1996 and the ] 1.7 Ga Thackaringa group Willyama supergroup in New South Wales, Aus-
tralia Stevens et al., 1988; Slack et al., 1989, or the Neoproterozoic Duruchaus formation Nosib
group in the Damara belt, Namibia Behr et al., 1983, are believed to have been deposited in
non-marine, playa lake environments in rift grabens.
Within the Gariep belt, which forms a coast- parallel branch of the larger network of Pan-
African orogenic belts in southwestern Africa Fig. 1, we found a dolomite-dominated metased-
imentary succession that bears many similarities to the inferred former playa deposits in the Dam-
ara belt. The succession occurs, however, in a very different geologic setting compared with other
Proterozoic former evaporite deposits — it is not associated with rift sediments but with mafic and
ultramafic rocks which have been previously de- scribed as representing either an oceanic island or
an aseismic ridge Frimmel et al., 1996a.
One of the enigmas in the reconstruction of Neoproterozoic paleogeography in southwestern
Africa is the width, or existence at all, of oceanic basins between the various crustal fragments that
amalgamated during the Pan-African orogenies. The Marmora terrane in the Gariep belt Fig. 1
provides one of the very few examples of true oceanic crust in the Pan-African belts of south-
western Africa. It remains unclear whether this oceanic crust formed in a wide, open ocean basin
or in a narrow sea. Thus, the question arises whether inferred former evaporite deposits there
are of marine or non-marine origin. To address this question we studied the whole rock geochem-
istry and Sr, O and C isotopic compositions of various dolomite beds, determined the mineral
chemistry and boron isotopic composition of tourmaline from stratiform tourmalinite, and
analysed the chemistry of fluid inclusions in tour- maline, early quartz and calcite veins.
2. Geologic setting and lithology
The Gariep belt evolved from an ocean basin, the so-called Adamastor ocean, that separated the
Kalahari craton of southern Africa from the Rio de la Plata craton in South America. The belt is
subdivided into an eastern para-autochthonous zone, consisting of rift graben and passive conti-
nental margin deposits, and a western, al- lochthonous
terrane Marmora
terrane of
oceanic provenance. Three sub-terranes have been distinguished within the latter Frimmel and Hart-
nady, 1992, two of which are dominated by mafic
Fig. 1. Distribution of tectonostratigraphic units in the western Gariep belt.
Fig. 2. Stratigraphic subdivision of the Chameis group in the Chameis sub-terrane.
on the geology of this unit and a detailed strati- graphic description is being provided elsewhere
Frimmel, 2000. A thick basal sequence of meta- basalt and tuff, metamorphosed to thinly lami-
nated
greenschist Dernburg
formation containing bodies of metagabbro and serpentinite
Bakers Bay suite is capped by carbonate rocks and pelitic to psammitic metasedimentary rocks
of the Bogenfels formation Fig. 2. Geochemi- cally, most of the mafic rocks bear all the hall-
marks of oceanic within-plate basalts, whereas, some are better compared with mid-ocean ridge
basalt Frimmel et al., 1996a. Submarine extru- sion of the mafic volcanics is indicated by the
evidence of pillow lavas, whereby relatively shal- low water depths are implied by the occurrence of
hyaloclastite and mafic breccias. The Dernburg formation is overlain by laminated to massive
limestone and dolomite Dreimaster member, lower Bogenfels formation, in which high Sr
contents indicate the original presence of arago- nite Frimmel, 2000. A relatively deep water
anoxic or restricted lagoonal setting is inferred for these carbonate rocks from their high Sr, but also
conspicuously high H
2
S contents. The predomi- nantly siliciclastic upper Bogenfels formation is
interpreted to represent flysch sediments that were laid down during the closure of the Adamastor
ocean.
Within the upper Dernburg formation, we found at numerous localities exotic dropstones
and lonestones, reaching up to 1.5 m in length, embedded in a greenschist metatuff matrix Fig.
3. The most plausible explanation for the pres- ence of these exotic clasts is that they represent
very coarse-grained ice-rafted detritus. Conse- quently, the mafic matrix of this diamictite
Chameis Gate member is indicative of volcanic activity during times of glaciation.
The rocks of interest to this study occur below the diamictite but within the mafic Dernburg for-
mation. They were found at several localities throughout the Chameis sub-terrane Fig. 1 and
comprise a mixed sequence of thinly laminated, highly magnesian calcpelite with stratiform layers
and boudins of up to 1.5 m thick tourmalinite, thin chert bands, massive light to medium grey
dolomicrite, very coarse-grained, sugary textured,
Fig. 3. Diamicite of the Chameis gate member with exotic granite gneiss dropstones in greenschist matrix; above, metre-
size clast, 500 m east of Bogenfels; below, 20 cm long clast pierced into underlying mafic tuff layer younging direction is
towards the fore, i.e. bottom of photo.
rocks. All of them underwent regional lower greenschist facies metamorphism Frimmel, 1995
accompanying intense southeast- and eastward folding and thrusting during continent collision
dated at 545 Ma Frimmel and Frank, 1998. The rocks of interest here come from the northern-
most of these tectonostratigraphic units, the Chameis sub-terrane Fig. 1. Due to severe access
restrictions, very little work had been carried out
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
