In order to shed some new light on the question of the absolute and relative ages of the Ancient Gneiss Complex and Onverwacht Group, a Sm-Nd whole-rock and mineral isochron study of the AGC was begun. At this point, the whole-rock study of samples from the Bimodal Suite selected from those studied for their geochemical characteristics by Hunter et al., is completed. These results and their implications for the chronologic evolution of the Kaapvaal craton and the sources of these ancient rocks are discussed

Barker, F.

Carlson, R. W.

Hunter, D. R.

English

NASA Technical Reports Server

24
Sm-Nd ISOTOPIC SYSTEMATICS OF THE ANCIENT GNEISS
COMPLEX, SOUTHERN AFRICA. R. W. Carlson1, D. R. Hunter2, and
F. Barker3; Department of Terrestrial Magnetism, 5241 Broad
Branch Rd., Washington, D.C., 20015, U.S.A.; 2University of
Natal, Pietermaritzburg, 3200, Natal, South Africa; 3U. S.
Geological Survey, 4200 University Dr.,Anchorage, Al., 99508
The igneous core of southwestern Africa's Kaapvaal craton
consists of the Onverwacht Group of mafic to ultramafic volcanics
of the Barberton greenstone belt and a complex gray gneiss
terrain called the Ancient Gneiss Complex(AGO (e.g. 1,2). Until
recently, precise geochronologic information for these two units
has been difficult to obtain due to the effects of post-formation
metamorphism. Even the assignment of relative ages between the
AGC and the Onverwacht is complicated by the lack of direct
contact between these two units in the field(l).
Recently, by applying the Sm-Nd radiometric system, Hamilton
et. al.(3) determined a whole-rock age of 3,530±50 Ma for the
lower ultramafic unit of the Onverwacht Group. Compared to this
age, Rb-Sr dates for gneisses of the oldest unit of the AGC, the
Bimodal Suite (BMS), tend to be slightly younger(3,200-3,300 Ma;
4). However, these Rb-Sr ages most likely reflect later meta-
morphic episodes rather than the emplacement ages of the inter-
layered metabasalts and tonalite-trondhjemite gneisses that make
up the BMS. Based on a correlation between initial 87Sr/86Sr and
age of individual gneiss units within the AGC, Davies and Allsopp
(4) suggested an emplacement age of about 3,400 Ma for the AGC
parental materials, some 100 Ma younger than the Onverwacht
volcanics. In contrast, Barton et al.(5) reported a Rb-Sr whole
rock age of 3,555±111 Ma for the BMS placing its formation at
about the same time as the Onverwacht Group.
In order to shed some new light on the question of the
absolute and relative ages of the AGC and Onverwacht Group, a
Sm-Nd whole-rock and mineral isochron study of the AGC was begun.
At this point, the whole-rock study of samples from the BMS
selected from those studied for their geochemical characteristics
by Hunter et al.(6) has been completed. We discuss here these
results and their implications for the chronologic evolution of
'SheTKaapvaal-craton and the s©urces~~of-~-these ancient: rocksvr:"----
Sm-Nd data for samples of the BMS (Table 1) precisely
define a line on the isochron diagram shown in the figure. The
line corresponds to an age of 3,417±34 Ma with an initial lH3Nd/
l4ltNd = 0.508149±31 or initial eNd = +1.1±0.6 (using the "bulk-
earth" Sm-Nd parameters of Jacobsen and Wasserburg(7)). All data
points lie within analytical uncertainty of the best fit line
with the exception of the data for sample SWZ-6 which lies below
the isochron by only 5 parts in 105. The excellent colinearity
of these data is surprising given the wide variation in chemical
composition of the samples from siliceous gneiss (SWZ-5; SiOo =
76%) to tonalite(SWZ-6; SiO? = 66%), diorite(SWZ-3; Si02 = 57%),
and metabasalt(SWZ-10 and 12; Si02 = 49%). Even the data for the
stratigraphically younger quartz-monzonite gneiss, SWZ-4, lie
on the same line defined by the remainder of the data. If the
data for SWZ-4 are left out of the line regression, the isochron
https://ntrs.nasa.gov/search.jsp?R=19840012882 2020-03-20T23:51:40+00:00Z
Sm-Nd SYSTEMATICS-ANCIENT GNEISS COMPLEX
Carlson R. W. et. al. 25
shifts to only a slightly older age of 3.45±0.04 Ga with the
initial e , increasing by only 0.1 unit.
Because of the excellent colinearity of the Sm-Nd data, the
age indicated is interpreted as the time of extraction of the
parental materials of the BMS from a common, isotopically homo-
geneous, source. The positive initial e , for the AGC indicates
that its source was in the mantle and not in a much older, LREE
enriched, crustal section. This is not to say that all the
varied chemical species that make up the AGC were derived
directly from the mantle in a single igneous event. Rather, the
Sm-Nd data allow for a variety of petrogenetic mechanisms, from
direct partial melting of the mantle for the basaltic components
to anatectic melting of slightly older crustal materials for the
more silicic rocks as long as these events occurred within the
time interval specified by the isochron.
The age determined for the BMS is distinct and some 100 Ma
younger than that of the Onverwacht volcanics(S) in accord with
the suggestion of Davies and Allsopp (4). Though the similar
initial isotopic compositions of the AGC and Onverwacht allow
for some of the more silicic members of the AGC to have been
derived by remelting of a basaltic crust of Onverwacht age, the
presence of basaltic members in the BMS also indicates a
significant contribution to the AGC from the mantle. Thus the
Kaapvaal craton appears to have originated by at least two
episodes of mantle derived mafic volcanism occurring over a
period of about 100 Ma.
The positive initial e , of the BMS,and its agreement with
that determined for the Onverwacht Group(3), shows the presence
of a relatively homogeneous mantle source for the oldest units
of the Kaapvaal craton. This source appears to have been
relatively depleted in the LREE for a considerable time prior to
the formation of the AGC and the Onverwacht volcanics. The
increasing occurrence of positive initial £.,, values for the
oldest crustal rocks (e.g. 8-11) implies eitner that
differentiation events within the earth occurred well before
(i.e. several hundred million years before) the time of preserv-
ation of the oldest observed crustal sections, or that a
"chondritic" model for the evolution of the Sm-Nd system of the
bulk-earth is not appropriate. The answer to this question
carries with it important consequences for models of the bulk
composition and early evolution of the earth. However, because
of the possible homogenizing effects of convective mixing within
the mantle over earth history, further constraints on the early
geochemical evolution of the earth clearly must be sought in
more complete and precise isotopic data for the ancient rocks
of the continental crust.
Sm-Nd SYSTEMATICS-ANCIENT GNEISS COMPLEX
26 Carlson R. W. et. al.
References;
(1) Hunter, D. R., The granitic rocks of the Precambrian in
Swaziland, Geol. Soc. South Africa Spec. Pub. 3, 131-148,
1972.
(2) Anhaeusser, C. R., The evolution of the early Precambrian
crust of southern Africa, Philos. Trans. R. Soc. Lond.
A273, 359-388, 1973.
(3) Hamilton, P. J., N. M. Evensen, R. K. O'Nions, H. Smith, and
A. J. Erlank, Sm-Nd dating of Onverwacht Group volcanics,
southern Africa, Nature 279, 298-300, 1979.
(4) Davies, R. D., and H. L. Allsopp, Strontium isotopic
evidence relating to the evolution of the lower Pre-
cambrian crust in Swaziland, Geology 4, 553-556, 1976.
(5) Barton, Jr., J. M., D. R. Hunter, M. P. A. Jackson, and
A. C. Wilson, Rb-Sr age and source of the Bimodal Suite
of the Ancient Gneiss Complex, Swaziland, Nature 283,
756-758, 1980.
(6) Hunter D. R., F. Barker, and H. T. Millard, The geochemical
nature of the Archean Ancient Gneiss Complex and
granodiorite suite, Swaziland: A preliminary study, Prec.
Res. 7, 105-127, 1978.
(7) Jacobsen, S. B., and G. J. Wasserburg, Sm-Nd isotopic
evolution of chondrites, Earth Planet. Sci. Lett. 50,
139-155, 1980.
(8) McCulloch, M. T., and W. Compston, Sm-Nd age of Kambalda and
Kanowa greenstones and heterogeneity in the Archean mantle
Nature 294, 322-327, 1981.
(9) Basu, A. R., S. L. Ray, A. K. Sana, and S. N. Sarkar,
Eastern Indian 3800-million year old crust and early
mantle differentiation, Science 212, 1502-1506, 1981.
(lO)Wooden J. L., B. Bansal, and H. Wiesmann, Archean crust-
mantle reservoirs: Sr and Nd evidence from the Superior
Province, Trans. Amer. Geophys. Union 64, 340, 1983,
(abstract)
(11)Macdougall, J. D., K. Gopalan, G. W. Lugmair, and A. B. Ray,
The banded gneiss complex at Rajasthan, India: Early
crust from depleted mantle at ^ 3.5 AE?, Trans. Amer.
Geophys. Union 64, 351, 1983, (abstract)
Sm-Nd SYSTEMATICS-ANCIENT GNEISS COMPLEX
Carlson, R. W. et. al.
Table 1: Sm-Nd Results for samples of the Bimodal Suite
Sample
SWZ-4
SWZ-6
SWZ-5
SWZ-3
SWZ-10
SWZ-12
Sm*
2.39
3.20
9.66
2.07
1.68
0.772
Nda
16.1
20.0
45.5
7.89
5.21
2.25
1
*
7Sm/1'"'Ndb
0.08982
0.09661
0.1282
0.1584
0.1953
0.2078
1
"
3Nd/1'"tNdC
0.510293±18
0.510403±25
0.511146±20
0. 511822124
0.512668±29
0. 512946129
a) Concentrations expressed in ppm. Sm and Nd blanks of 20 pg.
and 80 pg. respectively, are negligible for the sample sizes
analysed.
b) Determined with tracers calibrated with AMES Sm and Nd metal
standard solutions and cross-checked against the CIT n(Sm/Nd)
3 standard. Uncertainty ^0.1%.
c) Measured as NdO , fractionation corrected to l * 6NdO/1 H "*NdO =
0.722251 (1"6Nd/1UHNd = 0.7219). Data reported relative to
a value of x" 3Nd/' ""Nd = 0.511860 for the La Jolla Nd
standard.
0.513
0.512
5
0.511 -
0.510 -
l I
SWAZILAND
ANCIENT GNEISS COMPLEX
T- 3,417: 34 M<
I0- 0.508249 ± 31
£(!„). +1.U 0.6
0.08 0.12
I I
147^144
I
0.16 0.20
Nd
I
0.10 0.12 0.14 0.16
wSm/«*Nd
0.18 0.20
Figure 1: Sm-Nd isochron diagram for samples of the Bimodal
Suite. Inset shows deviations (6Y) in parts in
10,000 of the data from the best fit line.
27


Sm-Nd isotopic systematics of the ancient Gneiss complex, southern Africa

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