white dolosparite, breccia with irregularly shaped clasts of dolomite of the latter type set in an
equally coarse-grained,
dark grey
to pink
dolosparite matrix, and massive albitite. This se- quence Sholtzberg member, Fig. 2 is laterally
not continuous but occurs over a strike length of only a few hundred metres. Locally, massive stro-
matolitic, Fe-rich dolomite, intercalated within the metabasalt sequence, is developed in the vicin-
ity of the mixed sequence. A similar association of oceanic metabasalt, hyaloclastite and, in places,
stromatolitic and oolitic dolomite is also known from the Schakalsberge sub-terrane Fig. 1. That
tectono-stratigraphic unit is made up of a thick sequence of greenschist Grootderm formation
which is capped by dolomite of the Gais member Frimmel et al., 1996a — a likely correlative of
the Sholtzberg member in the Chameis sub- terrane.
No estimates can be made on the total thick- ness of the various stratigraphic units in the
Chameis sub-terrane because of intense folding and thrusting. Although most contacts are tec-
tonic, a few examples exist of gabbro having intruded the Sholtzberg member causing decime-
ter-thick contact metamorphic aureoles in calc- pelite. In most cases, however, it appears as if the
metagabbro bodies were tectonically emplaced with preferential movement along these dolomite-
rich strata, which gave rise to a previous interpre- tation
of the
whole Chameis
sub-terrane representing a tectonic melange zone Frimmel
and Hartnady, 1992. The dominant minerals in the mixed calcareous
succession of
the Sholtzberg
member are
dolomite, albite, and quartz. In addition, magne- sioriebeckite, talc, clinochlore, phlogopite, tour-
maline, and hematite replacing either magnetite or pyrite occur in effectively all rock types but in
highly variable proportions. Tourmaline-bearing mineral assemblages found include dolomite –
talc – quartz – albite – tourmaline,
dolomite – tour- maline – magnesioriebeckite, and dolomite – talc –
chlorite – tourmaline. Magnesioriebeckite and al- bite are ubiquitous phases also in many of the
mafic rocks in the Chameis sub-terrane and have been previously ascribed to extensive Na-metaso-
matism Frimmel and Hartnady, 1992. The tim- ing of this metasomatism must have been prior to
or during the main phase of orogenic deformation as the sodic amphibole has grown syn-tectonically
with respect to the major phase of folding Frim- mel, 1995. Sodium salts in a sediment, dominated
by Mg-carbonate and enriched in B, provide a likely source for the Na-metasomatism, and by
analogy with dolomitic rocks containing similar mineral associations in the Duruchaus formation
of the Damara belt Behr et al., 1983, the Sholtzberg member is interpreted as representing
a low grade metamorphosed former evaporite de- posit.
3. Dolomite geochemistry
Eleven samples of the various dolomite types in the
suspected meta-evaporite
sequence were
analysed for their trace and rare earth element REE contents using an ELAN 6000 ICP-MS
for analytical procedures see Frimmel, 2000 and for their Sr isotopic composition Table 1 using
conventional ion-exchange techniques and a VG Sector 7-collector mass spectrometer see Frimmel
et al., 1996a at the Department of Geological Sciences, University of Cape Town UCT.
Dolomite samples representing the inferred for- mer evaporite sequence within the Dernburg for-
mation from an area 5 km north of Bakers Bay, at co-ordinates 27°40.11S, 15°32.35E samples
HFG211-214 are compared with dolomite and limestone that cap that formation HFG192-204,
and that lack any relationship to evaporite de- posits, but represent diagenetically modified shal-
low marine carbonate deposits from 27°35.59S, 15°32.20E. All the dolomite samples from the
inferred evaporite sequence display a generally flat chondrite-normalised REE pattern LaYb = 1.6 –
6.7, and no Ce and Eu anomalies Fig. 4. A succession of limestone and dolomite, represent-
ing the lower Bogenfels formation 7 km to the south 27°41.58S, 15°32.48E, differs by display-
ing a light REE enrichment LaYb = 11.7 – 14.5 and 43.6, respectively.
We chose a relatively pure dolomite sequence from the Dreimaster member in the Bogenfels
formation samples HFG192 and HFG193 as
H .E
. Frimmel
, S
.- Y
. Jiang
Precambrian
Research
105 2001
57 –
71
61 Table 1
Trace, rare earth element and Sr isotope data for carbonate rocks from the Chameis sub-terrane
a
HFG c Dernburg formation
Bogenfels formation 204
193 192
211 212
213a 224
213b 223
214 222
Do Lst
Do-matrix Do
Lst Do-clast
Do Do
Do Do
Do 0.70
0.91 0.56
7.92 9.73
0.75 Li
2.09 1.26
0.41 0.34
0.34 2.53
5.56 1.15
2.86 3.28
1.90 0.22
0.94 1.97
Sc 1.11
0.80 8.59
10.68 3.25
97.71 8.08
19.92 23.63
1.52 8.80
6.99 2.38
V 6.37
14.60 2.26
31.82 17.50
20.60 76.61
2.33 11.98
3.34 1.44
Cr 2.60
16.03 3.47
2.81 4.49
3.29 1.42
Co 1.98
1.27 1.98
2.40 Ni
8.08 6.20
17.15 9.95
9.03 13.12
5.72 5.55
7.97 9.82
6.05 0.06
0.13 0.02
35.53 39.55
0.22 0.17
12.38 Rb
0.01 1.25
11.65 211.28
71.65 202.21
258.10 290.95
814.03 1412.26
182.96 69.27
174.63 217.89
Sr 8.16
7.20 22.87
26.44 9.28
9.14 14.27
1.00 5.57
6.60 3.08
Y 0.69
45.34 2.67
32.75 101.71
13.13 1.34
Zr 0.50
3.14 28.37
31.51 1.38
1.79 0.09
– –
3.69 6.00
0.20 1.28
0.25 0.14
Nb 0.01
0.01 0.00
1.05 1.52
0.00 0.01
0.00 Cs
0.20 0.17
0.04 8.35
5.50 3.10
147.79 147.69
Ba 1.65
97.58 82.22
1.31 1.47
0.74 2.89
2.95 1.69
10.29 14.29
3.72 2.18
La 0.80
1.44 2.89
3.29 11.07
Ce 10.28
7.77 5.04
18.89 26.60
3.57 6.32
4.22 2.44
10.24 2.03
1.90 0.89
2.54 3.66
1.58 0.41
0.68 0.39
Pr 1.18
0.95 5.86
4.38 9.36
8.87 3.95
8.80 12.61
1.32 3.50
2.77 1.59
Nd 1.31
0.97 3.01
2.89 1.08
1.68 2.49
0.20 0.76
0.74 0.40
Sm 1.24
1.04 0.40
0.35 0.54
0.36 0.08
Eu 0.13
0.28 0.21
0.23 4.25
3.74 1.40
1.60 2.25
Gd 0.18
0.97 0.78
0.98 0.49
1.45 0.75
0.72 0.25
0.27 0.39
0.24 0.03
0.17 0.08
Tb 0.16
0.13 1.39
0.97 4.42
4.11 1.41
1.40 2.14
0.14 0.77
0.96 0.51
Dy 0.30
0.22 0.94
0.91 0.31
0.31 0.49
0.03 0.17
0.21 0.11
Ho 2.37
2.30 0.78
0.80 1.31
0.79 0.07
Er 0.31
0.55 0.47
0.60 0.05
0.12 0.34
0.34 0.11
0.12 0.21
0.01 Tm
0.09 0.07
0.09 1.77
1.88 0.58
0.71 1.22
0.69 0.05
0.53 Yb
0.27 0.50
0.43 0.11
0.09 0.25
0.28 0.08
0.11 0.20
0.01 0.07
0.08 0.04
Lu 0.37
0.79 0.03
1.15 0.06
0.87 2.53
0.03 0.71
0.04 0.02
Hf 0.53
0.99 0.68
1.12 1.27
0.93 0.45
Pb 0.46
2.16 2.20
3.48 0.01
0.73 0.03
0.24 0.11
3.79 5.75
0.21 Th
1.27 0.92
0.11 0.02
0.45 0.43
1.53 1.52
0.27 0.15
0.03 U
0.41 0.50
0.22
87
Sr
86
Sr 0.70780
0.71805 0.70751
0.71104 n.d.
n.d. 0.71053
0.71798 0.71308
0.70879 0.71026
a
Do, dolomite; Lst, limestone; n.d., not determined.
Fig. 4. Chondrite-normalised rare earth element distribution in carbonate rocks from the Chameis sub-terrane.
rock. All of the meta-evaporite samples show a strong depletion in Rb, Cs, and Ba, associated
with a slight enrichment in Sr, and some of them are also depleted in Zr, Hf, Th and U. The
Dreimaster member dolomite from Bakers Bay follows a similar pattern as the meta-evaporite,
whereas an associated limestone is characterised by a marked enrichment in Sr.
The
87
Sr
86
Sr ratios determined for most of the carbonate samples are influenced by their Rb
contents and the addition of radiogenic
87
Sr 0.71053 – 0.71805, and they are therefore of no
further interest here. However, three meta-evapor- itic dolomite samples HFG211, 213a, 213b have
extremely low RbSr ratios of B 0.0005. They yielded
87
Sr
86
Sr ratios of 0.70879 9 0.00009, 0.7078 9 0.0001 and 0.7075 9 0.0001, respectively.
In particular the results obtained on the latter two samples, which come from a massive dolomite
breccia with HFG213a representing a dolomite clast
and HFG213b
coarsely recrystallised
dolomite matrix, are considered to be least influ- enced by any external radiogenic Sr and are there-
fore interpreted as approximating the initial ratio. reference for the trace element distribution in the
inferred meta-evaporite Fig. 5. That unit lacks mineralogical or textural evidence of a former
evaporite but has the appearance and composition of an ordinary dolomitised marine carbonate
Fig. 5. Trace element distribution in meta-evaporites and a limestone-succession from the lower Bogenfels formation normalised against the mean composition of a local dolomite sequence which shows no relation to evaporitic origin but appears to be a
diagenetically dolomitised shallow marine carbonate sequence.
Fig. 6. Stratigraphic correlation between units of the Marmora terrane and the Port Nolloth zone. Reference d
13
C curve for the latter from Fo¨lling et al. 1998.
4. Chemical and boron isotopic composition of tourmaline
