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RbSr age of Pan-African migmatites in the Oban massif of southeastern Nigeria
Journal of African Earth Sciences, Vol. 15, No. 1, pp. 65-72, 1992. Printed in Greet Britain
0899-5362/92 $5.00 + 0.00 © 1992 Pergamon Press Ltd
Rb-Sr age of Pan-African migmatites in the Oban Massif of southeastern Nigeria BARTH.N. EKWUEME Department of Geology, University of Calabar, P. O. Box 3651, Calabar, Nigeria (First received 15th November, 1989; revised form received 24th February, 1992)
Abstract - Migmatites consisting mainlyof lit-par-lit gneisses form one of the most dominant metamorpho-
sed rocks in the Oban massifof southeastern Nigeria. Banded schists and agmatites are the other mixed rocks recognized in the area. The lit-par-hi gneisses are composed of quartz, feldspar 0eucocratic portion) and biotite, chlorite, garnet (melanocratic portion). They are extensively sheared, fractured and folded. The dominant fold axes trend N-S parallel to the regional foliationof the rocks. Chemical data show that the litpar-lit gneiss is granodioritic in composition. The agmatite is restricted in occurrence. Banding is rare in the agmatite but dissectionof the paleosome by quartzo-feldspathic veins is common. The agmatite is quartz-dioritic in composition. The banded schist is coarse-grained and contains index minerals: garnet, kyanite and sillimanite, indicating that the area underwent high-grade metamorphism of the Barrovian type. It is politic in composition. These migmatitic rocks were formed by partial melting, metamorphic segregation and injection due to high-grade metamorphism. The migmatitic gneiss yielded a Rb-Sr isochron age of 510-~_11Ma whilst the migmatitic schists gave an age of 527+16 Ma.
(1982), E k w u e m e a n d O n y e a g o c h a (1986) a n d H o c k e y et al. (1986). R a h a m a n et aL (1983), Oshi (1984), O n y e a g o c h a (1986), E k w e r e a n d E k w u e m e ( 1991) a n d I m e o k p a r i a a n d E m o f u r i e t a (1991) are however, of t h e view t h a t s o m e of t h e g n e i s s e s are of i g n e o u s parentage. In spite of t h e divergent views o n t h e progenitors of t h e b a n d e d g n e i s s e s , m o s t of t h e w o r k e r s believe t h a t t h e y were overprinted b y t h e h i g h e s t g r a d e of t h e amphibolite facies regional m e t a m o r p h i s m (Oyawoye, 1972; O n y e a g o c h a , 1984). T h e y agree t h a t partial melting, m e t a m o r p h i c segregation a n d injection w h i c h a c c o m p a n y s u c h high-grade m e t a m o r p h i s m formed t h e b a n d e d g n e i s s e s a n d t h e agmatites. The age of m i g m a t i s a t i o n , however, r e m a i n s u n k n o w n for t h e Nigerian b a s e m e n t . In this paper, t h e a u t h o r e x a m i n e s t h e c h a r a c t e ristic f e a t u r e s o f m i g m a t i t i c g n e i s s e s a n d s c h i s t s in t h e O b a n m a s s i f a n d r e p o r t s n e w ages w h i c h p o s s i b l y date m i g m a t i s a t i o n in t h e area.
INTRODUCTION
A b o u t fifty p e r c e n t of Nigeria is covered b y P r e c a m b r i a n b a s e m e n t rocks. Of these, "mixed" r o c k s o t h e r w i s e k n o w n a s m i g m a t i t e s are t h e m o s t c o m m o n . The m i x e d r o c k s include b a n d e d gneiss, b a n d e d schist a n d agmatite. Oyawoye (1972) n o t e d t h a t t h e b a n d e d g n e i s s e s in the Nigerian b a s e m e n t are m i g m a t i t e s a n d are p o s s i b l y t h e oldest r o c k s in t h e c o u n t r y . He p o i n t e d o u t t h a t t h e s e b a n d e d g n e i s s e s o n geological b a s i s are older t h a n t h e granite g n e i s s w h i c h yielded a w h o l e - r o c k R b - S r i s o c h r o n age of 2 1 9 0 +30 Ma (Grant, 1971). Possibly b e c a u s e of their m i x e d a n d therefore, c o m p l e x n a t u r e , it h a s b e e n difficult to a s c e r t a i n t h e a c t u a l p r o g e n i t o r s of the b a n d e d gneisses. Freeth (1971) a n d B u r k e et aL (1972) s h o w e d t h a t t h e b a n d e d g n e i s s e s in I b a d a n SW Nigeria were derived from g r e y w a c k e a n d a g r e y w a c k e - s h a l e s e q u e n c e . O n t h e o t h e r h a n d , R a h a m a n (1978) p o i n t e d o u t t h a t g e o c h e m i c a l evidence s h o w s t h a t t h e s a m e b a n d e d g n e i s s e s are similar in composition to g r e y w a c k e s , s h a l e s a n d r o c k s of t h e dioritem o n z o n i t e range. He, n o n e t h e l e s s , f a v o u r e d a s e d i m e n t a r y origin for t h e b a n d e d g n e i s s e s b a s e d on t h e i n t e r b a n d i n g of t h e g n e i s s e s with quartzites a n d l e n s e s of calc-silicate rocks. O t h e r w o r k e r s w h o h a v e a r g u e d for a s e d i m e n t a r y origin for t h e g n e i s s e s in t h e Nigerian b a s e m e n t i n c l u d e Elueze
GEOLOGICAL S E T T I N G
E k w u e m e a n d O n y e a g o c h a (1985, 1986) a n d Ekwere a n d E k w u e m e (1991) have d i s c u s s e d t h e occurrence, p e t r o g r a p h y a n d g e o c h e m i s t r y o f r o c k s in the O b a n M a s s i f including migmatites. A brief s u m m a r y is given here. Migmatites in t h e O b a n m a s s i f are a s s o c i a t e d
65
66
B. N. EKWUEME
with pegmatitic granites, pegmatitic boulders and plane foliation trending N-S. In most cases, the pegmatitic veins. These pegrnatites which form the b a n d s are parallel b u t occasionally they are conneosome dissect the m e t a m o r p h i c host (paleo- torted and complexedly folded. The folds are genesome)• Banding is not c o m m o n in the agmatite- rally tight to isoclinal b u t chevron, r e c u m b e n t and type of migmatites in Oban massif a n d where ptygmatic folds also occur. Boudinage a n d pinchpresent, it is not conspicuous. The agmatite is best and-swell s t r u c t u r e s are c o m m o n on the limbs of exposed at Mbarakpa where the rock is highly the folds confirming that there was a drag on these foliated in the N-S to N4°E direction. It is often limbs. The b a n d in the gneiss cleaves along smooth Jointed and major fractures in the rock trend 20 ° chloritic surfaces, which are interpreted as s h e a r whilst minor ones t r e n d northwest. Ptygmatic folds planes. This is similar to the s h e a r reported in migmatitic gneisses (banded) of the Lokoja-Auchi are c o m m o n in the agmatite. The best exposures of lit-par-lit gneiss in the area (Hockey et al., 1986) where the rocks appear Oban massif occur at the now-abandoned Nigerian to have been sheared along sub-parallel planes Mining Corporation Q u a r r y in Oban. The rock is and the s h e a r planes according to Hockey et al. coarse-grained a n d consists of b a n d s ranging from (1986) afforded easy p a t h s of ingress to the migma1 c m to over 3 c m thick. The leucocratic b a n d is tisation. c o m p o s e d d o m i n a n t l y of quartz and feldspar Coarse-grained schists in the Oban massif are whereas the melanocratic portion contains phyllo- b a n d e d and constitute the migrnatitic schists in sflicates, (chlorite a n d biotite). The latter m a y also the area. They are restricted to the Kwa Falls and contain garnet. The minerals forming the d a r k Abbiatl areas (Fig. 1). Minerals in these schists portion of the b a n d e d gneiss show strong axial include garnet, kyanite and sflllmanite (Table 1) 5o48'~
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sement Complex Oban Massif Sediments.
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EXPLANATIONS S
Sedimentary Rocks
MgG
Medium grade
Pegmatite
MG GO
Migmatitic Gneiss Granite Gneiss
•B A
Banded Amphibolite
P PG
Pegmatitic
e
Granite
GD
Gronodiorite
LgS MS
Low grade Schist
Gr oeite
Migmatite Schist
A
A -I {.2 e
Gneiss
Amphibolite
Kwa falls Location of analysed sample
Towns/Villages
Fig. 1. Simplified geological map of the Oban massif showing locations of analyzed samples.
Rb-Sr age of Pan-African migmatites in the Oban Massif of southeastern Nigeria Table 1,. Average modal compositions of mlgmatltlcgnelsses and migmaUtlc schists in the Oban massif Quartz Plagioklase K-feldspar Biotite Muscovite Garnet Kyanite Sfllimanite Tourmaline Chlorite Epidote Hornblende Sphene Myrmekite Sericite Calcite Opaques Average of:
1 44.0 14.0 13.0 25.0
2 30.0 12.0 21.0 22.0 I 0.0 2.0
3 20.0 15.0 15.0 24.0
x 3.0
4 17.0 5.0 2.0 32.0 4.0 9.0 6.0 7.0 -
x (I0)
18.0 x (20)
15.0 x 10.0 1.0
3.0
1.0
(6)
(12)
I. = Banded gneiss at Oban 2. = Agmatite at Mbarakpa 3. = Garnet Sllllmanlte Schist (mlgmatitic) at Abblatl. 4 = Kyanite Sillirnanite Schist(migmatitic) at Kwa Falls x = Trace a n d t h e b a n d i n g of t h e s c h i s t s is a t t r i b u t e d to highg r a d e m e t a m o r p h i s m . C hl or i t e a n d s e c o n d a r y calcite h o w e v e r o c c u r in s o m e s c h i s t o u t c r o p s in Kwa Falls p o i n t i n g to local r e t r o g r e s s i o n . T h e m i g m a t i t e s s h o w c o n s i d e r a b l e v a r i a t i o n in their average major and trace element composit i o n s (Table 2). T h e migmaUtic g n e i s s e s have h i g h e r SiO 2, Na20 a n d K20 b u t lower Al203, Fe203 (Total) a n d MgO t h a n t h e m i g m a t i t i c schists. Fe203 (Total) is p a r t i c u l a r l y high (10.40%) in t h e m i g m a t i t i c s c h i s t at Abbiati a n d s u g g e s t s t h a t t h e p a r e n t r o c k
67
m a y be p y r i t e - b e a r i n g . T h i s s c h i s t h a s also t h e h i g h e s t AI203 a n d T102 c o n t e n t a m o n g all t h e a n a l y s e d rocks. T h i s is a r e f l e c t i o n of t h e m o d a l c o m p o s i t i o n (Table 1) w h i c h s h o w s t h a t It ls enr i c h e d in K-feldspar, biotite, g a r n e t a n d sflllmanite relative to o t h e r r o c k s u n d e r s t u d y . (Table 1). T h e b a n d e d g n e i s s is d e p l e t e d in Rb w h e n c o m p a r e d with o t h e r a n a l y s e d r o c k s (Table 3). Cr a n d NI a re m o r e a b u n d a n t in t h e m i g m a t i t l c s c h i s t s t h a n in t h e g n e i s s e s . T h e l a t t e r is r i c h e r In La, Y, Nb a n d B a t h a n t h e former. C o n s e q u e n t l y , t h e m i g m a t l t l c gneisses have higher K / R b , K / B a a n d B a / R b ratios. T h e g e o c h e m i c a l c h a r a c t e r i s t i c s of t h e m l g m a t l tic g n e l s s e s are si m i l ar to t h o s e of o r t h o g n e i s s whereas the migmatltlc schists have chemical f e a t u r e s o f a m e t a s e d i m e n t (cf. G a r r e l s a n d Mackenzie, 1971). T h e a v e r a g e c o m p o s i t i o n of t h e b a n d e d g n e i s s is slrnflar to t h a t of g r a n o d l o r i t e w h e r e a s t h a t of a g m a t i t e is close to t h a t of q u a r t z diorite of Condi e (1967). T h e c o m p o s i t i o n of t h e m i g m a t l t i c s c h i s t Is si m i l ar to t h a t of a n a v e ra g e hi gh g r a d e pelitic r o c k given b y S h a w (1956) (Table 2). Rb-Sr STUDIES
S a m p l e localities are s h o w n in Fig. 1. Ca re w a s t a k e n to collect large (5-10 kg) b l o c k s of t h e fre sh est available m a t e r i a l s to r e p r e s e n t t r u e wholerock systems. The agmatite, b a n d e d gneiss and b a n d e d schist were analysed. E a c h s a m p l e w a s c r u s h e d to p a s s t h r o u g h 80 m e s h a n d a q u a r t e r e d p a r t w a s analyzed. T h e 87Sr/ 86Sr r a t i o s w ere n o r m a l i z e d to a v a l u e of 0 . 1 1 9 4 a f t e r w h i c h AEI-MS2S a n d V G - M a s s 5 4 E s p e c t r o -
Table 2. Average major element composition of migmatites in the Oban massif. I SiO 2
2
67.52
TiO2 0.50 A1203 14.32 Fe203 4.38 MnO 0.40 MgO 1.28 CaO 2.19 Na20 3.18 I~O 4.61 P2Os 0.20 LOI 1.42 Total I00.00 Average of 25 * = Total Fe as 1 = Mlgmatitlc 2 = Migmatitic 3 = Mlgmatitic 4 = Mlgmatitic
64.91
0.65 15.73 4.12 0.06 2.08 3.67 3.68 3.65 0.29 1.16 100.00
3
4
58.70 I. 11 17.30 10.40 0.20 4.29 1.90 2.21 3.28 0.15 1.29 100.83 10
Fe20 a Gneiss (Banded Gneiss) Gneiss (Agmatite} Schist (Abbiati) Schist (Kwa Falls)
5 6 7 8
60.45 0.46 13.05 3.90 0.20 5.05 5.40 0.68 3.90 0.38 5.46 98.93 12
5
66.09 0.54 15.73 4.11 0.08 1.74 3.83 3.75 2.73 0.18
6
65.50 0.6 15.65 4.43 0.06 1.86 4.10 3.84 3.01 0.23
20
7
8
63.51 0.79 17.35 6.71 2.31 1.24 1.96 3.35 -
-
= Granodiorite (Cox et aL, 1979) = Homblende-biotite Granodiorite (Nockolds, 1954) = High grade pelitic rock (Shaw, 1956) = Tonalite (Cox et al., 1979)
6 I. 52
0.73 16.48 5.65 0.00 2.80 5.42 3.63 2.09 0.25
B. N. EKWUEME
68
m e t e r s were u s e d to analyze for Rb and Sr concen- gneiss n e a r Uwet (Ekwueme, 1987). The 676 +26 trations respectively. Age calculaUons were m a d e Ma age dates a maj or tectonothermal event in Uwet following Williams (1968) p r o g r a m m e s and using and the mlgmatisation of the schists appears to kSTRb = 1.42 x 10~a -~. Errors are 2 % for 87Rb/SSSr have occurred after t h a t event. a n d 0 . 0 1 % for STSr/SSSr and the uncertainties of DISCUSSION results are given at the 28 level.
Migmatitlc G a e l u e s a) A g m a t l t e - The data for the agmatite are plotted in the S~Rb/~Sr v e r s u s STRb/~Sr diagram (Fig. 2). Four data points yield a regression line giving a n age of 510 + 11 Ma and a n intercept of 0.71121 + 0.00025. MSWD is 0.6. Three other samples of the agmatite plot above the isochron and are s o m e w h a t scattered. A conventional age of 946 + 26 Ma was calculated for samples 6 and 7 using the classical 87Sr/~Sr initial raUo of 0.712. b) B a n d e d Gneiss - Eight samples of b a n d e d gneiss from Oban Q u a r r y (Fig. 1) were analysed. Each sample was separated into leucocratic and melanocratic parts and t h e n analysed. The aim is to establish the age of the restite and the granitic portions. It was also to avoid obtaining "mixed" ages with the a t t e n d a n t difficulties in their interpretations for the b a n d e d gneiss. As shown in Fig. 3, no reliable isochron can be drawn. The melanocratic (OBM-) and leucocratic (OBL-) portions cluster at different sections of the diagram. A useful isochron cannot be drawn in both cases b e c a u s e the points are scattered. It is also not possible to draw an isochron using the points defined by both the melanocratic and leucocratic portions of the b a n d e d gneiss.
Migmatltlc Schists Five samples of the coarse-grained schist at the Kwa Falls produced a regression line which yielded an age of 527 +16 Ma (Fig. 4). One of the samples (14) without retrograde chlorite gave a conventional age dates of 627 +24 Ma which is similar to the age of 676 +26 Ma obtained from m e d i u m grade ['0~ 6*
w .900
-
-e00
" t
'"'
•'TO 0
5
j
,
I0
15
87Rb
Fig. 2. Plot o f S r /
, 20
/ 86Sr
S r v e r s u s r q ~ b / S r for a g m a t i t e [MSWD = 0.6)
The Pan-African orogeny (1100-450 Ma) is presently the only u n d i s p u t e t tectonothermal event believed to have affected the Nigerian basement. A major event marking the Pan African orogeny in Nigeria was the intrusion of Older Granites into the b a s e m e n t complex. It is generally believed that the Older Granites were emplaced between col 500 and col 692 Ma and that the peak of the intrusion occurred at col 610 Ma (Van Breemen e t a / . , 1977). But the plutonic episode represented by the m a s s e s of Older Granites is but one aspect of a major orogeny. It also involved m e t a m o r p h i s m , migmatisation and tectonism, The p e a k of metamorphic event or migmatisation is not clearly known. Odeyemi (1982) following Grant (1971) suggested that the peak m e t a m o r p h i c event occurred at co_ 450-500 Ma. Harper et aL (1973) however, considered these ages dated epeirogenic uplift and cooling. Ekwueme (1985) proposed that the m a i n Pan African tectonothermal event occurred in the Oban massif 676 +26 Ma ago. Attempt h a s also been m a d e to correlate the col 675 Ma tectonothermal event in Nigeria with the intrusion of the syntectonic Older Granite ca. 655-677 Ma (Ekwueme, 1987b). Cahen et al. (1984) were of the view that the syntectonic Older Granite intrusive phase was almost c o n t e m p o r a n e o u s with a m e t a m o r p h i c Table 3. Average trace element composition of migmatites in Oban massif
Nb Zr Y Sr Rb Zn Cu Ni Cr Ce Nd V La Ba Sc K/Rb K/Ba Ba/Rb Rb/Sr
1
2
3
4
20 291 49 3 91 76 49 58 64 136 67 36 73 1495 II 420 26 16.42 0.27
16 225 37 213 193 80 30 50 59 131 63 34 70 1395 9 156 22 7.23 0.91
l0 200 15 249 120 i17 26 49 135 68 26 43 39 1400 7 227 19 II 0.48
14 311 20 433 ]45 131 30 165 180 190 55 40 49 1480 12 223 27 10.20 0.33
I = M i g m a t i t i c G n e i s s ( B a n d e d ) 3 = M i g m a t i t i c S c h i s t (Abbiatl) 2 = M i g m a t i t i c G n e i s s [Agrnatite) 4 = M i g m a t i t i c S c h i s t (Kwa Falls)
Rb-Sr age of Pan-African migmatitcs in the Oban Massif of southeasternNigeria
69
.850 OBM
~o
~.800
-
g, b**| •7 5 0
-
OBL m ~,D ql,
-700
0
I
I
I
I
z
I
I
I
I
I
2
3
4
5
6
7
8
9
87Rb/86Sr
Fig. 3. Plot of ~Sr/~Sr versus ~Rb/eeSr for banded gneiss (OBL = leucocratlc portion; OBM = Melanocratic portion of banded gneiss). t e m p e r a t u r e high at a b o u t 650 Ma or slightly earlier. E k w u e m e (1987a) a r g u e d t h a t t h e co. 675 Ma tectonic event p r o d u c e d t h e distinct N-S to NESW (0-50 °) stretching lineation a n d penetrative foliation in t h e O b a n m a s s i f a n d that the co. 675 Ma t h e r m a l event resulted in m e t a m o r p h i s m which attained the k y a n i t e - a l m a n d i n e amphibolite facies grade. The relationship between t h e e m p l a c e m e n t of Older Granites a n d m i g m a t i s a t i o n in the Nigerian
.T30
-720
'710 •7 0 8 B ~;
-7oo-
o
'~ o.s lrl g. 3
'to
I ~s
2'0
;.s
8 7 r c b / ' 815 S r
Fig. 4. Plot of ~Srl~Sr versus ~Rb/mSr for m l g m a t i t i c s c h i s t (MSWD = 0.7}
-~--
3.0
B a s e m e n t h a s r e m a i n e d controversial. There are two schools of t h o u g h t . One a r g u e s t h a t the Older Granites were p r o d u c t s of anatexis of t h e surr o u n d i n g c o u n t r y rocks, therefore a c o n s e q u e n c e of high-grade regional m e t a m o r p h i s m (McCurry, 1971; Odeyemi, 1978; R a h m a n et al., 1981; Onyeagocha, 1984). The s e c o n d school a r g u e s t h a t there is no special relationship between t h e intensity of m i g m a t i s a t i o n a n d t h e formation of the Older Granites (AJibade, 1982; E k w u e m e , 1985; Hockey et al., 1986). Field relationships between Older Granites a n d m i g m a t i t e s as well as age d e t e r m i n a t i o n s are n e c e s s a r y to resolve this controversy. If the Older Granites are the result Of high-grade regional m e t a m o r p h i s m of t h e c o u n t r y rocks, it follows t h a t these intrusive rocks s h o u l d only occur at the zones of t h e highest m e t a m o r p h i c grade where m i g m a t i t e s are f o u n d (Onyeagocha a n d Ekwueme, 1990). According to Wyllie (1977), s u c h granites are fused portions of migmatites. But this is not always the case, at least in t h e Oban massif. In this area, except for the granites (sensu stricto) t h e u s u a l p r o g r e s s i o n : l o w - g r a d e to m e d i u m - g r a d e to high-grade rocks to m i g m a t i t e s
70
B. N. EzwtrEl~m
to granitoid rocks recorded in some Precambrian terrains is not applicable. The granodiorites in the area are associated with low-, m e d i u m and highgrade rocks (Fig. i). This indicates t h a t whereas granite ( s e n s u stricto) especially those associated with high-grade rocks (for example, migmatitic schists and gneisses) m a y be product of partial melting of the c o u n t r y rocks due to high-grade regional m e t a m o r p h i s m (S-type of granite) the granodiorites m a y be intrusives from the lower crust (l-type) r a t h e r t h a n a consequence of regional metamorphism. These interpretations are similar to those of AJibade (1982) for Zungeru area and Hockey et al. (1986) for Lokoja-Auchi area both in Central Nigeria. These a u t h o r s agree that the Older Granites in their areas post-date migmatisation. Hockey et al. (1986) f u r t h e r suggested that the migmatisation could be a higher level manifestation of a process which at deeper level resulted in the formation of large m a s s e s of Older Granites. An alternative hypothesis for the Oban massif is that these granitoid rocks which have been shown to be calc-alkaline intrusives (Ekwueme, 1985) were products of the subduction which occurred east of the West African c r a t o n a b o u t 650 Ma ago (Bertrand and Caby, 1978). A date of 510 + 11 Ma h a s b e e n obtained from the agmatite in Mbarakpa area. It is interpreted here as the age ofmigmatisation in the Oban massif. It is Pan-African and the high value of the initial 87Sr/ S6Sr ratio (0.71121 -+0.00025) indicates that these materials were of crustal origin or were derived from the reworking of older material. A date of 510 + 11 Ma is close to ca. 500 Ma suggested as dating the peak of Pan-African m e t a m o r p h i s m in the Nigerian b a s e m e n t (Odeyemi, 1982). It is also correlatable with the age of 565-+22 Ma regarded as the date of migmatisation in S o u t h e r n Cameroon which is adjacent to the Oban massif (Toteu et al., 1987). 510 -+11 Ma is however closer to 523-+10 Ma widely believed to be the age of emplacement of late-tectonic granites e. g. Kusheriki granite t h a n the 655 -+15 Ma reported for syntectonic granite e.g. Maiinchi granitoids (see Cahen et al., 1984). The age of 523 _+10 Ma is even closer to the 527 _+16 Ma obtained from the migmatitic schist at the Kwa Falls. The 527 _+16 Ma date is close to those of agmatite in Mbarakpa as well as the age ofmigmatisation in s o u t h e m Cameroon mentioned earlier. It therefore, dates the migmatisation of the b a n d e d schist in the Kwa Falls whilst the 510 _+11 Ma dates the migmatisation of the gneiss in Mbarakpa. Migmatites occur at the highest grade of regional m e t a m o r p h i s m and involve anatexis, injection or segregation of the paleosome. This m e a n s that 510-527 Ma dates the peak of thermal event in the Oban massif. It therefore, appears that the 676 +26 Ma event was more pervasively tectonic t h a n ther-
mal and produced the penetrative N-S to NE-SW trending foliations and lineations in rocks of Oban massif. On the other hand, m e t a m o r p h i s m associated with the 676 + 26 Ma tectonic event attained Just the kyanite-almandine subfacies grade of the amphibolite facies. The significance of a conventional age of 946 +26 Ma obtained for some samples which plot above the 510 _+11 Ma isochron is not clear. Ekwueme (1985) reported a single whole-rock Rb-Sr age of 922 _+20 Ma from the same agmatite. That age is not significantly different from the 946 +26 Ma reported here. Ekwueme (1985) considered the 922 -+20 Ma age as tentative b u t hinted t h a t it m a y be dating a Kibaran event. The 946-+26 Ma conventional age obtained for this s t u d y s e e m s to suggest that a geologic event took place in the Oban massif then. Cahen et al. (1984) mentioned that ages in a 950 + 50 Ma range h a s been obtained in m a n y regions of Africa and that m a n y of these ages have been obtained on granites and pegmatites. They were of the opinion that ages in this range cover the manifestations of an important tectonothermal event that was mostly magmatic in Southwest Mrica. The 946 +_26 Ma is unlikely to be Kibaran but it is possible that it dates (i) the age of emplacement of the quartz dioritic rocks that was rnigmatised 510 _+11 Ma ago. The high initial aVSr/aSSr ratio (0.71121) of the 510 + 11 Ma isochron indicates that a n older material h a s been reworked during this time. This older material might be the quartz diorite that was emplaced 946 -+ 26 Ma ago; (ii) it could be dating a destabfllsation/resetting of the Kibaran clock in the Oban massif by the Pan African event (cf Ekwueme a n d Caen-Vachette, 1989); (ill) it m a y be dating an open system behaviour in the progenitor of the migmatites and therefore reflects no geological event (cf. Haslam et aL, 1980). The first case is most likely considering the findings of other workers in Africa (see for instance, Cahen et al., 1984; Toteu et al., 1987). The b a n d e d gneiss of Oban massif yielded neither an isochron nor a reliable conventional age. Rather, the melanocratic and leucocratic portion of the rock plot in separate sections of the diagram and scatter widely (Fig. 3). The wide scatter indicates a severe disturbance of isotopic systems (cf. Eaton, 1986; Haslam et al., 1986). This disturbance of the Rb-Sr systematics might be due to (i) metasomatism (ii) shearing and (iii} fracturing of these rocks. Oyawoye (1972) suggested that K, Na and B metasomatism affected most parts of the Nigerian basement. The m e t a s o m a t i s m m a y be due to rejuvenation of the b a s e m e n t during the intrusion of the Older Granite and it is most pronounced in the b a n d e d gneiss terrains. It is generally believed that m e t a s o m a t i s m and anatexis were the m a i n pro-
Rb-Sr age of Pan-African migmatites in the Oban Massif of southeastern Nigeria c e s s e s t h a t f o r m e d t h e Nigerian b a n d e d g n e i s s e s (Oyawoye, 1972; H o c k e y e t a l . , 1986). T h e b a n d e d g n e i s s in t h e O b a n m a s s i f c o n t a i n s a b u n d a n t o r t h o c l a s e a n d m i c r o c l i n e (Table 1) i n d i c a t i n g t h a t it c o u l d h a v e b e e n a f f e c t e d b y K ÷ m e t a s o m a t i s m . T h e high p r o p o r t i o n of K20 ( 4 . 6 1 % ) in t h e r o c k s (Table 2) c o n f i r m s this. In a d d i t i o n , t h e r o c k s a s Table 4. Rb-Sr whole rock isotopic data for migmatites in the Oban massif Sample Rb (ppm) No.
Sr (ppm) aTRb/S6Sr S~Sr/a6Sr Normalized
MIGMATITIC GNEISS(AGMATITE) 1 2 3 4 5 6 7 8
196 248 232 149 125 160 213 227
223 207 330 302 474 25 77 75
2.5415 3.4798 2.0363 1.4320 0.7636 18.680 8.4943 8.8593
0.72979 0.73602 0.72617 0.71962 0.71672 0.96467 0.82672 0.82688
4-pointisochron ( 1 , 2 , 3 , 5): T = 5 1 0 ± 11Ma Io= 0. 71121 ± 0.00025 MSWD= 0.6 MIGMATITIC SCHIST (Kyanite-Sffiimani~ schis0: 9 208 613 0.9842 0.71601 10 204 788 0.7491 0.71455 II 138 170 2.3610 0.72684 12 140 860 0.4714 0.71238 13 75 155 1.4193 0.93925 14 106 13 27.7000 0.93925 5-point isochron (9, 10, 11, 12, 13): T = 5 2 7 + 16Ma Io = 0.70883 + 0. 00018 MSWD = 0.7 MIGMATITIC GNEISS (BANDED GNEISS): (a) (MELANOCRATICPORTIONS): 95 149 91 69 145 97 112 141
15 16 17 18 19 20 21 22 b) 15 16 17 18 19 20 21 22
43 56 43 43 69 41 40 69
6.5543 7.8054 6.1897 4.6617 6.6577 6.8614 8.2332 5.9680
0.83845 0.84679 0.85734 0.85464 0.82442 0.85688 0.86189 0.85018
(LEUCOCRATIC PORTIONS): 81 137 132 99 93 91 83 130
176 375 374 247 302 241 193 315
1.3311 1.0593 1.0257 1.1661 0.8887 1.1029 1.2447 1.1977
0.75684 0.73286 0.73233 O.76016 0.73525 0.75609 0.75707 0.73852
71
m e n t i o n e d earlier, w e r e e x t e n s i v e l y s h e a r e d a n d a c c o r d i n g to E a t o n (1986) s h e a r i n g a n d m e t a s o m a t i s m c a u s e d e s t a b i l i s a t i o n of t h e R b - S r s y s t e m . W h e n t h e R b - S r d a t a for t h e b a n d e d g n e i s s a re c o m p a r e d w i t h t h o s e of a g m a t i t e , it is f o u n d t h a t b o t h t h e l e u c o c r a t i c (OBL) a n d t h e m e l a n o c r a t i c p o r t i o n s (OBM) are d e p l e t e d in Rb relative to t h e a g m a t i t e (Tables 3 a n d 4). T h i s implied t h a t t h e r e w a s a loss in r u b i d i u m , w h i c h c a n be i n t e r p r e t e d to i m pl y t h a t s u b s e q u e n t geological e v e n t s h a v e c a u s e d s o m e o p e n s y s t e m b e h a v i o u r in t h e wholer o c k s y s t e m s . T h e s e s u b s e q u e n t e v e n t s for t h e b a n d e d g n e i s s in t h e O b a n m a s s i f c o u l d b e r e l a t e d to s h e a r i n g , f r a c t u r i n g a n d m e t a s o m a t i s m . CONCLUSION T h e P a n - A f r i c a n t e c t o n o t h e r m a l e v e n t s in t h e Oban massif were m a r k e d by penetrative deformat i o n a n d p r o g r e s s i v e m e t a m o r p h i s m u p to t h e k y a n i t e - a l m a n d i n e - m u s c o v i t e s u b f a c i e s of t h e a m p h i b o l i t e facies 6 7 6 +26 Ma ago ( E k w u e m e , 1987a). Progressive m e t a m o r p h i s m of s c h i s t s a n d g n e i s s e s u p to t h e s i l l i m a n i t e - a l m a n d i n e - o r t h o clase a m p h i b o l i t e facies o c c u r r e d 527 + 16 Ma to 5 1 0 + 11 Ma ago a n d p o s s i b l y m a r k e d t h e p e a k t h e r m a l e v e n t t h a t r e s u l t e d in m i g m a t i s a t i o n of s c h i s t s a n d g n e i s s e s in t h e O b a n m a s s i f . F u r t h e r s t u d i e s u s i n g U-Pb m e t h o d of d a t i n g z i r c o n s a re r e q u i r e d to c o n f i r m t h i s age o f m i g m a t i s a t i o n in t h e O b a n m assi f.
Acknowledgement - Encouragement received from Dr. M. Caen-Vachette in dating rocks of Oban massif is gratefully acknowledged. I am grateful to all the reviewers whose suggestions greatly improved the quality of this paper. A research grant from the University of Calabar Senate covered the field work. REFERENCES Ajibade, A. C. 1982. Origin and emplacement of the Older Granites of Nigeria: some evidence from the Zungeru region. NigerlanJ. Min. Geol. 19(1), 221-230. Bertrand, J. M. L. and Caby, R. 1978. Geodynamic evolution of Pan-African orogenic belt: a new Interpretation of the Hoggar shield (Algerian Sahara). Geo/. Rundsch. 67, 357-388. Burke, K. C., Freeth, S. J. and Grant, N. K. 1972. Granite gneiss in the Ibadan area, Nigeria. In: African Geology (edited by Dessauvagie, T. F. J. and Whiteman, A. J.), p. 103-104, Ibadan. Cahen, L., Snelling, N. J., Delhal, J. and Vail, J. R. 1984. The Geochronology and Evolution o f Africa. Oxford Univ. Press, Oxford, 511 p. Condie, K. C. 1967. Geochemistry of early Precambrian greywackes fromWyoming. Geochim. Cosmochlm. Acta 31, 2135-2149. Cox, K. G., Bells, J. D. and Pankhurst, R. J. 1979. The Interpretations of Igneous Rocks. George Allen and Unwin, London.
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Eaton, IL M. 1986. Geochronology of the Grenville Province. In: The Grenville Province (edited by Moore, J . M., Davidson, A. a n d Baer, A. J.). Geol. Assoc. Canada Spec. P a p e r 3 1 , 127-173. Ekwere, S. J. a n d Ekwueme, B. N. 1991. G e o c h e m i s t r y of P r e c a m b r i a n gneisses in the e a s t e r n p a r t of the O b a n massif, s o u t h e a s t e r n Nigeria. GeoL Nynb. 70,105114. E k w u e m e , B. N. 1985. Petrology, geochemistry a n d RbSr geochronology of m e t a m o r p h o s e d rocks of Uwek a r e a s o u t h e a s t e r n Nigeria. Unpublished P h . D . Thesis, Univ. Nigeria, Nsukka. Ekwueme, B. N. 1987a. S t r u c t u r a l orientations a n d P r e c a m b r i a n deformational episodes of Uwet area, O b a n massif, SE Nigeria. Precambrian Res. 34, 269289. Ekwueme, B. N. 1987b. The C. 675 Ma tectonothermal event in Nigeria. In: Tectonothermal evolution of the WestAfrlcan orogens and clrctun-A tlantic terrane linkages. IGCP Project 233 Conf., Nouakchott Mauritania, p.85-86 (Abstract}. Ekwueme, B. N. 1988. The Kibaran rocks of Nigeria, West Africa. IGCP 255, Newsletter I , 15-18. E k w u e m e , B. N. 1990. R b - S r a g e s a n d petrologic features of P r e c a m b r i a n rocks from the O b a n massif, s o u t h e a s t e r n Nigeria. Precambrtan Res. 47, 271286. Ekwueme, B. N. a n d Caen-Vachette, M. 1989. Kibaran ages on amphibolites from the O b a n m a s s i f in the Pan African belt of Nigeria. J. Afr. Earth ScL 9, 719-723. Ekwueme, B. N. a n d Onyeagocha, A. C. 1985. Metamorphic Isograds of Uwet area, s o u t h e a s t e r n Nigeria. J. Afr. E a r t h ScL 8, 443-454. Ekwueme, B. N. a n d Onyeagocha, A. C. 1986. Geoc h e m i s t r y of m e t a s e d i m e n t a r y rocks of Uwet area, O b a n massif, s o u t h e a s t e r n Nigeria. Geol. Rundsctu 75, 411-420. Elueze, A. A. 1982. G e o c h e m i s t r y of the llesha granite gneiss in the b a s e m e n t complex of s o u t h w e s t e r n Nigeria. Precambrian Res. 19, 167-177. Freeth, S. J. 1971. Geochemical a n d related studies of West African igneous and metamorphic rocks. Unpublished P h . D . Thesis, Univ. Ibadan, Nigeria. Garrels, R. M. a n d Mackenzie, F. T. 1971. Evolution of metasedimentary rocks. Northon a n d Co. New York, 394 p. Grant, N. K. 1971. A compilation of radiometric ages from Nigeria. NigerianJ. Min. Geol. 6, 37-54. H a s l a m , H. W., Brewer, M. S., Davis, A. E. a n d Darbyshire, D. P. F. 1980. Anatexis and high-grade m e t a m o r p h i s m in the C h a m p i r a Dome, Malawi: petrological a n d Rb-Sr studies. M/n. Mag. 43, 701-714. Haslam, H. W., Rundle, C. C. a n d Brewer, M. S. 1986. Rb-Sr studies of m e t a m o r p h i c a n d igneous events in e a s t e r n Zambia. J. Afr. Earth ScL 5, 447-453. Harper, C. T., Sherrer, G., McCurry, P. a n d Wright, J. B. 1973. K-Ar retention ages from the Pan-Afrlcan of n o r t h e r n Nigeria. Geol. Soc. Am. Bull. 84, 919-926. Hockey, R. D., Sacchi, R., de Graaff, W. P. F. H. a n d Muotoh, E. O. C. 1986. The geology of Lokoja-Auchi. Geol. Surv. Nigerian BUlL 39, 71.
Imeokparia, E. G. a n d Emofurieta, W. O. 1991. Protoliths a n d petrogonesis of P r e c a m b r l a n gneisses from the Igbeti area, SW Nigeria. Chem. E r d e 51, 39-54. McCurry, P. 1971. Pan-African orogeny in n o r t h e r n Nigeria. Geo/. Soc. Am. BUlL 82. 3251-3262. Nockolds, S. R. 1954. Average chemical composition of s o m e igneous rocks. Geol. Soc. Am. Bull. 66, 10071032. Odeyemi, I. B. 1978. On the Petrology oflgalTa granite. 4 ° Conf. Geol. Afrique et S y m p o s i u m (Abstract). Odeyemi, I. B. 1982. A review of the orogenic events in the P r e c a m b r i a n b a s e m e n t of Nigeria, West Africa. Geo/. Rundsch. 70, 897-909. Onyeagocha, A. C. 1984. Petrology a n d geologic history ofNW Akwanga a r e a in n o r t h e r n Nigeria. J. Afrt. Earth ScL 2(I], 41-50. Onyeagocha, A. C. 1986. G e o c h e m i s t r y of b a s e m e n t granitic rocks from n o r t h c e n t r a l Nigeria. J. Afr. Earth ScL 5(6], 651-657. Onyeagocha, A, C. a n d E k w u e m e , B. N. 1982. The prePan- African structural features in northcentral Nigeria. Nigerian J. Min. Geol 19(2), 74-77. Onyeagocha, A. C. a n d Ekwueme, B. N. 1990. T e m p e r a t u r e - P r e s s u r e d i s t r i b u t i o n p a t t e r n s in m e t a m o r phosed rocks of the Nigerian b a s e m e n t complex: a preliminary analysis. J. Aft. Earth ScL I I, 83-93. Oshin, O. 1984. Distribution of u r a n i u m a n d thorium in the b a s e m e n t complex rocks of SW Nigeria. NigerianJ. M/rL Geo/. 21, 27-33 Oyawoye, M. O. 1972. The B a s e m e n t Complex of Nigeria. In: African Geology, (edited by Dessauvagie, T. F. J., Whiteman, A.J.), 67-97, Geology Dept. Univ. Ibadan. R a h a m a n , M. A. 1978. G e o c h e m i s t r y of s o m e gneisses from p a r t s of s o u t h e r n Nigeria. Nigerian J. Min. GeoL 15(I), 38-45. R a h a m a n , M. A., Olarewaju, V. O., Ocan, O. O. a n d Oshin I. O. 1983. Crustal evolution during the Proterozoic in s o u t h - w e s t e r n Nigetla: Indication from the migmatite-gneiss-quartzite complex. Bull. ScL Assoc. Nigeria, 9(I), 135-136. R a h m a n , A. A. M. S., Ukpong E. E. a n d A z m a t u a l l a h M. 1981. Geology of p a r t s of the O b a n Massif, southeastern Nigeria. Nigerian J. Min. Geol. I 0 ( I ] , 60-65. Shaw, D. M. 1956. G e o c h e m i s t r y ofpelitic rocks. Part Ill, Major elements a n d general geochemistry. Bull. Geol. Soc. Am. 87, 919-934. Toteu, S. F., Michard, A., Bertrand, J. M. a n d Rocci, G. 1987. U / P b d a t i n g of P r e c a m b r i a n r o c k s f r o m n o r t h e r n Cameroon, orogenic evolution a n d chronology of the Pan-African Belt of Central Africa. Precambrian Res. 37, 71-87. Van Breemen, O., Pidgeon, R. I. a n d Bowden, P. 1977. Age a n d isotopic studies of s o m e Pan-African granites from northcentral Nigeria. Precambrian Res. 4, 307319. Williamson, J. H. 1968. Least s q u a r e s fitting of a straight line. Canadian J. Physics 46, 1847. Wyllie, P. J. 1977. From Crunclbles t h r o u g h s u b d u c t i o n to batholiths. In: Energetics of Geological Processes, Saxena SK, Bhattacharji (eds.}, Springer-Verlag, New York, p. 389-433.
0899-5362/92 $5.00 + 0.00 © 1992 Pergamon Press Ltd
Rb-Sr age of Pan-African migmatites in the Oban Massif of southeastern Nigeria BARTH.N. EKWUEME Department of Geology, University of Calabar, P. O. Box 3651, Calabar, Nigeria (First received 15th November, 1989; revised form received 24th February, 1992)
Abstract - Migmatites consisting mainlyof lit-par-lit gneisses form one of the most dominant metamorpho-
sed rocks in the Oban massifof southeastern Nigeria. Banded schists and agmatites are the other mixed rocks recognized in the area. The lit-par-hi gneisses are composed of quartz, feldspar 0eucocratic portion) and biotite, chlorite, garnet (melanocratic portion). They are extensively sheared, fractured and folded. The dominant fold axes trend N-S parallel to the regional foliationof the rocks. Chemical data show that the litpar-lit gneiss is granodioritic in composition. The agmatite is restricted in occurrence. Banding is rare in the agmatite but dissectionof the paleosome by quartzo-feldspathic veins is common. The agmatite is quartz-dioritic in composition. The banded schist is coarse-grained and contains index minerals: garnet, kyanite and sillimanite, indicating that the area underwent high-grade metamorphism of the Barrovian type. It is politic in composition. These migmatitic rocks were formed by partial melting, metamorphic segregation and injection due to high-grade metamorphism. The migmatitic gneiss yielded a Rb-Sr isochron age of 510-~_11Ma whilst the migmatitic schists gave an age of 527+16 Ma.
(1982), E k w u e m e a n d O n y e a g o c h a (1986) a n d H o c k e y et al. (1986). R a h a m a n et aL (1983), Oshi (1984), O n y e a g o c h a (1986), E k w e r e a n d E k w u e m e ( 1991) a n d I m e o k p a r i a a n d E m o f u r i e t a (1991) are however, of t h e view t h a t s o m e of t h e g n e i s s e s are of i g n e o u s parentage. In spite of t h e divergent views o n t h e progenitors of t h e b a n d e d g n e i s s e s , m o s t of t h e w o r k e r s believe t h a t t h e y were overprinted b y t h e h i g h e s t g r a d e of t h e amphibolite facies regional m e t a m o r p h i s m (Oyawoye, 1972; O n y e a g o c h a , 1984). T h e y agree t h a t partial melting, m e t a m o r p h i c segregation a n d injection w h i c h a c c o m p a n y s u c h high-grade m e t a m o r p h i s m formed t h e b a n d e d g n e i s s e s a n d t h e agmatites. The age of m i g m a t i s a t i o n , however, r e m a i n s u n k n o w n for t h e Nigerian b a s e m e n t . In this paper, t h e a u t h o r e x a m i n e s t h e c h a r a c t e ristic f e a t u r e s o f m i g m a t i t i c g n e i s s e s a n d s c h i s t s in t h e O b a n m a s s i f a n d r e p o r t s n e w ages w h i c h p o s s i b l y date m i g m a t i s a t i o n in t h e area.
INTRODUCTION
A b o u t fifty p e r c e n t of Nigeria is covered b y P r e c a m b r i a n b a s e m e n t rocks. Of these, "mixed" r o c k s o t h e r w i s e k n o w n a s m i g m a t i t e s are t h e m o s t c o m m o n . The m i x e d r o c k s include b a n d e d gneiss, b a n d e d schist a n d agmatite. Oyawoye (1972) n o t e d t h a t t h e b a n d e d g n e i s s e s in the Nigerian b a s e m e n t are m i g m a t i t e s a n d are p o s s i b l y t h e oldest r o c k s in t h e c o u n t r y . He p o i n t e d o u t t h a t t h e s e b a n d e d g n e i s s e s o n geological b a s i s are older t h a n t h e granite g n e i s s w h i c h yielded a w h o l e - r o c k R b - S r i s o c h r o n age of 2 1 9 0 +30 Ma (Grant, 1971). Possibly b e c a u s e of their m i x e d a n d therefore, c o m p l e x n a t u r e , it h a s b e e n difficult to a s c e r t a i n t h e a c t u a l p r o g e n i t o r s of the b a n d e d gneisses. Freeth (1971) a n d B u r k e et aL (1972) s h o w e d t h a t t h e b a n d e d g n e i s s e s in I b a d a n SW Nigeria were derived from g r e y w a c k e a n d a g r e y w a c k e - s h a l e s e q u e n c e . O n t h e o t h e r h a n d , R a h a m a n (1978) p o i n t e d o u t t h a t g e o c h e m i c a l evidence s h o w s t h a t t h e s a m e b a n d e d g n e i s s e s are similar in composition to g r e y w a c k e s , s h a l e s a n d r o c k s of t h e dioritem o n z o n i t e range. He, n o n e t h e l e s s , f a v o u r e d a s e d i m e n t a r y origin for t h e b a n d e d g n e i s s e s b a s e d on t h e i n t e r b a n d i n g of t h e g n e i s s e s with quartzites a n d l e n s e s of calc-silicate rocks. O t h e r w o r k e r s w h o h a v e a r g u e d for a s e d i m e n t a r y origin for t h e g n e i s s e s in t h e Nigerian b a s e m e n t i n c l u d e Elueze
GEOLOGICAL S E T T I N G
E k w u e m e a n d O n y e a g o c h a (1985, 1986) a n d Ekwere a n d E k w u e m e (1991) have d i s c u s s e d t h e occurrence, p e t r o g r a p h y a n d g e o c h e m i s t r y o f r o c k s in the O b a n M a s s i f including migmatites. A brief s u m m a r y is given here. Migmatites in t h e O b a n m a s s i f are a s s o c i a t e d
65
66
B. N. EKWUEME
with pegmatitic granites, pegmatitic boulders and plane foliation trending N-S. In most cases, the pegmatitic veins. These pegrnatites which form the b a n d s are parallel b u t occasionally they are conneosome dissect the m e t a m o r p h i c host (paleo- torted and complexedly folded. The folds are genesome)• Banding is not c o m m o n in the agmatite- rally tight to isoclinal b u t chevron, r e c u m b e n t and type of migmatites in Oban massif a n d where ptygmatic folds also occur. Boudinage a n d pinchpresent, it is not conspicuous. The agmatite is best and-swell s t r u c t u r e s are c o m m o n on the limbs of exposed at Mbarakpa where the rock is highly the folds confirming that there was a drag on these foliated in the N-S to N4°E direction. It is often limbs. The b a n d in the gneiss cleaves along smooth Jointed and major fractures in the rock trend 20 ° chloritic surfaces, which are interpreted as s h e a r whilst minor ones t r e n d northwest. Ptygmatic folds planes. This is similar to the s h e a r reported in migmatitic gneisses (banded) of the Lokoja-Auchi are c o m m o n in the agmatite. The best exposures of lit-par-lit gneiss in the area (Hockey et al., 1986) where the rocks appear Oban massif occur at the now-abandoned Nigerian to have been sheared along sub-parallel planes Mining Corporation Q u a r r y in Oban. The rock is and the s h e a r planes according to Hockey et al. coarse-grained a n d consists of b a n d s ranging from (1986) afforded easy p a t h s of ingress to the migma1 c m to over 3 c m thick. The leucocratic b a n d is tisation. c o m p o s e d d o m i n a n t l y of quartz and feldspar Coarse-grained schists in the Oban massif are whereas the melanocratic portion contains phyllo- b a n d e d and constitute the migrnatitic schists in sflicates, (chlorite a n d biotite). The latter m a y also the area. They are restricted to the Kwa Falls and contain garnet. The minerals forming the d a r k Abbiatl areas (Fig. 1). Minerals in these schists portion of the b a n d e d gneiss show strong axial include garnet, kyanite and sflllmanite (Table 1) 5o48'~
s
9 o ? Ip=m
.i\•
(
/....~ .
.j
.~f f"
GG
-~,.
~.
"x
/ '
(
.
ugo
X ' - ' J ~" T,
.~.
t
\,
LgS •
J
l "
uw.t
,A. x"A vfI •q
^
BA
OG
\
ra,,,,=
~,,~" "~
o
Me
\
"~z:l.~,,us ':
sement Complex Oban Massif Sediments.
8°O0'E
~g' \
~1~_'~ "-" "A t 1 7 .'IS @,--,-_ " 2t ,Oban /.e Z2 • zO
. . . / r.pG • .~,,~yleU.am \
q.t_~ K~z~-'~
~..._.
\'~J
Abbiatti~,..t. "N "~
1 8 ° 30'
, I 8045 `
5°00~1 80 5 5 ' E
EXPLANATIONS S
Sedimentary Rocks
MgG
Medium grade
Pegmatite
MG GO
Migmatitic Gneiss Granite Gneiss
•B A
Banded Amphibolite
P PG
Pegmatitic
e
Granite
GD
Gronodiorite
LgS MS
Low grade Schist
Gr oeite
Migmatite Schist
A
A -I {.2 e
Gneiss
Amphibolite
Kwa falls Location of analysed sample
Towns/Villages
Fig. 1. Simplified geological map of the Oban massif showing locations of analyzed samples.
Rb-Sr age of Pan-African migmatites in the Oban Massif of southeastern Nigeria Table 1,. Average modal compositions of mlgmatltlcgnelsses and migmaUtlc schists in the Oban massif Quartz Plagioklase K-feldspar Biotite Muscovite Garnet Kyanite Sfllimanite Tourmaline Chlorite Epidote Hornblende Sphene Myrmekite Sericite Calcite Opaques Average of:
1 44.0 14.0 13.0 25.0
2 30.0 12.0 21.0 22.0 I 0.0 2.0
3 20.0 15.0 15.0 24.0
x 3.0
4 17.0 5.0 2.0 32.0 4.0 9.0 6.0 7.0 -
x (I0)
18.0 x (20)
15.0 x 10.0 1.0
3.0
1.0
(6)
(12)
I. = Banded gneiss at Oban 2. = Agmatite at Mbarakpa 3. = Garnet Sllllmanlte Schist (mlgmatitic) at Abblatl. 4 = Kyanite Sillirnanite Schist(migmatitic) at Kwa Falls x = Trace a n d t h e b a n d i n g of t h e s c h i s t s is a t t r i b u t e d to highg r a d e m e t a m o r p h i s m . C hl or i t e a n d s e c o n d a r y calcite h o w e v e r o c c u r in s o m e s c h i s t o u t c r o p s in Kwa Falls p o i n t i n g to local r e t r o g r e s s i o n . T h e m i g m a t i t e s s h o w c o n s i d e r a b l e v a r i a t i o n in their average major and trace element composit i o n s (Table 2). T h e migmaUtic g n e i s s e s have h i g h e r SiO 2, Na20 a n d K20 b u t lower Al203, Fe203 (Total) a n d MgO t h a n t h e m i g m a t i t i c schists. Fe203 (Total) is p a r t i c u l a r l y high (10.40%) in t h e m i g m a t i t i c s c h i s t at Abbiati a n d s u g g e s t s t h a t t h e p a r e n t r o c k
67
m a y be p y r i t e - b e a r i n g . T h i s s c h i s t h a s also t h e h i g h e s t AI203 a n d T102 c o n t e n t a m o n g all t h e a n a l y s e d rocks. T h i s is a r e f l e c t i o n of t h e m o d a l c o m p o s i t i o n (Table 1) w h i c h s h o w s t h a t It ls enr i c h e d in K-feldspar, biotite, g a r n e t a n d sflllmanite relative to o t h e r r o c k s u n d e r s t u d y . (Table 1). T h e b a n d e d g n e i s s is d e p l e t e d in Rb w h e n c o m p a r e d with o t h e r a n a l y s e d r o c k s (Table 3). Cr a n d NI a re m o r e a b u n d a n t in t h e m i g m a t i t l c s c h i s t s t h a n in t h e g n e i s s e s . T h e l a t t e r is r i c h e r In La, Y, Nb a n d B a t h a n t h e former. C o n s e q u e n t l y , t h e m i g m a t l t l c gneisses have higher K / R b , K / B a a n d B a / R b ratios. T h e g e o c h e m i c a l c h a r a c t e r i s t i c s of t h e m l g m a t l tic g n e l s s e s are si m i l ar to t h o s e of o r t h o g n e i s s whereas the migmatltlc schists have chemical f e a t u r e s o f a m e t a s e d i m e n t (cf. G a r r e l s a n d Mackenzie, 1971). T h e a v e r a g e c o m p o s i t i o n of t h e b a n d e d g n e i s s is slrnflar to t h a t of g r a n o d l o r i t e w h e r e a s t h a t of a g m a t i t e is close to t h a t of q u a r t z diorite of Condi e (1967). T h e c o m p o s i t i o n of t h e m i g m a t l t i c s c h i s t Is si m i l ar to t h a t of a n a v e ra g e hi gh g r a d e pelitic r o c k given b y S h a w (1956) (Table 2). Rb-Sr STUDIES
S a m p l e localities are s h o w n in Fig. 1. Ca re w a s t a k e n to collect large (5-10 kg) b l o c k s of t h e fre sh est available m a t e r i a l s to r e p r e s e n t t r u e wholerock systems. The agmatite, b a n d e d gneiss and b a n d e d schist were analysed. E a c h s a m p l e w a s c r u s h e d to p a s s t h r o u g h 80 m e s h a n d a q u a r t e r e d p a r t w a s analyzed. T h e 87Sr/ 86Sr r a t i o s w ere n o r m a l i z e d to a v a l u e of 0 . 1 1 9 4 a f t e r w h i c h AEI-MS2S a n d V G - M a s s 5 4 E s p e c t r o -
Table 2. Average major element composition of migmatites in the Oban massif. I SiO 2
2
67.52
TiO2 0.50 A1203 14.32 Fe203 4.38 MnO 0.40 MgO 1.28 CaO 2.19 Na20 3.18 I~O 4.61 P2Os 0.20 LOI 1.42 Total I00.00 Average of 25 * = Total Fe as 1 = Mlgmatitlc 2 = Migmatitic 3 = Mlgmatitic 4 = Mlgmatitic
64.91
0.65 15.73 4.12 0.06 2.08 3.67 3.68 3.65 0.29 1.16 100.00
3
4
58.70 I. 11 17.30 10.40 0.20 4.29 1.90 2.21 3.28 0.15 1.29 100.83 10
Fe20 a Gneiss (Banded Gneiss) Gneiss (Agmatite} Schist (Abbiati) Schist (Kwa Falls)
5 6 7 8
60.45 0.46 13.05 3.90 0.20 5.05 5.40 0.68 3.90 0.38 5.46 98.93 12
5
66.09 0.54 15.73 4.11 0.08 1.74 3.83 3.75 2.73 0.18
6
65.50 0.6 15.65 4.43 0.06 1.86 4.10 3.84 3.01 0.23
20
7
8
63.51 0.79 17.35 6.71 2.31 1.24 1.96 3.35 -
-
= Granodiorite (Cox et aL, 1979) = Homblende-biotite Granodiorite (Nockolds, 1954) = High grade pelitic rock (Shaw, 1956) = Tonalite (Cox et al., 1979)
6 I. 52
0.73 16.48 5.65 0.00 2.80 5.42 3.63 2.09 0.25
B. N. EKWUEME
68
m e t e r s were u s e d to analyze for Rb and Sr concen- gneiss n e a r Uwet (Ekwueme, 1987). The 676 +26 trations respectively. Age calculaUons were m a d e Ma age dates a maj or tectonothermal event in Uwet following Williams (1968) p r o g r a m m e s and using and the mlgmatisation of the schists appears to kSTRb = 1.42 x 10~a -~. Errors are 2 % for 87Rb/SSSr have occurred after t h a t event. a n d 0 . 0 1 % for STSr/SSSr and the uncertainties of DISCUSSION results are given at the 28 level.
Migmatitlc G a e l u e s a) A g m a t l t e - The data for the agmatite are plotted in the S~Rb/~Sr v e r s u s STRb/~Sr diagram (Fig. 2). Four data points yield a regression line giving a n age of 510 + 11 Ma and a n intercept of 0.71121 + 0.00025. MSWD is 0.6. Three other samples of the agmatite plot above the isochron and are s o m e w h a t scattered. A conventional age of 946 + 26 Ma was calculated for samples 6 and 7 using the classical 87Sr/~Sr initial raUo of 0.712. b) B a n d e d Gneiss - Eight samples of b a n d e d gneiss from Oban Q u a r r y (Fig. 1) were analysed. Each sample was separated into leucocratic and melanocratic parts and t h e n analysed. The aim is to establish the age of the restite and the granitic portions. It was also to avoid obtaining "mixed" ages with the a t t e n d a n t difficulties in their interpretations for the b a n d e d gneiss. As shown in Fig. 3, no reliable isochron can be drawn. The melanocratic (OBM-) and leucocratic (OBL-) portions cluster at different sections of the diagram. A useful isochron cannot be drawn in both cases b e c a u s e the points are scattered. It is also not possible to draw an isochron using the points defined by both the melanocratic and leucocratic portions of the b a n d e d gneiss.
Migmatltlc Schists Five samples of the coarse-grained schist at the Kwa Falls produced a regression line which yielded an age of 527 +16 Ma (Fig. 4). One of the samples (14) without retrograde chlorite gave a conventional age dates of 627 +24 Ma which is similar to the age of 676 +26 Ma obtained from m e d i u m grade ['0~ 6*
w .900
-
-e00
" t
'"'
•'TO 0
5
j
,
I0
15
87Rb
Fig. 2. Plot o f S r /
, 20
/ 86Sr
S r v e r s u s r q ~ b / S r for a g m a t i t e [MSWD = 0.6)
The Pan-African orogeny (1100-450 Ma) is presently the only u n d i s p u t e t tectonothermal event believed to have affected the Nigerian basement. A major event marking the Pan African orogeny in Nigeria was the intrusion of Older Granites into the b a s e m e n t complex. It is generally believed that the Older Granites were emplaced between col 500 and col 692 Ma and that the peak of the intrusion occurred at col 610 Ma (Van Breemen e t a / . , 1977). But the plutonic episode represented by the m a s s e s of Older Granites is but one aspect of a major orogeny. It also involved m e t a m o r p h i s m , migmatisation and tectonism, The p e a k of metamorphic event or migmatisation is not clearly known. Odeyemi (1982) following Grant (1971) suggested that the peak m e t a m o r p h i c event occurred at co_ 450-500 Ma. Harper et aL (1973) however, considered these ages dated epeirogenic uplift and cooling. Ekwueme (1985) proposed that the m a i n Pan African tectonothermal event occurred in the Oban massif 676 +26 Ma ago. Attempt h a s also been m a d e to correlate the col 675 Ma tectonothermal event in Nigeria with the intrusion of the syntectonic Older Granite ca. 655-677 Ma (Ekwueme, 1987b). Cahen et al. (1984) were of the view that the syntectonic Older Granite intrusive phase was almost c o n t e m p o r a n e o u s with a m e t a m o r p h i c Table 3. Average trace element composition of migmatites in Oban massif
Nb Zr Y Sr Rb Zn Cu Ni Cr Ce Nd V La Ba Sc K/Rb K/Ba Ba/Rb Rb/Sr
1
2
3
4
20 291 49 3 91 76 49 58 64 136 67 36 73 1495 II 420 26 16.42 0.27
16 225 37 213 193 80 30 50 59 131 63 34 70 1395 9 156 22 7.23 0.91
l0 200 15 249 120 i17 26 49 135 68 26 43 39 1400 7 227 19 II 0.48
14 311 20 433 ]45 131 30 165 180 190 55 40 49 1480 12 223 27 10.20 0.33
I = M i g m a t i t i c G n e i s s ( B a n d e d ) 3 = M i g m a t i t i c S c h i s t (Abbiatl) 2 = M i g m a t i t i c G n e i s s [Agrnatite) 4 = M i g m a t i t i c S c h i s t (Kwa Falls)
Rb-Sr age of Pan-African migmatitcs in the Oban Massif of southeasternNigeria
69
.850 OBM
~o
~.800
-
g, b**| •7 5 0
-
OBL m ~,D ql,
-700
0
I
I
I
I
z
I
I
I
I
I
2
3
4
5
6
7
8
9
87Rb/86Sr
Fig. 3. Plot of ~Sr/~Sr versus ~Rb/eeSr for banded gneiss (OBL = leucocratlc portion; OBM = Melanocratic portion of banded gneiss). t e m p e r a t u r e high at a b o u t 650 Ma or slightly earlier. E k w u e m e (1987a) a r g u e d t h a t t h e co. 675 Ma tectonic event p r o d u c e d t h e distinct N-S to NESW (0-50 °) stretching lineation a n d penetrative foliation in t h e O b a n m a s s i f a n d that the co. 675 Ma t h e r m a l event resulted in m e t a m o r p h i s m which attained the k y a n i t e - a l m a n d i n e amphibolite facies grade. The relationship between t h e e m p l a c e m e n t of Older Granites a n d m i g m a t i s a t i o n in the Nigerian
.T30
-720
'710 •7 0 8 B ~;
-7oo-
o
'~ o.s lrl g. 3
'to
I ~s
2'0
;.s
8 7 r c b / ' 815 S r
Fig. 4. Plot of ~Srl~Sr versus ~Rb/mSr for m l g m a t i t i c s c h i s t (MSWD = 0.7}
-~--
3.0
B a s e m e n t h a s r e m a i n e d controversial. There are two schools of t h o u g h t . One a r g u e s t h a t the Older Granites were p r o d u c t s of anatexis of t h e surr o u n d i n g c o u n t r y rocks, therefore a c o n s e q u e n c e of high-grade regional m e t a m o r p h i s m (McCurry, 1971; Odeyemi, 1978; R a h m a n et al., 1981; Onyeagocha, 1984). The s e c o n d school a r g u e s t h a t there is no special relationship between t h e intensity of m i g m a t i s a t i o n a n d t h e formation of the Older Granites (AJibade, 1982; E k w u e m e , 1985; Hockey et al., 1986). Field relationships between Older Granites a n d m i g m a t i t e s as well as age d e t e r m i n a t i o n s are n e c e s s a r y to resolve this controversy. If the Older Granites are the result Of high-grade regional m e t a m o r p h i s m of t h e c o u n t r y rocks, it follows t h a t these intrusive rocks s h o u l d only occur at the zones of t h e highest m e t a m o r p h i c grade where m i g m a t i t e s are f o u n d (Onyeagocha a n d Ekwueme, 1990). According to Wyllie (1977), s u c h granites are fused portions of migmatites. But this is not always the case, at least in t h e Oban massif. In this area, except for the granites (sensu stricto) t h e u s u a l p r o g r e s s i o n : l o w - g r a d e to m e d i u m - g r a d e to high-grade rocks to m i g m a t i t e s
70
B. N. EzwtrEl~m
to granitoid rocks recorded in some Precambrian terrains is not applicable. The granodiorites in the area are associated with low-, m e d i u m and highgrade rocks (Fig. i). This indicates t h a t whereas granite ( s e n s u stricto) especially those associated with high-grade rocks (for example, migmatitic schists and gneisses) m a y be product of partial melting of the c o u n t r y rocks due to high-grade regional m e t a m o r p h i s m (S-type of granite) the granodiorites m a y be intrusives from the lower crust (l-type) r a t h e r t h a n a consequence of regional metamorphism. These interpretations are similar to those of AJibade (1982) for Zungeru area and Hockey et al. (1986) for Lokoja-Auchi area both in Central Nigeria. These a u t h o r s agree that the Older Granites in their areas post-date migmatisation. Hockey et al. (1986) f u r t h e r suggested that the migmatisation could be a higher level manifestation of a process which at deeper level resulted in the formation of large m a s s e s of Older Granites. An alternative hypothesis for the Oban massif is that these granitoid rocks which have been shown to be calc-alkaline intrusives (Ekwueme, 1985) were products of the subduction which occurred east of the West African c r a t o n a b o u t 650 Ma ago (Bertrand and Caby, 1978). A date of 510 + 11 Ma h a s b e e n obtained from the agmatite in Mbarakpa area. It is interpreted here as the age ofmigmatisation in the Oban massif. It is Pan-African and the high value of the initial 87Sr/ S6Sr ratio (0.71121 -+0.00025) indicates that these materials were of crustal origin or were derived from the reworking of older material. A date of 510 + 11 Ma is close to ca. 500 Ma suggested as dating the peak of Pan-African m e t a m o r p h i s m in the Nigerian b a s e m e n t (Odeyemi, 1982). It is also correlatable with the age of 565-+22 Ma regarded as the date of migmatisation in S o u t h e r n Cameroon which is adjacent to the Oban massif (Toteu et al., 1987). 510 -+11 Ma is however closer to 523-+10 Ma widely believed to be the age of emplacement of late-tectonic granites e. g. Kusheriki granite t h a n the 655 -+15 Ma reported for syntectonic granite e.g. Maiinchi granitoids (see Cahen et al., 1984). The age of 523 _+10 Ma is even closer to the 527 _+16 Ma obtained from the migmatitic schist at the Kwa Falls. The 527 _+16 Ma date is close to those of agmatite in Mbarakpa as well as the age ofmigmatisation in s o u t h e m Cameroon mentioned earlier. It therefore, dates the migmatisation of the b a n d e d schist in the Kwa Falls whilst the 510 _+11 Ma dates the migmatisation of the gneiss in Mbarakpa. Migmatites occur at the highest grade of regional m e t a m o r p h i s m and involve anatexis, injection or segregation of the paleosome. This m e a n s that 510-527 Ma dates the peak of thermal event in the Oban massif. It therefore, appears that the 676 +26 Ma event was more pervasively tectonic t h a n ther-
mal and produced the penetrative N-S to NE-SW trending foliations and lineations in rocks of Oban massif. On the other hand, m e t a m o r p h i s m associated with the 676 + 26 Ma tectonic event attained Just the kyanite-almandine subfacies grade of the amphibolite facies. The significance of a conventional age of 946 +26 Ma obtained for some samples which plot above the 510 _+11 Ma isochron is not clear. Ekwueme (1985) reported a single whole-rock Rb-Sr age of 922 _+20 Ma from the same agmatite. That age is not significantly different from the 946 +26 Ma reported here. Ekwueme (1985) considered the 922 -+20 Ma age as tentative b u t hinted t h a t it m a y be dating a Kibaran event. The 946-+26 Ma conventional age obtained for this s t u d y s e e m s to suggest that a geologic event took place in the Oban massif then. Cahen et al. (1984) mentioned that ages in a 950 + 50 Ma range h a s been obtained in m a n y regions of Africa and that m a n y of these ages have been obtained on granites and pegmatites. They were of the opinion that ages in this range cover the manifestations of an important tectonothermal event that was mostly magmatic in Southwest Mrica. The 946 +_26 Ma is unlikely to be Kibaran but it is possible that it dates (i) the age of emplacement of the quartz dioritic rocks that was rnigmatised 510 _+11 Ma ago. The high initial aVSr/aSSr ratio (0.71121) of the 510 + 11 Ma isochron indicates that a n older material h a s been reworked during this time. This older material might be the quartz diorite that was emplaced 946 -+ 26 Ma ago; (ii) it could be dating a destabfllsation/resetting of the Kibaran clock in the Oban massif by the Pan African event (cf Ekwueme a n d Caen-Vachette, 1989); (ill) it m a y be dating an open system behaviour in the progenitor of the migmatites and therefore reflects no geological event (cf. Haslam et aL, 1980). The first case is most likely considering the findings of other workers in Africa (see for instance, Cahen et al., 1984; Toteu et al., 1987). The b a n d e d gneiss of Oban massif yielded neither an isochron nor a reliable conventional age. Rather, the melanocratic and leucocratic portion of the rock plot in separate sections of the diagram and scatter widely (Fig. 3). The wide scatter indicates a severe disturbance of isotopic systems (cf. Eaton, 1986; Haslam et al., 1986). This disturbance of the Rb-Sr systematics might be due to (i) metasomatism (ii) shearing and (iii} fracturing of these rocks. Oyawoye (1972) suggested that K, Na and B metasomatism affected most parts of the Nigerian basement. The m e t a s o m a t i s m m a y be due to rejuvenation of the b a s e m e n t during the intrusion of the Older Granite and it is most pronounced in the b a n d e d gneiss terrains. It is generally believed that m e t a s o m a t i s m and anatexis were the m a i n pro-
Rb-Sr age of Pan-African migmatites in the Oban Massif of southeastern Nigeria c e s s e s t h a t f o r m e d t h e Nigerian b a n d e d g n e i s s e s (Oyawoye, 1972; H o c k e y e t a l . , 1986). T h e b a n d e d g n e i s s in t h e O b a n m a s s i f c o n t a i n s a b u n d a n t o r t h o c l a s e a n d m i c r o c l i n e (Table 1) i n d i c a t i n g t h a t it c o u l d h a v e b e e n a f f e c t e d b y K ÷ m e t a s o m a t i s m . T h e high p r o p o r t i o n of K20 ( 4 . 6 1 % ) in t h e r o c k s (Table 2) c o n f i r m s this. In a d d i t i o n , t h e r o c k s a s Table 4. Rb-Sr whole rock isotopic data for migmatites in the Oban massif Sample Rb (ppm) No.
Sr (ppm) aTRb/S6Sr S~Sr/a6Sr Normalized
MIGMATITIC GNEISS(AGMATITE) 1 2 3 4 5 6 7 8
196 248 232 149 125 160 213 227
223 207 330 302 474 25 77 75
2.5415 3.4798 2.0363 1.4320 0.7636 18.680 8.4943 8.8593
0.72979 0.73602 0.72617 0.71962 0.71672 0.96467 0.82672 0.82688
4-pointisochron ( 1 , 2 , 3 , 5): T = 5 1 0 ± 11Ma Io= 0. 71121 ± 0.00025 MSWD= 0.6 MIGMATITIC SCHIST (Kyanite-Sffiimani~ schis0: 9 208 613 0.9842 0.71601 10 204 788 0.7491 0.71455 II 138 170 2.3610 0.72684 12 140 860 0.4714 0.71238 13 75 155 1.4193 0.93925 14 106 13 27.7000 0.93925 5-point isochron (9, 10, 11, 12, 13): T = 5 2 7 + 16Ma Io = 0.70883 + 0. 00018 MSWD = 0.7 MIGMATITIC GNEISS (BANDED GNEISS): (a) (MELANOCRATICPORTIONS): 95 149 91 69 145 97 112 141
15 16 17 18 19 20 21 22 b) 15 16 17 18 19 20 21 22
43 56 43 43 69 41 40 69
6.5543 7.8054 6.1897 4.6617 6.6577 6.8614 8.2332 5.9680
0.83845 0.84679 0.85734 0.85464 0.82442 0.85688 0.86189 0.85018
(LEUCOCRATIC PORTIONS): 81 137 132 99 93 91 83 130
176 375 374 247 302 241 193 315
1.3311 1.0593 1.0257 1.1661 0.8887 1.1029 1.2447 1.1977
0.75684 0.73286 0.73233 O.76016 0.73525 0.75609 0.75707 0.73852
71
m e n t i o n e d earlier, w e r e e x t e n s i v e l y s h e a r e d a n d a c c o r d i n g to E a t o n (1986) s h e a r i n g a n d m e t a s o m a t i s m c a u s e d e s t a b i l i s a t i o n of t h e R b - S r s y s t e m . W h e n t h e R b - S r d a t a for t h e b a n d e d g n e i s s a re c o m p a r e d w i t h t h o s e of a g m a t i t e , it is f o u n d t h a t b o t h t h e l e u c o c r a t i c (OBL) a n d t h e m e l a n o c r a t i c p o r t i o n s (OBM) are d e p l e t e d in Rb relative to t h e a g m a t i t e (Tables 3 a n d 4). T h i s implied t h a t t h e r e w a s a loss in r u b i d i u m , w h i c h c a n be i n t e r p r e t e d to i m pl y t h a t s u b s e q u e n t geological e v e n t s h a v e c a u s e d s o m e o p e n s y s t e m b e h a v i o u r in t h e wholer o c k s y s t e m s . T h e s e s u b s e q u e n t e v e n t s for t h e b a n d e d g n e i s s in t h e O b a n m a s s i f c o u l d b e r e l a t e d to s h e a r i n g , f r a c t u r i n g a n d m e t a s o m a t i s m . CONCLUSION T h e P a n - A f r i c a n t e c t o n o t h e r m a l e v e n t s in t h e Oban massif were m a r k e d by penetrative deformat i o n a n d p r o g r e s s i v e m e t a m o r p h i s m u p to t h e k y a n i t e - a l m a n d i n e - m u s c o v i t e s u b f a c i e s of t h e a m p h i b o l i t e facies 6 7 6 +26 Ma ago ( E k w u e m e , 1987a). Progressive m e t a m o r p h i s m of s c h i s t s a n d g n e i s s e s u p to t h e s i l l i m a n i t e - a l m a n d i n e - o r t h o clase a m p h i b o l i t e facies o c c u r r e d 527 + 16 Ma to 5 1 0 + 11 Ma ago a n d p o s s i b l y m a r k e d t h e p e a k t h e r m a l e v e n t t h a t r e s u l t e d in m i g m a t i s a t i o n of s c h i s t s a n d g n e i s s e s in t h e O b a n m a s s i f . F u r t h e r s t u d i e s u s i n g U-Pb m e t h o d of d a t i n g z i r c o n s a re r e q u i r e d to c o n f i r m t h i s age o f m i g m a t i s a t i o n in t h e O b a n m assi f.
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