0.64 6.8 2.57 0.91 2.14
3* 16.5 10* 4* 11
21 16.3 20 4.4 6.1
18.35 18.42 18.45 18.29 18.29
15.61 15.63 15.65 15.61 15.62
38.48 38.49 38.97 30.84 38.38
39.508 38.433 38.780 40.617 38.523 38.733 38.811
3.4 1.55 3.1 3.9 1.98 2.59 6.6
19.1 3.5 21 52 8.2 11.6 15.0
20.2 13.0 37 38 16.3 19.8 21
18.42 17.64 17.91 17.61 17.86 17.73 17.54
15.65 15.56 15.61 15.59 15.57 15.56 15.55
38.97 38.29 38.47 39.84 38.25 38.4 I 38.42
38.073 37.676 39.446 38.727 38.533 38.924
2.13 5.0 1.28 2.38 1.27 0.71
8* 20* 5* lO* 5* 3*
42 34 30 40 21 20
19.32 17.12 19.46 18.44 19.66 18.63
15.51 15.36 15.83 15.64 15.78 15.71
37.97 37.36 39.35 38.59 38.40 38.84
A mo comple,u: 369
18.533
Jos complex: 473 474
18.378 18.193
Pankshin complex: 451 470
18.079 18.200
6.1 1.05
Ririwai complex: 417 422 412 414 424 427
18.238 18.341 18.702 18.900 18.291 17.842
102 63 102 46 230 376
Shere Hills complex: 362 360 355
18.201 18.017 17.830
Zaranda complex: 790 387 388 385 389
18.409 18.214 18.692 18.678 18.959
Pan-African granites: 336 375 325 331 377 436 438
18.710 17.837 18.053 17.782 18.066 17.953 18.069
Malumfashi gneisses: 295 297 298 671 673 674
19.402 17.362 19.530 18.542 19.768 18.694
t = age for Mesozoic granite, else 170 Ma. Average within run precision on Pb isotope ratios = 0.003 (2~). Estimated absolute accuracy of Pb isotope ratios=O.Ol, 0.007 and 0.02 ( lo) for 206Pb/204Pb, 207Pb/2WPb and 2osPb/204Pb, respectively. *Estimated concentration.
BASEMENT
AND MESOZOIC
RING
COMPLEXES
29
OF THE JOS PLATEAU
Breemen et al., 1975). However, initial ratios of 0.735 and 0.722 are distinct. The arfvedsonite albite apogranite also yields a consistent age of 16627 Ma (I.R.=0.773) when an alkali amphibole separate (427A) is included in the whole-rock isochron and a sample with anomalously high Sr isotope ratio (426) is excluded. Overall, the different intrusive units of the Ririwai complex display gross initial Sr isotope heterogeneity (Table I), but were probably emplaced in a brief time interval near 166 Ma ago. 4. Isotope geochemistry of Mesozoic ring complexes Initial Sr and Nd isotope ratios are expressed as e(‘) and compared with e(“’ Ma) compositions of possible mantle and crustal sources in Fig. 2. The Mesozoic ring complexes have a wide range of eSrcompositions (from + 9 to + 978) but a relatively restricted rangein eNd( - 5.6 to + 0.9 ) , which is more radiogenic than any of the crustal samples analysed (eNd= -24.3 to -8.3 at 170 Ma). The positive tNd composition of the Zaranda syenite ( +0.9) provides good evidence that this +I0
I\
I
,
I
,
I
0 ENd
-10 Jos PLATEAU
-20
0
,
I +100
NIGERIA &merowl gmnulne eflclave x I I
+200
+ml
4.400
6.3
Fig. 2. eNdvs. tsr diagram showing the compositions of Nigerian Mesozoic ring complexes (Z= Zaranda; A = Amo; J= Jos; P= Pankshin; R = Ririwai; S= Shere Hills) and possible source compositions. The composition of basement forming the Jos Plateau is defined by Pan-African granites and the Malumfashi gneisses.Crosses represent granulite enclaves in Cameroon Line volcanics from Bambouto (Fig. 1).
I
I
16
19
1
1
20
(‘D’Pb/‘o’Pbl,,,,M,
Fig. 3. Initial Pb isotope compositions of Mesozoic ring complexes (symbols as in Fig. 2) and composition of possible sourcecompositions at 170 Ma (average age of magmatism ) ( x = Pan-African granites; * = Malumfashi gneisses). Encircled R symbols = Ririwai hydrothermal apogranite samples. SK indicates Pb isotope evolution lines of the Staceyand Kramers ( 1975) model.
contains a mantle-derived component, since it must come from a reservoir with time-integrated light rare-earth depletion relative to the bulk Earth. However, significant crustal contamination has occurred. The range of composition from eNdof - 5.6 to - 1.3 in the remaining centres is consistent with the conclusion of van Breemen et al. ( 1975), based on Sr isotope evidence, that “syenite-granite occurrences in the Nigerian Younger Granite province... may be attributed to crustal enrichment of syenitic liquids whose source lies in the mantle”.
,Initial Pb isotope compositions of Mesozoic rocks and Pb isotope compositions of possible mantle and crustal sourcesat 170 Ma (Table
30
A.P. DICKIN
PAN AFRICAN BASEMENT
17.0 0.70
I 0.71
I 0.72
I 0.72
1 0.74
0.75
(87s&sr),
Fig. 4. Initial Pb vs. Sr isotope compositions of Mesozoic ring complexes and field for Pan-African basement at 170 Ma (symbols as in Fig. 2). Brackets represent ranges of Pb isotope compositions for individual units and estimated errors on initial Sr isotope compositions.
III) areplotted on 207Pb/204Pb vs. 206Pb/204Pb and 208Pb/204Pbvs. 206Pb/204Pbdiagrams in Fig. 3. The mantle Pb field, as inferred from uncontaminated Cameroon Line volcanics (Halliday et al., 1988)) is age corrected using a mantle p-value ( 238U/204Pb)of 9.0. With the exception of the Ririwai arfvedsonite-albite apogranite (samples 424 and 427)) the Nigerian Mesozoic ring complexes form a short array on the Pb/Pb isochron diagram (Fig. 33)) whose slope correspondsto an apparent ageof - 1750 Ma. When the Mesozoic ring complex array is compared with possible magma sources, it is seen that it barely overlaps with the mantle field estimated from uncontaminated Cameroon Line volcanisms. However, the array does lie between the Pan-African granite and Malumfashi gneisscrustal fields in both Fig. 3A and B. When this observation is coupled with the 1750-Ma apparent ageof the ring complexes it suggeststhat Pb in theserocks is derived largely by mixing of crustal Pb sources with an averageEarly Proterozoic formation age. 5. Discussion Some of the Mesozoic ring complexes show evidence of intense hydrothermal activity (Bowden et al., 1979; Bowden and Kinnaird,
ET AL.
1984). Consequently it is important to establish the relative importance of crustal contamination at the magmatic and post-consolidation hydrothermal stages. The evidence for hydrothermal overprinting of isotope systematics in the Ririwai arfvedsonite-albeit apogranite is very strong. In addition to gross initial Sr isotope heterogeneity (Table I), this unit displays Pb isotope compositions significantly different from the remaining Mesozoic rocks and has Pb, U and Th contents of several hundreds of ppm (Table III). However, Pb isotope systematics appear not to have been hydrothermally perturbed in the other Mesozoic granites, including the biotite and arfvedsonite granites from Ririwai. The moderately elevated and heterogeneous initial Sr isotope compositions of the Shere Hills granites may also be attributed to sub-solidus alteration. This interpretation is supported by the minimal changesin eNdbetween this body and the granites with unradiogenic Sr, yielding a horizontal vector in Fig. 2. This is typical of the known relative mobility of Sr and immobility of rare earthsin hydrous fluids. The Ririwai apogranite shows a deviation in Nd as well asSr isotope composition, but Bowden et al. ( 1979) have shown that even rareearth elements were disturbed by hydrothermal activity in this grossly altered body. Our interpretation of the isotopic data can be summarised with the aid of Fig. 4. Assimilation of lower-crustal melts by mantle-derived differentiating magmas yields a trend from radiogenic to less radiogenic Pb, which is however accompanied by only minor increases in initial *‘Sr/*%r. This trend may result from crustal contamination by Rb-poor lower-crustal basement, and is observed in the Zaranda syenite and the Amo, Pankshin and Jos complexes. Hydrothermal overprinting of the Ririwai complex and one or more shereHills unit then superimposed a trend almost at right-angles to the magmatic contamination trend, in which 87Sr/86Srrose very rapidly. Only in the caseof extreme alteration suffered by the Rir-
BASEMENT
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OF THE JOS PLATEAU
iwai apogranite were Pb isotope compositions significantly affected by this process. 6. Conclusions Collins et al. ( 1982) regard A-type granites, along with I- and S-type, as almost exclusively the products of crustal melting, while Didier et al. ( 1982) distinguish C (crustal) and M (mantle-derived or mixed crustal + mantle) -types within each of the A-, I- and S-types. The evidence which we have presentedhere continues to support the original conclusion of van Breemen et al. ( 1975) that the Nigerian Mesozoic granites are the product of mantlederived magmas which have suffered a significant amount of contamination during their uprise and differentiation in the crustal basement of the Jos Plateau. The extent of this contamination is relatively minor in the Zaranda syenite, but increases to a more significant fraction in the Amo, Jos and Pankshin comnlexes. . Pb isotope data are consistent with assimilation of Early Proterozoic crust, which is the averageformation age found by Sm/Nd analysis of both Malumfashi gneissesand Pan-African granites of the Jos Plateau. Evidence hasbeen presentedfor extreme hydrothermal Sr and Pb contamination of the Ririwai arfvedsonite-albite apogranite. Other Ririwai units and one Shere Hills pluton appear to have suffered significant hydrothermal Sr contamination but little disturbance of Pb. Acknowledgements Isotope studies at East Kilbride are supported by the NERC and the Scottish Universities. Field work was supported by the NERC and the Overseas Development Administration. We thank J. Hutchinson and J. Jocelyn for technical assistance.We thank Dr. 0. van Breemen for the use of his unpublished Rb/Sr data on the Malumfashi samples.
References Black, R., 1985. The Pan-African event in the geological framework of Africa. Pangea, 1: 6- 16. Bowden, P. and Kinnaird, J.A., 1978. Younger granites of Nigeria - a zinc-rich tin province. Trans. Inst. Min. Metall., 87: B66-B69. Bowden, P. and Kinnaird, J.A., 1984. The petrology and geochemistry of alkaline granites from Nigeria. Phys. Earth Planet. Inter., 35: 199-211. Bowden, P., Bennet, J.N., Whitley, J.E. and Moyes, A.B., 1979. Rare earths in Nigerian Mesozoic granites and related rock. In: L.H. Ahrens (Editor), Origin and Distribution of the Elements (2nd Symposium). Phys. Chem. Earth, 11: 479-49 1. Bowden, P., Batchelor, R.A., Chappell, B.W., Didier, J. and Lameyre, J., 1984. Petrological, geochemical and source criteria for the classification of granitic rocks: a discussion. Phys. Earth Planet. Inter., 35: l-l 1. Caen-Vachette, M. and Umeji, A.C., 1987. Geology and geochronology of the Okene area: evidence for an Eburnean erogenic cycle in south-central Nigeria. J. Afr. Earth Sci., 7: 121-126. Collins, W.J., Beams, S.D., White, A.J.R. and Chappel, B.W., 1982. Nature and origin of A-type granites with particular reference to south eastern Australia. Contrib. Mineral. Petrol., 80: 189-200. DePaolo, D.J., 1981. Neodymium isotopes in the Colorado Front Range and crust-mantle evolution in the Proterozoic. Nature (London), 291: 193-196. Didier, J., Duthou, J.L. and Lameyre J., 1982. Mantle and crustal granites: genetic classification of erogenic granites and the nature of their enclaves. J. Volcanol. Geotherm. Res., 14: 125-132. Grant, N.K., 1970. The geochronology of Precambrian basement rocks from Ibadan, south-western Nigeria. Earth Planet. Sci. Lett., 10: 29-38. Halliday, A.N., Fallick, A.E., Dickin, A.P., MacKenzie, A.B., Stephens, W.E. and Hildreth, W., 1983. The isotopic and chemical evolution of Mount St. Helens. Earth Planet. Sci. Lett., 63: 241-256. Halliday, A.N., Dickin, A.P., Fallick, A.E. and Fitton, J.G., 1988. Mantle dynamics: a Nd, Sr, Pb and 0 isotopic study of the Cameroon Line volcanic chain. J. Petrol., 29: 181-211. Lemoine, S., Tempier, P., Basso&J.P., Caen-Vachette, M., Viaette, Y., Wenmenga, U. and Toure, S., 1985. Le Burkinien, cycle orogenique precurseur de I’EbumCen en Afrique de l’ouest. 13th Colloq. Afr. Geol., St. Andrews, Scotland, Sept. 10-l 3, 1985. Loiselle, MC. and Wones,D.R., 1979. Characteristicsand origin of anorogenic granites. Geol. Sot. Am. Abstr. Prog., 11: 468. Oversby, V.M., 1975. A lead isotope study ofaplites from the Precambrian basement rocks near Ibadan, SW Nigeria. Earth Planet. Sci. Lett., 27: 177-I 80.
32 Stacey, J.S. and Kramer, J.D., 1975. Approximation of terrestrial lead isotope evolution by a two-stage model. Earth Planet. Sci. Lett., 26: 207-22 1, van Breemen, O., Hutchinson, J. and Bowden, P., 1975. Age and origin of the Nigerian Mesozoic granites - A
A.P. DICKIN
ET AL.
Rb-Sr isotope study. Contrib. Mineral. Petrol., 50: 157-172. van Breemen, O., Pidgeon, R.T. and Bowden, P., 1977. Age and isotope studies of some Pan-African granites from north-central Nigeria. Precambrian Res., 4: 307319.
23
A Pb, Sr and Nd isotope study of the basement and Mesozoic ring complexes of the Jos Plateau, Nigeria A.P. Dickin”@, A.N. Halliday”,@and P. Bowdenb aScottish Universities Research and Reactor Centre, East Kilbride, Glasgow G75 OQU, Scotland, UK bDepartment of Geography and Geology University of St. Andrews, Fife KY16 9ST, Scotland, UK
(ReceivedOctober 23, 1989; revisedand acceptedJune 5, 1991)
ABSTRACT Dickin, A.P., Halliday, A.N. and Bowden, P., 1991I A Pb, Sr and Nd isotope study of the basement and Mesozoic ring complexesof the JosPlateau, Nigeria. Chem. Geol. (Isot. Geosci.Sect.), 94: 23-32. Combined Pb, Sr and Nd isotope determinations on severalNigerian Mesozoic ( w 170 Ma) ring complexes,studied previouslyby van Breemenand co-workers,indicate a multistage petrogeneticprocess.Mantle-derived differentiated magmas assimilatedcrustal basementof averageEarly Proterozoicage.After crystallisation,someplutons were subjectedto a secondstageof crustal contamination by circulating hydrothermal fluids. Crustal compositions were constrained by isotopic analysisof the Proterozoic basement of the Jos Plateau. Sm/Nd analysisof six gneissesyielded an averagecrustalresidenceageof 2 Ga, correspondingto the Burkinian event recognised elsewherein western Africa. However, one sampleyields a model age of 3 Ga, suggestingthe presenceof Archean crustal remnants. Pan-Africangranitoids yield a similar rangeof Nd model agesto the gneisses,suggestingthat they were largely generatedby crustalmelting. The Zaranda anorogeniccomplex hasrelatively radiogenicinitial Nd and Pb isotope compositionsand unradiogenic Sr (-0.5126, + 18.4 and +,0.705, respectively),attributed to a mantle-derived differentiated magma which suffered moderate contamination during ascentthrough the crust.Other ring complexestrend toward lessradiogenicNd and Pb isotope ratios and more radiogenicSr,indicative of an increasingcrustalcontribution. Initial Pb isotope compositionsyield a welldefined Pb/Pb isotope array with a slope ageof w 1.8 Ga which is consistentwith the averageNd crustalresidenceagesof basementgneissesand granitoids. The arfvedsonite albite apogranite from the Ririwai anorogenic complex has isotope ratios resembling Pan-African basement,probably resulting from hydrothermal overprinting with fluids equilibrated in the continental crust. Other Ririwai intrusions and one unit from the ShereHills display evidenceof hydrothermal overprinting of Sr and to someextent Nd isotope compositions,but only the Ririwai apogranite hasbeen significantly overprinted by hydrothermal Pb. The isotopic evidencesupports a model for the Mesozoicanorogenic (“A-type”) granitesof Nigeria in which mantlederived magmas suffered crustal contamination during magmatic differentiation to syenitic compositions, followed by sub-solidushydrothermal alteration in the continental crust.
1. Introduction The Nigerian Mesozoic granites form a series of anorogenicring complexeswhich cut the exposedPrecambrian basement of the Jos PlaPresent addresses:
aDepartment of Geology, McMaster University, Hamilton, Ont. L8S 4M 1, Canada. @Department of Geological Sciences,University of Michigan, Ann Arbor, MI 48109, U.S.A.
0168-9622/91/$03.50
teau in central Nigeria (Fig. 1) . They were regardedby Collins et al. ( 1982) and Bowden et al. ( 1984) as typical examples of A-type granites, a terminology first applied to anorogenic granites of distinctive mineralogy forming small post-tectonic intrusions in New England, U.S.A. (Loiselle and Wones, 1979). Different workers have conflicting views on the origins of A-type granites, and the petrogenesis of the Nigerian anorogenic granites is there-
0 1991 Elsevier SciencePublishers B.V. All rights reserved.
4.1’. DIC‘KIN
I I
IOOkm
ET Al.
I
N
Zaranda
) Amc BASEMENT PLATEAU
OF *
CAMEROON
I
SOOkm
5'
i
IO'
15-E
7’
Fig. 1. A. Location map of Nigeria showing areas of Precambrian basement outcrop (shaded)
9’
and Phanerozoic cover
(white).
B. Map of the eastern half of the Jos Plateau, showing locations of Mesozoic ring complexes enclosing plutonic rocks (black) and lavas (white). Labels indicate localities of sampled ring complexes and Precambrian basement rocks.
fore of generalas well as regional significance. The Nigerian ring complexes are thought to be the plutonic equivalents of chains of volcanoeserupting minor quantities of mildly alkaline basalt lavas, occasionally intercalated with hawaiite, trachyandesite and trachytes, but normally interbedded with peralkaline ignimbritic rhyolites which ultimately dominated the volcanism (Bowden et al., 1984). Bowden and Kinnaird ( 1978, 1984) identified a liquid line of descent from a syenite parent liquid (itself possibly derived by fractionation of a basic magma) to fayalite granite compositions. However, an early peralkaline trend leading to riebeckite granites and a late peraluminous trend leading to biotite granites were attributed to sub-solidus hydrothermal alteration. This alteration limits the reliability of much of the elemental chemistry of the plutons as petrogenetic indicators. In this situation, radiogenic isotope tracers may be more diagnostic. A detailed Rb-Sr study of the Nigerian Mesozoic granites was made by van Breemen et
al. ( 1975). Suites were analysed from six ring complexes in order to determine the agesand initial ratios of different units within each complex. Most of the rocks analysed had high Rb/Sr ratios, resulting in high-precision radiometric ages.However, in some casesthe error on initial Sr isotope ratios was very large, leading to uncertainties in petrogenetic interpretations basedon the data. In order to overcome these problems, further study is made here of the same sample suite analysed by van Breemen et al., using mineral separatesto establish more accurate initial Sr isotope ratios, and analysing selected samplesfor Pb and Nd isotope ratios. New isotopic analysesare also presented for samples of crustal basement from the Jos Plateau, including material studied by van Breemen et al. ( 1977). Most isotopic measurements were made on a VG@ 54E mass spectrometer at the Scottish Universities Research and Reactor Centre, using published techniques (Halliday et al., 1983). Five further Sm/Nd analyses were performed on a VG@ 354 mass spectrom-
BASEMENT
AND
MESOZOIC
RING
COMPLEXES
OF THE JOS PLATEAU
eter at McMaster University. All quoted Rb/ Sr agesare recalculated using a decay constant of 1.42.10-r’ a-’ for 87Rb.A decay constant of 6.54. lo-l2 a-’ was used for 14’Sm. 2. The Benin-Nigeria crustal basement The Benin-Nigeria shield forms part of the Pan-African belt of western Africa. Studies on the Tuareg shield to the north (Black, 1985) indicate ocean closure by continental collision in late Pan-African times. The continents involved in collision were essentially the 2-Gaold cratons of the Hoggar-Air domain and the Benin-Nigeria shield, but considerable rejuvenation of old agesoccurred during thrusting and folding. In SW Nigeria the Precambrian basement consists in part of charnockitic rocks of unknown ageand a gneissic complex comprising banded gneisses,migmatites, quartzites, schist, biotite -t amphibole gneisses,amphibolites and marbles metamorphosed to almandine-amphibolite facies. Caen-Vachette and Umeji ( 1987) attempted to datetheseparagneissesby the Rb/Sr method. Nineteen analysesdefined a scatterchron with an upper bound of 2202 + 3 1Ma and a lower bound of N I 700 Ma. Caen-Vachette and Umeji interpreted the 2202-Ma ageas a major metamorphic event in the Eburnean orogeny, renamed “Burkinian” by Lemoine et al. ( 1985). This interpretation is supported by an Rb-Sr ageof 2278 t- 34 Ma on foliated granitesfrom SW Nigeria which cut the paragneissic rocks (Grant, 1970). However, a crustal prehistory for the paragneissic rocks dating back to the Archean is indicated by agesof > 2600 Ma from the basement complex of southern Nigeria (Grant, 1970; Oversby, 1975). Pan-African rejuvenation of the basementproduced biotite Rb-Sr agesaround 500-600 Ma (Caen-Vachette and Umeji, 1987). In order to characterise the composition of
25
the crustal basement of the Jos Plateau, two migmatites (295, 297) and four granitoid orthogneisses ( 298, 671, 6 73, 674) were analysed from Malumfashi, north of Zaria (Fig. 1b, Tables I-III). The samples display a complete scatteron an Rb-Sr isochron diagram and yield depleted-mantle model ages ( tDM; DePaolo, 1981) from 1.3 to 3.0 Ga, suggestinga complex evolutionary history commencing in the Archean. However, the suite yields an average crustal residence age of 2 Ga and suggests that the Early Proterozoic “Burkinian” event was an important component in the crust-forming process. The continental crust of the Jos Plateau is largely composed of Pan-African granites with crystallisation agesranging from 596 to 677 Ma (van Breemen et al., 1977). Where possible, the same rocks with the extremes of Sr isotope composition (highest and lowest Rb/Sr) were analysed for Pb, though only one sample was analysed for Nd from each locality. When Nd isotope compositions are age corrected using the Rb-Sr dates, the initial ‘43Nd/144Ndratios, expressedas E(‘) are -2.3, - 10.5 and - 13.2 for samples of the Panyam (336)) Rahama (438) and Bauchi (331) plutons, respectively. These enriched sourcecompositions suggestthat the Pan-African granites were largely derived by melting of older crustal basement. Depleted-mantle Nd model agesfor these samples are 1.29, 1.92 and 1.96 Ga, respectively, again suggestingthat “Burkinian” basement played a role in the genesisof PanAfrican granites. Support for theseconclusions is provided by the Cameroon Line volcanics from Bambouto (Fig. la), which contain granulite enclaves (Halliday et al., 1988) which sample a segment of crustal basement similar to that forming the Jos Plateau. Two granulite enclaves yield tDM agesof 1.47 and 2.09 Ga, which are within the same range as the Jos Plateau samples. However, the Sr isotope composition of the enclaves is less radiogenic, possibly becausethey representRb-depleted lower crust.
26
A.P. DICKIN
ET AL.
TABLE 1 Rb/Sr data Sample No.
Lithology
Age (MaI
biotite granite
162
5.25
biotite granite
161
biotite granite syenite
151 151
biotite granite arfvedsonite granite apogranite
165.8 168 166
129.2 13.27
arfvedsonite granite riebeckite-biotite fayalite granite
160.6 160.6 161
trachy-rhyolite syenite syenite
186 186 186
&I
( 87Sr/86Sr),
20
0.71902
0.7069
0.0005
+37
16.77
0.74497
0.7066
0.0010
+33
13.99 0.31
0.73614 0.70544
0.7062 0.7048
0.0013 0.0002
+27 +7
1.04143 0.80456
0.7345 0.7220 0.7732
0.0070 0.0120 0.0040
+ 429 +251 +978
15.91 172.0 21.57
0.75181 1.10790 0.75942
0.7155 0.7156 0.7100
0.000 1 0.0010 0.0010
+158 +160 +81
85.55 2.33 0.17
0.93815 0.71105 0.70535
0.7150 0.7049 0.7049
0.0100 0.0001 0.000 I
+152 +9 +9
1.66 0.67 7.74 0.30 0.73 0.95 2.09
0.7217 0.7162 0.7763 0.7108 0.7149 0.7173 0.7276
0.7177 0.7146 0.7576 0.7101 0.7131 0.7150 0.7226
+190 +146 +757 +82 +125 +152 +260
3.35 1.25 4.47 2.43 6.35 2.25
0.8210 0.7226 0.7831 0.7286 0.7863 0.7324
0.8129 0.7196 0.7723 0.7227 0.7710 0.7270
+1542 +217 +965 +261 +947 + 322
Amo: 369WR Jos: 473 WR Pankshin: 451 WR 470 WR Ririwai: 412 WR 417-422 427As3 Shere Hills: 35 9 R*“ 361 R*’ 362 R Zaranda: 387 WR 38.5 WR 790 WR
Pan-African granites: isotope ratios at 170 Ma: 336 32s 331 375 377 436 438
Panyam Bauchi Bauchi Bauchi Bauchi Rahama Rahama
Maiumfashi gneisses: 295 297 298 671 673 674
migmatite migmatite granitic gneiss granitic gneiss granitic gneiss dioritic gneiss
*‘Errors t 1% (2~). **Errors average + 0.000 1 (20). *3Arfvedsonite separate. *4Arfvedsonite separate from same pluton as sample 355 analysed for Pb. *‘Riebeckite separate from same pluton as sample 360 analysed for Pb.
BASEMENT
AND MESOZOIC
RING
COMPLEXES
27
OF THE JOS PLATEAU
TABLE II Sm/Nd data Sample No.
Sm
Nd
(ppm)
(wm)
147Sm/144Nd
‘43Nd/‘44Nd
20
‘43Nd/‘44Nd
(170Ma)(.) %d
tDM (Gal
Mesozoic ring complexes: 369 473 451 412 417 360 385
10.69 36.98 68.97 33.21 18.48 14.49 18.79
50.62 163.7 338.2 137.1 82.36 62.62 116.8
0.1276 0.1365 0.1233 0.1464 0.1357 0.1399 0.0973
0.512501 0.512410 0.512361 0.512298 0.512436 0.512405 0.512564
0.000016 0.000030 0.000043 0.000018 0.000025 0.000023 0.000019
0.51237 0.5 1227 0.5 1224 0.51214 0.51229 0.51226 0.51245
-1.3 -3.2 -4.0 -5.6 -2.7 -3.4 +0.9
54.65 36.38 54.85
0.1006 0.1022 0.1091
0.511588 0.512101 0.511726
0.000017 0.000016 0.000019
0.51148 0.51200 0.51161
-18.4 -8.3 - 15.9
1.96 1.29 1.92
25.34 19.40 31.06 36.71 12.70 24.86
0.1248 0.0962 0.1221 0.1056 0.1442 0.1189
0.511314 0.511333 0.511967 0.512130 0.512039 0.5 12076
0.000018 0.000011 0.000011 0.000011 0.000011 0.000011
0.51118 0.51123 0.51183 0.51201 0.51188 0.51195
- 24.3 -23.3 -11.5 -1.9 - 10.6 -9.3
3.00 2.22 1.70 1.29 2.21 1.55
Pan-African granitoids: 331 336 438
9.096 6.148 9.896
Malumfbshi 295 297 298 671 673 674
gneisses:
5.232 3.086 6.269 6.414 3.026 4.892
* c(‘) =parts per ten thousand deviation from bulk Earth evolution line at time of emplacement, 1.
3. Revisedchronology of the Mesozoic ring complexes Rocks from the Amo, Jos, Pankshin, Ririwai, Shere Hills and Zaranda anorogenic complexes were analysed from the sample suite of van Breemen et al. ( 1975). When selecting samples for Pb and Nd isotope determination, those with the lowest Rb/Sr ratio from each intrusive unit (according to van Breemen et al. ) were chosenfor analysis. In the caseof the Amo, Jos, Pankshin and Zaranda plutonites, such samples had sufficiently low Rb/Sr ratio for the calculation of accurate initial 87Sr/86Sr ratios using the isochron agesof van Breemen et al. (Table I). Hence in these casesthe initial Pb, Nd, and Sr ratios are based on the same sample. In the Ririwai and Shere Hills complexes,whole-rock Rb/Sr ratios are too high to usethis procedure,and lower-Rb/Sr separated mineral phaseswere analysedin order to more
accurately constrain initial 87Sr/86Srratios. The Zaranda trachy-rhyolite initial ratio cannot be accurately determined in this manner becausethe rock is too fine-grained to allow mineral separation. For the Shere Hills complex an alkali amphibole separatewas analysedfrom eachof the three intrusive phases.Amphibole and wholerock samples from the arfvedsonite granite and riebeckite-biotite granites are colinear, yielding an ageof 160.62 1.2Ma and initial Sr ratio (I.R. ) of 0.715562 0.00009 (20). However, the amphibole separatefrom the arfvedsonite granite with remnant fayalite (362 R) yields a distinctly lower initial ratio of 0.7100 at 161 Ma (Table I). Re-analysis of the least radiogenic wholerock samples of the Ririwai biotite and arfvedsonite granites yielded isochrons with agesof 165.8_+1.6 and 168+ 6 Ma, respectively, in better agreement than in previous work (van
28
A.P. DICKIN
ET AL.
TABLE III Pb isotope data Sample “‘Pb Jzo4Pb No.
207Pb/204Pb
2osPb/‘04Pb
15.591
39.300
5.2
31
15.567 15.572
30.273 39.227
10.8 12.9
15.548 15.556
39.138 38.417
15.582 15.599 15.618 15.645 15.537 15.61 I
39.438 39.314 39.666 39.982 38.769 38.685
9.8 12.3 19.7 8.3 78 198
15.562 15.559 15.555
39.156 38.929 38.574
7.8 8.8 6.9
15.616 15.670 15.665 15.631 15.648
38.566 39.116 39.274 39.404 39.497
15.665 15.568 15.618 15.594 15.584 15.569 15.578
15.510 15.375 15.830 15.645 15.789 15.710
U (mm)
Th (mm)
Pb (wm)
(206Pb/204Pb),
( 204Pb/204Pb),
(20*Pb/204Pb),
14.2
17.93
15.56
38.13
48 53
24 33
17.63 17.55
15.56 15.54
38.13 38.45
36 4*
17.5 7.8
17.56 18.00
15.52 15.55
38.12 38.17
43 40 42 17.3 274 465
17.85 17.81 17.88 18.06 17.65 17.84
15.56 15.57 15.58 15.60 15.51 15.61
38.11 38.41 38.28 38.46 38.27 38.69
32 40 7.8
21 49 74
17.61 17.72 17.68
15.53 15.54 15.55
38.36 38.50 38.52
0.64 6.8 2.57 0.91 2.14
3* 16.5 10* 4* 11
21 16.3 20 4.4 6.1
18.35 18.42 18.45 18.29 18.29
15.61 15.63 15.65 15.61 15.62
38.48 38.49 38.97 30.84 38.38
39.508 38.433 38.780 40.617 38.523 38.733 38.811
3.4 1.55 3.1 3.9 1.98 2.59 6.6
19.1 3.5 21 52 8.2 11.6 15.0
20.2 13.0 37 38 16.3 19.8 21
18.42 17.64 17.91 17.61 17.86 17.73 17.54
15.65 15.56 15.61 15.59 15.57 15.56 15.55
38.97 38.29 38.47 39.84 38.25 38.4 I 38.42
38.073 37.676 39.446 38.727 38.533 38.924
2.13 5.0 1.28 2.38 1.27 0.71
8* 20* 5* lO* 5* 3*
42 34 30 40 21 20
19.32 17.12 19.46 18.44 19.66 18.63
15.51 15.36 15.83 15.64 15.78 15.71
37.97 37.36 39.35 38.59 38.40 38.84
A mo comple,u: 369
18.533
Jos complex: 473 474
18.378 18.193
Pankshin complex: 451 470
18.079 18.200
6.1 1.05
Ririwai complex: 417 422 412 414 424 427
18.238 18.341 18.702 18.900 18.291 17.842
102 63 102 46 230 376
Shere Hills complex: 362 360 355
18.201 18.017 17.830
Zaranda complex: 790 387 388 385 389
18.409 18.214 18.692 18.678 18.959
Pan-African granites: 336 375 325 331 377 436 438
18.710 17.837 18.053 17.782 18.066 17.953 18.069
Malumfashi gneisses: 295 297 298 671 673 674
19.402 17.362 19.530 18.542 19.768 18.694
t = age for Mesozoic granite, else 170 Ma. Average within run precision on Pb isotope ratios = 0.003 (2~). Estimated absolute accuracy of Pb isotope ratios=O.Ol, 0.007 and 0.02 ( lo) for 206Pb/204Pb, 207Pb/2WPb and 2osPb/204Pb, respectively. *Estimated concentration.
BASEMENT
AND MESOZOIC
RING
COMPLEXES
29
OF THE JOS PLATEAU
Breemen et al., 1975). However, initial ratios of 0.735 and 0.722 are distinct. The arfvedsonite albite apogranite also yields a consistent age of 16627 Ma (I.R.=0.773) when an alkali amphibole separate (427A) is included in the whole-rock isochron and a sample with anomalously high Sr isotope ratio (426) is excluded. Overall, the different intrusive units of the Ririwai complex display gross initial Sr isotope heterogeneity (Table I), but were probably emplaced in a brief time interval near 166 Ma ago. 4. Isotope geochemistry of Mesozoic ring complexes Initial Sr and Nd isotope ratios are expressed as e(‘) and compared with e(“’ Ma) compositions of possible mantle and crustal sources in Fig. 2. The Mesozoic ring complexes have a wide range of eSrcompositions (from + 9 to + 978) but a relatively restricted rangein eNd( - 5.6 to + 0.9 ) , which is more radiogenic than any of the crustal samples analysed (eNd= -24.3 to -8.3 at 170 Ma). The positive tNd composition of the Zaranda syenite ( +0.9) provides good evidence that this +I0
I\
I
,
I
,
I
0 ENd
-10 Jos PLATEAU
-20
0
,
I +100
NIGERIA &merowl gmnulne eflclave x I I
+200
+ml
4.400
6.3
Fig. 2. eNdvs. tsr diagram showing the compositions of Nigerian Mesozoic ring complexes (Z= Zaranda; A = Amo; J= Jos; P= Pankshin; R = Ririwai; S= Shere Hills) and possible source compositions. The composition of basement forming the Jos Plateau is defined by Pan-African granites and the Malumfashi gneisses.Crosses represent granulite enclaves in Cameroon Line volcanics from Bambouto (Fig. 1).
I
I
16
19
1
1
20
(‘D’Pb/‘o’Pbl,,,,M,
Fig. 3. Initial Pb isotope compositions of Mesozoic ring complexes (symbols as in Fig. 2) and composition of possible sourcecompositions at 170 Ma (average age of magmatism ) ( x = Pan-African granites; * = Malumfashi gneisses). Encircled R symbols = Ririwai hydrothermal apogranite samples. SK indicates Pb isotope evolution lines of the Staceyand Kramers ( 1975) model.
contains a mantle-derived component, since it must come from a reservoir with time-integrated light rare-earth depletion relative to the bulk Earth. However, significant crustal contamination has occurred. The range of composition from eNdof - 5.6 to - 1.3 in the remaining centres is consistent with the conclusion of van Breemen et al. ( 1975), based on Sr isotope evidence, that “syenite-granite occurrences in the Nigerian Younger Granite province... may be attributed to crustal enrichment of syenitic liquids whose source lies in the mantle”.
,Initial Pb isotope compositions of Mesozoic rocks and Pb isotope compositions of possible mantle and crustal sourcesat 170 Ma (Table
30
A.P. DICKIN
PAN AFRICAN BASEMENT
17.0 0.70
I 0.71
I 0.72
I 0.72
1 0.74
0.75
(87s&sr),
Fig. 4. Initial Pb vs. Sr isotope compositions of Mesozoic ring complexes and field for Pan-African basement at 170 Ma (symbols as in Fig. 2). Brackets represent ranges of Pb isotope compositions for individual units and estimated errors on initial Sr isotope compositions.
III) areplotted on 207Pb/204Pb vs. 206Pb/204Pb and 208Pb/204Pbvs. 206Pb/204Pbdiagrams in Fig. 3. The mantle Pb field, as inferred from uncontaminated Cameroon Line volcanics (Halliday et al., 1988)) is age corrected using a mantle p-value ( 238U/204Pb)of 9.0. With the exception of the Ririwai arfvedsonite-albite apogranite (samples 424 and 427)) the Nigerian Mesozoic ring complexes form a short array on the Pb/Pb isochron diagram (Fig. 33)) whose slope correspondsto an apparent ageof - 1750 Ma. When the Mesozoic ring complex array is compared with possible magma sources, it is seen that it barely overlaps with the mantle field estimated from uncontaminated Cameroon Line volcanisms. However, the array does lie between the Pan-African granite and Malumfashi gneisscrustal fields in both Fig. 3A and B. When this observation is coupled with the 1750-Ma apparent ageof the ring complexes it suggeststhat Pb in theserocks is derived largely by mixing of crustal Pb sources with an averageEarly Proterozoic formation age. 5. Discussion Some of the Mesozoic ring complexes show evidence of intense hydrothermal activity (Bowden et al., 1979; Bowden and Kinnaird,
ET AL.
1984). Consequently it is important to establish the relative importance of crustal contamination at the magmatic and post-consolidation hydrothermal stages. The evidence for hydrothermal overprinting of isotope systematics in the Ririwai arfvedsonite-albeit apogranite is very strong. In addition to gross initial Sr isotope heterogeneity (Table I), this unit displays Pb isotope compositions significantly different from the remaining Mesozoic rocks and has Pb, U and Th contents of several hundreds of ppm (Table III). However, Pb isotope systematics appear not to have been hydrothermally perturbed in the other Mesozoic granites, including the biotite and arfvedsonite granites from Ririwai. The moderately elevated and heterogeneous initial Sr isotope compositions of the Shere Hills granites may also be attributed to sub-solidus alteration. This interpretation is supported by the minimal changesin eNdbetween this body and the granites with unradiogenic Sr, yielding a horizontal vector in Fig. 2. This is typical of the known relative mobility of Sr and immobility of rare earthsin hydrous fluids. The Ririwai apogranite shows a deviation in Nd as well asSr isotope composition, but Bowden et al. ( 1979) have shown that even rareearth elements were disturbed by hydrothermal activity in this grossly altered body. Our interpretation of the isotopic data can be summarised with the aid of Fig. 4. Assimilation of lower-crustal melts by mantle-derived differentiating magmas yields a trend from radiogenic to less radiogenic Pb, which is however accompanied by only minor increases in initial *‘Sr/*%r. This trend may result from crustal contamination by Rb-poor lower-crustal basement, and is observed in the Zaranda syenite and the Amo, Pankshin and Jos complexes. Hydrothermal overprinting of the Ririwai complex and one or more shereHills unit then superimposed a trend almost at right-angles to the magmatic contamination trend, in which 87Sr/86Srrose very rapidly. Only in the caseof extreme alteration suffered by the Rir-
BASEMENT
AND
MESOZOIC
RING
COMPLEXES
31
OF THE JOS PLATEAU
iwai apogranite were Pb isotope compositions significantly affected by this process. 6. Conclusions Collins et al. ( 1982) regard A-type granites, along with I- and S-type, as almost exclusively the products of crustal melting, while Didier et al. ( 1982) distinguish C (crustal) and M (mantle-derived or mixed crustal + mantle) -types within each of the A-, I- and S-types. The evidence which we have presentedhere continues to support the original conclusion of van Breemen et al. ( 1975) that the Nigerian Mesozoic granites are the product of mantlederived magmas which have suffered a significant amount of contamination during their uprise and differentiation in the crustal basement of the Jos Plateau. The extent of this contamination is relatively minor in the Zaranda syenite, but increases to a more significant fraction in the Amo, Jos and Pankshin comnlexes. . Pb isotope data are consistent with assimilation of Early Proterozoic crust, which is the averageformation age found by Sm/Nd analysis of both Malumfashi gneissesand Pan-African granites of the Jos Plateau. Evidence hasbeen presentedfor extreme hydrothermal Sr and Pb contamination of the Ririwai arfvedsonite-albite apogranite. Other Ririwai units and one Shere Hills pluton appear to have suffered significant hydrothermal Sr contamination but little disturbance of Pb. Acknowledgements Isotope studies at East Kilbride are supported by the NERC and the Scottish Universities. Field work was supported by the NERC and the Overseas Development Administration. We thank J. Hutchinson and J. Jocelyn for technical assistance.We thank Dr. 0. van Breemen for the use of his unpublished Rb/Sr data on the Malumfashi samples.
References Black, R., 1985. The Pan-African event in the geological framework of Africa. Pangea, 1: 6- 16. Bowden, P. and Kinnaird, J.A., 1978. Younger granites of Nigeria - a zinc-rich tin province. Trans. Inst. Min. Metall., 87: B66-B69. Bowden, P. and Kinnaird, J.A., 1984. The petrology and geochemistry of alkaline granites from Nigeria. Phys. Earth Planet. Inter., 35: 199-211. Bowden, P., Bennet, J.N., Whitley, J.E. and Moyes, A.B., 1979. Rare earths in Nigerian Mesozoic granites and related rock. In: L.H. Ahrens (Editor), Origin and Distribution of the Elements (2nd Symposium). Phys. Chem. Earth, 11: 479-49 1. Bowden, P., Batchelor, R.A., Chappell, B.W., Didier, J. and Lameyre, J., 1984. Petrological, geochemical and source criteria for the classification of granitic rocks: a discussion. Phys. Earth Planet. Inter., 35: l-l 1. Caen-Vachette, M. and Umeji, A.C., 1987. Geology and geochronology of the Okene area: evidence for an Eburnean erogenic cycle in south-central Nigeria. J. Afr. Earth Sci., 7: 121-126. Collins, W.J., Beams, S.D., White, A.J.R. and Chappel, B.W., 1982. Nature and origin of A-type granites with particular reference to south eastern Australia. Contrib. Mineral. Petrol., 80: 189-200. DePaolo, D.J., 1981. Neodymium isotopes in the Colorado Front Range and crust-mantle evolution in the Proterozoic. Nature (London), 291: 193-196. Didier, J., Duthou, J.L. and Lameyre J., 1982. Mantle and crustal granites: genetic classification of erogenic granites and the nature of their enclaves. J. Volcanol. Geotherm. Res., 14: 125-132. Grant, N.K., 1970. The geochronology of Precambrian basement rocks from Ibadan, south-western Nigeria. Earth Planet. Sci. Lett., 10: 29-38. Halliday, A.N., Fallick, A.E., Dickin, A.P., MacKenzie, A.B., Stephens, W.E. and Hildreth, W., 1983. The isotopic and chemical evolution of Mount St. Helens. Earth Planet. Sci. Lett., 63: 241-256. Halliday, A.N., Dickin, A.P., Fallick, A.E. and Fitton, J.G., 1988. Mantle dynamics: a Nd, Sr, Pb and 0 isotopic study of the Cameroon Line volcanic chain. J. Petrol., 29: 181-211. Lemoine, S., Tempier, P., Basso&J.P., Caen-Vachette, M., Viaette, Y., Wenmenga, U. and Toure, S., 1985. Le Burkinien, cycle orogenique precurseur de I’EbumCen en Afrique de l’ouest. 13th Colloq. Afr. Geol., St. Andrews, Scotland, Sept. 10-l 3, 1985. Loiselle, MC. and Wones,D.R., 1979. Characteristicsand origin of anorogenic granites. Geol. Sot. Am. Abstr. Prog., 11: 468. Oversby, V.M., 1975. A lead isotope study ofaplites from the Precambrian basement rocks near Ibadan, SW Nigeria. Earth Planet. Sci. Lett., 27: 177-I 80.
32 Stacey, J.S. and Kramer, J.D., 1975. Approximation of terrestrial lead isotope evolution by a two-stage model. Earth Planet. Sci. Lett., 26: 207-22 1, van Breemen, O., Hutchinson, J. and Bowden, P., 1975. Age and origin of the Nigerian Mesozoic granites - A
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Rb-Sr isotope study. Contrib. Mineral. Petrol., 50: 157-172. van Breemen, O., Pidgeon, R.T. and Bowden, P., 1977. Age and isotope studies of some Pan-African granites from north-central Nigeria. Precambrian Res., 4: 307319.