Keywords

1 Introduction

Almeida et al. (1973) defined the Río de la Plata Craton to include the ‘ancient cratonic areas’ of the southernmost South American Platform, ‘already consolidated in the upper Precambrian’. The main outcrop regions of the craton are the Piedra Alta Terrane in central and southwestern Uruguay (Oyhantçabal et al. 2011) and the Tandilia System in Argentina (Cingolani 2011), but most of its assumed regional extension is covered by Phanerozoic sediments (Fig. 4.1). The westernmost area of the craton reaches the eastern border of the Pampean ranges near the city of Córdoba, as demonstrated by U–Pb SHRIMP data in zircon from drill cores (Rapela et al. 2007) and magnetotelluric investigations that allow the tracing of the unexposed boundary (Peri et al. 2013; Favetto et al. 2015). To the south, Tohver et al. (2008, 2012) report a Paleoproterozoic age for the Agua Blanca Granite, suggesting that the Río de la Plata Craton may be extended to the Ventania System, approximately 200 km south of the Tandilia outcrops. This interpretation is based on two zircon grains from an unfoliated two-mica-granite, assuming they are not inherited from the source. To the east, the Sarandí del Yí Shear Zone separates the craton from the Nico Pérez Terrane, an allochthonous block with African isotopic signature, amalgamated to the Río de la Plata Craton during the Ediacaran (Rapela et al. 2011; Oriolo et al. 2016a). The northern extension of the craton is also a matter of debate. The Paleoproterozoic basement of the Río Tebicuary area (Paraguay) could represent the northernmost outcrop of the Río de la Plata Craton (Rapela et al. 2007), though Dragone et al. (2017) based on geophysical arguments claim this should be considered a different craton.

Fig. 4.1
figure 1

The main outcrop areas of the Río de la Plata Craton and its boundaries. Interpretation based on Cingolani (2011), Oyhantçabal et al. (2011), Rapela et al. (2011), Peri et al. (2013), Favetto et al. (2015), Tohver et al. (2008, 2012) and Dragone et al. (2017)

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The Río de la Plata Craton is a key piece in the reconstructions of western Gondwana, but its paleogeographic position during the Precambrian is far from being resolved (Rapalini et al. 2015). The collisions with Congo and Kalahari cratons on its eastern margin are related to the evolution of the Dom Feliciano, Kaoko and Gariep Belts, while the collision against the Pampia Terrane on its western margin resulted in the Eastern Sierras Pampeanas. This chapter reviews the recent additions to the knowledge of the craton and discusses its tectonic evolution.

2 Geological Overview

2.1 The Río de la Plata Craton in Uruguay (Piedra Alta Terrane)

The Piedra Alta Terrane of southwestern Uruguay is the region where the craton is best exposed and the only one where its boundary is not hidden by Phanerozoic sediments. The Sarandí del Yí Shear Zone is the tectonic boundary of the craton with the Archean-Paleoproterozoic Nico Pérez Terrane (Hartmann et al. 2001). The latter was formerly thought to be part of the Río de la Plata Craton (Oyhantçabal et al. 2011) (Fig. 4.2).

Fig. 4.2
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Geological map of the Piedra Alta Terrane (Uruguay). Based on Preciozzi et al. (1985), Bossi and Ferrando (2001), Oyhantçabal et al. (2007, 2011)

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The Piedra Alta Terrane is composed of vast granitic gneiss areas separated by supracrustal metamorphic belts. Two main belts have been recognized: the Arroyo Grande Belt (Bossi and Ferrando 2001) which crops out in the northern part of the terrane; and the San José Belt (Bossi et al. 1993b; Oyhantçabal et al. 2003, 2007, 2011) located in southernmost Uruguay (Fig. 4.2).

2.1.1 Arroyo Grande Belt

This belt is a low metamorphic grade supracrustal block bounded by faults, about 15 km in width and 50 km in length. It strikes east–west and is located at the northern edge of the exposed area of the Piedra Alta Terrane (Ferrando and Fernández 1971; Fernández and Preciozzi 1974; Bossi et al. 1993b; Preciozzi 1993). The belt contains the Arroyo Grande Formation, a greenschist facies folded volcanosedimentary assemblage, with the base of the succession in the north and cut across by the Paso de Lugo fault in the south. Siliciclastic rocks predominate, including metarenites, quartzites, metarkoses, metapelites and metaconglomerates. Primary structures are frequently preserved. The metavolcanic rocks are restricted to the southern zone and include metabasalts and meta-andesites, with the paragenesis chlorite + epidote + albite + amphibole + quartz + opaque minerals ± calcite, and metadacites. Observed mineral assemblages indicate greenschist facies metamorphism (Fernández and Preciozzi 1974).

A felsic metavolcanic rock of this formation was dated at 2113 ± 8 Ma, while the age of intrusive post-orogenic granites range between 2108 ± 23 Ma and 2076 ± 18 Ma (U–Pb in zircon, Ferrando, pers. comm., cited by Bossi and Piñeyro 2014; see Table 4.1). LA-ICPMS U–Pb detrital zircon data in metasediments from the Arroyo Grande Formation show a rather simple age pattern with only one maximum at around 2.1 Ga (Basei et al. 2016). Thus a Rhyacian age of ca. 2.1 Ga is indicated for this formation, based on U–Pb data constraints.

Table 4.1 Selection of published zircon and titanite U-Pb SHRIMP, U-Pb LA-ICPMS and U-Pb multigrain geochronology data from the Río de la Plata Craton in the Tandilia Belt (Argentina) and the Piedra Alta Terrane (Uruguay)
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2.1.2 San José Belt

The San José Belt is a low- to medium-grade metamorphic supracrustal sequence exposed in the central and southern area of the Piedra Alta Terrane. It is composed of two formations running nearly east–west separated by the Cufré Shear Zone (Fig. 4.2).

The Paso Severino Formation occurs to the north of the shear zone and is a greenschist facies folded volcano-sedimentary succession including predominant metapelites, and rarely dolomitic marbles and banded iron formations; while orthoderived rocks include metabasalts and meta-andesites, metadacites and metatuffs. U–Pb SHRIMP ages in zircon from a metadacite of the Paso Severino Formation yielded an age of 2146 ± 7 Ma (Santos et al. 2003; see Table 4.1). This is similar to the crystallization age of the felsic volcanic protoliths in the Arroyo Grande Belt. Positive values of δ13C seem to confirm that the dolomitic limestones of Paso Severino formation were deposited during the global Lomagundi isotope excursion and are consistent with the age indicated for this succession by the interbedded felsic volcanic rocks (Maheshwari et al. 2010).

The Montevideo Formation crops out south of the Cufré Shear Zone and includes rocks formed under amphibolite-facies metamorphic conditions. It comprises ortho- and para-amphibolites, paragneisses and micaschists. The paragneisses and garnet and staurolite-bearing micaschists of the San José Formation of Preciozzi et al. (1985) are considered here part of the Montevideo Formation. The orthoamphibolites show geochemical signature similar to mid-oceanic ridge basalts (N-MORB) and partially preserved pillow structures (Oyhantçabal et al. 2003; Pascale and Oyhantçabal 2016).

The Montevideo Belt is divided into northeast and northwest-trending sectors owing to the influence of conjugate north-northwest dextral and east-northeast sinistral shear zones and its easternmost extreme is rotated clockwise as a result of the influence of the Sarandí del Yí Shear Zone. At least two deformation events are recognized in the belt (Campal 1990; Preciozzi 1993; Oyhantçabal et al. 2007). The last event is associated with syntectonic granites and the abovementioned conjugate shear system.

2.1.3 Florida Central Granitic Gneiss Belt

Gneisses and granites make up the bulk of the central area located between Arroyo Grande and San José belts. Decametre- to kilometre-sized blocks of supracrustal rocks are common within the granites and gneisses. They include micaschists, paragneisses and amphibolites. Orthogneisses with similar features are also observed invading the supracrustals of the Montevideo Formation in southernmost Uruguay. These have slightly peraluminous medium-K calc-alkaline compositions (Pascale and Oyhantçabal 2016) and have been dated at 2170 ± 10 Ma (LA-ICPMS in zircon; Basei et al. 2016). A similar age was obtained in the mesosome of migmatites in southwestern Uruguay (2170 ± 24 Ma; LA-ICPMS in zircon; Basei et al. 2016). Ages and field observations indicate that most of these orthogneisses correspond to intrusions coeval with volcanosedimentary basins and that do not represent a basement related to a previous orogeny.

2.1.4 Paleoproterozoic Late- to Post-orogenic Granitoids and Gabbros

Several late- and post-orogenic plutons intrude the Piedra Alta Terrane. Available major element data summarized in Fig. 4.3 indicates that most of the intrusions are calc-alkaline with medium- to high-K content, although some alkaline plutons have also been recognized.

Fig. 4.3
figure 3

Geochemistry of the Paleoproterozoic late- to post-orogenic magmatism in the Río de la Plata Craton. SiO2 versus FeOt/MgO (Miyashiro 1974) a, A/NK versus A/CNK diagram (Shand 1943) b, AFM diagram (Irvine and Baragar 1971) c, SiO2 versus K2O plot (Peccerillo and Taylor 1976) d. Data for the granitoids and gabbros of the Piedra Alta Terrane (Oyhantçabal et al. 2011 and references therein). Tandilia granites (Pankhurst et al. 2003). Tandilia calc-alkaline Dyke Swarm (Iacumin et al. 2001)

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In the San José Belt, SHRIMP U–Pb ages of 2065 ± 9 and 2074 ± 6 Ma were obtained for the Isla Mala pluton (Hartmann et al. 2000) and 2086 ± 4 Ma for the Cufré Granite (LA-ICPMS U–Pb age in zircon, Basei et al. 2016). In the Arroyo Grande Belt, the Marincho Complex yielded a concordia age of 2081 ± 1 Ma (Basei et al. 2016). These similar ages point to a single magmatic event in both areas.

Post-orogenic gabbros are also frequently associated in both supracrustal belts (e.g., Mahoma, Rospide and Carreta Quemada gabbros). U–Pb SHRIMP ages for zircons from the Rospide Gabbro yielded 2076 ± 6 and 2086 ± 7 Ma (Hartmann et al. 2008a). The similar ages of gabbros and granites and the signature of the gabbros in the FeOt/MgO versus SiO2 plot of Miyashiro (1974) point to coeval calc-alkaline magmatism (Fig. 4.3).

Several post-tectonic granites intrude the central granitic gneiss belt. The Cerro Colorado Granite yielded a concordia age of 2078 ± 5 Ma and two inherited concordant zircon grains show ages around 2.2 Ga (LA-ICPMS U–Pb in zircon; Oriolo et al. 2016a). This age is similar to the Rb-Sr whole-rock age of 2071 ± 70 Ma reported for this granite by Cingolani et al. (1990). The Piedra Alta granodiorite yielded a 2110 ± 4 Ma age (LA-ICPMS U–Pb in zircon, Basei et al. 2016). Additionally, Rb-Sr whole-rock data gave similar ages (2015 ± 40 Ma, Piedra Alta granodiorite; 2030 ± 75 Ma, Carmelo granodiorite; 1970 ± 55 Ma, Conchillas granitic gneiss; Umpierre and Halpern 1971). These age constraints and the field relationships (Oyhantçabal et al. 2007) indicate that these intrusions correspond to the same magmatic event responsible for the post-orogenic plutons that intruded into the Arroyo Grande and San José belts.

The Soca granite has compositions similar to that of the rapakivi granites from southern Finland, including high contents of LIL and HFS elements and high FeO*/[FeO* + MgO] ratios (Oyhantçabal et al. 1998), but Nb, Y and Ce contents show it belongs to the A2-type post-collisional granites of Eby (1992). Zircon U–Pb age constraints include a SHRIMP age of 2056 ± 6 Ma (Santos et al. 2003) and a LA-ICPMS age of 2079 ± 8 Ma (Basei et al. 2016). These ages are similar to those obtained for calc-alkaline granites (e.g., Isla Mala, Cufré and Marincho plutons) and confirm that the Soca granite should not be considered an example of anorogenic rapakivi association, but an A2-type rapakivi granite formed after continent collision (Eby 1992; Larin 2009).

In summary, this late- to post-orogenic association of granite and gabbro magmatism occurred at 2080–2050 Ma and includes medium to high K2O calc-alkaline mafic to felsic and felsic alkaline rapakivi intrusions.

2.1.5 Mylonites and Shear Zones

Sinistral shear zones trending 090°–060° and dextral shear zones trending ca. 340° build up a system of conjugate strike-slip faults that controls the architecture of the Piedra Alta Terrane. Sinistral shear zones prevail and some of the best-known examples include the Colonia (Bossi et al. 2005; Ribot et al. 2005; Gianotti et al. 2010) and the Mosquitos shear zones.

The Colonia Shear Zone (Fig. 4.2) comprises two parallel branches striking ca. 090º and cropping out ca. 4 km apart in Colonia City and its surroundings. Subvertical foliations with subhorizontal stretching lineations and kinematic indicators point to a strike-slip wrench component, but frequent sinistral and dextral indicators are found, suggesting pure shear dominated deformation. Microstrucures in quartz and feldspar indicate deformation conditions of c. 450–550º C. The deformation age is bracketed by a K–Ar cooling age of 1796 ± 16 Ma in muscovite (Gianotti et al. 2010) and a U–Pb LA-ICPMS age in magmatic zircon of 2078 ± 9 Ma considered as the age of the protolith (Ribot et al. 2005; Ribot et al. 2013).

Similar time constraints were obtained for the Mosquitos Shear Zone (Fig. 4.2). K–Ar data in muscovite yielded cooling ages that range between 1909 ± 23 Ma and 2049 ± 25 Ma (Oyhantçabal et al. 2006).

2.1.6 Florida Dyke Swarm

The Florida Dolerite Dyke Swarm (also known as Uruguayan Dike Swarm; Teixeira et al. 1999) is more than 100 km in width, trends ca. 060º and extends for more than 250 km from westernmost Uruguay to the Sarandí del Yí Shear Zone in the east. When the dykes approach the shear zone, they are rotated clockwise, being a sense indicator of the first shear event (Oriolo et al. 2015 and references therein).

Individual dykes are subvertical, with a thickness attaining 50 m and a length up to 20 km. Whole-rock geochemistry and petrographic features allow the distinction of high and low TiO2 tholeiitic dykes of andesitic-basalt and andesite composition, respectively (Bossi et al. 1993a). The swarm displays positive εSr and negative εNd values typical of an EM1-type mantle source (Mazzucchelli et al. 1995). A U–Pb age on baddeleyite of 1790 ± 5 Ma is the best estimate of the crystallization age of the high Ti dykes, considered younger than the low Ti dykes (Halls et al. 2001).

2.1.7 Neoproterozoic Record, Sedimentary Successions and Granites

A Neoproterozoic cover is usually lacking in the Piedra Alta Terrane. An exception is a small outcrop area corresponding to the Piedras de Afilar Formation (see Fig. 4.2). The formation comprises low-grade arenites, shales and limestones intruded by dolerite sills (Coronel et al. 1982; Pecoits et al. 2008; Pamoukaghlian 2012). It lies in sedimentary contact above the Coronilla and Soca granites. The detrital zircon pattern shows dominantly Paleo- and Mesoproterozoic sources with the youngest peak at ca. 1009 Ma, which constrains the upper stratigraphic age limit of this unit (Gaucher et al. 2008). The Archean, Paleo- and Mesoproterozoic zircons observed in these sediments suggest that the Nico Pérez Terrrane was most probably the main source. A Neoproterozoic age is also consistent with δ13C values (+5 to +6‰ V-PDB) reported by Pamoukaghlian et al. (2006) and the lack of Ediacaran zircons could indicate that the age is >650 Ma (see discussion in Pecoits et al. 2016).

The sequence strikes north-northwest, with the monoclinal structure dipping towards the southeast, and is intruded by diabase sills showing greenschist facies mineral assemblage (Coronel et al. 1982).

The La Paz Granite is a small intrusion located some kilometres north of the city of Montevideo. It is the only Neoproterozoic granite recognized in the Río de la Plata Craton in Uruguay. Two petrographic facies are recognized in this pluton: porphyritic granite with K-feldspar megacrysts and equigranular granite. The presence of miarolitic cavities and hypersolvus textures in alkali feldspar is indicative of shallow emplacement. The mineralogy is K-feldspar, quartz albite, biotite and amphibole, with epidote, apatite and zircon as accessory minerals. Whole-rock geochemistry data presented by Oyhantçabal et al. (1990) and Abre et al. (2014) show the typical signature of the post-collisional A2-type granites of Eby (1992): metaluminous, high Na2O + K2O content, Eu negative anomaly and low Nb/Y ratio. The La Paz Granite was dated by U–Pb LA-ICPMS in zircon at 587 ± 8 Ma (Cingolani et al. 2012), roughly similar to a previous Rb–Sr whole-rock isochron age of 547 ± 15 Ma (Umpierre and Halpern 1971).

2.2 The Río de la Plata Craton in Argentina: The Tandilia System

2.2.1 Paleoproterozoic Basement: The Buenos Aires Complex

The Buenos Aires Complex (Marchese and Di Paola 1975) is a basement association including large areas of granitic gneisses and small occurrences of amphibolites, high-grade schists and marbles. Mylonite belts up to several kilometres wide crosscut this basement. Additionally, post-orogenic granite intrusions are frequent.

Metamorphic rocks different from the otherwise dominant granitic gneiss are conspicuous in the Balcarce area. These include granulite facies gneisses with orthopyroxene and hornblende, schists, olivine marbles (Punta Tota, Balcarce area), pyroxene-rich ultramafic rocks (Cinco Cerros and Punta Tota, Balcarce area) and migmatites (Fig. 4.4). Metamorphic conditions were estimated at 750–800 °C and 5–6 kb, followed by near isobaric cooling to about 500–450 °C and 5.5–6.5 kb (Delpino et al. 2001; Delpino and Dristas 2008; Massonne et al. 2012). The studied reactions indicate a metamorphic evolution along a counterclockwise P–T path.

Fig. 4.4
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Geological map of the Tandilia System showing the main outcrops of the Paleoproterozoic basement (Buenos Aires Complex) and the Neoproterozoic to lower Paleozoic sedimentary cover. Note the location of the Mar del Plata Terrane and the Punta Mogotes borehole. Based on Cingolani (2011) and references therein

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In contrast, granitic gneisses dominate in the Tandil area, and only scarce outcrops of amphibolites, granulites and charno-enderbites have been found (Dalla Salda et al. 1988, 2005; Hartmann et al. 2002).

2.2.1.1 Orthogneisses

The orthogneisses occur in the Balcarce and Tandil areas, show a distinct foliation and the composition ranges from tonalitic to granodioritic and granitic. The mineral association comprises quartz + K-feldspar + plagioclase + biotite ± amphibole ± garnet ± epidote ± pyroxene ± muscovite ± sillimanite (Dalla Salda et al. 2006). Major and trace element data indicate that the basement orthoderived rocks are of calc-alkaline composition (Dalla Salda et al. 1988, 2006). On the other side, Rapela et al. (2007) present a complete set of geochemical and isotopic data, but from rocks obtained in boreholes located more than 800 km to the northwest of the Tandilia System in the western edge of the Río de la Plata Craton, and they indicate that these rocks are associated with intraoceanic subduction systems or primitive continental arc settings. U–Pb SHRIMP ages in zircon for the orthogneisses of the Buenos Aires Complex range from 2228 ± 6 Ma to 2113 ± 12 Ma (Cingolani et al. 2002; Hartmann et al. 2002). Despite their geochemical features being different, the ages from boreholes near Córdoba (Rapela et al. 2007) are similar to those that have been determined for the Buenos Aires Complex, pointing to events that occurred in a quite narrow timespan.

2.2.1.2 High-Grade Supracrustal Rocks

Two marble bodies were identified within the basement of the Tandilia System. One is in the Punta Tota area, near Balcarce city, described by Delpino and Dristas (2008 and references therein) where thin beds of dolomitic marble are intercalated in amphibolites and constitute the upper part of a layered basement sequence, which starts at the base with garnet migmatites showing a great abundance of pegmatitic segregates. Another marble crops out at the San Miguel quarry, near the town of Barker, and was revisited by Lajoinie et al. (2013, 2014). It occurs associated with prevalent garnet-biotite gneiss and both rocks are intruded by Paleoproterozoic granites. The marbles are whitish and coarse-grained; the calcite + diopside + quartz mineral association indicates upper amphibolite metamorphic facies conditions. The intrusion of thin granitic bodies composed of quartz + plagioclase + K-feldspar generated a wollastonite + vesuvianite + grossular mineral association in the contact aureole. Isotopic δ13C and δ18O data in calcite grains revealed important positive anomalies. Values of δ18O obtained from two diopside populations, proximal and distal to the contact marble-granite, together with the δ18O values in calcite, allow the determination of temperatures of 716 °C for the metamorphic and 451 °C for the metasomatic processes. The concordance of the geological and isotopic characteristics of the marble from the San Miguel area (Lajoinie et al. 2014) with the worldwide record of Paleoproterozoic carbonates, together with the estimated age of the protolith, identify it as a marine carbonate deposited during the ‘Lomagundi-Jatuli event’. This event is also recorded in Uruguay in the San José Belt (Maheshwari et al. 2010), in South Africa (Schröder et al. 2008) and in other Paleoproterozoic areas worldwide.

2.2.1.3 Mylonites and Shear Zones

Shear zones are widespread along the Tandilia System and the associated mylonites are derived mainly from granitic protoliths. In the Azul region, a mylonite belt is developed with east–west strike for about 40 km long and 3 km wide (Fig. 4.4) (Gonzalez Bonorino et al. 1956; Frisicale 1999). Deformation microstructures observed in quartz and feldspars allow an estimation of upper greenschist to amphibolite facies conditions for the deformation (Frisicale et al. 2001, 2005). Analysis of structures and microstructures in this shear zone indicate the deformation regime as pure shear dominated with an important component of flattening and minor wrenching (Frisicale et al. 2001). Other shear belts were recognized south of the Tandil city and in the Alta de Vela Range (Dalla Salda 1981).

2.2.1.4 Post-collisional Granites

The abovementioned metamorphic rocks have been intruded by several granite plutons. These are typically grey granitoids, except in the northernmost part of the complex where the granitoid rocks are reddish, as observed in the Sierra Chica and other quarries near Olavarría city. In the central region of the complex (Tandil area) a wide belt consisting of tonalite, granodiorite and granite occurs. These rocks show magmatic arc affinity, possibly representing syn- to post-tectonic phases of the Paleoproterozoic evolution. The Alta de Vela and Montecristo leucogranites of the Tandil area (Fig. 4.4) are heterogeneous with high radiogenic, typically post-collisional signature (Dalla Salda et al. 2006). The undeformed tonalite in the Chacofi quarry (Balcarce area) was dated at 2073 ± 7 Ma and shows clear intrusive relationships with the country rock (mafic garnet orthogneiss) dated at 2194 ± 6 Ma (Hartmann et al. 2002), thus being a clear indication of the timing of the post-orogenic magmatism.

A garnet-bearing leucogranite and two country rocks from the El Cristo—San Verán hills (Balcarce area) were studied recently by Martínez et al. (2017) using mineral chemistry to decipher the P–T evolution. The leucogranite records an isothermal decompression from 5.3 to 3.8 kbar at 665 °C. The garnet-biotite migmatite was exhumed from 5.5 kbar at 630 °C to 4.3 kbar at 615 °C. Several analyses of monazite grains of the country rocks yielded three groups of U-Th–Pb ages, which these authors relate to a collisional event (ca. 2.13–2.14 Ga.), a post-collisional thermal overprint (ca. 2.01 Ga) and slow cooling of the orogen (1.80–1.90 Ga). Inherited ages of 2.28 and 2.25 Ga could refer to an early accretionary stage of the orogen. These results are consistent with mentioned SHRIMP ages older than 2.1 Ga for the orthogneisses and younger than 2.1 Ga for the post-collisional granites.

2.2.1.5 The Case of Punta Mogotes Formation on the Eastern Side of the Tandilia System

The Punta Mogotes Formation was defined in a borehole near the city of Mar del Plata and includes more than 80 m of mainly slightly deformed metapelites beneath a ca. 400 m-thick quartz-arenite cover (Borrello 1962). The metamorphism in the metapelites was dated by K–Ar in illite fine fraction at around 600 Ma (Cingolani and Bonhomme 1982). These metapelites are the only rocks that could be correlated with the Brasiliano cycle (Ramos 1988) of eastern Uruguay. After Rapela et al. (2011) the Punta Mogotes Formation at the bottom of the borehole contains a 740–840 Ma detrital zircon age peak (obtained in two samples) that is assigned to a widespread Neoproterozoic rifting event. The Punta Mogotes Formation could be correlated with other units that crop out in the Dom Feliciano Belt of Uruguay. These authors suggested, based on isotopic data, the existence of a new terrane, the ‘Mar del Plata Terrane’, about 20 km west of the homonymous city, that would have drifted away from the southwestern corner of the Angola block at ca. 780 Ma. Negative εHf(t) and δ18O > 6.5% values suggest derivation by melting of old crust during a protracted extensional episode.

2.2.2 Neoproterozoic—Lower Paleozoic Sedimentary Cover

Sedimentary processes in the Tandilia System began at ca. 800 Ma with an unconsolidated arkosic saprolite that records a paleoweathering basement surface (Zalba et al. 1993; Dristas et al. 2003). In the currently accepted stratigraphic scheme (see Poiré and Spalletti 2005; Gómez Peral et al. 2007; Poiré and Gaucher 2009; Cingolani 2011 and references therein) the Sierras Bayas Group (ca. 185 m thick) is a Neoproterozoic siliciclastic-carbonate sedimentary sequence. It is overlain by the siliciclastic Cerro Negro Formation (ca. 150–400 m thick) containing the first reliable record of Ediacaran soft-bodied organisms in South America (Arrouy et al. 2016) and the final Lower Paleozoic sedimentary strata, the 100–400 m thick Balcarce Formation. Rapela et al. (2007) reported from this unit new U–Pb detrital zircon age patterns showing detrital zircons as young as 475–480 Ma (Early Ordovician), suggesting a Late Ordovician to Lower Silurian sedimentation age. After detailed provenance studies including mineralogy and geochemistry by Zimmermann and Spalletti (2009), the Balcarce Formation comprises mainly detrital material derived from the basement of the Río de la Plata Craton and Upper Neoproterozoic to Lower Paleozoic igneous rocks. High concentrations of tourmaline and Ti-rich heavy minerals, including zircon and nearly euhedral chromite, are common. The source of chromite may be associated with convergent tectonics causing the obduction of oceanic crust during pre-Upper Ordovician times. Trace element geochemistry of recycled pyroclastic material, associated with the quartz-arenites, also suggests volcanic arc sources.

Diamictite levels were recognized at the Sierra del Volcán (Balcarce area) in between the crystalline basement and the Balcarce Formation. The diamictite is 4 m thick and bears polyhedral dropstones, often in a vertical position, affecting both ripple bedding and lamination structures. Zimmermann and Spalletti (2009) suggested a possible Hirnantian age for this glacial event, as this member of the Balcarce Formation can be correlated with an important glacial event in southern South America and South Africa. Some subalkaline dolerite sills intruded in kaolinitic shales assigned to the Balcarce Formation in the Los Barrientos area (Rapela et al. 1974) yielded whole-rock K–Ar ages of 450 and 490 Ma and represent the youngest event recorded.

2.2.3 Diagenesis and Hydrothermal Activity

In the Tandilia System, evidence for hydrothermal activity (clay deposits, quartz crystals, alunite and iron-rich levels) was recognized several times (Dristas and Frisicale 1988; Dristas et al. 2003). These deposits were interpreted to occur along faults, fractures and breccias through which the hydrothermal fluids percolated. The consequent mixing of hydrothermal fluids with meteoric water gave rise to more oxidizing conditions and to the formation of alunite veins. The iron-rich deposits of the Barker region were studied by Dristas and Martínez (2007) and Martínez et al. (2010). These authors concluded that the deposits were formed at an unconformity by associated large-scale hydrothermal activity under low temperature conditions. Angeletti et al. (2014) obtained a Neoproterozoic U–Pb SHRIMP age of 652 ± 37 Ma in zircon rims and overgrowths interpreted as recording the hydrothermal event. Martínez and Dristas (2006, 2008) demonstrated that the lower part of the Neoproterozoic sequence, along the contact with the basement, was intensively altered by hydrothermal activity (Cuchilla de las Águilas and Cerrito La Cruz). On the other hand, Gómez Peral et al. (2007) divided the diagenetic processes that affected the dolostones from the lower part of the sedimentary cover (Villa Mónica and Loma Negra formations) into several stages. More recently, Martínez et al. (2010, 2013) presented K–Ar ages in clay minerals that confirm the Late-Neoproterozoic age of the hydrothermal fluid activity in the Tandilia Belt.

As recently established by Dristas et al. (2017) in the Barker area, the megabreccias, limestone breccias and phosphate-bearing breccias hosted in black limestone and along the contact with the Cerro Negro Formation are the result of extensive hydrothermal alteration of the original micritic limestone and other fine-grained clastic sediments. Quartz and calcite cements from hydraulic breccias in the limestone contain low-salinity aqueous fluid inclusions. Corresponding homogenization temperatures display 200–220 °C and 110–140 °C in hydrothermal quartz, and 130–150 °C in late calcite cement. Carbon and oxygen stable isotope analyses of carbonates from the Loma Negra Formation support the major role of hydrothermal activity dated ca. 590–620 Ma.

3 Geophysics

The isostatic residual Bouguer anomaly map of the Río de la Plata Craton is shown in Fig. 4.5. Gravity signature confirms most structural features of the Precambrian Basement of Argentina and Uruguay. The Piedra Alta Terrane shows east–west oriented gravity highs matching the San José and Arroyo Grande supracrustal Belts. The Sarandí del Yí Shear Zone coincides with a low gravity lineament, which prolongation can be traced with northwest trend to Argentina. In eastern Uruguay, the gravity signature is characterized by ca. north–south trend, in concordance with the structure of the southern Dom Feliciano Belt and a roughly east–west gravity high related to the Mesozoic Laguna Merín Basin (Fig. 4.5).

Fig. 4.5
figure 5

Bouguer gravity anomaly map of the Río de la Plata Craton. WGM2012 Bouguer anomalies were obtained with GeoMapApp (Ryan et al. 2009; http://www.geomapapp.org)

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A similar east–west pattern is observed in the Buenos Aires province in residual gravity (Miranda et al. 2015) and magnetic maps (Chernicoff et al. 2014). Again, these geophysical features match the structural trend in the basement. In the Tandil area, a magnetic low has been interpreted as a suture zone, the El Cortijo Suture Zone, by Chernicoff et al. (2014). Another possible suture zone, the Salado Suture Zone, was put forward by Pángaro and Ramos (2012) based on Bouguer gravity anomalies. Nevertheless, the similarity of isotope data in all areas of the Río de la Plata Craton indicates that, in case these sutures were confirmed, the involved terranes would be only para-autochthonous.

4 U–Pb Geochronology

Selected U–Pb ages, most of them SHRIMP ages in zircon, from the basement of the Río de la Plata Craton are presented in Table 4.1. The magmatic crystallization ages obtained are always Paleoproterozoic (Rhyacian), in the range 2234–2065 Ma, but some inherited ages from the cores of a few zircon crystals indicate older events: 2368 Ma (Siderian) to 2185 Ma (Early Rhyacian) and only one Neoarchean age of ca. 2657 Ma in a tonalite from the Tandil region. The data suggests that crustal growth started in the Río de la Plata during the Neoarchean, while most of the magmatism is juvenile and occurred during the Rhyacian, in agreement with Hf-Sr-Nd isotope data (see Sect. 4.5). The lack of recrystallization or new zircon growth during the Meso- and Neoproterozoic suggests that the Río de la Plata Craton was preserved from younger events such as those of the Greenville or Brasiliano orogenies.

The U–Pb ages obtained point to two main magmatic events at 2.25–2.12 Ga and 2.1–2.06 Ga. These have been interpreted as two different orogenies—Encantadas and Camboriú, respectively (Hartmann et al. 2002; Santos et al. 2003)—but most probably represent the accretionary and post-collisional phases of the same orogeny (Oyhantçabal et al. 2011; Martínez et al. 2017). In the latter model, ages in the range 2.22–2.1 Ga are considered crystallization ages of the protoliths, generated during the pre- to syncollisional stage, while 2.0–2.1 Ga ages, in agreement with field observations, correspond to the late- to post-orogenic igneous activity. A similar tectonic evolution scheme considering subduction, collision and post-collision stages is envisaged by Chernicoff et al. (2016).

5 Nd and Hf Isotope Data

Sm–Nd model ages show similar values in the Piedra Alta Terrane and the Tandilia System (average 2.43 Ga) and indicate that the main crustal growth episode happened at the Neoarchean–Paleoproterozoic boundary (Fig. 4.6). The Rhyacian U–Pb crystallization ages and the highly evolved signature of gneissic and granitic rocks, derived mostly indirectly from mantle sources, point to a short period of crustal recycling between the Neoarchean and the Rhyacian. The slightly negative εNd(t) values (−0.7 to −3.30) in Tandilia are another proof of a crustal source for most of the granitoids in the Río de la Plata Craton (Hartmann et al. 2002; Pankhurst et al. 2003). Still scarce zircon Hafnium isotope data is consistent with the observed neodymium results (Cingolani et al. 2010; Oriolo et al. 2016a); εHf(t) values are positive to slightly negative and Hf model ages are slightly older than U–Pb ages.

Fig. 4.6
figure 6

εNd(t) versus age plot showing Nd isotopic data from granitoid and orthogneisses from the Piedra Alta Terrane (data from Peel and Preciozzi 2006; Basei et al. 2016) and from the Tandilia Belt (data from Hartmann et al. 2002; Pankhurst et al. 2003)

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The similarity in crustal residence times, together with the abovementioned similarity in U–Pb geochronology, indicates that Piedra Alta and Tandilia terranes are most probably not allochthonous as considered by Bossi and Cingolani (2009), but rather should represent the accretion of para-autochthonous terranes.

6 Constraints on Metamorphic Events, Exhumation and Cooling

As mentioned above, the age of the main metamorphic event in the Río de la Plata Craton is constrained between the age of the metamorphic protoliths (2.2–2.1 Ga) and the age of the undeformed (post-collisional) granites dated at ca. 2.07 Ga. Chernicoff et al. (2015, 2016) reported the first direct dating of the metamorphism at 2120 ± 11 Ma (metamorphic titanite) and interpreted it as corresponding to the collisional stage. Additionally, shear zones in Tandilia and in southern Uruguay evolved under a comparable deformation regime and temperature conditions, and show similar time constraints (Frisicale et al. 2001; Ribot et al. 2005; Gianotti et al. 2010).

K–Ar cooling ages along a transect across the major tectonic units of the Precambrian Uruguayan Shield (Piedra Alta Terrane, Nico Pérez Terrane and Dom Feliciano Belt) were presented by Oyhantçabal et al. (2011). The transect (Fig. 4.7) shows Paleoproterozoic cooling ages for the Piedra Alta Terrane in sharp contrast with the Neoproterozoic cooling ages from the Nico Pérez Terrane and the Dom Feliciano Belt. Likewise, in the Tandilia System the K–Ar and Ar–Ar cooling ages are Paleoproterozoic (Teixeira et al. 2002). The results demonstrate that the Río de la Plata Craton was not thermally overprinted during the Meso- or Neoproterozoic in contrast to the Nico Pérez and Mar del Plata terranes and the Dom Feliciano Belt, which show widespread Neoproterozoic reworking (Cingolani and Bonhomme 1982; Cingolani 2011; Oyhantçabal et al. 2011; Rapela et al. 2011).

Fig. 4.7
figure 7

Time-space diagram of muscovite K–Ar cooling ages across the Piedra Alta Terrane (Río de la Plata Craton), the Nico Pérez Terrane and the Dom Feliciano Belt (east–west profile). X coordinates in kilometres. Origin of coordinates 55º48′W

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7 Proterozoic Mafic Dyke Swarms

Available petrological and geochronological data for the Precambrian dyke swarms of the Río de la Plata Craton is presented in Table 4.2. A calc-alkaline dyke swarm intruded in Tandilia is near-coeval with the post-orogenic granitic plutonism (Teixeira et al. 2002). In Uruguay, a dyke swarm of similar age has not been recognized yet but numerous calc-alkaline gabbros could be the equivalent of this mafic post-orogenic magmatism.

Table 4.2 Summary of available petrological and geochronological data of the Precambrian dyke swarms of the Río de la Plata Craton
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Despite some geochemical similarities, the Florida dyke swarm of Uruguay (1.79 Ga, Halls et al. 2001) and the dykes of Tandilia (1.59 Ga) are not coeval (Teixeira et al. 2013). Nevertheless, both swarms reflect anorogenic extension after the Paleoproterozoic orogeny in the Río de la Plata Craton.

8 Tectonic Evolution

The tectonostratigraphic chart in Fig. 4.8 shows the main events on the different domains of the Río de la Plata Craton. Neoarchean to Early Paleoproterozoic crustal growth is documented by Nd and Hf model ages ranging between 2.6 and 2.1 Ga, while epsilon-(Nd)(t) and -(Hf)(t) positive to slightly negative values suggest a short period of crustal reworking. In an overview of the Tandilia System, Cingolani (2011) showed that the evolution could be represented by a first stage (ca. 2.2 Ga) during which Neoarchean juvenile separated continental blocks (Buenos Aires, El Cortijo and Tandilia blocks) converge. Calc-alkaline orthogneisses dated at ca. 2.2–2.1 Ga are widespread in all areas of the Río de la Plata Craton and should be the result of subduction-related magmatism. A similar model, but implying two subductions, was presented by Chernicoff et al. (2014), who suggested that extension of Neoarchean crust during Siderian times (2500–2300 Ma) caused the separation of the Balcarce, Tandilia and Buenos Aires terranes, and the development of narrow oceans. An island arc represented by the El Cortijo Formation was developed at this time. Evidence for relics of oceanic crust and suture zones is elusive in Precambrian terranes, and other possible suture zones have been suggested in the craton in Uruguay (e.g., the Ojosmin Complex or the Colonia Shear Zone; Bossi and Piñeyro 2004). Whether this magmatism reflects subduction resulting from reamalgamation between para-autochthonous terranes or not is a topic for future research. In any case, if the proposed suture zones were confirmed, the isotopic similarity of involved terranes indicates that they were para-autochthonous.

Fig. 4.8
figure 8

Tectonostratigraphic chart of the Rio de la Plata Craton in Argentina and Uruguay

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During the Late Rhyacian (ca. 2.1 Ga), continent–continent collision occurred, as suggested by thick mylonite belts, and suspected relics of ocean floor rocks. The collision caused thrusting and transcurrent tectonics favouring the anatexis of the crustal rocks. The age of the main metamorphism has been dated at ca. 2.1 Ga in metamorphic titanites from the basement in Buenos Aires city (Chernicoff et al. 2016). This age is bracketed by the protolith age of the orthogneisses (2.2–2.1 Ga) and the age of the post-orogenic granites and gabbros (ca. 2.07 Ga), and it is similar to the age of the shear zones, thus further confirming the timing of the collision. Late- to post-collision granitoids and gabbros (ca. 2.1–2.0 Ga) represent post-orogenic crustal extension.

K–Ar cooling ages indicate Paleoproterozoic cratonization (Hart 1966; Cingolani 2011; Oyhantçabal et al. 2011), consistent with subsequent crustal extension and emplacement of anorogenic tholeiitic dyke swarms (at ca. 1.8 Ga in Uruguay and ca. 1.6 Ga in Argentina). These dykes do not show any metamorphic overprint or deformation, except at the edge of the craton in Uruguay, because of the influence of the Sarandí del Yí Shear Zone (Oriolo et al. 2015).

Afterwards, the basement suffered a long period of cooling, uplift, weathering and peneplanization, and the geological record begins again during the Neoproterozoic with deposits on a stable platform (the Villa Mónica Formation of the Sierras Bayas Group in Argentina and the Piedras de Afilar Formation in Uruguay).

Rapela et al. (2011) indicate, based on U–Pb, Hf and O isotope data in zircons, that the Río de la Plata Craton abuts against a distinct terrane to the east during the Ediacaran: the ‘Mar del Plata Terrane’. As they point out, the Río de la Plata Craton is bounded by transcurrent faults and no Neoproterozoic belts were developed. The assembly of this part of Southwest Gondwana was accomplished before the deposition of the Ordovician (to Silurian?) siliciclastic platform sediments of the Ba1carce Formation in the Tandilia System. A comparable situation is observed in Uruguay, where the Nico Pérez Terrane was accreted to the eastern Río de la Plata Craton along the Sarandí del Yí Shear Zone (Oriolo et al. 2016a, b; Chap. 7) without the formation of a Neoproterozoic belt.

9 Conclusions

A review of the geology of the Río de la Plata Craton, including geochronological, isotopic data and geophysics, reveal the following main conclusions:

  • Nd and Hf isotopic data for the Río de la Plata Craton is characterized by Nd and Hf model ages between 2.7 and 2.1 Ga, and positive to slightly negative epsilon values, thus indicating a single event of crustal growth and a short period of crustal recycling. Eo- Paleo- or Mesoarchean crust-forming events are absent in the craton.

  • Granitic gneisses dated at 2.2–2.1 Ga predominate in the craton and are evidence of a large magmatic event, probably subduction related. The supracrustal successions (e.g., San José and Arroyo Grande belts of Uruguay) are roughly coeval with this magmatism as indicated by detrital zircon data and the age of interbedded lavas, and they may represent intra-arc basins.

  • Crystallization ages of 2.2–2.1 Ga for the metamorphic protoliths and 2.1–2.05 Ga for the granitoids represent pre- and post-collisional magmatism of a Rhyacian orogeny.

  • The uniformity of isotopic signature indicates that if suspected Paleoproterozoic suture zones are confirmed inside the craton, they should represent the accretion of para-autochthonous terranes.

  • Cratonization occurred in Paleoproterozoic times (Statherian) and a lack of Meso- or Neoproterozoic overprinting is a distinctive feature, pointing out that this craton had a thick and strong lithosphere when it was amalgamated with Gondwana in the Neoproterozoic.