Zimmermann and Bahlburg 2003[1]

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Provenance analysis and tectonic setting of the Ordovicianclastic deposits in the southern Puna Basin, NW Argentina

UDO ZIMMER MANN* and HEINRICH BAHLBUR G

*Department of Geology, RAU University, PO Box 524, 2006 Auckland Park, Johannesburg,South Africa (E-mail: [email protected])Geologisch-Palaontologisches Institut, Westfalische Wilhelms-Universitat Munster,Corrensstrasse 24, 48149 Munster, Germany (E-mail: [email protected])

ABSTRACT

Provenance studies on Early to Middle Ordovician clastic formations of thesouthern Puna basin in north-western Argentina indicate that the sedimentarydetritus is generally composed of reworked crustal material. Tremadoc quartz-rich turbidites (Tolar Chico Formation, mean composition Qt89 F7 L4) arefollowed by volcaniclastic rocks and greywackes (Tolillar Formation, mean Qt33

F42 L25). These are in turn overlain by volcaniclastic deposits (mean Qt24 F30L46) of the Diablo Formation (late Arenigearly Llanvirn) that are intercalated bylava flows. All units were deformed in the Ocloyic Orogeny during the Middleand Late Ordovician. Sandstones of the Tolar Chico Formation are characterizedby Th/Sc ratios > 1, La/Sc ratios 10, whereas associated fine-grained wackesshow slightly lower values for both ratios. LREE (light rare earth elements)enrichment of the arenites is 50 chondrite, Eu/Eu* values are between 072and092, and flat HREE(heavyrare earth elements) patternsindicate a derivationfrom mostly felsic rocks of typical upper crustal composition. The eNd(t sed)values scatter around )11 to )9. The calculated Nd-TDM residence ages varybetween 18 and 20 Ga indicating contribution by a Palaeoproterozoic crustalcomponent. The Th/Sc and La/Sc ratios of the Tolillar Formation are lower than

those of the Tolar Chico Formation. Normalized REE (rare earth elements)patterns display a similar shape to PAAS (post-Archaean average Australianshale) but with higher abundances of HREEs. Eu/Eu* values range between 044and 117, where the higher values reflect the abundance of plagioclase andfeldspar-bearing volcanic lithoclasts. Average eNd(t sed) values are less negativeat)51, and Nd-TDM are lower at 16 Ga. This is consistent with characteristics ofregional rocks of upper continental crust composition, which most probablyrepresent the sources of the studied detritus. The rocks of the Diablo Formationhave the lowestTh/Sc and La/Sc ratios, lower LREEabundances than the averagecontinental crust and are slightly enriched in HREEs. Eu/Eu* values are between063 and 117. The Nd isotopes (eNd(t sed) )3 to )1; TDM 12 Ga) indicatethat one source component was less fractionated than both the underlying Early

Ordovician and the overlying Middle Ordovician units. Synsedimentaryvulcanites in the Diablo Formation show the same isotopic composition. Ourdata indicate that the sedimentary detritus is generally composed of reworkedcrustal material, but that the Diablo Formation appears to contain80%ofalessfractionated component, derived from a contemporaneous continental volcanicarc. There are no data indicating an exotic detrital source or the accretion of anexotic block at this part of the Gondwana margin during the Ordovician.

Keywords Geochemistry, isotope geochemistry, Ordovician, provenanceanalysis, Puna, retroarc basin deposits.

Sedimentology (2003) 50, 10791104 doi: 10.1046/j.1365-3091.2003.00595.x

2003 International Association of Sedimentologists 1079


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INTRODUCTION

Provenance studies of sedimentary rocks aim todecipher the composition and geological evolu-tion of the sediment source areas and to constrainthe tectonic setting of the depositional basin.Here, results are presented of a provenance studyof the Lower to Middle Ordovician clastic Tolar

Chico, Tolillar and Diablo Formations of thecoeval southern Puna Basin of north-westernArgentina.

A meaningful provenance analysis necessarilyconcentrates on the evaluation of indicators thatare inherited from the original source areas.However, the data need to be scrutinized for theeffects of secondary factors that have the potential

Fig. 1. (A) Geological map of the Puna with important outcrop regions: 1, Salar de Pocitos, including ComplejoIgneo Pocitos and Complejo Basico Ojo de Colorados; 2, Central Sierra de Calalaste (Quebrada El Diablo); 3, SouthernSierra de Calalaste (SC); 4, Huaitiquina and Aguada de la Perdz; 5, Salar de Rincon; 6, Salar de Antofalla;7, Cuchiyacu pluton, modified after Rapela et al. (1992). (B and C) Study areas in the southern Puna of north-westernArgentina. Outcrops: 1, Vega Quiron; 2, Quebrada Honda; 3, Quebrada Carro Grande; 4, Falda Cienaga; 5, Los

Nacimientos; 6, Antofagasta de la Sierra; 7, Quebrada del Diablo.

1080 U. Zimmermann & H. Bahlburg

2003 International Association of Sedimentologists, Sedimentology, 50, 10791104


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to obscure this information, i.e. sorting, weather-ing, diagenesis, metamorphism and recent alter-ation processes (Morton & Hallsworth, 1999).Accordingly, a combination of several indepen-dent analytical approaches is required to haveconfidence in the provenance indicators. In viewof the Early Palaeozoic age of the studied rocks

and the effects of two orogenic overprints, i.e.during the end-Ordovician Ocloyic and theTertiary Andean orogenies, a combination offieldwork, petrographical, geochemical and Ndisotope data is used to relate the composition andevolution of the source regions to the platetectonic setting of the poorly understood southernPuna Basin, and to evaluate the potential role ofexotic terranes in the development of this part ofthe Western Gondwana margin during the EarlyPalaeozoic.

GEOLOGICAL SETTING

The Argentinean Puna forms the southern con-tinuation of the Bolivian highland plateau, situ-ated at an altitude of4000 m. Sedimentary andmagmatic rocks of Ordovician age are widelyexposed in a system of horsts and grabens formedsince the middle Tertiary as a consequence of theAndean orogeny (Zeil, 1979; Allmendinger et al.,1997). This study concentrates on two represen-tative outcrop areas of Lower Palaeozoic rocks inthe southern Puna, namely at Salar de Pocitos and

the middle to southern part of the Sierra deCalalaste (Fig. 1).

The association of Ordovician sedimentaryunits with mafic to ultramafic rocks in thesouthern Puna has been interpreted as the rem-nants of an Ordovician ocean (e.g. Ramos et al.,1986) that closed during the collision of one ofseveral proposed westerly terranes with thewestern Gondwana margin (Fig. 2). These terra-nes include the Puna Terrane (e.g. Coira et al.,1982), the composite PunaFamatina Terrane(Conti et al., 1996) and the ArequipaAntofalla

Terrane (e.g. Ramos et al., 1986; Forsythe et al.,1993; Bahlburg & Herve, 1997), partly interpretedas derived from Laurentia (Dalla Salda et al.,1992; Dalziel, 1997). Other authors assume thatthe continental margin of Gondwana in thisregion alternated between compressive andextensional tectonic regimes (e.g. Mon & Hongn,1991). These different tectonic scenarios implyvery different palaeotectonic positions for thePuna Basin (oceanic, back-arc, retroarc or intra-cratonic basin). In this provenance study of

Ordovician clastic formations, new petrographic,geochemical and isotope geochemical analysesare presented for the southern Puna Basin. Thesedata are the key to the interpretation of the source

area evolution and establishing the basinspalaeotectonic setting.

STRATIGRAPHY

The Lower Ordovician clastic sedimentary suc-cessions in the southern Puna include synsedi-mentary lava flows. The formations arefurthermore tectonically associated with maficto ultramafic rocks of poorly defined pre-Arenigage; the youngest age limit is given by felsic to

intermediate intrusives of the Complejo IgneoPocitos (Fig. 1A, no. 1) dated at 476 2 Ma (earlyArenig; Kleine et al., 1999). Ordovician depositsto the east of Salar de Pocitos and south of theSalar de Hombre Muerto (Fig. 1B and C: 1, VegaQuiron; 2, Quebrada Honda; 3, Quebrada CarroGrande; 4, Falda Cienaga; 5, Los Nacimientos;6, Antofagasta de la Sierra) preserve no recordof contemporaneous magmatism.

The plutonic rocks of the Complejo IgneoPocitos intruded pre-Arenig mafic to ultramafic

Fig. 2. Proposed geotectonic units between 15S and40S including the assumed Puna and PunaFamatinaTerranes. The black spots indicate the alleged ophio-litic rocks in the southern Puna (e.g. Ramos et al.,1986). The Famatina Terrane represents the southernpart of a hypothetical PunaFamatina Terrane (modi-fied after Bahlburg & Herve, 1997); STG, Santiago deChile; BUE, Buenos Aires.

Provenance of sedimentary rocks, Puna Basin 1081



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bodies and show tectonic contacts with thesedimentary rocks. Tectonic juxtaposition of themafic to ultramafic rocks most likely occurredduring the Ocloyic Orogeny in the Late Ordovi-cian. Geochemical data characterize the maficbodies as calc-alkaline rocks related to a mag-matic arc environment (Zimmermann et al.,

1999).An improved stratigraphy (Fig. 3) for the Lower

Ordovician units in the southern Puna is based onnew finds of graptolites (Zimmermann, 2000).The oldest Ordovician sedimentary rocks arerepresented by the Tolar Chico Formation (afterZappettini et al., 1994; redefined here). Thisunfossiliferous formation contains thick quartzarenites as well as thin layers of silty wackesdisplaying grading and ripple cross-lamination.The presence of thick turbidite packages and theabsence of shallow-water deposits indicate a

marine environment below wave base. The TolarChico Formation is conformably overlain by theTolillar Formation (after Zappettini et al., 1994;redefined here). At the base, it consists of rare

medium-grained quartz arenites, which giveway to volcaniclastic sandstones, quartz-richsiltstones and shales. Fine- to coarse-grainedvolcaniclastic greywackes and massive feldspa-thic greywackes are the most abundant litholo-gies. Flute marks, ripple, cross- and parallellamination are consistent with deposition from

turbidity currents. Debris-flow deposits consist-ing of immature detritus are interbedded with theturbidites and shale horizons. The immaturematerial points to only minor reworking betweenthe source and the site of deposition.

In the central part of the Sierra de Calalaste(Quebrada Diablo, Fig. 1A, no. 2; Fig. 1C, no. 7), avolcanogenic unit of Arenig to Lower Llanvirnage crops out, which is informally named theDiablo Formation after its typical outcrop in theQuebrada El Diablo (260335,0S, 673639,3W;Fig. 3; Zimmermann, 2000). This formation is

dominated by very coarse-grained debris-flowdeposits and fine- to coarse-grained greywackesand siltstones representing turbidites. Rhyoliticto andesitic lavas, including some pillowed

Fig. 3. Stratigraphic table of the Early Palaeozoic of NW Argentina. The grey shading marks the stratigraphy dis-cussed in this paper (compilation of stratigraphic references in Acenolaza & Baldis, 1987; Bahlburg & Herve, 1997;Zimmermann et al., 2002).




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andesites, form occasional intercalations. Chilledmargins mark the contacts of the lava flows withthe underlying sedimentary rocks. Rare peperitesand volcaniclastic flows are the result of sub-aqueous synsedimentary volcanism. This associ-ation of volcanogenic sedimentary rocks andinterbedded lava flows could be interpreted as

having been deposited in a volcanic apron (e.g.White & Busby-Spera, 1987). Equivalent associa-tions are also exposed in the northern Puna(Bahlburg, 1991). These units of volcanogenicfacies contrast markedly with the overlyingsiliciclastic Coquena and Falda Cienaga Forma-tions exposed to the north-east and east (outcrops16 in Figs 1B and C and 3; Acenolaza et al.,1975; Zimmermann et al., 2002).

Flute marks (n 104) in the Lower Ordovicianformations indicate a uniform axial sedimenttransport from the S and SSW to the N and

NNE, which is also dominant in the continuationof the Ordovician basin in the northern Puna.Here, volcaniclastic detritus originated in a west-ern volcanic arc source, whereas more maturebasement detritus was derived from the eastward-lying Late ProterozoicEarly Cambrian Puncovis-cana orogen (Bahlburg, 1991, 1998).

The entire Lower Ordovician sedimentary suc-cession from the Tolar Chico Formation to theDiablo Formation underwent only a very low-grade metamorphism, as indicated by analyses ofthe illite and chlorite crystallinities (Zimmer-mann, 1999). The sedimentary successions are

isoclinally folded with folds commonly vergingwestwards. Towards the south, the deformationclearly increases, and the vergence becomes morevariable. The cleavage is predominantly parallelto the bedding in the coarse units. In the peliticrocks, two cleavages may be developed locally.

PREVIOUS STUDIES

Geological studies of the Ordovician in theremote southern Puna are rare. Only local studies

on the metamorphic basement, early Palaeozoicmagmatic rocks and Ordovician sedimentaryrocks of the southern Puna have been published,including preliminary palaeontological work (seecompilations in Ramos et al., 1986; Rapela et al.,1992; Bahlburg & Herve, 1997; Zimmermann,2000; Zimmermann et al., 2002). However, Ordo-vician sedimentary rocks are extensively exposedin this region and are the key to interpreting theactive margin evolution of the southern centralAndes during the Early Palaeozoic.

PROVENANCE STUDY

Analytical methods

Framework mineral composition was quantifiedusing the point-counting method of Gazzi andDickinson as described by Ingersoll et al. (1984)

with a Swift model F counter. The thin sectionswere partly stained to identify the differentfeldspars. A total of 350530 counts were execu-ted in traverses to obtain a sufficient database.

Heavy mineral composition was quantifiedusing the counting method of Boenick (1983).The separated heavy minerals (60150 lm) weredivided in two fractions (6090 lm and 90120 lm) and counted in traverses up to 250counts. The opaque minerals were not consideredin the calculation of the mean percentages.

X-ray diffraction (XRD) was carried out on 43

samples (< 2 lm) to determine the illite crystal-linity. The samples were dried on a sampleholder (2 2 cm) and measured in four directionsglycolized and not glycolized. A Philips X-raydiffractometer was operated at 30 mA and 40 kV.The sample were measured from 2 to 45 2h, insteps of 002 per 2 s. The measurements of illitecrystallinity were analysed using diffrak (Sie-mens, version 30). The analytical process isdescribed in Warr & Rice (1994), and thestandards of Warr & Rice (1994) were used.

Scanning electron microscope (SEM) studieswere carried out on light and heavy minerals. The

separated minerals were mounted with a currentconducting glue on sample holders (diameter12 cm) and coated with gold. The samples wereanalysed using a SEM 505 (Philips).

Cathodoluminescence (CL) studies were per-formed using an ASK-SEM-CL (ASK-Wesel) at theDepartment of Mineralogy (University of Heidel-berg, Germany). The CL analyses were comparedwith backscatter electron images of the samesamples using a BSE microscope LEO 440 (FirmaLEO-Zeiss/Leica-Oberkochem, Cambridge).

X-ray fluorescence (XRF) analyses for major and

trace elements were carried out using a SRS 303(Siemens) wavelength-dispersive XRF spectro-meter operating at 50 kV and 50 mA. Only freshand homogeneous samples were selected,crushed and milled. Fudion disks were preparedwith Spektroflux 100 (Lithiumborate, Li2B4O7) asflux. Detection limits for major elements arerelated to the atomic number, and are between 1and 10 lg for medium heavy elements. Precisionis 05 (1 r); accuracy was controlled by repetit-ive measurements of standards, and each sample




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was measured twice. The errors of major elementsvary between 05% and 2%.

INAA (instrumental neutron activation analy-sis) was performed by Actlabs (Ontario, Canada).Samples were crushed and milled in an agatemill. The powder was dissolved in lithium meta-borate flux fusion, and the resulting molten

bead was rapidly digested in a weak nitric acidsolution. Detection limits are between 05 p.p.m.and 5 p.p.m. for elements such as Ag and Cr.INAA precision and accuracy based on replicateanalysis of international rock standards are 25%(1 r) for most elements and 10% for U, Sr, Ndand Ni.

TIMS (thermalionization mass spectrometry) forSm-Nd analyses were performed at the NIGL (BGS,Keyworth, UK) by isotope dilution with a mixed149Sm150Nd spike, using HDEHP-coated polysty-rene columns for the REE separation. Measure-

ments were carried out on a fully automated VG354 mass spectrometer, using static collection todetermine the Sm and Nd concentrations, andmixed static/peak-jumping to determine the143Nd/144Nd ratios with an internal precision of 10 p.p.m. (1 SD). Long-term reproducibility of143Nd/144Nd ratios both on the La Jolla andin-house standards is better than 15 p.p.m. (1 r).Sm/Nd ratios on rock standards are reproducibleto 0102% (1 r). La Jolla standard measured0511900/)0000008 of seven analyses. Valuesused for calculations: Sm/NdCHUR 01966,147Sm/144Ndcrust 0115,

147Sm/144Ndmantle

0222, eDMt 0 86, Q 2513, f 0415, three-stage model Sm/Ndlimit 0300, equiv. TDM 2063,1993896236. Nd model ages calculated afterDePaolo et al. (1991).

Petrofacies

Tolar Chico FormationThe Tolar Chico Formation (Figs 3 and 4) iscomposed mainly of quartz arenites intercalatedwith some fine-grained wackes. Besides quartz,the quartz arenites contain slightly more alkali

feldspar (microcline) than plagioclase (Fig. 4D).Rock fragments are usually rare but, in somesamples, sedimentary lithoclasts, with internalsilt to fine-sand grain size, are abundant (Fig. 4C,Table 1). Accessory phases include biotite, mus-covite and calcite. XRD analyses show the pres-ence of chlorite and illite. The pseudomatrix(Dickinson, 1970) has the same qualitativemineralogical composition as the framework frac-tion. The wackes are similar in their matrixcomposition, but show a higher amount of

sedimentary lithoclasts (e.g. sample A84, Table 1).Cathodoluminescence analysis of quartz grainsindicates a dominance of metamorphic quartz(mean: Qmet 75%, Qplut 20%, Qvolc 5%;Zimmermann, 1999). Towards the top of theformation, volcanic quartz becomes more abun-dant. SEM pictures of individual quartz grains

show v-shaped indentations/impact marks,which suggest a subaqueous turbulent transportregime (Krinsley & Marshall, 1987). Some feld-spars show alteration to sericite and others, asobserved with SEM, are fresh. Lithoclasts aresubordinate and, in most cases, they are ofphyllitic or other (meta-)sedimentary origin(Table 1). The heavy mineral composition of theTolar Chico Formation is dominated by zircon,tourmaline and rutile in the 60150 lm fraction.Metamorphic minerals such as sillimanite, epi-dote, zoisite and pumpellyite are subordinated.

Amphiboles and other mafic minerals are extre-mely rare (< 2%; Table 2).

Tolillar FormationThe sandstones and wackes of the Tolillar For-mation are relatively rich in feldspar includingboth plagioclase and potassium feldspar (Fig. 4Aand D). The abundance of framework clast typesis variable in the greywackes and volcaniclasticsandstones (Fig. 4; Table 1). Nearly 70% of thequartz grains are of volcanic origin according toCL analysis (mean: Qmet 10%, Qplut 20%,Qvolc 70%; Zimmermann, 1999). Quartz grains

have ragged form and display resorption embay-ments probably indicating a rapid cooling processin volcanic rocks (Matter & Ramseyer, 1985).Also, some resorption embayments may be theresult of solution processes (e.g. Schneider, 1993).In the upper part of the succession, massivefeldspar-rich greywackes occur (plg + kf > 50%;A223, A273, A296, A309, X27; Table 1). Theamount of plagioclase in relation to potassiumfeldspar increases slightly from the Tolar ChicoFormation (Fig. 4D). Microcline is absent.Volcanic lithoclasts are abundant. They are

most commonly of felsic origin (Lv(felsic)/Lv(intermed) 3:1). Metamorphic lithoclasts areinfrequent (Table 1). The arrow in Fig. 4A and Bshows the stratigraphic trend in the TolillarFormation to an increasingly volcanic composi-tion, overlapping with the Diablo Formation. Inthe heavy mineral population, tourmaline andrutile are less abundant, and zircon is onlyslightly more frequent than in the Tolar ChicoFormation. Apatite and amphibole abundanceincreases markedly from the Tolar Chico Forma-




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tion. Metamorphic minerals, apart from epidote,are very rare (Table 2).

Diablo FormationThis formation is also relatively feldspathic withslightly more plagioclase than potassium feldspar

(Fig. 4, Table 1). Plagioclase abundances areroughly similar to those in the Tolillar Formation.Plagioclase occurs as large grains (> 1 cm), andmany preserve a magmatic zonation. In thefeldspar-rich greywackes of this formation, quartzabundances vary between 18% and 46% (Fig. 4;

Fig. 4. Framework mineral composition of Lower Ordovician sedimentary rocks in the southern Puna. (AD)Framework mode diagrams according to Dickinson & Suczek (1979) and Dickinson et al. (1983): Q, quartz; F, feldspar;L, lithoclasts; Qm, monocrystalline quartz; Qp, polycrystalline quartz; Qt, total quartz Qm + Qp; P, plagioclase; K,potassium feldspar; Lt, total lithoclasts including Qp; Lv, volcanic lithoclasts; Ls, sedimentary lithoclasts. (A) Qt-F-Ldiagram: the arrow indicates the trend from bottom to top in the Tolillar Formation. (B) Qm-F-Lt diagram: the arrowindicates the trend from bottom to top in the Tolillar Formation. (C) Qp-Lv-Ls diagram: note the mixed composition ofthe Tolillar Formation and the relatively high amount of Ls in the Diablo Formation. (D) Qm-P-K: note that albitizationmay have reduced the P/F ratio. For comparison, the averages of Lower to Middle Ordovician sedimentary rocks andturbiditic sandstones of the northern Puna: VS, volcanosedimentary successions; LTS, lower turbidite system; UTS,upper turbidite system (Fig. 3); Qm-P-K data for the northern Puna rocks and Coquena Formation are not available.

Falda Cienaga Formation from Zimmerman et al. (2002); northern Puna rocks from Bahlburg (1990) were counted afterSuttner & Basu (1985) and corrected for comparison with the GazziDickinson method described by Ingersoll et al.(1984). UTS, LTS, VS and Coquena Formation averages are not available for (D) and Coquena Formation for (C).




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Table 1). Quartz grains often show resorptionembayments pointing to derivation from volcanicsources. CL analysis shows that more than 80% of

the quartz grains are originally from volcanicrocks. Quartz derived from metamorphic rocks isnearly absent (mean: Qmet 2%, Qplut 13%,Qvolc 85%; Zimmermann, 1999). The amountof volcanic lithoclasts increases, including anotable population of intermediate composition.Even though there is some overlap with somevolcanogenic samples of the Tolillar Formation,the Diablo Formation contains the strongestinput of volcanogenic material of all threesuccessions (Fig. 4; Table 1). Relicts of alteredglass shards, peperites and massive fine-grainedsilicified layers interpreted as recrystallized

ashes are abundant in the Diablo Formation.XRD analysis reveals that the siltstones andshales have a similar qualitative mineralogicalcomposition. In the heavy mineral fraction(Table 2), elongated idiomorphic zircons withgas bubble inclusions point to a volcanic origin(Kostov, 1973). The amount of amphibole increa-ses markedly over the Tolillar Formation(Table 2). Neither metamorphic minerals norheavy minerals indicating a mafic source (e.g.Morton et al., 1992; Schafer & Dorr, 1997) wereobserved.

Major element geochemistry

The geochemical analysis of sedimentary rocks isa valuable tool for provenance studies of matrix-rich sandstones as long as the bulk composition isnot strongly affected by diagenesis, metamorph-ism or other alteration processes (McLennanet al., 1993). Abundances and ratios of majorelements thus need to be checked for mobility,especially during diagenesis (e.g. Boles & Franks,

1979; McLennan et al., 1980; McLennan, 2001). Agood measure of the degree of chemical weather-ing can be obtained by calculation of the chemical

index of alteration (CIA; Nesbitt & Young, 1982)using the molar proportions of Al2O3, CaO*,Na2O and K2O, where CaO* is CaO in silicatesonly. The resultant value (CIA) is a measure of theproportion of Al2O3 vs. the mobile oxides in theanalysed samples typically representing the alter-ation of feldspars and volcanic glass to clayminerals.

An ideal weathering trend for a rock of rhyoliticto dacitic composition is indicated by a solidarrow, subparallel to the line linking the CaO* +Na2O and Al2O3 apices of the triangular diagramin Fig. 5 (A-CN-K diagram after Nesbitt & Young,

1984; Fedo et al., 1995). However, whereas somesamples follow this ideal trend, others deviatefrom it along a weathering trend towards an illitecomposition, possibly as a result of a metasomaticincrease in K during diagenesis, caused by eitherthe conversion of aluminous clay minerals toillite or transformation of plagioclase to K-feld-spar (Fedo et al., 1995). Alternatively, this patternmay indicate mixing of a moderately weatheredsource with an unweathered one of differentprimary composition, or a secondary gain orloss especially of Na and K, and potentially Ca,

in the silicate fraction, e.g. during albitization(McLennan et al., 1993).

The average CIA value of all formations is 58(SD 68), with broadly similar formation aver-ages between 57 and 63 (Table 3). Variationsresulting from different grain sizes are minor(Fig. 5). The quartz arenites of the Tolar ChicoFormation have relatively low CIA values becauseof low Al2O3 concentrations, and do not seem tofollow a clear weathering trend. Nesbitt et al.(1996) demonstrated that a probable in situ

Table 1. Selected framework mineral data of the Ordovician Puna Basin deposits [data this study, Bahlburg (1998)for the southern Puna Basin and Zimmermann et al. (2002) for the Falda Cienaga Formation].

Basin Qt F L Matrix (%) Lv/L P/F

Ordovician southern Puna BasinTolar Chico Formation (n 23) 888 67 45 854 006 036Tolillar Formation (n 26) 33 42 251 214 03 04

Diablo Formation (n

14) 245 30 45

7 16

88 0

3 0

6Falda Cienaga Formation 78 12 9 85 0 023

Ordovician northern Puna BasinVolcanosedimentary successions 49 17 34 098 081Lower turbidite system 49 23 28 085 064Upper turbidite system 67 8 25 062 072

Complete point-counting data for the Tolar Chico, Tolillar and Diablo Formations are available in Supplementarymaterial, Table S1.




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alteration of feldspar could produce a quartz-richcomposition causing difficulties in the interpre-tation of the CIA. Also, a few samples (e.g. A193,coarse grained greywacke; Table 3) have lowconcentrations of CaO (022%) and K2O (039%)and high Na2O values, which may reflect albiti-zation (e.g. Milliken, 1988). Major element abun-dances may in some cases reflect the composition

of the weathering profile mantling the sourcerocks rather than the bedrocks themselves (Nes-bitt et al., 1996). The major element data presen-ted here thus indicate that the rocks were affectedby alteration to a minor degree.

Provenance classification diagrams after Bhatia(1983) and Roser & Korsch (1986, 1988) using themajor element chemistry show a wide spread forthe three formations and cannot be used tointerpret the tectonic setting successfully becauseof the mobility of K and Na, particularly during

Fig. 5. (A) The relation CaO* + Na2OAl2O3K2O isplotted in combination with the chemical index ofalteration (CIA) to show alteration trends (after Nesbitt& Young, 1984; Fedo et al., 1995). The CIA is calculatedusing molar proportions (Nesbitt & Young, 1982):CIA [Al2O3/(Al2O3 + CaO* + Na2O + K2O)]100; CaO*,only the CaO in silicates. An ideal alteration trend(straight line arrow) would be (sub)parallel to the ACNline (McLennan et al., 1990; Panahi & Young, 1997).K-metasomatism in the form of replacement of plagio-clase by potassium feldspar greatly influenced the

alteration history (after Fedo et al., 1995). Kao, kaolin-ite; ill, illite; plag, plagioclase; ksp, potassium feldspar.

Table2.

Abundancesoftheh

eavymineralpopulationintheLowerOrdoviciansedimentaryrocksofthesouthernPuna(theopaquefractio

nwasdominatedby

pyriteandhaematite).

Zircon

total

Zircon

rounded

Zircon

broken

Zircon

euhedralTourmalineRutileMonaziteApatiteAmphiboleZo

isiteSillimaniteEpidoteOthersO

paque*ZTR

Sum

TolarChicoFm.

Counts

920

580

313

27

323

80

15

29

24

11

8

8

62

312

1792

Mean(%)

622

630

340

30

218

54

10

20

16

07

05

05

42

174

894

TolillarFm.

Counts

714

172

464

78

45

34

8

146

92

0

2

5

33

167

1246

Mean(%)

662

240

650

110

42

32

07

135

85

0

02

05

31

134

735

DiabloFm.

Counts

718

165

460

93

49

39

14

157

223

0

0

0

42

325

1567

Mean(%)

578

230

640

130

39

31

11

126

180

0

0

0

34

207

649

*AfterBoenick(1983):opaque

mineralsarenotconsideredinthe

calculationofmeanpercentages.

AfterHubert(1962):percentageofzircon,tourmalineandrutile

tototalheavyminerals.

Notetheconstantpercentageo

fzirconinallthreeformations,and

thechangeintheamountofidiomor

phicgrains.Thenumberofapatiteandamphibolegrains

increasesfromtheTolarChico

totheDiabloFormation,metamorp

hicheavymineralswerenotfoundintheDiabloFormation.Thirty-eightsampleswithgrain

sizesbetween60and250lmwereseparated,andthefractionbetw

een60and150lmwasanalysed.S

eparationtechniquesandcountingmethodaccordingto

Boenick(1983)andMange&Maurer(1992).




10/26

weathering of feldspar. This is consistent with theresults of Bahlburg (1998) obtained from Ordovi-cian rocks of the northern Puna.

Trace element geochemistry

Trace elements, such as the high field strengthelements Th, Sc and Zr and REE (rare earthelements), are particularly useful for provenanceanalysis as they are insoluble and usuallyimmobile under surface conditions. On accountof their typical behaviour during fractionalcrystallization, weathering and recycling, theypreserve characteristics of the source rocks in the

sedimentary record (e.g. Taylor & McLennan,1985; Bhatia & Crook, 1986; McLennan, 1989;McLennan et al., 1993; Roser et al., 1996).

Trace elements such as Cr are useful in iden-tifying accessory detrital components such aschromite, commonly derived from mafic to ultra-mafic sources including ophiolites, not readilyrecognized by petrography alone. The average Crcontent of the upper continental crust is83 p.p.m. (McLennan, 2001). Average Cr valuesof the studied formations decrease from theTolar Chico Formations upper crustal value of 81 p.p.m. (n 9; SD 501) to 46 p.p.m.in the Diablo Formation (n 20; SD 415;

Table 3. Summary of selected major and trace element data for the Ordovician southern Puna Basin deposits.

Tolar Chico Fm.(n 9)

Tolillar Fm.(n 15/10)(major/traceelement samples)

Diablo Fm.(n 20/10)

Mean SD Mean SD Mean SD UCC*

CIA 627 63 573 77 563 112SiO2 8776 887 7307 568 7294 546 66CaO 023 010 110 120 156 169 42Na2O 021 013 321 160 331 201 39K2O 203 092 287 156 217 113 34Cr 8078 5011 5580 2092 4643 4152 83Ni 1176 1291 1098 1340 926 1225 44Ta 067 032 146 053 061 038 1Nb 618 413 1359 414 861 233 12La 2322 1228 3690 1414 2660 1175 30Nd 1878 986 2680 769 2370 955 26Sm 291 157 471 095 404 163 45Yb 171 108 420 128 403 141 22

Zr

19900 50

72 129

00 40

82 153

00 35

15 190Th 648 326 1403 356 1013 254 107

Sc 387 390 873 224 1281 447 14Ce/Ce* 090 006 089 008 107 014 107Eu/Eu* 082 008 067 028 079 014 066Sm/Nd 016 002 018 003 017 002 017LaN/YbN 956 115 683 448 409 203 505La/Sc 805 252 435 172 235 152 214Zr/Sc 8180 4047 1390 275 1310 422 1357Th/Sc 228 082 151 054 085 032 076La/Th 359 038 310 142 263 087 280Th/U 522 388 433 172 400 142 382Zr/Th 3385 832 943 261 1589 396 1776Cr/Th 1493 992 435 227 593 618 776Cr/V 319 269 138 136 052 142 078Y/Ni 137 078 635 475 025 377 050

Major elements in percentage, trace elements in p.p.m.Other trace elements were measured by INAA.Major and some trace elements were measured by XRF.SD, standard deviation (1 r) ; UCC after McLennan (2001).Further major (TiO2, Al2O3, Fe2O3, MnO and P2O5) and trace element data (REE, Sr, U, Rb, Cs, Ba, V, Hf, Y, Cu, Pband Co) are available in Supplementary material, Table S2.




11/26

Table 3). One sample from the Tolar Chico For-mation and two samples from the Diablo Forma-tion have Cr concentrations reaching 200 p.p.m.Cr-bearing minerals were not found in the studiedheavy mineral concentrates (60150 lm), but maypotentially be present as a minor constituent inthe fraction < 60 lm. On the other hand, the

relatively high average Cr/Th ratio (mean 149,SD 99) of the Tolar Chico Formation is mostlydue to low Th concentrations. All the othersamples have Cr/Th ratios lower than 78, theaverage upper continental crust (UCC) value(McLennan, 2001). Input from mafic sourceswould also result in an enrichment of V and Ni.The respective values are much lower than in theupper continental crust leading to relatively highCr/V and Y/Ni ratios in most of the samples(Table 3). This underlines the insignificance of amafic or ultramafic contribution to the deposits

(Floyd & Leveridge, 1987; McLennan et al., 1993).Another good tracer of mafic source componentsis the compatible element Sc, particularly whencompared with Th, which is incompatible andthus enriched in felsic rocks. Both elements aregenerally immobile under surface conditions andtherefore preserve the characteristics of theirsource. Thus, the Th/Sc ratio is considered arobust provenance indicator (Taylor & McLennan,1985; McLennan et al., 1990). The respectiveratio in average upper continental crust is 079(McLennan, 2001).

From the Tolar Chico to the Diablo Formation,

the average Th/Sc ratios decrease from 228 to085 (Fig. 6, mean values from Table 3), close toor greater than the average crustal value. In theTolar Chico Formation, both Th and Sc abun-dances are significantly less than upper crustalvalues (Th/Sc mean 228; SD 082). This is

almost certainly caused by quartz enrichment inthe quartz arenites, producing a dilution effectalso seen in the respective REE data. The TolillarFormation shows a wide spread of valuesbetween 092 and 25 (mean 151; SD 054)resulting from the heterogeneous mineralogicalcomposition. In the Diablo Formation, the average

values (average 085; SD 032) obscure the factthat some samples have Th/Sc ratios between 056and 070, which are lower than upper continentalcrustal values. Significantly, these samples rep-resent deposits intercalated with rhyolitic toandesitic lava flows (samples B44, B75, B86,B90, B96 and B130; Table 3).

Th/Sc and Zr/Sc element ratios can reveal acompositional heterogeneity in the source(s), if thesamples show Th/Sc and Zr/Sc values along thetrend from mantle to upper continental crustcompositions (McLennan et al.,1993;Fig. 6).Even

though the data for the three studied formationscluster almost exclusively in the upper continentalcrust compositional field, the lower Th/Sc valuesin the Diablo Formation reflect a stronger inputfrom a less evolved source. The Zr/Sc ratio iscommonly used as a measure of the degree ofsediment recycling leading to the enrichment ofthe stable mineral zircon in the deposits (McLen-nan, 1989; McLennan et al., 1993). The Zr/Sc ratiois highest in the Tolar Chico Formation (mean - 818; SD 405) and far above the UCC value(Fig. 6; Table 3). Zr values range from 145 to293 p.p.m. (average 199 p.p.m.) and scatter

around the UCC average of 190 p.p.m. (Table 3;McLennan, 2001). The high Zr/Sc ratios are causedmainly by a Sc concentration far below the value ofaverage upper crust (14 p.p.m.; McLennan, 2001).

An average Zr/Th ratio of 3385 (SD 83),another measure of the degree of recycling, is well

Fig. 6. Plot of Th/Sc vs. Zr/Scincluding the averages of the sedi-mentary rocks of the northern Punafor comparison. The values of theTolar Chico Formation have to beinterpreted carefully, as discussedin the text (modified after McLen-nan et al., 1993); , average; VS,volcanosedimentary successions;LTS, lower turbidite system; UTS,upper turbidite system (see alsoFigs 3 and 4).




12/26

above the upper crustal average of 1776 andresults from low Th concentrations. Th is presentin many minerals but is commonly abundant inthe heavy minerals monazite, zircon, titanite andin minerals of the epidote group such as epidoteand allanite. Concentration of these heavy min-erals during recycling would accordingly lead to

an increase in Zr and Th abundances in therespective deposits. Low values of both Th and Zrtherefore corroborate the evidence of the heavymineral petrography, indicating that the maturequartz arenites of the Tolar Chico Formation donot show the zircon enrichment expected, giventhe maturity of the deposits. In contrast, the Zr/Scand Zr/Th values of the Tolillar and DiabloFormations are very similar to the respectivevalues of the upper crust (Table 3).

Rare earth elements

The REEs represent well-established provenanceindicators (McLennan et al., 1990, 1993; McLen-nan, 2001). However, some mobility of REE mayoccur during weathering and diagenesis (Milo-dowski & Zalasiewicz, 1991; Zhao et al., 1992;Bock et al., 1994; McDaniel et al., 1994; Utzmannet al., 2002).

The REE abundances and patterns of all threeformations are characterized by an enrichment ofLREE, pronounced negative Eu anomalies inalmost all samples and relatively flat HREEpatterns. Most of the samples show patterns

similar to the post-Archaean average Australianshale (PAAS) composite representing the post-Archean upper continental crust (Fig. 7A and B;Taylor & McLennan, 1985).

The quartz-rich rocks of the Tolar ChicoFormation have patterns with similar shapes toPAAS but lower abundances due to a dilutioneffect by quartz (e.g. Cullers, 1995; Bock et al.,2000). This effect is borne out in Fig. 7A, whichdemonstrates that the Tolar Chico very fine-grained wackes have essentially the same REEpatterns as PAAS, whereas the abundances in the

quartz arenites are significantly lower. The aver-age LaN/YbN ratios (where the subscript N refersto chondrite-normalized abundances) of the sam-ples decrease from 956 in the Tolar ChicoFormation (SD 115) to 46 in the Diablo For-mation (SD 203). The relatively high LaN/YbNratio in the Tolar Chico Formation appears to becaused by a slight depletion of the HREEs. Thisindicates that the formation of these maturedeposits has not been accompanied by an enrich-ment of heavy minerals, especially zircon, which

would have led to an increase in HREE (McLen-nan et al., 1993). This is supported by the heavymineral analyses, which show nearly similarabundances of zircon in all three formations(Table 2), whereas concentrations of Zr and Hfare close to the average upper continental crustalvalue (Table 3). Eu/Eu* values lie between 069

and 097 (mean 082; SD 008), which reflectthe modest negative Eu anomaly.

The REE patterns of the Tolillar and DiabloFormations show only small differences in theconcentrations between fine- and coarse-grainedrocks (Table 3; Fig. 7A). The samples of theTolillar Formation show slight differences in theLREE and HREE concentrations. Two sampleshave slightly positive Eu anomalies with high Eu/Eu* values (112 and 117) and reflect a higherplagioclase concentration according to petro-graphic results (samples A296 and X27 in

Table 1). Eu/Eu* values show a wide rangebetween 044 and 117 (mean 067; SD 028).The LaN/YbN ratios are in general lower (mean 686; SD 448), with the exception of one sam-ple and reflect average UCC composition (505after McLennan, 2001).

Most samples of the Diablo Formation areslightly depleted in LREE and weakly enriched inHREE. The volcanic ash sample B44 of the DiabloFormation has a lower average abundance of LREE(Fig. 7A), which may have been caused by the lossof LREE during weathering of volcanic glass (e.g.Wood et al., 1976; Taylor & McLennan, 1985;

Utzmann et al., 2002). One sample from the DiabloFormation has a slight positive Eu anomaly(Fig. 7A; Table 3, sample B90). The Eu/Eu* valuesof the Diablo Formation are generally higher(mean 079, SD 014) than in the TolillarFormation. The LaN/YbN ratios decrease relativeto the Tolillar Formation with all samples onlyslightly below average UCC (mean 409,SD 203; Table 3).

Implications of the trace and rare earthelement ratios

Comparison of the studied units with the averagecomposition of the upper continental crust(McLennan, 2001) shows that the quartz arenitesof the Tolar Chico Formation have lower values inrelation to UCC, with the exception of Cr, Ba, Hfand Zr (Table 3). Cr appears in three samples inhigher concentrations but, as shown above,Cr-bearing heavy minerals were not found in theheavy mineral spectra. In addition, the Cr/Thratios are too low to point to an input from maficsources (Table 3). The low trace element concen-




13/26

trations may result from the high quartz content ofthe samples (e.g. McLennan et al., 1993; Cullers,1995; Tables 1 and 3). Feldspar is rare in thisformation as shown above (Table 1; Fig. 4A and B).This is consistent with low REE, Nb, Ta, Sc and Thcontents typical of derivation from mostly grani-toid sources (Cullers, 1995). The fine-grained

wackes of the same formation have higher concen-trations of nearly all trace elements (Table 3), andof the REEs in particular (Fig. 7A). The fine-grained samples from the Tolar Chico Formationare considered to reflect the composition of thesource, a signal that is diluted in thequartz arenitesby the high quartz content. Nearly all relevant

Fig. 7. (A) REE patterns of all samples of the three Lower Ordovician formations of the southern Puna. Stippledpatterns represent the fine-grained samples in each formation. (B) REE element patterns of Ordovician volcanic andsedimentary rocks; pattern of PAAS, quartzites of the Cambrian Meson Group and siliciclastic deposits of thePuncoviscana Formation (Figs 1A and 3) for comparison; data from Bahlburg (1998) and Bock et al. (2000).Chondritic normalization of the sedimentary rocks and PAAS according to Taylor & McLennan (1985). The insetshows the REE spectra of crustally derived magmatic rocks of the Sierra Famatina (according to Pankhurst et al.,1998, fig. 2).




14/26

trace element ratios (Sm/Nd, LaN/YbN, La/Sc, Zr/Sc, Th/Sc and La/Th) show evolved values inrelation to average continental crust (Table 3).

The volcanogenic Tolillar and Diablo Forma-tions most prominently display their overallupper continental crust composition in the traceelement concentrations with slight enrichments

in Yb and Y. Variations in both formations arepronounced (Table 3) and were most probablycaused by the mixing of different sources. Asshown in the Petrography section, the amount ofvolcanogenic material increased during theArenig and from the Tolillar to the DiabloFormation. When the data are compared withaverage upper crustal values (not shown here), amarked negative Nb-Ta anomaly becomes evidentin almost all samples but is most prominent in theDiablo Formation. Such a negative anomaly indi-cates the influence of a volcanic arc in the source

area (Hofmann, 1988, 1997).Despite the intraformational variations in thetrace element ratios, there is a clear trend from theTolillar to the Diablo Formation from a recycledsedimentary composition to one dominated by acontinental arc source (Figs 4 and 8). This shift isobservable using La-Th-Zr-Sc relations (seebelow) and Eu/Eu*. Most of the rocks in theTolillar Formation show Eu/Eu* values between044 and 063, and three samples have highervalues. With very few exceptions, the Th/Scratios are uniformly 1 (mean 151, SD 054).According to McLennan et al. (1993), these data

are in accordance with a dominant recycledsedimentary source (Eu/Eu* 060070; Th/Sc

1) and a probably young undifferentiated arcinfluence (Eu/Eu* 100, Th/Sc < 10; McLennanet al., 1993). The Diablo Formation has slightlyhigher Eu/Eu* values (range 063107; mean 079; SD 014), but markedly lower Th/Sc ratios(mean 085; SD 032; Table 3). The REEpatterns and the trace element ratios can be best

explained by derivation from an evolved vol-canic arc of largely UCC composition. This arcinfluence is most dominant in the Diablo Forma-tion, which is also consistent with the petro-graphic data.

Trace element ratios such as La/Th, La/Sc,Zr/Sc and Th/Sc have been used successfully todiscriminate tectonic settings (Bhatia & Crook,1986). However, such an approach has to be usedwith caution because it has been shown thatspecific tectonic settings do not necessarilyproduce sedimentary rocks with unique geo-

chemical signatures (McLennan et al., 1990;Bahlburg, 1998). La-Sc-Th and Th-Sc-Zr/10 ratios(Fig. 8) of the Tolar Chico Formation are typicalof rifted margins. Those of the Tolillar Formationscatter over the active continental margin field,whereas the Diablo Formation shows a continen-tal island arc character.

Isotope geochemistry

Input of detrital material into a basin from differ-ent terranes with variable crustal compositionsand histories can potentially be distinguished by

analysing the Nd isotope systems of sedimentaryrocks (e.g. McCulloch & Wasserburg, 1978; DePa-olo et al., 1991; McLennan et al., 1993). The TolarChico Formation shows the most negativeeNd(t sed) between )11 and )88 (mean )974;Table 4). In the Tolillar Formation, at theTremadocArenig boundary, the mean eNd(t sed)rises to )51. In the Diablo Formation, in turn, themean eNd(t sed) value is only )195 (Table 4).

Figure 9 plots Sm/Nd (the fractional deviationof the sample 147Sm/144Nd from a chondriticreference) of the southern Puna samples vs.

eNd(t 470), where eNd(t 470) is eNd at 470 Ma, thesedimentation age. This allows a comparison ofSm/Nd and Nd isotope systematics at the time ofsedimentation. The eNd values (Table 4) of theTolar Chico, Diablo Formation and Falda Cienaga(Middle Ordovician rocks) are associated withSm/Nd values between )043 and )031. However,two samples from the Tolillar Formation (A208and A211) show clearly less negative Sm/Ndvalues ()021 and )0164 respectively) and maybe the result of a resetting of the Nd isotopic

Fig. 8. (A and B) Tectonic discrimination diagrams forsandstones after Bhatia & Crook (1986). Note the loss ofLREE, here La, in sample B44 in (A), where the samesample plots in the Th-Sc-Zr/10 triangle in the con-tinental island arc field similar to nearly all samplesfrom the Diablo Fm. (B) A, oceanic island arc; B, con-tinental island arc; C, active continental margin,including in (A) passive or rifted margin setting;D, passive or rifted margin.




15/26

Table

4.

IsotopegeochemistryoftheOrdoviciansedimentaryroc

ksandassociatedmagmaticrocks.

Samples

Rocktype

Age(Ma

)Sm

(p.p.m.)Nd(p.p.m.)

147Sm/144Nd

143Nd/144Nd

143Nd/144Ndi

eNd(t

sed)*

eNd(t

470Ma)

Sm/Nd

TCHUR

TDM

TDM

TolarChicoFm.

A6

qa

489

263

1408

01129

0511890

0511528

)94

)96

)0429

1360

1906

1839

A18

qa

489

235

1234

01151

0511883

0511514

)96

)98

)0418

1411

1925

1890

A76

qa

492

247

1297

01151

0511869

0511498

)99

)101

)0418

1436

1943

1910

A82

qa

488

298

1554

01159

0511817

0511447

)110

)112

)0413

1547

2013

2003

A88

qa

488

261

1422

01109

0511836

0511481

)103

)105

)0439

1425

1968

1883

A90

qa

488

233

1272

01105

0511890

0511537

)92

)94

)0441

1323

1896

1799

A92

g

492

757

4023

01138

0511920

0511553

)88

)90

)0424

1320

1870

1812

Mean

328

1744

01135

0511872

0511508

)974

)994

)0426

1403

1931

1877

TolillarFm.

A194

g

485

652

2887

01364

0512198

0511765

)49

)5

)0310

1114

1586

1796

A208

a

485

393

1519

01562

0512283

0511787

)44

)45

)0210

1338

1554

2165

A211

a

484

478

1750

01652

0512215

0511691

)63

)64

)0164

2044

1691

2733

A299

g

482

689

4155

01002

0512088

0511772

)48

)49

)0493

870

1579

1384

A303

g

480

694

3350

01252

0512176

0511782

)46

)47

)0366

986

1566

1614

M5

g

480

629

3845

00989

0512082

0511771

)49

)50

)0499

867

1583

1376

M7

g

480

709

4282

01001

0512075

051176

)51

)52

)0493

890

1598

1400

CU4

p

485

481

2125

01369

0512161

0511726

)56

)57

)0307

1218

1641

1877

CU6

a

485

432

1993

01310

0512157

0511741

)53

)54

)0337

1117

1620

1755

Mean

573

2879

01278

0512159

0511755

)510

)520

)0353

1160

1602

1789

DiabloFm.

B19

p

477

246

1299

01143

0512270

0511913

)22

)22

)0422

683

1376

1305

B35

la

478

395

2013

01186

0512268

0511897

)25

)25

)0400

724

1400

1365

B44

a

478

093

473

01193

0512159

0511785

)46

)47

)0396

945

1564

1544

B86

g

476

549

2561

01295

0512386

0511982

)08

)09

)0345

573

1269

1329

B90

la

476

500

2325

01299

0512348

0511943

)16

)17

)0343

664

1331

1402

B96

g

476

739

3265

01368

0512374

0511948

)15

)16

)0308

673

1324

1472

B119

c

475

344

1619

01286

0512385

0511985

)08

)09

)0349

568

1267

1318

B130

la

475

736

3276

01359

0512369

0511946

)16

)17

)0312

676

1327

1465

Mean

450

2104

01266

0512320

0511925

)195

)203

)0359

688

1357

1400

CoquenaandFaldaCienagaF

m.

QH45

g

473

423

2023

01262

0512100

0511709

)63

)63

)0361

1165

1679

1759

VQ22

are

468

435

2238

01174

0512093

0511733

)59

)59

)0406

1049

1650

1615

171

are

468

666

3290

01224

0512100

0511725

)61

)61

)0381

1104

1662

1687

191

g

473

530

2640

01213

0512158

0511782

)48

)48

)0386

972

1575

1577

LN2

are

468

367

1838

01206

0512143

0511773

)51

)51

)0390

993

1593

1589

Mean

484

2406

01216

0512119

0511744

)564

)564

)0385

1057

1632

1645




16/26

Table4.

(Continued).

Samples

Rocktype

Age(Ma)Sm

(p.p.m.)Nd(p.p.m.)

147Sm/144Nd

143Nd/144Nd

143Nd/144Ndi

eNd(t

sed)*

eNd(t

470Ma)

Sm/Nd

TCHUR

TDM

TDM

Volcanicrocks(DiabloFm.)

B13

rhyolite

476

553

3901

00857

0512284

0512017

)02

)04

)0566

488

1215

928

B63

andesite

476

767

2830

01638

0512390

0511879

)28

)30

)0171

1153

1429

1893

B66

andesite

476

373

3042

00741

0512412

0512181

30

28

)0625

282

937

1163

Plutonicrocksfrom

SalardePocitos(CBOdC)

Poc5-5

gabbro

500

661

2851

01402

0512658

0512199

40

)0290

)54

867

A272

gabbro

500

295

1136

01570

0512831

0512317

63

)0205

)747

642

811

A220715

gabbro

500

270

1049

01556

0512789

0512279

56

)0213

)564

6

896

A4-6

gabbro

500

448

1382

01960

0512913

0512271

54

)0008

ND

731

1715

A325

cumulate

500

529

398

08036

0512703

0510071

)376

3067

16

3477

)117

A44

gabbro

500

164

564

01758

0512828

0512252

50

)0110

)1403

767

1190

Plutonicrocksfrom

thesouth

ernSierradeCalalaste(SC)

C15

gabbro

468

742

3162

01419

0512824

0512389

69

)0282

)521

554

662

C62

gabbro

468

780

2751

01714

0512866

0512341

60

)0133

)1391

652

950

C137

cumulate

468

067

160

02532

0513171

0512395

70

0281

1433

542

89

P450/44

829

6016

00833

0512507

0512507

)26

177

1077

733

*eNd(t

sed)referstothepalaeontologicalcontrolledsedimentationage.

Referstoasedimentationage

of470Ma,theageoftheyoungestsedimentationeventdiscussedinthestudy,forcomparisonswithotherformationsinFigs9,

10and11.

CalculatedaccordingDePaol

oetal.(1991).

CalculatedaccordingDePaolo(1981).

SC,southernSierraCalalastem

aficrocks;CBOdC,ComplejoBasicoOjodeColorado;qa,quartzarenite;

g,greywacke;a,ash;are,arenite;la,

litharenite;p,pelite;

c,conglomerate;ND,notdefined.

Dataforthemagmaticrocksfrom

Bocket

al.(2000).




17/26

system. Also, the Sm/Nd ratios of these twosamples are much higher (023) than the mean ofthe Tolillar Formation (018; SD 003). The Sm/Nd ratios of the remaining samples correspond tothe mean of the formation and show PAAS-likeREE patterns (Fig. 7A). Samples A208 and A211were taken from feldspathic turbidites (A294, M5,M7) intercalated with quartz-rich siltstones (CU6,CU4) and pelites (A208 and A211). A large spreadin Sm/Nd and a uniform isotopic composition canbe noted at the time of sedimentation (470 Ma).

Only the loss of a phase crystallized at the time ofdeposition will create a vertical array on a Sm/Ndvs. eNd(t) plot, whereas loss of older LREE-enriched minerals will produce diagonal arrays(Bock et al., 1994). Fractionation of LREE duringdiagenesis can occur especially on the boundaryof clay minerals during early diagenesis of unsta-ble FeMn oxyhydroxides and volcanic phases(Milodowski & Zalasiewicz, 1991; Utzmann et al.,2002). Ce/Ce* anomalies are an indicator of anoxidizing environment, which could cause LREEmobility (McDaniel et al., 1994). Such anomalies

were not observed, although A211 shows Ce/Ce*values of about 109 (Table 3). An early diageneticrather than a weathering environment is favouredas the cause for the REE redistribution.

The Diablo Formation is characterized by anaverage eNd(t sed) value of only )195 (Table 4),resembling isotopic data from the synsedimentarylava flows of the Diablo Formation, and consistentwith a significant input of less evolved material.Sample B44, interpreted as a volcanic ash, prob-ably suffered the loss of LREE by weathering of

high glass content (e.g. Wood et al., 1976; Utz-mann et al., 2002). In the Mid-Ordovician depos-its, the influence of younger material disappeared(Zimmermann et al., 2002; Falda Cienaga Forma-tion, mean )564), and the formations returnedto eNd(t sed) values typical of the Tolillar Forma-tion and other Lower Palaeozoic sedimentary and

magmatic rocks of this region (Figs 10 and 11;Tables 4 and 5; Bock et al., 2000).

Implications of the isotope dataThe Tolar Chico Formation has the strongestnegative eNd values at the time of deposition(Table 4; Figs 911). Calculation of model ages(TDM) from the Nd isotope data reveals that theTolar Chico Formation has the oldest model ages,which cluster around values of 19 Ga. This indi-cates that at least one of the sources of this quartzarenite formation is considerably older than a large

amount of the detrital material of the analysedexposed basement rocks of the Sierras Pampeanasand the Puncoviscana Formation in the neighbour-ing regions (Rapela et al., 1998; Bock et al., 2000;Figs 1A, 2, 3 and 10). As sedimentary rocks usuallyrepresent mixtures of several sources, a significantpart of the detritus must still be older than 19 Ga,i.e. early Palaeoproterozoic or even older.

Potential source rocks or regions registeringdated Palaeoproterozoic or even Archaean mag-matic or metamorphic events do not occur innorth-western Argentina but are present in theArequipa Massif in southern Peru and northern-

most Chile (compilation in Bahlburg & Herve,1997; Bock et al., 2000). They are also abundantlycommonly recorded on the Brazilian shield (Sato,1999) and occur in basement rocks of the BuenosAires region of the Rio de la Plata Craton (seeFigs 2 and 12B). Regions to the north of theOrdovician basin can be excluded as a source forthe Ordovician rocks because of the sedimentarytransport directions indicating a northward trans-port in the basin (see above; Bahlburg, 1990;Zimmermann, 1999; Zimmermann et al., 2002).The Tolillar Formation had negative eNd values at

deposition ranging from)

44 to)

63. Calculatedmodel ages (TDM) cluster around 16 Ga. Using thetwo-stage model of DePaolo (1981), the twodisturbed samples (A211 and A208) show older,unrealistic values. With the exception of theDiablo Formation, the Arenig and Middle Ordo-vician formations of the southern and northernPuna have similar model ages to the basementrocks of the Pampeanas Terrane and the pre-Ordovician sedimentary rocks (Meson Group andPuncoviscana Formation), which have TDM ages

Fig. 9. Plot of Sm/Nd vs. eNdt for samples from theOrdovician of the southern Puna, where eNdt is theisotopic composition at the approximate ages ofdeposition for each formation (see Fig. 3; Table 4). Notethe differences between the arrays in the Tolillar For-mation and the three other successions. This points to aredistribution of Nd isotopes at approximately the time

of deposition (470 Ma).




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that range from 14 to 18 Ga. More than 70% ofthese values fall in the range 14 t o 16 Ga

(Bahlburg, 1998; Pankhurst et al., 1998; Rapelaet al., 1998; Bock et al., 2000; Lucassen et al.,2000; Zimmermann et al., 2002). This is consis-tent with the petrographic record of these forma-tions, which shows a mixing of a minor volcaniccomponent and a major amount of (recycled)detritus of cratonic provenance (Bahlburg, 1990,1998; Zimmermann et al., 2002; this study).

The model ages of the Diablo Formation rangebetween 12 and 14 Ga (eNd(t sed) mean )195).They are significantly younger than those of the

underlying formations and those of the entireOrdovician stratigraphy of the Puna. The input ofless evolved material may have been derived fromcontemporary intermediate to felsic volcanicrocks, which have similar model ages (Table 4;Bock et al., 2000; Zimmermann, 2000). Aco-variation of eNd data with Th/Sc ratios isconsistent with dilution from young arc material

(Fig. 11). Other potential source rocks with sim-ilar mixed Nd isotopic characteristics do notoccur in the region.

Mixing scenarios for the Tolillarand Diablo FormationsMixing models after DePaolo et al. (1991) havebeen used to model the probable sources for theTolillar and Diablo Formations (Table 5). Thedebris constituting the Tolillar and DiabloFormations represents a mixture of an oldermetasedimentary source and a younger, mag-

matic one. Geochemical data for probable meta-sedimentary sources such as the PuncoviscanaFormation (Late Proterozoic to Early Cambrian;Acenolaza et al., 1988) show comparable REEpatterns to the Tolillar Formation (Bock et al.,2000; Zimmermann & van Staden, 2002). There-fore, the older source could in fact be repre-sented by the metasedimentary rocks of thePuncoviscana Formation and mature rocks simi-lar to sample A92 from the Tolar Chico Forma-tion. The less evolved source was predominantly

Fig. 10. Histogram of the Nd model ages (TDM) calculated according to the three-stage model of DePaolo et al. (1991).Included for comparison are data from Late Proterozoic to Cambrian sedimentary and magmatic rocks of SierrasPampeanas and Sierra Famatina (Figs 1A, 2 and 3). The samples from the Diablo Formation are characterized by aninput of slightly less fractionated, i.e. younger, material. Typical model ages of about 1 518 Ga were found in all

formations except the Tolar Chico and Diablo Formations and may indicate a western Gondwana crustal formationevent presented by Sato (1999). Data for the Sierras Pampeanas from Rapela et al. (1998); for the Sierra Famatina fromPankhurst et al. (1998); for the Falda Cienaga Formation from Zimmermann et al. (2002); for the volcanic rocks of theDiablo Formation from Bock et al. (2000).

Fig. 11. Th/Sc vs. eNd(t 470) plot. The results areconsistent with those evaluated by petrological as wellas geochemical methods (Figs 4 and 8). Diagram afterMcLennan et al. (1993); data for the Falda CienagaFormation from Zimmermann et al. (2002).




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volcanic, as shown by the petrographic andgeochemical data.

With respect to the Tolillar Formation, theOrdovician volcanic rocks of the Puna are not

possible source rocks because they are younger.Late Cambrian to very Early Ordovician volcanicand, to a lesser degree, plutonic rocks exposed inthe southern Puna and to the south of the Puna inthe Sierras Pampeanas and Sierra Famatina (Rape-la et al., 1998; Table 5A) are more likely. Thisvolcanic source, which was originally connectedto the plutonic rocks, has been eroded away andmay be preserved only as detrital grains in theTolillar, and perhaps the Diablo, Formations.Deposits of the Tolillar Formation are therefore

modelled using the older low-grade to high-grademetamorphic source and a younger source thatincludes Late Cambrian to very Early Ordovicianplutonic and volcanic rocks of the Sierras Pam-

peanas and Sierra Famatina (Rapela et al., 1998;Table 5A and B). The data are best modelled by amix of Sierras Pampeanas-type metasedimentarybasement material as well as Late Cambrian to veryEarly Ordovician felsic to intermediate magmaticdebris (9095%) with 510% quartz-rich sedimen-tary material similar to the Tolillar Formation(Table 5B). This is consistent with the petrograpicas well as the geochemical data.

A mixing model for the Diablo Forma-tion (Table 5C) requires as the older source

Table 5. Mixing models after DePaolo et al. (1991) based on eNd (t sed) values and Nd (p.p.m.) concentrations.

ASource Symbol Age (approx.) Ma eNd(t sed) Nd (p.p.m.)

Tolar Chico Fm. (A92) TC 490 )88 4023Tolillar Fm. TOL 485 )51 2881Puncoviscana Fm. PV 560 )70 3803

Sierras Pampeanas SP 520)

49 28

28Sierra Famatina SF 500 )50 3263

Diablo Fm. DB 476 )20 2104Lava of Diablo Fm. DBLAVA 476 )13 3255CBOdC CBOdC 472 58 1396

B

Source 1 (s1)high fractionated

Source 2 (s2)low fractionated

Tolillar Fm.mix

Beta Alpha % of source 2

TC (A92) SP Average 143 093 93PV SP Average 131 090 90

C

Source 1 (s1)high fractionated

Source 2 (s2)low fractionated

Diablo Fm.mix

Beta Alpha % of source 2

TC DBLAVA Average 124 093 93SF DBLAVA Average 100 082 82PV DBLAVA Average 117 090 90SP DBLAVA Average 087 080 80TC CBOdc Average 288 072 72SF CBOdc Average 234 048 48SP CBOdc Average 203 044 44PV CBOdc Average 272 064 64

A. The data represent the basis of the mixing calculations in B and C. The shale sample A92 was taken as repre-sentative for the Tolar Chico Formation because this fine-grained sample is considered to reflect the source com-position better than the quartz arenites of this formation (as discussed in the text). CBOdC, Complejo Basico Ojo de

Colorado; unknown formation age. 472 Ma is the minimum intrusion age.B. Mixing models for the Tolillar Formation using as end-members the Tolar Chico Formation (sample A92) and thePuncoviscana Formation as old end-members. The younger end-member is represented by the magmatic rocks of theSierras Pampeanas (Rapela et al., 1998). Beta(b) Nd(p.p.m)s1/Nd(p.p.m)s2; Alpha(a) b/(b + [(eNd(s2) ) eNd(mix))/(eNd(mix) ) eNd(s1))].C. Mixing models for the Diablo Formation. The Tolar Chico Formation, the Puncoviscana Formation and themagmatic rocks of the Sierras Pampeanas were selected as representative of the highly fractionated sources. Thesesources are mixed with the synsedimentary lava flows of the Diablo Formation and with mafic and ultramafic rocks(data in Table 4) exposed in the southern Puna (Zimmermann, 1999).




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end-member rocks such as those of (1) the TolarChico Formation; (2) the metamorphic basementand plutonites of the Sierras Pampeanas; (3) thefelsic to intermediate magmatic rocks of the SierraFamatina to the south; or (4) the PuncoviscanaFormation (Figs 1 and 2). For the other, lessevolved and probably younger source, the Late

Cambrian to very Early Ordovician eruptive rocksof the region could be considered and/or themafic to ultramafic rocks of pre-Arenig age,including the Complejo Basico Ojo de Colorados(CBOdC) exposed in the southern Puna (Table 5Aand C). However, an input of between 44% and72% of this mafic material is necessary to obtainthe required isotopic values when combined witheither of the older end-members (Table 5A andC). As shown above, neither the framework andheavy mineral petrography nor the trace elementdata including REE and Eu/Eu* values (Table 3;

Figs 7, 8 and 11) indicate a significant input ofmafic material. In view of this and the isotopicevidence, the absence of a significant mafic inputfor the mixing models is inferred. The best resultsare obtained when a basement source representedby the Sierras Pampeanas and/or the Puncovis-cana Formation is mixed with the synsediment-ary lava flows of the Diablo Formation. Thismixture coincides very well with our petro-graphic data indicating a < 20% influence ofmetasedimentary and magmatic basement materi-al and 80% of felsic to intermediate volcanicdetritus.

INTERPRETATION

The Tolar Chico Formation is quartz rich andcomposed of mature recycled and well-roundedgrains, a small fraction of angular quartz grainsand rare alkali feldspar. Zircon abundances arelower than expected in quartz sandstones. Severalfeatures point to derivation of the quartz arenitesfrom felsic metamorphic sources, including meta-sedimentary rocks and paragneisses, namely (1)

the abundance of metamorphic quartz; (2) thepresence of microcline and metamorphic litho-clasts; (3) the occurrence of metamorphic heavyminerals such as rutile, which is a widespreadaccessory mineral in metamorphic rocks, andtourmaline, which is abundant in granites andregion and contact metamorphic rocks (Mange &Maurer, 1992); (4) the conspicuously low concen-tration of amphiboles; and (5) the fractionatedgeochemical characteristics comparable withaverage continental crustal compositions. CL

analyses of individual quartz grains suggest aminor volcanic input of the order of 5%. Felsicsynsedimentary volcanism is recorded in lowerTremadocian sedimentary rocks north of thestudy area (Las Vicunas Formation; Fig. 3).

The constant element ratios in the quartzarenites reflect either a homogeneous mixing or

a strong influence of physical sorting and/orweathering. The fine-grained rocks, in turn, pre-serve the geochemical and isotope geochemicalcharacteristics of the source regions (Cullers,1995). The chemical and isotopic geochemicalsignatures of the siltstones are consistent with aderivation from mostly (old) upper crustalsources (Figs 9 and 11).

The most likely source region is represented bythe Puncoviscana orogenic belt of Late Neopro-terozoic to Early Cambrian age to the east of thePuna basin. This fold belt is composed of felsic

plutonic, metasedimentary and subordinate vol-canic rocks of upper crustal composition. Palaeo-currents are also consistent with this fold belt as asource region (Fig. 1A; Bahlburg, 1991; Zimmer-mann, 2000). Low-grade metaturbidites of thePuncoviscana Formation contain a significantamount of metamorphic material and have gen-erally younger Nd model ages and less negativeeNd values than the Tolar Chico Formation (Bocket al., 2000; Lucassen et al., 2000). Preliminarygeochemical and Nd isotope data on medium- tohigh-grade basement rocks show similar toslightly older model ages (Lucassen et al., 2000)

than the low-grade metasedimentary rocks of thePuncoviscana Formation and the Lower Palaeo-zoic granitoids of the Sierras Pampeanas, whichwere used in the calculation of the mixingmodels.

The Tolillar Formation received a larger inputof detrital volcanic material of felsic and inter-mediate composition than the underlying TolarChico Formation and does not show significantinput from a metamorphic source. This isexpressed in (1) higher abundances of volcanicquartz and lithoclasts and amphiboles; (2) a

decrease in heavy minerals, metamorphic quartzand lithoclasts; (3) feldspar-rich arenites; and(4) geochemical characteristics pointing to anactive continental margin (Fig. 8, McLennanet al., 1990, 1993). The geochemical and isotopiccharacteristics of the Tolillar Formation are sim-ilar to the Neoproterozoic Puncoviscana Forma-tion (Bock et al., 2000; Zimmermann & vanStaden, 2002), the Mid-Ordovician Falda CienagaFormation (Zimmermann et al., 2002), the base-ment of the Pampeanas Terrane (Rapela et al.,




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1998) and crustally derived plutonic rocks of theFamatinian volcanic arc (Pankhurst et al., 1998).This arc evolved on the basement of the Pampea-nas Terrane (Pankhurst et al., 1998), today loca-ted south of the Puna (Fig. 1A, south of 27S;Fig. 12B). Also, the Tolillar Formation receivedmost of its relatively immature detritus from

nearby source regions by northward-directedaxial palaeocurrents (present co-ordinates;Fig. 12B). A similar dispersal has been identifiedin the northern Puna Basin by Bahlburg (1990).

The Diablo Formation contains the highestamount of detrital volcanic material (Fig. 4;Table 1), as well as, on average, the youngestdetrital material (Figs 911; Table 4). The geo-chemical composition of the sedimentary rocks is

similar to the synsedimentary volcanic rockswithin the formation (Table 3). Nd isotope mixingmodels suggest that the latter may account for 8085% of volcanic input in the Diablo Formation,assuming that the other mixing end-member isrepresented by the basement of the PampeanasTerrane and/or the Puncoviscana Formation. The

geochemical signature of the sedimentary rocks(Figs 8 and 11), including a negative Nb-Taanomaly, points to a significant influence ofmaterial originating in a volcanic arc setting(e.g. Hofmann, 1988). The sedimentological fea-tures, the petrological, geochemical and isotopegeochemical similarities of coeval sedimentaryand volcanic rocks in this formation indicate thatthe Diablo Formation formed as a marine volcanic

A

B

Fig. 12. (A) Palaeotectonic reconstruction of the Ordovician active continental margin in the southern centralAndes. No oceanic crust is formed in the Gondwana related retroarc basin. (B) Active margin setting during LateArenig time (Diablo Formation). In the southern Puna, a retroarc basin evolved to the E of the NW- to SE-orientatedPunaFamatinian volcanic arc (active since the lower Tremadoc) on continental crust. There is no geologicalevidence for allochthonous or even exotic crustal blocks (sketch modified after Rapela et al., 1998). AAT, ArequipaAntofalla Terrane; RPC, Rio de la Plata Craton; the white arrow shows the dominant sedimentation direction, and theshadowed oval indicates the intensity of deformation from high (darker part) to very low (white part).




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apron close to an eruption centre. Minor amountsof older crustal material were also transportedinto the depositional area.

The deposits of the Tolillar and Diablo Forma-tions are compositionally immature. The preser-vation of detrital material of plutonic, volcanic,sedimentary and, to a lesser extent, metamorphic

origin in the same strata is indicative of only aminor weathering and sorting influence acting onthe detritus on its way from source to sink. Inturn, this implies (1) relatively short transportpaths and thus potentially marked relief; and (2)only minor intermediate storage of sediment.These features are typical of tectonically activebasins at active continental margins or of strike-slip basins (e.g. Nilsen & Sylvester, 1995).

PALAEOTECTONIC EVOLUTION

The collision of the Pampeanas Terrane withGondwana between 535 and 515 Ma in the Lowerto Middle Cambrian (Rapela et al., 1998; Keppie& Bahlburg, 1999) led to the deformation andmetamorphism of the Puncoviscana Formationand medium- to high-grade metamorphic base-ment rocks. Subsequently, the siliciclastic andshallow-marine Meson Group was depositedunconformably above the metaturbidites of thePuncoviscana Formation in a small intracratonicbasin during the Late Cambrian (Sanchez &Salfity, 1990). During the Tremadoc, a subduction

zone was initiated on the western margin of thePampeanas Terrane, representing the westernborder of Gondwana (Fig. 12; Pankhurst et al.,1998). In the Puna Basin, the first felsic volcanicrocks formed during the early Tremadoc in theLas Vicunas Formation (Salar de Rincon area;Fig. 1A, no. 5; Fig. 3; Moya et al., 1993). At theTremadocArenig boundary, volcanic input in-creased markedly in the Tolillar Formation. Theoverlying Diablo Formation, like the volcanosed-imentary successions in the northern Puna (Huai-tiquina and Aguada de la Perdz; Fig. 1A, no. 4) is

dominated by volcaniclastic input and the depos-ition of primary volcanic material with a volcanicarc signature (this paper, e.g. Breitkreuz et al.,1989; Bahlburg, 1990).

In the early Ordovician, the PunaFamatinianvolcanic arc was built on the western border ofthe Pampeanas Terrane as represented by theSierras Pampeanas (Fig. 12A and B), and a retro-arc basin evolved to the east and north-east of thisarc (Bahlburg, 1991; Zimmermann, 2000). Thedeformation of the basin fill increases from south-

west to north-east (Mon & Hongn, 1991; Zimmer-mann, 2000; Fig. 12B). At the ArenigLowerLlanvirn transition, the volcanic activity endedin the northern and southern Puna (Bahlburg,1990; Bahlburg & Furlong, 1996; Moya, 1997;Zimmermann et al., 2002). Subsequent magmaticactivity occurred in a large belt along the eastern

border of the Puna in the Faja Eruptiva de laPuna Occidental (Fig. 1A, Early Palaeozoic plu-tonics; Mendez et al., 1973). This activity wasmost probably related to Late Ordovician strike-slip tectonics connected to the Ocloyic Orogeny(Bahlburg, 1990; Coira et al., 1999). During theOcloyic Orogeny, the successions in the Punawere strongly deformed and subjected to greatstrain (Hongn & Mon, 1999).

The interpretation of the plate tectonic evolu-tion of the Puna basin is controversial and partlyrests on the significance of mafic and ultramafic

rocks of assumed Ordovician age in the southernPuna (e.g. Ramos et al., 1986). These have beeninterpreted as oceanic crust flooring a basin,which became progressively oceanic southward.Accordingly, the end-Ordovician closure of thebasin during the Ocloyic Orogeny should haveresulted in the accretion of the western area of thePuna as a separate, potentially exotic terrane andin the preservation of the intervening ocean flooras an ophiolite in the southern Puna (Forsytheet al., 1993; Conti et al., 1996; Rapalini et al.,1999).

However, the alleged ophiolites exposed in the

southern Puna do not show the geochemical andpetrological characteristics typical of oceaniccrust (Nicolas, 1989; Zimmermann et al., 1999).The assumption of a southward-deepening and -opening basin favoured by the interpretation ofpalaeomagnetic data is unrealistic as 90% of theflute mark palaeocurrent measurements (n 227)distributed over the entire basin and its stratigra-phy indicate uniform axial transport towards thenorth (Bahlburg, 1990; Zimmermann, 2000;Zimmermann et al., 2002).

Finally, the present study demonstrates that the

detritus constituting the Ordovician sedimentaryrocks of the Puna were probably supplied fromneighbouring sources to the south and east of thebasin. There are so far no indications of the inputof exotic detrital material.

CONCLUSIONS

Petrographic, geochemical and isotopic data fromthe Tolar Chico through to the Diablo Formations




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define a temporal compositional trend from abasement source to an arc volcanic source (Figs 8and 11) in the southern Puna. This reflects achange in provenance from an upper crustalsource in the Tolar Chico Formation (Tremadoc),which was deposited at a rifted margin, to anoverall andesitic arc composition in the Diablo

Formation (Arenig). The amount of volcanigenicmaterial delivered to the basin increased close tothe TremadocArenig boundary. In the TolillarFormation, the input of old basement materialdecreases correspondingly, but it preserves min-eralogical, geochemical and isotopic evidence fora provenance in Precambrian to Cambrian (meta-)sedimentary successions (Meson Group andPuncoviscana Formation) as well as of the base-ment of the Pampeanas Terrane. The peak ofactive margin volcanism was reached during theArenig in the Diablo Formation. It shows an input

of slightly less evolved material mixed withdetritus typical of the Tolillar and PuncoviscanaFormations, the basement of the PampeanasTerrane as well as the Famatinian magmatic arc.The immature volcanic debris with less negativeeNd values was probably eroded from synsedi-mentary lava flows intercalated in the DiabloFormation, as it shows similar isotopic and traceelement characteristics.

The input of volcaniclastic material with itsspecific geochemical and isotopic compositionwas short lived and disappeared at the beginningof the Llanvirn when the volcanic sources of the

Diablo Formation were recycled into the Mid-Ordovician Coquena and Falda Cienaga Forma-tions in the southern Puna and into the PunaTurbidite Complex of the northern Puna. Thisstudy shows that erosional debris with modelages over 20 Ga contributed significantly to thedetritus deposited in the Ordovician basin ofnorth-western Argentina particularly as the TolarChico Formation of Tremadoc age.

Input of mafic material, implied by differentpalaeotectonic models, is neither indicated by theframework and heavy mineral compositions nor by

geochemical and Nd isotope data. Average contin-ental crust composition represents the provenanceof most of the sedimentary rocks in the OrdovicianPuna basin. Consequently, the orogenic processesat this active plate margin were dominated byrecycling of pre-existing Proterozoic basementrocks of the Pampeanas Terrane. There are noindications of terrane suture zones, oceanic crustor the accretion of allochthonous or even exoticblocks. As there is no evidence of exotic sources,the Puna deposits reflect the composition of the

adjacent continental regions represented by theSierras Pampeanas basement, i.e. the PampeanasTerrane including the Puncoviscana fold belt.

ACKNOWLEDGEMENTS

This study was funded by Deutsche Forschungsg-emeinschaft grant Ba 1011/11-1 and DeutscherAkademischer Austauschdienst grant D/98/04324. We thank B. Bock for the geochemicaldata of the magmatic rocks, R. J. Pankhurst for hisconstructive comments on the isotopic data, aswell as organizing perfect working conditions atthe NIGL (Keyworth), Fernando Hongn for help intectonic problems, and Cristina Moya as well asJorg Maletz for the determination of the fossils.We also thank Diane McDaniel, Robert L. Cullersand Scott M. McLennan for their helpful and

inspiring reviews. This paper is a contribution toIGCP projects 436 Pacific Gondwana margin and453 Uniformitarianism revisited: a comparisonbetween modern and ancient orogens.

SUPPLEMENTARY MATERIAL

The following material is available from http://www.blackwellpublishing.com/products/journals/suppmat/sed/sed595/sed595sm.htm

Table S1. Abundances of the framework min-erals of the studied formations. All samples were

counted according to the GazziDickinson meth-od described by Ingersoll et al. (1984). is meanvalue.

Table S2. Geochemical data of the Lower Ordo-vician sedimentary rocks. Major and some traceelements (1) were measured by XRF, other traceelements by INAA (2). SD, standard deviation(1 r); qu, quartz arenite; g, greywacke; a, ash; la,litharenite; p, pelite; c, conglomerate; phi, grainsize; UCC * after McLennan (2001).

REFERENCES

Acenolaza, F.G. and Baldis, B. (1987) The Ordovician Systemof South America. Int. Union Geol. Sci., Spec. Publ., 22,168.

Acenolaza, F.G., Toselli, A.J. and Durand, F.R. (1975) Estra-tigrafa y paleontologa de la region de Hombre Muerto,Provincia de Catamarca, Argentina. I. Congr. ArgentinoPaleontol. Bioestratigr., I, 109111.

Acenolaza, F.G., Miller, H. and Toselli, A.J. (1988) ThePuncoviscana Formation (Late PrecambrianEarly Cam-brian). Sedimentology, tectonometamorphic history and age




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of the oldest rocks of NW Argentina. In: The SouthernCentral Andes: Contributions to Structure and Evolution ofan Active Continental Margin (Eds H. Bahlburg, C. Breitk-reuz and P. Giese), Lect. Notes Earth Sci., 17, 2538.

Allmendinger, R.W., Jordan, T.E., Kay, S.M. and Isacks,B.L. (1997) The evolution of the Altiplano-Puna plateauof the Central Andes. Annu. Rev. Earth Planet Sci., 25,139174.

Bahlburg, H. (1990) The Ordovician basin in the Puna of NWArgentina and N Chile: geodynamic evolution from back-arcto foreland basin. Geotekton. Forsch., 75, 1107.

Bahlburg, H. (1991) The Ordovician back-arc to foreland suc-cessor basin in the ArgentineanChilean Puna: tectonosed-imentary trends and sea-level changes. In: Sedimentation,Tectonics and Eustasy (Ed. D.I.W. Macdonald), I

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