Chemostratigraphy of Mesoproterozoic and Neoproterozoic ... filea NEG-LABISE, Departamento de...

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Precambrian Research 182 (2010) 313–336 Contents lists available at ScienceDirect Precambrian Research journal homepage: www.elsevier.com/locate/precamres Chemostratigraphy of Mesoproterozoic and Neoproterozoic carbonates of the Nico Pérez Terrane, Río de la Plata Craton, Uruguay Leticia Chiglino a,, Claudio Gaucher b,c , Alcides N. Sial a , Jorge Bossi d , Valderez P. Ferreira a , Márcio M. Pimentel e a NEG-LABISE, Departamento de Geologia, Universidade Federal de Pernambuco (UFPE), C.P. 7852, Recife, PE 50670-000, Brazil b Departamento de Geología, Facultad de Ciencias, Iguá 4225, 11400 Montevideo, Uruguay c Institute of Geography and Geology and Nordic Center for Earth Evolution (NordCEE), University of Copenhagen, Øster Voldgade 10, 1350 Copenhagen, Denmark d Cátedra de Geología, Facultad de Agronomía, Garzón 780, Montevideo, Uruguay e Instituto de Geociencias, Universidade de Brasilia, Brasília, DF 70910-900, Brazil article info Article history: Received 8 June 2009 Received in revised form 14 December 2009 Accepted 2 June 2010 Keywords: Mesoproterozoic Carbon isotopes Strontium isotopes Rodinia Uruguay abstract Increasing evidence shows that Mesoproterozoic rocks are widespread in the Río de la Plata Craton. Car- bon and strontium isotope analyses were carried out for three different, carbonate-bearing successions in the southern Nico Pérez Terrane. The Parque UTE Group is erected, comprising (from base to top) the mainly volcanogenic Ca ˜ nada Espinillo Formation, the dolomitic Mina Valencia Formation and the mixed carbonate-siliciclastic Cerro del Mástil Formation. A 13 C curve was obtained for carbonates of the Parque UTE Group, which is characterized by a plateau at +1 to +1.6V-PDB, bracketed between two negative excursions (1.8V-PDB at the base and 3.3V-PDB at the top). These values are consistent with a Mesoproterozoic depositional age for the unit, as indicated by U–Pb ages of synsedimentary volcanics and gabbros of 1429 ± 21 and 1492 ± 4 Ma, respectively. The Mataojo Formation and the Marcos de los Reyes Formation were previously thought to be of similar age. The chemostratigraphic data, however, show that there is a hiatus of at least 700 Myr between these units, and that neither is correlative of the Parque UTE Group. Dolomitic marbles of the Mataojo Formation yielded a 13 C curve that varies within a narrow range around 0V-PDB (0.6 to +0.4). This, along with the youngest U–Pb detrital zircon ages of 1802 Ma constrains deposition between 1.8 and 1.5 Ga (late Palaeoproterozoic–early Mesoproterozoic). In contrast with the latter unit, the Marco de los Reyes Formation yielded 13 C values defining moderate to high-amplitude 13 C excursions between +4.4V- PDB and 3.2V-PDB. Corresponding 87 Sr/ 86 Sr values range between 0.7070 and 0.7080. Both C and Sr chemostratigraphy point to a later Neoproterozoic, possibly Ediacaran age for the Marco de los Reyes Formation. It is here proposed that modest 13 C seawater enrichment and low-amplitude 13 C excursions, following a long period of isotopic invariance around 0V-PDB, began already around 1.50–1.45 Ga. Thereafter, the amplitude of 13 C excursions increased gradually until the late Neoproterozoic. One important conclusion emerging from the data presented in this paper is that Mesoproterozoic rocks are common in the Río de la Plata Craton, and probably fringe it at both its western and eastern side. This suggests a rather central position of the craton within Rodinia, as also suggested by other lines of evidence, such as detrital zircon ages and a widespread thermal overprint at 1.25 Ga. © 2010 Elsevier B.V. All rights reserved. 1. Introduction The Río de la Plata Craton (Almeida et al., 1971) is a conti- nental block occurring in Uruguay (Fig. 1), eastern Argentina and southernmost Brazil which was likely accreted during the Meso- proterozoic (Gaucher et al., 2009a). The application of the terrane Corresponding author. Tel.: +55 81 2126 8242; fax: +55 81 2126 8242. E-mail addresses: [email protected] (L. Chiglino), [email protected] (C. Gaucher). concept (Coney et al., 1980; Howell, 1989) to the Precambrian basement of Uruguay has led to several studies elucidating the inde- pendent evolution of each tectonostratigraphic unit, thus providing a means for deciphering the long and complicated geological his- tory of the larger region. Three tectonostratigraphic terranes are currently recognized in the Río de la Plata Craton (e.g. Bossi and Cingolani, 2009; Fig. 1): (a) the Nico Pérez Terrane defined by Bossi and Campal (1992) and located east of the Sarandí del Yí Shear Zone (SYSZ); (b) the Piedra Alta Terrane defined by Bossi et al. (1993a) and consisting of the mainly Palaeoproterozoic (2.000 ± 100 Ma) block located west of the SYSZ and north of the Colonia Shear 0301-9268/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.precamres.2010.06.002

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Precambrian Research 182 (2010) 313–336

Contents lists available at ScienceDirect

Precambrian Research

journa l homepage: www.e lsev ier .com/ locate /precamres

hemostratigraphy of Mesoproterozoic and Neoproterozoic carbonates of theico Pérez Terrane, Río de la Plata Craton, Uruguay

eticia Chiglinoa,∗, Claudio Gaucherb,c, Alcides N. Sial a, Jorge Bossid,alderez P. Ferreiraa, Márcio M. Pimentele

NEG-LABISE, Departamento de Geologia, Universidade Federal de Pernambuco (UFPE), C.P. 7852, Recife, PE 50670-000, BrazilDepartamento de Geología, Facultad de Ciencias, Iguá 4225, 11400 Montevideo, UruguayInstitute of Geography and Geology and Nordic Center for Earth Evolution (NordCEE), University of Copenhagen, Øster Voldgade 10, 1350 Copenhagen, DenmarkCátedra de Geología, Facultad de Agronomía, Garzón 780, Montevideo, UruguayInstituto de Geociencias, Universidade de Brasilia, Brasília, DF 70910-900, Brazil

r t i c l e i n f o

rticle history:eceived 8 June 2009eceived in revised form4 December 2009ccepted 2 June 2010

eywords:esoproterozoic

arbon isotopestrontium isotopesodiniaruguay

a b s t r a c t

Increasing evidence shows that Mesoproterozoic rocks are widespread in the Río de la Plata Craton. Car-bon and strontium isotope analyses were carried out for three different, carbonate-bearing successionsin the southern Nico Pérez Terrane. The Parque UTE Group is erected, comprising (from base to top) themainly volcanogenic Canada Espinillo Formation, the dolomitic Mina Valencia Formation and the mixedcarbonate-siliciclastic Cerro del Mástil Formation. A �13C curve was obtained for carbonates of the ParqueUTE Group, which is characterized by a plateau at +1 to +1.6‰ V-PDB, bracketed between two negativeexcursions (−1.8‰ V-PDB at the base and −3.3‰ V-PDB at the top). These values are consistent with aMesoproterozoic depositional age for the unit, as indicated by U–Pb ages of synsedimentary volcanicsand gabbros of 1429 ± 21 and 1492 ± 4 Ma, respectively.

The Mataojo Formation and the Marcos de los Reyes Formation were previously thought to be of similarage. The chemostratigraphic data, however, show that there is a hiatus of at least 700 Myr between theseunits, and that neither is correlative of the Parque UTE Group. Dolomitic marbles of the Mataojo Formationyielded a �13C curve that varies within a narrow range around 0‰ V-PDB (−0.6 to +0.4‰). This, alongwith the youngest U–Pb detrital zircon ages of 1802 Ma constrains deposition between 1.8 and 1.5 Ga(late Palaeoproterozoic–early Mesoproterozoic). In contrast with the latter unit, the Marco de los ReyesFormation yielded �13C values defining moderate to high-amplitude �13C excursions between +4.4‰ V-PDB and −3.2‰ V-PDB. Corresponding 87Sr/86Sr values range between 0.7070 and 0.7080. Both C andSr chemostratigraphy point to a later Neoproterozoic, possibly Ediacaran age for the Marco de los ReyesFormation.

13 13

It is here proposed that modest C seawater enrichment and low-amplitude � C excursions, followinga long period of isotopic invariance around 0‰ V-PDB, began already around 1.50–1.45 Ga. Thereafter,the amplitude of �13C excursions increased gradually until the late Neoproterozoic.

One important conclusion emerging from the data presented in this paper is that Mesoproterozoicrocks are common in the Río de la Plata Craton, and probably fringe it at both its western and eastern

r cental zi

side. This suggests a ratheof evidence, such as detri

. Introduction

The Río de la Plata Craton (Almeida et al., 1971) is a conti-ental block occurring in Uruguay (Fig. 1), eastern Argentina andouthernmost Brazil which was likely accreted during the Meso-roterozoic (Gaucher et al., 2009a). The application of the terrane

∗ Corresponding author. Tel.: +55 81 2126 8242; fax: +55 81 2126 8242.E-mail addresses: [email protected] (L. Chiglino),

[email protected] (C. Gaucher).

301-9268/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.precamres.2010.06.002

tral position of the craton within Rodinia, as also suggested by other linesrcon ages and a widespread thermal overprint at 1.25 Ga.

© 2010 Elsevier B.V. All rights reserved.

concept (Coney et al., 1980; Howell, 1989) to the Precambrianbasement of Uruguay has led to several studies elucidating the inde-pendent evolution of each tectonostratigraphic unit, thus providinga means for deciphering the long and complicated geological his-tory of the larger region. Three tectonostratigraphic terranes arecurrently recognized in the Río de la Plata Craton (e.g. Bossi and

Cingolani, 2009; Fig. 1): (a) the Nico Pérez Terrane defined by Bossiand Campal (1992) and located east of the Sarandí del Yí Shear Zone(SYSZ); (b) the Piedra Alta Terrane defined by Bossi et al. (1993a)and consisting of the mainly Palaeoproterozoic (2.000 ± 100 Ma)block located west of the SYSZ and north of the Colonia Shear

314 L. Chiglino et al. / Precambrian Research 182 (2010) 313–336

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ig. 1. Geological sketch map of the Nico Pérez Terrane and surrounding areas, moBSZ: Sierra Ballena shear zone; SYSZ: Sarandí del Yí Shear Zone (dextral shear sensPT: Nico Pérez Terrane, CDT: Cuchilla Dionisio Terrane. The map presented in Fig.

one; and (c) the Tandilia Terrane (Bossi et al., 2005; Bossi andingolani, 2009), which crops out south of the Colonia Shear Zone

n Uruguay and in the homonymous region of the Buenos Airesrovince (Argentina) and includes a Transamazonian basement2.200–2.000 Ma), albeit slightly older than the Piedra Alta Terrane.

Large, continental-scale shear zones separate the above men-ioned terranes. While mapping in detail the Piedra Alta maficike swarm, dated at 1790 ± 5 Ma (U–Pb on baddeleyite: Halls etl., 2001), Bossi and Campal (1992) recognized and described theextral SYSZ with mylonites up to 8 km wide. The dike swarm

s clearly affected by this shear zone (Fig. 1), and the curvaturef the eastern part of the dike swarm is consistent with dex-

ral shear. Furthermore, no dikes occur east of the SYSZ. The agef the SYSZ is probably late Mesoproterozoic as shown by post-mplacement thermal overprinting of the Piedra Alta mafic dikewarm (Bossi et al., 1993b) between 1370 and 1170 Ma, reflectedn initial release portions of 40Ar–39Ar spectra and Rb–Sr mineral

from Bossi and Cingolani (2009) and references therein. CSZ: Colonia shear zone;inistral reactivation, see text). Inset: PAT: Piedra Alta Terrane, TT: Tandilia Terrane,cated at point 1 (stratotype of the PUG).

isochrons (Teixeira et al., 1999). Bossi and Navarro (2001) reportan 40Ar–39Ar age of 1240 ± 5 Ma for the same event. The SYSZwas reactivated sinistrally in the Cambrian, during the accretion ofGondwana (Bossi and Gaucher, 2004; Gaucher et al., 2008a, 2009a).

Bossi et al. (2005) and Ribot et al. (2005) recognized anddescribed a significant mylonite band, the sinistral Colonia shearzone, that they interpreted as a terrane boundary between thenorthern Piedra Alta Terrane and the southern Tandilia Ter-rane (Fig. 1). On the basis of cross-cutting relationships betweenmylonites and well-dated dike swarms, Bossi et al. (2005) sug-gested that the age of the Colonia shear zone is between 1790 ± 5and 1588 ± 11 Ma (Halls et al., 2001; Iacumin et al., 2001).

Finally, Gaucher et al. (1998) and Bossi et al. (1998) concluded,on the basis of paleontological and geological data, that the sinis-tral Sierra Ballena shear zone represents the eastern boundary ofthe Nico Pérez Terrane and consequently of the Río de la Plata Cra-ton (Fig. 1). This shear zone formed in the Cambrian (ca. 530 Ma)

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s a result of the collision of the allochthonous Cuchilla Dionisio-elotas Terrane (“Arachania”: Gaucher et al., 2009b) with the Río dea Plata Craton during the final stages of Gondwana amalgamationBossi and Gaucher, 2004; Basei et al., 2005; Gaucher et al., 2008a,009a,b).

The Nico Pérez Terrane is the oldest and most complex part ofhe Río de la Plata Craton. Hartmann et al. (2001) reported Archeanges as old as 3.4 Ga for the La China Complex, which is in turn over-ain by stromatolite-bearing sedimentary cover (Cerro de Villalbaormation and associated units), which was probably deposited inhe Neoarchean (2.75 Ga; Gaucher et al., 1996, 2006a; Hartmann etl., 2001). Granulite-facies BIF and associated rocks comprise thealentines Formation, of Neoarchean (Bossi and Ferrando, 2001;artmann et al., 2001) or lower Palaeoproterozoic (Santos et al.,003) age. Several orogenic events are recorded as metamorphicims in zircons from different units of the Nico Pérez Terrane, cen-ered at 3.1, 2.7 and 2.2–2.1 Ga (Hartmann et al., 2001; Santos et al.,003). Mesoproterozoic orogenic events, on the other hand, wereot intense enough to lead to zircon recrystallization, although

ower intercepts at 1.25 Ga were recently found in gneisses of thea China Complex (F. Chemale, pers. comm., 2008). As mentionedbove, a number of Mesoproterozoic K–Ar and Ar–Ar ages cen-ered at 1.25 Ga are interpreted as reflecting Grenvillian tangentialectonics affecting the Río de la Plata Craton (see Gaucher et al.,009a, and references therein). However, until recently little evi-ence existed of the occurrence of rocks actually deposited duringhe Grenvillian orogenic cycle in the Nico Pérez Terrane (Bossit al., 1998), with a number of authors favouring a Neoprotero-oic age for these units (e.g. Sánchez Bettucci and Ramos, 1999;asei et al., 2000). Oyhantcabal et al. (2005) reported U–Pb zir-on ages of 1429 ± 21 Ma for metarhyolites of the upper Fuenteel Puma Group (here redefined as Parque UTE Group; Fig. 1), and492 ± 4 Ma for metagabbros at the base of the same successionlsewhere. These ages support the existence of a volcanosedimen-ary succession deposited between 1.49 and 1.43 Ga. Moreover,–Pb detrital zircon ages of sandstones of the Carapé Group (Fig. 1)re consistent with a Mesoproterozoic depositional age for part ofhis unit (Basei et al., 2008).

In this paper, we report isotope chemostratigraphic data (C, Ond Sr) for several carbonate-bearing successions of the Nico Pérezerrane, which strongly suggest that Mesoproterozoic volcanosed-mentary successions are an integral part of the Río de la Plataraton.

. Lithostratigraphy and age constraints

.1. Carapé Tectonic Slab and Carapé Group

Bossi (1983) erected the Carapé Group to include a medium-rade, fault-bounded metamorphic complex in the southeasternart of the Nico Pérez Terrane. This complex is composed ofarbles, BIFs, amphibolites, gneisses and micaschists (Bossi andavarro, 2001). Detrital zircon ages (Basei et al., 2008) show that

he Carapé Group was sourced in and is thus autochthonous tohe Nico Pérez Terrane (see Section 5.2.2). Our chemostratigraphicata show that the Carapé Group likely includes successions ofery different ages, thus demanding a redefinition of this importantithostratigraphic unit. Other lithostratigraphic schemes proposedor the area (e.g. Sánchez Bettucci and Ramos, 1999; Oyhantcabal etl., 2005) will be also discussed below in light of the chemostrati-

raphic results presented here.

After detailed geological mapping (Fig. 2), Bossi et al. (2007) sub-ivided the Carapé Group into three formations, namely the Marcoe los Reyes, Mataojo and Edén Formations, which are part of thearapé Tectonic Slab. This tripartite lithostratigraphic scheme is

earch 182 (2010) 313–336 315

adopted here and the different units, especially the carbonates, arebriefly described below.

2.1.1. Marco de los Reyes FormationThe Marco de los Reyes Formation comprises gray limestones

(Fig. 3F–G), BIF, muscovite micaschists, quartzites, amphibolites,and fine-grained gneisses (Figs. 6–8). Subvertical dips are char-acteristic of the unit, and deformation diminishes from north tosouth. The occurrence of sub-concordant, boudinaged granitic sillsis a characteristic feature of limestones and BIF of the CarapéGroup (Bossi and Navarro, 1991; Rossini, 2002; Bossi et al., 2007).Disrupted quartz-arenite layers also occur within the limestones(Fig. 8), resulting in boudinaged quartzite blocks within the lesscompetent, sheared limestone. Deformation of the limestones isductile and very significant (Rossini, 2002; Bossi et al., 2007), asshown by: (a) vicinity (ca. 10 km) to the continental-scale SierraBallena Shear Zone (Fig. 2), (b) preservation of limestones as thinbasement inliers between large granite intrusions (Fig. 2), (c) tightanisopachous folding with subvertical axial planes, and (d) oblit-eration of all primary sedimentary structures except in the south(Bulldog Quarry).

Limestones of the Marco de los Reyes Formation are finely lam-inated and dark gray to black in colour due to high organic mattercontents (Fig. 3G). Geochemically they are Mg-poor limestones,MgO ranging between 0.25 and 10.9 wt% (mean = 2.5%, N = 53). Srconcentrations are usually high (up to 3050 ppm, see below), sug-gesting an originally aragonitic mineralogy (Tucker and Wright,1990). The depositional environment inferred for the limestoneson the basis of lithofacies (BIF-limestone assemblage) and thefew sedimentary structures preserved (Fig. 3F) is a deep shelfcharacterized by dysoxic or anoxic conditions that led to organicmatter preservation. The organic- and Sr-rich, Mg-poor natureof the limestones, their association with BIF and their structuralcharacteristics are diagnostic features that clearly separate themfrom carbonates of either the Mataojo Formation (i.e. Mg-rich,organic- and Sr-poor) or the PUG (i.e. Mg- and organic-rich, Sr-poor,Mn-rich).

No reliable age constraints are available for the Marco delos Reyes Formation, hence the importance of obtaining high-resolution chemostratigraphic data.

2.1.2. Mataojo FormationThe Mataojo Formation comprises whitish dolomitic mar-

bles, quartzose metasandstones, amphibolites and biotite-schists(Bossi et al., 2007; Fig. 11). Coarse metasandstones occur at thebase, passing into schists and dolostones up section. The lat-ter are, in turn, overlain by heterolithic facies composed of finemetasandstone–metapelite intercalations (Fig. 3E). The MataojoFormation is usually shallowly dipping (<50◦) and shows lessdeformation than the Marco de los Reyes Formation, sedimentarystructures being largely preserved. No boudinaged granites occurin the Mataojo Formation. The latter unit is overthrust onto theMarco de los Reyes Formation, which can be clearly seen at RobainaQuarry.

Dolomitic marbles of the Mataojo Formation are typicallywhite and thick-bedded to massive. Geochemically, they con-tain 20.5–23.8 wt% MgO (mean = 22.1%, N = 11) and 30.8–33.5 wt%CaO (mean = 32.1%, N = 11), close to stoichiometric burial dolomite(Tucker and Wright, 1990). Metapelite intercalations are common.

The age of the Mataojo Formation is loosely constrained by:

(a) U–Pb SHRIMP detrital zircon ages reported by Basei et al. (2008)for basal sandstones of the unit, with the youngest grain datedat 1802 ± 59 Ma.

(b) An U–Pb zircon age 530 ± 14 Ma for the Cuchillita Granite(Oyhantcabal et al., 2005), which intrudes the succession. It

316 L. Chiglino et al. / Precambrian Research 182 (2010) 313–336

et al.,

2

a

Fig. 2. Geological map of the Carapé Tectonic Slab (Bossi

is worth noting that the same granite yielded a K–Ar age of571 ± 10 Ma (Sanchez Bettucci et al., 2003), difficult to reconcile

with the more reliable U–Pb age.

.1.3. Edén FormationThe Edén Formation is made up of paragneisses, para-

mphibolites, micaschists and rare pyroxenites and migmatites

2007), showing the location of different sections studied.

(Bossi et al., 2007; Fig. 2). Metamorphism is typically medium-grade, possibly reaching high-grade in the eastern part of

the unit.

The Edén Formation is in tectonic contact with the MataojoFormation, being thrust with northern vergence onto the latterunit (Fig. 2). An important structural feature is a large-scale drag fold observed at the eastern boundary of the

L. Chiglino et al. / Precambrian Research 182 (2010) 313–336 317

Fig. 3. Sedimentary structures and typical lithofacies of the studied units. (A) Graded sandstone beds showing erosive base (arrowed) intercalated with marls, PUG at FPU11. (B) Overturned, graded calcarenite beds, exhibiting hummocky cross-bedding at the base (arrowed), PUG at FPU 144. (C) Graded calcirudite-calcarenite bed, stratotype ofthe Mina Valencia Formation (PUG). (D) Lapilli-tuff (T) interbedded with shale beds (Sh), stratotype of the Cerro del Mástil Formation (PUG). (E) Metasandstones interbeddedwith metapelites and dolostones. Note sandstone layer with flat base and rippled top (arrowed). (F) Calcarenites showing trough cross-bedding and truncations, Marco del e Marc

EtZ2

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os Reyes Formation at PIR 51. (G) Laminated, gray, high-Sr limestones typical of th

dén Formation (Fig. 2), which can be easily explained ashe result of sinistral shear along the Sierra Ballena Shear

one at ca. 530 Ma (Bossi and Gaucher, 2004; Gaucher et al.,008a).

No carbonates or other suitable sedimentary rocks wereescribed so far from the Edén Formation, making it unsuitable forhemostratigraphic studies.

o de los Reyes Formation (section AIG 11).

2.2. Parque UTE Group

The Parque UTE (UTE: Usinas y Transmisiones Eléctricas)Group (PUG), which occurs in the southern Nico Pérez Terrane(Figs. 1 and 4), was erected on the basis of geological mapping ofkey areas, chemostratigraphy and available U–Pb ages by Chiglinoet al. (2008) and Bossi et al. (2008). In its original definition par-

318 L. Chiglino et al. / Precambrian Research 182 (2010) 313–336

Park a

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ially includes the former Lavalleja Group (Bossi, 1966) or ComplexSánchez Bettucci and Ramos, 1999) and the Fuente del Puma GroupBossi et al., 1998) or Formation (Sánchez Bettucci and Ramos,999), all terms rendered obsolete by the new data. The Lavallejaroup/Complex comprised unrelated, Neoarchean, Mesoprotero-oic, Neoproterozoic and Cambrian successions in tectonic contactsee Bossi et al., 2002; Gaucher et al., 2004a; Basei et al., 2008;ossi and Cingolani, 2009), thus prompting a revision of this lithos-ratigraphic term. The name “Fuente del Puma Group/Formation”s not appropriate either, because its type area is located on Edi-caran limestones of the Arroyo del Soldado Group (Gaucher et al.,004a).

The group is composed, from base to top, of the following unitsFigs. 4 and 13): (a) the mainly volcanogenic Canada Espinillo For-

ation, (b) the predominantly dolomitic Mina Valencia Formation,nd (c) the mixed carbonate-siliciclastic Cerro del Mástil Formation.brief description of each of these units is presented below.The PUG yielded U–Pb zircon ages of 1429 ± 21 Ma for rhyolites

ear the top and 1492 ± 4 Ma for gabbros at the base (Oyhantcabalt al., 2005). These ages may be interpreted as representing anvolving bimodal magmatism, with basic terms at the base andcid at the top. Pb–Pb ages on galenas of the Mina Valencia For-ation yielded ages between 1500 and 1200 Ma (Garau, in Bossi

nd Navarro, 2001). The succession was deformed and metamor-hosed (greenschist facies) at 1210–1250 Ma, as shown by K–Arating of basalts and gabbros belonging to the unit (Gómez Rifas,995). A second metamorphic event is evidenced by partial reset-ing of the K–Ar system, yielding apparent ages between 750 and00 Ma (Gómez Rifas, 1995; Sánchez Bettucci et al., 2004). Detritalircon U–Pb ages show that the PUG was sourced in the Nico Pérezerrane (see Section 5.2.2).

.2.1. Canada Espinillo FormationAs stratotype of the Canada Espinillo Formation, Chiglino

2008) designated the section exposed between the homonymousreek and the Mina Valencia Quarry, inside the UTE Park (seeig. 4). The base of the unit is not exposed, its thickness possiblyxceeding 2 km (Fig. 4). The unit comprises basic metavolcanics“prasinites”), metagabbros and intervening metapelites. Volcanic

nd surroundings) of the Parque UTE Group.

rocks are mainly tholeiitic basalts with normative quartz andhypersthene (Midot, 1984; Mallmann et al., 2007), but more silicicterms (andesites, dacites) were also described by Sánchez Bettucciet al. (2001). These, including subordinate acid tuffs, occur near thecontact with the overlying carbonates. Oyhantcabal et al. (2005)reported a U–Pb zircon concordant age of 1492 ± 4 Ma for gab-bros of the Canada Espinillo Formation, providing the currently bestavailable age constraint. We assume that the dated gabbros formedroughly coeval to the extensive basaltic magmatism recorded inthe unit. TDM (depleted mantle) Sm–Nd model ages for basalts andgabbros yielded Archean and Palaeoproterozoic values between 2.9and 1.7 Ga (Mallmann et al., 2007). K–Ar ages of metagabbros inthe type area range between 1210 and 630 Ma, which are inter-preted here as reflecting greenschist metamorphism at ca. 1.2 Gaand partial resetting due to earliest Cambrian tangential tectonics(e.g. Bossi and Gaucher, 2004; Gaucher et al., 2008a, 2009a).

It is worth noting that Mallmann et al. (2007) identified twodifferent groups of mafic rocks in the former Lavalleja Group, theyoungest of which yielded an U–Pb zircon age of 590 ± 2 Ma, andhas been related by Gaucher et al. (2008b) to basic magmatism ofthe basal Las Ventanas Formation. These rocks are obviously notincluded in the Canada Espinillo Formation, but caution is requiredbecause both units are tectonically interleaved and have been oftenmistaken in the past. Finally, it is evident that metagabbro sills alsointrude the overlying units, especially the Mina Valencia Formation(e.g. Figs. 15 and 16). No ages are currently available for these rocks.

2.2.2. Mina Valencia FormationThe Mina Valencia Formation comprises dolomitic calcirudites

(flake-breccias, conglomerates: Fig. 3C) and calcarenites (Fig. 3B)at the base, passing into pure, fine-grained, massive dolostoneswhich have been extensively mined at the homonymous quarrywhere the stratotype is located (Chiglino, 2008; Figs. 4 and 13).Clastic limestones at the base exhibit large-scale (� up to 3.5 m)

hummocky cross-stratification and normal grading (Figs. 3B andC), showing that they were formed by storm processes. The basal40–60 m are less dolomitic and often quite pure, being mined forthe cement industry at least at one quarry (Fig. 17). The clasticcarbonates represent a shallowing-upward sequence passing into

L. Chiglino et al. / Precambrian Research 182 (2010) 313–336 319

Table 1Geochemical and isotopic analyses organized according to geological unit and section. Str. m*: stratigraphic distance (m) to the previous sample.

Sample Str. m* �13C V-PDB �18O V-PDB 87Sr/86Sr Ca (ppm) Mg (ppm) Mn (ppm) Sr (ppm) Rb Mg/Ca Mn/Sr

Parque UTE GroupFPU 144

1 0 1.13 −12.96 213,914 11,680 263 190 66 0.055 1.382 2.8 1.47 −12.06 259,530 9799 225 218 45 0.038 1.033 17.1 1.54 −12.45 0.71172 ± 3 296,203 7103 310 347 26 0.024 0.895 7.7 0.40 −14.24 178,421 11,125 3989 259 30 0.062 15.46 2.4 −0.08 −14.27 0.71842 ± 10 266,245 8116 705 493 13 0.030 1.437 1.0 0.84 −14.75 0.71069 ± 3 295,810 8659 767 521 15 0.029 1,478 7.0 1.29 −14.16 0.71565 ± 1 264,035 14,230 689 482 19 0.054 1.439 3.3 1.59 −14.05 0.71102 ± 9 269,162 9395 488 474 27 0.035 1.0310 1.9 −0.08 −14.58 0.71567 ± 1 321,779 6431 596 478 19 0.020 1.25

FPU 1531 0 −3.31 −12.60 295,080 21,346 255 185 10 0.072 1.382 14 −2.59 −9.43 0.70831 ± 3 296,296 10,613 387 109 4 0.036 3.553 7 −0.83 −12.44 266,981 17,064 170 97 3 0.064 1.754 10 −0.84 −12.15 0.71091 ± 2 334,549 10,130 201 137 4 0.030 1.475 7 −0.28 −13.37 0.71614 ± 5 345,631 9467 185.8 125 2 0.027 1.49

FPU 1541 0 2.29 −10.962 6 −1.45 −11.04 229,372 33,828 317 136 4 0.15 2.333 4 0.59 −15.12 306,664 31,235 209 167 4 0.10 1.254 20 −1.75 −12.32 0.71254 ± 2 219,934 82,671 593 195 10 0.38 3.04FPU 150-1 15 −2.13 −7.99 0.72272 ± 9 259,688 15,678 487 235 3 0.060 2.07

FPU 111 0 −1.80 −17.72 0.71229 ± 44 365,866 8864 775 185 2 0.0242 4.192 17 0.23 −12.82 89,804 66,993 852 25 64 0.746 34.14 12 −0.69 −10.49 92,235 55,355 775 9 51 0.600 86.15 3.5 0.62 −12.27 103,604 65,606 929 32 54 0.633 29.06 2.5 −0.57 −12.24 108,609 67,476 1162 34 53 0.621 34.27 2 nd nd 53,554 48,662 620 nd nd 0.9098 5 1.12 −12.12 111,111 71,817 1239 32 55 0.646 38.79 4.5 1.02 −12.09 105,248 70,310 1084 25 57 0.668 43.310 4 0.86 −12.43 95,095 64,099 1472 24 58 0.674 61.311 2 0.74 −12.70 96,668 65,486 2014 35 58 0.678 57.5

VAL1 0 −0.10 −11.792 3 −0.33 −11.72 190,530 92,436 2246 256 32 0.485 8.773 3.5 −0.59 −12.62 140,150 91,350 1007 76 60 0.652 13.24 13.5 0.78 −8.06 180,170 147,360 5730 14 0 0.818 4095 2 0.58 −8.146 5 0.58 −8.88 233,370 124,810 387 6 1 0.535 64.57 5 0.81 −8.39 240,630 109,440 542 175 3 0.457 3.108 5 1.02 −9.239 5 0.73 −8.71 180,100 152,970 4569 18 1 0.849 25410 5 1.32 −8.70 242,200 158,760 542 34 1 0.655 15.911 5 1.19 −9.3712 5 1.06 −8.45 187,320 157,910 620 30 1 0.843 20.713 5 0.99 −10.4214 5 1.60 −7.22 197,320 131,140 4259 42 3 0.665 10115 5 1.47 −8.2016 5 1.57 −8.2317 10 1.24 −10.7718 5 1.49 −6.5719 5 1.58 −7.05 198,320 143,740 54 51 10 0.725 10.6

ANCAP N51 0 −0.73 −11.522 4 −0.51 −11.263 3 −0.65 −10.434 8 −0.60 −11.16 372,210 13,326 775 35 5 0.036 22.15 8 −0.88 −11.926 15 −0.86 −11.87 372,420 4523 77.5 110 6 0.012 0.7057 2 −0.83 −12.56 371,350 8261 232 58 5 0.022 4.008 2 −0.67 −14.29 344,330 12,180 5034 17 3 0.035 2969 2 −0.83 −10.80 375,710 5547 77.5 62 5 0.015 1.2510 2 −0.74 −12.4711 2 −0.60 −14.51 347,910 11,094 620 47 11 0.032 13.212 2 −0.77 −12.88 369,130 9673 77.5 65 5 0.026 1.1913 2 −0.82 −11.8914 2 −0.69 −11.7715 2 −0.61 −13.9716 2 −0.61 −13.97

320 L. Chiglino et al. / Precambrian Research 182 (2010) 313–336

Table 1 (Continued )

Sample Str. m* �13C V-PDB �18O V-PDB 87Sr/86Sr Ca (ppm) Mg (ppm) Mn (ppm) Sr (ppm) Rb Mg/Ca Mn/Sr

Mataojo FormationFPU 136

3 0 0.07 −5.66 221,793 143,695 387 60 <10 0.65 6.454 7.3 −0.01 −9.675 8 −0.12 −7.24 216,216 131,153 542 47 <10 0.61 11.56 12 0.18 −8.887 12.4 −0.42 −10.65 219,648 132,359 1007 105 <10 0.60 9.598 8.8 0.20 −8.189 7.8 −0.29 −10.97 230,659 143,936 852 124 <10 0.62 6.8710 6 1.26 −11.4111 7 0.18 −7.8412 5 −0.46 −10.0913 5 −0.12 −7.71 234,306 123,615 542 107 <10 0.53 5.0714 2 −2.13 −7.99 220,721 134,831 309 72 <10 0.61 4.30

LCN31 0 0.38 −6.01 239,770 132,710 696 23 3 0.55 30.32 4.7 −0.59 −7.92 233,910 130,780 542 22 3 0.56 24.64 3.7 0.44 −6.27 237,770 132,410 774 31 3 0.56 25.05 5 0.43 −5.39 234,060 130,060 774 34 1 0.56 22.86 5 0.34 −5.38 238,200 131,870 774 34 3 0.55 22.8

Marco de los Reyes FormationLCN2

1 0 0.61 −7.48 0.70709 ± 2 386,887 2714 0 2238 2 0.0070 02 5 −0.59 −7.92 0.70714 ± 1 36,249 361 0 2244 3 0.010 03 10 0.7 −7.37 0.70704 ± 1 38,293 265 0 2450 1 0.0053 04 15 0.75 −7.11 0.70709 ± 2 38,164 530 0 2590 5 0.014 05 20 0.56 −7.38 0.70723 ± 2 38,071 289 0 2301 3 0.0076 0

AIG 12A 0 0.2 −11.62 0.71082 ± 4 326,250 38,950 154 383 6 0.12 0.404B 3.44 −0.15 −8.9 232,340 54,870 929 313 30 0.24 2.97C 6.5 0.69 −12.57 0.71040 ± 2 299,090 59,630 77.5 499 9 0.13 0.155D 11 2.24 −13.41 0.70830 ± 3 325,470 54,810 77.5 560 3 0.17 0.138E 17 0.79 −17.03 0.70991 ± 3 308,530 60,530 77.5 589 3 0.20 0.132F 23 0.79 −17.03 311,670 65,960 77.5 628 3 0.21 0.112

AIG 111 0 0.58 368,654 22,311 77.5 1332 3 0.0605 0.05822 5 0.69 −9.71 0.70786 ± 2 362,270 5306 77.5 1802 4 0.0146 0.04303 5 0.67 −8.93 0.70730 ± 3 370,710 5849 0 1926 7 0.0157 04 2 0.56 −8.5 0.70711 ± 2 384,813 4342 0 1850 3 0.0113 05 3 0.68 −8.83 0.70707 ± 2 380,523 4342 0 1580 3 0.0114 06 5 0.5 −8.84 0.70724 ± 3 376,000 12,783 0 1704 3 0.0339 07 5 0.5 −7.48 0.70718 ± 2 385,930 3678 0 2018 3 0.0095 08 1.5 0.48 −7.12 0.70752 ± 4 387,290 6814 0 1438 2 0.0175 09 8 0.58 −8.15 0.70716 ± 2 377,420 4764 0 2052 2 0.0126 010 7 0.43 −7.85 0.70732 ± 1 378,850 7599 0 1489 3 0.0200 011 6 0.35 −8.26 0.70709 ± 2 324,390 6754 77.5 2019 6 0.0208 012 6 0.62 −7.73 0.70717 ± 2 375,490 3317 0 2291 4 0.0088 013 5 0.69 −7.33 0.70697 ± 2 391,860 2472 0 2081 2 0.0063 014 1.5 0.69 −8.3 197,040 35,517 387 34 62 0.1802 11.4

PIR 511 0 −3.13 −23.92 304,240 24,421 77.5 2169 2 0.08026 0.03572 4.5 −3.2 −18.03 307,960 39,010 232 1555 1 0.12667 0.14933 3 4.08 −20.17 0.70756 ± 1 389,430 3136 0 2455 2 0.00803 04 2 0.83 −21.73 0.70757 ± 1 354,700 4700 154 1416 3 0.01325 0.108765 2.5 3.13 −20.97 0.70764 ± 1 374,140 7170 0 2579 4 0.01916 06 4.8 2.6 −18.98 0.70765 ± 2 351,620 21,118 0 2294 7 0.06023 07 5.2 3.72 −18.26 0.70767 ± 1 359,340 13,320 77.5 2308 7 0.03706 0.03358 5.2 3.17 −20.63 365,350 10,250 77.5 2441 8 0.02805 0.03179 5.2 3.97 −12.07 0.70770 ± 3 375,640 6750 77.5 2071 4 0.01796 0.037310 5 3.95 −12.35 370,210 8140 0 2571 6 0.02198 011 6.4 4.42 −11.73 0.70772 ± 2 374,490 8080 77.5 2026 5 0.02157 0.038212 4 4.23 −12.85 0.70755 ± 1 390,070 3670 0 2553 3 0.00940 013 3.5 3.92 −10.87 0.70771 ± 3 378,000 4820 77.5 2355 4 0.01275 0.032814 8 3.39 −16.59 0.70938 ± 2 369,280 2110 77.5 2240 8 0.00571 0.034515 11 −1.94 −16.09 308,030 31,990 154 1816 7 0.10385 0.084816 5 −2.27 −14.64 329,470 22,310 154 1891 7 0.06771 0.081417 5.5 −1.92 .155.7 316,600 32,560 154 1880 8 0.10284 0.081918 5 1.66 −16.59 345,910 26,530 77.5 2439 2 0.07669 0.031719 4 2.83 −17.92 383,790 4820 0 2679 3 0.01255 020 5 2.81 −19.61 380,210 7230 0 2463 5 0.01901 0

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assive dolostone deposits that characterize the middle of theormation at its stratotype (Fig. 13). At the top, dolostones arenely bedded/laminated, marking the beginning of a new trans-ression. Black, organic-rich, pyritic metamarls occur both at thease (Figs. 3A and 14) and at the transition to the overlying Cerroel Mástil Formation, the latter representing an important trans-ressive surface.

Carbonates at the base of the Mina Valencia Formation are com-osed of 85% carbonates (dolomite and calcite) and 15% terrigenouslastics (quartz, phyllosilicates and opaque minerals). Geochem-cally, they contain 0.75–15.3 wt% MgO, 7.5–52.6 wt% CaO, andariable amounts of SiO2 and Al2O3 reflecting different amounts ofalcite, dolomite, quartz and phyllosilicates. Up section, carbonatesecome pure dolostones (up to 99.94% dolomite), with less than 5%errigenous clastics. At its stratotype, the Mina Valencia Formationeaches 180 m in thickness, showing a transitional contact to theverlying Cerro del Mástil Formation.

.2.3. Cerro del Mástil FormationThe Cerro del Mástil Formation conformably overlies the Mina

alencia Formation, its stratotype being located at the homony-

ous hill in the UTE Park (Chiglino, 2008; Figs. 4 and 13). At

he base, quartzose sandstones are overlain by black, organic-richhales 50–60 m in thickness (Fig. 13). The middle part of the units characterized by dark gray, fine dolomitic limestones. At theop, black shales are interbedded with acid lapilli-tuffs bearing �

Fig. 5. Crossplots of �13C and 87Sr/86Sr against common alteration proxies (�18O, M

earch 182 (2010) 313–336 321

quartz phenocrysts, total thickness reaching 50 m (Fig. 13). Theupper contact of the unit is not exposed. Fine, cm-scale intercala-tions between tuffs and shales (Fig. 3A) show the synsedimentarycharacter of volcanism.

Carbonates of the Cerro del Mástil Formation are more cal-citic than the underlying unit, comprising 1.7–13.7 wt% MgO(mean = 4.3%, N = 9) and 30.8–48.3 wt% CaO (mean = 39.7%, N = 9),the rest being mostly SiO2.

U–Pb zircon ages of 1429 ± 21 Ma for rhyolites occurring 4 kmto the SE of the type area (Fig. 1) were reported by Oyhantcabalet al. (2005). These rocks are here assigned to the Cerro del MástilFormation, suggesting an early Mesoproterozoic depositional agefor the top of the PUG.

It is worth noting that the type area of the three units that makeup the PUG (Fig. 4) is only a fragment of its real extent. The PUGlikely continues to the NE and SW of its type area, but detailedgeological mapping is required to elucidate this point.

3. Materials and methods

Standard thin sections of carbonates were prepared, stained

(Alizarin Red-S) and carefully analyzed under a petrographic micro-scope. Domains consisting of pure primary carbonates and showingno recrystallization were selected. Least altered samples (lack-ing veins, discoloration, weathered rinds, recrystallization features,and silicification) were microdrilled with 1 mm drill at the LABISE,

n/Sr, Sr concentration) for limestones of the Marco de los Reyes Formation.

3 ian Re

Ft2otls(blVsTitaafrstwtfRitac

22 L. Chiglino et al. / Precambr

ederal University of Pernambuco, Brazil. CO2 was extracted fromhese carbonate samples in a high vacuum line after reaction of0 mg with phosphoric acid at 25 ◦C (12 h of reaction for limestonesr 3 days for dolostones), and cryogenically cleaned, according tohe method described by Craig (1957). CO2 gas released was ana-yzed for O and C isotopes in a double inlet, triple collector masspectrometer (VG Isotech SIRA II), using the BSC reference gasBorborema skarn calcite) that is calibrated against NBS-18 (car-onatite), NBS-19 (toilet seat limestone) and NBS-20 (Solenhofen

imestone), has a �18O value of −11.3‰ V-PDB and �13C = −8.6‰-PDB. The external precision based on multiple standard mea-urements of NBS-19 was better than 0.1‰ for carbon and oxygen.he results are expressed in the �-notation in parts per thousandn relation to international V-PDB scale. Samples chosen for Sr iso-opic analyses were pre-treated with ammonium acetate to removedsorbed Sr (Montanez et al., 1996) and then leached in 0.5 Mcetic acid and centrifuged to separate the dissolved and insolubleractions. Sr was eluted from solutions by ion exchange chromatog-aphy using Sr-Spec resin. 87Sr/86Sr values were determined intatic mode using a Finnigan MAT 262 seven-collector mass spec-rometer at the University of Brasilia, Brazil. The isotopic ratiosere normalized to 86Sr/88Sr value of 0.1194 and the 2� uncer-

ainties on Sr isotope measurements was less than 0.00017, exceptor two samples (030130/8 and 10, Table 1) with 2� = 0.00037.

epeated analyses of NIST 987 standard over the analysis period

ndicated a value of 0.71028 ± 0.00002 (2�) for the 87Sr/86Sr ratio,he certified value being 0.71034 ± 0.00026. Whole-rock chemicalnalyses were carried out on fused beads by XRF (X-Ray Fluores-ence) at the LABISE, Recife.

Fig. 6. Litho- and chemostratigraphic profile of the Marco de los Reyes Fo

search 182 (2010) 313–336

4. Chemostratigraphy and age

A total of 149 �13C and �18O analyses, and 43 87Sr/86Sr analyseson carbonates of all studied units were carried out (see Table 1). Toassess the nature of the isotopic signals, 110 analyses of major andtrace elements (especially Mn, Rb and Sr; Table 1) were performedon the same samples by XRF. The results for each of the units studiedare presented below.

4.1. Marco de los Reyes Formation

Limestones of the Marco de los Reyes Formation are char-acterized by high-Sr concentrations (maximum: 3056 ppm;mean = 1882 ppm; N = 53; Fig. 5) that suggest an original aragoniticprecursor (Tucker and Wright, 1990). Mn/Sr values are correspond-ingly very low (<0.2 for all but three samples; Fig. 5, Table 1),strongly suggesting that isotopic values reflect seawater composi-tion (Montanez et al., 1996; Jacobsen and Kaufman, 1999; Melezhiket al., 2001). This view is strengthened by the lack of correlationbetween �13C, �18O, 87Sr/86Sr, Mn/Sr and Sr concentrations (Fig. 5;Jacobsen and Kaufman, 1999).

We studied six different sections of the Marco de los Reyes For-mation, of which the most representative are presented in Figs. 6–8.In the southern part of the Carapé Tectonic Slab (Bossi et al., 2007),

preservation of the succession, texture and sedimentary structuresis better than in the northern part, where only isolated tectonicslices of limestones occur. Thus, the section studied at Bull DogQuarry (Fig. 6) is here consider to be the most representative of allprofiles.

rmation at Bull Dog Quarry (point PIR 51); 34◦49′09′′S, 55◦05′47′′W.

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The �13C curve obtained for the Marco de los Reyes Forma-ion is characterized by positive values of up to +4.4‰ V-PDB for

ost of the section. Two negative excursions down to −3.2‰ V-DB characterize the lower and upper parts of the unit at Bull Doguarry (Fig. 6). In the other sections studied, only partial curvesere obtained due to the fact that they represent incomplete slices

f limestone, with the base and/or top missing (Figs. 7 and 8). Thenferred correlation between the different sections of the Marco deos Reyes Formation is shown in Fig. 9.

87Sr/86Sr values vary consistently between 0.7070 and 0.7079 inll sections, with only five anomalous values mostly encounteredn a single section (Floridán Quarry; Fig. 8). Even there, a singlenalysis yielding 0.7083 might still be considered as a near-primaryalue.

Both the �13C and 87Sr/86Sr curves obtained for the Marco deos Reyes Formation are best explained if the unit was deposited inhe Ediacaran. According to Halverson et al. (2007, 2009) 87Sr/86Sralues between 0.7070 and 0.7080 characterize the late Cryogeniannd most of the Ediacaran, and have been also reported from latediacaran successions (Gaucher et al., 2004b, 2009c). However, theate Cryogenian is characterized by elevated �13C ratios of up to10‰ V-PDB (Jacobsen and Kaufman, 1999; Halverson et al., 2005,

009), unlike the curve obtained for the Marco de los Reyes For-ation. Therefore, we favour an Ediacaran depositional age for the

nit. We are confident that the 87Sr/86Sr values obtained representear-seawater composition because of the high-Sr nature of the

imestones, the consistency of 87Sr/86Sr in all sections except one

Fig. 7. Litho- and chemostratigraphic profile of the Marco de los Reyes Forma

earch 182 (2010) 313–336 323

(Fig. 9), and the non-altered nature suggested by different alterationproxies (Fig. 5).

4.2. Mataojo Formation

Two representative sections of the Mataojo Formation werestudied (Fig. 11), the most complete being the Piedrahíta Quarrysection (section FPU 136). Mn/Sr values obtained for dolostonesin these sections range between 4.3 and 30, mostly outside thefield of “unaltered” limestones with respect to �13C (Jacobsenand Kaufman, 1999), but not unusual considering the dolomiticcomposition of the samples (Gaucher et al., 2007a, and ref-erences therein). No co-variation of �13C with �18O or otheralteration proxies is observed (Fig. 10), confirming the near-primary nature of �13C values for the Mataojo Formation. On theother hand, samples analyzed contain in average only 60 ppmSr (N = 11, maximum = 124 ppm), which is characteristic of dolo-stones (Gaucher et al., 2007a). Therefore, we did not attempt tomeasure 87Sr/86Sr ratios, because these are likely altered by post-depositional processes, unlike the more robust carbon isotopecomposition.

�13C values for the Mataojo Formation are almost invariably

around 0‰ V-PDB, although a shift to negative values down to−2.1‰ V-PDB may be recorded at the top of the unit (Fig. 11). How-ever, only one sample shows negative �13C values (−2.1‰ V-PDB),which is likely related to a higher proportion of silicates (Fig. 11)leading to a metamorphic shift in �13C (Nascimento et al., 2007).

tion at Los Portugueses Quarry (point AIG 11); 34◦18′12′′S, 55◦55′28′′W.

324 L. Chiglino et al. / Precambrian Research 182 (2010) 313–336

Fig. 8. Litho- and chemostratigraphic profiles of the Marco de los Reyes Formation at Floridán Quarry (point AIG 12; 34◦18′19′′S, 55◦56′02′′W) and Robaina Quarry (pointLCN 2; 34◦33′57′′S, 54◦55′02′′W).

L. Chiglino et al. / Precambrian Research 182 (2010) 313–336 325

F es Fos

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ig. 9. Preferred correlation between the different sections of the Marco de los Reyection AIG 12.

The �13C curve obtained is very different from that observed forhe Marco de los Reyes Formation, which is characterized by bothositive and negative values with a total amplitude of ca. 8‰. �13Calues around 0‰ characterize Archean (3500–2450 Ma) and latealaeoproterozoic–early Mesoproterozoic (2000–1300 Ma) car-onates (Veizer et al., 1989; Kah et al., 1999; Bartley et al., 2001,007; Lindsay and Brasier, 2002). Given that the youngest detritalircon dated by Basei et al. (2008) in the Mataojo Formation yieldedn U–Pb SHRIMP age of 1802 ± 59 Ma, we envisage a late Palaeo-roterozoic to early Mesoproterozoic age (1800–1300 Ma) for thenit. This age assignment is further supported by other lines of evi-ence, such as detrital zircon spectra and Nd isotopes (Mallmannt al., 2007; Basei et al., 2008).

.3. Parque UTE Group

Four sections of the PUG were studied: the stratotype, Negroamara Quarry, ANCAP 5 Quarry and an unnamed metamarl quarrypoint FPU 11).

Dolostones of the middle and upper Mina Valencia Forma-ion exhibit relatively low Sr concentrations generally less than00 ppm. On the other hand, dolomitic limestones of the lowerina Valencia and Cerro del Mástil Formations show higher Sr con-

entrations of up to 520 ppm. Mn contents are anomalously high

mean = 1072 ppm, maximum = 5730 ppm), and may represent arimary feature of these carbonates. Mn/Sr ratios are correspond-

ngly high, the samples with Mn/Sr > 3 being mostly dolostonesFig. 12; Table 1). Thus their elevated Mn/Sr ratios do not neces-arily imply post-depositional alteration (Kah et al., 1999; Gaucher

rmation. Note 87Sr/86Sr values consistently between 0.7070 and 0.7079, except for

et al., 2007a). �13C vs Mn/Sr, �13C vs �18O and �13C vs Sr con-centration crossplots (Fig. 12) do not reveal any co-variation ofthese parameters, thus showing that �13C values for carbonatesof the PUG reflect near-seawater values. On the other hand, of 1287Sr/86Sr analyses performed, 11 analyses yielded values between0.7107 and 0.7227, too radiogenic to represent seawater values.A single 87Sr/86Sr determination for limestones of the Cerro delMástil Formation at the stratotype yielded a ratio of 0.7083, whichis here taken as the least altered value, but probably still reflect-ing significant post-depositional resetting (Mn/Sr = 3.6). Therefore,available 87Sr/86Sr determinations cannot be used to constrain thedepositional age of the PUG.

The �13C curve thus obtained for carbonates of the PUG is char-acterized by a brief but well-documented negative excursion to−1.8‰ V-PDB associated to limestones at the base of the MinaValencia Formation (Figs. 13, 14 and 16). A steady rise to posi-tive values of up to +1.6‰ is recorded in the rest of the unit. Theplateau around +1‰ encompasses 150 m of section at the stra-totype (Fig. 13), being the most prominent chemostratigraphicfeature of the PUG (Fig. 17). A single positive �13C value of +2.3‰occurs at the base of gray limestones of the Cerro del Mástil For-mation, followed by progressive decrease to −3.3‰ V-PDB (Fig. 13).These data suggest that the positive �13C plateau of the Mina Valen-cia Formation continues into the lower Cerro del Mástil Formation

and is followed by a well-defined negative excursion up section.The correlation between the studied sections is shown in Fig. 17.

As mentioned above, the age of the PUG is constrained between1429 ± 21 and 1492 ± 4 Ma (Oyhantcabal et al., 2005). It has beenargued that the onset of Mesoproterozoic low-amplitude �13C sec-

326 L. Chiglino et al. / Precambrian Research 182 (2010) 313–336

18O, M

u(stp

5

5

5

tdAdo

Fig. 10. Crossplots of �13C against common alteration proxies (�

lar variations to values near +3.5‰ V-PDB took place after 1300 MaKah et al., 1999; Bartley et al., 2001; Bartley and Kah, 2004). Theections of the PUG described here suggest, however, that depar-ures from the 0‰ baseline were already achieved 150 Myr earlier,ossibly heralding the onset of larger excursions (see below).

. Discussion

.1. Regional geology

.1.1. Lithostratigraphic conundrums: a new approachChemostratigraphic data presented here strongly suggest that

he Mataojo Formation is more than 700 Myr older than the Marcoe los Reyes Formation. Both units are in tectonic contact (Fig. 2).ttempts to date the pre-tectonic boudinaged granites in the Marcoe los Reyes Formation have failed due to the metamictic naturef zircons (Robert Frei, personal communication). If the Neopro-

n/Sr, Sr concentration) for dolostones of the Mataojo Formation.

terozoic age of the latter unit is confirmed, the Carapé Group inthe sense of Bossi et al. (2007) becomes obsolete. According tothe International Stratigraphic Guide, a Group is “a sequence oftwo or more contiguous or associated formations with significantand diagnostic lithologic properties in common” (Salvador, 1994,p. 35). None of these criteria apply to the Carapé Group in viewof the new data. The term “Zanja del Tigre Formation”, introducedby Sánchez Bettucci and Ramos (1999), and “Arroyo Molles Forma-tion” (Zanja del Cerro Grande Group, Oyhantcabal et al., 2005) alsogroup the successions here assigned to the Marco de los Reyes andMataojo Formations in a single unit of formational hierarchy. Forthe same reasons explained above, these terms become also obso-lete. There is no unifying, traditional lithostratigraphic term that

can be applied to the Marco de los Reyes and Mataojo Formations,given the huge hiatus that separates them and the tectonic nature oftheir contact (Salvador, 1994). Bossi et al. (2007) proposed the name“Carapé Tectonic Slab” as a tectonostratigraphic term to group allthe units within that fault-bounded block (see Bossi and Cingolani,

L. Chiglino et al. / Precambrian Research 182 (2010) 313–336 327

Fig. 11. Litho- and chemostratigraphic profile of the Mataojo Formation at Piedrahíta Quarry (FPU 136; 34◦31′37′′S, 55◦11′15′′W) and at San Agustín Quarry (LCN 3; 34◦31′56′′S,55◦06′12′′W).

328 L. Chiglino et al. / Precambrian Research 182 (2010) 313–336

18O, M

2nEtt

oGdm2vMipa

Fig. 12. Crossplots of �13C against common alteration proxies (�

009). In our view, this term continues to be valid, as well as theames of the individual formations (Marco de los Reyes, Mataojo,dén). As we will discuss below, the Carapé Tectonic Slab is a sub-errane of the Nico Pérez Terrane (Figs. 1 and 2), and not a separateectonostratigraphic terrane (see Section 5.2.2).

As discussed in Section 2.2, the Parque UTE Group partly replaceslder, now obsolete lithostratigraphic terms, such as “Lavallejaroup” (Bossi, 1966; Sánchez Bettucci and Ramos, 1999), “Fuenteel Puma Group” (Bossi et al., 1998) and “Fuente del Puma For-ation” (Sánchez Bettucci and Ramos, 1999; Oyhantcabal et al.,

005). All these units have been shown to include successions of

ery different ages in tectonic contact (e.g. Gaucher et al., 2004a;allmann et al., 2007; Basei et al., 2008), greatly hindering research

n that area. The new approach emphasizes lithostratigraphy sup-orted by geochronology (radiometric dating, chemostratigraphy)s the only way of unravelling the geology of a very complex area.

n/Sr, Sr concentration) for carbonates of the Parque UTE Group.

The PUG and its three formations are clearly defined in terms oftheir petrography, structures, geochemistry, isotopic compositionof carbonates and age. The stratotypes of each formation are eas-ily accessible and well exposed within a national park, enablingfuture correlation attempts (e.g. Fig. 17). We are fully aware thatthe extent of the PUG is much larger than shown in Fig. 4, butthis can be improved in the future. The emphasis of future map-ping should be placed at distinguishing Mesoproterozoic rocks ofthe PUG from tectonically interleaved Neoproterozoic volcanicsand metasediments, as well as post-PUG basic sills, as implied bythe U–Pb ages and Nd isotopic data presented by Mallmann et al.

(2007).

5.1.2. CorrelationsThe diagnostic characteristics of the units studied here are sum-

marized in Table 2.

L. Chiglino et al. / Precambrian Research 182 (2010) 313–336 329

c profi

NatTmMds

Fig. 13. Litho- and chemostratigraphi

Other Precambrian carbonate-bearing successions occur in theico Pérez Terrane, for which chemostratigraphic data are avail-ble. The oldest carbonates in the Nico Pérez Terrane were assignedo the Cerros de Villalba Formation (Gaucher et al., 1996, 2006a).

13

his unit is characterized by invariant � C values near 0‰ V-PDB,uch like the Mataojo Formation. In both the Cerros de Villalba andataojo Formations quartz-arenites occur at the base, passing into

olostones up section. On the other hand, a number of differencesuggest that these units are different, namely:

le of the PUG at its stratotype (Fig. 4).

(a) the Cerros de Villalba Formation has been assigned to theNeoarchean, on the basis of U–Pb SHRIMP datings of theyoungest detrital zircon (2762 ± 8 Ma) and zircon overgrowthsof 2721 ± 7 Ma, interpreted to reflect the metamorphic over-

print of the unit (Hartmann et al., 2001);

(b) the Arroyo Perdido Granite intrudes the Cerros de Villalba For-mation and yielded a Rb–Sr isochron of 2001 ± 117 Ma (Gaucheret al., 2006a), consistent with a Neoarchean age for the latterunit;

330 L. Chiglino et al. / Precambrian Research 182 (2010) 313–336

Fig. 14. Litho- and chemostratigraphic profile of the PUG at a metamarl quarry along road 81 (point FPU 11); 34◦33′38′′S, 55◦17′06′′W.

Table 2Typical features and distinguishing criteria of the PUG, Marco de los Reyes and Mataojo Formations.

Feature Mataojo Formation Marco de los Reyes Formation Parque UTE Group

Facies association(protolite names)

Quartz-arenite-dolostone-pelite Limestone-BIF-pelite Basic/acidvolcanics-limestone-dolostone-marl-blackshale

Carbonate type White dolomitic marble Dark gray, laminated, pure metalimestone Gray, dolomitic, clastic limestone and pure,massive dolostone

Mean Mg/Ca carbonate 0.58 (N = 11) 0.053 (N = 45) 0.32 (N = 45)Metamorphism Amphibolite facies Amphibolite facies Greenschist faciesDeformation, dip Moderate, <50◦ High, subvertical Moderate-low, subverticalBoudinaged granite sills No Yes NoLeast altered 87Sr/86Sr Not measured 0.7070–0.7079 0.7083 (secondary)[Sr] ppm Mean = 60 Maximum = 124 Mean = 1882 Maximum = 3056 Mean = 141 Maximum = 521�13C ‰ V-PDB 0 ± 0.5 −3.2 to +4.4 −3.3 to +1.6U–Pb age constraints <1802 ± 59 Ma None 1492 ± 4 Ma (base) 1429 ± 21 Ma (top)Preferred depositional age 1.8–1.3 (1.5) Ga 635–550 Ma 1.5–1.4 Ga

L. Chiglino et al. / Precambrian Research 182 (2010) 313–336 331

Negro

(

tr

iN2Ms+(sf

Fig. 15. Litho- and chemostratigraphic profile of the PUG at

(c) dolostones of the upper Cerros de Villalba Formation are stro-matolitic (Gaucher et al., 1996, 2006a), which has not beenobserved for the Mataojo Formation; and

d) whereas detrital zircon ages for sandstones beneath the Cer-ros de Villalba Formation show two peaks at 2.97 and 2.76 Ga(Hartmann et al., 2001), the Mataojo Formation is characterizedby peaks at 3.2, 3.0–2.8, 2.3–2.2 and 1.8 Ga (Basei et al., 2008).

Thus, despite similar �13C curves, available evidence suggesthat the Cerros de Villalba and Mataojo Formations are not cor-elative, but more work is needed to solve this issue.

The Mina Verdún Group is a late Mesoproterozoic volcanosed-mentary, carbonate-bearing succession occurring in the southernico Pérez Terrane (Poiré et al., 2005; Gaucher et al., 2006b, 2007b,009c). Gaucher et al. (2006b, 2009c) presented a �13C curve for theina Verdún Group, which is characterized by a negative excur-

ion to −3‰ V-PDB at the base, followed by a positive plateau at2‰ V-PDB. The highest �13C values reported by Gaucher et al.2006b, 2009c) are +2.5‰ V-PDB, with a single anomalous analy-is yielding +4‰ V-PDB. This curve resembles that presented hereor the PUG, although in the latter unit �13C is generally lower. In

Tamara Quarry (point FPU 144); 34◦33′59′′S, 55◦15′51′′W.

the chemostratigraphic schemes of Kah et al. (1999), Bartley et al.(2001) and Bartley and Kah (2004), the Mina Verdún Group fits wellinto the group of post-1300 Ma Mesoproterozoic carbonates, sug-gesting that it is somewhat younger than the PUG. Other importantdifferences between the two units are: (a) the stromatolitic, bio-genic nature of carbonates of the Mina Verdún Group, dominatedby Conophyton (Poiré et al., 2005; Gaucher and Poiré, 2009); (b) thescarcity of basic volcanic rocks in the Mina Verdún Group, unlikethe PUG; and (c) the common occurrence of pure and thick lime-stones with up to 95% calcite in the Mina Verdún Group (Poiré et al.,2005; Poiré and Gaucher, 2009), virtually unknown from the PUG.

The chemostratigraphic features that characterize the Marco delos Reyes Formation are virtually identical to those reported for thePolanco Formation of the Arroyo del Soldado Group (Fig. 1; Gaucheret al., 2004c, 2009c). The lower Arroyo del Soldado Group, includ-ing the Polanco Formation, has been assigned to the late Ediacaran

on the basis of acritarch biostratigraphy, occurrence of Cloud-ina, and radiometric ages of the basement and intrusive granites(Gaucher, 2000; Gaucher et al., 2008a; Gaucher and Poiré, 2009).The Polanco Formation records two �13C positive excursions to+5.5‰ V-PDB and +3.3‰ V-PDB, separated by a negative excursion

332 L. Chiglino et al. / Precambrian Research 182 (2010) 313–336

the PU

te�(aF0t0FPseaslecaM2cg

(

(

Fig. 16. Litho- and chemostratigraphic profile of

o −4.5‰ V-PDB (Gaucher et al., 2004c, 2009c). A second negativexcursion is recorded in the uppermost Polanco Formation. The13C curve presented here for the Marco de los Reyes FormationFigs. 6 and 9) is remarkably similar, except for the fact that neg-tive values occur also at the base of the unit, unlike the Polancoormation. Whereas 87Sr/86Sr values range between 0.7073 and.7086 in the Polanco Formation (Gaucher et al., 2004b, 2009c), inhe Marco de los Reyes Formation they vary between 0.7070 and.7079. 87Sr/86Sr is thus marginally lower in the Marco de los Reyesormation, suggesting that the unit may be slightly older than theolanco Formation (Halverson et al., 2007, 2009). However, suchmall differences in 87Sr/86Sr could also be explained by differ-nt degrees of post-depositional alteration of the original ratios,lthough no geochemical or isotopic evidence supports this. Otherimilarities between both units include: (a) the high-Sr nature ofimestones, suggesting an originally aragonitic precursor (Gauchert al., 2009c); (b) the association of limestones with BIF in both suc-essions (Gaucher, 2000; Gaucher et al., 2004c; Bossi et al., 2007);

nd (c) occurrence of thick chert deposits in both the Polanco andarco de los Reyes Formations (Bossi and Navarro, 1991; Gaucher,

000). The main lithological difference is the higher metamorphiconditions attained in the Marco de los Reyes Formation (upperreenschist to amphibolite facies: Rossini, 2002), contrasting with

G at ANCAP 5 Quarry; 34◦30′43′′S, 55◦10′55′′W.

only an advanced diagenesis for the Polanco Formation (Gaucher,2000).

5.2. Global implications

5.2.1. Onset of Mesoproterozoic ı13C oscillationsCarbonates of the PUG record modest 13C enrichment to +1.6‰

V-PDB, with one value reaching +2.3‰. Although these values arelower than those typical of post-1300 Ma carbonates (Kah et al.,1999; Bartley et al., 2001, 2007), they are also distinctively differ-ent from late Palaeoproterozoic–early Mesoproterozoic carbonatescharacterized by �13C invariably around 0‰ V-PDB. Other coevalcarbonate successions also show a similar pattern, such as:

a) The lower Belt Supergroup in the northwestern US yielded pos-itive �13C values of up to +2.5‰ V-PDB for carbonates depositedbetween 1.5 and 1.43 Ga (Hall and Veizer, 1996).

b) Dolostones of the lower Bangemall Supergroup of northwestern

Australia show low-amplitude (<3‰) �13C secular variationsbetween −2 and +2‰ V-PDB (Buick et al., 1995), similar to thatrecorded in the PUG. The age of the lower Bangemall Group isconstrained between 1.62 and 1.47 Ga (Wingate et al., 2002 andreferences therein), thus roughly coeval to the PUG.

L. Chiglino et al. / Precambrian Research 182 (2010) 313–336 333

ons o

(

atKroh

5

sdaea(amPs

Fig. 17. Correlation between the studied secti

c) The upper Espinhaco Supergroup (São Francisco Craton, Brazil)yielded modestly enriched �13C values of up to +1.9‰ V-PDB(Santos et al., 2004). The carbonates there occur stratigraphi-cally on top of dykes dated at 1514 ± 22 Ma (U–Pb on zircon:Pedreira and De Waele, 2008; Danderfer et al., 2009, and refer-ences therein).

The modest �13C excursions at 1.5–1.4 Ga are consistent withgradual, stepwise increase in the amplitude of �13C excursions

hroughout the Mesoproterozoic and Neoproterozoic (Bartley andah, 2004). Thus, it might be stratigraphically meaningful to sepa-ate the low-amplitude �13C interval (ca. 1.5–1.4 Ga) from both thelder static period (0‰ V-PDB) and the younger, post-1.300 Ma,igher-amplitude interval.

.2.2. The Río de la Plata Craton: at the heart of Rodinia?A growing amount of evidence has been recently presented

uggesting the occurrence of Mesoproterozoic rocks in the Ríoe la Plata Craton. This includes Mesoproterozoic detrital zirconges in autochthonous Neoproterozoic sandstone units (Gauchert al., 2008a; Blanco et al., 2009), stromatolite biostratigraphynd carbon isotope chemostratigraphy of the Mina Verdún Group

Poiré et al., 2005; Gaucher et al., 2006b, 2007b, 2009c; Poirénd Gaucher, 2009) and Mesoproterozoic U–Pb ages of synsedi-entary volcanics (Oyhantcabal et al., 2005) here assigned to the

UG. The chemostratigraphic data presented in this paper clearlyhow that two more sedimentary successions should be added

f the PUG. CEFm: Canada Espinillo Formation.

to the list of Mesoproterozoic units: the PUG and the MataojoFormation.

A valid question that needs to be answered is if these sedimen-tary successions are autochthonous to the Río de la Plata Craton. Anumber of authors considered the Río de la Plata Craton devoid ofMesoproterozoic rocks, thus casting doubt on whether the palaeo-continent was part of the supercontinent Rodinia. Detrital zirconages of metasandstones of the Mataojo Formation and PUG, how-ever, clearly show that they were sourced in the Río de la PlataCraton. These are:

(a) U–Pb zircon age spectra for sandstones at the base of theMataojo Formation show a prominent peak at 2.2 Ga, with sub-ordinate peaks at 2.7–3.0, 3.2–3.4 and 1.8 Ga (Basei et al., 2008).All this ages are characteristic of basement rocks of either theNico Pérez Terrane (Archean peaks) or the whole Río de la PlataCraton (2.2 and 1.8 Ga; see Gaucher et al., 2008a, 2009a; Blancoet al., 2009; Bossi and Cingolani, 2009). Thus, it is clear that theMataojo Formation was sourced in the Río de la Plata Craton,mainly in the Nico Pérez Terrane, and is autochthonous to thattectonostratigraphic unit.

(b) Detrital zircon ages are available for two samples of sandstones

now assigned to the PUG. Mallmann et al. (2007) presentedU–Pb SHRIMP ages for 24 zircon grains. Of the total popula-tion, 37.5% are within 2.75–2.65 Ga, 33% between 2.2 and 2.0 Ga,17% between 2.9 and 3.55 Ga, a few ages around 2.55 Ga andthe youngest zircon dated at 1963 ± 10 Ma. All these ages agree

3 ian Re

fCaomor2

arstCK

zdpP

6

frd

P+aaUSBBo�cc

w

34 L. Chiglino et al. / Precambr

well with provenance from the Río de la Plata Craton (NicoPérez Terrane, e.g. Hartmann et al., 2001; Gaucher et al., 2008a,2009a; Blanco et al., 2009). Basei et al. (2008) presented 18U–Pb SHRIMP zircon ages for a sandstone sample just 10 kmto the NE (nearby point FPU 144, Fig. 1) of the outcrop studiedby Mallmann et al. (2007). 39% of the total population yieldedArchean ages between 3.4 and 2.6 Ga, 33% yielded Palaeopro-terozoic ages (2.4–1.8 Ga) and the rest (5 grains) gave youngerages grouped around 1.1 Ga and 700 Ma (Basei et al., 2008). Ofthe latter population the Neoproterozoic detrital zircon ages aremoderately to highly discordant, except for only one grain thatyielded 715 ± 26 Ma. It is interesting to note that the detrital zir-con age spectra obtained by Mallmann et al. (2007) and Basei etal. (2008) for the PUG are very similar, except for the youngerages reported by Basei et al. (2008). The latter are also in con-tradiction with early Mesoproterozoic U–Pb ages obtained byOyhantcabal et al. (2005) for synsedimentary volcanics in thePUG. Regardless of how these younger ages are interpreted,detrital zircon ages reported by Basei et al. (2008) also supportthe autochthonous nature of the PUG with respect to the Río dela Plata Craton.

On the basis of abundant Mesoproterozoic detrital zircon agesound in Ediacaran sandstones autochthonous to the Río de la Plataraton in Argentina and Uruguay, Gaucher et al. (2008a) postulatedproto-Andean, Mesoproterozoic belt fringing the western part

f the craton as the source of these zircons. The hypothetical beltay represent the southward extension of Mesoproterozoic units

f the Amazonian Craton (Sunsás Orogenic Cycle) and/or be directlyelated to the Grenvillian orogen of Laurentia (Gaucher et al., 2008a,009a,b).

The successions studied in this paper were likely unavailables a major source for Ediacaran units, as shown by palaeocur-ents (Gaucher, 2000; Gaucher et al., 2008a). Therefore, it is hereuggested that they represent the remnants of a separate Mesopro-erozoic belt marking the eastern boundary of the Río de la Plataraton, maybe related to the Namaqua-Natal Belt of the westernalahari Craton (Gaucher et al., 2009b).

If the Río de la Plata Craton was fringed by Mesoprotero-oic belts, which are in part characterized by typical Grenvillianeformation ages (1.25 Ga), it may have occupied a quite centralosition within Rodinia, as envisaged by Meert and Torsvik (2003),isarevsky et al. (2008) and Gaucher et al. (2009b).

. Conclusions

Detailed litho- and chemostratigraphic profiles are presentedor three Proterozoic, carbonate-bearing units of the Nico Pérez Ter-ane: the Parque UTE Group, the Mataojo Formation and the Marcoe los Reyes Formation.

A �13C curve was obtained for carbonates of the newly erectedarque UTE Group, which is characterized by a plateau at +1 to1.6‰ V-PDB, bracketed between two negative excursions (−1.8‰t the base and −3.3‰ at the top). This values are consistent withMesoproterozoic depositional age for the unit, as indicated by–Pb ages of synsedimentary volcanics (1492 ± 4 to 1429 ± 21 Ma).imilar �13C curves were reported from the roughly coeval lowerangemall Supergroup in Australia, the Espinhaco Supergroup inrazil and the lower Belt Supergroup in the USA. Thus, a periodf modest 13C enrichment characterized by low-amplitude (<4‰)

13C excursions might herald the stronger �13C oscillations thatharacterize post-1300 Ma Mesoproterozoic and Neoproterozoicarbonates.

The Mataojo Formation (age < 1.8 Ga) is characterized byhite dolomitic marbles associated with quartz-arenites and

search 182 (2010) 313–336

metapelites. In all studied sections �13C varies within a narrowrange around 0‰ V-PDB (−0.6 to +0.4‰). This curve is consistentwith an age between 1800 and 1300 Ma, or if we use the new datafrom the Parque UTE Group, between 1800 and 1500 Ma. The men-tioned time interval is characterized by carbonates with �13C of0 ± 1‰ V-PDB.

In contrast to the other units studied, limestones of the Marcode los Reyes Formation yielded �13C values defining moderate tohigh-amplitude �13C excursions between +4.4‰ and −3.2‰ V-PDB.Corresponding 87Sr/86Sr values range between 0.7070 and 0.7079.Both C and Sr chemostratigraphy point to a later Neoproterozoic,possibly Ediacaran age for the Marco de los Reyes Formation, inagreement with the occurrence of BIF in that unit. A correlation withthe Polanco Formation of the Arroyo del Soldado Group seems plau-sible given the similar chemostratigraphic features of both units,but awaits confirmation through other methods (e.g. detrital zircondating).

The different, Mesoproterozoic and Neoproterozoic ages ofthe Mataojo and Marco de los Reyes Formations indicated bychemostratigraphic data, and the tectonic nature of their con-tact render lithostratigraphic terms that have been used to groupthese units obsolete. Among these are “Carapé Group”, “Zanja delTigre Formation”, “Arroyo Molles Formation”, “Fuente del PumaGroup/Formation” and the already discredited term “LavallejaGroup”.

Finally, one important corolary of the data presented in thispaper is the common occurrence of Mesoproterozoic rocks in theRío de la Plata Craton, which probably fringe it at both its westernand eastern side. This further emphasizes a rather central positionof the craton within Rodinia.

Acknowledgements

Thorough, constructive and very helpful reviews by two anony-mous referees are gratefully acknowledged. Field work waspartially financed by a research project (“Estratigrafía y carac-terización de carbonatos neoproterozoicos del Terreno Nico Pérez,Uruguay”) of CSIC, Uruguay, to C.G. This is a contribution to projectIGCP 478 “Neoproterozoic-Early Palaeozoic Events in SouthwesternGondwana”.

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