257
S. Afr.J. Geol., 1988,91 (2) ,257-263
40 Ar/
39
Ar dating of the Cambro-Ordovician Vanrhynsdorp tectonite in southern
Namaqualand
P.G. Gresse
Geological Survey of South Africa, P.O. Box 572, Bellville 7535, Republic of South Africa
F.J. Fitch
FM Consultants Limited, 21 Harcourt Drive, Herne Bay, Kent CT6 8DJ, England
J.A. Miller
Bullard Laboratories, Department of Earth Sciences, University of Cambridge, Madingley Rise, Madingley Road, Cambridge
C83 DEZ, England
Accepted 29 February 1988
40 ArP9 Ar
plateau ages are reported from metasediments and biotite concentrates from the late Precambrian/
Cambrian Vanrhynsdorp Group in southern Namaqualand. The results show a span of 75 Ma from about 552
Ma to 476 Ma. Progressively younger ages were obtained from the northeastern marginal area towards the
southwestern central area of the Vanrhynsdorp orogen. A single major dynamothermal metamorphic event
near to 496 Ma is postulated for the Vanrhynsdorp tectonite. Relatively high ages recorded in the northeast (553
Ma and 527 Ma) are interpreted as resulting from the interference of inherited excess argon contained within
original detrital grains still preserved in middle an chi-zone phyllites. Fully recrystalline phyllites southwest of
the Arizona Fault have ages close to 496 Ma. A relatively low value obtained from biotite-grade rocks in the
southwest (476 Ma) is regarded as registering the delayed cooling age of biotite in this zone as a result of slow
.
uplift and continued deformation in the more deeply buried central part of the orogen.
40 ArP9 Ar
plato-ouderdomme van metasedimente en biotietkonsentrate van die laat-Prekambriese/Kambriese
Groep Vanrhynsdorp in sui del ike Namaqualand word gerapporteer. Die resultate toon 'n verspreiding oor
75 Ma vanaf omtrent 552 Ma to 476 Ma. Progressiewe jonger ouderdomme is verkry vanaf die noordoostelike
marginale gebied na die suidwestelike sentrale gebied van die Vanrhynsdorp-orogeen. 'n Enkele hoof
dinamotermiese metamorfe gebeurtenis van om en by 496 Ma word gepostuleer vir die Vanrhynsdorptektoniet. Relatiewe hoe ouderdomme verkry in die noordooste (553 Ma en 527 Ma) word toegeskryf aan die
effek van geerfde oortollige argon in oorspronklike detritale korrels wat behoue gebly het in middel
anchisonefilliete. Ten volle gerekristalliseerde filliete suidwes van die Arizonaverskuiwing gee ouderdomme
naby aan 496 Ma. 'n Relatiewe lae waarde verkry van biotietgraadrotse in die suidweste (476 Ma) word
bestempel as 'n vertraagde afkoelouderdom van biotiet as gevolg van stadige opheffing en aanhoudende
vervorming in hierdie dieper gelee sentrale gedeelte van die orogeen.
Introduction
The Vanrhynsdorp Group was named and defined by
Gresse (1986) as a low-grade foreland thrust-fold belt
along the west coast of southern Africa (Figure 1). It
consists of a late Precambrian/Cambrian sequence of
sediments with northwesterly grain which shows
increasing deformation and grade of metamorphism
from northeast to southwest. Temperatures increase
from diagenetic to biotite-grade greenschist facies
towards the southwest. Deformation was effected and
controlled by thrusts (F I ) steepening out of a sole. thrust.
Deformational intensification therefore takes place
zonally in association with successive ramps. A younger
phase of backfolding affected one half of the
Vanrhynsdorp Group southwest of a steepened F 1
thrust, the Arizona Fault. One-cleavage phyllite (Sl) is
replaced by two-cleavage phyllite and schist (Sl plus S2)
across this fault.
This sudden deformational
intensification led the earliest workers to correlate
sediments northeast of the Arizona Fault with the Nama
Group and those on the southwest with the Malmesbury
Group. The entire exposure is now correlated with the
Nama (Kroner, 1968; Gresse, 1986).
Stratigraphy
A generalized stratigraphic succession for the southern
Vanrhynsdorp Group as defined by Gresse (1986) is
provided in Table 1.
Tectonic uplift increases from northeast to southwest
in the tectonite, with the result that the upper Brandkop
Subgroup is only present in the undeformed
northeastern part of the exposure whilst the middle and
lower Knersvlakte and Gifberg subgroups outcrop in the
southwest. Diagenetic and anchi-grade mudstone,
siltstone, and sandstone of the Brandkop and upper
Knersvlakte Subgroups make way for sericitic and
chloritic shale and phyllite (lower Knersvlakte
Subgroup) and greenschist-grade biotite-schist and
marble (Gifberg Subgroup) towards the southwest.
Increasing metamorphic grade is therefore both a
stratigraphic and tectonically related feature.
Sample description and analytical methods
Four whole rock samples (crushed, 60/85 #) and two
biotite concentrates (601120 #) were dated by the 40 Ar/
39 Ar step heating technique (Fitch et at., 1969). The
whole rock samples were all collected in the
Besonderheid and Gannabos formations (see Figure 1
S .-Afr. Tydskr. Geol., 1988,91 (2)
258
and Table 1); the biotites were extracted from Aties
Formation schistose quartzites. Sample freshness and
microscopic investigation of cleavage morphology and
mineralogy served as criteria for selection. These
characteristics are listed in Table 2. Samples GV 328 and
325 contained only one penetrative transverse cleavage
Sl' GV 285 and 280 also displayed a prominent S2
crenulation cleavage. By dating the three groups of
samples it was hoped to determine the exact ages of the
two cleavages and perhaps confirm one or both with the
biotite age.
Figure 1 Locality map and sample positions.
The analyses were carried out in the Cambridge (UK)
dating laboratory as described in Mitchell (1968); Fitch
et al. (1969); Brereton, (1970; 1972); Fitch et al. (1974).
The petrographic character of the samples is such that
recoil effects (e.g. as described in Fitch et al. (1978) are
not thought to be a significant source of error. In 40 Arl
39 Ar dating no absolute values of potassium or argon
content are measured, nor is it necessary to weigh out
the sample. Errors arise only from the scatter in the
measured isotopic rations and from uncertainties in the
value of J (the neutron absorption factor). The
magnitude of the experimental error is calculated from
twice the standard deviation generated by the argon
isotope rations when the extreme values are substituted
in the following equation:
Table 1 Stratigraphic nomenclature of the Vanrhynsdorp Group and the lithology of
4oArP9Ar samples
Formation
Lithology
Klipbak
Sandstone, shale
Stofkraal
Mudstone, sandstone
Van Zylkop
Conglomerate, sandstone, mudstone
Astynskloof
Sandstone
Dolkraals
Siltstone, shale, sandstone
Kalk Gat
Mudstone, siltstone
I
Besonderheid
Subgroup
Brandkop
Knersvlakte
Vanrhynsdorp
Shale, siltstone, sandstone, grit, conglomerate
2Gannabos
Shale, siltstone
Flaminkberg
Sandstone, conglomerate
3 Aties
Shale, limestone, dolomite
Widouw
Limestone, dolomite
Gifberg
2 _
GV 280,285,325
GV 328
3 _
GV 307,308
I _
Group
259
S.Afr.J. Geol. ,1988,91(2)
Table 2
Description of samples used for age determinations
Sample
no.
Locality
La.OS
Lo.OE
GV 328
31°26'
18°49'
Gannabos
green phyllitic
shale
GV325
31°29'
18°56'
GV285
31°21'
GV280
Formation
( + lithology)
Structure
cl.(Sn),lin.(8 n),bed.(So)
Mineralogy
Remarks
S1 =FSO,(S2~S1)
81(SoIS1),83(S1 /S 2)
Q.,ser.,chl.
Detrital chI., S1 = penetrative slaty
cl., S2=zonal discrete solution cl.
Besonderheid
grey phyllitic
shale
S1 .& So,8 1(SoIS1)
Q.,ser.,chl.
Detrital chI., diag.ser. on SO,S1 =
penetrative slaty cl.
18°37'
Besonderheid
green phyllite
S1 d SO,S2~ S1
81(SoIS1) ,83(S1/S 2)
Q.,ser.,chl.
S1 = penetrative slaty cl.
S2=zonal crenulation cl.
31°28'
18°42'
Besonderheid
green phyllite
S1 L1. SO,S2~S1
81(SoIS1),8iSoIS2)
83(S1 /S 2)
Q.,ser.,chl.
S1 = penetrative slaty cl.
S2=zonal crenulation cl.
GV 308
31°32'
18°29'
Aties
white biotite-quartzite
porphyroclastic bio.
porphyroblastic Ksp.
Q. ,ser. ,bio. ,Ksp.
2 biotites, one slightly younger,
with ser. (Old biotite separated?)
GV 307
31°32'
18°29'
Aties
white biotite-quartzite
porphyroclastic bio.
porphyroblastic Ksp.
Q. ,ser. ,bio. ,Ksp.
2 biotites, one slightly younger,
with ser. (Old biotite separated?)
cl. - cleavage; lin. - lineation; bed. - bedding; Q. - quartz; ser. - sericite; chI. - chlorite; Ksp. - potassium feldspar; bio. - biotite.
J
40ArP9Ar
where J is a measure of the neutron absorption; t is
apparent age; r is decay constant of 4oK-1; and 40 ArP9Ar
is measured isotope ratio.
The uncertainty in J is estimated from its change
across the irradiation can. Up to 8 standards can be run
from each can and variations in flux are minimized by a
rotation of the can half way through the irradiation. Flux
changes from end to end of the can rarely exceed a few
per cent while the deviation of the individual standards
from the average is normally less than 0,5% and
frequently as low as 0,05%. Uncertainties in the age of
the standard used to establish J are neglected. Standard
ages, like decay constants, are arrived at by consensus
and whilst any changes in them would cause large blocks
of data to be revised upwards or downwards, this would
not affect the magnitude of the internal precision of a
particular age determination.
Results
The experimental results of the 40 ArP9Ar step heating
analyses are summarized briefly in Table 3 (a photocopy
of the full 60-page data report is available at cost by
application to the authors) and age spectrum plots are
presented in Figure 2.
The major events recorded in these samples occurred
in Lower Palaeozoic times, within the age· range
560-475 Ma. Also seen in all six plots is evidence of
minor argon loss disturbances during the subsequent age
range 350-175 Ma. In at least three plots (GV 280, GV
285, and GV 325) an excess argon disturbance (most
likely related to the same subsequent thermal events)
influences the values obtained from the major age
component over the early low temperature steps. Biotite
sample GV 307 produced a good example of an
'upwardly convex' plateau (Tetley & McDougall, 1978)
in which the ages obtained from steps lying in the middle
part of the plateau feature are marginally in excess of
their true value. Nevertheless, good plateau sectors
revealing the age of the major component can be
identified with confidence in all six age spectra.
Comparison between the plateau and summation
(equivalent to total degassing) ages as listed in Table 4
confirms the need for age spectrum rather than
conventional K-Ar dating in Orogenic belts. The good
plateau ages quoted in Table 4 were calculated by taking
an average of the individual, virtually identical apparent
ages obtained through sequences of adjacent steps - i.e.
from the 'plateau-forming' steps, as defined in Fitch et
al. (1969) - that can be identified in each diagram.
When treated in this way, the results show a grouping
according to the geographical distribution of the samples
(Figure 1). Samples GV 325 and GV 328 northeast of the
Arizona Fault have older, non-coincident, plateau ages
of 527 Ma and 552 Ma respectively. The plateau sectors
of the age spectra from samples GV 280 and GV 285
southwest of the fault are younger but are virtually
coincident at 495 and 499 Ma respectively. The plateau
ages obtained from samples GV 307 and 308 in the
extreme southwest are younger still and also virtually
coincident at 476-488 Ma and 476 Ma, respectively.
However, because it is upwardly convex, the true
plateau value obtained from biotite sample GV 307 is
most probably closer to 476 than to 488 Ma.
The six ages define an overall younging trend in a
S.-Afr. Tydskr. Geol., 1988,91 (2)
260
Table 3 Summary of argon-40/argon-39 step heating
analyses
Table 3
continued
Step No.
Step No.
Age
Error
Age
Error
Ar39 %release
Ar39 % release
SAMPLE (4)
FM8426 TR
1
178,53
14,24
1,59
1,46
GV 328
2
373,54
6,72
3,94
60/85 #
SAMPLE (1)
FM8423 TR
152,16
15,00
GV280
2
275,66
12,18
1,68
3
312,39
2,87
9,49
60/85 #
3
281,92
7,15
3,05
4
·344,21
1,61
18,02
5
527,94
2,29
12,67
good
plateau
at 495±2
4
271,22
3,94
6,59
5
502,99
2,11
13,41
6
512,05
2,16
13,06
7
528,52
1,86
16,06
8
496,07
1,75
17,00
9
488,11
1,72
17,28
10
499,72
2,77
9,68
11
447,07
25,59
0,73
6
559,53
2,90
9,71
good
7
551,01
2,61
10,93
plateau
8
545,63
2,90
9,64
at 552±3
9
552,76
2,24
13,19
10
551,02
3,27
8,44
11
490,40
8,11
2,38
SAMPLE (5)
15,86
41,70
0,88
109,48
28,50
1,15
GV 307
2
167,97
8,55
3,89
2
196,36
22,90
1,37
biotite
3
376,85
3,31
10,37
3
300,26
9,23
3,21
4
202,21
2,99
12,94
12,85
SAMPLE (2)
FM8415 TR
FM8424 TR
GV285
60/85 #
5
396,20
6
546,65
2,51
3,04
12,86
7
518,73
2,78
14,12
15,38
4
475,69
2,80
upwardly
5
484,90
3,96
8,86
convex
6
486,44
3,49
10,15
plateau
7
8
487,84
4,12
8,52
485,46
4,45
7,85
between
8
495,80
3,27
11,72
9
505,07
4,07
9,29
plateau
10
494,32
3,54
10,76
at 499±6
11
496,02
8,03
3,35
good
12
502,34
8,36
476-488±4
3,86
9
485,23
4,72
7,37
10
479,29
2,95
12,15
11
461,33
2,38
15,35
12
479,88
13,45
-232,54
59,06
1,04
56,55
13,16
3,98
1,748
SAMPLE (6)
SAMPLES
GV280-8
496,07
20,50
FM8416 TR
9
488,11
20,84
GV 308
2
10
499,72
11,67
biotite
3
183,37
3,90
12,88
4
436,09
6,15
8,17
GV280
& GV285
combined
plateau-
GV285-8
495,80
14,14
5
473,55
5,16
9,87
features
9
505,07
11,20
6
486,70
3,85
13,46
10
494,32
12,97
11
496,02
4,04
7
8
474,28
7,18
7,04
good
476,77
5,35
9,53
plateau
9
475,08
3,71
13,92
10
478,28
3,66
14,17
11
461,28
7,07
5,95
12
502,34
4,65
Average (weighted by % Ar39 released)
at 476±5
495,34
SAMPLE (3)
FM8425 TR
201,60
10,50
2,61
Ages and errors in Ma
GV 325
2
358,73
4,12
7,99
A verage plateau ages unweighted except where indicated
60/85 #
3
346,73
2,91
11,44
4
510,17
2,84
12,01
5
611,47
2,74
13,05
6
562,36
3,04
11,32
good
7
528,11
2,03
18,16
plateau
8
525,77
3,28
10,29
at 527±3
9
525,69
3,25
10,40
10
493,40
11,85
1,99
11
330,17
34,24
0,75
See text for availability of full experimental data
southwesterly direction, in sympathy with increasing
metamorphic grade and intensity of tectonism. The
oldest ages were obtained from middle anchi-zone
phyllites showing evidence of a single very low-grade
metamorphic overprint associated with the Sl slaty
cleavage. The non-coincident age spectrum from these
rocks suggest that variable amounts of inherited excess
argon - derived from the continued presence of original
S.Afr.l .Geol. ,1988,91(2)
261
eoo
600
200
d. GV 328
a. GV 280
0
0
60
400
ti
-
400
:i
w
CJ
c:t
200
200
e. GV 307
b. GV 285
0
0
60
400
400
200
20
c. GV 325
o.-------------~--~-----------------o
20
40
80
80
100
1. GV 308
o--------------~--~-----------------0
20
40
60
80
100
CUM. Ar39 %
Figure 2
40 Ar/39 Ar
age spectra plots for Vanrhynsdorp Group samples. a-d: whole rock; e-f: biotite.
detrital grains of mica and chlorite - partially mask the
true metamorphic age of the new S1 cleavage micas and
consequently increase the apparent ages of these rocks.
Southwest of the Arizona Fault the strongly cleaved
phyllites of upper anchi-grade most probably cooled
quite rapidly after being completely outgassed of
previously accumulated radiogenic argon during a
complex high level dynamothermal event. Thus, the
plateau ages obtained from this zone of the Orogen,
which record major sheet silicate crystallization along S1
at close to 496 Ma, are regarded as indicating the true
age of the major Orogenic tectonism. The two
southwesternmost samples, which also date S1 micas,
reveal a mean biotite cooling age of 476 Ma. The 20-Ma
difference between the apparent ages obtained in the
higher level Besonderheid/Gannabos phyllites and the
stratigraphically and tectonically lower Aties biotitequartzite is interpreted as indicating delayed cooling of
the latter during slow uplift of the more deeply buried
part of the Orogen.
The argon loss disturbances in the first three to five
heating steps in the diagram of Figure 2 suggest that
there may be one or more rather weak metamorphic
(thermal) overprints between ± 350 Ma and 175 Ma.
Any age in this range is, however, incompatible with the
F2 tectonic event, which everywhere can be shown to be
closely related to F l' The late events fall partly within the
age span of the Cape Orogeny (Halbich et al., 1983), but
S.-Afr. Tydskr. Geol., 1988,91 (2)
262
Table 4 4°ArP9Ar plateau and summation ages from
the Vanrhynsdorp Group
Sample
No.
Type
GV 328
GV 325
GV285
GV 280
GV 308
GV 307
T.R.
T.R.
T.R.
T.R.
Bio
Bio
Heating steps
Unweighted
Summation age
used
age & error (Ma) for comparison
6-10
7-9
8-12
8-10
7-10
4-10
552 ± 3
527 ± 3
499 ± 6
495 ± 2
476 ± 5
476(-488) ± 4
474
495
438
473
411
452
T.R. = whole rock
Bio = biotite
this orogeny has had no discernible tectonic effect on the
Vanrhynsdorp Group; alternatively, and perhaps more
likely, the disturbances may reflect purely thermal
events such as the intrusion of Karoo-age dolerite dykes
in the area which, according to Fitch & Miller (1983),
display a magmatic maximum at 193 Ma in the Central
Karoo Province.
Regional geological significance
All six samples appear to have been argillaceous
sediments of late Precambrian or Cambrian age that
were involved in a late Cambrian/early Ordovician
Orogeny around 496 Ma ago. Younger ages are
displayed by rocks in the more central, southwestern
part of the orogen than along the northeastern margin.
This apparent cooling pattern across strike is comparable
to the one described for the Damara Orogen by
Hawkesworth et ai. (1983). Late Precambrian,
Cambrian, and Ordovician events have been dated
extensively along the west coast of Southern Africa, but
the assessment and regional implications of these results
have proved both difficult and controversial. Originally,
two tectono-metamorphic events were postulated,
namely one at around 650 Ma (Pan-African) and one
around 550 Ma (Damaran; e.g. Clifford, 1967; Kroner
et ai., 1978). Subsequently a near 450-Ma event was
added (e.g. Clauer & Kroner, 1979; Ahrendt et ai.,
1977; 1983) and the polycyclic tectono-thermal evolution
of the Damara and Gariep Orogens (e.g. Allsopp et ai.,
1979) became evident. Hawkesworth et ai. (1983)
reviewed much of the available material and concluded
that there were major tectono-thermal events at
650--620 Ma and at 560-550 Ma. The latter was the last
and most significant regional event, related at least
partly to crustal thickening. Subsequent tectonic and
metamorphic events in the period 550-450 Ma such as
the movement of the Naukluft gravity nappes at around
500 Ma which also involved Nama sediments, are
regarded as consequent upon continued uplift in the
Central Zone of the Damara Orogen.
Ages in the central part of the Vanrhynsdorp Orogen
are up to 75 Ma younger than those on the northeastern
margin, a pattern very similar to the cooling history
identified in the Damara. Nevertheless, as the plateau
ages obtained from the extremely low-grade rocks
outcropping northeast of the Arizona Fault are noncoincident, an interpretation of the oldest dates based
upon inherited argon is preferred. Ahrendt et ai. (1983)
reported preliminary ages of 530 Ma and 630 Ma for
ditrital micas in respectively the Fish River and Upper
Schwarzrand sediments of the Nama Group, an
observation which lends credence to the above
assumption. Considering the low closure temperature of
biotite (300°), the 20-Ma 40 ArP9 Ar plateau age range
from 496 Ma to 476 Ma seen between samples GV 280,
GV 285 and GV 307 and GV 308 is regarded as
indicating the maximum extent of prolonged cooling
across the strike of the orogen.
Thus, on the basis of the presently available evidence,
a single major dynamothermal metamorphic event (F 1),
which culminated at 496 Ma, with continued cooling
until about 476 Ma in the deeper, central parts of the
orogen is postulated for the Vanrhynsdorp Group. It is
possible that this prolongation may have been
associated, in part, with F2 backfolding in the
southwestern part of the orogen, but further evidence is
required before this hypothesis can be properly assessed.
At present, there is insufficient evidence to propose the
existence of a second metamorphic event younger than
496 Ma in the Vanrhynsdorp Group. The detailed
tectonic history of the Vanrhynsdorp Group will be
discussed in a separate publication.
Acknowledgements
The contents of this text, excluding the technical data
concerning analytical methods and results, forms part of
a Ph.D. thesis completed in 1986 at the University of
Stellenbosch. Prof. I.W. Hiilbich was the promotor.
Financial assistance for dating the samples was provided
by the University of Stellenbosch (Queen Victoria
Stipendium), O'okiep Copper Company (OCC) and the
Geological Survey of South Africa. These institutions
are thanked for their co-operation and interest. Mrs. C.
Roestoff of the Geological Survey, Bellville, was
responsible for typing the manuscript.
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