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RbS0r and KAr age measurements on the Modipe gabbro of Bechuanaland and South Africa
EARTH AND PLANETARY SCIENCE LETTERS 1 (1966) 439-442. NORTH-HOLLAND PUBL.COMP.,
AMSTERDAI~
R b - S r A N D K - A t AGE M E A S U R E M E N T S O N T H E M O D I P E G A B B R O OF BECHUANALAND AND SOUTH AFRICA M. W. McELHINNY
*
Department of Geophysics and Geochemistry, Australian National University, Canberra, A. C. T. Received 6 S e p t e m b e r 1966
R b - S r total rock m e a s u r e m e n t s a r e r e p o r t e d for six s a m p l e s of the Modipe gabbro. Although the rubidium concentration in the samples is low, the points on the isochron d i a g r a m lie on a straight tree within the e x p e r i m e n t a l e r r o r and an age of 2630 ± 470 m.y. is obtained with initial r a t i o 0.7010 ± 0,0010. The gabbro is intruded by the Gaberones granite and total rock m e a s u r e m e n t s on t h r e e samples of the granite give an age of 2340 * 50 m.y., which although not significantly different from the gabbro age is consistent with the known geological relationship of t h e m . K - A r m e a s u r e m e n t s on s e p a r a t e d m i n e r a l fractions f r o m t h r e e of the gabbro s a m p l e s give consistent plagioclase ages at 1980 m.y., which s e e m s to indicate the gabbro suffered a mild r e h e a t i n g at this time, which probably r e l a t e s to the intrusion of the Bushveld Complex. The pyroxene ages fall in the range 2670 to 3000 re.y., and this may r e p r e s e n t another instance of excess argon in pyroxenes. Alternatively the pyroxenes might still r e c o r d the date of intrusion of the gabbro, and this is consistent with the upper range given by the R b - S r age.
1. G E O L O G Y T h e M o d i p e g a b b r o l i e s s o m e 150 m i l e s w e s t of J o h a n n e s b u r g a n d i s e x p o s e d a s a l i n e of h i l l s 10 m i l e s l o n g a n d 4 m i l e s w i d e . H a l f of t h e e x p o sure lies in the Beehuanaland Protectorate and h a l f i n t h e T r a n s v a a l , S o u t h A f r i c a (fig. 1). J o n e s [1] r e p o r t s t h a t t h e g a b b r o t o t h e n o r t h w e s t h a s been intruded by the Gaberones granite; near the c o n t a c t t h e g a b b r o i s p e r m e a t e d b y a s e r i e s of thin microgranitic veinlets and the granite is contaminated and contains numerous inclusions of b a s i c m a t e r i a l . T o t h e n o r t h of M o d i p e a c o a r s e r a p a k i v i g r a n i t e g i v e s way to a f i n e g r a i n e d c h i l l p h a s e of t h e g r a n i t e a g a i n s t t h e gabbroic rocks although the actual contact is not exposed. Apart from its relationship to the Gaberones granite there is no other evidence for the a g e of t h e g a b b r o . The Gaberones granite has been divided into f o u r p l u t o n s [2] (fig. 1) a n d c o n s i s t s of a c e n t r a l r a p a k i v i t i c g r a n i t e s u r r o u n d e d by two r o u g h l y parallel border phases comprising an inner zone of r e d o r g r e y g r a n i t e a n d a n o u t e r z o n e of p o r p h y r i t i c g r a n o p h y r e s [3]. T h e g r a n i t e i s p r e V e n t e r s d o r p i n a g e [3], w h i c h p l a c e s i t b e t w e e n 2200 a n d 3000 m . y . [4]. E v a n s a n d M c E l h i n n y [5] h a v e m a d e a p a l a e o m a g n e t i c s t u d y of t h e M o d i p e g a b b r o a n d t h e a d j a c e n t G a b e r o n e s g r a n i t e p l u t o n . T h e p u r p o s e of
I --24°30"S
X /
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GA 1~1 GAlg65 [ O A l g ~
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X _ _ GABERONES~rL,,~,'-..2. ...~."
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AFRICA
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Fig. 1. Geological sketch map of the Modipe gabbro and the Gaberones granite, showing the location of the samples. * On leave f r o m Department of Physics, University College of Rhodesia, Salisbury, Rhodesia.
440
M.W. McE LHINNY
this i n v e s t i g a t i o n was to obtain the age of the Modipe gabbro to s u p p l e m e n t the p a l a e o m a g n e t i c m e a s u r e m e n t s and the six g a b b r o s a m p l e s anal y s e d f o r m e d p a r t of the p a l a e o m a g n e t i c c o l l e c tion. In addition two s a m p l e s of the g r a n i t e c o l l e c t e d f o r p a l a e o m a g n e t i c p u r p o s e s and a t h i r d c o l l e c t e d f o r this study w e r e a n a l y s e d to s e r v e as a ch eck on the gabbro age.
2. Rb-Sr MEASUREMENTS
2.1. Experimental procedures The s a m p l e s w e r e f i r s t p u r i f i e d using the cation exchange c o l u m n technique [6]. Rb and Sr c o n c e n t r a t i o n s w e r e d e t e r m i n e d by i s o t o p e dilution using a 87Rb and a m i x e d 8 6 S r - 8 4 S r spike r e s p e c t i v e l y . Owing to the low abundance of Rb in the gabbro, s a m p l e s of between 0.5 and 1.5 g w e r e used and e x c e s s i v e amounts of i r o n affected the p e r f o r m a n c e of the ion exchange c o l u m n s in the c a s e of the Rb s e p a r a t i o n . T h e s e aliquots w e r e t h e r e f o r e c o n v e r t e d back to p e r c h l o r a t e s a f t e r spiking and heated strongly to c o n v e r t m o s t of the iron and a l u m i n i u m to oxides. The a l k a l i s w e r e d i s s o l v e d in hot d e m i n e r a l i s e d w a t e r and the r e s i d u e obtained a f t e r e v a p o r a t i o n could then be e f f i ci en t l y s e p a r a t e d by the ion exchange p r o cess. Rb i s o t o p e r a t i o s w e r e m e a s u r e d with a 6-in., 90 ° m a s s s p e c t r o m e t e r (MS2-SG) using a t r i p l e f i l a m e n t s o u r c e , w h e r e a s the Sr isotope r a t i o s w e r e m e a s u r e d with a 12-in., 60 ° t r i p l e f i l a m e n t m a c h i n e (Nuclide) e m p l o y i n g switching of the a c c e l e r a t i n g v o l t ag e [7]. E x c e p t in the c a s e of s a m p l e GA1985, f r o m the G a b e r o n e s g r a n i t e , all 8 7 S r / 8 6 S r r a t i o s w e r e m e a s u r e d f r o m unspiked runs, and all Sr isotope r a t i o s w e r e n o r m a l i s e d to c o n f o r m with a 8 8 S r / 8 6 S r r a t i o of 8.340.
2.2. Modipe gabbro results The R b - S r total r o c k data f o r the Modipe gabb r o s a m p l e s a r e s u m m a r i z e d in table 1, and t h e s e w e r e a n a l y s e d using the usual i s o c h r o n method shown in fig. 2. It is e s t i m a t e d that a single 8 7 S r / 8 6 S r m e a s u r e m e n t f a l l s within +0.0006 of the t r u e value at the 95% confidence l e v e l [8], w h e r e a s the p r e c i s i o n of the 8 7 R b / 8 6 S r r a t i o s is about +1% at the 95% confidence l e v e l . The i s o c h r o n was a n a l y s e d s t a t i s t i c a l l y using a r e g r e s s i o n method [9], which a l l o w s f o r known experimental error in both 8 7 S r / 8 6 S r and 87Rb/86Sr and which weighs the data to allow f o r the n o n - u n i f o r m v a r i a n c e of 87Rb/86Sr. The r e g r e s s i o n line taking into account t h e s e i n s t r u m e n t a l v a r i a t i o n s has a m e a n s q u a r e of weighted
Table 1 Rb-Sr total rock results - Modipe gabbro. Normalised to 88Sr/86Sr = 8.340. Sample
Rb (ppm)
Sz (ppr
87Rb/86Sr
87Sr/86Sr
GA1980 GA1983 GA1984 GA2051 GA2052 GA2053
9.38 6.39 29.79 6.23 12.59 8.53
285 267 295. 266. 192 275
0.0944 0.0688 0.2907 0.0671 0.1882 0.0892
0.7051 0.7034 0.7120 0.7074 0.7074 0,7041
0.712
G~, 19B~ j . -
~
0.708
' ~ 2o52
0704
0700
~
I
I OJO
I
I O.L~O
]
I 030
Fig. 2. Rb-Sr isochron diagram for the Modipe gabbro total rock data. d e v i a t e s of 0.92, which i n d i c a t e s the data l i e on a s t r a i g h t line within the e x p e r i m e n t a l e r r o r . U s ing decay constant f o r 87Rb of X = 1.39 X 10 -11 y r -1, the c a l c u l a t e d age is 2630 + 470 m .y ., and the initial 8 7 S r / 8 6 S r r a t i o is 0.7010 + 0.0010, w h e r e the e r r o r s quoted a r e the 95% confidence intervals. The 470 m.y. u n c e r t a i n t y i n t e r v a l for the age is a c o n s e q u e n c e of the v e r y n a r r o w r a n g e and low v a l u e s of the R b / S r r a t i o s in the gabbro (table 1). P r e s e n t day 8 7 S r / 8 6 S r r a t i o s r a n g e f r o m 0.7034 to 0.7120, and to obtain an i s o c h r o n f r o m t h e s e s a m p l e s t h e r e f o r e made e x t r e m e d e m a n d s f r o m i n s t r u m e n t a l t e c h n i q u e s in the m e a s u r e ment of the 8 7 S r / 8 6 S r r a t i o s . The i n i t i al r a t i o is the s a m e as that obtained by Allsopp [10] f o r the G r e a t Dyke of Southern Rh o d esi a, which is of s i m i l a r age, and lends f u r t h e r support to the view that the i n i t i al r a t i o f o r a n c i e n t m a f i c r o c k s a p p r o a c h e s the a c h o n d r i t e value [11]. 2.3. Gaberones granite samples The R b - S r total r o c k data f o r the t h r e e g r a n ite s a m p l e s a r e s u m m a r i s e d in table 2, and the r e s u l t s a r e shown on the i s o c h r o n d i a g r a m in fig. 3. The r e g r e s s i o n line through t h e s e t h r e e points g i v e s an age of 2340 + 180 m.y. with initial r a t i o 0.714 + 0.031 at the 95% confidence level. The l a r g e u n c e r t a i n t y in the age a r i s e s f r o m the
441
Rb-Sr AND K-Ar AGE MEASUREMENTS ON THE MODIPE GABBRO
Table 2 R b - S r total rock r e s u l t s - Gaberones granite. Normalised to 88Sr/86Sr = 8.340. Rb Sr Sample (ppm) (ppm) 87Rb/86Sr
el, but the results are consistent with the known g e o l o g i c a l r e l a t i o n s h i p of t h e g r a n i t e to t h e g a b bro.
87Sr/86Sr 3. K - A r M E A S U R E M E N T S
GA1981 283.0 GA1985 426.9 GA1986 313.2
92.0 24.0 85.8
9.100 61.446 10.890
1.0108 2.7359* 1.0804
* Calculated value from spiked Sr run.
2C
~L t.c
]
I 20
I
] 4O
Rb87/Sr 86
I
el
Fig. 3. R b - S r isochron d i a g r a m for the Gaberones granite total rock data. u s e of a s t a t i s t i c a l a n a l y s i s w i t h o n l y t h r e e p o i n t s . It m a y b e m o r e r e a l i s t i c to u s e t h e d a t a f o r s a m p l e G A 1 9 8 5 a l o n e in w h i c h c a s e t h e a g e i s c o n s e r v a t i v e l y e s t i m a t e d a t 2340 ± 50 m . y . T h e s l o p e s of t h e t w o r e g r e s s i o n l i n e s f o r t h e gabbro and the granite cannot be shown to be s i g n i f i c a n t l y d i f f e r e n t a t t h e 95% c o n f i d e n c e l e v -
K-Ar age measurements were made on sepa r a t e d m i n e r a l f r a c t i o n s f r o m t h r e e of t h e g a b b r o s a m p l e s u s i n g t h e m e t h o d of M c D o u g a l l [12]. Ar isotope ratios were measured on a Reynoldstype mass spectrometer using the static method and employing an 38Ar spike. K concentrations w e r e d e t e r m i n e d by t h e f l a m e p h o t o m e t e r t e c h n i q u e [13] a n d t h e s e w e r e c h e c k e d f o r t h e p y r o x e n e s u s i n g t h e n e u t r o n a c t i v a t i o n m e t h o d [14]. The results are summarised in table 3 and it is g r a t i f y i n g t o n o t e t h e a g r e e m e n t b e t w e e n t h e two methods, which encourages the view that the flame photometer technique may be used for K c o n c e n t r a t i o n s a s l o w a s 150 p p m . The three plagioclase ages are consistently low and are incompatible with the Rb-Sr age, whereas the pyroxene ages are consistent with it. T h i s s u g g e s t s t h a t t h e g a b b r o m u s t h a v e s u f f e r e d a m i l d r e h e a t i n g 1980 m . y . ago. It i s s i g n i f i c a n t t h a t t h e n e a r e s t e x p o s u r e of t h e B u s h v e l d C o m p l e x , d a t e d a t 1970 + 70 m . y . [15], l i e s o n l y 10 m i l e s a w a y a n d i t i s p o s s i b l e t h a t t h e p l a g i o c l a s e a g e s r e c o r d t h e d a t e of i t s i n t r u s i o n . T h e r e a r e s e v e r a l e x p l a n a t i o n s of t h e p y r o x ene ages. They may represent another instance of e x c e s s a r g o n i n p y r o x e n e s f r o m r o c k s w h i c h have been deeply buried and thermally metamorp h o s e d [16, 17]. It i s s u r p r i s i n g t h a t t h e r a d i o -
Table 3 K-Ar results on separated minerals - Modipe gabbro.
Sample
Mineral
Potassium (%)
Flame photometry GA1980
Plagioclase
GA1980
Pyroxene
GA1983
Plagioclase
GA1983
Pyroxene
GA2051
Plagioclase
GA2051
Pyroxene
0.354 0.356
*40Ar ( s c c / g m × 1 0 -6)
0.355
0.0189 0.0217 0.0203 0.291 0.299
Neutron activation
0.0197 0.0198 0.0198
4.64
0.295
0.0199 0.0205 0.0206 0.0206
0.309 0.310
4.83
0.309
0.0156 0.0162 0.0169
0.0138 0.0142 0.0140
4.48
*40Ar/40K
Atm. A r (%)
Calculated age (m.y.)
0.2049
18.8
1981 i 20
0.3354 0.3438
37.6
2635 -~100 2670 ± 100
0.1900
4.9
1934± 30
0.3540 0.3440
35.3
2713 ± 100 2670 ± 100
0.2052
2.3
1983 ± 25
0.4059 0.4696
27.4
2912 ± 100 3130±100
442
M. W, McE LHINNY
g e n i e a r g o n p e r g r a m in t h e p y r o x e n e s i s a p proximately the same for all three samples, w h i c h l e a d s to t h e s u g g e s t i o n t h a t t h e p y r o x e n e s were "saturated" with argon for the particular pressure and temperature conditions prevailing a t t h e t i m e of i n t r u s i o n of t h e B u s h v e l d C o m p l e x . H o w e v e r , i n v i e w of t h e c o n s i s t e n c y b e t w e e n t h e p y r o x e n e a g e s a n d t h e R b - S r a g e , it i s p o s s i b l e t h a t t h e p y r o x e n e s s t i l l r e c o r d t h e t i m e of i n t r u s i o n of t h e g a b b r o . E i t h e r t h e two p y r o x e n e a g e s of 2670 m . y . f r o m GA1980 a n d G A 1 9 8 3 r e c o r d t h e true age and GA2051 contains excess argon for it a l s o h a s t h e l o w e s t K c o n t e n t , o r e l s e t h e a g e of 300D m . y . f r o m t h e G A 2 0 5 1 p y r o x e n e r e p r e s e n t s the true age and the others have suffered argon l o s s . On t h i s i n t e r p r e t a t i o n t h e R b - S r a g e m u s t b e r e g a r d e d a s a m i n i m u m e s t i m a t e a n d it i s s u g g e s t e d t h a t t h e a g e of t h e g a b b r o l i e s b e t w e e n t h e l i m i t s 2670 a n d 30D0 m . y . , w h i c h i n c l u d e s t h e upper range given by the Rb-Sr age.
ACKNOWLEDGEMENTS It i s a p l e a s u r e t o a c k n o w l e d g e t h e a s s i s t a n c e of D r . I. M c D o u g a l l w h o i n t r o d u c e d m e to t h e t e c h n i q u e s of K - A r d a t i n g . I w o u l d l i k e to t h a n k M r . J. A. C o o p e r w h o i n s t r u c t e d m e on t h e f l a m e photometer technique and Dr. J.W. Morgan who kindly carried out the neutron activation determ i n a t i o n s on t h e p y r o x e n e s . M r . I. I n g r a m h e l p e d w i t h t h e a r g o n e x t r a c t i o n . D r . P . A. A r r i e n s a n d M r . A. R. C r a w f o r d g a v e i n s t r u c t i o n on R b - S r m a s s s p e c t r o m e t r y a n d M r . M. J. V e r n o n h e l p e d m e w i t h t h e c h e m i c a l p u r i f i c a t i o n of t h e s a m p l e s . Their assistance is gratefully acknowledged. Mr. R. R u d o w s k i c a r r i e d o u t t h e m i n e r a l s e p a r a t i o n s . I wish to thank the Royal Society and Nuffield Foundation for a travel grant and the Australian National University for a Visiting Fellowship.
[2] A . P o l d e r v a a r t , The G a b e r o n e s granite, XIX Int. Geol. Cong. A l g i e r s , F a s c . XX (1952) 315. [3] E . P . W r i g h t , Geology of the G a b e r o n e s d i s t r i c t , Geol. Surv. Bechuanaland, R e c o r d s 1956 (1958) 12. [4] L.O. Nicolaysen, S t r a t i g r a p h i c i n t e r p r e t a t i o n of age m e a s u r e m e n t s in Southern Africa, Petrologic studies, Buddington Volume (Geol. Soc. A m e r , New York) (1962) 569. [5] M . E , E v a n s and M . W . M c E l h i n n y , The p a l e o m a g n e t i s m of the Modipe gabbro, J. Geophys. Res., in press. [6] W. Compston, J. F. Lovering and M. J. Vernon, The r u b i d i u m - s t r o n t i u m age of the Bishopville aubrite and its component enstatite andt'eldspar, Geochim. Cosmochim. Acta 29 (1965) 1085, [ 7] P. A. A r r i e n s and W. Compston, in p r e p a r a t i o n , [8] I.McDougall and W. Compston, Strontium isotope composition and p o t a s s i u m - r u b i d i u m r a t i o s in some r o c k s from Reunion and Rodriguez, Indian Ocean, Nature 207 (1965) 252. [9] G.A. McIntyre, C . B r o o k s , W. Compston and A. Turek, The s t a t i s t i c a l a s s e s s m e n t of R b - S r isochrons, J. Geophys. Res., in p r e s s . [10] H . L . A l l s o p p , R b - S r and K-Ar age m e a s u r e m e n t s on the G r e a t Dyke of Southern Rhodesia, J . G e o phys. Res. 70 (1965) 977. [11] C . E . H e d g e and F . G . W a l t h a l l , Radiogenic s t r o n tium-87 as an index to geological p r o c e s s e s , Science 140 (1963) 1214. [12] I.McDougall, P o t a s s i u m - a r g o n age m e a s u r e m e n t s on d o l e r i t e s from A n t a r c t i c a and South Africa, J. Geophys.Res. 68 (1963) 1535. [13] J , A . Cooper, The flame photometric d e t e r m i n a t i o n of potassium in geological m a t e r i a l s used for pot a s s i u m - a r g o n dating, Geochim. C o s m o c h i m . A c t a 27 (1963) 525. [14] J.W. Morgan and A. D. T. Goode, Potassium abundances in some ultrabasic and basic rocks, Earth Planet. Sci. Letters 1 (1966) 110. [15] L.O. Nicolaysen, J.W.L.de Villiers, A.J.Burger and F.W.E.Strelow, New measurements relating to the absolute age of the Transvaal System and the Bushveld Igneous Complex, Trans. Geol. Soc. S.Africa 61 (1958) 137. [16] S.R. Hart and R. T. Dodd, Excess radiogenic argon in pyroxenes, J.Geophys.Res.67 (1962) 2998. [17] I.McDougall and D.H.Green, Excess radiogenic argon in pyroxenes and isotopic ages on minerals from Norwegian eclogites, Norsk Geol. Tidsskr. 44
(1964) 183. RE FERENCES [1] M . T . J o n e s , The geology of the a r e a to the east of Gaberones, Geol. Surv. Bechuanaland, R e c o r d s 1959/1960 (1963) 17.
AMSTERDAI~
R b - S r A N D K - A t AGE M E A S U R E M E N T S O N T H E M O D I P E G A B B R O OF BECHUANALAND AND SOUTH AFRICA M. W. McELHINNY
*
Department of Geophysics and Geochemistry, Australian National University, Canberra, A. C. T. Received 6 S e p t e m b e r 1966
R b - S r total rock m e a s u r e m e n t s a r e r e p o r t e d for six s a m p l e s of the Modipe gabbro. Although the rubidium concentration in the samples is low, the points on the isochron d i a g r a m lie on a straight tree within the e x p e r i m e n t a l e r r o r and an age of 2630 ± 470 m.y. is obtained with initial r a t i o 0.7010 ± 0,0010. The gabbro is intruded by the Gaberones granite and total rock m e a s u r e m e n t s on t h r e e samples of the granite give an age of 2340 * 50 m.y., which although not significantly different from the gabbro age is consistent with the known geological relationship of t h e m . K - A r m e a s u r e m e n t s on s e p a r a t e d m i n e r a l fractions f r o m t h r e e of the gabbro s a m p l e s give consistent plagioclase ages at 1980 m.y., which s e e m s to indicate the gabbro suffered a mild r e h e a t i n g at this time, which probably r e l a t e s to the intrusion of the Bushveld Complex. The pyroxene ages fall in the range 2670 to 3000 re.y., and this may r e p r e s e n t another instance of excess argon in pyroxenes. Alternatively the pyroxenes might still r e c o r d the date of intrusion of the gabbro, and this is consistent with the upper range given by the R b - S r age.
1. G E O L O G Y T h e M o d i p e g a b b r o l i e s s o m e 150 m i l e s w e s t of J o h a n n e s b u r g a n d i s e x p o s e d a s a l i n e of h i l l s 10 m i l e s l o n g a n d 4 m i l e s w i d e . H a l f of t h e e x p o sure lies in the Beehuanaland Protectorate and h a l f i n t h e T r a n s v a a l , S o u t h A f r i c a (fig. 1). J o n e s [1] r e p o r t s t h a t t h e g a b b r o t o t h e n o r t h w e s t h a s been intruded by the Gaberones granite; near the c o n t a c t t h e g a b b r o i s p e r m e a t e d b y a s e r i e s of thin microgranitic veinlets and the granite is contaminated and contains numerous inclusions of b a s i c m a t e r i a l . T o t h e n o r t h of M o d i p e a c o a r s e r a p a k i v i g r a n i t e g i v e s way to a f i n e g r a i n e d c h i l l p h a s e of t h e g r a n i t e a g a i n s t t h e gabbroic rocks although the actual contact is not exposed. Apart from its relationship to the Gaberones granite there is no other evidence for the a g e of t h e g a b b r o . The Gaberones granite has been divided into f o u r p l u t o n s [2] (fig. 1) a n d c o n s i s t s of a c e n t r a l r a p a k i v i t i c g r a n i t e s u r r o u n d e d by two r o u g h l y parallel border phases comprising an inner zone of r e d o r g r e y g r a n i t e a n d a n o u t e r z o n e of p o r p h y r i t i c g r a n o p h y r e s [3]. T h e g r a n i t e i s p r e V e n t e r s d o r p i n a g e [3], w h i c h p l a c e s i t b e t w e e n 2200 a n d 3000 m . y . [4]. E v a n s a n d M c E l h i n n y [5] h a v e m a d e a p a l a e o m a g n e t i c s t u d y of t h e M o d i p e g a b b r o a n d t h e a d j a c e n t G a b e r o n e s g r a n i t e p l u t o n . T h e p u r p o s e of
I --24°30"S
X /
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GA 1~1 GAlg65 [ O A l g ~
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Fig. 1. Geological sketch map of the Modipe gabbro and the Gaberones granite, showing the location of the samples. * On leave f r o m Department of Physics, University College of Rhodesia, Salisbury, Rhodesia.
440
M.W. McE LHINNY
this i n v e s t i g a t i o n was to obtain the age of the Modipe gabbro to s u p p l e m e n t the p a l a e o m a g n e t i c m e a s u r e m e n t s and the six g a b b r o s a m p l e s anal y s e d f o r m e d p a r t of the p a l a e o m a g n e t i c c o l l e c tion. In addition two s a m p l e s of the g r a n i t e c o l l e c t e d f o r p a l a e o m a g n e t i c p u r p o s e s and a t h i r d c o l l e c t e d f o r this study w e r e a n a l y s e d to s e r v e as a ch eck on the gabbro age.
2. Rb-Sr MEASUREMENTS
2.1. Experimental procedures The s a m p l e s w e r e f i r s t p u r i f i e d using the cation exchange c o l u m n technique [6]. Rb and Sr c o n c e n t r a t i o n s w e r e d e t e r m i n e d by i s o t o p e dilution using a 87Rb and a m i x e d 8 6 S r - 8 4 S r spike r e s p e c t i v e l y . Owing to the low abundance of Rb in the gabbro, s a m p l e s of between 0.5 and 1.5 g w e r e used and e x c e s s i v e amounts of i r o n affected the p e r f o r m a n c e of the ion exchange c o l u m n s in the c a s e of the Rb s e p a r a t i o n . T h e s e aliquots w e r e t h e r e f o r e c o n v e r t e d back to p e r c h l o r a t e s a f t e r spiking and heated strongly to c o n v e r t m o s t of the iron and a l u m i n i u m to oxides. The a l k a l i s w e r e d i s s o l v e d in hot d e m i n e r a l i s e d w a t e r and the r e s i d u e obtained a f t e r e v a p o r a t i o n could then be e f f i ci en t l y s e p a r a t e d by the ion exchange p r o cess. Rb i s o t o p e r a t i o s w e r e m e a s u r e d with a 6-in., 90 ° m a s s s p e c t r o m e t e r (MS2-SG) using a t r i p l e f i l a m e n t s o u r c e , w h e r e a s the Sr isotope r a t i o s w e r e m e a s u r e d with a 12-in., 60 ° t r i p l e f i l a m e n t m a c h i n e (Nuclide) e m p l o y i n g switching of the a c c e l e r a t i n g v o l t ag e [7]. E x c e p t in the c a s e of s a m p l e GA1985, f r o m the G a b e r o n e s g r a n i t e , all 8 7 S r / 8 6 S r r a t i o s w e r e m e a s u r e d f r o m unspiked runs, and all Sr isotope r a t i o s w e r e n o r m a l i s e d to c o n f o r m with a 8 8 S r / 8 6 S r r a t i o of 8.340.
2.2. Modipe gabbro results The R b - S r total r o c k data f o r the Modipe gabb r o s a m p l e s a r e s u m m a r i z e d in table 1, and t h e s e w e r e a n a l y s e d using the usual i s o c h r o n method shown in fig. 2. It is e s t i m a t e d that a single 8 7 S r / 8 6 S r m e a s u r e m e n t f a l l s within +0.0006 of the t r u e value at the 95% confidence l e v e l [8], w h e r e a s the p r e c i s i o n of the 8 7 R b / 8 6 S r r a t i o s is about +1% at the 95% confidence l e v e l . The i s o c h r o n was a n a l y s e d s t a t i s t i c a l l y using a r e g r e s s i o n method [9], which a l l o w s f o r known experimental error in both 8 7 S r / 8 6 S r and 87Rb/86Sr and which weighs the data to allow f o r the n o n - u n i f o r m v a r i a n c e of 87Rb/86Sr. The r e g r e s s i o n line taking into account t h e s e i n s t r u m e n t a l v a r i a t i o n s has a m e a n s q u a r e of weighted
Table 1 Rb-Sr total rock results - Modipe gabbro. Normalised to 88Sr/86Sr = 8.340. Sample
Rb (ppm)
Sz (ppr
87Rb/86Sr
87Sr/86Sr
GA1980 GA1983 GA1984 GA2051 GA2052 GA2053
9.38 6.39 29.79 6.23 12.59 8.53
285 267 295. 266. 192 275
0.0944 0.0688 0.2907 0.0671 0.1882 0.0892
0.7051 0.7034 0.7120 0.7074 0.7074 0,7041
0.712
G~, 19B~ j . -
~
0.708
' ~ 2o52
0704
0700
~
I
I OJO
I
I O.L~O
]
I 030
Fig. 2. Rb-Sr isochron diagram for the Modipe gabbro total rock data. d e v i a t e s of 0.92, which i n d i c a t e s the data l i e on a s t r a i g h t line within the e x p e r i m e n t a l e r r o r . U s ing decay constant f o r 87Rb of X = 1.39 X 10 -11 y r -1, the c a l c u l a t e d age is 2630 + 470 m .y ., and the initial 8 7 S r / 8 6 S r r a t i o is 0.7010 + 0.0010, w h e r e the e r r o r s quoted a r e the 95% confidence intervals. The 470 m.y. u n c e r t a i n t y i n t e r v a l for the age is a c o n s e q u e n c e of the v e r y n a r r o w r a n g e and low v a l u e s of the R b / S r r a t i o s in the gabbro (table 1). P r e s e n t day 8 7 S r / 8 6 S r r a t i o s r a n g e f r o m 0.7034 to 0.7120, and to obtain an i s o c h r o n f r o m t h e s e s a m p l e s t h e r e f o r e made e x t r e m e d e m a n d s f r o m i n s t r u m e n t a l t e c h n i q u e s in the m e a s u r e ment of the 8 7 S r / 8 6 S r r a t i o s . The i n i t i al r a t i o is the s a m e as that obtained by Allsopp [10] f o r the G r e a t Dyke of Southern Rh o d esi a, which is of s i m i l a r age, and lends f u r t h e r support to the view that the i n i t i al r a t i o f o r a n c i e n t m a f i c r o c k s a p p r o a c h e s the a c h o n d r i t e value [11]. 2.3. Gaberones granite samples The R b - S r total r o c k data f o r the t h r e e g r a n ite s a m p l e s a r e s u m m a r i s e d in table 2, and the r e s u l t s a r e shown on the i s o c h r o n d i a g r a m in fig. 3. The r e g r e s s i o n line through t h e s e t h r e e points g i v e s an age of 2340 + 180 m.y. with initial r a t i o 0.714 + 0.031 at the 95% confidence level. The l a r g e u n c e r t a i n t y in the age a r i s e s f r o m the
441
Rb-Sr AND K-Ar AGE MEASUREMENTS ON THE MODIPE GABBRO
Table 2 R b - S r total rock r e s u l t s - Gaberones granite. Normalised to 88Sr/86Sr = 8.340. Rb Sr Sample (ppm) (ppm) 87Rb/86Sr
el, but the results are consistent with the known g e o l o g i c a l r e l a t i o n s h i p of t h e g r a n i t e to t h e g a b bro.
87Sr/86Sr 3. K - A r M E A S U R E M E N T S
GA1981 283.0 GA1985 426.9 GA1986 313.2
92.0 24.0 85.8
9.100 61.446 10.890
1.0108 2.7359* 1.0804
* Calculated value from spiked Sr run.
2C
~L t.c
]
I 20
I
] 4O
Rb87/Sr 86
I
el
Fig. 3. R b - S r isochron d i a g r a m for the Gaberones granite total rock data. u s e of a s t a t i s t i c a l a n a l y s i s w i t h o n l y t h r e e p o i n t s . It m a y b e m o r e r e a l i s t i c to u s e t h e d a t a f o r s a m p l e G A 1 9 8 5 a l o n e in w h i c h c a s e t h e a g e i s c o n s e r v a t i v e l y e s t i m a t e d a t 2340 ± 50 m . y . T h e s l o p e s of t h e t w o r e g r e s s i o n l i n e s f o r t h e gabbro and the granite cannot be shown to be s i g n i f i c a n t l y d i f f e r e n t a t t h e 95% c o n f i d e n c e l e v -
K-Ar age measurements were made on sepa r a t e d m i n e r a l f r a c t i o n s f r o m t h r e e of t h e g a b b r o s a m p l e s u s i n g t h e m e t h o d of M c D o u g a l l [12]. Ar isotope ratios were measured on a Reynoldstype mass spectrometer using the static method and employing an 38Ar spike. K concentrations w e r e d e t e r m i n e d by t h e f l a m e p h o t o m e t e r t e c h n i q u e [13] a n d t h e s e w e r e c h e c k e d f o r t h e p y r o x e n e s u s i n g t h e n e u t r o n a c t i v a t i o n m e t h o d [14]. The results are summarised in table 3 and it is g r a t i f y i n g t o n o t e t h e a g r e e m e n t b e t w e e n t h e two methods, which encourages the view that the flame photometer technique may be used for K c o n c e n t r a t i o n s a s l o w a s 150 p p m . The three plagioclase ages are consistently low and are incompatible with the Rb-Sr age, whereas the pyroxene ages are consistent with it. T h i s s u g g e s t s t h a t t h e g a b b r o m u s t h a v e s u f f e r e d a m i l d r e h e a t i n g 1980 m . y . ago. It i s s i g n i f i c a n t t h a t t h e n e a r e s t e x p o s u r e of t h e B u s h v e l d C o m p l e x , d a t e d a t 1970 + 70 m . y . [15], l i e s o n l y 10 m i l e s a w a y a n d i t i s p o s s i b l e t h a t t h e p l a g i o c l a s e a g e s r e c o r d t h e d a t e of i t s i n t r u s i o n . T h e r e a r e s e v e r a l e x p l a n a t i o n s of t h e p y r o x ene ages. They may represent another instance of e x c e s s a r g o n i n p y r o x e n e s f r o m r o c k s w h i c h have been deeply buried and thermally metamorp h o s e d [16, 17]. It i s s u r p r i s i n g t h a t t h e r a d i o -
Table 3 K-Ar results on separated minerals - Modipe gabbro.
Sample
Mineral
Potassium (%)
Flame photometry GA1980
Plagioclase
GA1980
Pyroxene
GA1983
Plagioclase
GA1983
Pyroxene
GA2051
Plagioclase
GA2051
Pyroxene
0.354 0.356
*40Ar ( s c c / g m × 1 0 -6)
0.355
0.0189 0.0217 0.0203 0.291 0.299
Neutron activation
0.0197 0.0198 0.0198
4.64
0.295
0.0199 0.0205 0.0206 0.0206
0.309 0.310
4.83
0.309
0.0156 0.0162 0.0169
0.0138 0.0142 0.0140
4.48
*40Ar/40K
Atm. A r (%)
Calculated age (m.y.)
0.2049
18.8
1981 i 20
0.3354 0.3438
37.6
2635 -~100 2670 ± 100
0.1900
4.9
1934± 30
0.3540 0.3440
35.3
2713 ± 100 2670 ± 100
0.2052
2.3
1983 ± 25
0.4059 0.4696
27.4
2912 ± 100 3130±100
442
M. W, McE LHINNY
g e n i e a r g o n p e r g r a m in t h e p y r o x e n e s i s a p proximately the same for all three samples, w h i c h l e a d s to t h e s u g g e s t i o n t h a t t h e p y r o x e n e s were "saturated" with argon for the particular pressure and temperature conditions prevailing a t t h e t i m e of i n t r u s i o n of t h e B u s h v e l d C o m p l e x . H o w e v e r , i n v i e w of t h e c o n s i s t e n c y b e t w e e n t h e p y r o x e n e a g e s a n d t h e R b - S r a g e , it i s p o s s i b l e t h a t t h e p y r o x e n e s s t i l l r e c o r d t h e t i m e of i n t r u s i o n of t h e g a b b r o . E i t h e r t h e two p y r o x e n e a g e s of 2670 m . y . f r o m GA1980 a n d G A 1 9 8 3 r e c o r d t h e true age and GA2051 contains excess argon for it a l s o h a s t h e l o w e s t K c o n t e n t , o r e l s e t h e a g e of 300D m . y . f r o m t h e G A 2 0 5 1 p y r o x e n e r e p r e s e n t s the true age and the others have suffered argon l o s s . On t h i s i n t e r p r e t a t i o n t h e R b - S r a g e m u s t b e r e g a r d e d a s a m i n i m u m e s t i m a t e a n d it i s s u g g e s t e d t h a t t h e a g e of t h e g a b b r o l i e s b e t w e e n t h e l i m i t s 2670 a n d 30D0 m . y . , w h i c h i n c l u d e s t h e upper range given by the Rb-Sr age.
ACKNOWLEDGEMENTS It i s a p l e a s u r e t o a c k n o w l e d g e t h e a s s i s t a n c e of D r . I. M c D o u g a l l w h o i n t r o d u c e d m e to t h e t e c h n i q u e s of K - A r d a t i n g . I w o u l d l i k e to t h a n k M r . J. A. C o o p e r w h o i n s t r u c t e d m e on t h e f l a m e photometer technique and Dr. J.W. Morgan who kindly carried out the neutron activation determ i n a t i o n s on t h e p y r o x e n e s . M r . I. I n g r a m h e l p e d w i t h t h e a r g o n e x t r a c t i o n . D r . P . A. A r r i e n s a n d M r . A. R. C r a w f o r d g a v e i n s t r u c t i o n on R b - S r m a s s s p e c t r o m e t r y a n d M r . M. J. V e r n o n h e l p e d m e w i t h t h e c h e m i c a l p u r i f i c a t i o n of t h e s a m p l e s . Their assistance is gratefully acknowledged. Mr. R. R u d o w s k i c a r r i e d o u t t h e m i n e r a l s e p a r a t i o n s . I wish to thank the Royal Society and Nuffield Foundation for a travel grant and the Australian National University for a Visiting Fellowship.
[2] A . P o l d e r v a a r t , The G a b e r o n e s granite, XIX Int. Geol. Cong. A l g i e r s , F a s c . XX (1952) 315. [3] E . P . W r i g h t , Geology of the G a b e r o n e s d i s t r i c t , Geol. Surv. Bechuanaland, R e c o r d s 1956 (1958) 12. [4] L.O. Nicolaysen, S t r a t i g r a p h i c i n t e r p r e t a t i o n of age m e a s u r e m e n t s in Southern Africa, Petrologic studies, Buddington Volume (Geol. Soc. A m e r , New York) (1962) 569. [5] M . E , E v a n s and M . W . M c E l h i n n y , The p a l e o m a g n e t i s m of the Modipe gabbro, J. Geophys. Res., in press. [6] W. Compston, J. F. Lovering and M. J. Vernon, The r u b i d i u m - s t r o n t i u m age of the Bishopville aubrite and its component enstatite andt'eldspar, Geochim. Cosmochim. Acta 29 (1965) 1085, [ 7] P. A. A r r i e n s and W. Compston, in p r e p a r a t i o n , [8] I.McDougall and W. Compston, Strontium isotope composition and p o t a s s i u m - r u b i d i u m r a t i o s in some r o c k s from Reunion and Rodriguez, Indian Ocean, Nature 207 (1965) 252. [9] G.A. McIntyre, C . B r o o k s , W. Compston and A. Turek, The s t a t i s t i c a l a s s e s s m e n t of R b - S r isochrons, J. Geophys. Res., in p r e s s . [10] H . L . A l l s o p p , R b - S r and K-Ar age m e a s u r e m e n t s on the G r e a t Dyke of Southern Rhodesia, J . G e o phys. Res. 70 (1965) 977. [11] C . E . H e d g e and F . G . W a l t h a l l , Radiogenic s t r o n tium-87 as an index to geological p r o c e s s e s , Science 140 (1963) 1214. [12] I.McDougall, P o t a s s i u m - a r g o n age m e a s u r e m e n t s on d o l e r i t e s from A n t a r c t i c a and South Africa, J. Geophys.Res. 68 (1963) 1535. [13] J , A . Cooper, The flame photometric d e t e r m i n a t i o n of potassium in geological m a t e r i a l s used for pot a s s i u m - a r g o n dating, Geochim. C o s m o c h i m . A c t a 27 (1963) 525. [14] J.W. Morgan and A. D. T. Goode, Potassium abundances in some ultrabasic and basic rocks, Earth Planet. Sci. Letters 1 (1966) 110. [15] L.O. Nicolaysen, J.W.L.de Villiers, A.J.Burger and F.W.E.Strelow, New measurements relating to the absolute age of the Transvaal System and the Bushveld Igneous Complex, Trans. Geol. Soc. S.Africa 61 (1958) 137. [16] S.R. Hart and R. T. Dodd, Excess radiogenic argon in pyroxenes, J.Geophys.Res.67 (1962) 2998. [17] I.McDougall and D.H.Green, Excess radiogenic argon in pyroxenes and isotopic ages on minerals from Norwegian eclogites, Norsk Geol. Tidsskr. 44
(1964) 183. RE FERENCES [1] M . T . J o n e s , The geology of the a r e a to the east of Gaberones, Geol. Surv. Bechuanaland, R e c o r d s 1959/1960 (1963) 17.