Fig. 10. Na – Cl – Br systematics of fluid inclusion leachates from various vein types in the meta-evaporites sequence of the
Dernburg formation for legend see Fig. 9. Analytical uncer- tainties correspond to size of symbols.
6. Discussion
Extensive alkali-metasomatism in the whole Chameis sub-terrane is indicated by the wide-
spread occurrence of magnesioriebeckite and al- bite in proportions which cannot be explained by
any common igneous or sedimentary bulk rock composition Frimmel and Hartnady, 1992. The
source of the alkalis is most readily found in the dolomitic rocks that enclose, in most places tec-
tonically, large blocks of mafic and ultramafic rocks. A Na-rich sedimentary precursor of the
dolomite is postulated because of the widespread presence
of massive
albitite and
albite-rich dolomite, and the ubiquitous occurrence of mag-
nesioriebeckite and phlogopite in the dolomite. From the lack of calcite in the presence of
dolomite + talc + quartz, a very Mg-rich pre- metamorphic carbonate precursor rock, rich in
magnesite, is inferred. The presence of massive tourmalinite implies a major B source which,
apart from graniticpegmatitic melts, is most read- ily provided by a high evaporation rate of seawa-
ter. No granitic or pegmatitic bodies are known from the whole of the Chameis sub-terrane and its
exposed neighbouring tectonic units. Although there are few isolated occurrences of post-oro-
genic carbonatite bodies in the wider region e.g. Cooper and Reid, 1998, their small size and age
cannot be reconciled with the regional 100 km scale of alkali-metasomatism. The presence of an
initially highly magnesian, alkali-rich sediment with mobilisation of the alkalis in particular dur-
ing diagenesis and regional, syn-orogenic meta- morphism
is therefore
considered the
most plausible explanation for the mineral distribution
observed. Although reflecting the composition of a post-
depositional fluid, the limited fluid inclusion data obtained on the various vein samples, in particu-
lar their high overall salinity and their Na:Cl:Br proportions point toward a strong involvement of
seawater in the make-up of the post-depositional local pore water. Seawater is characterised by a
negative Ce anomaly because of its oxidative re- moval Elderfield and Greaves, 1982, whereas
positive Ce anomalies may be suggestive of either a reducing marine environment De Baar et al.,
volved in the make up of a fluid, the Na – Cl – Br relations are particularly useful Kesler et al.,
1995; Viets et al., 1996. Due to the very small partition coefficient of Br into the major evaporite
minerals, a sensitive monitor of the degree of evaporation is given by the ClBr ratio of the
residual brine. As seawater evaporates, the ClBr ratio of the residual brine remains similar to that
of seawater ClBr = 662 until halite saturation is reached. Further evaporation and precipitation of
halite leads to a progressive decrease of the ClBr and NaBr ratios. Residual brines that become
physically separated from their evaporite minerals will have ClBr ratios that reflect the degree of
evaporation. Similarly, dissolution of halite and other evaporite minerals in meteoric water or
seawater will cause ClBr ratios that are very high in comparison to those of the residual brines.
The fluid inclusions from the quartz – tourma- line veins have a composition almost identical to
that of seawater Fig. 10 and the aqueous inclu- sions in the quartz vein within the chert, which
are halite-undersaturated, plot on the line that delineates the trend of evaporated seawater. In
contrast, the calcite vein hosts saline aqueous inclusions whose ClBr ratio is, on average, higher
than that of seawater, thus indicating dissolution of halite. Furthermore, a shift in their NaBr ratio
away from the seawater line suggests some Na – Ca exchange.
1988 or an evaporative lacustrine environment Mo¨ller and Bau, 1993 and are typically associ-
ated with negative Eu anomalies. The lack of a positive Ce anomaly and a slightly negative to
absent Eu anomaly in the inferred meta-evaporite samples may be viewed, therefore, as pointing
against a lacustrine but toward a marine, slightly reducing environment. Such a mildly reducing
environment might be indicated also by the lack of NO
3 −
in those fluid inclusion leachate samples that contain small amounts of NH
4 +
as the NH
4 +
NO
3 −
ratio seems to be a monitor of redox poten- tial Frimmel et al., 1999.
A continental influence should be reflected by a strong light REE enrichment. Of all the carbonate
rocks analysed, the evaporite-derived dolomite samples display the least light REE enrichment.
In fact their REE patterns are markedly flat Fig. 4 and conform to those of the oceanic island
basalts in the Dernburg formation Frimmel et al., 1996a. Also, the strong depletion of the evap-
orite-derived dolomite in Rb, and to a lesser extent in Ba, Zr, Hf, and Th, reflects the lack of
detrital components from a continental source. Further evidence of a lack of continental input
comes from the Sr isotope data. Although the
87
Sr
86
Sr ratio of seawater fluctuated greatly dur- ing the Neoproterozoic era, the near-primary
87
Sr
86
Sr ratios obtained for the evaporite-derived dolomite are close to the lower end of the poten-
tial range of Neoproterozoic seawater composi- tion. Any continental influence should be reflected
by an increase in
87
Sr
86
Sr. A marine origin of the inferred meta-evaporite
is also supported by the B isotopic composition. The possible B sources for the tourmalines are
either hydrothermal fluids related to the mafic oceanic volcanism or seawater. Taking the maxi-
mum d
11
B value measured + 27.5‰ and allow- ing for isotope fractionation between tourmaline
and aqueous fluid Palmer et al., 1992, a tourma- line formation temperature of 200°C would corre-
spond to a d
11
B
fluid
value around 40‰ which is typical of seawater. The minimum d
11
B value obtained for tourmalinite in the meta-evaporite
succession 10.7‰ can be explained by either a lower d
11
B
fluid
24‰ at the same temperature as above or by a lower temperature 50°C and an
isotopic composition typical of seawater, or by a combination of both possibilities. The theoretical
temperatures calculated are in perfect agreement with an expected tourmaline formation during
diagenesis. As there is no obvious reason for tourmaline formation to take place over a temper-
ature interval of some 150°C, the lower d
11
B values obtained may be indicative of mixing of
seawater with submarine hydrothermal fluids as- sociated with the igneous activity that is manifest
in the Dernburg formation. By analogy, the high proportions of SO
4 2 −
detected in some of the tourmalinite-hosted fluid inclusions could be re-
lated either to evaporitic sulphates or to sulphuric volatiles exhaled by hydrothermal vents on the
volcanic edifice.
In conclusion, all our new data are in line with the presence of marine evaporites within a forma-
tion that is otherwise dominated by oceanic mafic rocks. As most of these mafic rocks represent
either former oceanic islands or an aseismic ridge Frimmel et al., 1996a, and because of the associ-
ation with stromatolitic dolomite, the most likely depositional environment we envisage for the for-
mation of the evaporite is that of an atoll, sur- rounded by stromatolitic reef mounds on top of a
guyot.
7. Paleogeographic and climatic implications
