Alene et al. (2006)

21 Pages·2006·1 MB·English

PrecambrianResearch147(2006)79–99
The Tambien Group, Ethiopia: An early Cryogenian
(ca. 800–735Ma) Neoproterozoic sequence in the
Arabian–Nubian Shield
Mulugeta Alenea, Gawen R.T. Jenkinb,∗, Melanie J. Lengc,d, D.P. Fiona Darbyshirec
aDepartmentofEarthSciences,AddisAbabaUniversity,P.O.B.1176,AddisAbaba,Ethiopia
bDepartmentofGeology,UniversityofLeicester,Leicester,LE17RH,UK
cNERCIsotopeGeosciencesLaboratory,BritishGeologicalSurvey,Nottingham,NG125GG,UK
dSchoolofGeography,UniversityofNottingham,Nottingham,NG72RD,UK
Received19October2005;receivedinrevisedform2February2006;accepted12February2006
Abstract
TheTambienGroupinTigrai,Ethiopia,comprisesanumberofinlierseachcontaining2–3kmthicknessofinterbeddedcarbonate
andclasticsediments,cappedinoneinlierbyaglaciogenicdiamictite.Arangeofgeochemicalindicessuggestnear-pristineC-and
Sr-isotopevaluesarepreservedandthese,togetherwithlithologicalvariations,allowlocalcorrelationbetweentheseinliersand
correlationwiththeglobalNeoproterozoicisotopestratigraphy.AcompositesectionoftheTambienGroupshows(cid:1)13Cincarbonate
of∼+6‰atitsbase,decreasingupwardstotwinlowsof∼–4‰separatedbyabriefexcursionbacktopositivevalues,thenrisesagain
toaplateauof∼+6‰beforefinallydecreasingsharplyto−2‰beneaththeNegashdiamictiteatitstop.Noglaciogenicsediments
areobservedassociatedwiththelowertwinnednegativeanomalies.The87Sr/86Srvaluesincarbonateschangefrom∼0.7063in
thelowerunitsto∼0.7067intheupperunits.The(cid:1)13Coforganicmatterchangeslittlethroughthesequence(−24.2±1.3‰),
withtheresultthattheC-isotopefractionationbetweencarbonateandorganicmatterdecreasesfrom+26to+21‰upthroughthe
secondlowincarbonate(cid:1)13Cbeforeincreasingto∼+29.5‰intherestofthesequence.Togetherwithexistingradiometricage
constraints,theSr-isotopedataindicatethattheNegashdiamictiteisSturtianinage,andthelowernegativeC-isotopeanomalies
appeartocorrelatewiththenon-glaciogenicBitterSpringsStagerecognisedinAustraliaandSvalbard.Accordingtothiscorrelation
theTambienGroupwasdepositedintheintervalca.800–735Ma.OurdatareinforcetheemergingviewthatnotallNeoproterozoic
negativeC-isotopeanomaliesareassociatedwithglaciation.ThevariationsinC-isotopefractionationswithinoursequencecontrast
withthosefromAustraliaandareattributedtodifferencesinlocalenvironmentalvariables,probablytemperature.Thesedifferences
areconsistentwiththerecentproposalthattheBitterSpringsStageanomalyistheresultofapairofInertialInterchangeTruePolar
Wanderevents[Halverson,G.P.,Maloof,A.,Schrag,D.,Dudas,F.,Hurtgen,M.,inpress.Stratigraphyandgeochemistryofaca800
ManegativecarbonisotopestageinnortheasternSvalbard.Chem.Geol.][Maloof,A.C.,Halverson,G.P.,Kirschvink,J.L.,Schrag,
D.P.,Weiss,B.P.,Hoffman,P.F.,inpress.Combinedpaleomagnetic,isotopicandstratigraphicevidencefortruepolarwanderfrom
theNeoproterozoicAkademikerbreenGroup,Svalbard.GSABull.],andfurtherhigh-resolutiondatingandpalaeomagneticstudies
oftheTambienGroupshouldallowtestingofthishypothesis.
©2006ElsevierB.V.Allrightsreserved.
Keywords: Carbonate;Carbon;Oxygen;Strontium;Isotopes;Sturtianglaciation;BitterSpringsStage;TruePolarWander
∗ Correspondingauthor.Tel.:[phone];fax:[phone].
E-mailaddress:[email protected](G.R.T.Jenkin).
0301-9268/$–seefrontmatter©2006ElsevierB.V.Allrightsreserved.
doi:10.1016/j.precamres.2006.02.002
80 M.Aleneetal./PrecambrianResearch147(2006)79–99
1. Introduction extendingdeepbelowtheSturtianglacial,perhapsonly
recorded elsewhere in Svalbard/East Greenland, Aus-
The Neoproterozoic was a critical phase in Earth tralia and Canada (Hill and Walter, 2000). We present
evolution marking the transition from an oxygen defi- new C- and Sr-isotope data that define a more com-
cient(possiblysulphidic)oceantoanoxygenatedocean pleteisotopestratigraphyfortheserocks,includingatthe
moresimilartothepresentday(AnbarandKnoll,2002; base of the sequence a pronounced negative C-isotope
Hurtgenetal.,2005).Thiswasaccompaniedbydrastic anomaly that is not associated with glaciogenic rocks.
climatechange,possiblyincludingatleasttwo‘Snow- These data allow us to make an improved correlation
ballEarth’globalglaciationseachfollowedbyextreme withtheglobalisotopestratigraphy,extendingthisNeo-
greenhouseconditions(Hoffmanetal.,1998b;Hoffman proterozoic sequence downwards to at least 800Ma in
andSchrag,2002).Whilerecentdatinghasshownasig- age,andprovidesupportforanewmodelfornon-glacial
nificanttimegapbetweenthelastinferredglobalglacia- carbonisotopeanomaliesintheNeoproterozoic.
tionandthedevelopmentofmulticellularorganismsand
theensuingCambrianradiation(Hoffmannetal.,2004; 2. Regionalsetting
Condonetal.,2005),itisconceivablethatglobalglacia-
tionsmayhavebeencriticalinsettingthesceneforlater The Neoproterozoic basement of central Tigrai,
evolutionaryevents(HoffmanandSchrag,2002;Knoll northern Ethiopia (Fig. 1a), is composed of low-grade
and Carroll, 1999). However, there is still controversy metavolcanics,slate,carbonatesandplutonics(Arkinet
as to the exact number and intensity of glaciations in al.,1971;Beyth,1971;Garland,1972;Alene,1998).It
the Neoproterozoic, let alone their causes and effects. is in the south (Nafka terrane) of the Arabian–Nubian
Theinferredglobalglaciationsaretemporallyassociated Shield(ANS)thatdevelopedduringtheNeoproterozoic
withlargenegativecarbonisotopeanomaliesinthesed- byrifting,arcaccretionandterraneamalgamationpro-
imentaryrecord,withtheanomalyeitherstartingatthe cessesintheEastAfricanOrogen(Stern,1994;Meert,
top of the glacial deposits, or just beneath them. Inter- 2003; Johnson and Woldehaimanot, 2003). The base-
pretations of these carbon isotope anomalies vary, and mentisoverlainbyflat-lyingPhanerozoicsedimentsand
they may well be the result of more than one process, volcanics.
(HoffmanandSchrag,2002;Schragetal.,2002;Pavlov Two major sequences are recognised in the base-
et al., 2005; Kennedy et al., 2001; Jiang et al., 2003; ment(Beyth,1971):theTsalietGroup,anoldermainly
HigginsandSchrag,2003).Irrespectiveoftheirorigin, metavolcanic/metavolcanoclasticsequenceandtheTam-
thecombinationoftheseanomalieswiththeworldwide bien Group, a younger metasedimentary slate and car-
depositionofglacialdepositsprovidesthepotentialfor bonate succession. Beyth (1971) described the bound-
globalchemostratigraphiccorrelationintheNeoprotero- ary between the two sequences as ‘probably uncon-
zoic(Knolletal.,1986;JacobsenandKaufman,1999; formable’.However,Alene(1998)hasdescribedagra-
Walteretal.,2000;Melezhiketal.,2001b;Halversonet dationalcontactbetweentheTsalietGroupandbasalpart
al.,2005).However,itisincreasinglybeingrecognised oftheTambienGroup,althoughhigherunitsoftheTam-
that some Neoproterozoic sequences contain negative bienGrouponlaptheTsalietGroupindicatingtiltingof
carbon isotope anomalies that are not associated with depocentres located on the present inliers during Tam-
glaciogenic rocks (e.g. Hill and Walter, 2000; Corsetti bienGroupdeposition.Bothsequenceshavebeensub-
and Kaufman, 2003; Zhang et al., 2005; Condon et jectedtotwomajorphasesoffoldingasaresultofN–S
al., 2005; Melezhik et al., 2005, Halverson et al., in and E–W regional compression, respectively (Alene,
press,Dehleretal.,2005).Theseanomaliescomplicate 1998;AleneandSacchi,2000).D foldedbeddingand
1
chemostratigraphiccorrelationsanddemandalternative producedtightminorfolds(wavelengthofmmtodm),
explanationsfortheirorigin. elongationlineationandpervasiveregionalfoliation.D
2
PartoftheNeoproterozoicTambienGroupinnorth- causedlongwavelength(upto8km),upright,openpar-
ern Ethiopia was recently remapped by Alene (Alene, allelfoldswithoutproducingasignificantcleavage.D is
2
1998;AleneandSacchi,2000;Aleneetal.,2000).This consideredtobeduetoE–Wdirectedshorteningassoci-
Neoproterozoic sequence is one of the least studied in atedwiththeend-phasecollisionbetweenEastandWest
Africa,yetliesatacriticalgeotectonicboundarybetween Gondwanaland,andcorrelateswithpost-accretionstruc-
the Arabian–Nubian Shield (mostly juvenile crust) to turesdescribedelsewhereintheANS(Abdelsalamand
thenorthandMozambiquebelt(mostlyreworkedolder Stern,1996).
crust)tothesouth.Existingdata(Milleretal.,2003)sug- Mineral assemblages in the Tsaliet metavol-
gesttheTambienGroupcouldrepresentararesequence canics indicate that peak regional metamorphism
M.Aleneetal./PrecambrianResearch147(2006)79–99 81
Fig.1. (a)GeologicalmapoftheMaiKenetal–NegashstudyareaincentralTigraimodifiedafterAlene(1998).Insetmapshowsthelocationofthe
studyareainnorthernEthiopia.ThePhylliteUnitisthoughtbealateralequivalentoftheTambienGroup.Insomeliteraturethegranitesarenamed
astheMarebgranitesuite,butweprefernottousethisnomenclatureasitimpliesapost-tectonictiming,whereasthegranitesherearepre-D2.Age
datesareshownonpre-D2granites;sources:NegashandHauzien—Milleretal.(2003),single-zirconPb/Pbevaporationmethod;MaiKenetal
—Beyth(1972),K–Arage.(b)DetailedgeologyoftheMaiKenetal–Tsediaareaandsamplelocations(opensquarein(a)),basedon1:50,000
mappingbyAlene(1998).Way-upcriteriaincludestromatoliticlamination,channel-fillandslumpstructuresinthecarbonates.
82 M.Aleneetal./PrecambrianResearch147(2006)79–99
at pumpellyite–actinolite to lower greenschist facies methodageof854±3Maonlow-grademetavolcanics
(∼245–375◦C by chlorite thermometry) was attained (Teklay,1997).AsimilarsequencetotheTsalietGroup
coevalwithD (Alene,1998;AleneandSacchi,2000). to the NW in Axum yielded ages of 756–806Ma on
1
Webelievethatmaximumtemperaturesexperiencedby syntectonic granites (Sm–Nd and chemical Th–U–Pb
the Tambien Group are at the low end of this range zirconisochronmethods;Tadesseetal.,2000)suggest-
◦
<250 C,onthebasisof(a)O-andC-isotopefractiona- ingTsalietGroupdepositionbeforethattime.
tionsbetweencalciteanddolomite(Aleneetal.,1999),
(b)closetodepositionalcalcite-organicmatterC-isotope 3. TheTambienGroup
fractionations(reportedbelow),and(c)theblackcolour
of palynomorphs preserved in some samples (Mullins, TheTambienGroupisexposedinfourinliersformed
UniversityofLeicester,personalcommunication). byD synclinoria,fromwesttoeast:MaiKenetal,Tse-
2
Radiometric ages of the granites (Fig. 1a), locally dia, Chehmit and Negash (Beyth, 1971; Fig. 1a). Each
intruded between D and D (Alene, 1998; Alene and sequencecontains∼2–3kmtruethicknessofsediment
1 2
Sacchi,2000),constraindepositionandearlymetamor- afterdipsfromD foldsareremoved(Fig.2).D folds
2 1
phism of the Tambien Group to >613Ma. The age of causeonlylocalrepetitionsofstratigraphy
the Tsaliet Group, and hence a maximum depositional IntheMaiKenetalsection(Figs.1band2)thelitho-
age of the Tambien Group, is less well constrained, logic sequence comprises (from bottom to top): Lower
with no local radiometric data available. A metavol- Slate (Werii Slate), Lower Limestone (Assem Lime-
canic sequence similar to the Tsaliet Group in Eritrea stone),UpperSlate(TsediaSlate)andUpperLimestone
to the north gave a single-zircon Pb/Pb evaporation (MaiKenetalLimestone)—namesinbracketsarethose
Fig.2. StratigraphicsequenceoftheTambienGroupmetasedimentsintheMaiKenetal,TsediaandNegashsections,northernEthiopia.Unitnames
giveninparenthesesarethoseusedbyBeyth(1971).Therangeofthicknessofunitsineachinlierisgiven;thethicknessesplottedarethoseinthe
areainwhichsamplesweretaken.
M.Aleneetal./PrecambrianResearch147(2006)79–99 83
used by Beyth (1971). In the Tsedia and Chehmit sec-
tions,thelowerthreeunitsarerepresentedbuttheUpper
Limestone is not present. Although the correlation of
theLowerLimestoneisnotstraightforward,therelative
stratigraphicpositionandlithologic(composition,grain
size) and textural features (degree of deformation and
fabric development) suggest that it is the same unit. In
allthreesections,theLowerLimestoneisfoundbetween
theUpperandLowerSlates.
IntheNegashsection(Fig.2)thesequenceisdiffer-
ent and it includes (from bottom to top) Negash Slate,
Dolomite and Slate, limestone (Negash Limestone),
Slate/Pebbly slate (Diamictite Slate). The composition
andgrainsizewithonlyweakfabricdevelopmentsug-
gestthattheNegashLimestonewouldbebestcorrelated
withtheUpperLimestoneintheMaiKenetalsectionand
weprovideisotopedatatosupportthisbelow.Garland
(1972)groupedthetwolowermostunitsoftheNegash
sequence as the Didikama Formation and the three top
unitsastheMatheosFormationanddescribedbothfor-
mationsasunitsyoungerthantheTambienGroup.How-
ever,Beyth(1971),Alene(1998)andBeythetal.(2003)
all consider the Negash section as part of the Tambien
Group.
3.1. LowerSlate(WeriiSlate)
Ablack,brownorgreenishgrey,well-laminatedand
stronglyfoliatedslateintheMaiKenetalandTsediasec-
tions(Fig.1b).Itcontainsgreywackeandgraphiticbeds.
The basal contact with the underlying Tsaliet Group is
gradationalwithnomajordiscontinuitybetweenthetwo
groups.Greenishgreyslateandgreywackelayersoccur
bothsidesoftheboundary,andthereisnomarkeddefor-
mationalormetamorphiccontrastacrossthecontact.
3.2. LowerLimestone(AssemLimestone)
In the Mai Kenetal synclinorium this is exposed on
the western limb as a sub-vertical layer ∼700m thick
Fig.3. FieldphotographsandCLimagesofMaiKenetallimestone.
(Fig.1b).Itisgreyorblacklimestonewithstromatolitic
(a)StromatoliticlaminationintheLowerLimestoneoftheMaiKene-
lamination (Fig. 3a), calcite veins and dark stylolites.
talsection;MK72samplelocality.(b)ParasiticD2 foldswithsub-
Stromatolitic lamination and slump structures indicate horizontal fold axes in Lower Limestone of the Tsedia section. (c)
younging towards the Upper Limestone. It has fine- to CathodoluminescenceimageofsampleMK100(LowerLimestoneof
medium-grainedcrystallinetextureandis∼90%calcite; theTsediasection),mostofthesampleshowslittleluminescence,but
orangeluminescencepicksoutazoneincalcitemicrospar.Widthof
the remainder is dolomite, detrital quartz and feldspar,
image1.6mm.
andminorpyrite.Incathodeluminescence(CL)itshows
predominantlydullbrownanhedraltosubhedralcalcite
grains,andlesseramountofredluminescentdolomite. Limestonehereismainlyblacktodarkgrey,commonly
In the Tsedia synclinorium the Lower Limestone is intercalated with slate and well bedded and laminated;
exposed in both limbs. It contains D folds with NNE insomeplacesapliticsillsintrudeit.Thecompositionis
2
trendingsub-horizontalfoldaxes(Fig.3b).TheLower predominantly crystalline calcite (>90%), with detrital
84 M.Aleneetal./PrecambrianResearch147(2006)79–99
albite and quartz. The texture is similar to that of the notmatrixsupportedanddoesnotcontainanyfeatures
Lower Limestone in the Mai Kenetal section. In CL diagnosticofglaciogenicorigin,suchasstriatedpebbles.
it shows a largely dark matrix containing orange lumi-
nescentandzonedcalcitehavingrhombic,rectangular, 3.6. Dolomite–Slateintercalation
andlesscommonlysub-circulargrainshapesoftenwith
dark cores (Fig. 3c). Non-luminescent, coarse-grained, A 1200m thick unit comprising white to pink
sub-idiomorphicalbiteandquartzshowingpreferredori- dolomite intercalated with dark grey, black and green
entationarealsocommon. slateatNegash.Thedolomiteisfinetocoarsegrained,
consistingofover90%dolomitewithcalciteanddetri-
3.3. UpperSlate(TsediaSlate) talquartzandalbite.Itdisplaystightfoldandrefolded
structuresaswellasstromatoliticlaminations.
Alighttodarkgrey,thinlybeddedandwell-laminated
andfoliatedslate.AtTsediaitoccursinthecoreofthe 3.7. NegashLimestone
syncline overlying the Lower Limestone. The onlap of
theUpperSlateontotheTsalietGroupontheeastside Thisisamassiveblacklimestone.Ithasgradational
oftheMaiKenetalsynclinorium(Fig.1b)mustrelateto contacts with the underlying dolomite and overlying
tilting of the depocentre during deposition of the Tam- slate.Itisfine-grained,laminated,fractured,locallystro-
bienGroupand/orfaultingofthecontact. matolitic,andcontainsquartzandcalciteveins.InCLit
showsdarktodarkbrownluminescentmicrocrystalline
3.4. UpperLimestone(MaiKenetalLimestone) zones (or bands) strongly speckled with black (non-
luminescent) dots alternating with zones without black
This forms a sub-vertical NNE trending, massive dots.Thepresenceoftheblackdotsandtheabsenceof
black limestone layer in the core of the Mai Kenetal well-recrystallisedcalcitegrainsaresimilarwiththatof
synclinorium;youngingisdemonstratedbyslumpstruc- theUpperLimestoneatMaiKenetal.
tures.Itcontainsminorsub-verticalfoldaxes,andatthe
baseitisintercalatedwiththeUpperSlate.NeartheN–S 3.8. DiamictiteSlate
trending fault (Fig. 1b) it is locally brecciated. It con-
tainsdarkcarbonaceousseamsandcalcareousfragments Overlying the Negash Limestone is a light to dark
probably representing redeposited carbonate flakes. It greyslatethatformsthecoreoftheNegashsyncline.At
is fine-grained and contains abundant calcite veins. It thebasetheslateisintercalatedwithlimestonebedsand
comprises mainly calcite (∼95%) with detrital quartz towardsthetopitbecomesdiamictiticcontainingabun-
andalbite,pyriteand±hydratedironoxide.InCL,sam- dant matrix-supported pebbles and cobbles of variable
pleMK77showsamicriticcalcitematrix,whileMK78 size (mostly 1–15cm diameter) and shape, sometimes
shows‘fragmental’calcitetexturewithmainlybrownto with striated surfaces (Miller et al., 2003). The clasts
dark,andtoalesserextentorange,calciteluminescence arepolymict,includingblacklimestone,metavolcanics,
occurring in different forms including dark long frag- greywacke,quartzitesandgraniticrocks.Thefeaturesof
ments and circular shapes mostly speckled with black thediamictiteareconsistentwithaglacialorigin.
dotsandhavingorangerims.Thehighlyveinedsample
MK79containssporadicpatchesoforangeluminescent 4. Analyticalmethods
calciteaswellasafewfine-grained,circularcalcitesthat
exhibitbrightorangeluminescence;calciteveinsbelong 4.1. Sampling
toatleastfourgenerations.
Because all samples have been metamorphosed and
3.5. NegashSlate some parts of the sequence are veined, we have sam-
pled in order to characterise any post-D vein material
1
Adarkgreyandgrey–greenslateoverlyingtheTsaliet present,identifiedasbeingcoarselycrystalline(typically
Group in the Negash synclinorium. It contains 1–2m 1–5mm grain size) and cross-cutting the metamorphic
thickbedsofspottypurpleslate,well-laminatedbands, fabric, as well as the least altered rock away from the
graphiticlayers,quartzite,conglomerateandgreywacke. veins.Thisstrategyenablesustoidentifytheleastaltered
Beythetal.(2003)suggestedapolymictconglomerate samples at any horizon by monitoring the direction in
in this unit could represent a lower glacial diamictite, which the geochemistry of samples was altered by the
butwedonotconcurwiththissuggestionbecauseitis lateveinfluids.Weatheredcrustsandmarkerpensample
M.Aleneetal./PrecambrianResearch147(2006)79–99 85
numberswereremovedfromallsamplespriortocrush- eter.Isotopevalues((cid:1)13C,(cid:1)18O)arereportedaspermil
ing, ensuring no modern contamination. Samples for (‰)deviationsoftheisotopicratios(13C/12C,18O/16O)
analysisofhomogeneousrockswereobtainedbycrush- relative to V-PDB using a within-run laboratory stan-
ing small volumes, whereas veined or laminated rocks dardcalibratedagainstNBSstandards.Overallanalyti-
were sampled on unweathered cut or broken surfaces calreproducibilityisnormallybetterthan0.1‰for(cid:1)13C
withadiamond-tippeddrilltoobtain∼200mgsamples and(cid:1)18O(2σ).
ofveinandhostrockmaterial,andanydifferentcoloured
laminae.Thesesampleswerethensplitforchemical,and
4.5. Organicmatterδ13C
C-,O-andSr-isotopeanalysis.
Carbonatewasremovedfromsamplesbyreactionof
4.2. Calcitechemistry
between19and∼100gofpowderedrockwith5%HCl
Powderedsamples(∼100mg)wereweighedandthen overnight.Theresiduecontainingtheorganicmatterwas
washed and dried. In some samples a tiny amount of
leachedin1Maceticacidfor2h—thisisbelievedto
organic scum was released on dissolution and this was
quantitativelydissolvecalcite,butleavedolomiteunre-
decantedoff.Lossofthis,probablyinsignificantamount
acted.Solutionswerequantitativelypipettedoffforanal-
of material, which is likely to have had lower (cid:1)13C
ysisandresiduesweredriedandweighedtocalculatethe
thanthebulksample,meansthatmeasured(cid:1)13Cvalues
weightofsampledissolved.Thisweightwasthenusedto
ontheresiduecouldrepresentmaximumvaluesforthe
calculateconcentrationsinthedissolvedcalcite.Acetic
total organic matter. 13C/12C analyses were performed
acidsolutionsweredrieddownandredissolvedin10%
by combustion in a Carlo Erba NA1500 on-line to a
HClforanalysiswithaJobinYvonHoribaUltima2opti-
VGTripleTrapandOptimadual-inletmassspectrometer,
calemissionICP.Precisionandaccuracyareestimated
at±5%. with(cid:1)13CvaluescalculatedtotheV-PDBscaleusinga
within-runlaboratorystandardscalibratedagainstNBS-
19 and -22. Replicate analysis of well-mixed samples
4.3. Calcite87Sr/86Sr
indicatedaprecisionof±<0.1‰(1S.D.).
Approximately 20mg of powdered sample were
treatedwithRomil©ultrapure1Maceticacid.Aftercen- 5. Preservationofprimaryisotopesignatures
trifuging, leachates were dried down and the residue
dissolvedin2.5MHCl.Strontiumwasseparatedbycon- Because our samples have experienced deformation
ventionalcationexchangetechniquesusingBiorad©AG and low-grade metamorphism it is imperative that we
50W-X8ionexchangeresin.Strontiumwasloadedonto demonstratewearerecordingdepositionalisotopesigna-
rhenium (Re) filaments together with a tantalum oxide turesratherthanalteredvaluesimposedpost-deposition.
(TaO) activator following the method of Birck (1986). ThisproblemappliestoallNeoproterozoicsamplesbut
Isotope ratios were measured on a Finnegan MAT Tri- isoftenperceivedtobemoreacuteforsamplesthathave
ton multicollector mass spectrometer operated in static beenmetamorphosed.However,thismaynotnecessarily
mode. Nineteen analyses of NBS 987 measured dur- bethecasebecause,oncetheyhaverecrystallisedduring
ing the period of study yielded a mean 87Sr/86Sr of theearlieststagesofmetamorphism,itiswidelyagreed
0.7102542±0.0000092 (2σ). For consistency all the that limestones and marbles exhibit some of the low-
measuredratiosarenormalisedrelativetotheaccepted est permeabilities of any metamorphic rocks (Holness
value of 0.710248 for NBS 987. Replicate determina- and Graham, 1995) and are often metamorphosed as
tions (n=11) of the North Atlantic seawater standard closed systems. Even when infiltrated by fluids along
yielded0.7091753±0.0000062(2σ). veinsthedegreeoffluid–rockinteractionandexchange
maybesmallandlocalisedsothatisotopesystemsinthe
4.4. Calciteδ13Candδ18O rockarerelativelyresistanttoexchange.Decarbonation
reactionsbetweensilicatesandcarbonatesareknownto
Samples were ground in agate and equivalent of causedecreasesin(cid:1)13Cand(cid:1)18Oinhighergrademeta-
10mg of carbonate was reacted with anhydrous phos- morphicrocks(Schidlowski,1987;Nabelek,1991),but
◦
phoricacidinvacuoovernightataconstant25 C.The providedthatlimestonescontainlowconcentrationsof
liberated CO was cryogenically separated from water silicatematerialand/orhavenotbeenheatedtotemper-
2
vapour under vacuum and collected for analysis. Mea- aturesatwhichdecarbonationtakeplace,thentheyare
surementsweremadeonaVGOptimamassspectrom- likely to retain isotope signatures from the diagenetic
86 M.Aleneetal./PrecambrianResearch147(2006)79–99
realm. Indeed, Melezhik et al. (2001a) detailed exam- tinevalues.Theselowvaluesareduetointeractionwith
ples of amphibolite grade marbles that have retained low (cid:1)18O meteoric or metamorphic fluids that formed
closetopristineC-andSr-isotopesignatures.Therefore, post-D vein samples MK72v and MK79v (Table 1).
1
the end result may be that most of the processes that Two of the lowest (cid:1)18O samples in the Upper Lime-
couldhavealteredtheisotopecompositionsinourrocks stones(MK79and80)areclosetotheprominentpost-D
2
probablyoccurduringdiagenesis—incommonwithall N–Sfault(Fig.1b)suggestingthatthiswasaconduitfor
Neoproterozoiccarbonates.Manyofthevariablescon- low(cid:1)18Ofluidsandprovidingafurtherconstraintonthe
trolling the degree of alteration of isotope ratios, such timingoffluidalteration.The(cid:1)18OvalueofMK72vcal-
as effective fluid/rock ratios, are the same whether in cite of –20.6‰ V-PDB (+9.68‰ V-SMOW) would be
themetamorphicordiageneticrealms,andmonitorsof inequilibriumwithfluidof∼0‰V-SMOWat∼200◦C
alteration that are proxies for these variables, such as (Zheng,1999),butveintemperatureswouldhavetohave
◦
Mn/Sr,shouldbeexpectedtoidentifyalteredisotopeval- beenaslowas100 Cforthecalcitetobedepositedin
uesirrespectiveoftheirtimings.Theoreticaloverviews equilibriumwithafluidof−8‰V-SMOW—theupper
of the relative robustness of carbonate isotope systems limitofNeoproterozoicmeteoricwaterifseawaterwas
duringfluid–rockinteraction(BannerandHanson,1990; −8‰.ThuseventhefluidthatformedMK72vwasprob-
JacobsenandKaufman,1999)showthat,onthebasisof ably not dominated by seawater or meteoric water, but
typicalconcentrationsofC,OandSrinrocksandfluids, waspositivelyshiftedbyfluidrockexchange;thissug-
itistobeexpectedthat(cid:1)13Cshouldbemorerobustthan gestsrelativelylowfluid/rockratios.NotethattheMK72
87Sr/86Sr,whichisinturnmorerobustthan(cid:1)18O.Thus rockhasretainedahigher(cid:1)18Ovaluethantheveinsin
if(cid:1)18Ocanbeshowntobepristine,thenitislikelythat the same rock, indicating that either water/rock ratios
Sr and C isotopes will also be, etcetera; therefore, we were low (<1; Jacobsen and Kaufman, 1999), or that
dealwith(cid:1)18OvaluesfirstandthenproceedtoSrandC exchangewaskineticallylimited,orsomecombination
isotopes. ofthetwo.SinceSr-isotopeexchangerequiressimilaror
largerwater/rockratioscomparedtoO-isotopeexchange
5.1. δ18O to become significant (Jacobsen and Kaufman, 1999),
carb
andSr-exchangekineticsarelikelytobethesamerate
Rock (cid:1)18O values vary between −4.7 and orslowerthanoxygenisotopeexchange,thislendssup-
carb
−13.9‰ with an average of −9.6‰ (Table 1). Vein porttopreservationofdepositionalSr-isotopesignatures
(cid:1)18O values are lower on average (−12.6‰), rang- inthesesamples.
carb
ing between −7.9 and −20.6‰. Rock (cid:1)18O values
carb
are distinctly lower than in modern marine limestones 5.2. 87Sr/86Sr
(∼0‰), but both measurements on pristine carbonates
throughthePhanerozoic(Veizeretal.,1999)andmod- Two Upper Limestone samples ∼600m apart
ellingstudies(Wallmann,2001)areincreasinglyindicat- (∼225mstratigraphically)havestrontiumisotoperatios
ingthatNeoproterozoicseawaterwassignificantly18O- thatareidentical(mean:0.706732±0.000002;Table1)
depleted compared to modern seawater. It is suggested withinanalyticalerror.TwosamplesoftheLowerLime-
that pristine Neoproterozoic carbonates may have had stone at Mai Kenetal and Tsedia have a mean ratio
(cid:1)18Oaslowas−8‰,withvariationsaroundthisvalue of 0.7062±0.0001; one vein in the Lower Limestone
resulting from local depositional environment, temper- showsaslightlylowervalue(0.70603).
ature and glaciation. Certainly Hoffman and Schrag Several criteria have been used to assess the preser-
(2002) and Kasemann et al. (2005) present data from vation of Sr-isotope ratios in Neoproterozoic carbon-
carbonates around the Ghaub glacial in Namibia that ates (e.g. Asmerom et al., 1991; Kaufman et al., 1993;
show parallel secular trends in (cid:1)18O and (cid:1)13C, where Kaufman and Knoll, 1995; Jacobsen and Kaufman,
the(cid:1)18Odataareinterpretableintermsofdepositional 1999; Fairchild et al., 2000; Melezhik et al., 2001b;
temperature variations and thus could be primary, and Thomasetal.,2004).Low(cid:1)18O valuesmayindicate
carb
thelatterauthorsarguethatthisisthecase.These(cid:1)18O exchangewithmeteoricormetamorphicwater,although
valuesareintherange−2to−11‰,similartoestimates asdiscussedabove,theinitialstartingpoint,andthere-
ofMesoproterozoicprimarymarinecalcite(cid:1)18Oof−6 fore what represents a ‘low’ value is still debatable.
to−10‰(FrankandLyons,2000;Kah,2000).Thusitis Furthermore,meteoricwaterwillnotcontainsignificant
possiblethatsomeofourhigher(cid:1)18Ovaluesarecloseto Srunlessithasinteractedwithrocksearlierinitsflow-
primaryvalues,butmanyarebelowtypicalcut-offs(e.g. path,andthenitsacquired87Sr/86Srratiomayormaynot
>−11‰;Kaufmanetal.,1993)usedtodistinguishpris- bedifferentfromtherocksthatweareexamining.Hence
Table1
IsotopedataandelementconcentrationsinaceticacidsolublefractionofTigraiNeoproterozoiccarbonates
SampleNo. Unit Description Heighta (cid:1)13Ccarb (cid:1)18Ocarb (cid:1)13Corg εTOCc TOCd 87Sr/86Sr Ca Fe Mg Mn Sr Mn/Sr Mg/Ca Residuee
(m) (‰b) (‰b) (‰b) (‰) (%) (wt.%) (ppm) (ppm) (ppm) (ppm) (%)
MaiKenetal
MK77 U.Lst. Whole-rockblack 2550 +5.8 −8.5 −23.7 +30.2 0.23 0.7067329±24 40.23 2045 2229 160 2450 0.065 0.006 7.2
limestone
MK76v U.Lst. Veincontaining 2325 +6.0 −7.9 38.91 4363 3008 235 5362 0.044 0.008 30.7
calciteandquartz
MK76r U.Lst. Whole-rockblack 2325 +5.8 −9.6 −22.1 +28.4 0.14 0.7067306±24 40.60 3621 2838 283 2958 0.096 0.007 11.7
limestone
MK78v U.Lst. Veincontaining 2250 +5.0 −8.7 38.40 2967 3272 327 7087 0.046 0.009 24.0
calciteandquartz
MK78r U.Lst. Whole-rockblack 2250 +5.2 −9.4 −23.8 +29.7 0.06 39.07 2125 2039 529 2752 0.192 0.005 14.2
limestone M
MK80 U.Lst. Whole-rock 2025 +5.3 −11.4 .
A
brecciatedblack le
limestone ne
MK79vl U.Lst. Latercalciteveinin 1950 +4.8 −14.4 36.79 1204 2992 93 2360 0.039 0.008 2.2 et
MK79ev U.Lst. sEaamrlpielrecMalKci7te9veinin 1950 +4.5 −14.1 41.39 1620 3758 109 3996 0.027 0.009 0.5 al./
P
sampleMK79 re
MK79r U.Lst. Whole-rockblack 1950 +5.7 −13.9 −23.2 +29.6 0.08 39.02 1996 7769 104 2578 0.041 0.020 11.1 ca
m
limestone b
MK75gl L.Lst. Whole-rocklight-grey 1250 −0.8 −9.2 40.46 354 2470 84 189 0.443 0.006 1.0 ria
limestone n
MK75b L.Lst. Blacklaminain 1250 −1.0 −9.7 Re
s
sampleMK75 ea
MK74 L.Lst. Whole-rockgrey 1075 −1.0 −9.5 0.003 41.11 227 2924 90 160 0.565 0.007 1.7 rc
h
limestone 1
MK73 L.Lst. Whole-rockgrey 1040 −0.9 −8.1 −24.7 +24.5 0.01 0.7061116±25 41.31 346 3598 80 165 0.483 0.009 3.0 47
limestone (2
MK72v L.Lst. Calciteveininsample 1005 −1.9 −20.6 0.7060252±27 42.44 206 1762 245 295 0.829 0.004 2.4 00
6
MK72 )
MK72r L.Lst. Whole-rockgrey 1005 −1.0 −11.5 38.13 450 2042 148 156 0.949 0.005 3.2 79
–
limestone 9
9
MK71 L.Lst. Whole-rockblack 900 −2.0 −8.9 −27.0 +25.7 0.09 38.25 4486 89190 122 229 0.532 0.233 19.6
limestone
Tsedia
MK97 L.Lst. Whole-rockblack 1329 −3.0 −11.3 −24.6 +22.2 0.04 39.24 1527 3373 181 2733 0.066 0.009 9.6
limestone
MK98b L.Lst. Blacklaminain 1280 −4.3 −11.9 −24.9 +21.0 0.10 37.66 2394 3939 1015 2052 0.495 0.010 52.6
sampleMK98
MK98g L.Lst. Greylaminainsample 1280 −4.5 −11.7
MK98
MK99 L.Lst. Whole-rockgrey 1159 −2.9 −8.1 40.01 1739 2585 725 1050 0.691 0.006 5.4
limestone
MK96 L.Lst. Whole-rockblack 1086 −3.9 −11.1
limestone
MK100 L.Lst. Whole-rockblack 964 −0.7 −12.2 −24.1 +24.0 0.09 0.7063091±20 40.65 1086 3678 87 2034 0.043 0.009 2.3
limestone
8
7
88 M.Aleneetal./PrecambrianResearch147(2006)79–99
eue
d
Resi(%) 1.9 4.1 9).
9
Ca 4 6 19
Mg/ 0.00 0.00 al.,
et
Sr 1 6 es
Mn/ 0.02 0.02 Hay
(
m) 5 3 Corg
Sr(pp 279 267 (cid:1)13
−
Mnppm) 59 69 Ccarb
( (cid:1)13
≈
m) 9 1 }
Mg(pp 170 224 −1
0)]
Fe(ppm) 662 770 +100
Ca(wt.%) 41.66 36.51 (cid:1)130)/(Corg
0
0
1
+
8786Sr/Sr {(cid:1)13[(Ccarb Ftiiogn.a4l.dCatraos(ssmplaoltlsoopfecnarsbyomnbatoels(cid:1))18frOomvsM.(ail)le8r7Sert/a86l.S(r2,0(b0)3(cid:1))1a3nCd.AAldednie-
dOC%) 3=10 etal.(1999).
T( 1)
− oxygenisotopeevidenceofinteractionwithmeteoricor
εcTOC(‰) α(TOC imneStar-misoortpohpiecrwataiotesr.iOsunrortoackprsiaomripelveisdfernocmeMofaailKteeranteitoanl
3
(cid:1)13Corgb(‰) er=10 aitneddTrasnegdeiaothfa(cid:1)t1w8Oerevamlueeass,uwreidthfoornSlyr-itshoetoTpseedhiaavesaamlipmle-
(cid:1)18Ocarbb(‰) −10.0 −4.7 −9.7 −7.5 −8.8 −6.8 −6.6 ganicmatt b(f∼reoi0mn.g0M0<0a−0i91K)1e‰nleest(saFlri(agMd.i4Koag7)e2.nvTi)chhetahhsaiang8h7MlSyKr1/8876O3S,-rdtrheapetiloevteseildnig-vhferteilnye
(cid:1)13Ccarbb(‰) +4.0 +6.8 +3.7 +5.3 +4.9 +6.6 +6.3 eandor rinogckthsaatmthpelevjeuisntflaubiodvceoiutldinhtahveesptrraetviigoruasplhyy,instuergagcetsetd-
nat withthelow87Sr/86SrvolcanicsoftheunderlyingTsaliet
aHeight(m) 2360 2320 2240 2240 2200 2200 2120 group. encarbo aGlr.o(2u0p0.N3)ehgaavsheLraitmheersthoingehesarm(cid:1)1p8lOestahnaanlyosuerdsbaymMplielslearnedt
en we 87Sr/86SrvaluesoverlapwithourUpperLimestonesam-
ContinuedTable1() SampleNo.UnitDescription NegashNW121NegashWhole-rockblackLst.limestoneNW122NegashWhole-rockblackLst.limestoneNW123vNegashCalciteveinLst.NW123rNegashWhole-rockblackLst.limestoneNW124fNegashFine-grainedblackLst.limestoneNW124cNegashCoarse-grainedblackLst.limestoneNW125NegashWhole-rockblackLst.limestone aCalculatedheightabovebaseofTambibRelativetoV-PDBstandard.εcistheC-isotopefractionationbetTOCdTotalorganiccarboninrocksample.eAceticacidinsolubleresidue. 8hpsaciamiss7asoneloeeSmyiuuftIgstrloenafo,p/hdlmp8lnllbdti6ieerkeumsoSeoseprtu.rlfraelepahnytsfsarhiaiattssotoitrvleco8aslnaes7tonoerbiSawegrsdfercrelitc/arpaths8xoyhpr6ihecnggsihSynhtctneyralaaariltldifinorh(lnrboiFgsceasoiehaeinalrgsnniitagSgc.iantneoahrt4oganetiwaettRen,()scilJhb.ooto8eihe/fT7rwtSnnrSohhikercsrueitlied/geanri8premht6iLoeetnSdr8ptsowcior7seatwrSaliirleoobt.laerahv,ml/trpse2e8utihL6irh0rnttaSeiehe0agmssra1aessttd)measahi.smuntmesoThtrdouiuiepShlnncsinalirnhgeees-rt