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Lead isotopic study of aplites from the Precambrian basement rocks near Ibadan, Southwestern Nigeria
Earth and Planetary Science Letters, 27 (1975) 177-180
© Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
LEd
LEAD ISOTOPIC STUDY OF APLITES FROM THE PRECAMBRIAN BASEMENT ROCKS NEAR IBADAN, SOUTHWESTERN NIGERIA V.M. OVERSBY Research School o f Earth Sciences, Australian National University, Canberra, A.C.T. (.4 ustralia)
Received April 8, 1975 Revised version received June 16, 1975
Pb isotopic compositions for three total-rock samples of aplite and their constituent K-feldspars from the Nigerian basement assemblage near Ibadan show lead homogenization during the Pan-African thermo-tectonic event. A secondary isochron formed by the K-feldspars data points is used to calculate a primary age of about 2750 m.y. for the aplites. The aplites do not register any Pb isotopic effect from the intrusion of granite gneiss in the area at 2330 m.y.
1. Introduction
2. Analytical methods
The Precambrian basement rocks of southwestern Nigeria were studied by Grant [ 1] using whole-rock R b - S r isochron methods. The area consists of banded gneisses and refolded quartzites into which a grossly concordant granite gneiss has been emplaced. The banded gneiss contains an aplite sheet which is folded and semi-concordant to the banded gneiss. Grant [1 ] obtained a whole-rock R b - S r isochron age of 2330 -+70 m.y. (~,Rb = 1.39 X 10 -1I yr -x) for the granite gneiss. K - A r ages on biotite and amphibole from the granite gneiss gave ages of 481 -+ 19 m.y. and 499 + 20 m.y. [1 ] showing response of the area to the Pan-African thermo-tectonic event described by Kennedy [2]. Data for the aplite sheet and banded gneiss were inconclusive, but suggested that these units might be significantly older than the granite gneiss. Grant [ 1] postulated that the banded gneiss and aplite might have formed during the 2800 -+ 200-m.y. Liberian cycle. This study presents Pb isotopic analyses on three aplite total rock samples and their K-feldspars and one banded gneiss total rock. All of the samples were collected by N.K. Grant from the quarry at Ojo rock, 6 miles north of Ibadan on the lbadan-Oyo road (3°50'E, 7°29'N). Two of the aplites were included in Grant's original R b - S r study.
Rock chips weighing approximately 30 g were cleaned ultrasonically with 2N HC1 to remove surface contamination, and rinsed several times with triple distilled water. Samples were crushed to provide a representative total-rock sample and then sized for mineral separation of K-feldspar. Chemical separation of Pb and U are described in Oversby [3]. Pb isotopic compositions were determined by silica gel mass spectrometry with double spike fractionation control of all samples except 68BG65. Precision of analyses is better than 0.1% (20) for all ratios (cf. Oversby [3], for data on standard samples and duplicate analyses). Chemical blanks were 0.5 ng per analysis for U and 5.1 ng for Pb.
3. Results and discussion Pb isotopic data and Pb and U concentrations are given in Table 1. Samples marked AWKF are K-feldspars which were treated with hot 6N HC1 for 20 minutes followed by hot NHO3 for 20 minutes before dissolution. The banded gneiss sample is 68BG65; the other samples are aplites. The Pb isotopic data are plotted as 2°TPb/2°4Pb vs. 2°6pb/2°4pb in Fig. 1. The K-feldspar isotopic compositions are all extremely radiogenic and enriched in 2°~Pb relative to
178 TABLE 1 Ibadan, Nigeria Sample No. 2°8pb/2°6pb
2°Tpb/2°6pb
2°6pb/2°4pb
2°TPb/2°4pb 2°spb/2°apb Pb(ppm) U(ppm)
U(238U/2°4pb)
68BG62 TR1 TR2 KF
: 2.1624 2.1618 2.1783
0.81075 0.80931 0.88368
19.480 19.469 17.708
15.794 15.757 15.648
42.125 42.091 38.573
24.47 24.21 52.47
8.75 <0.002
24.22 0
68BG67 TR KF AWKF
2.1416 2.1508 2.1462
0.79282 0.81991 0.82265
20.207 19.564 19.475
16.020 16.041 16.021
43.276 42.079 41.799
40.52 79.99 83.36
4.38 0.718 0.326
7.52 0.610 0.264
68BG66 TR KF AWKF
2.1312 2.1430 2.1360
0.80971 0.83964 0.84216
19.628 18.939 18.933
15.893 15.902 15.945
41.831 40.588 40.442
39.48 66.87 67.73
4.54 0.790 0.202
7.79 0.780 0.196
68BG65 TR
2.2403
0.90220
17.334
15.639
38.833
n.d.
0.496
On this ground alone, it is unlikely that their isotopic composition could represent a lead that was in existence 2330 m.y. ago at the time of granite gneiss emplacement. The linear array of radiogenic K-feldspar compositions suggests that some metamorphic redistri2°6pb.
IBADAN, Nigeria 16'1
bution of Pb occurred similar to that discussed by Rosholt et al. [4]. Confirmation of a metamorphic event is found by plotting the 2°6pb/2°4Pb ratios against 238U/2°4pb (Fig. 2). U - P b ages calculated for the individual aplites are 68BG62, 452 m.y.; 68BG66, 588 m.y.;68BG67, 592 m.y. Whole-rock samples are frequently found to
~,~,J 210
0[] 68BG 68BG 62 67 O 68BG66 ~. 68BG65
£ ) o ~ O ~
ii 20 0
159
j~
pb 207 ' pb ~ 02~ 4
190
15"7
15"517'0
1810
l 19"0 pb 206
201.0
J
21I' 0
V,
Fig. 1. Pb isotopic compositions for total rocks and K-feldspars from Ibadan, Nigeria. The regression line shown is for four Kfeldspar points. Solid points are total-rock data; open points are K-feldspars.
lr.o
J
J
5!0 - -
J
J
10Lo
15L. . . .
10
250
U 238
Fig. 2. U-Pb isochron plot for aplite total rocks and K-feldspars Symbols as for Fig. 1.
179
have lost U due to recent weathering [4,5]. However, this is not always the case. Farquharson and Richards [6] obtained concordant R b - S r and U - P b ages on the Sybella microgranite, and Rosholt et al. [7] found concordant R b - S r and U-Pb ages for a granite from Saskatchewan. The general agreement of the aplite U - P b ages with the K - A r measurements of the PanAfrican event at Ibadan [1] suggests that we are measuring a true time of Pb homogenization. Assuming that the U - P b ages reflect the Pan-African event, two explanations are possible for the range of ages found. The true time of metamorphism could be 452 m.y. for all three samples, and 68BG66 and 68BG67 would then have lost about 25% of their U in some recent weathering episode. The agreement between their ages makes this interpretation unattractive. Alternatively, 68BG66 and 68BG67 could have become closed systems to Pb diffusion at a higher temperature than 68BG62, and its younger age merely represents a later stage in the cooling subsequent to the PanAfrican event. As will be shown below, the exact time of the metamorphic redistribution of Pb makes very little difference in the calculation of a primary age for the aplite. In order to further test the age of metamorphism, the total-rock and feldspar compositions were calculated back in time using the measured U - P b age and measured ~t values for each sample. The nature of the calculation requires that the 2°6pb/2°4Pb ratios of K-feldspars and total rocks agree; however, any large discrepancy in metamorphic age would appear as a systematic difference in Z°VPb/Z°4pb. The "age corrected" ratios are given in Table 2 and plotted in Fig. 3. It is clear that use of a different metamorphic age would not improve the agreement of the age corrected compositions. The difference in :°TPb/ 2°4pb for age corrected ratios is about twice the experimental error, perhaps indicating a slight lack of equilibration during the Pan-African event. A secondary isochron has been drawn through four of the K-feldspar points, excluding 68BG66 AWKF which falls clearly above the perfectly fitted line formed by the other four points. If a metamorphic age is assumed, we may calculate the primary age of the feldspars from the slope of the secondary isochron (cf. Rosholt et al. [4] for equations and theoretical discussion). If we take 590 m.y. as the metamorphic age, the primary age of the feldspars (and by necessity,
TABLE 2 Pb isotopic compositions corrected for last stage of growth using measured/~ and 238U/2°4pb-2°6pb/2°aPb age Sample No.
(2°6pb/2°4pb)*
(2°Tpb/2°4pb)*
68BG62 TR KF
17.717 17.708
15.677 15.648
68BG67 TR KF AWKF
19.487 19.506 19.450
15.977 16.038 16.019
68BG66 TR KF AWKF
18.882 18.864 18.914
15.849 15.898 15.944
the aplites) would be 2710 m.y. If the metamorphic age is taken as 450 m.y., the primary age would be 2775 m.y. Inclusion of the 68BG66 AWKF point would slightly increase the slope of the feldspar line and the calculated primary age. The Pb isotope data thus support the suggestion by Grant [1] that the Nigerian basement was formed during the 2800 + 200 m.y. Liberian cycle. The distribution of Pb and U during subsequent events can be summarized as follows. The aplites were emplaced into the banded gneiss prior to 2750 m.y., and then metamorphosed at about 2750 m.y. The degree of metamolphism was sufficient to totally equilibrate Pb
16'1
15"9 p b 207
15'7
15'5 17.5
I
J 18-5
I
I 19.5
V"
p b 206
Fig. 3. Pb isotopic data corrected for radiogenic increase since the time determined from their U - P b ages. Solid points are total-rock data; open points are K-feldspars.
180 isotopes within the aplites. Thus, at 2750 m.y. the aplites began a stage of Pb isotopic evolution starting from a single homogeneous isotopic composition. At 2330 m.y., according to R b - S r data, the granite gneiss was emplaced [ 1]. Internal equilibration o f Pb between aplite whole rocks and feldspars is permitted at this time only if the whole rocks remained closed systems. In particular, the whole rocks must retain their U/Pb ratios without change. This seems a highly unlikely event, in view of the changes in whole-rock U/Pb ratios during the Pan-African event. It seems more likely that the U - P b systems of the rocks and minerals remained closed systems from 2750 m.y. until the Pan-African event. At about 590 m.y., the aplites were subjected to the Pan-African thermo-tectonic event. Each wholerock sample equilibrated isotopically with its enclosed K-feldspar. The whole rocks did not equilibrate with each other. Thus, the K-feldspars preserve a picture of the whole-rock Pb isotopes 590 m.y. ago. This picture will not change between 590 m.y. and the present, since the K-feldspars have very low U/Pb ratios. The K-feldspar data points, therefore, define an isochron for lead isotopic growth starting at 2750 m.y. and ending at 590 m.y. After the Pan-African event each aplite remained a closed system with its own unique "initial" Pb isotopic composition. The rock with the highest U/Pb ratio after the Pan-African event contains the least radiogenic K-feldspar lead. Thus, it must have had the lowest U/Pb ratio from 2750 m.y. to 590 m.y. This clearly shows change of U/Pb ratios during the PanAfrican event. This strongly suggests that the U - P b system was undisturbed by the 2330-m.y. granite gneiss intrusion, since no change in U/Pb ratios occurred then. The R b - S r system shows a quite different
response. The granite gneiss samples appear to be unaffected by the Pan-African event. The reason for this difference in response is not understood.
Acknowledgements I am grateful to Dr. N.K. Grant for supplying the samples used in this study, to Mrs. R. Maier who performed the chemical separations, and to Mr. R. Rudowski for mineral separations. On line computer acquisition and reduction of data was developed by Dr. P.A. Arriens.
References 1 N.K. Grant, Geochronology of Precambrian basement rocks from Ibadan, Southwestern Nigeria, Earth Planet. Sci. Lett. 10 (1970) 29. 2 W.Q. Kennedy, The structural differentiation of Africa in the Pan-African (±500 m.y.) tectonic episode, Res. Inst. Afr. Geol. (Leeds, U.K.), 8th Ann. Rept. (1964) 48. 3 V.M. Oversby, Lead isotopic study of acid igneous rocks from the Pilbara, Western Australia, submitted to Geochim. Cosmochim. Acta. 4 J.N. Rosholt, R.E. Zartman and I.T. Nkomo, Lead isotopic systematics and uranium depletion in the Granite Mountains, Wyoming, Geol. Soc. Am. Bull. 84 (1973) 989. 5 V.M. Oversby, Lead isotopic systematics and ages of Archaean acid intrusions in the Kalgoorlie-Norseman area, Western Australia, Geochim. Cosmochim. Acta 39 (1975) in press. 6 R.B. Farquharson and J,R. Richards, Whole-rock U - T h - P b and Rb-Sr ages of the 8ybella microgranite and pegmatite, Mount lsa, Queensland, J. Geol. Soc. Aust. 17 (1970) 53. 7 J.N. Rosholt, Z.E. Peterman and A.J. Bartel, U - T h - P b and Rb-Sr ages in granite reference sample from southwestern Saskatchewan, Can. J. Earth Sci. 7 (1970) 184.
© Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
LEd
LEAD ISOTOPIC STUDY OF APLITES FROM THE PRECAMBRIAN BASEMENT ROCKS NEAR IBADAN, SOUTHWESTERN NIGERIA V.M. OVERSBY Research School o f Earth Sciences, Australian National University, Canberra, A.C.T. (.4 ustralia)
Received April 8, 1975 Revised version received June 16, 1975
Pb isotopic compositions for three total-rock samples of aplite and their constituent K-feldspars from the Nigerian basement assemblage near Ibadan show lead homogenization during the Pan-African thermo-tectonic event. A secondary isochron formed by the K-feldspars data points is used to calculate a primary age of about 2750 m.y. for the aplites. The aplites do not register any Pb isotopic effect from the intrusion of granite gneiss in the area at 2330 m.y.
1. Introduction
2. Analytical methods
The Precambrian basement rocks of southwestern Nigeria were studied by Grant [ 1] using whole-rock R b - S r isochron methods. The area consists of banded gneisses and refolded quartzites into which a grossly concordant granite gneiss has been emplaced. The banded gneiss contains an aplite sheet which is folded and semi-concordant to the banded gneiss. Grant [1 ] obtained a whole-rock R b - S r isochron age of 2330 -+70 m.y. (~,Rb = 1.39 X 10 -1I yr -x) for the granite gneiss. K - A r ages on biotite and amphibole from the granite gneiss gave ages of 481 -+ 19 m.y. and 499 + 20 m.y. [1 ] showing response of the area to the Pan-African thermo-tectonic event described by Kennedy [2]. Data for the aplite sheet and banded gneiss were inconclusive, but suggested that these units might be significantly older than the granite gneiss. Grant [ 1] postulated that the banded gneiss and aplite might have formed during the 2800 -+ 200-m.y. Liberian cycle. This study presents Pb isotopic analyses on three aplite total rock samples and their K-feldspars and one banded gneiss total rock. All of the samples were collected by N.K. Grant from the quarry at Ojo rock, 6 miles north of Ibadan on the lbadan-Oyo road (3°50'E, 7°29'N). Two of the aplites were included in Grant's original R b - S r study.
Rock chips weighing approximately 30 g were cleaned ultrasonically with 2N HC1 to remove surface contamination, and rinsed several times with triple distilled water. Samples were crushed to provide a representative total-rock sample and then sized for mineral separation of K-feldspar. Chemical separation of Pb and U are described in Oversby [3]. Pb isotopic compositions were determined by silica gel mass spectrometry with double spike fractionation control of all samples except 68BG65. Precision of analyses is better than 0.1% (20) for all ratios (cf. Oversby [3], for data on standard samples and duplicate analyses). Chemical blanks were 0.5 ng per analysis for U and 5.1 ng for Pb.
3. Results and discussion Pb isotopic data and Pb and U concentrations are given in Table 1. Samples marked AWKF are K-feldspars which were treated with hot 6N HC1 for 20 minutes followed by hot NHO3 for 20 minutes before dissolution. The banded gneiss sample is 68BG65; the other samples are aplites. The Pb isotopic data are plotted as 2°TPb/2°4Pb vs. 2°6pb/2°4pb in Fig. 1. The K-feldspar isotopic compositions are all extremely radiogenic and enriched in 2°~Pb relative to
178 TABLE 1 Ibadan, Nigeria Sample No. 2°8pb/2°6pb
2°Tpb/2°6pb
2°6pb/2°4pb
2°TPb/2°4pb 2°spb/2°apb Pb(ppm) U(ppm)
U(238U/2°4pb)
68BG62 TR1 TR2 KF
: 2.1624 2.1618 2.1783
0.81075 0.80931 0.88368
19.480 19.469 17.708
15.794 15.757 15.648
42.125 42.091 38.573
24.47 24.21 52.47
8.75 <0.002
24.22 0
68BG67 TR KF AWKF
2.1416 2.1508 2.1462
0.79282 0.81991 0.82265
20.207 19.564 19.475
16.020 16.041 16.021
43.276 42.079 41.799
40.52 79.99 83.36
4.38 0.718 0.326
7.52 0.610 0.264
68BG66 TR KF AWKF
2.1312 2.1430 2.1360
0.80971 0.83964 0.84216
19.628 18.939 18.933
15.893 15.902 15.945
41.831 40.588 40.442
39.48 66.87 67.73
4.54 0.790 0.202
7.79 0.780 0.196
68BG65 TR
2.2403
0.90220
17.334
15.639
38.833
n.d.
0.496
On this ground alone, it is unlikely that their isotopic composition could represent a lead that was in existence 2330 m.y. ago at the time of granite gneiss emplacement. The linear array of radiogenic K-feldspar compositions suggests that some metamorphic redistri2°6pb.
IBADAN, Nigeria 16'1
bution of Pb occurred similar to that discussed by Rosholt et al. [4]. Confirmation of a metamorphic event is found by plotting the 2°6pb/2°4Pb ratios against 238U/2°4pb (Fig. 2). U - P b ages calculated for the individual aplites are 68BG62, 452 m.y.; 68BG66, 588 m.y.;68BG67, 592 m.y. Whole-rock samples are frequently found to
~,~,J 210
0[] 68BG 68BG 62 67 O 68BG66 ~. 68BG65
£ ) o ~ O ~
ii 20 0
159
j~
pb 207 ' pb ~ 02~ 4
190
15"7
15"517'0
1810
l 19"0 pb 206
201.0
J
21I' 0
V,
Fig. 1. Pb isotopic compositions for total rocks and K-feldspars from Ibadan, Nigeria. The regression line shown is for four Kfeldspar points. Solid points are total-rock data; open points are K-feldspars.
lr.o
J
J
5!0 - -
J
J
10Lo
15L. . . .
10
250
U 238
Fig. 2. U-Pb isochron plot for aplite total rocks and K-feldspars Symbols as for Fig. 1.
179
have lost U due to recent weathering [4,5]. However, this is not always the case. Farquharson and Richards [6] obtained concordant R b - S r and U - P b ages on the Sybella microgranite, and Rosholt et al. [7] found concordant R b - S r and U-Pb ages for a granite from Saskatchewan. The general agreement of the aplite U - P b ages with the K - A r measurements of the PanAfrican event at Ibadan [1] suggests that we are measuring a true time of Pb homogenization. Assuming that the U - P b ages reflect the Pan-African event, two explanations are possible for the range of ages found. The true time of metamorphism could be 452 m.y. for all three samples, and 68BG66 and 68BG67 would then have lost about 25% of their U in some recent weathering episode. The agreement between their ages makes this interpretation unattractive. Alternatively, 68BG66 and 68BG67 could have become closed systems to Pb diffusion at a higher temperature than 68BG62, and its younger age merely represents a later stage in the cooling subsequent to the PanAfrican event. As will be shown below, the exact time of the metamorphic redistribution of Pb makes very little difference in the calculation of a primary age for the aplite. In order to further test the age of metamorphism, the total-rock and feldspar compositions were calculated back in time using the measured U - P b age and measured ~t values for each sample. The nature of the calculation requires that the 2°6pb/2°4Pb ratios of K-feldspars and total rocks agree; however, any large discrepancy in metamorphic age would appear as a systematic difference in Z°VPb/Z°4pb. The "age corrected" ratios are given in Table 2 and plotted in Fig. 3. It is clear that use of a different metamorphic age would not improve the agreement of the age corrected compositions. The difference in :°TPb/ 2°4pb for age corrected ratios is about twice the experimental error, perhaps indicating a slight lack of equilibration during the Pan-African event. A secondary isochron has been drawn through four of the K-feldspar points, excluding 68BG66 AWKF which falls clearly above the perfectly fitted line formed by the other four points. If a metamorphic age is assumed, we may calculate the primary age of the feldspars from the slope of the secondary isochron (cf. Rosholt et al. [4] for equations and theoretical discussion). If we take 590 m.y. as the metamorphic age, the primary age of the feldspars (and by necessity,
TABLE 2 Pb isotopic compositions corrected for last stage of growth using measured/~ and 238U/2°4pb-2°6pb/2°aPb age Sample No.
(2°6pb/2°4pb)*
(2°Tpb/2°4pb)*
68BG62 TR KF
17.717 17.708
15.677 15.648
68BG67 TR KF AWKF
19.487 19.506 19.450
15.977 16.038 16.019
68BG66 TR KF AWKF
18.882 18.864 18.914
15.849 15.898 15.944
the aplites) would be 2710 m.y. If the metamorphic age is taken as 450 m.y., the primary age would be 2775 m.y. Inclusion of the 68BG66 AWKF point would slightly increase the slope of the feldspar line and the calculated primary age. The Pb isotope data thus support the suggestion by Grant [1] that the Nigerian basement was formed during the 2800 + 200 m.y. Liberian cycle. The distribution of Pb and U during subsequent events can be summarized as follows. The aplites were emplaced into the banded gneiss prior to 2750 m.y., and then metamorphosed at about 2750 m.y. The degree of metamolphism was sufficient to totally equilibrate Pb
16'1
15"9 p b 207
15'7
15'5 17.5
I
J 18-5
I
I 19.5
V"
p b 206
Fig. 3. Pb isotopic data corrected for radiogenic increase since the time determined from their U - P b ages. Solid points are total-rock data; open points are K-feldspars.
180 isotopes within the aplites. Thus, at 2750 m.y. the aplites began a stage of Pb isotopic evolution starting from a single homogeneous isotopic composition. At 2330 m.y., according to R b - S r data, the granite gneiss was emplaced [ 1]. Internal equilibration o f Pb between aplite whole rocks and feldspars is permitted at this time only if the whole rocks remained closed systems. In particular, the whole rocks must retain their U/Pb ratios without change. This seems a highly unlikely event, in view of the changes in whole-rock U/Pb ratios during the Pan-African event. It seems more likely that the U - P b systems of the rocks and minerals remained closed systems from 2750 m.y. until the Pan-African event. At about 590 m.y., the aplites were subjected to the Pan-African thermo-tectonic event. Each wholerock sample equilibrated isotopically with its enclosed K-feldspar. The whole rocks did not equilibrate with each other. Thus, the K-feldspars preserve a picture of the whole-rock Pb isotopes 590 m.y. ago. This picture will not change between 590 m.y. and the present, since the K-feldspars have very low U/Pb ratios. The K-feldspar data points, therefore, define an isochron for lead isotopic growth starting at 2750 m.y. and ending at 590 m.y. After the Pan-African event each aplite remained a closed system with its own unique "initial" Pb isotopic composition. The rock with the highest U/Pb ratio after the Pan-African event contains the least radiogenic K-feldspar lead. Thus, it must have had the lowest U/Pb ratio from 2750 m.y. to 590 m.y. This clearly shows change of U/Pb ratios during the PanAfrican event. This strongly suggests that the U - P b system was undisturbed by the 2330-m.y. granite gneiss intrusion, since no change in U/Pb ratios occurred then. The R b - S r system shows a quite different
response. The granite gneiss samples appear to be unaffected by the Pan-African event. The reason for this difference in response is not understood.
Acknowledgements I am grateful to Dr. N.K. Grant for supplying the samples used in this study, to Mrs. R. Maier who performed the chemical separations, and to Mr. R. Rudowski for mineral separations. On line computer acquisition and reduction of data was developed by Dr. P.A. Arriens.
References 1 N.K. Grant, Geochronology of Precambrian basement rocks from Ibadan, Southwestern Nigeria, Earth Planet. Sci. Lett. 10 (1970) 29. 2 W.Q. Kennedy, The structural differentiation of Africa in the Pan-African (±500 m.y.) tectonic episode, Res. Inst. Afr. Geol. (Leeds, U.K.), 8th Ann. Rept. (1964) 48. 3 V.M. Oversby, Lead isotopic study of acid igneous rocks from the Pilbara, Western Australia, submitted to Geochim. Cosmochim. Acta. 4 J.N. Rosholt, R.E. Zartman and I.T. Nkomo, Lead isotopic systematics and uranium depletion in the Granite Mountains, Wyoming, Geol. Soc. Am. Bull. 84 (1973) 989. 5 V.M. Oversby, Lead isotopic systematics and ages of Archaean acid intrusions in the Kalgoorlie-Norseman area, Western Australia, Geochim. Cosmochim. Acta 39 (1975) in press. 6 R.B. Farquharson and J,R. Richards, Whole-rock U - T h - P b and Rb-Sr ages of the 8ybella microgranite and pegmatite, Mount lsa, Queensland, J. Geol. Soc. Aust. 17 (1970) 53. 7 J.N. Rosholt, Z.E. Peterman and A.J. Bartel, U - T h - P b and Rb-Sr ages in granite reference sample from southwestern Saskatchewan, Can. J. Earth Sci. 7 (1970) 184.