Aetas II South Americ an Symposium on Isotope Geology I .) '\ (.. c,

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PRELIMINARY CHEMOSTRATIGRAPHIC INSIGHTS ON CARBONATE ROCKS FROMNICO PEREZ TERRANE (URUGUAy).

, \K Kawashita' , C. Gaucher' , P. Sprechrnan", W. Teixeira' andR. Victo ria.'

l -Inst. de Geociencias, USP. R. do Lago 562, CEP 05508-900, S. Paulo, Brazil. kaka@usp.br2-Depto. de Paleontologia, Facultad de Ciencias. Montevideo, Uruguay. gaucher@mailer.um-marburg.de3-CENA-USP. Av. Centemlrio,303 , CEP 13400, Piracicaba, S. Paulo, Brazil. reyna@cena.usp.br

Keywords: Chemostratigraphy, Carbonate Rocks , Stable Isotopes, Radiogenic

Limestones and carbonate rocks, under certain unit, including stromatolites, stromatolitic brecciascircunstances, can be dated successfully by and thick intercalations of carbonate rocks, wasradiometric methods (Moorbath et al., 1987; afffected by two deformational events, and theThoulkerids et al, 1994; Kawashita et al., 1997). paragenesis indicate a low-to medium grade ofWhen using "ages" derived from 87Sr/86Sr ratios, a metamorphism. Conversely, the overlying Arroyo delsecular variation of Sr composition on marine Soldado Group was deformed only by the youngercarbonates is relatively well known for Phanerozoic tectonism, implying a younger age for this rock unit.Eon oceans. Precambrian carbonates, however, show The 87Sr/86Sr and 8%0 determinations performedan irregular curve ascending from ca. 0.702 to the on 4 limestones (see Table 1) from Polanco Formationvalue of 0.709. However (Ghorokov et al. 1995), at two different localities and 6 dolomitic carbonateshows that this approach is applicable to rocks from Cerro Villalba Formation. As we expected,Neoproterozoic carbonate rocks, although there is among ' 4 limestones from Polanco Formation , thesome limitation due to the lack of biostratigraphic samples from Puntas de Verbal with around 1400 ppmcriteria to erect a well-defined curve. of Sr exhibit lower and concordant ratios compared to

This paper deals with the the possibilities of those from two samples of the Cierro de las Cuentasapplying chemostratigraphy concept on two carbonate that shows Sr contents otherwise lesser than 600formations of Nico Perez Terrane, Uruguay. 1- The ppm. According to the criteria suggested by some ofPolanco Formation belongs to the Arroyo del Sodado above cited 'works, the ratio 'of 0.70780 can beGroup and contains a low-diversity microfossil- accepted as the probable or maximum value for theassemblages (Gaucher et al., 1998; Gaucher and coexistent sea. According to the data available for theSprechmann, 1999). The microfilaments and interval 610 - ca. 540 Ma (Vendian) and the curveacritarchs found in the Polanco Limestone Formation proposed by Kaufman et ai, 1983, the ratio of 0.70780and elsewhere in the Arroyo del Soldado Group have signs an age of ca. 590 Ma ( Fig.I), which is morebeen assigned to the Upper Vendian (Valdaian), on compatible' with Lower Vendian than Upper Vendian,the basis of low diversity, small size of spheromorphs as it has been stated in terms of Vendian microfossil«lSO/lm), absence of acantomorphs and abundance assemblages. This ratio, with :a possible trend for 'aof Bavlinella fa~olata. Similar assemblages have lower ratio, suggests to be more concordant with thebeen found in Namibia (Nama Group), Brazil 0:70740 ratio assumed by Kawashita et al., 1993, for(Corumba Group) and East European Platform Bambui and Una limestones (Neoproterozoic Sao(Valdai Series, Kotlin Horizon). The age of the Francisco Basin) or reported by Kawashita et al.,Polanco Formation is const rained by Cloudina 1996, for Araras Group limestones (Neoproterozoicriemkeae GERMS occurring in the underlying Verbal Paraguay Belt). The 87Sr/86Sr ratios found in Nama,Formation (Gaucher et al., 1998; Gaucher and Witvley and Corumba Group limestones areSprechmann, 1999). 2- The CelTO de Villalba significantly higher, in the range 0.70813 to 0.70892Carbonate Formation belongs to the informal Basal pointing to an age interval of around 590 to 550Ma.Group (Gaucher et al., 1996), for which the These examples reported by Kawashita et al. (1993),geochronological evidences suggest a Meso- Derry et al. (1992) and Kawashita et al. (1996), haveproterozoic age (Teixeira et al., 1999). The whole Sr contents higher than 1000 ppm. Moreover, a RbiSI'

399

Actus II South American Symposium on Isotope Geology

age of 532 ± 11 Ma (Fig.2) based on 4 samples fromthe Guazunambi Granite that cuts the PolancoFormation impl ies for a rn inmimum deposi tional agefor it.

Th e carbonate sampl es ' from 'Cerro de VillalbaFormation (Basal Group), sampled in two extremestratigraphic positions (bas e and top), were not whollyanalysed because the samples were dolomitic andshowed low Sr contents. The Sr ratios for twodolomitic stromatolites from top with unusual ratiosof ca. 0.725 clearly indicate the high contribution ofradiogenic 87Sr. This process probably took placeduring either metamorphism or dolomitization of theFormation.

Because of scarcity of Ol3C and 0180 dataperformed, we cannot make firm conclusions derivedfrom these results. Based on the 4 data available forPolanco Format ion, it seems that the most probableprimary val ues for Ol3C and 0 180 , both versus PDB ,would be close to +2.8%0 and -12.3%0, respectively.These values are in accordance w ith the. reportedvalues for Nama and Witvley groups. It is noteworthythat the resul ts availabl e for Bambui and Una Grouplimestones fall into three distinct domains (Fig.3 ). Wecan argue that the data from Polanco, Nama andWitvley limestones are fit ing better because of similarpaleogeographical and paleotectonic situations durin gNeoproterozoic times. More global studies andadditional data have to be performed iri order toconfirm this argument. The other 0%0 results obtainedon Villalba Formation and illustrated in the satnediagram plot in two separate fields, showing that theyare uncorrelatable with the limestones from PolancoFormation. It is proposed that the two tectonic eventsmentioned above lead to disturbance of the C and aprimary isotope ratios of these carbonates, as issupported by the Sr ratios.

REFERENCES

Derry , L.A. , Kaufman, AJ. and Jacobsen, S.B ., 1992.Sedimentary cycling and environmental change in theLate Proterozoic: Evidence from stable andradiogenic isotopes. Geochimica et CosmochimicaActa. 56: 1317-1329.Gaucher, C. and Sprechmann., P., 1995.Paleontologia, sedimentologia y paleogeografia delproterozoico medio y superior del terreno Nico Perez,Uruguay. In: VI ' Simp. Sul-Brasileiro Geol . Bol.Resumos Expandidos . Porto Alegre, Brazil. 101-104.Gaucher, C., Sprechmann, P. and Schiplov, A., 199~OO '

Upper and Middle proterozoic fossiliferoussedimentary sequences of the Nico Perez Terrane ofUruguay: litrostratigraphic uni ts, paleontology,deposit ional envi ronments and correlations. NewesJah rbruch fur Geologie and Paleontologie.Abhandlun g. 199(3): 339,-367. .Gaucher, C., Sprechmann, P., Montana, 1. andMartinez, S., 1998. Lito estratigrafia, Sedimentologiay Paleogeografia del. Grupo Arroyo de Soldado(Vendiano-Cambrico, Uruguay). Aetas del IICongresso Uruguayo de Geologia. 24.31.Gorokhov, L.M. , Semikhatov, M.A., Baskakov, A.V.,Kutyaven, E.P. , Mel'nikov, N.N., Sochava, A.V. andTurchenko, T.L. 1995. Sr Isotopic composition inRiphean, Vendian, and Lower Cambrian carbonatesfrom Siberia. Stratigraphy and Geological Correlation.3(1): 1-28.Kufman, AJ., Jacobsen, S.B . .and Knoll, A.H. , 1993.The Vendian record of Sr and C isotopic variation inseawater: Implications for tectonics and paleoclimate.Earth and Planetary Science Letters. 120: 409-430.Kawashita, K., Kawashita, M.Y., Petronilho, L., Sato ,K., and Rodri gues, . M.S ., 1994. LixiviacaoProgressiva e Selcti va de Sr em Carbonatos: Sua 'importancia e Apl icacao em Carbonatos do GrupoBambui. 38° Congreso Brasileiro De Geologia.Resumos Expandidos SBG. 2: 399-400.Kawashita, K., Zaine , M., Petronilho, L. andFernandes, V.V ., 1996. Rochas Carbonaticas da FaixaPara~uai: idades e inferencias com base em razces87Sr/ 6Sr. Anais XXXIX Congreso Brasileiro DeGeologia, Salvador, Ba. 6:531-532.Kawashita, K., Babinski, M., Van Schmus, W.R . andBarros, G., (1997). Calcarios dos grupos Bambui eUna: Novo enfoque com base em dados Pb/U, Sm/Nde 87Sr/86Sr. Aetas X Semana de Geoquimica/Cong.Geoq. Paises de Lingua Portuguesa, Braga, Portugal.393-396 . .Moorbath, S., Taylor, P.N., Orpen, l.L. , Treolar, P.and Wilson, l .F., 1987. First direct radiometric datingof Archaean Stromatolitic limestrone. Nature. 326(61 16): 865-867.Teixeira, W., Renne, ·P., Bossi, L, Campal, N. andD' Agrella FHho, 1999. 40Ar/39AI' and Rb-Srgeochronology of the ,Uruguayan dike swarm, Rio deLa Plata Craton and implications for Proterozoicinterplate activity in western Gondwana. PrecambrianResearch. 93: 153-180.Torquato, l .R. and Misi , A., 1977. Medidas isotopicasde carbono e oxigenio em carbonatos do GrupoBambui na regiao centro-norte do estado da Bahia.

Aetas II South American Symposium on Isotop e Geology

0.70

7e-3

"""'10"' . "" ( lUll)

, (II. ) • 532.0 +-11.0 (2~III • 0.70624 +- 0.0000b ( I f)WSWD :0. '7 .7

0.45

87 /6Rb 51'

f1g.2. Rb-Sr isochron for Infruslve Gucsuncmb j gronite Inthe Arroyo del Soldodo Group .

0.718"",uulI~tl OR.lNIT£

0.711

0 .709~

",111

---------<O~

"VI 0.700«>

0.7OJ .

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0.15 0.30

TVR1lJ.HVENDIANCAlI 81l1AH0.7100 ~'Tf"'~.......j'~,...,...,..~r-,.,.........-....,.,+~,........j

00.10~' . ~-- -- ..-~.......~~~: , ,

a 0' ",

0.7090 'b ,a

,v , 40 '

\\ <:. - ,,l; < :: 00 a0.70aa \

'" \£0 0 ,

<, \.. \r-.Vl 0.70BO \eo \0

_PolQnco \ 0

I,

0.7075,

I '~I

,,, Um..'an. 0 '

0.7070 . f . 'a Llm..la no, altorod ,,a Dolomite I• 9ur ko ,I 01. (198 2)

0.70l15 • k.la ~ Jacob..n (19117)1- -A.merom 0' 01. (1991) I

~ ..6.100.70'~'''::0''''''~530::-··''''·;~·~~'''''el'''0......- J

Age (1.40 )

ng. t . Eollmoted ag e lor Polonoo rormatl on bas ed In . enulor

'Jorlolion in 87Sr 86gr of 'Jendla n end caMbrian seowat.r

lugg••1 by Kaufman 01 01. ( 1993 ) from pronrvod Itmlston • •.

13, c(PIl8 )X.

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Flg.3 . Croup'ol of ,Uc and ,,60 lor Iho carbonato rocks from

Nlco P' rez Terren e. Also I"c ludod lho flold valu n for Nom~ a nd

Wltvl oy, groupo (Homo and Soulh Africa ) compllod from Kaufman

01 01. ( 1977) and Derry et 0 1. {1993}, a nd lor Bombul, Una ond

CurumbO gr oups (Bro:II), complied from KowoBhlla el 01. (1994),

Torquato and I.Ils l (1977) and Kawa.hlta 01 01. (199& ) rupoollv. IV·

Tho dom ain. lor mod.m and f..... hwa l. r IImoslono . or. olso plaited

lo r eempertson.

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Aetas II South American Symposium on Isotope Geology

Rev. Bras. de Geoc . 7,14-24. Africa. South African Geology. 22: 190-202.Toulkeridis, T., Goldstein , S.L., Clauer, N., Kroner,A. and Lower, D.R., 1994. Sm-Nd dating of Fig. TreeClay minerals of the Barbeton Greenstone belt, South

Table 1. Analytical Data and 87Sr/86Sr Values of Carbonate Rocks from Nico Perez Terrane (Uruguay) . .

Field N. Sample Group/Form Rb Sr 87Sr/86Sr 81:ic 8180.Type & local (ppm) (ppm) (PDB) (PDB)

PYE- Limestone Polanco F., 4 1380 0.70780 2.82 -12.332/1 P.YerbalPYE- Limestone Polanco F., 14 1460 0.70785 2.86 -11.032/2 P.YerbalCCU- Limestone Polanco F., 3 575 0.70801 2.56 -11.44/12 C.CuentasCCU- Limestone Polanco F., 6 580 0.70816 2.60 -11.34/13 C.CuentasMIN-31/5 Dolomitic Basal G., C. 17 40 n.a. -0.71 -5.2

rock del UmbuMIN-31/6 Dolomitic Basal G., C. 8 40 n.a -0.89 -5.7

rock del UmbuMIN-31/7 Dolomitic Basal G., C. 4 55 n.a -0.09 -8.5

rock del UmbuMIN-31/8 Dolomitic Basal G., C. ~3 40

..n.a -0.45 -5.8

rock del UmbuAFL-85/1 Stromatolite Basal/C.de ~3 165 0.72365 -2.18 -9.8

VillalbaAFL-85/2 Strom atolite Basal/C .de ~3 105 0.72531 -2.55 -13.5

Villalba

* All determinations were .performed on soluble fractions using 1N HCI acid for Sr/Sr ratiodeterminations or 100% phosphoric acid for CO2 extraction to obtain 8%0 values versus PDB. The Srisotopic compositions were obtained in a single collector VG-354 instrument whereas the 813C and8180 were get in' a multiple collector Delta E mass spectrometer. The external error for Sr/Sr ratios isestimated as 0.00003, based on 9 analyses done on the standard NBS-987 which yield an averagevalue of 0.71025 ± 0.00002 (10-) . The estimated error for 8 determinations is better than 0.3%0.

Abbreviations : P. :: Puntas de; C~ = Cerro (de las); G. =Group ; F. = Formationn.a.= not analysed

402

Aetas II SouthAmerican Symposium on Isotope Geology

GEOCHRONOLOGY AND CHEMOSTRATIGRAPHY OF "LA TINTA"NEOPROTEROZOIC SEDIMENTARY ROCKS, BUENOS AIRES PROVINCE,

ARGENTINA. .

K. Kawashita' , R. Varela' , C. Cingolani / , E. Soliani Jr.3, E. Linares", S.A. Valencio",. A.V. Ramos" and M. Do Campo"

1 Instituto de Geociencias, USP. Rua do Lago, 562; 05508-900, S. Paulo, Brasil. kaka@usp.br2 Centro de Investigaciones Geologicas, UNLP-CONICET. Calle I N° 644". 1900 La Plata, Argentina.

varela@cig.museo.unlp.edu .ar; ccingola@cig.museo.unlp.edu.ar3 Instituto de Geociencias, UFRGS. Av. Bento Goncalves, 9000, Poa, RS, Brasil. soliani@if.ufrgs.br4 INSTITUTO DE GEOCRONOLOGIA Y GEOLOGIA ISOTOPICA (INGElS). Pabe1l6n INGElS, Ciudad

Universitaria, 1428 Buenos .Aires, Argentina. linares@ingeis.uba.ar; susana@ingeis.uba.ar; adriana@ingeis.uba.ar ; marga@ingeis.uba.ar

Keywords: La Tinta Group, Chemostratigraphy, Geochro nology, Rio de la Plata cratonic region.

INTRODUCTION"

The Tandilia (Buenos Aires Province) Neo­proterozoic sedimentary cover largelly known as LaTinta Group (Amos et al., 1972) was redefined(Iniguez et al., 1989; Poire, 1993) to integrate a lowersection as Sierras Bayas Group and the upper part asCerro Negro Formation. These Neoproterozoic unitsrepresents a Southern portion of the Rio de la Platacratonic region (Fig. 1), in the SouthwesternGondwana (Dalla Salda et al., 1988). In an attempt toobtain better constrained ages for Sierras BayasGroup, new whole rock RblSr determinations onargillites and strontium isotopic measurements onlimestones were carried out in order to compare withprevious fine fractions RblSr isochronic ages and alsowith inferred secular variation curve proposed byGorokhov et al. (1995). Ol3C and 8180 measurementswere also performed on the same carbonates with thepurpose to compare with others of similar agesreported in the literature and to constrain the 87Sr;86Srdeterminations.

PREVIOUS WORKS

The Sierras Bayas Group in Olavarria area is welldescribed by Poire (1993) . The three transgressive­regressive sedimentary sequences reported by lfiiguezet al. (1989) comprise from base to top by VillaM6nica, Cerro Largo and Lama Negra Formations.The Loma Negra Formation is overlain uncon­formably by Cerro Negro Formation, which is

403

followed , also unconformably, by .quartzites ofOrdovician Balcarce Formation. The Cerro NegroFormation was previously studied by Bonhomme andCingolani (1980) and Cingolani and Bonhomme(1982), who obtained an isochronic RblSr age of 723± 21 Ma, established on fine fractions separated fromCerro Negro quarry argillites. This age was tentatively ·assumed as early diagenesis time. Other twosiliciclastic Cerro Largo and Villa M6nica Formationsof Sierra Bayas Group were also studied by RblSrmethod on fine fractions (Bonhomme and Cingolani,1980; Cingolani and Bonhomme, 1988). Theirrespective ages of 769 and 797 Ma, consistent withthe stratigraphic positions, were interpreted as theminimum ages for sedimentation. Among the SierrasBayas Group, only the uppermost carbonate sequence,the Lorna Negra Formation, -was .not dated byradiometrically methods , until the present. Carbon andoxygen isotopic data in limestone rocks were obtainedby Valencio et al. (1985).

RESULTS AND DISCUSSION

In order to evaluate the possible age ofsedimentation for Cerro Negro and Lorna NegraFormations and to check the validity of the concept ofmechanical mixing (Cordani et al., 1978) whole rockRblSranalysis were performed on argillites sampledfrom two different localities in Olavarria area (Fig. 2).The first set of 7 selected greenish argillites fromCerro Negro Formation supplied a well aligned

Aetas II South Amer ican Symposium on Isotope Geolo gy

LOWERPALEOZOIC

1

SE

PRECAMBRIANI

ro:'l~

1~=1 Dolomlc

~ Limcstoncs

I II co II, , "KnI.

f .. t+ ii\

SIERRAS

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i Buenosf , A~es

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o

61 ' A'.. t.:::. 51'....- " ~• 0<1'0;: _" .... Si rras

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Figure 1 : Geological sketch map from Tandilia Region, Correlation ofthc units.

(MSWD = O.J0) points in an isochron diagram with ago, the clay minerals were homogeneized duringan age of 734 ± 48 Ma and initial ratio of 0.7330 ± diagenesis. Both examples confirm the validity of0.008 (Fig.3). This age performed on whole rock using total rock analyses on diagenetic rocks tosamples would support the previous interpretation of evaluate the probable age of sedimentation, and theearly diagenesis for the age of 723 Ma obtained in fine present data sign the possible age limit of ca. 730 Mafractions. Nevertheless, the sedimentation of Cerro for Sierras Bayas Group. This means that the CerroNegro Formation could started earlier, around 770 Ma Negro Formation was deposited during or laterbecause of large uncertainty of 48 Ma in the age. The Sturtian glacial epoch, established as around 780 Mawhole rock analysis performed on 10 varied by Knoll et al. (1986).assortment of argillites (pinky to greenish) sampled at The micritic black limestones sampled 111

Lorna Negra quarry, Olavarria area, suggest an Olavarria and Barker area, at Loma Negra and EIisochronic age of 725 ± 36 Ma and estimated initial Infierno quarries, respectively, are exhibiting H7Sr/86Srratio of 0.707 (Fig. 3). Although the points are not in a wide range, between 0.70584 and 0.71409 (Figurewell aligned (MSWD = 7.55), this calculated age is 4). Using the criteria currently adopted (Kaufman etcoherent with two other estimated ages for overlying al .,1993; Kawashita et al ., 1994), the most probableFormation. Thus, these ages support an hypothesis ratios can be assumed as 0.7059 or 0.7068, inferredthat the Lama Negra and Cerro Negro Formations respectively from EI Infierno and Lama Negra blackwere deposited at ca. 770 Ma and, at around 730 Ma limestones. Based in the Rb content, the best estimate

404

Aetas II South American Symposium on Isotope Geology

collaborators. The first inferred age of ca. 800 Ma ismore compatible and is in accordance with the abovementioned RbiSI' ages for overlying Cerro NegroFormation, whose minimun age was estimated by

for Loma Negra 51' "sea" would be 0.70584 ±0.00010 . This value points to two possibleaj es, 800ancl 1100 Ma, when plottecl in the secular 7Sr/ 86Srratio variation curve ill the Neoproterozoic ancl EarlyC bri d b G kl dam nan seawater propose )y oro 10V an

Lab.N ° Field W Rb (ppm)* Sr (ppm)* 87Rb/86Sr** 87Sr/86Sr***SPR12952 CNO~12A 229 .7 48 .8 . 13.85 . 0.87833SPR12953 CN O-12C 230.6 47.1 14.42 0.88448SPR12954 CNO-12D 190.4 53.0 10.54 0.84410SPR12955 CNO-12E 157.0 52.5 8.75 0.82433SPR 13109 CNO-12B 209 .2 53.0 11.60 0.85475SPR13110 CNO-12F 221.8 56.5 11.53 0.85197SPR13111 CNO-121 221 .6 48.1 13.55 0.87342

CIG 845 LNO-1B 229.3 39.1 17.28 0.88904CIG 846 LNO -1C 229 .6 40.9 16.54 0.89 116CIG 847 LNO-1D 227.4 37.6 17.86 0.91277CIG 848 LNO-1E 224.2 38.0 17.37 0.88314CIG 849 LNO-1F 220 .3 51.8 12.45 0.82453CIG 850 LNO-1G 228 .0 39.4 17.04 0.88600CIG 851 LNO-2A 221 .3 45 .6 14.22 0.83568CIG 852 LNO-2B-1 229.7 41.6 16.23 0.86541CIG 853 LNO~2B-7 223 .3 . 44.4 14.77 0.85428CIG 854 l.NO-2C 223 .1 39.8 16.50 0.88264

Figure 2. RblSr who le rock analytical data; CNO (Cerro Negro Formation); LNO (Cerro Largo Formation); *X-rayfluorescence, error ± 2%; **Error ± 2.8%; *"'*Erroi' ± 0.000 10

12 16 . 20 0 4 8

F igure 3: Isochron diagrams from argillites of Olavarria.

16

B7~Sr

12

Age: 725 ± 36 Ma(Wendt.1I Model)R,: 0.7066 ± 0.0026MSWD: 7.55

CERRO LARGO FORMATION

B7SrfGSr •

0.76

0.88

0.92

0.84

. 0.80

B7~Sr

Age: 734 ± 48 Ma(York Model)

R1: 0.733 ± 0.008MSWD: 0.10

84o

0.72

0.76

0.84

0.80

CERRO NEGRO FORMATION0.92 B7Sr/B6Sr

0.88

Bonhomme and Cingolani (1980) as 769 ± 12 Ma.Assumption of the age of 800 Ma for Loma NegraFormation implies that either this age or 793 ± 32 Maobtained by Cingolani and Bonhomme (1988) for thelowermost Villa Monica Formation are relatively

younger or concordant, respectively, with the probableexpected age of sedimentation.

Despite some restrictions to use the stableisotope data (C and 0) on Neoproterozoic limestones,because both elements can be more affected than

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Aetas II South American Symposium on Isotope Geology

Barker and Olavarria area, the primary 813C would be3.4 ± OJ %0, while the primary 81 80PDB would bediffe rent in these studied areas . A value of -8.0 %0, anaverage value on four similar black limestones fromEI Infierno quarry, is suggested through cross plotwith 87Sr/86Sr ratios. Using the same proceeding forLoma Negra Formation limeston es, a primary value ofaround -5.0%0 can be suggested. This is the minimumvalue recorded on one profile sampled at Loma Negraquarry, Olavarria. The sampl es with lower valuesmust be influence of post-depositional effect. Thisaffecte d the 87Srl6Sr ratios and is well evident ongreenish and brownish limestones that constitute thebase of LNO (Lorna Negra , Olavarrfa) profile. Theyshow a negative correlation: decreasing in 8180 values

d . . I 87S /86S . .an mcrease II) t ie I' r rati os, III response toincreasing alteration .

In summary, the concepts o f mechanical mixingand chemos tratigraphy applied to silicic lastic rocksand limestone s of some of La Tinta neoproterozoicunits has revealed as very useful and valid tools.

modern ones by post-depositional processes, most ofthe limestones were analized for carbon and oxygenisotopic compos ition. The 813CPDB %0 determi nationsperformed on the Loma Negra Formation limestones(Fig. 4) show no significant variation s as is usuallyobserved in the Neoproterozoic basins (Kaufman et al., 1991; 1995). With one exception, the results are inthe range 3.0 - 4.6 %0. It is noteworthy to observe thatin the lower 87Sr/86Sr ratios limestones, the respective

813C values are lower than 4.0%0. Conversely, in thelimestones exhibiting 8180 ~ -9.0%0the 87Sr/86Sr ratiosare significantly higher, denoting influence ofdiagenetic process or effect of meteoric fluid.Anyway, it seems that the same process wasresponsable for changing the primary isotopesignatures. Since, there is facies control in carbo nisotopic carbonate composition within the basin and,oxygen isotopes are also sensitive indicators ofdiagenesis. is not easy to eva luate the primary isotopecompositions.

One key to infer the possible values is to use87S /86S ' I I' I .t le r r ratios as support. t seems t rat eit ier 111

FieldN° Type of sample Local/Area Rb Sr 5/Srr "Sr . 8DCI'DB OIK/'DB(ppm) (ppm) (%0) (%0)

LNO-3A Greenish limestone Lorna Negra/Olavarria 11 0 290 0.70981±12 + 5.0 -10.1LNO-3B Greenish limestone Lorna Negra/Olavarria 40 270 0.711 53±7 + 4.6 - 9.7LNO-3C Greenish limestone Lorna Negra/Olavarria 25 295 0.71135±7 + 4.3 - 9.5LNO-3D Brownish limestone Loma Negra/Olavarria 25 305 O.71030±1 2 + 4.4 - 9.4LNO-3I Black limestone Lorna Negra/Olavarria 9 325 0.70690±1 2 + 3.7 - 5.7LNO-7A Black limestone Lorna Negra/Olavarria 7 355 .0.70717±6 + 3.7 - 5.5LNO-7Cl Black limestone Lorna Negra/Olavarria 9 330 0.70697±5 + 3.1 - 5.1LNO-7C2 Black limestone Lorna Negra/Olavarria 8 325 0.70752±8 + 3.1 - 5.5LNO-7E Black limestone Lorna Negra/Olavarria II 325 0.70725±11 +3.4 -5.4 .--LNO-7G Black limestone Lorna Negra/Olavarria 10 31 5 0.70712±8 + 3.6 - 4.9LNO-5A Black limestone LornaNegra/Olavarria 6 320 0.70680±7 + 3.9 - 6.6LNO-5D Black limestone LomaNegra/Olavarria 5 330 0.70689±8 + 4.0 - 6.8LNO-5G Black limestone Lorna Negra/Olavarria 8 310 0.7071 2±6 + 4.1 -7.0LNO-51 Black limestone LornaNegra/OJavarria 8 325 0.70746±6 +4.0 - 7.2Av- l Black limestone Avellaneda/Olavarria 20 320 0.708I0±6 O .8±0.4) (-10.5±2,5)Av-2 Greenish limestone Avellaneda/Olavarria 30 335 0.70873±6 n.a. n.a.YC-I(top) Black limestone Yilla Cacique/Barker 35 560 0.71323±7 (3.4±O.7) (-9.0~:0.7)

YC-2 (base) Black limestone Yilla Cacique/Barker 15 270 0.71409±1 2 n.a. n.a.El lnf.I(Y) Black limestone EI lnflerno/Barker 5 238 O.70666±13 (3.7±O.7) (-8.0:1:1,2)EIInf.2(Y) Black limestone EI lnfierno/Barker < 3 254 0.70584:1:I0 (3.7±0.7) (-8.0±1 ,2)El lnf.3(Y) Black limestone EI Infierno/Barker :;;3 281 0.70604±11 (3.7±0.7) (-8.0±1,2)

Figure 4. Isotopic compositions and Rb and Sr contents on Loma Negra Formation limestones from Barker and Olavarriaareas. The 87Sr/K6Sr ratios were normalized assuming 86Sr/88Sr = 0.1194 and were performed on soluble fractions using INHCI acid. 8DC and 8180 determinations, both versus PDB, were carried 'out on soluble fractions using 100% phosphoricacid for CO2 extractions. The 87Sr/86Sr ratios were obtained in a single collector VG-354 mass spectrometer, while the 8DC

and 8180 were obtai~ed in a MM-602 instrument. The estimated errors in 8 determinations are ± 0.2 '}{III. The &values inparentheses are those from similar samples from the same locality.

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