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
Tourmaline occurs in the form of massive, up to 1 m thick, stratiform tourmalinite layers, lenses
and boudins within a calcpelite horizon or at the interface with overlying dolomite. In addition,
tourmaline is found in bedding-parallel and cross- cutting quartz veins. Tourmaline is a most useful
petrogenetic indicator mineral whose chemical composition and boron isotopic composition have
been used successfully in the past to discriminate between different environments of tourmaline for-
mation and possible boron sources Henry and Guidotti, 1985; Jiang, 1998. We therefore,
analysed the mineral chemistry of tourmaline, us- ing conventional electron microprobe techniques
at the Department of Geological Sciences, UCT for further analytical details see Frimmel et al.,
1995, and the boron isotopic composition by negative ion thermal ionisation mass spectrometry
at the Max-Planck-Institut fu¨r Chemie in Mainz. For the latter type of analysis, tourmalinite pow-
ders were decomposed in tightly-capped Teflon vials with a mixture of HF + HNO
3
at tempera- tures of 100°C for about 1 week until they were
completely digested. Mannitol was added to the samples before decomposition in order to sup-
press boron volatilisation. The boron fractions in the samples were extracted and purified by using a
cation-exchange resin AG 50WX12, 200 – 400 mesh and a boron-specific resin Amberlite 743,
40 – 80 mesh; Aggarwall and Palmer, 1995. Dur- ing the period of this study, 25 analyses of the
NIST boric acid SRM 951 standard yielded an average
11
B
10
B ratio of 4.0087 9 0.7 2s. The external 2s precision of the measurements for all
samples is estimated to be better than 9 1.0‰ based on duplicate analyses. The boron isotope
data are reported in conventional per mil d nota- tion as d
11
B = [
11
B
10
B
sample 11
B
10
B
standard
− 1] × 1000.
Tourmaline has
an ideal
formula of
XY
3
Z
6
BO
3 3
Si
6
O
18
OH, F
4
, where X = Na, Ca, K, or vacancy; Y = Mg, Fe, Mn, Al, and Li; and
Z = Al, Fe, Cr, and V. Formulae were calculated on the assumptions that B and OH + F are
present in stoichiometric quantities, possible Si- In addition, d
18
O and d
13
C values for the dolomite samples were used, in conjunction with a larger
database for all the carbonate sequences in the Gariep and other Pan-African belts in southwest-
ern Africa, for stratigraphic correlation of these units Frimmel, 2000. The stromatolitic dolomite
within the Dernburg formation differs with d
13
C values of up to 2.82‰ from the carbonate rocks
of the Dreimaster member whose d
13
C values range
from −
2.37 to
− 0.18‰.
Using a
chemostratigraphic profile through the passive continental margin sequence in the external
Gariep belt Port Nolloth zone as reference Fo¨lling et al., 1998, the d
13
C of the marine carbonates in the Marmora terrane compare best
with those immediately underlying and overlying the glaciogenic Numees formation diamictite Fig.
6. Thus, a correlation between the stromatolitic Gais and Sholtzberg members in the Marmora
terrane with the upper Dabie River formation, which contains similar stromatolites resembling
Conophyton, in the Port Nolloth zone, is envis- aged Frimmel, 2000. This is supported by the Sr
isotope data as the Dabie River formation dolomite and limestone is characterised by rela-
tively low
87
Sr
86
Sr, as found in the evaporitic dolomite studied here, in contrast to those car-
bonate rocks that follow above the Numees for- mation diamictite, all of which have
87
Sr
86
Sr ratios significantly higher than 0.708.
deficiency in the tetrahedral site is balanced by Al, with the remaining Al occupying the Z-site. Fur-
thermore, it was assumed that no significant amounts of Li are present as there are no granite
or pegmatite to which the tourmaline could be related, and the associated dolomite-rich rocks
display very low Li concentrations B 1 ppm. The analysis of some 80 tourmaline grains re-
vealed that all of these grains are of dravitic composition Table 2 with NaNa + Ca of
0.81 – 1.00. No relation exists between composi- tion and mode of occurrence or mineral assem-
blage. All analyses show an Al-deficiency in the Z-site and this site was then filled with Fe
3 +
and Cr. If any Fe was left, it was assigned as Fe
2 +
to the Y-site, resulting in Fe
2 +
Fe
2 +
+ Mg ratios
of up to 0.43, but also in an over-occupancy of the Y site which suggests that some of that Fe is
also present as Fe
3 +
. The real Fe
2 +
Fe
2 +
+ Mg
is more likely close to zero, in which case the bulk of the analyses would plot into the field for meta-
carbonate rocks as defined by Henry and Guidotti 1985 Fig. 7. Thus, the tourmaline composition
is compatible with an evaporitic origin.
To distinguish between a marine and a non- marine precursor, B isotope ratios can be particu-
larly useful. Marine evaporites are characterised by distinctly higher d
11
B values + 18.2 to +
31.7‰ than non-marine evaporites Swihart et al., 1986 whose d
11
B values − 30.1 to + 7.0‰ span the range typical of most other rock types
Jiang, 1998. Four samples of stratiform tourma- linite layers and nodules from the inferred meta-
evaporite sequence near Bakers Bay gave very
Table 2 Representative electron microprobe analyses of tourmaline from the Dernburg formation
215 Rim 216 Core
HFG c 216 Rim
153 Core 217 Core
217 Rim 153 Rim
215 Core 33.87
35.44 35.45
36.11 36.20
33.75 34.37
SiO
2
34.52 Al
2
O
3
30.21 31.82
26.68 31.13
30.16 28.52
28.02 25.53
FeO
a
5.70 8.52
9.59 8.52
7.02 4.27
6.05 9.33
10.66 6.33
10.37 6.72
4.75 Fe
2
O
3 a
7.80 9.46
9.47 0.27
1.64 1.14
1.58 TiO
2
0.09 0.09
0.22 1.85
Cr
2
O
3
0.03 0.00
0.00 0.01
0.41 0.00
0.00 0.02
0.01 0.04
0.03 0.01
0.00 0.02
0.04 0.06
MnO MgO
11.62 8.14
10.72 9.41
8.99 11.87
10.95 10.18
0.03 0.09
0.01 0.07
CaO 0.16
0.49 0.31
0.47 K
2
O 0.02
0.01 0.00
0.02 0.00
0.02 0.03
0.03 Na
2
O 2.47
2.55 2.89
3.06 3.34
3.20 2.76
2.95 0.00
0.00 0.04
0.00 F
0.00 0.00
0.30 0.00
0.00 0.00
0.00 0.00
0.00 0.00
0.00 Cl
0.00 10.68
10.53 10.41
10.46 10.49
B
2
O
3 b
10.33 10.49
10.49 97.24
96.48 95.93
96.42 96.21
96.35 97.15
95.19 Total
c
5.81 5.61
5.88 5.88
5.88 Si
5.97 5.64
5.71 0.03
0.36 0.29
IV
Al 0.12
0.12 0.12
0.39 0.19
5.95 5.77
4.87 5.84
5.35 5.35
5.52 5.11
VI
AlZ 0.05
0.23 0.16
0.65 0.58
0.48 Fe
3+
1.13 0.89
0.00 0.19
0.16 0.31
0.46 0.81
0.96 1.13
Fe
2+
0.00 0.00
0.00 0.05
0.00 0.00
0.00 0.00
Cr 0.20
0.01 0.01
0.03 0.23
0.03 0.20
0.14 Ti
0.01 0.01
0.00 0.00
0.01 0.00
0.00 0.00
Mn 2.51
2.02 2.87
2.69 2.34
2.21 2.88
2.71 Mg
0.08 0.06
0.09 0.03
0.01 Ca
0.00 0.02
0.01 0.00
0.00 0.01
0.01 K
0.00 0.00
0.00 0.00
Na 1.02
0.89 0.81
0.82 0.93
0.98 0.95
1.05
a
Calculated Fe
3+
Fe
2+
minimum ratio as required to completely fill Z-site.
b
Calculated assuming 3 B per formula unit.
c
Corrected for FO equivalent.
Fig. 7. Cation variation diagrams for tourmalines from the Dernburg formation, Marmora terrane; data fields after Henry
and Guidotti 1985, 1, Li-rich felsic intrusives; 2, Li-poor felsic intrusives; 3, hydrothermally altered granites; 4 and 5,
Al-saturated and Al-undersaturated, respectively, metapelities and metapsammites, 6, Fe
3 +
-rich quartz-tourmaline rocks, calc-silicates, and metapelites; 7, low-Ca ultramafic rocks; 8,
metacarbonate rocks and metapyroxenites.
low d
11
B of + 4.0‰ Fig. 8. All in all, the d
11
B values for tourmaline grains from the Chameis
sub-terrane, though spanning over a relatively large interval, are, in general, very high and
provide strong evidence of a marine evaporitic origin, although a continental setting with highly
evolved
seawater-derived brines
cannot be
excluded.
5. Fluid inclusion chemistry
