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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 Geoscience Section), 94 ( 1991 ) 23-32 Elsevier Science Publishers B.Y., 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. Hallidaya.P and P. Bowden" 'Scottish UniversitiesResearch and Reactor Centre, East Kilbride, GlasgowG75 OQU, Scotland, UK "Depanment ofGeography and Geology, UniversityofSt. Andrews, Fife KYl6 9ST, Scotland. UK (Received October 23, 1989; revised and accepted June 5, 1991)
ABSTRACT Dickin, A.P., Halliday, A.N. and Bowden, P., 1991. A Pb, Sr and Nd isotope study of the basement and Mesozoic ring complexes of the Jos Plateau, Nigeria. Chem. Geo1. (Isot. Geosci. Sect.), 94: 23-32. Combined Pb, Sr and Nd isotope determinations on several Nigerian Mesozoic (- 170 Ma) ring complexes, studied previously by van Breemen and co-workers, indicate a multistage petrogenetic process. Mantle-derived differentiated magmas assimilated crustal basement ofaverage Early Proterozoic age. After crystallisation, some plutons were subjected to a second stage of crustal contamination by circulating hydrothermal fluids. Crustal compositions were constrained by isotopic analysis of the Proterozoic basement of the Jos Plateau. Sm/Nd analysis of six gneisses yielded an average crustal residence age of 2 Ga, corresponding to the Burkinian event recognised elsewhere in western Africa. However, one sample yields a model age of 3 Ga, suggesting the presence of Archean crustal remnants. Pan-African granitoids yield a similar range of Nd model ages to the gneisses, suggesting that they were largely generated by crustal melting. The Zaranda anorogenic complex has relatively radiogenic initial Nd and Pb isotope compositions and unradiogenic Sr ( - 0.5126, - 18.4 and -0.705, respectively), attributed to a mantle-derived differentiated magma which suffered moderate contamination during ascent through the crust. Other ring complexes trend toward less radiogenic Nd and Pb isotope ratios and more radiogenic Sr, indicative of an increasing crustal contribution. Initial Pb isotope compositions yield a welldefined Pb/Pb isotope array with a slope age of - 1.8 Ga which is consistent with the average Nd crustal residence ages of basement gneisses and 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 Shere Hills display evidence of hydrothermal overprinting of Sr and to some extent Nd isotope compositions, but only the Ririwai apogranite has been significantly overprinted by hydrothermal Pb, The isotopic evidence supports a model for the Mesozoic anorogenic ("A-type") granites of Nigeria in which mantlederived magmas suffered crustal contamination during magmatic differentiation to syenitic compositions, followed by sub-solidus hydrothermal alteration in the continental crust.
1. Introduction
The Nigerian Mesozoic granites form a series of anorogenic ring complexes which cut the exposed Precambrian basement of the Jos PlaPresent addresses: "Department of Geology, McMaster University, Hamilton, Ont. L8S 4Ml, Canada. Pnepartment of Geological Sciences, University of Michigan, Ann Arbor, MI 48109, U.S.A.
teau in central Nigeria (Fig. 1). They were regarded by 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 ofA-type granites, and the petrogenesis of the Nigerian anorogenic granites is there-
0168-9622/91/$03.50 © 199 I Elsevier Science Publishers B.Y. All rights reserved.
24
A.I'. PICKIN ET AL.
15·
B
A 100km N
t·
11·
~\.P
•
Q
RAHAMAfX
10·
BASEMENT OF THE JOS PLATEAU 4
iJ' .
Zaranda
J t
BAUCHI X)( x
'Amo~.· ~
_
..
Shere Hills
Jos \
.• • I x
S·
~~
Ririwa i
ZARI A
•
PANYAM
S·
Panks~ in
r.S.::F'v.
\~
'O'i,~~~\v.'i,~\"-' S·
10'
lS·E
1·
S·
Fig. I. A. Location map of Nigeria showing areas of Precambrian basement outcrop (shaded) 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 general as well as regional significance. The Nigerian ring complexes arc thought to be the plutonic equivalents of chains of volcanoes erupting 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 aI., 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 ages and 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 cases the error on initial Sr isotope ratios was very large, leading to uncertainties in petrogenetic interpretations based on the data. In order to overcome these problems, further study is made here of the same sample suite analysed by van Breemen et aI., using mineral separates to establish more accurate initial Sr isotope ratios, and analysing selected samples for Pb and Nd isotope ratios. New isotopic analyses are also presented for samples of crustal basement from the los Plateau, including material studied by van Breemen et aI. ( 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-
R\SEl-IENT AND MESOZOIC RI NG
CO~IPLEXES
OFTII EJOS PLATEAU
eter at McMastcr University. All quoted Rb/ Sr ages are recalculated using a decay constant of 1.42 .10- 1 1 a-I for 87Rb. A decay constant of6.54·10- 12 a-I was used for 147Sm.
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 ages occurred during thrusting and folding. In SW Nigeria the Precambrian basement consists in part of charnockitic rocks of unknown age and a gneissic complex comprising banded gneisses, migmatites, quartzites, schist, biotite ± amphibole gneisses, amphibolites and marbles metamorphosed to almandine-amphibolite facies. Caen-Vachette and Umeji ( 1987) attempted to date these paragneisses by the Rb/Sr method. Nineteen analyses defined a scatterchron with an upper bound of 2202 ± 31 Ma and a lower bound of - 1700 Ma, Caen-Vachette and Umeji interpreted the 2202-Ma age as a major metamorphic event in the Eburnean orogeny, renamed "Burkinian" by Lemoine et al. (1985). This interpretation is supported by an Rb-Sr age of 2278 ± 34 Ma on foliated granites from 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 ages of > 2600 Ma from the basement complex of southern Nigeria (Grant, 1970; Oversby, 1975). Pan-African rejuvenation of the basement produced biotite Rb-Sr ages around 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, 673, 674) were analysed from Malumfashi, north of Zaria (Fig. Ib, Tables I-III). The samples display a complete scatter on an Rb-Sr isochron diagram and yield depleted-mantle model ages (tDM; DePaolo, 1981) from 1.3 to 3.0 Ga, suggesting a 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 ages ranging 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 143Nd/ 144Nd ratios, expressed as <:(1) are -2.3, -10.5 and -13.2 for samples of the Panyam (336), Rahama (438) and Bauchi (331) plutons, respectively. These enriched source compositions suggest that the Pan-African granites were largely derived by melting of older crustal basement. Depleted-mantle Nd model ages for these samples are 1.29, 1.92 and 1.96 Ga , respectively, again suggesting that "Burkinian" basement played a role in the genesis of PanAfrican granites. Support for these conclusions 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 t D M ages of 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 because they represent Rb -depleted lower crust.
A.P. DICKIN ET AL
26 TA BLE ! Rb /Sr data Sample No.
(87SrJS6Sr)l*21
( 87Sr /8 6Sr),
20
5.25
0.71902
0.7069
0.0005
+37
161
16.77
0.74497
0.7066
0.00 10
+33
biotite granite syen ite
151 lSI
13.99 0.31
0 .736 14 0.7 0544
0.7062 0.7048
0 .00 13 0 .0002
+27 +7
biot ite granite ar fvedsonite gra nite apog ranitc
165.8 168 166
129.2
1.04143
13.27
0.80456
0.7345 0.7220 0.7732
0.0070 0.0 120 0.0040
+429 +25 1 +978
arfvedsonite granite riebeckite- biotite fayal ite granite
160.6 160.6 161
15.9 1 172.0 21.57
0.7518 1 1.10790 0.75942
0.7155 0.7156 0.7100
0.00 01 0.0010 0.0010
+158 + 160 +8 1
trachy -rhyolite syenite syenite
186 186 186
85.55 2.33 0.17
0.938 15 0.7 1105 0.70535
0.7150 0.7049 0.7049
0.0100 0.0001 0.0001
+152 +9 +9
1.66 0.67 7.74 0.30 0.73 0.95 2.09
0.7217 0.7 162 0.7763 0.7 108 0.7 149 0.7173 0.7276
0.7 177 0.7146 0.7576 0. 7101 0.7131 0.71 50 0.7226
+190 +146 +757 +82 +125 +152 +260
3.35 1.25 4.47 2.43 6.35 2.25
0.82 10 0.7226 0.7831 0.7286 0.7863 0.7324
0.8 129 0.7 196 0.7723 0.7227 0.7710 0.7270
+ 1542 +217 +965 +261 +947 +322
Litho logy
Age (Ma)
b iot ite granite
162
biotite granite
87Rb /86Sr(O, )
(II
Amo:
369WR
Jos: 473WR
Panksliin: 45 1WR 470WR
Ririwai: 412 W R 417·422 427 A*J
Shere Hills: 359 R*' 36 1 R*) 362 R
Zarallda: 387WR 385WR 790 W R
Pan-African granites: isotoperatiosat 170 Ma: 336 325 33 1 375 377 436 438
Panyam Bauchi Bauchi Bauc hi Bauchi Rahama Rahama
Malutnfashi gneisses: 295 297 298 671 673 674
migmati te
rnigrnatite granitic gne iss granitic gneiss granitic gneiss dioritic gneiss
*'Errors ± 1% (20). *2Errors average ± 0.0001 ( 2a ) . *JArfvedsonite separate. *'Arfvcdsonitc separate from same pluton as sample 355 analysed for Pb. *)R iebeckite separate from same pluton as sample 360 analysed for Pb .
BASEMENT AND MESOZOIC RING CO~tPLEXES OFTHE 10S PLATEAU
27
TABLE II Sm/Nd data Sample No.
Sm (ppm)
Nd (ppm)
141Sm/'''Nd
143Nd/'''Nd
2a
143Nd/I"Nd
E(l10M.)(.) Nd
tDM
(Ga)
Mesozoicring 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 ' 0.5124 10 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 0.51227 0.51224 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 0.512076
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 -7.9 -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
Malumfashi gneisses:
295 297 298 671 673 674
5.232 3.086 6.269 6.414 3.026 4.892
* Ell):::; parts per ten thousand deviation from bulk Earth evolution line at time of emplacement, t.
3. Revised chronology 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 chosen for analysis. In the case of the Amo, Jos , Pankshin and Zaranda plutonites, such samples had sufficiently low RbjSr ratio for the calculation of accurate initial 87Sr j 86 Sr ratios using the isochron ages of van Breemen et al. (Table I). Hence in these cases the 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 use this procedure, and lower-Rb/Sr separated mineral phases were analysed in order to more
accurately constrain initial 87Srj 86 S r ratios. The Zaranda trachy-rhyolite initial ratio cannot be accurately determined in this manner because the rock is too fine-grained to allow mineral separation. For the Shere Hills complex an alkali amphibole separate was analysed from each ofthe three intrusive phases. Amphibole and wholerock samples from the arfvedsonite granite and riebeckltc-biotite granites are colinear, yielding an age of 160.6 ± 1.2 Ma and initial Sr ratio (I.R.) of 0.71556±0.00009 (20'). However, the amphibole separate from 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 ages of 165.8±1.6 and 168±6 Ma, respectively, in better agreement than in previous work (van
A.I'. DICKIN ET AL.
28 TABLE III Pb isotope data Sample No.
~ o6I'b(O~Pb
e06Pb/~~Pb ),
e~Pb( O~Pb),
eosPb/ ~o~Pb
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 17.8 1 17.88 18.06 17.65 17.84
15.56 15.57 15.58 15.60 15.51 15.61
38. 11 38.4 1 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* II
21 16.3 20 4.4 6. 1
18.35 18.42 18.45 18.29 18.29
15.61 15.63 15.65 15.6 1 15.62
38.48 38.49 38.97 30.84 38.38
39.508 38.433 38.780 40.617 38.523 38.733 38.8 11
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 38.41 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*
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
~ o7Pbr o~Pb
2osPb/2 0~Pb
15.591
39.300
5.2
31
15.567 15.572
30.2 73 39.22 7
10.8 12.9
15.548 15.556
39. 138 38.4 17
15.582 15.599 15.618 15.645 15.537 15.6 11
39.438 39.3 14 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 39.49 7
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
Pb Th (ppm) (ppm) (ppm)
U
Amo camp/ex:
369
18.533
los camp/ex:
473 474
18.378 18. 193
Panks liin camp/ex:
451 470
18.079 18.200
6. 1 1.05
Ririwai camp/ex:
417 422 4/2 4/4 424 427
18.238 18.341 18.702 18.900 18.291 17.842
102 63 102 46 230 376
Shere lIills camp/ex:
362 360 355
18.201 18.017 17.830
Zarandacamp/ex:
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 67/ 673 674
19.402 17.362 19.530 18.542 19.768 18.694
10*
5* 3*
t =age for Mesozoic granite, else 170 Ma, Average within run precision on Pb isotope ratios=0.003 (20') . Estimated absolute accuracy of Pb isotope ratios= 0.0 1,0.007 and 0.02 (I a ) for 206 P bFo~Pb , 207Pb/2 0~Pb and 2osPb/ 20~Pb, respectively. *Estimated concentration .
29
BASEMENT AND MESOZOIC RING COMPLEXES OF THE JOS PLATEAU
Breemen et aI., 1975). However, initial ratios of 0.735 and 0.722 are distinct. The arfvedsonite albite apogranite also yields a consistent age of 166::t 7 Ma (I.R. = 0.77 3) 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 Maago. 4. Isotope geochemistry of Mesozoic ring complexes
.~
395
AFRICAN
390
~fj*o
385
b\ ("'P 20'PbZ
+10
*
70 "'1a
380
* 500M.a
37.5
*
'o~
*
u-
158
("'Pb) 20'Pb 110M
~J\l\C"~,.z
*
*
,,~~z ~ R--.. /z. ZI
156
Initial Sr and Nd isotope ratios are expressed as E(t) and compared with E(l70 Ma) compositions of possible mantle and crustal sources in Fig. 2. The Mesozoic ring complexes have a wide range of fS r compositions (from +9 to +978) but a relatively restricted range in fNd( -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 ENd composition of the Zaranda syenite (+ 0.9) provides good evidence that this
*
"l
s~
!if!'3"feV
~/
*
154
17
*
18
19
1'''Pb/'''Pbl17•
20 Ma
Fig. 3. Initial Pb isotope compositions of Mesozoic ring complexes (symbols as in Fig. 2) and composition ofpossible source compositions 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 Stacey and Kramers (1975) model.
~MIRB CAMEROON LINE
VOLCANICS
CD
o
NIGERIANMESOZOICANOROGENIC COMPLEXES
®
J
'\
'i1\
·10
@lLArlvedSOnile)· J ~®
(Arlvedsonite)
P
.",
~
(C!-~~~'!.l!:~n~!!.t!!~!!l:!~
x:
I I
BASEMENT OF THE JOS PLATEAU
I
NIGERIA X-cameroon granuliteenclave
~:
o
LB.!.o.!i~l
PRECAMBRIAN •
+200
+300
+400
ESr
Fig. 2. fNd vs. ESr diagram showing the compositions of Nigerian Mesozoic ring complexes (Zee 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. I).
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 ENd of - 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 sources at 170 Ma (Table
30
A.P. DICKIN ET AL.
18.5
r----.---.---.,....---.,.----, "P'
18.0
T Siom.
'
'""
206Pb) ( 204Pb I
> -(R r - - - <
/ 1
17.5
=-_
17.0 0.70
__=_~-__=_-:l:-::-----.L.---...L-------l 0.71 0.72 0.73 0.74 0.75 (87SrJ66Srll
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) are plotted on 207Pbj204Pb vs. 206Pb j204Pb and 208Pb/204Pb vs. 206Pb/204Pb diagrams in Fig. 3. The mantle Pb field, as inferred from uncontaminated Cameroon Line volcanics (Halliday et al., 1988), is age corrected using a mantle Ji-value 38U /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 arrayon the Pb/Pb isochron diagram (Fig. 3B), whose slope corresponds to an apparent age of '" 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 gneiss crustal fields in both Fig. 3A and B. When this observation is coupled with the l750-Ma apparent age of the ring complexes it suggests that Pb in these rocks is derived largely by mixing of crustal Pb sources with an average Early Proterozoic formation age.
e
5. Discussion Some of the Mesozoic ring complexes show evidence of intense hydrothermal activity (Bowden et al., 1979; Bowden and Kinnaird,
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 changes in ENd between 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 ofrare earths in hydrous fluids. The Ririwai apogranite shows a deviation in Nd as well as Sr 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 87Sr/ 86S 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 los complexes. Hydrothermal overprinting of the Ririwai complex and one or more shere Hills unit then superimposed a trend almost at right-angles to the magmatic contamination trend, in which 87Sr/ 86S r rose very rapidly. Only in the case of extreme alteration suffered by the Rir-
31
BASEMENT AND MESOZOIC RING CO:'1PLEXES OFTHE 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-, 1- and S-types. The evidence which we have presented here 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 ofthe Jos Plateau. The extent ofthis contamination is relatively minor in the Zaranda syenite, but increases to a more significant fraction in the Amo, Jos and Pankshin complexes. Pb isotope data are consistent with assimilation of Early Proterozoic crust, which is the average formation age found by Sm/Nd analysis of both Malumfashi gneisses and Pan-African granites of the Jos Plateau. Evidence has been presented for 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. O. 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, I: 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. Chern. Earth, II: 479-491. Bowden, P., Batchelor, R.A., Chappell, B.W., Didier, J. and Lameyre, J., 1984. Petrological, geochemical and source criteria for the classification ofgranitic rocks: a discussion. Phys. Earth Planet. Inter., 35: I-II. Caen-Vachette, M. and Umeji, AiC; 1987. Geology and geochronology of the Okene area: evidence for an Eburnean orogenic 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.1., 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 oforogenic 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, AP., MacKenzie, A.B., Stephens, W.E. and Hildreth, W., 1983. The isotopic and chemical evolution of Mount St. Helens. Earth Planet. Sci. Len., 63: 241-256. Halliday, AN., Dickin, A.P., Fallick, A.E. and Fitton, J.G., 1988. Mantle dynamics: aNd, Sr, Pb and 0 isotopic study of the Cameroon Line volcanic chain. J. Petrol., 29: 181-211. Lemoine, S., Tempier, P., Bassot, J.P., Caen-Vachette, M., Viaette, Y., Wenmenga, U. and Toure, S., 1985. Le Burkinien, cycle orogenique precurseur de l'Eburnecn en Afrique de l'ouest. 13th Colloq. Afr. Geol., St. Andrews, Scotland, Sept. 10-13, 1985. Loiselle, M.e. and Wones, D.R., 1979. Characteristics and origin of anorogenic granites. Geol. Soc. 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: 177-180.
32 Stacey, J.S. and Kramer, J.D., 1975. Approximation of terrestria11ead isotope evolution by a two-stage model. Earth Planet. Sci. Lett., 26: 207-221. van Breernen, 0., Hutchinson, J. and Bowden, P., 1975. Age and origin of the Nigerian Mesozoic granites - A
A.I'. D1CKtN ET AL.
Rb-Sr isotope study. Contrib. Mineral. Petrol., 50: 157-172. van Breemcn, 0 ., 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. Hallidaya.P and P. Bowden" 'Scottish UniversitiesResearch and Reactor Centre, East Kilbride, GlasgowG75 OQU, Scotland, UK "Depanment ofGeography and Geology, UniversityofSt. Andrews, Fife KYl6 9ST, Scotland. UK (Received October 23, 1989; revised and accepted June 5, 1991)
ABSTRACT Dickin, A.P., Halliday, A.N. and Bowden, P., 1991. A Pb, Sr and Nd isotope study of the basement and Mesozoic ring complexes of the Jos Plateau, Nigeria. Chem. Geo1. (Isot. Geosci. Sect.), 94: 23-32. Combined Pb, Sr and Nd isotope determinations on several Nigerian Mesozoic (- 170 Ma) ring complexes, studied previously by van Breemen and co-workers, indicate a multistage petrogenetic process. Mantle-derived differentiated magmas assimilated crustal basement ofaverage Early Proterozoic age. After crystallisation, some plutons were subjected to a second stage of crustal contamination by circulating hydrothermal fluids. Crustal compositions were constrained by isotopic analysis of the Proterozoic basement of the Jos Plateau. Sm/Nd analysis of six gneisses yielded an average crustal residence age of 2 Ga, corresponding to the Burkinian event recognised elsewhere in western Africa. However, one sample yields a model age of 3 Ga, suggesting the presence of Archean crustal remnants. Pan-African granitoids yield a similar range of Nd model ages to the gneisses, suggesting that they were largely generated by crustal melting. The Zaranda anorogenic complex has relatively radiogenic initial Nd and Pb isotope compositions and unradiogenic Sr ( - 0.5126, - 18.4 and -0.705, respectively), attributed to a mantle-derived differentiated magma which suffered moderate contamination during ascent through the crust. Other ring complexes trend toward less radiogenic Nd and Pb isotope ratios and more radiogenic Sr, indicative of an increasing crustal contribution. Initial Pb isotope compositions yield a welldefined Pb/Pb isotope array with a slope age of - 1.8 Ga which is consistent with the average Nd crustal residence ages of basement gneisses and 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 Shere Hills display evidence of hydrothermal overprinting of Sr and to some extent Nd isotope compositions, but only the Ririwai apogranite has been significantly overprinted by hydrothermal Pb, The isotopic evidence supports a model for the Mesozoic anorogenic ("A-type") granites of Nigeria in which mantlederived magmas suffered crustal contamination during magmatic differentiation to syenitic compositions, followed by sub-solidus hydrothermal alteration in the continental crust.
1. Introduction
The Nigerian Mesozoic granites form a series of anorogenic ring complexes which cut the exposed Precambrian basement of the Jos PlaPresent addresses: "Department of Geology, McMaster University, Hamilton, Ont. L8S 4Ml, Canada. Pnepartment of Geological Sciences, University of Michigan, Ann Arbor, MI 48109, U.S.A.
teau in central Nigeria (Fig. 1). They were regarded by 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 ofA-type granites, and the petrogenesis of the Nigerian anorogenic granites is there-
0168-9622/91/$03.50 © 199 I Elsevier Science Publishers B.Y. All rights reserved.
24
A.I'. PICKIN ET AL.
15·
B
A 100km N
t·
11·
~\.P
•
Q
RAHAMAfX
10·
BASEMENT OF THE JOS PLATEAU 4
iJ' .
Zaranda
J t
BAUCHI X)( x
'Amo~.· ~
_
..
Shere Hills
Jos \
.• • I x
S·
~~
Ririwa i
ZARI A
•
PANYAM
S·
Panks~ in
r.S.::F'v.
\~
'O'i,~~~\v.'i,~\"-' S·
10'
lS·E
1·
S·
Fig. I. A. Location map of Nigeria showing areas of Precambrian basement outcrop (shaded) 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 general as well as regional significance. The Nigerian ring complexes arc thought to be the plutonic equivalents of chains of volcanoes erupting 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 aI., 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 ages and 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 cases the error on initial Sr isotope ratios was very large, leading to uncertainties in petrogenetic interpretations based on the data. In order to overcome these problems, further study is made here of the same sample suite analysed by van Breemen et aI., using mineral separates to establish more accurate initial Sr isotope ratios, and analysing selected samples for Pb and Nd isotope ratios. New isotopic analyses are also presented for samples of crustal basement from the los Plateau, including material studied by van Breemen et aI. ( 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-
R\SEl-IENT AND MESOZOIC RI NG
CO~IPLEXES
OFTII EJOS PLATEAU
eter at McMastcr University. All quoted Rb/ Sr ages are recalculated using a decay constant of 1.42 .10- 1 1 a-I for 87Rb. A decay constant of6.54·10- 12 a-I was used for 147Sm.
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 ages occurred during thrusting and folding. In SW Nigeria the Precambrian basement consists in part of charnockitic rocks of unknown age and a gneissic complex comprising banded gneisses, migmatites, quartzites, schist, biotite ± amphibole gneisses, amphibolites and marbles metamorphosed to almandine-amphibolite facies. Caen-Vachette and Umeji ( 1987) attempted to date these paragneisses by the Rb/Sr method. Nineteen analyses defined a scatterchron with an upper bound of 2202 ± 31 Ma and a lower bound of - 1700 Ma, Caen-Vachette and Umeji interpreted the 2202-Ma age as a major metamorphic event in the Eburnean orogeny, renamed "Burkinian" by Lemoine et al. (1985). This interpretation is supported by an Rb-Sr age of 2278 ± 34 Ma on foliated granites from 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 ages of > 2600 Ma from the basement complex of southern Nigeria (Grant, 1970; Oversby, 1975). Pan-African rejuvenation of the basement produced biotite Rb-Sr ages around 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, 673, 674) were analysed from Malumfashi, north of Zaria (Fig. Ib, Tables I-III). The samples display a complete scatter on an Rb-Sr isochron diagram and yield depleted-mantle model ages (tDM; DePaolo, 1981) from 1.3 to 3.0 Ga, suggesting a 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 ages ranging 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 143Nd/ 144Nd ratios, expressed as <:(1) are -2.3, -10.5 and -13.2 for samples of the Panyam (336), Rahama (438) and Bauchi (331) plutons, respectively. These enriched source compositions suggest that the Pan-African granites were largely derived by melting of older crustal basement. Depleted-mantle Nd model ages for these samples are 1.29, 1.92 and 1.96 Ga , respectively, again suggesting that "Burkinian" basement played a role in the genesis of PanAfrican granites. Support for these conclusions 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 t D M ages of 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 because they represent Rb -depleted lower crust.
A.P. DICKIN ET AL
26 TA BLE ! Rb /Sr data Sample No.
(87SrJS6Sr)l*21
( 87Sr /8 6Sr),
20
5.25
0.71902
0.7069
0.0005
+37
161
16.77
0.74497
0.7066
0.00 10
+33
biotite granite syen ite
151 lSI
13.99 0.31
0 .736 14 0.7 0544
0.7062 0.7048
0 .00 13 0 .0002
+27 +7
biot ite granite ar fvedsonite gra nite apog ranitc
165.8 168 166
129.2
1.04143
13.27
0.80456
0.7345 0.7220 0.7732
0.0070 0.0 120 0.0040
+429 +25 1 +978
arfvedsonite granite riebeckite- biotite fayal ite granite
160.6 160.6 161
15.9 1 172.0 21.57
0.7518 1 1.10790 0.75942
0.7155 0.7156 0.7100
0.00 01 0.0010 0.0010
+158 + 160 +8 1
trachy -rhyolite syenite syenite
186 186 186
85.55 2.33 0.17
0.938 15 0.7 1105 0.70535
0.7150 0.7049 0.7049
0.0100 0.0001 0.0001
+152 +9 +9
1.66 0.67 7.74 0.30 0.73 0.95 2.09
0.7217 0.7 162 0.7763 0.7 108 0.7 149 0.7173 0.7276
0.7 177 0.7146 0.7576 0. 7101 0.7131 0.71 50 0.7226
+190 +146 +757 +82 +125 +152 +260
3.35 1.25 4.47 2.43 6.35 2.25
0.82 10 0.7226 0.7831 0.7286 0.7863 0.7324
0.8 129 0.7 196 0.7723 0.7227 0.7710 0.7270
+ 1542 +217 +965 +261 +947 +322
Litho logy
Age (Ma)
b iot ite granite
162
biotite granite
87Rb /86Sr(O, )
(II
Amo:
369WR
Jos: 473WR
Panksliin: 45 1WR 470WR
Ririwai: 412 W R 417·422 427 A*J
Shere Hills: 359 R*' 36 1 R*) 362 R
Zarallda: 387WR 385WR 790 W R
Pan-African granites: isotoperatiosat 170 Ma: 336 325 33 1 375 377 436 438
Panyam Bauchi Bauchi Bauc hi Bauchi Rahama Rahama
Malutnfashi gneisses: 295 297 298 671 673 674
migmati te
rnigrnatite granitic gne iss granitic gneiss granitic gneiss dioritic gneiss
*'Errors ± 1% (20). *2Errors average ± 0.0001 ( 2a ) . *JArfvedsonite separate. *'Arfvcdsonitc separate from same pluton as sample 355 analysed for Pb. *)R iebeckite separate from same pluton as sample 360 analysed for Pb .
BASEMENT AND MESOZOIC RING CO~tPLEXES OFTHE 10S PLATEAU
27
TABLE II Sm/Nd data Sample No.
Sm (ppm)
Nd (ppm)
141Sm/'''Nd
143Nd/'''Nd
2a
143Nd/I"Nd
E(l10M.)(.) Nd
tDM
(Ga)
Mesozoicring 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 ' 0.5124 10 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 0.51227 0.51224 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 0.512076
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 -7.9 -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
Malumfashi gneisses:
295 297 298 671 673 674
5.232 3.086 6.269 6.414 3.026 4.892
* Ell):::; parts per ten thousand deviation from bulk Earth evolution line at time of emplacement, t.
3. Revised chronology 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 chosen for analysis. In the case of the Amo, Jos , Pankshin and Zaranda plutonites, such samples had sufficiently low RbjSr ratio for the calculation of accurate initial 87Sr j 86 Sr ratios using the isochron ages of van Breemen et al. (Table I). Hence in these cases the 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 use this procedure, and lower-Rb/Sr separated mineral phases were analysed in order to more
accurately constrain initial 87Srj 86 S r ratios. The Zaranda trachy-rhyolite initial ratio cannot be accurately determined in this manner because the rock is too fine-grained to allow mineral separation. For the Shere Hills complex an alkali amphibole separate was analysed from each ofthe three intrusive phases. Amphibole and wholerock samples from the arfvedsonite granite and riebeckltc-biotite granites are colinear, yielding an age of 160.6 ± 1.2 Ma and initial Sr ratio (I.R.) of 0.71556±0.00009 (20'). However, the amphibole separate from 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 ages of 165.8±1.6 and 168±6 Ma, respectively, in better agreement than in previous work (van
A.I'. DICKIN ET AL.
28 TABLE III Pb isotope data Sample No.
~ o6I'b(O~Pb
e06Pb/~~Pb ),
e~Pb( O~Pb),
eosPb/ ~o~Pb
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 17.8 1 17.88 18.06 17.65 17.84
15.56 15.57 15.58 15.60 15.51 15.61
38. 11 38.4 1 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* II
21 16.3 20 4.4 6. 1
18.35 18.42 18.45 18.29 18.29
15.61 15.63 15.65 15.6 1 15.62
38.48 38.49 38.97 30.84 38.38
39.508 38.433 38.780 40.617 38.523 38.733 38.8 11
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 38.41 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*
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
~ o7Pbr o~Pb
2osPb/2 0~Pb
15.591
39.300
5.2
31
15.567 15.572
30.2 73 39.22 7
10.8 12.9
15.548 15.556
39. 138 38.4 17
15.582 15.599 15.618 15.645 15.537 15.6 11
39.438 39.3 14 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 39.49 7
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
Pb Th (ppm) (ppm) (ppm)
U
Amo camp/ex:
369
18.533
los camp/ex:
473 474
18.378 18. 193
Panks liin camp/ex:
451 470
18.079 18.200
6. 1 1.05
Ririwai camp/ex:
417 422 4/2 4/4 424 427
18.238 18.341 18.702 18.900 18.291 17.842
102 63 102 46 230 376
Shere lIills camp/ex:
362 360 355
18.201 18.017 17.830
Zarandacamp/ex:
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 67/ 673 674
19.402 17.362 19.530 18.542 19.768 18.694
10*
5* 3*
t =age for Mesozoic granite, else 170 Ma, Average within run precision on Pb isotope ratios=0.003 (20') . Estimated absolute accuracy of Pb isotope ratios= 0.0 1,0.007 and 0.02 (I a ) for 206 P bFo~Pb , 207Pb/2 0~Pb and 2osPb/ 20~Pb, respectively. *Estimated concentration .
29
BASEMENT AND MESOZOIC RING COMPLEXES OF THE JOS PLATEAU
Breemen et aI., 1975). However, initial ratios of 0.735 and 0.722 are distinct. The arfvedsonite albite apogranite also yields a consistent age of 166::t 7 Ma (I.R. = 0.77 3) 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 Maago. 4. Isotope geochemistry of Mesozoic ring complexes
.~
395
AFRICAN
390
~fj*o
385
b\ ("'P 20'PbZ
+10
*
70 "'1a
380
* 500M.a
37.5
*
'o~
*
u-
158
("'Pb) 20'Pb 110M
~J\l\C"~,.z
*
*
,,~~z ~ R--.. /z. ZI
156
Initial Sr and Nd isotope ratios are expressed as E(t) and compared with E(l70 Ma) compositions of possible mantle and crustal sources in Fig. 2. The Mesozoic ring complexes have a wide range of fS r compositions (from +9 to +978) but a relatively restricted range in fNd( -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 ENd composition of the Zaranda syenite (+ 0.9) provides good evidence that this
*
"l
s~
!if!'3"feV
~/
*
154
17
*
18
19
1'''Pb/'''Pbl17•
20 Ma
Fig. 3. Initial Pb isotope compositions of Mesozoic ring complexes (symbols as in Fig. 2) and composition ofpossible source compositions 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 Stacey and Kramers (1975) model.
~MIRB CAMEROON LINE
VOLCANICS
CD
o
NIGERIANMESOZOICANOROGENIC COMPLEXES
®
J
'\
'i1\
·10
@lLArlvedSOnile)· J ~®
(Arlvedsonite)
P
.",
~
(C!-~~~'!.l!:~n~!!.t!!~!!l:!~
x:
I I
BASEMENT OF THE JOS PLATEAU
I
NIGERIA X-cameroon granuliteenclave
~:
o
LB.!.o.!i~l
PRECAMBRIAN •
+200
+300
+400
ESr
Fig. 2. fNd vs. ESr diagram showing the compositions of Nigerian Mesozoic ring complexes (Zee 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. I).
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 ENd of - 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 sources at 170 Ma (Table
30
A.P. DICKIN ET AL.
18.5
r----.---.---.,....---.,.----, "P'
18.0
T Siom.
'
'""
206Pb) ( 204Pb I
> -(R r - - - <
/ 1
17.5
=-_
17.0 0.70
__=_~-__=_-:l:-::-----.L.---...L-------l 0.71 0.72 0.73 0.74 0.75 (87SrJ66Srll
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) are plotted on 207Pbj204Pb vs. 206Pb j204Pb and 208Pb/204Pb vs. 206Pb/204Pb diagrams in Fig. 3. The mantle Pb field, as inferred from uncontaminated Cameroon Line volcanics (Halliday et al., 1988), is age corrected using a mantle Ji-value 38U /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 arrayon the Pb/Pb isochron diagram (Fig. 3B), whose slope corresponds to an apparent age of '" 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 gneiss crustal fields in both Fig. 3A and B. When this observation is coupled with the l750-Ma apparent age of the ring complexes it suggests that Pb in these rocks is derived largely by mixing of crustal Pb sources with an average Early Proterozoic formation age.
e
5. Discussion Some of the Mesozoic ring complexes show evidence of intense hydrothermal activity (Bowden et al., 1979; Bowden and Kinnaird,
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 changes in ENd between 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 ofrare earths in hydrous fluids. The Ririwai apogranite shows a deviation in Nd as well as Sr 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 87Sr/ 86S 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 los complexes. Hydrothermal overprinting of the Ririwai complex and one or more shere Hills unit then superimposed a trend almost at right-angles to the magmatic contamination trend, in which 87Sr/ 86S r rose very rapidly. Only in the case of extreme alteration suffered by the Rir-
31
BASEMENT AND MESOZOIC RING CO:'1PLEXES OFTHE 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-, 1- and S-types. The evidence which we have presented here 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 ofthe Jos Plateau. The extent ofthis contamination is relatively minor in the Zaranda syenite, but increases to a more significant fraction in the Amo, Jos and Pankshin complexes. Pb isotope data are consistent with assimilation of Early Proterozoic crust, which is the average formation age found by Sm/Nd analysis of both Malumfashi gneisses and Pan-African granites of the Jos Plateau. Evidence has been presented for 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. O. van Breemen for the use of his unpublished Rb/Sr data on the Malumfashi samples.
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32 Stacey, J.S. and Kramer, J.D., 1975. Approximation of terrestria11ead isotope evolution by a two-stage model. Earth Planet. Sci. Lett., 26: 207-221. van Breernen, 0., 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 Breemcn, 0 ., Pidgeon, R.T. and Bowden, P., 1977. Age and isotope studies of some Pan -African granites from north-central Nigeria. Precambrian Res., 4: 307319.