Marine evaporites from an oceanic island in the ...directory.umm.ac.id/Data...

Precambrian Research 105 (2001) 57–71

Marine evaporites from an oceanic island in theNeoproterozoic Adamastor ocean

H.E. Frimmel a,*, S.-Y. Jiang b

a Department of Geological Sciences, Uni6ersity of Cape Town, Pri6ate Bag, Rondebosch 7701, South Africab State Key Laboratory of Mineral Deposit Research, Department of Earth Sciences, Nanjing Uni6ersity,

Nanjing 210093, People’s Republic of China

Received 28 October 1999; accepted 17 July 2000

Abstract

We report a hitherto unknown occurrence of ancient Neoproterozoic evaporite deposits from an allochthonousterrane in the Pan-African Gariep belt in Namibia. Low contents of Rb, Cs, Ba, Zr, Hf, Th, and U, flatchondrite-normalised rare earth element (REE) patterns, 87Sr/86Sr ratios as low as 0.7075, Na–Cl–Br systematics offluid inclusion leachates, and high d11B values for stratiform tourmalinites, together with geologic evidence, such asassociation with oceanic basalt, gabbro, and stromatolitic dolomite, point to a marine evaporitic origin. An atollenvironment on an oceanic island is envisaged as a likely depositional setting. In the associated mafic sequence wefound a diamictite with metre-sized ice rafted detritus, suggesting the presence of sea ice cover at relatively lowlatitude around the time of evaporite deposition. Based on chemostratigraphic (87Sr/86Sr, d13C) comparison withpassive continental margin sediments in the para-autochthonous external part of the Gariep belt, a correlation of themafic diamicitite with the global Varangian (590–560 Ma) glaciation is proposed. © 2001 Elsevier Science B.V. Allrights reserved.

Keywords: Gariep belt; Marmora terrane; Neoproterozoic; Evaporite; Boron isotopes; Varangian glaciation

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1. Introduction

The record of Precambrian evaporite deposits issparse which is largely due to the fact that pri-mary evaporite minerals do not survive even low-grade metamorphism easily and post-depositionalmetasomatism in these rocks often obliterates pri-

mary geochemical signatures. Yet, knowledge ofthe distribution of evaporite deposits is pivotal forthe reconstruction of stratigraphic correlation, pa-leogeography and paleoclimate. The presence ofArchean evaporite deposits in the Barbertongreenstone belt, South Africa, has been inferredfrom d11B values for tourmaline (Byerly andPalmer, 1991). Only few Proterozoic examples offormer evaporite occurrences exist. Most of them,such as the 2.1 Ga borate deposits in the Liaohegroup of eastern Liaoning, China (Jiang et al.,

* Corresponding author. Tel.: +24-21-6502901; fax: +24-21-6503783.

E-mail address: [email protected] (H.E. Frimmel).

0301-9268/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved.

PII: S 0301 -9268 (00 )00104 -2

H.E. Frimmel, S.-Y. Jiang / Precambrian Research 105 (2001) 57–7158

1996) and the ]1.7 Ga Thackaringa group(Willyama supergroup) in New South Wales, Aus-tralia (Stevens et al., 1988; Slack et al., 1989), orthe Neoproterozoic Duruchaus formation (Nosibgroup) in the Damara belt, Namibia (Behr et al.,1983), are believed to have been deposited innon-marine, playa lake environments in riftgrabens.

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 similaritiesto the inferred former playa deposits in the Dam-ara belt. The succession occurs, however, in a verydifferent geologic setting compared with otherProterozoic former evaporite deposits — it is notassociated with rift sediments but with mafic andultramafic rocks which have been previously de-scribed as representing either an oceanic island oran aseismic ridge (Frimmel et al., 1996a).

One of the enigmas in the reconstruction ofNeoproterozoic paleogeography in southwesternAfrica is the width, or existence at all, of oceanicbasins between the various crustal fragments thatamalgamated 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 thisoceanic crust formed in a wide, open ocean basinor in a narrow sea. Thus, the question ariseswhether inferred former evaporite deposits thereare of marine or non-marine origin. To addressthis question we studied the whole rock geochem-istry and Sr, O and C isotopic compositions ofvarious dolomite beds, determined the mineralchemistry and boron isotopic composition oftourmaline from stratiform tourmalinite, andanalysed 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 theKalahari craton of southern Africa from the Riode la Plata craton in South America. The belt issubdivided into an eastern para-autochthonouszone, consisting of rift graben and passive conti-nental margin deposits, and a western, al-lochthonous terrane (Marmora terrane) ofoceanic provenance. Three sub-terranes have beendistinguished 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.

H.E. Frimmel, S.-Y. Jiang / Precambrian Research 105 (2001) 57–71 59

Fig. 2. Stratigraphic subdivision of the Chameis group in theChameis 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 rocksand pelitic to psammitic metasedimentary rocksof 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 ridgebasalt (Frimmel et al., 1996a). Submarine extru-sion of the mafic volcanics is indicated by theevidence of pillow lavas, whereby relatively shal-low water depths are implied by the occurrence ofhyaloclastite and mafic breccias. The Dernburgformation is overlain by laminated to massivelimestone and dolomite (Dreimaster member,lower Bogenfels formation), in which high Srcontents indicate the original presence of arago-nite (Frimmel, 2000). A relatively deep wateranoxic or restricted lagoonal setting is inferred forthese carbonate rocks from their high Sr, but alsoconspicuously high H2S contents. The predomi-nantly siliciclastic upper Bogenfels formation isinterpreted to represent flysch sediments that werelaid down during the closure of the Adamastorocean.

Within the upper Dernburg formation, wefound at numerous localities exotic dropstonesand 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 representvery coarse-grained ice-rafted detritus. Conse-quently, the mafic matrix of this diamictite(Chameis Gate member) is indicative of volcanicactivity during times of glaciation.

The rocks of interest to this study occur belowthe diamictite but within the mafic Dernburg for-mation. They were found at several localitiesthroughout the Chameis sub-terrane (Fig. 1) andcomprise a mixed sequence of thinly laminated,highly magnesian calcpelite with stratiform layersand boudins of up to 1.5 m thick tourmalinite,thin chert bands, massive light to medium greydolomicrite, very coarse-grained, sugary textured,

Fig. 3. Diamicite of the Chameis gate member with exoticgranite gneiss dropstones in greenschist matrix; above, metre-size clast, 500 m east of Bogenfels; below, 20 cm long clastpierced into underlying mafic tuff layer (younging direction istowards the fore, i.e. bottom of photo).

rocks. All of them underwent regional lowergreenschist facies metamorphism (Frimmel, 1995)accompanying intense southeast- and eastwardfolding and thrusting during continent collisiondated at 545 Ma (Frimmel and Frank, 1998). Therocks of interest here come from the northern-most of these tectonostratigraphic units, theChameis sub-terrane (Fig. 1). Due to severe accessrestrictions, very little work had been carried out


white dolosparite, breccia with irregularly shapedclasts of dolomite of the latter type set in anequally coarse-grained, dark grey to pinkdolosparite matrix, and massive albitite. This se-quence (Sholtzberg member, Fig. 2) is laterallynot continuous but occurs over a strike length ofonly a few hundred metres. Locally, massive stro-matolitic, Fe-rich dolomite, intercalated withinthe metabasalt sequence, is developed in the vicin-ity of the mixed sequence. A similar association ofoceanic metabasalt, hyaloclastite and, in places,stromatolitic and oolitic dolomite is also knownfrom the Schakalsberge sub-terrane (Fig. 1). Thattectono-stratigraphic unit is made up of a thicksequence of greenschist (Grootderm formation)which is capped by dolomite of the Gais member(Frimmel et al., 1996a) — a likely correlative ofthe Sholtzberg member in the Chameis sub-terrane.

No estimates can be made on the total thick-ness of the various stratigraphic units in theChameis sub-terrane because of intense foldingand thrusting. Although most contacts are tec-tonic, a few examples exist of gabbro havingintruded the Sholtzberg member causing decime-ter-thick contact metamorphic aureoles in calc-pelite. In most cases, however, it appears as if themetagabbro bodies were tectonically emplacedwith preferential movement along these dolomite-rich strata, which gave rise to a previous interpre-tation of the whole Chameis sub-terranerepresenting a tectonic melange zone (Frimmeland Hartnady, 1992).

The dominant minerals in the mixed calcareoussuccession of the Sholtzberg member aredolomite, albite, and quartz. In addition, magne-sioriebeckite, talc, clinochlore, phlogopite, tour-maline, and hematite (replacing either magnetiteor pyrite) occur in effectively all rock types but inhighly variable proportions. Tourmaline-bearingmineral 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 themafic rocks in the Chameis sub-terrane and havebeen previously ascribed to extensive Na-metaso-matism (Frimmel and Hartnady, 1992). The tim-

ing of this metasomatism must have been prior toor during the main phase of orogenic deformationas the sodic amphibole has grown syn-tectonicallywith respect to the major phase of folding (Frim-mel, 1995). Sodium salts in a sediment, dominatedby Mg-carbonate and enriched in B, provide alikely source for the Na-metasomatism, and byanalogy with dolomitic rocks containing similarmineral associations in the Duruchaus formationof the Damara belt (Behr et al., 1983), theSholtzberg member is interpreted as representinga low grade metamorphosed former evaporite de-posit.

3. Dolomite geochemistry

Eleven samples of the various dolomite types inthe suspected meta-evaporite sequence wereanalysed for their trace and rare earth element(REE) contents using an ELAN 6000 ICP-MS(for analytical procedures see Frimmel, 2000) andfor their Sr isotopic composition (Table 1) usingconventional ion-exchange techniques and a VGSector 7-collector mass spectrometer (see Frimmelet al., 1996a) at the Department of GeologicalSciences, 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°40.11%S, 15°32.35%E (samplesHFG211-214) are compared with dolomite andlimestone 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°35.59%S,15°32.20%E. All the dolomite samples from theinferred evaporite sequence display a generally flatchondrite-normalised REE pattern (La/Yb=1.6–6.7), and no Ce and Eu anomalies (Fig. 4). Asuccession of limestone and dolomite, represent-ing the lower Bogenfels formation 7 km to thesouth (27°41.58%S, 15°32.48%E), differs by display-ing a light REE enrichment (La/Yb=11.7–14.5and 43.6, respectively).

We chose a relatively pure dolomite sequencefrom the Dreimaster member in the Bogenfelsformation (samples HFG192 and HFG193) as


Tab

le1

Tra

ce,

rare

eart

hel

emen

tan

dSr

isot

ope

data

for

carb

onat

ero

cks

from

the

Cha

mei

ssu

b-te

rran

ea

HF

Gc

Der

nbur

gfo

rmat

ion

Bog

enfe

lsfo

rmat

ion

204

193

192

211

212

213a

224

213b

223

214

222

Do

Lst

Do-

mat

rix

Do

Lst

Do-

clas

tD

oD

oD

oD

oD

o

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

Sc1.

110.

808.

5910

.68

3.25

97.7

18.

0819

.92

23.6

31.

528.

806.

992.

38V

6.37

14.6

02.

2631

.82

17.5

020

.60

76.6

12.

3311

.98

3.34

1.44

Cr

2.60

16.0

33.

472.

814.

493.

291.

42C

o1.

981.

271.

982.

40N

i8.

086.

2017

.15

9.95

9.03

13.1

25.

725.

557.

979.

826.

050.

060.

130.

0235

.53

39.5

50.

220.

1712

.38

Rb

0.01

1.25

11.6

521

1.28

71.6

520

2.21

258.

1029

0.95

814.

0314

12.2

618

2.96

69.2

717

4.63

217.

89Sr

8.16

7.20

22.8

726

.44

9.28

9.14

14.2

71.

005.

576.

603.

08Y

0.69

45.3

42.

6732

.75

101.

7113

.13

1.34

Zr

0.50

3.14

28.3

731

.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.

7914

7.69

Ba

1.65

97.5

882

.22

1.31

1.47

0.74

2.89

2.95

1.69

10.2

914

.29

3.72

2.18

La

0.80

1.44

2.89

3.29

11.0

7C

e10

.28

7.77

5.04

18.8

926

.60

3.57

6.32

4.22

2.44

10.2

42.

031.

900.

892.

543.

661.

580.

410.

680.

39P

r1.

180.

955.

864.

389.

368.

873.

958.

8012

.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

Sm1.

241.

040.

400.

350.

540.

360.

08E

u0.

130.

280.

210.

234.

253.

741.

401.

602.

25G

d0.

180.

970.

780.

980.

491.

450.

750.

720.

250.

270.

390.

240.

030.

170.

08T

b0.

160.

131.

390.

974.

424.

111.

411.

402.

140.

140.

770.

960.

51D

y0.

300.

220.

940.

910.

310.

310.

490.

030.

170.

210.

11H

o2.

372.

300.

780.

801.

310.

790.

07E

r0.

310.

550.

470.

600.

050.

120.

340.

340.

110.

120.

210.

01T

m0.

090.

070.

091.

771.

880.

580.

711.

220.

690.

050.

53Y

b0.

270.

500.

430.

110.

090.

250.

280.

080.

110.

200.

010.

070.

080.

04L

u0.

370.

790.

031.

150.

060.

872.

530.

030.

710.

040.

02H

f0.

530.

990.

681.

121.

270.

930.

45P

b0.

462.

162.

203.

480.

010.

730.

030.

240.

113.

795.

750.

21T

h1.

270.

920.

110.

020.

450.

431.

531.

520.

270.

150.

03U

0.41

0.50

0.22

87Sr

/86Sr

0.70

780

0.71

805

0.70

751

0.71

104

n.d.

n.d.

0.71

053

0.71

798

0.71

308

0.70

879

0.71

026

aD

o,do

lom

ite;

Lst

,lim

esto

ne;

n.d.

,no

tde

term

ined

.


Fig. 4. Chondrite-normalised rare earth element distribution incarbonate rocks from the Chameis sub-terrane.

rock. All of the meta-evaporite samples show astrong depletion in Rb, Cs, and Ba, associatedwith a slight enrichment in Sr, and some of themare also depleted in Zr, Hf, Th and U. TheDreimaster member dolomite from Bakers Bayfollows a similar pattern as the meta-evaporite,whereas an associated limestone is characterisedby a marked enrichment in Sr.

The 87Sr/86Sr ratios determined for most of thecarbonate samples are influenced by their Rbcontents and the addition of radiogenic 87Sr(0.71053–0.71805), and they are therefore of nofurther interest here. However, three meta-evapor-itic dolomite samples (HFG211, 213a, 213b) haveextremely low Rb/Sr ratios of B0.0005. Theyyielded 87Sr/86Sr ratios of 0.7087990.00009,0.707890.0001 and 0.707590.0001, respectively.In particular the results obtained on the latter twosamples, which come from a massive dolomitebreccia with HFG213a representing a dolomiteclast and HFG213b coarsely recrystalliseddolomite 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 theinferred meta-evaporite (Fig. 5). That unit lacksmineralogical or textural evidence of a formerevaporite but has the appearance and compositionof an ordinary dolomitised marine carbonate

Fig. 5. Trace element distribution in meta-evaporites and a limestone-succession from the lower Bogenfels formation normalisedagainst the mean composition of a local dolomite sequence which shows no relation to evaporitic origin but appears to be adiagenetically dolomitised shallow marine carbonate sequence.


Fig. 6. Stratigraphic correlation between units of the Marmoraterrane and the Port Nolloth zone. Reference d13C curve forthe latter from Folling et al. (1998).

4. Chemical and boron isotopic composition oftourmaline

Tourmaline occurs in the form of massive, upto 1 m thick, stratiform tourmalinite layers, lensesand boudins within a calcpelite horizon or at theinterface with overlying dolomite. In addition,tourmaline is found in bedding-parallel and cross-cutting quartz veins. Tourmaline is a most usefulpetrogenetic indicator mineral whose chemicalcomposition and boron isotopic composition havebeen used successfully in the past to discriminatebetween different environments of tourmaline for-mation and possible boron sources (Henry andGuidotti, 1985; Jiang, 1998). We therefore,analysed the mineral chemistry of tourmaline, us-ing conventional electron microprobe techniquesat the Department of Geological Sciences, UCT(for further analytical details see Frimmel et al.,1995), and the boron isotopic composition bynegative ion thermal ionisation mass spectrometryat the Max-Planck-Institut fur Chemie in Mainz.For the latter type of analysis, tourmalinite pow-ders were decomposed in tightly-capped Teflonvials with a mixture of HF+HNO3 at tempera-tures of �100°C for about 1 week until they werecompletely digested. Mannitol was added to thesamples before decomposition in order to sup-press boron volatilisation. The boron fractions inthe samples were extracted and purified by using acation-exchange resin (AG 50WX12, 200–400mesh) and a boron-specific resin (Amberlite 743,40–80 mesh; Aggarwall and Palmer, 1995). Dur-ing the period of this study, 25 analyses of theNIST boric acid SRM 951 standard yielded anaverage 11B/10B ratio of 4.008790.7 (2s). Theexternal 2s precision of the measurements for allsamples is estimated to be better than 91.0‰based on duplicate analyses. The boron isotopedata are reported in conventional per mil d nota-tion as d11B (= [(11B/10B)sample/(11B/10B)standard−1]×1000.

Tourmaline has an ideal formula of(XY3Z6BO3)3Si6O18(OH, F)4, where X=Na, Ca,K, or vacancy; Y=Mg, Fe, Mn, Al, and Li; andZ=Al, Fe, Cr, and V. Formulae were calculatedon the assumptions that B and (OH+F) arepresent in stoichiometric quantities, possible Si-

In addition, d18O and d13C values for the dolomitesamples were used, in conjunction with a largerdatabase for all the carbonate sequences in theGariep and other Pan-African belts in southwest-ern Africa, for stratigraphic correlation of theseunits (Frimmel, 2000). The stromatolitic dolomitewithin the Dernburg formation differs with d13Cvalues of up to 2.82‰ from the carbonate rocksof the Dreimaster member whose d13C valuesrange from −2.37 to −0.18‰. Using achemostratigraphic profile through the passivecontinental margin sequence in the externalGariep belt (Port Nolloth zone) as reference(Folling et al., 1998), the d13C of the marinecarbonates in the Marmora terrane compare bestwith those immediately underlying and overlyingthe glaciogenic Numees formation diamictite (Fig.6). Thus, a correlation between the stromatoliticGais and Sholtzberg members in the Marmoraterrane with the upper Dabie River formation,which contains similar stromatolites resemblingConophyton, in the Port Nolloth zone, is envis-aged (Frimmel, 2000). This is supported by the Srisotope data as the Dabie River formationdolomite and limestone is characterised by rela-tively low 87Sr/86Sr, as found in the evaporiticdolomite studied here, in contrast to those car-bonate rocks that follow above the Numees for-mation diamictite, all of which have 87Sr/86Srratios 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 significantamounts of Li are present as there are no graniteor pegmatite to which the tourmaline could berelated, and the associated dolomite-rich rocksdisplay very low Li concentrations (B1 ppm).The analysis of some 80 tourmaline grains re-vealed that all of these grains are of draviticcomposition (Table 2) with Na/(Na+Ca) of0.81–1.00. No relation exists between composi-tion and mode of occurrence or mineral assem-blage. All analyses show an Al-deficiency in theZ-site and this site was then filled with Fe3+ andCr. If any Fe was left, it was assigned as Fe2+ tothe Y-site, resulting in Fe2+/(Fe2+ +Mg) ratiosof up to 0.43, but also in an over-occupancy of

the Y site which suggests that some of that Fe isalso present as Fe3+. The real Fe2+/(Fe2+ +Mg)is more likely close to zero, in which case the bulkof the analyses would plot into the field for meta-carbonate rocks as defined by Henry and Guidotti(1985) Fig. 7). Thus, the tourmaline compositionis 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 characterisedby distinctly higher d11B values (+18.2 to+31.7‰) than non-marine evaporites (Swihart etal., 1986) whose d11B 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 2Representative electron microprobe analyses of tourmaline from the Dernburg formation

215 Rim 216 CoreHFGc 216 Rim153 Core 217 Core 217 Rim153 Rim 215 Core

33.8735.4435.4536.1136.2033.7534.37SiO2 34.52Al2O3 30.2131.82 26.6831.13 30.16 28.52 28.02 25.53FeOa 5.708.52 9.598.52 7.02 4.27 6.05 9.33

10.666.3310.376.724.75Fe2O3a 7.809.469.47

0.27 1.64 1.14 1.58TiO2 0.090.09 0.22 1.85Cr2O3 0.030.000.000.010.410.000.000.02

0.010.040.030.01 0.000.020.040.06MnOMgO 11.628.14 10.729.41 8.99 11.87 10.95 10.18

0.03 0.09 0.01 0.07CaO 0.16 0.49 0.31 0.47K2O 0.02 0.01 0.00 0.02 0.00 0.02 0.03 0.03Na2O 2.472.552.893.063.343.202.762.95

0.00 0.00 0.040.00F 0.00 0.000.30 0.000.00 0.000.000.000.000.000.00Cl 0.00

10.6810.5310.4110.46 10.49B2O3b 10.3310.4910.49

97.2496.48 95.93 96.4296.21 96.35 97.15 95.19Totalc

5.815.615.885.885.88Si 5.975.645.710.030.360.29IVAl 0.120.12 0.12 0.39 0.19

5.95 5.77 4.875.84 5.35 5.35 5.52 5.11VIAl(Z)0.05 0.23 0.16 0.650.58 0.48Fe3+ 1.13 0.89

0.00 0.19 0.16 0.31 0.460.810.961.13Fe2+

0.00 0.00 0.00 0.05 0.00 0.00 0.00 0.00Cr0.200.01 0.01 0.03 0.23 0.03 0.20 0.14Ti

0.010.01 0.00 0.000.01 0.00 0.00 0.00Mn2.512.02 2.87 2.692.34 2.21 2.88 2.71Mg

0.080.060.090.030.01Ca 0.000.020.010.00 0.00 0.01 0.01K 0.00 0.00 0.00 0.00

Na 1.020.89 0.810.820.930.980.95 1.05

a Calculated Fe3+/Fe2+ minimum ratio as required to completely fill Z-site.b Calculated assuming 3 B per formula unit.c Corrected for F�O equivalent.


Fig. 7. Cation variation diagrams for tourmalines from theDernburg formation, Marmora terrane; data fields after Henryand Guidotti (1985), 1, Li-rich felsic intrusives; 2, Li-poorfelsic intrusives; 3, hydrothermally altered granites; 4 and 5,Al-saturated and Al-undersaturated, respectively, metapelitiesand metapsammites, 6, Fe3+-rich quartz-tourmaline rocks,calc-silicates, and metapelites; 7, low-Ca ultramafic rocks; 8,metacarbonate rocks and metapyroxenites.

low d11B of +4.0‰ (Fig. 8). All in all, the d11Bvalues for tourmaline grains from the Chameissub-terrane, though spanning over a relativelylarge interval, are, in general, very high andprovide strong evidence of a marine evaporiticorigin, although a continental setting with highlyevolved seawater-derived brines cannot beexcluded.

5. Fluid inclusion chemistry

Eleven samples of quartz-chlorite-tourmalineveins, coarse-grained sparry, pink calcite veinswithin dolomite, a quartz vein within a blackchert at the base of the evaporite-deriveddolomite, and stratiform tourmalinite all ofwhich, based on microthermometric and texturalobservations, appear to be dominated by only asingle fluid inclusion generation were selected forcrush-leach analysis at the Department of Geolog-ical Sciences, UCT, to determine the relativeabundance of dissolved ionic species. The fluidinclusions are undersaturated to saturated two orthree-phase (vapour, liquid, with or without halitedaughter crystal) aqueous and CO2-rich. They are1–5 mm in size and display highly variable liquid,vapour ratios which is most likely due to neckingof these fluid inclusions after they have reachedthe liquid–vapour curve on their retrograde P–Tpath. About 500 mg of cleaned mineral separatewere crushed under doubly distilled de-ionised

high d11B values of +10.7 to +27.5‰ (Fig. 8).One tourmaline sample from a tourmaline-richdolomite from a similar succession at ChameisBay also yielded a high d11B value of +20.1‰. Incontrast, one tourmaline sample from a verycoarse-grained tourmaline aggregate from a mag-nesian calcpelite about 1 km to the east of thesample locality near Bakers Bay has a relatively

Fig. 8. Boron isotopic composition of tourmalines from the Dernburg formation, compared with those of marine and non-marineevaporites and seawater (after Jiang, 1998).


Table 3Sample average concentrations (in mg/l) of solutes in fluid inclusions from the Sholtzberg member

NaSample typea NH4 K Mg Mn Ca Cl Br SO4

1 29.18 0.00 7.54 4.87 0.00 29.02 72.58 0.13 1.900.00 7.06 4.40 0.0028.25 26.951 63.23 0.21 2.19

2 3.00 0.00 0.92 1.39 0.00 8.61 4.91 0.03 2.630.21 2.02 0.31 0.5416.30 27.903 28.45 0.10 17.490.27 2.43 0.92 0.853 63.9621.83 39.08 0.15 113.470.08 3.71 5.17 0.003.34 68.794 3.95 0.00 81.260.06 2.85 4.43 0.00 30.74 4.03 0.00 113.544 2.400.09 2.44 3.79 0.002.15 23.184 4.28 0.00 315.29

2.265 0.14 0.69 1.93 0.00 2.38 5.45 0.00 2.000.605 0.10 0.45 1.87 0.00 2.71 4.20 0.00 2.28

0.07 0.43 1.58 0.000.00 1.775 3.20 0.00 0.56

a 1, Sparry calcite vein; 2, quartz vein in chert; 3, quartz–chlorite–tourmaline vein in meta-evaporite; 4, tourmaline nodule; 5,stratiform tourmalinite layer.

water and the resultant leachate was analysedusing high performance ion chromatography(HPIC) for Na+, K+, NH4

+, Ca2+, Mg2+, Mn2+,Cl−, F−, NO3

−, SO42− (reflecting total dissolved

sulphur) and Br−. The choice of eluent (Na2CO3

and NaHCO3) for the analysis of anion concentra-tions precluded the determination of carbonic spe-cies in the leachates. Precision is estimated to bebetter than 92% for most ions except for K+, forwhich it is 94%. The lower limits of detection areon the order of 0.001 mg/l (for further details onthe technique see Frimmel et al., 1999). Eachsample was analysed at least twice and the meansof the concentrations obtained are reported (Table3). As the exact volume of fluid inclusions taken upinto the leachate during crushing is not known, theabsolute concentrations are of little significancebut molar element ratios (Fig. 9) are more informa-tive. No F− and NO3− were detected and NH4

+

was found only in small amounts. Similarly, noneof the samples analysed contained measurableMn2+, except for the quartz vein in the chert.Using HPIC, Br− was below the detection limitand was then analysed for using ICP-MS, which isby an order of magnitude more sensitive for thiselement. The Br− contents were found to be verylow and in the tourmalinite-hosted fluid inclusionsnot detectable. The dominant cation in all samplesis Ca2+, followed by Na+, Mg2+, and K+ (Fig.9a). Among the anions, Cl− dominates except forsome tourmaline-hosted inclusions in which SO4

2−

is present in higher concentrations (Fig. 9b).Charge balance calculation suggests the presenceof a major additional anion, most likely CO3

2−,which could not be analysed for directly.

In order to assess the extent to which waterderived from the evaporation of seawater is in-

Fig. 9. Sample-average molar cation and anion distribution ofleachates from fluid inclusions hosted by stratiform tourmalin-ite and various vein types in the meta-evaporite sequence ofthe Dernburg formation. Analytical uncertainties correspondto size of symbols.


Fig. 10. Na–Cl–Br systematics of fluid inclusion leachatesfrom various vein types in the meta-evaporites sequence of theDernburg formation (for legend see Fig. 9). Analytical uncer-tainties correspond to size of symbols.

6. Discussion

Extensive alkali-metasomatism in the wholeChameis sub-terrane is indicated by the wide-spread occurrence of magnesioriebeckite and al-bite in proportions which cannot be explained byany common igneous or sedimentary bulk rockcomposition (Frimmel and Hartnady, 1992). Thesource of the alkalis is most readily found in thedolomitic rocks that enclose, in most places tec-tonically, large blocks of mafic and ultramaficrocks. A Na-rich sedimentary precursor of thedolomite is postulated because of the widespreadpresence of massive albitite and albite-richdolomite, and the ubiquitous occurrence of mag-nesioriebeckite and phlogopite in the dolomite.From the lack of calcite in the presence ofdolomite+ talc+quartz, a very Mg-rich pre-metamorphic carbonate precursor rock, rich inmagnesite, is inferred. The presence of massivetourmalinite implies a major B source which,apart from granitic/pegmatitic melts, is most read-ily provided by a high evaporation rate of seawa-ter. No granitic or pegmatitic bodies are knownfrom the whole of the Chameis sub-terrane and itsexposed neighbouring tectonic units. Althoughthere are few isolated occurrences of post-oro-genic carbonatite bodies in the wider region (e.g.Cooper and Reid, 1998), their small size and agecannot be reconciled with the regional (100 km)scale of alkali-metasomatism. The presence of aninitially highly magnesian, alkali-rich sedimentwith mobilisation of the alkalis in particular dur-ing diagenesis and regional, syn-orogenic meta-morphism is therefore considered the mostplausible explanation for the mineral distributionobserved.

Although reflecting the composition of a post-depositional fluid, the limited fluid inclusion dataobtained on the various vein samples, in particu-lar their high overall salinity and their Na:Cl:Brproportions point toward a strong involvement ofseawater in the make-up of the post-depositionallocal pore water. Seawater is characterised by anegative Ce anomaly because of its oxidative re-moval (Elderfield and Greaves, 1982), whereaspositive Ce anomalies may be suggestive of eithera reducing marine environment (De Baar et al.,

volved in the make up of a fluid, the Na–Cl–Brrelations are particularly useful (Kesler et al.,1995; Viets et al., 1996). Due to the very smallpartition coefficient of Br into the major evaporiteminerals, a sensitive monitor of the degree ofevaporation is given by the Cl/Br ratio of theresidual brine. As seawater evaporates, the Cl/Brratio of the residual brine remains similar to thatof seawater (Cl/Br=662) until halite saturation isreached. Further evaporation and precipitation ofhalite leads to a progressive decrease of the Cl/Brand Na/Br ratios. Residual brines that becomephysically separated from their evaporite mineralswill have Cl/Br ratios that reflect the degree ofevaporation. Similarly, dissolution of halite andother evaporite minerals in meteoric water orseawater will cause Cl/Br ratios that are very highin comparison to those of the residual brines.

The fluid inclusions from the quartz–tourma-line veins have a composition almost identical tothat of seawater (Fig. 10) and the aqueous inclu-sions in the quartz vein within the chert, whichare halite-undersaturated, plot on the line thatdelineates the trend of evaporated seawater. Incontrast, the calcite vein hosts saline aqueousinclusions whose Cl/Br ratio is, on average, higherthan that of seawater, thus indicating dissolutionof halite. Furthermore, a shift in their Na/Br ratioaway from the seawater line suggests some Na–Ca exchange.


1988) or an evaporative lacustrine environment(Moller and Bau, 1993) and are typically associ-ated with negative Eu anomalies. The lack of apositive Ce anomaly and a slightly negative toabsent Eu anomaly in the inferred meta-evaporitesamples may be viewed, therefore, as pointingagainst a lacustrine but toward a marine, slightlyreducing environment. Such a mildly reducingenvironment might be indicated also by the lackof NO3

− in those fluid inclusion leachate samplesthat contain small amounts of NH4

+ as the NH4+/

NO3− 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 carbonaterocks analysed, the evaporite-derived dolomitesamples display the least light REE enrichment.In fact their REE patterns are markedly flat (Fig.4) and conform to those of the oceanic islandbasalts in the Dernburg formation (Frimmel etal., 1996a). Also, the strong depletion of the evap-orite-derived dolomite in Rb, and to a lesserextent in Ba, Zr, Hf, and Th, reflects the lack ofdetrital components from a continental source.Further evidence of a lack of continental inputcomes from the Sr isotope data. Although the87Sr/86Sr ratio of seawater fluctuated greatly dur-ing the Neoproterozoic era, the near-primary 87Sr/86Sr ratios obtained for the evaporite-deriveddolomite are close to the lower end of the poten-tial range of Neoproterozoic seawater composi-tion. Any continental influence should be reflectedby an increase in 87Sr/86Sr.

A marine origin of the inferred meta-evaporiteis also supported by the B isotopic composition.The possible B sources for the tourmalines areeither hydrothermal fluids related to the maficoceanic volcanism or seawater. Taking the maxi-mum d11B value measured (+27.5‰) and allow-ing for isotope fractionation between tourmalineand aqueous fluid (Palmer et al., 1992), a tourma-line formation temperature of 200°C would corre-spond to a d11Bfluid value around 40‰ which istypical of seawater. The minimum d11B valueobtained for tourmalinite in the meta-evaporitesuccession (10.7‰) can be explained by either alower d11Bfluid (24‰) at the same temperature asabove or by a lower temperature (50°C) and an

isotopic composition typical of seawater, or by acombination of both possibilities. The theoreticaltemperatures calculated are in perfect agreementwith an expected tourmaline formation duringdiagenesis. As there is no obvious reason fortourmaline formation to take place over a temper-ature interval of some 150°C, the lower d11Bvalues obtained may be indicative of mixing ofseawater with submarine hydrothermal fluids as-sociated with the igneous activity that is manifestin the Dernburg formation. By analogy, the highproportions of SO4

2− detected in some of thetourmalinite-hosted fluid inclusions could be re-lated either to evaporitic sulphates or to sulphuricvolatiles exhaled by hydrothermal vents on thevolcanic edifice.

In conclusion, all our new data are in line withthe presence of marine evaporites within a forma-tion that is otherwise dominated by oceanic maficrocks. As most of these mafic rocks representeither former oceanic islands or an aseismic ridge(Frimmel et al., 1996a), and because of the associ-ation with stromatolitic dolomite, the most likelydepositional environment we envisage for the for-mation of the evaporite is that of an atoll, sur-rounded by stromatolitic reef mounds on top of aguyot.

7. Paleogeographic and climatic implications

The presence of marine evaporites implies a lowto moderate latitude for the area of deposition. Arelatively warm water environment, permitting thegrowth of algal reef mounds, might be deducedfrom the stromatolitic dolomite surrounding themeta-evaporites. However, such an interpretationis not definitive as it is based on analogies withmodern carbonate facies which might differ fromthose in the Precambrian. Nevertheless, the rela-tively low latitude inferred for the carbonate rockswithin the Dernburg formation appears to be incontradiction to the presence of glaciogenic di-amictite in the upper section of the Dernburgformation. The lithology of this formation thusexemplifies the big enigma of NeoproterozoicEarth history, i.e. the close association of glacialand marine carbonate sediments. So far


Neoproterozoic glacial deposits have been de-scribed only from rift grabens or continental mar-gins (e.g. Young, 1995). If the depositionalenvironment that is preferred for the meta-eva-porite in the Dernburg formation of the Marmoraterrane is correct, a sea ice cover away from thecontinental margin and reaching to relatively lowlatitudes has to be inferred.

An unresolved problem is the age of this glacia-tion. No direct age data exist for the volcano-sed-imentary succession in the Chameis sub-terranebut, based on chemostratigraphic and biostrati-graphic (Conophyton-like stromatolites) correla-tion with the para-autochthonous passivecontinental margin sequence (Port Nolloth group)in the external Gariep belt, some constraints canbe set on the age of the Dernburg formation. Twostratigraphically different glaciogenic diamictitehorizons are distinguished in the Port Nollothgroup, namely the Kaigas and the Numees forma-tion. For the Kaigas formation, a correlation withthe global c. 750 Ma Sturtian glaciation is wellestablished by single zircon age data from overly-ing felsic rift volcanics (Frimmel et al., 1996b).The age of the younger Numees formation di-amictite is, however, problematic. The marine car-bonates above the Kaigas and Numees formationdiamictites, respectively, can be distinguished bytheir characteristic geochemical signatures. Thed13C of the carbonates above the Kaigas forma-tion diamictite follow, starting from depleted val-ues around −4‰, a pronounced positiveexcursion, reaching up to +8‰, just to declineagain immediately underneath the Numees forma-tion diamictite. The carbonates above the Numeesformation diamictite start again with negatived13C ratios of −4‰ but then remain fairly con-stant around 0‰ (Fig. 6). The pre-Numees car-bonates are characterised by low 87Sr/86Sr ratiosbetween 0.7071 and 0.7076 whereas those abovethis younger diamictite have relatively high 87Sr/86Sr ratios of 0.7083–0.7088 (Folling and Frim-mel, 1999). These chemical data would be in linewith a correlation of the Numees formation withthe global Varangian glaciation (Asmerom et al.,1991). Such a correlation has found further sup-port by recent Pb–Pb carbonate ages which areinterpreted to date early diagenesis (Folling et al.,

2000). Carbonates overlying the Kaigas formationdiamictite yielded an age of 728932 which isindistinguishable from a Pb–Pb single zircon ageof 74196 for associated felsic volcanics (Frimmelet al., 1996b). The cap carbonates above the Nu-mees formation diamictite gave an age of 555928Ma thus strongly supporting a correlation of theNumees glaciation with the global Varangianglacial epoch for which a time span between 590and 560 Ma has been suggested (Saylor et al.,1998).

The cap carbonates on top of the Dernburgformation mafic diamictite in the Marmora ter-rane have geochemical (high Sr content) and iso-topic (d13C) characteristics very similar to those ofthe cap carbonates above the Numees formationdiamictite (Fig. 6), for which a post-Varangianage has been inferred above. By analogy we as-sume that the diamictite in the Dernburg forma-tion is a reflection of Varangian glaciation. Adirect d13C correlation between the meta-evapor-ites described in this study and open marine car-bonates elsewhere was not attempted because ofthe restricted nature (and thus potential C iso-topic peculiarities) that is postulated for the depo-sitional environment of the former. However,d13C data for dolomite of the Gais member, whichis considered the best stratigraphic equivalent ofthe meta-evaporites available within the Marmoraterrane, are in good agreement with a strati-graphic position immediately below the glacialdeposits (Fig. 6).

Possibly more reliable than d13C data are thenear-primary 87Sr/86Sr ratios. For the Sholtzbergmember, this ratio was found to be similar to thattypical of pre-Varangian carbonate rocks else-where (e.g. Asmerom et al., 1991; Folling andFrimmel, 1999). The relatively low near-primary87Sr/86Sr ratios determined for the Sholtzbergmember not only provide additional support forits marine origin but are probably also the bestavailable evidence for a post-Sturtian/pre-Varangian age.

Glaciations of Varangian (Vendian) age at onlymoderately high latitudes (30–40°S) have beendocumented paleomagnetically by Torsvik et al.(1995) and strong evidence for equatorial glacia-tion at sea level around 600 Ma has been reported


from South Australia (Schmidt and Williams,1995). Our findings provide further support,though from a very different area, for the exis-tence of low-latitude glaciation, not only nearcoast lines but also in the oceanic realm for thistime.

Acknowledgements

This work was funded through a South AfricanNational Research Foundation grant to HEF anda China National Science Foundation grant (no.49925306) and an Alexander von HumboldtFoundation grant to SYJ. We thank NamdebLtd., in particular R. Burrell, J. Ward and R.Spaggiari for their permission to enter the re-stricted Diamond area and for their financial andlogistic support. Thanks are also due to A. Spathwho acquired the ICP-MS data and to B. Saylorwho provided useful comments on an earlier ver-sion of the manuscript. G.H. Swihart and J.Veizer are thanked for reviewing the manuscript.

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