- Email: [email protected]
Charnockite-granite association in SW Nigeria: rapakivi granite type and charnockitic plutonism in Nigeria?
Journal of African Earth Sciences,Vol. 6, No. 1, pp. 67-77, 1987
0731-7247/87 $3.00+ 0.00 Pergamon Journals Ltd.
Printed in Great Britain
Charnockite-granite association in SW Nigeria: rapakivi granite type and charnockitic plutonism in Nigeria? V . O . OLAREWAJU Department of Geology, University of Ife, Ile-Ife, Nigeria
(Received for publication 19 May 1986) Abstract--A plutonic complex containing both charnockitic and non-charnockitic granite rocks (Older Granites) occurs within the amphibolite facies rocks of gneisses and migmatites in the Ado Ekiti-Akure region of southwestern Nigeria. This complex comprises three petrographic types of charnockitic rocks and three of granitic rocks. These are the coarse-grained charnockitic variety, massive fine-grained and the gneissic finegrained types, while the granitic rocks consist of the fine-grained biotite granite, medium- to coarse-grained and the porphyritic biotite-hornblende granites. Field observation shows remarkable close association of these charnockitic and non-charnockitic components of the complex, and also geochemical evidence provides indications for a petrogenetic link between the rocks. The coarse charnockitic rock type and the granitic rocks in the area have high K20 levels relative to SiO2, high K20/Na20 and high FeO/MgO ratios. There are striking chemical similarities which also characterize rocks of the rapakivi suite. Comparable petrogenetic processes are therefore thought to have been in operation, and the granites of the area are likened to rapakivi granite types which could be a product of charnockitic plutonism. All the above features are reminiscent of the rapakivi granite-massive anorthosite-charnockitic rock series, the close association of which is well documented in some parts of the world. On the basis of trace element geochemistry, the charnockitic rocks are divided into two groups which are the 'normal'-LIL (large-ion-lithophile) and LIL-deficient types. The 'normal'-LIL type is represented by the coarse-grained charnockitic type while the LIL-deficient type is the massive fine-grained variety. The LILdeficient variety has low REE while the 'normal'-LIL charnockitic rock type and the granites are enriched in REE and exhibit fractionated patterns. Both the LIL and REE patterns are consistent with fractionation processes involving separation of LILdeficient phases from a basic magma emplaced under high grade conditions, and the 'normal'-LIL rock type with the granites represent equivalents of rapakivi granites that crystallized from the residual melt at higher structural levels.
INTRODUCTION
plutonism in the area after due comparison with what exists in other parts of the world.
THE CLOSE association of charnockitic and non-charnockitic granitic rocks (Older Granites) with respect to field relations is well documented in the Basement Complex of Nigeria, more especially in the Ado Ekiti region of SW Nigeria (Hubbard 1968, Cooray 1972, 1975). Similar observation has been made elsewhere in the world, for example, the admixture of charnockitic and non-charnockitic rocks of the plutonic association-charnockite/granite association in the Varberg region of southwest Sweden (Hubbard and Whitley 1978, 1979). The association, rapakivi granite-massive anorthositecharnockite has also been well studied in some other parts of the world like the Scandinavian countries (Kranck 1970) and Rap Farvel district of south Greenland (Bridgwater and Waterson 1967). In all the above associations from other parts of the world, a link between the charnockitic component and rapakivi type of granite has been proposed, with the latter being a high level crystallization component related to the charnockitic emplacement at greater depth. This study makes use of field and geochemical evidence and attempts to show the petrogenetic connection between the granites and the charnockitic rocks encountered in the Ado Ekiti-Akure region of southwest Nigeria. It suggests rapakivi granite-charnockite type
LITHOLOGY AND FIELD RELATIONSHIPS
Three petrographic types of charnockitic rocks are encountered and these include: the coarse-grained variety (quartz hypersthene syenite), massive and the gneissic fine-grained varieties (hypersthene granodiorite and quartz monzonorite, respectively, Figs. I and 2). By virtue of the field relationships and the structure of the gneissic fine-grained charnockitic rock type (quartz monzonorite) in the Ado Ekiti-Akure region, it is thought that this variety belongs to an older period of charnockitic rock formation within the Basement Complex of Nigeria, as previously suggested by other workers (Hubbard 1968, Cooray 1972). Therefore, this variety is not discussed any further in this paper. Three textural varieties of granite are also recognized in the Ado Ekiti-Akure region. They are: the fine- to mediumgrained biotite granite, medium- to coarse-grained nonporphyritic biotite-hornblende and the porphyritic biotite-hornblende granites (Figs. 1 and 2). Detailed petrography of all these rocks has already been given in Olarewaju (1986). Modal analysis is given in Table 1. The granites underlie about 65% of the region while the charnockitic rocks underlie the remaining 35%. 67
68
V.O.
OLAREWAJU
t /'o
S'NF~to m e O i o m - g l l k l l d biotite g r i n to.
~+
0
° 0
\¢~s: /
o'~ 0
00
!'-x
't
Canoe p o r p h y r l t l e b i o t i t e and btotite~.~."o oo~° ° ° - o o o . ~ . ~ berltbloogo | r o n i t e . : . o o °o o o Ceorle-~rllNd o b 4 t o o o k i t l n rook. ~o o o ~ ' ~ ' ~ + ~ Mllllivll fieo-grllllld eklrookltle rook~..~o Gnoteeio f i n e - - g r o i n e d chornokitic rook"...o o o o o Qvotzite. • . gnoionoa and Mlgmotlteo.
I
~ o °o o o :°oOO "o°o ." o o o
|
~..~/:0°o :o**" ° o ° . . **,
s,,i...d ,ig
/...**~ o 0 0 o o°
Folietlon
direction /..~ o o o "" ,, o o o o ./:o. o -. o o o Vein o " o o o o- ' o o o o o , • quortl o~o..;.. o o o °, o o o Geological b o u n d a r y (clpproa) - o o ° o o',O o o o Oeologiool u unoertl~'~ oOO ° o °oOo o o',°o o ° o o ~o o ooo o o oo. o o • +._..~OO~o u _~o°oOOOo~, o
°..o •
° v 0°o
.ooOO°0" o
°
° O o ¢
...'..%;;°: ;o oo ! ° . o o ,'~N
o
o%
*--2 ° ° o ~
o ~o o ° o ° . =o °o c o Jo 0 o o ' OoOoO ~o='O0° o .
o o
~:°o° o :
© o o
°oo ° o
o o% i
o og, o°°°o\
o
".0 ° 0 o . c :.Ooo° " ".o O o O
o oO
o,
o
o o o
o o o o °~OoO°
o o
o o.-
o o
~.
.
• -g."
° ~/Oo...... /°o %t~.-"
;
ERO
,o-o o~-0 . . o o°0°o . o ~
b~SE : o . . *o. ,/.~.~..~:.' \1:°° : ~'o Io o Jo ° o
3¢-
o °°o-= o o ° o -c ,o o,o ooo oo
oo
o
0
o:
o o o • o: e ,o°o o OoOo o o o o o • o o:c~.* oo o • o o o. "'..oo o o~ ooo ooooo~ 'q%~ o o , ©. . . . . . . . o ;.
AWULE
-0,,,0~~*** . **0:,". * * . . . . *,
rhs~ 0
1
2
3
I
I
I
i
~
$
6
7
8
K m I I I
I
Fig. 1. Geological map of Ado Ekiti-Akure area of SW Nigeria (mapped by the author between 1978 and 1980).
Charnockite-granite association in SW Nigeria Q /
~
intimate association suggests a c o m m o n time of emplacement. The coarse-grained and massive finegrained charnockitic rock types are believed to be of magmatic origin judging from their unfoliated, massive, homogeneous nature and the cross-cutting contacts. Inclusions or xenoliths of the massive fine-grained charnockitic variety are present in the porphyritic biotitehornblende granite near Ikere (Fig. 3) and also within the coarse charnockitic rock at A d o Ekiti. This relationship indicates that the fine-grained charnockitic rock pre-dates these rock types. There is also what appears to be a 'mixture' of the fine-grained charnockitic rock type and the coarse variety in some parts of the outcrop at A d o Ekiti. Some sharp contacts indicating intrusion are also present between these two rock varieties in the same locality.
3 = Granite 4 = Hypersthane grarlodiorite "f~ • Quartz hypersthene syenite 9 e • Quartz monzonofite
2
/ A
I
4
I
5
Chorn I ®thorn 3 & Chorrl 4 + Gron 1 X Gron 2
~
\
\
69
P
Fig. 2. CombinedQAP diagram for the granites and charnockiticrocks of Ado Ekiti-Akure.
GEOCHEMISTRY
Analytical methods Porphyritic biotite-hornblende granite is the most abundant (55%) while the medium- to coarse-grained biotitehornblende granite and the fine- to medium-grained biotite granite make up roughly 10%. The latter variety is, however, of very small occurrence (Fig. 1). The granites occur mainly as low-lying outcrops and inselbergs, while the charnockitic rocks are in the form of low outcrops which are boulder-like in some instances. The coarse-grained charnockitic variety is intimately associated or even 'intermixed' in some cases with the porphyritic biotite-hornblende granite and/or mediumto coarse-grained non-porphyritic granite in the field. This 'admixture' makes it difficult to draw precise boundaries between these rock groups in some cases. This
The major elements SiO2, TiO2, A1203, Fe203 (total), MnO, MgO, CaO, K20, and P205 with trace elements V, Cr, Ni, Cu, Zn, Th, Rb, Sr, Y, Zr, Nb and Ba were all determined by X R F using an automatic Philips 1212 Spectrometer at Imperial College, London. Norrish fusion pellets prepared from powder of the rock samples were used for the major elements while briquettes made from rock powder were used for the trace elements determination (see details of sample preparation and data processing in Parker 1980a,b). Wet chemical techniques were employed for determination of Na20 and FeO. Na20 was determined using an E E L Flame Photometer and FeO determined by
Table 1. Modal compositionby volume of granitic and charnockiticrocks from Ado Ekiti-Akure region
Quartz Alkali feldspar Plagioclase Hornblende Biotite Orthopyroxene Clinopyroxene Olivine (fayalite) Sphene Allanite Zircon Iron ores Apatite Total Q A P
Gran-1 (8)
Gran-2 (2)
27.5 34.4 22.7 4.8 8.8 ---0.8 0.2 0.1 0.4 0.3 100.0 32.5 40.6 26.9
25.5 40.8 19.0 7.3 6.3 ---. . 0.2 0.9 0.2 100.2 29.9 47.8 22.3
Gran-3 (2) 28.3 42.5 15.3 -11.9 ---. .
Charn-3 (2)
13.6 53.3 20.3 5.9 1.4 0.2 2.6
17.0 14.5 46.2 0.5 6.4 2.6 11.4
1.9
. . 1.5 0.3 0.1 99.9 32.9 49.4 17.7
Charn-1 (6)
. . 0.1 0.8 0.1 100.2 15.6 61.1 23.3
- -
11.9 12.2 39.8 3.7 14.8 2.8 12.9 - -
. . -1.6 -100.2 21.8 18.6 59.6
Number in brackets = number of analyses. Gran-1 = Porphyriticbiotite-hornblende granite. Gran-2 = Medium-to coarse-grainedbiotite-hornblende granite. Gran-3 = Fine- to medium-grainedbiotite granite. Charn-1 = Coarse-grainedcharnockiticrock type (quartz hypersthene syenite). Charn-3 = Gneissicfine-grained charnockiticrock type (quartz monzonorite). Charn-4 = Massivefine-grainedcharnockiticrock type (hypersthene granodiorite). ARS 6:l-E
Charn-4 (4)
-1.9 -100.0 18.7 19.0 62.3
70
V . O . OLAREWAJU Table 2. Averages of major and trace element data for the granitic and charnockitic rocks of Ado Ekiti-Akure Charn-1 (44)
Charn-3 (11)
Charn-4 (37)
Gran-1 (41)
Gran-2 (8)
Gran-3 (9)
Gneiss (20)
A
B
C
SiO2 TiO~ A1203 Fe203 FeO MnO MgO CaO Na20 K20 P205 H20 LOI Total
64.99 0.64 15.49 1.03 4.05 0.11 0.58 2.44 3.46 6.13 0.16 0.12 0.52 99.72
58.83 1.04 16.04 2.41 5.69 0.13 3.42 5.48 2.98 2.70 0.25 0.09 0.67 99.73
55.05 1.85 15.76 1.77 7.97 0.15 3.15 5.97 3.32 3.16 0.62 0.10 0.57 99.44
66.71 0.67 14.98 1.01 2.95 0.06 0.90 2.39 3.22 5.74 0.21 0.09 0.62 99.55
67.19 0.55 14.96 0.99 3.32 0.07 0.59 2.05 3.21 6.00 0.14 0.10 0.53 99.70
68.05 0.56 14.89 0.60 2.57 0.04 0.80 2.05 3.09 5.77 0.16 0.07 0.81 99.46
69.69 0.37 14.71 0.95 2.13 0.05 0.82 2.74 3.21 4.57 0.10 0.06 0.48 99.88
69.15 0.56 14.63 1.22 2.27 0.06 0.99 2.45 3.35 4.58 0.20 --99.46
68.16 0.46 14.37 1.95 2.07 0.04 0.50 1.47 1.83 6.89 0.11 0.23 1.20 99.58
62.51 1.13 15.07 2.18 3.70 0.13 1.80 3.54 3.87 4.57 0.44 0.42 -99.36
V Cr Ni Cu Zn Th Rb Sr Y Zr Nb Ba
24 11 9 14 88 19 138 182 47 742 38 1136
128 99 32 20 97 12 78 383 46 207 16 978
138 33 31 29 121 8 93 477 53 390 31 1421
37 9 7 10 64 43 218 287 22 404 19 1418
30 20 8 15 49 25 138 297 33 219 11 952
Trace elements(ppm) 41 27 13 14 9 9 13 12 70 81 24 27 176 167 313 182 44 46 385 563 26 34 1352 1073
Number in brackets = number of analyses. Charn-1 = Coarse-grained charnockitic rock (quartz hypersthene syenite). Charn-3 = Gneissic fine-grained charnockitic rock (quartz monzonorite). Charn-4 = Massive fine-grained charnockitic rock (hypersthene granodiorite). Gran-1 = Porphyritic biotite-hornblende granite. Gran-2 = Medium- to coarse-grained biotite-hornblende granite. Gran-3 = Fine-grained biotite granite. Gneiss = Granitic gneiss. A = Average of 121 adamellites (Nockolds 1954). B = Normal rapakivi granite (Kranck 1970). C = Average of 2 coarse-grained charnockites from Varberg region, southwest Sweden (Hubbard and Whitley 1979).
titration. Decomposition of the sample powder with HF/metavanadate solution was followed by titration with K2Cr207 for FeO determination. Analytical results
One hundred and seventy (170) representative samples of the different rock types including the granitic gneisses in the region were analyzed. The major and trace element data, averages only, are presented in Table 2, full data being available in Olarewaju (1981). The rare-earth element abundances are reported in Table 3. In general, geochemical studies of the charnockitic and granitic rocks of the area show trends comparable to those of calc-alkaline plutonic suites (Nockolds and Allen 1953, Saunders et al. 1980) with SiO2, K20, Rb and Th increasing with DI while TiO2, Fe203 (total), MgO, CaO, P205, V, Ni, Zn, Y and Sr decrease with Differentiation Index (DI) (Olarewaju 1981, 1986). Major element geochemistry reveals the predominantly adamellitic composition of the granites and coarse-grained charnockitic rocks, while the fine-
grained charnockitic variety is mainly of granodioritic composition (Figs. 4 and 5, Table 2). The high K20 levels relative to SiO2 (Fig. 6), high K20/Na20 ratios (Fig. 7), and high FeOT*/MgO (Fig. 8) are the salient features of the granites and coarse charnockitic rock that parallel those chemical compositions which characterize the rapakivi granites (Emslie 1973, Wilson 1980). Trace element geochemistry gives broad division of the charnockitic rocks into 'normal'-LIL (large-ionlithophile element) and LIL-deficient rocks, The 'normal'-LIL is represented by the coarse charnockitic rock (with K20 > 4.4% and 138 ppm Rb, 182 ppm Sr) while the LIL-deficient is exemplified by the fine-grained variety with lower K20 and Rb values but higher Sr contents (Table 2). These divisions are similar to those used by Field et al. (1980) for the charnockitic gneisses of south Norway. The charnockitic rocks generally have lower absolute REE abundances than the granites. The values range from 361 ppm in the massive fine-grained charnockitic
* FeOT is the total iron.
Chamockite-granite association in SW Nigeria
Fig. 3. Xeno!iths of massive fine-grained charnockitic rock type (hypersthene granodiorite) in porphyritic biotitehornblende granite near Ikere.
71
Charnockite-granite association in SW Nigeria CHARN 1 = C o a r s e - g r a l n e d charnockitic rock CHARN 3 = Gneissic fine-grained charnockitic rock CHARN 4 = Massive fine-grained charnGRAN 1 GRAN 2
73
CoO CHRRN 1 CHRRN 3 CHRRN 4 GRRN I GRRN 2 GRRN 3 GNEISS
ockitic rOCkbiotite_ / = Porphyritic hornblende granite ~/ , = Medium-to c o a r s e - g r a i n e ~ grantebi°tltehornblende /
GRAN 3
=
GNEISS
= Granitic
graniteFine-grainedbiotit 7 gneiss
'(9 + X
O ,p
q
G= Granite
Field
GO=Granodiorite Om=Quat I z monzonite
To=Tonalite To +
NazO
I-0
0-5 K20 /
~Xx K2 0
1.5
Nat0
Fig. 4. CaO-Na20-K20 diagram (after Condie and Hunter 1976) showing distribution of rocks in region of study.
variety to 474 ppm in the coarse type and 504 ppm in the porphyritic biotite-hornblende granite (Table 3). The degree of fractionation is not as highly pronounced as in the granites as the charnockitic rocks all display only mildly fractionated REE patterns (Figs. 9, 10 and 11). In general, there is light rare-earth element (LREE) enrichment in the rocks, negative europium anomalies especially in the granites and varying degrees of heavy rare-earth element (HREE) depletion. An
/
t.
DISCUSSION A petrogenetic link between the charnockitic and granitic rocks of Ado Ekiti-Akure is indicated by both field and geochemical evidence. Prominent in the evidence are: (i) the striking 'admixture' of these two groups of rocks in the field; (ii) fractionated REE patterns with LREE enrichment and also some negative Eu anomalies indicating igneous origin (Figs. 9, 10 and 11);
16
(9 Charn 1 ~ Charn 3
~t~
\
12 ¢
IQ.
~ga o 2
t,r"~- = ' ~
4 / -
To = Tonalife
eOA I
• .+" J ---"
G d - Eranodiorit"e Om
.
Tj-
-
0
=
Ouarfz monzonite Trondhjemite Granite
Fig. 5. A b - A n - O r (normative averages) diagram for the rocks studied. Fields after O'Connor (1965).
45
53
61 SIO I w e l g h t
69
77
per cent
Fig. 6. K20 vs SiO 2 diagram. The dashed lines show the field of calc-alkaline plutons of the Antarctic Peninsula (Saunders e t al. 1980). Symbols as in Fig. 4.
74
V . O . OLAREWAJU 10 in
c: Q
8 +
~
~
~
o
~
o. .
~a
+
t~
G =Granite
Field
o
if,
AD = A d a m e l l i t e
GDmtG ranodior ire
TO ==Tonaiite
-0
1.0
2..0
3 .O No=O
4.0
Weight
per
5.0
cent
Fig. 7. K20-Na20 relationships of the rocks in Ado Ekiti-Akure. Boundaries drawn after Viljoen and Viljoen (1969). Symbols as in Fig. 4.
(iii) a possible common parental liquid suggested by the distribution of the immobile transition elements (Olarewaju 1981); (iv) similar Pan-African ages of 631 + 18 Ma given by the rocks (Tubosun et al. 1984) suggesting synchronous emplacement. A similar link between non-charnockitic granitic and charnockitic rocks has been reported elsewhere by, for example, Kranck (1970) on the rapakivi plutons of Scandinavia, Hubbard and Whitley (1978, 1979) who established a link between Torpa granites of rapakivi type and their charnockitic source in southwest Sweden, and Field et al. (1980) on the charnockitic gneisses of south Norway. REE have been particularly useful in establishing the
fact that the rocks in the Ado Ekiti-Akure region have striking similarities with the rapakivi granites of Finland (Koljonen and Rosenberg 1974) and the granitic--charnockitic rocks of Hubbard and Whitley (1979, Figs. 9, 10 and 11, Table 3). Presumably, comparable petrogenetic processes have been in operation.
600.
A = Pot'phyriti¢ granite B = Medium-to coarse-grained granite
D= Torpo granite [Hubbard =, Wh trey 19?9 )
C = Fine grained granite
E = Rapakivi granite(Koljonen lit Rosenberg, IgT& ) F = Portugat granite ( de Albuquerque, 1978)
C .
~
%
G - NASC (Haskin et o1.,lC368) NASC " North Amerioan Shale
Composite
B F
100-
Fog(total)
~s0
~
,,~3
/ ~ _. ~ / / C A L C
-- A L K A L, N E
/~,+/+. A-:*¢,.F i~020 + K 2 0 /
v
w
v
v
v
w
10 --
~
\
v
\ v
\MgO
Fig. 8. AFM (total alkalies vs F o g t vs MgO) diagram showing the major chemical variations in the rocks of Ado Ekiti-Akure. The dashed line (A) is the divide between the tholeiitic (above) and calc-alkaline (below) trends as suggested by Irvine and Baragar (1971) with trends from the Peruvian Batholith B (Pitcher 1978) and the Lower California Batholith C (Carmichael et al. 1974). Symbols as in Fig. 4.
0
11oC~z
l~d
S'm ~u dd l'b
H'o
Tm '~b flu
Fig. 9. Comparison of chondrite-normalized REE distribution patterns of the granitic rocks of the Ado Ekiti-Akure r e # o n with published values.
Charnockite-granite association in SW Nigeria A = Massive fine-groined chartlockitic r o c k B = Gneissic fine--groined chornockitic rock C = Fine-groined charnockite, Sweden ( Hubbard & Whitley, lgTg)
A = Coarse--groined
D = Charnockitic granite gneiss, Sweden ( H u b b a r d 8. Whitley, lg79). E= Continental basalt,average of 52 ( Helmke & Hoskin, 1973 I F = NASC ( Haskin etal, 1968).
¢harnockitl¢
rock
B = Coarse charnockite,southwest Selden ( Hubbard IkWhitley, 11179) C= M a s s i v e c h a r n o c k i t e , Brazil (Gosparini 1~ Mantovan b 1979) D=NASC(Hclskin el a1,19681
300300-
A 200- C
75
A
100- F 100
u.J
E
°zS( 0 -r 0
~5C c_
v (J 0 {E
Fig. 10. Chondrite-normalized R E E comparison distribution patterns for the fine-grained charnockitic rocks.
Fig. 11. R E E comparison diagram for the coarse-grained charnockitic rock.
Table 3. Rare-earth element data (ppm) for the rocks from Ado Ekiti-Akure, SW Nigeria
La Ce Nd Sm Eu Tb Ha Yb Lu ~REE Y ~REE + Y ~LREE HREE ~ LREEPZ H R E E La/Yb +LaN/SmN +CeN/YbN Eu/Eu*
A
B
C
D
E
F
G
H
I
J
129.59 264.66 87.26 13.88 1.90 1.52 1.29 3.24 0.38 503.72 58.40 562.12 495.40 6.43 77.05 40.00 5.12 19.52 0.46
75.41 150.73 56.00 10.33 1.71 1.18 1.56 2.83 0.35 300.10 38.00 338.10 292.47 5.92 49.40 26.65 4.01 12.72 0.58
200.43 435.60 126.17 15.70 1.84 1.09 0.50 1.81 0.18 783.32 27.00 810.32 777.90 3.58 217.29 110.73 9.54 54.94 0.58
100.75 258.13 91.93 13.25 3.07 1.62 0.97 4.11 0.47 474.30 49.00 523.30 464.06 7.17 64.72 24.51 4.17 14.97 0.78
79.02 179.98 77.57 14.74 3.29 1.61 1.39 3.46 0.42 361.48 53.25 414.73 351.31 6.88 51.06 22.84 2.94 12.41 0.78
50.45 99.22 38.71 7.78 1.76 1.13 1.36 4.36 0.53 205.30 46.67 251.97 196.16 7.38 26.60 11.57 3.56 5.57 0.73
69.51 138.80 42.10 6.25 1.01 0.67 1.22 1.83 0.21 261.60 22.00 283.60 256.66 3.93 65.31 37.98 6.10 18.15 0.56
146.00 269.00 78.00 16.00 0.82 1.60 -6.30 1.33 519.05 ----------
152.00 334.00 106.00 17.00 2.20 1.90 -5.70 0.90 619.70 ----------
77.20 155.00 70.30 18.40 3.00 1.90 -6.50 1.00 333.30 ----------
+ = Chondrite-normalized La/Sm and Ce/Yb ratios. Eu/Eu* = Chondrite-normalized Eu value divided by the value obtained by drawing a straight line between Sm and Tb. G r a n - l - - A = Average of 10 coarse porphyritic biotite-hornblende granites. Gran-2--B = Medium- to coarse-grained biotite-hornblende granite. G r a n - 3 - - C = Average of 2 fine-grained biotite granites. C h a r n - l - - D = Averge of 7 coarse-grained charnockitic rocks (quartz hypersthene syenite). C h a r n - 4 - - E = Average of 4 fine-grained charnockitic rocks (hypersthene granodiorite). C h a r n - 3 - - F = Average of 3 gneissic fine-grained charnockitic rocks (quartz monzonorite). G n e i s s - - G = Granitic gneiss. H = Rapakivi granite, Kalanti Finland (Koljonen and Rosenberg 1974). I = Torpa granite, southwest Sweden (Hubbard and Whitley 1978). J = Average of 9 charnockitic phases of charnokite-granite association, southwest Sweden (Hubbard and Whitley 1978).
76
V . O . OLAREWAJU
It is significant to note that although the distinctive mantled megacryst feldspar rapakivi texture s e n s u s t r i c t o is not observed in the granites, m a j o r element chemistry is similar to that in rocks recognized as belonging to the rapakivi suite. For example, in the rapakivi suite, the SiO 2 range is 64-72%, K 2 0 is high ( > 6 % ) and M g O always very low ( < 1 % ) (Kranck 1970). The corresponding figures for a rapakivi granite from Satakunta, Finland (Koljonen and Rosenberg 1974) are: SiO 2 = 68.16%, K 2 0 - - 6 . 8 9 % and M g O = 0.50% and for T o r p a granite ( H u b b a r d and Whitley 1979); SiO 2 = 70.7%, K 2 0 = 5.58% and M g O = 0.52%. Ranges of these elements in the granites of A d o E k i t i - A k u r e are: SiO 2 = 66.71-68.05%, K 2 0 = 5.74--6.00% and MgO = 0.59-0.90%. The coarse-grained charnockitic rock in the region gives the average values SiO 2 = 64.99%, K 2 0 = 6.13% and MgO -- 0.58%, while the associated massive finegrained intermediate variety gives average values of SiO2 = 55.05%, K 2 0 = 3.16% and M g O - - 3.15% (Table 2). It is thus seen that chemically, the granites studied are similar to the rapakivi granite types. It has been shown earlier that the coarse-grained variety of the charnockitic rocks is enriched in L I L elements while the fine-grained variety is depleted in t h e m (Table 2). The coarse charnockitic rock and the massive fine-grained type therefore represent the ' n o r m a l ' - L I L and LILdeficient rocks. F r o m the foregoing discussion, it is seen that on the basis of the chemistry, the granites in the A d o E k i t i A k u r e region could be likened to rapakivi granite which has been suggested to be a product of charnockitic plutonism which in some cases could be triggered by anorthosite e m p l a c e m e n t ( H u b b a r d and Whitley 1978, 1979). In the Varberg region of southwest Sweden, no anorthosite is found but only inferred since it may exist at unexposed levels ( H u b b a r d and Whitley 1979). Likewise, no anorthosite is exposed in the A d o E k i t i A k u r e region but the occurrence of LIL-deficient charnockitic rock type in this area seems crucial in the link between this rock on one hand and the ' n o r m a l ' - L I L charnockitic rock together with the granites which might represent equivalents of rapakivi granites at higher structural levels on the other hand. The LIL-deficient charnockitic variety with its low K, Rb and high Sr levels (Table 2) indicates derivation from a source depleted in alkalis and enriched in Sr. These features correspond to those for granulites (Tarney e t al. 1972, Tarney 1976) and they would suggest a source region of deep crustal levels or u p p e r mantle. The composition of the L I L deficient type is therefore closer to that of the original m a g m a than the composition of the other rocks in the area. This parental m a g m a is believed to be andesiticdacitic in composition (Olarewaju 1981). This model does not require the involvement of an unseen anorthosite in the charnockite-granite association as H u b b a r d and Whitley (1978, 1979) suggested. Field et al. (1980) suggested a similar model to this one.
CONCLUSIONS Field and geochemical evidence show that the charnockite-granite association of A d o E k i t i - A k u r e in SW Nigeria forms a cogenetic sequence likened to rapakivi granite-anorthosite-charnockite group encountered in some other parts of the world. No anorthosite is exposed in the area, but the LIL-deficient charnockitic rock type forms an important link between itself on one hand, and the ' n o r m a l ' - L I L charnockitic rock together with granites on the other hand. Both the L I L and R E E patterns are suggestive of a fractionation process which involves LIL-deficient phases emplaced under high grade conditions and the ' n o r m a l ' - L I L rock type with the granites representing equivalents of rapakivi granites that possibly crystallized from residual melts at higher structural levels. Acknowledgements--Thanks are due to Dr N. W. Rogers for the
instrumental neutron activation analysis (INAA) at the University of London Reactor Centre. Discussions with Dr M. A. Rahaman and critical reviews by Mr O. Ocan greatly improved the content of the manuscript.
REFERENCES Albuquerque, C. A. R. de 1978. Rare earth elements in 'Younger' granites, Northern Portugal. Lithos 11,219-229. Bridgwater, D. and Waterson, J. S. 1967. Igneous intrusion and associated rocks of the mangerite-charnockite suite. Nature 213, 897. Carmichael, I. S. E., Turner, F. J. and Verhoogen, J. 1974. Igneous Petrology, p. 568. McGraw-Hill, New York. Condie, K. C. and Hunter, D. R. 1976. Trace element geochemistryof Archean granitic rocks from the Barberton region, South Africa. Earth Planet. Sci. Lett. 29,389-400. Cooray, P. G. 1972. Note on the charnockites of the Akure-Ado Ekiti area, Western Nigeria. In: African Geology (Edited by Dessauvagie, T. F. J. and Whiteman, A. J.), pp. 45-63. Ibadan University Press, Ibadan. Cooray, P. G. 1975. The charnockitic rocks of Nigeria. Pichamuthu volume, pp. 50-73. Bangalore University, India. Emslie, R. F. 1973. Some chemical characteristics of Anorthositic Suites and their significance. Can. J. Earth Sci. 10, 54-71. Field, D., Drnry, S. A. and Cooper, D. C. 1980. Rare-earth and LIL element fractionation in high grade charnockitic gneisses, South Norway. Lithos 13,281-289. Gasparini, P. and Mantovani, M. S. M. 1979. Geochemistry of charnockites from Sao Paulo State, Brazil. Earth Planet. Sci. Lett. 42, 311-320. Haskin, L. A., Haskin, M. A., Frey, F. A. and Wildeman, T. R. 1968. Relative and absolute terrestrial abundances of the rare earths. In: Origin and Distribution o f the Elements (Edited by Ahrens, L. H.), Vol. 30, pp. 889-912. Pergamon Press, Oxford. Helmke, P. A. and Haskin, L. A. 1973. Rare-earth elements Co, Sc and HF in the Steens Mountain basalts. Geochem. Cosmochim. Acta 37, 1513-1529. Hubbard, F. H. 1968. The association Charnockite-Old Granite in SW Nigeria. Jl Mining Geol. (Nig. ) 3, 25-32. Hubbard, F. H. and Whitley, J. E. 1979. REE in charnockite and associated rocks, southwest Sweden. Lithos 12, 1-11. Irvine, T. N. and Baragar, W. R. A. 1971. A guide to the chemical classification of the common volcanic rocks. Can. J. Earth Sci. 8, 523-548. Koljonen, T. and Rosenberg, R. J. 1974. Rare-earth elements in granitic rocks. Lithos 7,249-261. Kranck, E. H. 1970. Anorthosites and rapakivi, magmas from the lower crust. New York State Mus. Sci. Serv. Mere. 18, 93-97. Nockoids, S. R. 1954. Average chemicalcompositionsof some igneous rocks. Geol. Soc. Am. Bull. 65, 1007-1032.
Charnockite-granite association in SW Nigeria Nockolds, S. R. and Allen, R. 1953. The geochemistry of some igneous rock series. Geochim. Cosmochim. Acta 4, 105-142. O'Connor, J. T. 1965. A classification for quartz-rich igneous rocks based on feldspar ratios. U.S. Geol. Surv. ProL Pap. 5258, pp. 79-84. Olarewaju, V. O. 1981. Geochemistry of the charnockitic and granitic rocks of the basement complex around Ado Ekiti-Akure, S.W. Nigeria. Ph.D. thesis, Univ. Lond. Olarewaju, V. O. 1986. Petrology and geochemistry of charnockitic and associated granitic rocks of Ado Ekiti-Akure, S.W. Nigeria. Precambr. Nigeria 1. Parker, R. J. 1980a. Preparation of silicate rock samples for XRF analysis. Tech. Report No. XRF-3. Dept. Geol., Imperial College, Univ. of London. Parker, R. J. 1980b. Computer processing and evaluation of XRF analytical data for silicate rock samples. Tech. Report No. XRF-4, Dept. Geol., Imperial College, Univ. of London. Pitcher, W. S. 1978. The anatomy of a batholith. J. Geol. Soc. Lond. 135, 157-182.
77
Saunders, A. D., Tamey, J. and Weaver, S. D. 1980. Transverse geochemical variations across the Antarctic Peninsula: implications for the genesis of calc-alkaline magmas. Earth Planet. Sci. Lett. 46, 344-360. Tarney, J. 1976. Geochemistry of Archean high-grade gneisses, with implications as to the origin and evolution of the Precambrian crust. In: The Early History of the Earth (Edited by Windley, B. F.), pp. 405-424. Wiley, London. Tarney, J., Skinner, A. C. and Sheraton, J. W. 1972. A geochemical comparison of major Archean gneiss units from N.W. Scotland and East Greenland. Proc. 24th Int. Geol. Congr., Montreal, Vol. 1, pp. 162-174. Tubosun, I. A., Lancelot, J. R., Rahaman, M. A. and Ocan, O. 1984. U/Pb Pan-African ages of two charnockite-granite associations from south-western Nigeria. Contr. Miner. Petrol. 88, 188-195. Viljoen, M, J. and Viljoen, R. P. 1969. The geochemical evolution of the granitic rocks of the Barberton region. Geol. Soc. S. Afr. Sp. Publ. 2, 189-219. Wilson, M. R. 1980. Granite types in Sweden. Geologiska Foreningens i Stockholm Forhandlingar 102,167-176.
0731-7247/87 $3.00+ 0.00 Pergamon Journals Ltd.
Printed in Great Britain
Charnockite-granite association in SW Nigeria: rapakivi granite type and charnockitic plutonism in Nigeria? V . O . OLAREWAJU Department of Geology, University of Ife, Ile-Ife, Nigeria
(Received for publication 19 May 1986) Abstract--A plutonic complex containing both charnockitic and non-charnockitic granite rocks (Older Granites) occurs within the amphibolite facies rocks of gneisses and migmatites in the Ado Ekiti-Akure region of southwestern Nigeria. This complex comprises three petrographic types of charnockitic rocks and three of granitic rocks. These are the coarse-grained charnockitic variety, massive fine-grained and the gneissic finegrained types, while the granitic rocks consist of the fine-grained biotite granite, medium- to coarse-grained and the porphyritic biotite-hornblende granites. Field observation shows remarkable close association of these charnockitic and non-charnockitic components of the complex, and also geochemical evidence provides indications for a petrogenetic link between the rocks. The coarse charnockitic rock type and the granitic rocks in the area have high K20 levels relative to SiO2, high K20/Na20 and high FeO/MgO ratios. There are striking chemical similarities which also characterize rocks of the rapakivi suite. Comparable petrogenetic processes are therefore thought to have been in operation, and the granites of the area are likened to rapakivi granite types which could be a product of charnockitic plutonism. All the above features are reminiscent of the rapakivi granite-massive anorthosite-charnockitic rock series, the close association of which is well documented in some parts of the world. On the basis of trace element geochemistry, the charnockitic rocks are divided into two groups which are the 'normal'-LIL (large-ion-lithophile) and LIL-deficient types. The 'normal'-LIL type is represented by the coarse-grained charnockitic type while the LIL-deficient type is the massive fine-grained variety. The LILdeficient variety has low REE while the 'normal'-LIL charnockitic rock type and the granites are enriched in REE and exhibit fractionated patterns. Both the LIL and REE patterns are consistent with fractionation processes involving separation of LILdeficient phases from a basic magma emplaced under high grade conditions, and the 'normal'-LIL rock type with the granites represent equivalents of rapakivi granites that crystallized from the residual melt at higher structural levels.
INTRODUCTION
plutonism in the area after due comparison with what exists in other parts of the world.
THE CLOSE association of charnockitic and non-charnockitic granitic rocks (Older Granites) with respect to field relations is well documented in the Basement Complex of Nigeria, more especially in the Ado Ekiti region of SW Nigeria (Hubbard 1968, Cooray 1972, 1975). Similar observation has been made elsewhere in the world, for example, the admixture of charnockitic and non-charnockitic rocks of the plutonic association-charnockite/granite association in the Varberg region of southwest Sweden (Hubbard and Whitley 1978, 1979). The association, rapakivi granite-massive anorthositecharnockite has also been well studied in some other parts of the world like the Scandinavian countries (Kranck 1970) and Rap Farvel district of south Greenland (Bridgwater and Waterson 1967). In all the above associations from other parts of the world, a link between the charnockitic component and rapakivi type of granite has been proposed, with the latter being a high level crystallization component related to the charnockitic emplacement at greater depth. This study makes use of field and geochemical evidence and attempts to show the petrogenetic connection between the granites and the charnockitic rocks encountered in the Ado Ekiti-Akure region of southwest Nigeria. It suggests rapakivi granite-charnockite type
LITHOLOGY AND FIELD RELATIONSHIPS
Three petrographic types of charnockitic rocks are encountered and these include: the coarse-grained variety (quartz hypersthene syenite), massive and the gneissic fine-grained varieties (hypersthene granodiorite and quartz monzonorite, respectively, Figs. I and 2). By virtue of the field relationships and the structure of the gneissic fine-grained charnockitic rock type (quartz monzonorite) in the Ado Ekiti-Akure region, it is thought that this variety belongs to an older period of charnockitic rock formation within the Basement Complex of Nigeria, as previously suggested by other workers (Hubbard 1968, Cooray 1972). Therefore, this variety is not discussed any further in this paper. Three textural varieties of granite are also recognized in the Ado Ekiti-Akure region. They are: the fine- to mediumgrained biotite granite, medium- to coarse-grained nonporphyritic biotite-hornblende and the porphyritic biotite-hornblende granites (Figs. 1 and 2). Detailed petrography of all these rocks has already been given in Olarewaju (1986). Modal analysis is given in Table 1. The granites underlie about 65% of the region while the charnockitic rocks underlie the remaining 35%. 67
68
V.O.
OLAREWAJU
t /'o
S'NF~to m e O i o m - g l l k l l d biotite g r i n to.
~+
0
° 0
\¢~s: /
o'~ 0
00
!'-x
't
Canoe p o r p h y r l t l e b i o t i t e and btotite~.~."o oo~° ° ° - o o o . ~ . ~ berltbloogo | r o n i t e . : . o o °o o o Ceorle-~rllNd o b 4 t o o o k i t l n rook. ~o o o ~ ' ~ ' ~ + ~ Mllllivll fieo-grllllld eklrookltle rook~..~o Gnoteeio f i n e - - g r o i n e d chornokitic rook"...o o o o o Qvotzite. • . gnoionoa and Mlgmotlteo.
I
~ o °o o o :°oOO "o°o ." o o o
|
~..~/:0°o :o**" ° o ° . . **,
s,,i...d ,ig
/...**~ o 0 0 o o°
Folietlon
direction /..~ o o o "" ,, o o o o ./:o. o -. o o o Vein o " o o o o- ' o o o o o , • quortl o~o..;.. o o o °, o o o Geological b o u n d a r y (clpproa) - o o ° o o',O o o o Oeologiool u unoertl~'~ oOO ° o °oOo o o',°o o ° o o ~o o ooo o o oo. o o • +._..~OO~o u _~o°oOOOo~, o
°..o •
° v 0°o
.ooOO°0" o
°
° O o ¢
...'..%;;°: ;o oo ! ° . o o ,'~N
o
o%
*--2 ° ° o ~
o ~o o ° o ° . =o °o c o Jo 0 o o ' OoOoO ~o='O0° o .
o o
~:°o° o :
© o o
°oo ° o
o o% i
o og, o°°°o\
o
".0 ° 0 o . c :.Ooo° " ".o O o O
o oO
o,
o
o o o
o o o o °~OoO°
o o
o o.-
o o
~.
.
• -g."
° ~/Oo...... /°o %t~.-"
;
ERO
,o-o o~-0 . . o o°0°o . o ~
b~SE : o . . *o. ,/.~.~..~:.' \1:°° : ~'o Io o Jo ° o
3¢-
o °°o-= o o ° o -c ,o o,o ooo oo
oo
o
0
o:
o o o • o: e ,o°o o OoOo o o o o o • o o:c~.* oo o • o o o. "'..oo o o~ ooo ooooo~ 'q%~ o o , ©. . . . . . . . o ;.
AWULE
-0,,,0~~*** . **0:,". * * . . . . *,
rhs~ 0
1
2
3
I
I
I
i
~
$
6
7
8
K m I I I
I
Fig. 1. Geological map of Ado Ekiti-Akure area of SW Nigeria (mapped by the author between 1978 and 1980).
Charnockite-granite association in SW Nigeria Q /
~
intimate association suggests a c o m m o n time of emplacement. The coarse-grained and massive finegrained charnockitic rock types are believed to be of magmatic origin judging from their unfoliated, massive, homogeneous nature and the cross-cutting contacts. Inclusions or xenoliths of the massive fine-grained charnockitic variety are present in the porphyritic biotitehornblende granite near Ikere (Fig. 3) and also within the coarse charnockitic rock at A d o Ekiti. This relationship indicates that the fine-grained charnockitic rock pre-dates these rock types. There is also what appears to be a 'mixture' of the fine-grained charnockitic rock type and the coarse variety in some parts of the outcrop at A d o Ekiti. Some sharp contacts indicating intrusion are also present between these two rock varieties in the same locality.
3 = Granite 4 = Hypersthane grarlodiorite "f~ • Quartz hypersthene syenite 9 e • Quartz monzonofite
2
/ A
I
4
I
5
Chorn I ®thorn 3 & Chorrl 4 + Gron 1 X Gron 2
~
\
\
69
P
Fig. 2. CombinedQAP diagram for the granites and charnockiticrocks of Ado Ekiti-Akure.
GEOCHEMISTRY
Analytical methods Porphyritic biotite-hornblende granite is the most abundant (55%) while the medium- to coarse-grained biotitehornblende granite and the fine- to medium-grained biotite granite make up roughly 10%. The latter variety is, however, of very small occurrence (Fig. 1). The granites occur mainly as low-lying outcrops and inselbergs, while the charnockitic rocks are in the form of low outcrops which are boulder-like in some instances. The coarse-grained charnockitic variety is intimately associated or even 'intermixed' in some cases with the porphyritic biotite-hornblende granite and/or mediumto coarse-grained non-porphyritic granite in the field. This 'admixture' makes it difficult to draw precise boundaries between these rock groups in some cases. This
The major elements SiO2, TiO2, A1203, Fe203 (total), MnO, MgO, CaO, K20, and P205 with trace elements V, Cr, Ni, Cu, Zn, Th, Rb, Sr, Y, Zr, Nb and Ba were all determined by X R F using an automatic Philips 1212 Spectrometer at Imperial College, London. Norrish fusion pellets prepared from powder of the rock samples were used for the major elements while briquettes made from rock powder were used for the trace elements determination (see details of sample preparation and data processing in Parker 1980a,b). Wet chemical techniques were employed for determination of Na20 and FeO. Na20 was determined using an E E L Flame Photometer and FeO determined by
Table 1. Modal compositionby volume of granitic and charnockiticrocks from Ado Ekiti-Akure region
Quartz Alkali feldspar Plagioclase Hornblende Biotite Orthopyroxene Clinopyroxene Olivine (fayalite) Sphene Allanite Zircon Iron ores Apatite Total Q A P
Gran-1 (8)
Gran-2 (2)
27.5 34.4 22.7 4.8 8.8 ---0.8 0.2 0.1 0.4 0.3 100.0 32.5 40.6 26.9
25.5 40.8 19.0 7.3 6.3 ---. . 0.2 0.9 0.2 100.2 29.9 47.8 22.3
Gran-3 (2) 28.3 42.5 15.3 -11.9 ---. .
Charn-3 (2)
13.6 53.3 20.3 5.9 1.4 0.2 2.6
17.0 14.5 46.2 0.5 6.4 2.6 11.4
1.9
. . 1.5 0.3 0.1 99.9 32.9 49.4 17.7
Charn-1 (6)
. . 0.1 0.8 0.1 100.2 15.6 61.1 23.3
- -
11.9 12.2 39.8 3.7 14.8 2.8 12.9 - -
. . -1.6 -100.2 21.8 18.6 59.6
Number in brackets = number of analyses. Gran-1 = Porphyriticbiotite-hornblende granite. Gran-2 = Medium-to coarse-grainedbiotite-hornblende granite. Gran-3 = Fine- to medium-grainedbiotite granite. Charn-1 = Coarse-grainedcharnockiticrock type (quartz hypersthene syenite). Charn-3 = Gneissicfine-grained charnockiticrock type (quartz monzonorite). Charn-4 = Massivefine-grainedcharnockiticrock type (hypersthene granodiorite). ARS 6:l-E
Charn-4 (4)
-1.9 -100.0 18.7 19.0 62.3
70
V . O . OLAREWAJU Table 2. Averages of major and trace element data for the granitic and charnockitic rocks of Ado Ekiti-Akure Charn-1 (44)
Charn-3 (11)
Charn-4 (37)
Gran-1 (41)
Gran-2 (8)
Gran-3 (9)
Gneiss (20)
A
B
C
SiO2 TiO~ A1203 Fe203 FeO MnO MgO CaO Na20 K20 P205 H20 LOI Total
64.99 0.64 15.49 1.03 4.05 0.11 0.58 2.44 3.46 6.13 0.16 0.12 0.52 99.72
58.83 1.04 16.04 2.41 5.69 0.13 3.42 5.48 2.98 2.70 0.25 0.09 0.67 99.73
55.05 1.85 15.76 1.77 7.97 0.15 3.15 5.97 3.32 3.16 0.62 0.10 0.57 99.44
66.71 0.67 14.98 1.01 2.95 0.06 0.90 2.39 3.22 5.74 0.21 0.09 0.62 99.55
67.19 0.55 14.96 0.99 3.32 0.07 0.59 2.05 3.21 6.00 0.14 0.10 0.53 99.70
68.05 0.56 14.89 0.60 2.57 0.04 0.80 2.05 3.09 5.77 0.16 0.07 0.81 99.46
69.69 0.37 14.71 0.95 2.13 0.05 0.82 2.74 3.21 4.57 0.10 0.06 0.48 99.88
69.15 0.56 14.63 1.22 2.27 0.06 0.99 2.45 3.35 4.58 0.20 --99.46
68.16 0.46 14.37 1.95 2.07 0.04 0.50 1.47 1.83 6.89 0.11 0.23 1.20 99.58
62.51 1.13 15.07 2.18 3.70 0.13 1.80 3.54 3.87 4.57 0.44 0.42 -99.36
V Cr Ni Cu Zn Th Rb Sr Y Zr Nb Ba
24 11 9 14 88 19 138 182 47 742 38 1136
128 99 32 20 97 12 78 383 46 207 16 978
138 33 31 29 121 8 93 477 53 390 31 1421
37 9 7 10 64 43 218 287 22 404 19 1418
30 20 8 15 49 25 138 297 33 219 11 952
Trace elements(ppm) 41 27 13 14 9 9 13 12 70 81 24 27 176 167 313 182 44 46 385 563 26 34 1352 1073
Number in brackets = number of analyses. Charn-1 = Coarse-grained charnockitic rock (quartz hypersthene syenite). Charn-3 = Gneissic fine-grained charnockitic rock (quartz monzonorite). Charn-4 = Massive fine-grained charnockitic rock (hypersthene granodiorite). Gran-1 = Porphyritic biotite-hornblende granite. Gran-2 = Medium- to coarse-grained biotite-hornblende granite. Gran-3 = Fine-grained biotite granite. Gneiss = Granitic gneiss. A = Average of 121 adamellites (Nockolds 1954). B = Normal rapakivi granite (Kranck 1970). C = Average of 2 coarse-grained charnockites from Varberg region, southwest Sweden (Hubbard and Whitley 1979).
titration. Decomposition of the sample powder with HF/metavanadate solution was followed by titration with K2Cr207 for FeO determination. Analytical results
One hundred and seventy (170) representative samples of the different rock types including the granitic gneisses in the region were analyzed. The major and trace element data, averages only, are presented in Table 2, full data being available in Olarewaju (1981). The rare-earth element abundances are reported in Table 3. In general, geochemical studies of the charnockitic and granitic rocks of the area show trends comparable to those of calc-alkaline plutonic suites (Nockolds and Allen 1953, Saunders et al. 1980) with SiO2, K20, Rb and Th increasing with DI while TiO2, Fe203 (total), MgO, CaO, P205, V, Ni, Zn, Y and Sr decrease with Differentiation Index (DI) (Olarewaju 1981, 1986). Major element geochemistry reveals the predominantly adamellitic composition of the granites and coarse-grained charnockitic rocks, while the fine-
grained charnockitic variety is mainly of granodioritic composition (Figs. 4 and 5, Table 2). The high K20 levels relative to SiO2 (Fig. 6), high K20/Na20 ratios (Fig. 7), and high FeOT*/MgO (Fig. 8) are the salient features of the granites and coarse charnockitic rock that parallel those chemical compositions which characterize the rapakivi granites (Emslie 1973, Wilson 1980). Trace element geochemistry gives broad division of the charnockitic rocks into 'normal'-LIL (large-ionlithophile element) and LIL-deficient rocks, The 'normal'-LIL is represented by the coarse charnockitic rock (with K20 > 4.4% and 138 ppm Rb, 182 ppm Sr) while the LIL-deficient is exemplified by the fine-grained variety with lower K20 and Rb values but higher Sr contents (Table 2). These divisions are similar to those used by Field et al. (1980) for the charnockitic gneisses of south Norway. The charnockitic rocks generally have lower absolute REE abundances than the granites. The values range from 361 ppm in the massive fine-grained charnockitic
* FeOT is the total iron.
Chamockite-granite association in SW Nigeria
Fig. 3. Xeno!iths of massive fine-grained charnockitic rock type (hypersthene granodiorite) in porphyritic biotitehornblende granite near Ikere.
71
Charnockite-granite association in SW Nigeria CHARN 1 = C o a r s e - g r a l n e d charnockitic rock CHARN 3 = Gneissic fine-grained charnockitic rock CHARN 4 = Massive fine-grained charnGRAN 1 GRAN 2
73
CoO CHRRN 1 CHRRN 3 CHRRN 4 GRRN I GRRN 2 GRRN 3 GNEISS
ockitic rOCkbiotite_ / = Porphyritic hornblende granite ~/ , = Medium-to c o a r s e - g r a i n e ~ grantebi°tltehornblende /
GRAN 3
=
GNEISS
= Granitic
graniteFine-grainedbiotit 7 gneiss
'(9 + X
O ,p
q
G= Granite
Field
GO=Granodiorite Om=Quat I z monzonite
To=Tonalite To +
NazO
I-0
0-5 K20 /
~Xx K2 0
1.5
Nat0
Fig. 4. CaO-Na20-K20 diagram (after Condie and Hunter 1976) showing distribution of rocks in region of study.
variety to 474 ppm in the coarse type and 504 ppm in the porphyritic biotite-hornblende granite (Table 3). The degree of fractionation is not as highly pronounced as in the granites as the charnockitic rocks all display only mildly fractionated REE patterns (Figs. 9, 10 and 11). In general, there is light rare-earth element (LREE) enrichment in the rocks, negative europium anomalies especially in the granites and varying degrees of heavy rare-earth element (HREE) depletion. An
/
t.
DISCUSSION A petrogenetic link between the charnockitic and granitic rocks of Ado Ekiti-Akure is indicated by both field and geochemical evidence. Prominent in the evidence are: (i) the striking 'admixture' of these two groups of rocks in the field; (ii) fractionated REE patterns with LREE enrichment and also some negative Eu anomalies indicating igneous origin (Figs. 9, 10 and 11);
16
(9 Charn 1 ~ Charn 3
~t~
\
12 ¢
IQ.
~ga o 2
t,r"~- = ' ~
4 / -
To = Tonalife
eOA I
• .+" J ---"
G d - Eranodiorit"e Om
.
Tj-
-
0
=
Ouarfz monzonite Trondhjemite Granite
Fig. 5. A b - A n - O r (normative averages) diagram for the rocks studied. Fields after O'Connor (1965).
45
53
61 SIO I w e l g h t
69
77
per cent
Fig. 6. K20 vs SiO 2 diagram. The dashed lines show the field of calc-alkaline plutons of the Antarctic Peninsula (Saunders e t al. 1980). Symbols as in Fig. 4.
74
V . O . OLAREWAJU 10 in
c: Q
8 +
~
~
~
o
~
o. .
~a
+
t~
G =Granite
Field
o
if,
AD = A d a m e l l i t e
GDmtG ranodior ire
TO ==Tonaiite
-0
1.0
2..0
3 .O No=O
4.0
Weight
per
5.0
cent
Fig. 7. K20-Na20 relationships of the rocks in Ado Ekiti-Akure. Boundaries drawn after Viljoen and Viljoen (1969). Symbols as in Fig. 4.
(iii) a possible common parental liquid suggested by the distribution of the immobile transition elements (Olarewaju 1981); (iv) similar Pan-African ages of 631 + 18 Ma given by the rocks (Tubosun et al. 1984) suggesting synchronous emplacement. A similar link between non-charnockitic granitic and charnockitic rocks has been reported elsewhere by, for example, Kranck (1970) on the rapakivi plutons of Scandinavia, Hubbard and Whitley (1978, 1979) who established a link between Torpa granites of rapakivi type and their charnockitic source in southwest Sweden, and Field et al. (1980) on the charnockitic gneisses of south Norway. REE have been particularly useful in establishing the
fact that the rocks in the Ado Ekiti-Akure region have striking similarities with the rapakivi granites of Finland (Koljonen and Rosenberg 1974) and the granitic--charnockitic rocks of Hubbard and Whitley (1979, Figs. 9, 10 and 11, Table 3). Presumably, comparable petrogenetic processes have been in operation.
600.
A = Pot'phyriti¢ granite B = Medium-to coarse-grained granite
D= Torpo granite [Hubbard =, Wh trey 19?9 )
C = Fine grained granite
E = Rapakivi granite(Koljonen lit Rosenberg, IgT& ) F = Portugat granite ( de Albuquerque, 1978)
C .
~
%
G - NASC (Haskin et o1.,lC368) NASC " North Amerioan Shale
Composite
B F
100-
Fog(total)
~s0
~
,,~3
/ ~ _. ~ / / C A L C
-- A L K A L, N E
/~,+/+. A-:*¢,.F i~020 + K 2 0 /
v
w
v
v
v
w
10 --
~
\
v
\ v
\MgO
Fig. 8. AFM (total alkalies vs F o g t vs MgO) diagram showing the major chemical variations in the rocks of Ado Ekiti-Akure. The dashed line (A) is the divide between the tholeiitic (above) and calc-alkaline (below) trends as suggested by Irvine and Baragar (1971) with trends from the Peruvian Batholith B (Pitcher 1978) and the Lower California Batholith C (Carmichael et al. 1974). Symbols as in Fig. 4.
0
11oC~z
l~d
S'm ~u dd l'b
H'o
Tm '~b flu
Fig. 9. Comparison of chondrite-normalized REE distribution patterns of the granitic rocks of the Ado Ekiti-Akure r e # o n with published values.
Charnockite-granite association in SW Nigeria A = Massive fine-groined chartlockitic r o c k B = Gneissic fine--groined chornockitic rock C = Fine-groined charnockite, Sweden ( Hubbard & Whitley, lgTg)
A = Coarse--groined
D = Charnockitic granite gneiss, Sweden ( H u b b a r d 8. Whitley, lg79). E= Continental basalt,average of 52 ( Helmke & Hoskin, 1973 I F = NASC ( Haskin etal, 1968).
¢harnockitl¢
rock
B = Coarse charnockite,southwest Selden ( Hubbard IkWhitley, 11179) C= M a s s i v e c h a r n o c k i t e , Brazil (Gosparini 1~ Mantovan b 1979) D=NASC(Hclskin el a1,19681
300300-
A 200- C
75
A
100- F 100
u.J
E
°zS( 0 -r 0
~5C c_
v (J 0 {E
Fig. 10. Chondrite-normalized R E E comparison distribution patterns for the fine-grained charnockitic rocks.
Fig. 11. R E E comparison diagram for the coarse-grained charnockitic rock.
Table 3. Rare-earth element data (ppm) for the rocks from Ado Ekiti-Akure, SW Nigeria
La Ce Nd Sm Eu Tb Ha Yb Lu ~REE Y ~REE + Y ~LREE HREE ~ LREEPZ H R E E La/Yb +LaN/SmN +CeN/YbN Eu/Eu*
A
B
C
D
E
F
G
H
I
J
129.59 264.66 87.26 13.88 1.90 1.52 1.29 3.24 0.38 503.72 58.40 562.12 495.40 6.43 77.05 40.00 5.12 19.52 0.46
75.41 150.73 56.00 10.33 1.71 1.18 1.56 2.83 0.35 300.10 38.00 338.10 292.47 5.92 49.40 26.65 4.01 12.72 0.58
200.43 435.60 126.17 15.70 1.84 1.09 0.50 1.81 0.18 783.32 27.00 810.32 777.90 3.58 217.29 110.73 9.54 54.94 0.58
100.75 258.13 91.93 13.25 3.07 1.62 0.97 4.11 0.47 474.30 49.00 523.30 464.06 7.17 64.72 24.51 4.17 14.97 0.78
79.02 179.98 77.57 14.74 3.29 1.61 1.39 3.46 0.42 361.48 53.25 414.73 351.31 6.88 51.06 22.84 2.94 12.41 0.78
50.45 99.22 38.71 7.78 1.76 1.13 1.36 4.36 0.53 205.30 46.67 251.97 196.16 7.38 26.60 11.57 3.56 5.57 0.73
69.51 138.80 42.10 6.25 1.01 0.67 1.22 1.83 0.21 261.60 22.00 283.60 256.66 3.93 65.31 37.98 6.10 18.15 0.56
146.00 269.00 78.00 16.00 0.82 1.60 -6.30 1.33 519.05 ----------
152.00 334.00 106.00 17.00 2.20 1.90 -5.70 0.90 619.70 ----------
77.20 155.00 70.30 18.40 3.00 1.90 -6.50 1.00 333.30 ----------
+ = Chondrite-normalized La/Sm and Ce/Yb ratios. Eu/Eu* = Chondrite-normalized Eu value divided by the value obtained by drawing a straight line between Sm and Tb. G r a n - l - - A = Average of 10 coarse porphyritic biotite-hornblende granites. Gran-2--B = Medium- to coarse-grained biotite-hornblende granite. G r a n - 3 - - C = Average of 2 fine-grained biotite granites. C h a r n - l - - D = Averge of 7 coarse-grained charnockitic rocks (quartz hypersthene syenite). C h a r n - 4 - - E = Average of 4 fine-grained charnockitic rocks (hypersthene granodiorite). C h a r n - 3 - - F = Average of 3 gneissic fine-grained charnockitic rocks (quartz monzonorite). G n e i s s - - G = Granitic gneiss. H = Rapakivi granite, Kalanti Finland (Koljonen and Rosenberg 1974). I = Torpa granite, southwest Sweden (Hubbard and Whitley 1978). J = Average of 9 charnockitic phases of charnokite-granite association, southwest Sweden (Hubbard and Whitley 1978).
76
V . O . OLAREWAJU
It is significant to note that although the distinctive mantled megacryst feldspar rapakivi texture s e n s u s t r i c t o is not observed in the granites, m a j o r element chemistry is similar to that in rocks recognized as belonging to the rapakivi suite. For example, in the rapakivi suite, the SiO 2 range is 64-72%, K 2 0 is high ( > 6 % ) and M g O always very low ( < 1 % ) (Kranck 1970). The corresponding figures for a rapakivi granite from Satakunta, Finland (Koljonen and Rosenberg 1974) are: SiO 2 = 68.16%, K 2 0 - - 6 . 8 9 % and M g O = 0.50% and for T o r p a granite ( H u b b a r d and Whitley 1979); SiO 2 = 70.7%, K 2 0 = 5.58% and M g O = 0.52%. Ranges of these elements in the granites of A d o E k i t i - A k u r e are: SiO 2 = 66.71-68.05%, K 2 0 = 5.74--6.00% and MgO = 0.59-0.90%. The coarse-grained charnockitic rock in the region gives the average values SiO 2 = 64.99%, K 2 0 = 6.13% and MgO -- 0.58%, while the associated massive finegrained intermediate variety gives average values of SiO2 = 55.05%, K 2 0 = 3.16% and M g O - - 3.15% (Table 2). It is thus seen that chemically, the granites studied are similar to the rapakivi granite types. It has been shown earlier that the coarse-grained variety of the charnockitic rocks is enriched in L I L elements while the fine-grained variety is depleted in t h e m (Table 2). The coarse charnockitic rock and the massive fine-grained type therefore represent the ' n o r m a l ' - L I L and LILdeficient rocks. F r o m the foregoing discussion, it is seen that on the basis of the chemistry, the granites in the A d o E k i t i A k u r e region could be likened to rapakivi granite which has been suggested to be a product of charnockitic plutonism which in some cases could be triggered by anorthosite e m p l a c e m e n t ( H u b b a r d and Whitley 1978, 1979). In the Varberg region of southwest Sweden, no anorthosite is found but only inferred since it may exist at unexposed levels ( H u b b a r d and Whitley 1979). Likewise, no anorthosite is exposed in the A d o E k i t i A k u r e region but the occurrence of LIL-deficient charnockitic rock type in this area seems crucial in the link between this rock on one hand and the ' n o r m a l ' - L I L charnockitic rock together with the granites which might represent equivalents of rapakivi granites at higher structural levels on the other hand. The LIL-deficient charnockitic variety with its low K, Rb and high Sr levels (Table 2) indicates derivation from a source depleted in alkalis and enriched in Sr. These features correspond to those for granulites (Tarney e t al. 1972, Tarney 1976) and they would suggest a source region of deep crustal levels or u p p e r mantle. The composition of the L I L deficient type is therefore closer to that of the original m a g m a than the composition of the other rocks in the area. This parental m a g m a is believed to be andesiticdacitic in composition (Olarewaju 1981). This model does not require the involvement of an unseen anorthosite in the charnockite-granite association as H u b b a r d and Whitley (1978, 1979) suggested. Field et al. (1980) suggested a similar model to this one.
CONCLUSIONS Field and geochemical evidence show that the charnockite-granite association of A d o E k i t i - A k u r e in SW Nigeria forms a cogenetic sequence likened to rapakivi granite-anorthosite-charnockite group encountered in some other parts of the world. No anorthosite is exposed in the area, but the LIL-deficient charnockitic rock type forms an important link between itself on one hand, and the ' n o r m a l ' - L I L charnockitic rock together with granites on the other hand. Both the L I L and R E E patterns are suggestive of a fractionation process which involves LIL-deficient phases emplaced under high grade conditions and the ' n o r m a l ' - L I L rock type with the granites representing equivalents of rapakivi granites that possibly crystallized from residual melts at higher structural levels. Acknowledgements--Thanks are due to Dr N. W. Rogers for the
instrumental neutron activation analysis (INAA) at the University of London Reactor Centre. Discussions with Dr M. A. Rahaman and critical reviews by Mr O. Ocan greatly improved the content of the manuscript.
REFERENCES Albuquerque, C. A. R. de 1978. Rare earth elements in 'Younger' granites, Northern Portugal. Lithos 11,219-229. Bridgwater, D. and Waterson, J. S. 1967. Igneous intrusion and associated rocks of the mangerite-charnockite suite. Nature 213, 897. Carmichael, I. S. E., Turner, F. J. and Verhoogen, J. 1974. Igneous Petrology, p. 568. McGraw-Hill, New York. Condie, K. C. and Hunter, D. R. 1976. Trace element geochemistryof Archean granitic rocks from the Barberton region, South Africa. Earth Planet. Sci. Lett. 29,389-400. Cooray, P. G. 1972. Note on the charnockites of the Akure-Ado Ekiti area, Western Nigeria. In: African Geology (Edited by Dessauvagie, T. F. J. and Whiteman, A. J.), pp. 45-63. Ibadan University Press, Ibadan. Cooray, P. G. 1975. The charnockitic rocks of Nigeria. Pichamuthu volume, pp. 50-73. Bangalore University, India. Emslie, R. F. 1973. Some chemical characteristics of Anorthositic Suites and their significance. Can. J. Earth Sci. 10, 54-71. Field, D., Drnry, S. A. and Cooper, D. C. 1980. Rare-earth and LIL element fractionation in high grade charnockitic gneisses, South Norway. Lithos 13,281-289. Gasparini, P. and Mantovani, M. S. M. 1979. Geochemistry of charnockites from Sao Paulo State, Brazil. Earth Planet. Sci. Lett. 42, 311-320. Haskin, L. A., Haskin, M. A., Frey, F. A. and Wildeman, T. R. 1968. Relative and absolute terrestrial abundances of the rare earths. In: Origin and Distribution o f the Elements (Edited by Ahrens, L. H.), Vol. 30, pp. 889-912. Pergamon Press, Oxford. Helmke, P. A. and Haskin, L. A. 1973. Rare-earth elements Co, Sc and HF in the Steens Mountain basalts. Geochem. Cosmochim. Acta 37, 1513-1529. Hubbard, F. H. 1968. The association Charnockite-Old Granite in SW Nigeria. Jl Mining Geol. (Nig. ) 3, 25-32. Hubbard, F. H. and Whitley, J. E. 1979. REE in charnockite and associated rocks, southwest Sweden. Lithos 12, 1-11. Irvine, T. N. and Baragar, W. R. A. 1971. A guide to the chemical classification of the common volcanic rocks. Can. J. Earth Sci. 8, 523-548. Koljonen, T. and Rosenberg, R. J. 1974. Rare-earth elements in granitic rocks. Lithos 7,249-261. Kranck, E. H. 1970. Anorthosites and rapakivi, magmas from the lower crust. New York State Mus. Sci. Serv. Mere. 18, 93-97. Nockoids, S. R. 1954. Average chemicalcompositionsof some igneous rocks. Geol. Soc. Am. Bull. 65, 1007-1032.
Charnockite-granite association in SW Nigeria Nockolds, S. R. and Allen, R. 1953. The geochemistry of some igneous rock series. Geochim. Cosmochim. Acta 4, 105-142. O'Connor, J. T. 1965. A classification for quartz-rich igneous rocks based on feldspar ratios. U.S. Geol. Surv. ProL Pap. 5258, pp. 79-84. Olarewaju, V. O. 1981. Geochemistry of the charnockitic and granitic rocks of the basement complex around Ado Ekiti-Akure, S.W. Nigeria. Ph.D. thesis, Univ. Lond. Olarewaju, V. O. 1986. Petrology and geochemistry of charnockitic and associated granitic rocks of Ado Ekiti-Akure, S.W. Nigeria. Precambr. Nigeria 1. Parker, R. J. 1980a. Preparation of silicate rock samples for XRF analysis. Tech. Report No. XRF-3. Dept. Geol., Imperial College, Univ. of London. Parker, R. J. 1980b. Computer processing and evaluation of XRF analytical data for silicate rock samples. Tech. Report No. XRF-4, Dept. Geol., Imperial College, Univ. of London. Pitcher, W. S. 1978. The anatomy of a batholith. J. Geol. Soc. Lond. 135, 157-182.
77
Saunders, A. D., Tamey, J. and Weaver, S. D. 1980. Transverse geochemical variations across the Antarctic Peninsula: implications for the genesis of calc-alkaline magmas. Earth Planet. Sci. Lett. 46, 344-360. Tarney, J. 1976. Geochemistry of Archean high-grade gneisses, with implications as to the origin and evolution of the Precambrian crust. In: The Early History of the Earth (Edited by Windley, B. F.), pp. 405-424. Wiley, London. Tarney, J., Skinner, A. C. and Sheraton, J. W. 1972. A geochemical comparison of major Archean gneiss units from N.W. Scotland and East Greenland. Proc. 24th Int. Geol. Congr., Montreal, Vol. 1, pp. 162-174. Tubosun, I. A., Lancelot, J. R., Rahaman, M. A. and Ocan, O. 1984. U/Pb Pan-African ages of two charnockite-granite associations from south-western Nigeria. Contr. Miner. Petrol. 88, 188-195. Viljoen, M, J. and Viljoen, R. P. 1969. The geochemical evolution of the granitic rocks of the Barberton region. Geol. Soc. S. Afr. Sp. Publ. 2, 189-219. Wilson, M. R. 1980. Granite types in Sweden. Geologiska Foreningens i Stockholm Forhandlingar 102,167-176.