Reassessing the polyphase Neoproterozoic evolution of the Punta del Este Terrane, Dom Feliciano Belt, Uruguay

Abstract

Some recent models challenge the position and extension of the assumed oceanic basins formed through the break-up of Rodinia, and the tectonic processes involved in the Gondwana assembly, making the investigation of the Early Neoproterozoic record of great relevance. Within the South-American Atlantic margin, the Punta del Este Terrane (PET) of the Dom Feliciano Belt (DFB) comprises a unique Tonian to Ediacaran record, and has a strategic position to reconstruct spatio-temporal relationships with the southern African orogenic belts. Novel zircon U–Pb and Lu–Hf data from the PET basement orthogneisses display Tonian magmatic ages (805–760 Ma) and Hf isotopic signatures indicative of mainly crustal/metasedimentary sources, (Nd T_DM ages: 2.2–1.9 Ga, and εHf(t): − 12 to − 4). The basement paragneisses yielded late Paleoproterozoic to Neoproterozoic U–Pb ages, but dominantly positive εHf(t) values. The presented results confirm the correlation of the PET with the Coastal Terrane of the Kaoko Belt, and discard the idea of the Nico Pérez Terrane as a source. Detrital zircon U–Pb and Lu–Hf data from the Rocha Formation yielded a main peak at ca. 660 Ma, with the Neoproterozoic grains showing a εHf(t) between + 1 and + 14. The deposition age of the Rocha Formation is constrained by the youngest detrital zircon age peak (660 Ma), and the beginning of the deposition of the Sierra de Aguirre Formation (580 Ma). The data indicate common sources with the Marmora Terrane, and it is thus proposed that the Rocha Formation belongs to the Gariep Belt, and it was juxtaposed during the Ediacaran to the DFB.

Introduction

During the late Neoproterozoic, the interaction of the main cratons that constitutes the present day configuration of South America and Africa, together with several minor crustal blocks, led to the amalgamation of Southwest Gondwana (e.g. Heilbron et al. 2008; Basei et al. 2010; Frimmel et al. 2011; Brito Neves and Fuck 2014; Siegesmund et al. 2018; Konopásek et al. 2020). This process involved several orogenic events and is known as the protracted Pan—African/Brasiliano Orogeny, which extended up to the Cambrian (e.g. Kröner 1984; Rino et al. 2008). From a global perspective, the late Neoproterozoic to Cambrian assembly of Gondwana was the result of the convergence that followed the early Neoproterozoic Rodinia break-up (e.g. Oriolo et al. 2017). However, the participation of many southern South American and African crustal blocks in Rodinia has been challenged and, therefore, the early Neoproterozoic geologic record of these regions may not necessarily record Rodinia break-up (e.g. Cordani et al. 2003; Kröner and Cordani 2003; Oriolo et al. 2017). In this context, Tonian units in Western Gondwana are the key to understanding the transition from the break-up of Rodinia to the assembly of Gondwana.

The Punta del Este Terrane in the southern Brasiliano Dom Feliciano Belt, hosts the record of Tonian tectonomagmatic processes, which have alternatively been interpreted as the result of subduction or continental rifting (e.g. Lenz et al. 2011, 2012; Masquelin et al. 2012; Martil et al. 2017; Konopasek et al. 2017, 2020; Will et al. 2019; De Toni et al. 2021). In addition, this South American Terrane, located close to the African Gariep Belt before the opening of the Atlantic Ocean, is a fundamental piece for reconstructing the spatio-temporal relationships between these orogenic belts. Magmatic (805–760 Ma) and metamorphic ages (660–640 Ma) of the basement of the Punta del Este Terrane, resemble those of the Coastal Terrane of the Kaoko Belt. Both terranes have been previously correlated (e.g. Oyhantcabal et al. 2009; Kónopasek et al. 2017; Will et al. 2019). However, the tectonic setting of the Tonian magmatism has been discussed considerably, mostly based on different interpretations of its geochemical and isotopic signatures. On the other hand, the Punta del Este Terrane has the only metasedimentary successions on the eastern side of the Granite Belt of the Dom Feliciano Belt, among which the Rocha Formation is the most significant (Figs. 1, 2). This low-grade metasedimentary formation has been envisaged as a turbidite sequence, that may be correlated with the Oranjemund Group of the African Gariep Belt (Basei et al. 2005; Abre et al. 2020), but the nature of the basin to which it is associated, its isotopic signature, sources and depositional age are still under discussion.

Fig. 1

Simplified geological map of the Atlantic coastal regions of southern South America and Africa with relative positions reconstructed before the opening of the Atlantic Ocean. Main shear zones (SZs): SYSZ: Sarandí del Yí; SBSZ: Sierra Ballena; SSSZ: Sierra de Sosa; ISZ: Ibaré; CL: Caçapava Lineament; DCSZ: Dorsal do Canguçu; MGSZ: Major Gercino; IPSZ: Itajaí-Perimbó; PiSZ: Piˆen; LCSZ: Lancinha-Cubatão; TPSZ: Three Palms; PuSZ: Purros; ST: Sesfontein Thrust; AF: Autseib Fault; OTT: Otjohorongo Thrust; OL: Okahandja Lineament; SCT: Schakalsberge Thrust (modified after Hueck et al. 2022)

Fig. 2

Geological sketch of the Dom Feliciano Belt. SBSZ: Sierra Ballena Shear Zone, ACSZ: Alférez Cordillera Shear Zone, LRSZ: Laguna de Rocha Shear Zone, SYSZ: Sarandí del Yí Shear Zone, MASZ: María Albina Shear Zone, SSSZ: Sierra de Zosa Shear Zone, CASZ: Cerros de Amaro Shear Zone, RSZ: Rivera Shear Zone, ISZ: Ibaré Shear Zone, DCSZ: Dorsal do Canguçu Shear Zone, PMSZ: Paso do Marinheiro Shear Zone, CSSZ: Caçapava do Sul Shear Zone [modified after Masquelin el al. (2017) and Hueck et al. 2018, 2020a, b)]

In this study, we present novel zircon U–Pb and Lu–Hf data of ortho- and paragneisses of the Punta del Este Terrane basement and the overlying Rocha Formation, combined with geological and structural data. The results are evaluated to determine the timing and isotopic signatures of the main tectonomagmatic, sedimentary and metamorphic events in the Punta del Este Terrane and provide a comprehensive revised tectonic evolution from the Tonian to the Ediacaran.

Geological framework

On both sides of the South Atlantic Ocean, a series of orogenic systems record the amalgamation of Gondwana in the late Neoproterozoic during the Pan African/Brasiliano orogenic cycle (e.g. Heilbron et al. 2008; Basei et al. 2010; Frimmel et al. 2011; Brito Neves and Fuck 2014; Siegesmund et al. 2018; Konopasek et al. 2020; Caxito et al. 2021, 2022). In its southern portion, this system resulted from the interaction between the South American Rio de La Plata craton and the African Kalahari and Congo cratons, together with several minor continental fragments (Fig. 1). This process resulted in the formation of the Dom Feliciano Belt in South America, and the Kaoko, Damara and Gariep belts on the African side (Fig. 1, Frimmel et al. 2002, 2011; Passchier et al. 2002, 2016; Goscombe et al. 2005, 2017a, b, 2018; Konopasek et al. 2005, 2008, 2016; Goscombe and Gray 2008; Frimmel 2018).

The Dom Feliciano Belt (Fig. 2) extends along the South American eastern margin for over 1400 km, from the Florianópolis region in southern Brazil to southernmost Uruguay (e.g. Basei et al. 2000; Philipp et al. 2016; Hueck et al. 2018). Recent studies also show that this belt probably continues southwards in the Argentinian continental shelf, constituting the basement of the Ventania Belt (Rapela et al. 2011; Ballivian Justiniano et al. 2020; Christiansen et al. 2021). It is divided into an Eastern and a Western sector, separated by the Sierra Ballena–Major Gercino transcurrent system (e.g. Oriolo et al. 2016; Masquelin et al. 2017; Oyhantçabal et al. 2018; Hueck et al. 2019). This lineament marks the border between terranes with a series of differences in the geologic signatures, particularly in the ages of the basement, and the geochemistry and isotopic signatures of the Ediacaran magmatism (Lara et al. 2020). As with most of the orogenic belts along the Atlantic margins of South America and Africa, the evolution of the Dom Feliciano Belt has traditionally been interpreted in terms of subduction of oceanic crust and subsequent collision (Basei et al. 2000, 2005, 2008, 2018; Bitencourt and Nardi 2000; Goscombe and Gray 2008; Konopasek et al. 2008; Oyhantçabal et al. 2009; Saalmann et al. 2011; Frimmel et al. 2011; Oriolo et al. 2016; Passchier et al. 2016; Philipp et al. 2016; Goscombe et al. 2017a; Arena et al. 2016; Frimmel 2018; Hueck et al. 2019; Ramos et al. 2018, 2020; Will et al. 2019, 2020). More recently, however, tectonic models arguing intracontinental processes for the region, challenge the extent and location of the assumed oceanic basins between the different crustal blocks (Konopasek et al. 2017, 2020; Percival et al. 2021).

Several minor continental fragments involved in this large orogenic process are preserved as basement inliers, characterized by Archean to Paleoproterozoic ages (Figs. 1, 2, Hueck et al. 2022). Among them, the Nico Pérez Terrane is of particular interest for the evolution of the region. This Archean to Paleoproterozoic block (e.g. Hartmann et al. 2001; Santos et al. 2003; Gaucher et al. 2011, 2014; Oyhantçabal et al. 2012; Oriolo et al. 2016; Masquelin et al. 2021) records several deformation and metamorphic processes related to the amalgamation of southwest Gondwana (e.g. Oriolo et al. 2016; Masquelin et al. 2017; Oyhantçabal et al. 2018), being interpreted as a marginal piece of the Congo Craton during the Neoproterozoic (e.g. Oyhantçabal et al. 2011, 2018; Rapela et al. 2011; Oriolo et al. 2016; Percival et al. 2021; Hueck et al. 2022).

In contrast to these older basement fragments two Tonian–Cryogenian blocks are recognized within the Dom Feliciano Belt. They are the São Gabriel and Punta del Este Terrane, located, respectively, in the western and eastern sectors of the belt (Fig. 2). Both blocks comprise partially overlapping magmatic ages, but have contrasting isotopic signatures in which the first one is predominantly juvenile, whereas the second clearly involved the reworking of crustal sources. The São Gabriel Terrane comprises two main magmatic associations, interpreted as two different magmatic arcs, together with passive margin associations and relict ophiolite complexes (Philipp et al. 2018, 2021). The first magmatic association presents ages ranging from 890 to 860 Ma and is interpreted as an intraoceanic arc (Pasinho Arc, e.g. Phillip et al. 2018 and references therein). The second one represents a juvenile Cordilleran arc, with ages ranging from 770 to 720 Ma (São Gabriel Arc). This last arc magmatism is followed by post-collisional intrusions ranging from 700 to 680 Ma (Phillip et al. 2018).

The Punta del Este Terrane

This terrane represents the main expression of Tonian rocks in the eastern domain of the Dom Feliciano Belt (Figs. 2, 3). Since its original definition (Preciozzi et al. 1999), the boundaries, extension and units that comprise this block have been subject of considerable discussion. The modern definition of the terrane includes all units to the east of the Sierra Ballena Shear Zone (Bosi and Gaucher 2004; Oyhantçabal et al., 2010; Gaucher et al. 2014; Will et al. 2019). These are comprised of (a) a high-grade basement represented by the Cerro Olivo Complex and minor chrono-correlate gneissic inliers and xenolithes along the extension of the Dom Feliciano Belt, such as the Piratini and the Chacará das Pedras gneisses, and the Varzéa do Capivarita and Porto Belo Complexes (Fig. 1, Gross et al. 2006; Koester et al. 2016; Philipp et al. 2016; Martil 2016; Martil et al. 2011, 2017; Cruz et al. 2017; Tambara et al. 2019; De Toni et al. 2020); (b) the low-grade metasedimentary Rocha Formation, the main cover succession of the terrane; (c) the metavolcano-sedimentary Paso del Dragón—Arroio Grande Complex; (d) voluminous Ediacaran granitic magmatism, mostly grouped within the Aiguá Batholith; (e) minor Ediacaran volcanic deposits and basins, the most significant of which is the Sierra de Aguirre Formation; and (f) relics of late-orogenic volcano-sedimentary deposits, represented by the San Carlos Formation.

Fig. 3

Geological map and profile of the main exposures of the Punta del Este Terrane based on Spoturno et al. (2012) and Silva Lara et al. (2021). a Age data from Will et al. (2019), b after Oyhantçabal et al. (2009), c after Hartmann et al. (2002), d Basei et al. (2013). SBSZ: Sierra Ballena Shear Zone, PLPSZ: Puntas de las Palmas Shear Zone, PTSZ: Paso de los Talas Shear Zone, ASZ: Álferez Shear Zone, CSZ: Cordillera Shear Zone, LRSZ: Laguna de Rocha Shear Zone. Granites: γ1—Aiguá granite, γ2—Aiguá mylonitic granite, γ3—Valdivia granite, γ4—Puntas del Arroyo Manantiales mylonitic granite, γ5—Sierra de Garzón granite, γ6—Cerro Catedral Unit, γ7—José Ignacio Granite, γ8—Florencia granite, γ9—Rocha Granite, γ10—Maldonado granite, γ11—Santa Teresa Granitic Complex, γ12—Cerro Áspero Granite, γ13—Puntas del Arroyo Rocha granite, γ14—Sierra de los Caracoles mylonitic granite, γ15—El Pintor granite, γ16—Velázquez granite

The Basement

The Cerro Olivo Complex (Fig. 3) is a high-grade metamorphic complex, consisting of orthoderived and paraderived units (Masquelin et al. 2005, 2010). The complex shows a poliphasic evolution: The main magmatic ages of the orthogneisic rocks range from 800 to 760 Ma, with inherited zircon grains yielding Paleo- to Mesoproterozoic ages (e.g. Oyhantçabal et al. 2009; Lenz et al. 2011; Masquelin et al. 2012). Other minor orthogneissicse units comprised within this block, the Arroio Pedrado, Sarandí de Barceló and Mendizábal gneisses have magmatic ages between 680 and 665 Ma, though their tectonic significance is still unclear (Phillip and Machado 2002; Gaucher et al. 2014; Vieira et al. 2016, 2019; Will et al. 2019).

Granulite-facies metamorphism and subsequent migmatization affected the basement from ca. 660 to 640 Ma (Preciozzi et al. 1999; Gross et al. 2009; Oyhantçabal et al. 2009; Basei et al. 2011; Lenz et al. 2011; Masquelin et al. 2012; Peel et al. 2018; Will et al. 2019; Vieira et al. 2019). This event is coeval with minor mafic sin- to post-deformational magmatism, with ages ranging from 659 to 635 Ma (Lenz et al. 2014).

The stratigraphy of the complex is still controversial, and the relationship between the ortho- and paragneisses is complex. A compilation of the different stratigraphic proposals for the Cerro Olivo Complex is presented by Masquelin et al. (2005), summarizing the results of Masquelin (1990, 2001), Masquelin et al. (2002) and Preciozzi et al. (1993, 1999, 2002). Most of the models propose that the mafic and paraderived rocks are the oldest of the complex, based on limited structural field observations (Masquelin et al. 2001, 2005, 2006).

The Rocha Formation

The Rocha Formation (Betucci and Burgueño 1993) is the main low-grade supracrustal succession of the Punta del Este Terrane. It is a metasedimentary sequence built up of metapelites and metasandstones which underwent greenschist facies metamorphism (Betucci and Burgueño 1993; Menezes 2010). This unit is in tectonic contact with the Cerro Olivo Complex and the Sierra de Aguirre Formation through the sinistral Laguna de Rocha Shear Zone (Masquelin 2005; Silva Lara et al. 2021).

Based on locally preserved sedimentary structures, Basei et al. (2005) proposed a transitional fluvial to tidal plain environment for the deposition of the Rocha Formation. More recently, and based on lithological and geochemical similarities, this unit was interpreted as a turbidite sequence linked to a magmatic arc (Blanco et al. 2014; Abre et al. 2020). Tectonic reconstructions have correlated the Rocha Formation with the Oranjemund Group of the Gariep Belt (Basei et al. 2005; Gaucher et al. 2009; Abre et al. 2020). Constraints on the timing of deposition were first estimated at > 600 Ma based on the youngest recognized cluster of detrital zircon (Basei et al. 2005), whereas more recent contributions have proposed a maximum deposition age of 569 ± 10 Ma (Abre et al. 2020), determined by the youngest detrital zircon.

Though little studied, two contrasting models for the structural style of the unit have been proposed. Masquelin and Tabó (1990) proposed a structural evolution of four main deformation phases: D₁ represents the transposition of S₀ by the cleavage S₁, D₂ that developed isoclinal folds with sub-vertical axes plunging to N210°–230°; D₃ with open folds of sub-vertical axial planes with a N130° trend; and D₄ with chevron folds. On the other hand, Menezes (2010) proposed three main deformation phases: D₁ characterized by open to tight folds with sub-horizontal axes and axial planes with high-angle dips to the NW and SE, and associated S₁ cleavage development; D₂, characterized by open folds with sub-vertical fold axes, affecting the S₁; and D₃, characterized by crenulation cleavage with sub-horizontal axial planes and axes.

The Laguna de Rocha Shear Zone

This shear zone is the tectonic limit between the Cerro Olivo Complex and the Rocha Formation. The Sierra de Aguirre Formation, which overlies the Cerro Olivo Complex, is also in tectonic contact with the Rocha Formation through this shear zone. The shear zone was first described as a cataclastic zone between the Rocha and the Sierra de Aguirre Formation (Masquelin and Tabó 1990). Similarly, Bossi and Navarro (1991) and Campal and Gancio (1993) suggest that the tectonic limit between these two supracrustal units is defined by a sub-vertical brittle fault. Campal and Shipilov (2005) propose that the limit between these two formations is delineated by a mylonitic to cataclastic zone. Masquelin (2005) defines this tectonic contact as a shear zone named the Laguna de Rocha Shear Zone, and extended its significance to the region, proposing that the structure also represents the limit between the Cerro Olivo Complex and the Rocha Formation. More recently, Silva Lara et al. (2021) described the kinematics and time of deformation of the shear zone, based on field features, quartz CPO analysis and K–Ar ages. The structure is a sinistral transcurrent shear zone, with deformation conditions within the greenschist facies. K–Ar ages in syn-kinematic muscovites of the shear zone yielded an age of 564 ± 6.1 Ma, interpreted as syn-kinematic crystallization ages. Clay-sized white mica fractions yielding ages between 535 and 525 Ma, are interpreted as representative of the end of the ductile deformation within the shear zone. These results are similar to the ones reported for the neighboring shear zones (e.g. Hueck et al. 2017, 2020a, b).

The Paso del Dragón–Arroio Grande Complex

The Paso del Dragón–Arroio Grande Complex is another cover succession of the Punta del Este Terrane, classically correlated with the Rocha Formation (e.g. Peel et al. 2018). It comprises two tectonically intercalated units, a metavolcano-sedimentary succession, composed of mica-schists, quartzites and minor metavolcanic rocks, and a metamafic–ultramafic association, with subordinated marbles (Peel et al. 2018; Chaves Ramos et al. 2018). The depositional age of the metasediments of the complex remains controversial. The youngest detrital zircon peak reached a Cryogenian age (660–650 Ma, Gaucher et al. 2014; Beloni et al. 2016; Peel et al. 2018; Chaves Ramos et al. 2018), while zircon crystals younger than 650 Ma are interpreted as associated with metasomatic reactions related to the magmatic activity and the activity of the shear zones (Chaves Ramos et al. 2018). Different ages for the deposition and metamorphism of this unit have been suggested based on the results of both, metasedimentary and metamafic units (Klein et al. 2018; Chaves Ramos et al. 2018; Peel et al. 2018; Will et al. 2019).

The Ediacaran granitic magmatism

Felsic volcanic-intrusive magmatism constitutes an important part of the Ediacaran history of the Punta del Este Terrane, recording a protracted evolution (ca. 630–543 Ma) and frequent links with the large-scale shear zones of the Dom Feliciano Belt. Associated with this granitic magmatism, several minor volcanic units can be found. The main occurrence is the Sierra de Aguirre Formation (Masquelin and Tabó 1990), which mainly comprises volcaniclastic rocks intercalated with minor lava flows (Campal and Schipilov 2005; Silva Lara et al. 2021; Will et al. 2021). These were deposited between ca. 580 and 570 Ma (Hartmann et al. 2002; Silva Lara et al. 2021; Will et al. 2021) in a restricted basin bounded by the Laguna de Rocha Shear Zone (Silva Lara et al. 2021). This formation is interpreted to have been deposited in an extensional or transtensional setting, later inverted by a single-phase folding coeval to the transpressional activity of the Laguna de Rocha Shear Zone (Masquelin 2005; Silva Lara et al. 2021).

Analytical techniques

U–Pb geochronology

An extensive geochronological investigation was carried out to achieve a comprehensive evolution model of the Punta del Este Terrane and, in particular, constrain the deposition and deformation age of the Rocha Formation, one of the major outstanding questions in the region. For that, geochronological analyses were carried out in the basement of the sequence, in the meta sedimentary rocks themselves, and in key intrusive bodies with clear links to the structural evolution of the sequence.

SHRIMP and LA-ICP-MS U–Pb zircon analyses were carried out at the Geochronological Research Center of the Geosciences Institute in the São Paulo University (CPGeo—USP). A total of three SHRIMP analyses and eight LA-ICP-MS analyses were carried out. SHRIMP age determinations were performed in orthogneiss samples from the Cerro Olivo Complex as well as in samples from a rhyolithic dyke intrusive in the Rocha Formation and from the Rocha Granite, whereas the rest of the presented geochronological data correspond to LA-ICP-MS analyses. Analytical procedures are provided in Appendix A and the analytical data in Appendix B, together with the interpretation of each spot analysis. Concordia ages were calculated using Isoplot/Ex 4.15 (Ludwig 2008). For most of the samples, grains which produced > 10% discordant results were not taken into account for the age calculations.

The age spectra of detrital zircon of the analyzed samples in this work are presented as kernel density plots (KDP) and histograms plotted using DensityPlotter (Vermeesch 2012). To avoid acquiring ages younger than the true maximum depositional age due to laboratory or field contamination, as well as lead loss (Coutts et al. 2019; Andersen et al. 2019), we used the youngest peak age calculated considering age probability contributions from at least three analyses, calculated with the AgePick spreadsheet of the University of Arizona LaserChron Center (www.geo.arizona.edu/alc).

Lu–Hf isotopes

Lu–Hf isotopic measurements were carried out at the Geochronological Research Center of the Geosciences Institute of the São Paulo University (CPGeo—USP) using a LA-ICP-MS. Further analytical procedures and results are given in Appendix C. Spot analyses were performed as close as possible to the previous concordant U–Pb spots in the same crystals, and εHf and T_DM parameters were calculated after Yang et al. (2007) using the ²⁰⁶Pb/²³⁸U ages obtained from the corresponding U–Pb spot analyses.

Sm–Nd analysis

Sm–Nd analyses were carried out in three samples from the Rocha Formation at the Geochemistry and Isotope Geology Department of the Georg-August Universität Göttingen, using a thermal ionisation mass spectrometer (TIMS—Finnigan MAT 262) equipped with nine movable faraday collectors that can be paired with nine 1011 and one 1012 Ω amplifiers.

Lithology and structure of the studied units

Cerro Olivo Complex

The main exposures of the Cerro Olivo Complex crop out as an elongated body about 140 km long and 40 km wide at the southeastern portion of the Dom Feliciano Belt (Fig. 3). It is composed of ortho- and paraderived gneisses, which mostly show a metamorphic grade between granulite and upper amphibolite facies. The structural grain presents a general NNE trend, but large deviations from the general trend can be observed locally.

The main lithologies recognized and sampled in this work are: (a) banded felsic orthogneisses, composed of quartz, plagioclase and perthitic feldspar, with variable amounts of biotite and orthopyroxene, protomylonitic overprint; (b) mafic granulites, composed mostly of orthopyroxene, quartz and plagioclase, with hornblende, Ti–rich biotite and clinopyroxene as main accessories, characterized by a granoblastic texture; (c) pelitic gneisses, composed of quartz, plagioclase, microcline, orthoclase, garnet, biotite, and sillimanite, with accessory muscovite, opaque minerals, zircon, apatite, epidote and pyroxene, characterized by a diffuse and sometimes protomylonitic banding; (d) calc-silicate rocks, composed of calcite, wollastonite, quartz, clinopyroxene and garnet. Other lithologies are also present in this unit, such as quartzites, muscovite-biotite gneisses and different kinds of migmatites.

Rocha Formation

Lithological characterization

Outcrops of the formation are scarce and not continuous, with the best exposures being observed at the Uruguayan Atlantic shore. Metapelites are the dominant lithology of the unit, with significant occurrences of metasandstones and metawackes only in the coastal region.

At outcrop scale the metasandstones and metawackes show a general quartzitic composition, with variable amount of detrital muscovite. In the northern portion of the unit, a conspicuous carbonatic component is recognized within the matrix. Petrographically, they are poorly sorted, very fine- to fine-grained rocks with up to 30% of matrix (Fig. 4a, b). They are composed of subangular to subrounded clasts of quartz (70–85%), feldspar (10–20%) and lithics (< 5%). The quartz clasts are either monocrystalline or polycrystalline, showing different internal microstructures, such as undulose extinction, subgrains, and evidence of dynamic recrystallization by means of bulging and sub-grain rotation. The feldspar clasts are albitized plagioclase and K-feldspar. Some grains are also partly or completely replaced by sericite. The lithic clasts are mostly muscovite-quartz-schist fragments. Detrital biotite, totally or partially transformed to chlorite, as well as detrital and neoformed muscovite, were also recognized. The matrix is composed of fine-grained quartz, muscovite and minor feldspar. Epidote, titanite, apatite, tourmaline, zircon, rutile, calcite and magnetite are the main accessory minerals.

Fig. 4

Petrographic and microstructural aspects of the Rocha Formation: a fine-grained, poorly sorted quartz-feldspathic metasandstone, showing a cleavage made up of muscovite; b fine-grained quartz-feldspathic metasandstone showing compositional layering determined by the variation in the muscovite content; c intersection of the bedding and the cleavage in the hinge of a folded metapelite; d layers of different grain size and composition; e Quartz veins parallel to the cleavage showing boudinage in a metapelite; f laminated metapelite with brittle structures

The metapelites show a composition dominated by fine-grained quartz, minor feldspar and variable amounts of fine-grained white mica. This last feature, together with variations in the grain size, determines a primary lamination in the metapelites (Fig. 4c–f).

Structure

In spite of its deformation, some primary structures are still preserved, such as heterolithic structures, lamination, cross lamination and erosive surfaces (Fig. 5a–f). However, these structures are scarce and the whole sequence is strongly deformed, thus preventing the reconstruction of a full stratigraphic column. The Rocha Formation is characterized by two different fold geometries described in this work as F₁ and F₂. F₁ folds are isoclinal and affect the bedding S₀ (Fig. 6a, b). The cleavage (S₁) associated with these folds could not be recognized and is supposed to be transposed by the S₂ (Fig. 6c). The measured F₁ folds show fold axes plunging 70–90° to the 210°. F₂ folds re-fold the S₀ and show tight geometry, with an associated axial plane cleavage S₂. This cleavage constitutes the main reference surface of the formation in most parts of the outcrops, where it is slightly oblique or nearly parallel to the S₀. F₂ folds show axes plunging 5°–20° to the SSW, and axial planes with a general 030° trend, nearly vertical or dipping slightly to the NW or SE. F₂ folds get progressively tighter from E to W, and in the proximity of the Laguna de Rocha Shear Zone they acquire a near isoclinal geometry (Fig. 7).

Fig. 5

Sedimentary features of the Rocha Formation: a Erosive surfaces between sand beds; b heterolithic structures: lenses and intercalation between silt and clay fractions; c lamination defined by alternating layers of silt and fine-grained sand; d terminations of cross bedding sets; e metasandstone layer with diagonal lamination; f beds of sandstone with cross laminations

Fig. 6

Structural features of the Rocha Formation: a folds of the F₂ in metasandstones (dark layers) intercalated within metapelites; b isoclinal fold in metapelites (F₁), the middle band is a quartz vein folded together with the layers; c angular relationship between the cleavage and the bedding; d late En Echelon veins in metasandstones; e) Faulted metapelites; f structural relationship between the dacitic dyke dated in this work (UYS-06–18) and the F₂ of the Rocha Formation

Fig. 7

Structural sketch of the Sierra de Aguirre region, between the approximate coordinates (34°00′S, 54°00′W) and (34°15′S, 54°10 W), showing the geometrical relation between the Sierra de Aguirre and Rocha Formations. The sketch also shows the structural data for both units and the Laguna de Rocha Shear Zone (Modified after Silva Lara et al. 2021)

Quartz veins related to pressure-solution processes are conspicuous and at least two generations can be observed. The older one is strongly deformed and shows boudinage structures parallel to the S₂ (Fig. 4e), while the younger one cross-cuts the foliation and has an en echelon geometry (Fig. 6d). Late structures such as kink bands and faults are sometimes recognized crosscutting the former structures, both sharing a general N–S trend (Fig. 6e).

Relationship to the intrusive bodies

Granitoids that intrude the Rocha Formation cross-cut F₂ structures. On the other hand, no structures indicative of syn-deformation emplacement were recognized in the granites. Skarns are observed in the contact aureoles, showing a variable extension, with assemblages characterized by garnet + biotite + muscovite, biotite + chlorite, and chlorite + actinolite. In addition, a series of undeformed dacitic dykes were recognized discordantly cross-cutting the F₂ of the Rocha Formation (Fig. 6f), with a 045°–030° strike and variable dip/dip direction. They show a thickness of 1–15 m, and a maximum length of 3 km. They are porphyritic, with millimetric phenocrysts of quartz sometimes showing embayed structures, plagioclase and muscovite, and a matrix composed of fine-grained quartz and feldspar with sericitic alteration.

Metamorphic grade and microstructures

The metamorphic grade of the sequence is low, with muscovite as the main metamorphic mineral constituting the cleavage (Fig. 4). In the proximity of the Laguna de Rocha Shear Zone, the mineral assemblages in the metasediments are dominated by chlorite, and a ductile behavior of quartz is characterized by bulging and sub-grain rotation. Similar observations have been made on the neighboring Sierra de Aguirre Formation on the other side of the same shear zone (Silva Lara et al. 2021). The occurrence of biotite as a metamorphic mineral, described by Betucci and Burgueño (1993), was not corroborated.

The S₂ cleavage shows different characteristics, directly related to the grain size and the amount of matrix in the case of the metasandstones. The coarser-grained and matrix-poor sandstones show a less developed cleavage, which is irregular and discontinuous, whereas fine-grained metasandstones and metapelites show parallel and well-developed cleavage (Fig. 4a, b). The main mineral that constitutes the cleavage is fine- to medium-grained muscovite, which is accompanied by minor chlorite. Sometimes the cleavage also has important amounts of fine-grained opaque minerals, suggesting pressure-solution processes.

Quartz veins related to pressure-solution processes are common in the Rocha Formation. They are commonly boudinated and parallel to the cleavage, with dynamic recrystallization of quartz dominated by bulging (Fig. 4e). Some quartz clasts are stretched with the major axis parallel to the cleavage. The presence of calcite patches within the metasandstones is common in the northern portion of the formation. These calcite grains show type I and II twins (Bukhard 1993) and less frequently bent twins. All microstructures observed in the metasediments suggest that deformation was active under temperatures below 400 °C, which is in accordance with the observed lower greenschist facies.

Laguna de Rocha Shear Zone

The Laguna de Rocha Shear Zone is the tectonic contact between the Cerro Olivo Complex and the Rocha Formation. Associated with this tectonic contact is a restricted basin, where the Sierra de Aguirre Formation is deposited. This shear zone has a general NNE–SSW trend, and a sinistral shear sense (Silva Lara et al. 2021, Fig. 8a). It dips steeply to the W in its western portion and variably to the W and the E in the eastern portion. A stretching lineation of quartz and fine-grained white mica was recognized (Fig. 8b), varying from horizontal to plunging around 45° to SSW (Fig. 7). Four types of mylonites were recognized: (a) chlorite-rich mylonites: this lithology is the most common within the shear zone, representing mylonitized schists of the Rocha Formation. Strongly stretched quartz veins are common; (b) quartz mylonites: this lithology crops out mostly in the contact between the Sierra de Aguirre and the Rocha Formation, particularly on the southern portion of the first. It is composed mostly of quartz, with variable amounts of muscovite/chlorite, which determines the mylonitic foliation; (c) mylonitic metasandstones: fine-grained gray rocks, rich in deformed quartz veins and with common isoclinal folds; and (d) granitic and gneissic mylonites: they show porphyroclasts of quartz-feldspar aggregates and a fine-grained matrix composed of quartz, muscovite, biotite and feldspar (Fig. 8a).

Fig. 8

a Gneissic mylonite of the Laguna de Rocha Shear Zone showing sinistral kinematic indicators, looking down, top to the north; b Quartz stretching lineation within quartz mylonites of the Laguna de Rocha Shear Zone, approximately N–S lateral view, lineation dipping to the SW

Samples

A total of 11 samples were analyzed, 4 belonging to the Cerro Olivo Complex, 5 to the Rocha Formation and 2 to the intrusive bodies. The key information of the samples is shown in Table 1, and an extended description in the supplementary material.

Table 1 Sample list

Analytical results

Geochronology of the Cerro Olivo Orthogneisses

Sample UY-12–16 (Orthogneiss)—The zircon grains of the sample are elongated and show regular to well-formed bipyramidal terminations. Oscillatory zoning patterns are a common feature. Most grains show overgrowths with homogeneous textures, and also sometimes homogeneous cores with a stubby geometry. Of the 17 analyzed spots, 15 produced < 10% discordant ages, out of which 6 outlying young ages were interpreted as Pb-loss and discarded from the age calculations. An individual U–Pb age yielding a result of 846 ± 16 Ma is interpreted as inherited. From the remaining spots, two ages were obtained. A concordia age of 773.3 ± 6.7 Ma (95% confidence, MSWD = 0.27, probability of concordance = 0.60), based on four spots measured on the cores of the grains with a magmatic structure, is interpreted as representative of the magmatic crystallization age of the rock. From the other four spots, measured on the rims and cores of homogeneous grains, a concordia age of 650.1 ± 5.2 Ma (95% confidence, MSWD = 0.083, probability of concordance = 0.77) is interpreted as the age of the metamorphic event under granulite facies conditions (Fig. 9a, b).

Fig. 9

Left side: Cathodoluminescence images of representative zircon grains and U–Pb concordia plots for samples dated from the Cerro Olivo orthogneisses. Right side: Concordia diagrams showing the individual results used to calculate the age. The red ellipses represent the concordia age, the black ellipses the grains employed to calculate the concordia age. Error ellipses are plotted at the 2σ level. MSWD: mean square of weighted deviates

Sample UY-13–16 (Paleosome in migmatite)—The zircon grains of this sample show elongated to stubby shapes, with lengths between 250 and 120 µm. Bipyramidal terminations and magmatic zonings are developed in parts of the grains. Broken grains are common. Wide zircon overgrowths are absent, and for that reason the majority of the measurements were performed within the cores of the grains. All 12 analyzed spots produced concordant results, but due to a relatively large range of ages covered between the oldest and the youngest result (ca. 25 My), the mean ²⁰⁶Pb/²³⁸U ages were used as a guide to choose the best-defined population to calculate a concordia age of 781.9 ± 3.2 Ma (95% confidence, MSWD = 1.3, probability of concordance = 0.26), based on ten grains. This age is interpreted as the time of magmatic crystallization of the igneous protolith (Fig. 9c).

Detrital zircon results

The Cerro Olivo Paragneisses

Sample UY-14–16 (Metaquartzite)—The zircon grains of this sample have a size of 50–150 µm and are mostly stubby or fragments of larger crystals. Rounded shapes as well as overgrowths with homogeneous texture are common features. The main characteristics within the cores of the grains are complex oscillatory zoning. Seventy-five analyzed crystals produced concordant results, ranging from 663 to 1924 Ma (Fig. 10). The youngest observed peak is centered at 673 Ma (n = 3), and is interpreted as the age of metamorphism of the paragneiss. The remaining peaks represent main age populations in the Tonian (850–750 Ma, n = 16), Mesoproterozoic (1.3–1.0 Ga, n = 46) and late Paleoproterozoic (1.7 Ga; n = 11).

Fig. 10

Kernel density plots of detrital zircon age data for samples of the Chafalote Metamorphic Suite and the Rocha Formation (Bin width = 50 Ma)

Sample UY-15–16 (Metaquartzite)—The zircon grains of this sample are rounded and have a size of 70–130 µm. The cores of the grains are fragments of larger crystals, either stubby or elongated, with oscillatory zoning, while the rounded overgrowths have a homogenous texture. From this sample, 96 spot analyses produced concordant results ranging from 707 to 1841 Ma (Fig. 10), with the youngest age peak centered at an age of 716 Ma (n = 5). The remaining peaks represent age populations in the Tonian (900–790 Ma, n = 15), Mesoproterozoic (1.4–1.0 Ga, n = 55) and Late Paleoproterozoic (1.7 Ga, n = 6).

Sample UY-19–16 (Paragneiss)—From this sample a total of 58 concordant results were obtained in different zircon crystals, ranging from 2909 to 643 Ma (Fig. 10). The youngest peak yielded an age of 652 Ma (n = 12), followed by significant populations in the Tonian (930–770 Ma, n = 25); Mesoproterozoic (1.1–1.0 Ga, n = 6), and Paleoproterozoic (1.8 Ga; n = 3).

The Rocha formation

Sample UY-01–16 (Fine-grained metapelite)—The analyzed detrital zircon grains are 50–200 µm long and mostly show prismatic shapes with oscillatory zoning. From this sample, 59 concordant results were obtained from different zircon crystals, with ages ranging from 1743 to 645 Ma (Fig. 10). The resulting ages show a near-unimodal distribution, with a main peak at 664 Ma (n = 43), and a subordinated Tonian peak (867–771 Ma, n = 6).

Sample UY-05–16 (Laminated Metapelite)—The analyzed detrital zircon grains are prismatic and 50–150 µm long. They exhibit oscillatory zoning as their main feature. Rounded grains, broken grains and grains with patchy textures or inclusions also occur within the sample. Fifty-four crystals from this sample produced concordant results, with ages ranging from 2435 to 661 Ma (Fig. 10). The youngest age peak of this sample is centered at 668 Ma (n = 12), with subsequent age peaks in the Tonian (821–722 Ma; n = 26) and Mesoproterozoic (1.0 Ga, n = 6), accompanied by grains yielding disperse Meso- to Paleoproterozoic individual ages.

Sample UY-07–16 (Fine-grained metapelite)—Only three grains of this sample were recovered, yielding concordant individual ages of 657, 696 and 2064 Ma (LA-ICP-MS). These ages will be added to the compilation of all the results of detrital zircons obtained for the Rocha Formation in the discussion.

Sample UY-53–18 (Fine-grained metasandstone)—Only ten detrital zircon grains from this sample were recovered, with ages ranging from 1999 to 604 Ma. As in the case of the former sample, these results will be added to the compilation of detrital zircon ages in the discussion.

U–Pb geochronology of the Ediacaran magmatic rocks

Sample UY-06–18 (Dacitic dyke intrusive to the Rocha Formation)—Zircon grains belonging to this sample have a variable grain size between 70 and 350 µm. They mostly show prismatic elongated shapes, with bipyramidal terminations and oscillatory zoning. Some grains are broken or show cores with complex zoning. Out of the 14 analyzed spots, 11 produced < 10% discordant ages. Of these, three outlying young ages were discarded for the calculations, and interpreted as the result of Pb-loss. From the remaining points a concordia age of 565.1 ± 4.6 Ma was calculated (2σ, MSWD = 0.54, probability of concordance = 0.46), and interpreted as representative of the magmatic crystallization age of the sample (Fig. 11a). Two concordant grains yielding individual ²⁰⁶Pb/²³⁸U ages of 618 Ma and 648 Ma are interpreted as inherited.

Fig. 11

Concordia diagrams for analyzed zircons from samples belonging to the Ediacaran magmatism of the Punta del Este Terrane. Left diagrams: red ellipses represent the inherited grains, black ellipses the grains employed to calculate the concordia age, and the dotted black ellipses represent Pb-loss. Right diagrams: the dotted red circles represent the calculated concordia age. a Sample UY-06–18, dacitic dyke; b Sample UY-16–16, Rocha granite

Sample UY-16–16 (Rocha Granite—syn-kinematic to the Cordillera Shear Zone)—The zircon grains belonging to this sample are elongated and prismatic, with a size of 80–300 µm. They commonly show oscillatory zoning patterns, and broken terminations and crystal fragments are also common. From the 12 analyzed spots, only four gave < 10% discordant ages. From those points, two of them produced outlying ²⁰⁶Pb/²³⁸U individual ages. The younger individual outlying age of 441.4 ± 6.9 Ma was interpreted as the result of Pb-loss, and the oldest outlying age of 656.0 ± 11.3 Ma was interpreted as inherited. The remaining two concordant results were combined with two of the less discordant measured spots (< 20%), resulting in a concordia age of 557.6 ± 2.9 Ma (2σ, MSWD = 1.8, probability of concordance = 0.18) that is interpreted as the timing of the magmatic crystallization of the rock (Fig. 11b).

Zircon Lu–Hf isotopes

The Cerro Olivo Complex

The two analyzed orthogneiss samples (UY-12–16 and UY-13–16) show relative homogeneous results, with ¹⁷⁶Hf/¹⁷⁷Hf_(i) values ranging from 0.28198 to 0.28216, εHf_(t) from − 12 to − 4, and T_DM ages from 2.2 to 1.9 Ga. These values suggest a crustal signature for the magmatic protoliths of the gneisses (Fig. 12).

Fig. 12

New U–Pb versus εHf zircon data for the Cerro Olivo Complex, Ediacaran granitoids intrusive in the Punta del Este Terrane, and the Rocha Formation. Data of the Sierra de Aguirre Formation from Silva Lara et al. (2021)

On the other hand, the analyzed zircon grains in the paragneiss samples (UY-14–16 and UY-15–16) show two populations with contrasting characteristics. Tonian grains with U–Pb ages of 980 to 772 Ma yield ¹⁷⁶Hf/¹⁷⁷Hf_(i) values between 0.2818 and 0.28223, corresponding to slightly negative εHf(t) values between 0 and − 12 and Paleoproterozoic model ages (T_DM from 2400 to 1700 Ma). In contrast, zircon grains yielding Meso- and Paleoproterozoic U–Pb ages show ¹⁷⁶Hf/¹⁷⁷Hf_(i) values from 0.28169 to 0.28225, predominantly slightly positive to positive εHf_(t) values from 0 to 10 (except for two grains with negative values), and Paleoproterozoic to Neoarchean model ages (T_DM 2600–1600 Ma). The characteristics of the Tonian zircon grains point to a source with a more pronounced crustal signature, while the Meso to Paleoproterozoic zircon grains seem to be related with a dominantly juvenile source.

The Rocha formation

Lu–Hf data were obtained in zircons from two samples of the Rocha Formation (UY-01–16 and UY-05–16) to complement the regional whole-rock Sm–Nd database. The analyzed zircon crystals have ¹⁷⁶Hf/¹⁷⁷Hf_(i) values ranging from 0.28079 to 0.28276, showing a distribution in two groups of εHf_(t) values, depending on the individual U–Pb ages of the crystals: Neoproterozoic zircon grains show positive to strongly positive values, ranging from 1 to 14, while zircon grains yielding Paleoproterozoic U–Pb individual ages show slightly negative to negative values, from 0 to − 14 (Fig. 12). T_DM ages also show a similar contrast: Neoproterozoic zircon grains show either Neoproterozoic or Mesoproterozoic model ages, while model ages for the Paleoproterozoic grains are Neo- to Paleoarchean.

Whole-rock Sm–Nd isotopes of the Rocha formation

The isotopic whole-rock Sm–Nd ratios of three samples from the Rocha Formation were analyzed (UY-01–16, UY-03–16 and UY-05–16). Results are shown in Fig. 13 and listed in Appendix D. The samples show εNd₍₆₀₀₎ values between − 1.32 and 1.47, whereas T_DM model ages are between 1.30 and 1.52 Ga.

Fig. 13

U–Pb versus εNd data for different units of the Punta del Este terrane. Data from: a Abre et al. (2020), b Oyhantçabal et al. (2011), c Silva Lara et al. (2021), d Vieira et al. (2019), e Lenz et al. (2011), f Lara et al. (2020), g Martil et al. (2017)

Discussion

The Cerro Olivo Complex: origin and setting

Orthoderived rocks

Conflicting tectonic interpretations like rifting or a continental arc have been proposed for the Cerro Olivo Complex, mostly based on geochemical characteristics (Basei et al. 2010; Masquelin et al. 2012; Lenz et al. 2012; Koester et al. 2016; Konopasek et al. 2017; Martil et al. 2017; Tambara et al. 2019; Will et al. 2019; De Toni et al. 2021). Though not conclusive, the geochemical compositions show several characteristics suggestive of an arc setting, such as the general tonalitic–granodioritic nature of the rocks, moderate to high Ba, moderate to low Nb, the REE patterns and contents (ΣREE = 186 ppm in average), Ce_N (mean = 90), Ce_N/Yb_N (mean = 9), and La_N/Yb_N (mean = 11) values, as well as the average slightly negative to negative Eu anomalies indicative of processes of plagioclase fractionation in the source and/or assimilation (i.e. Lenz et al. 2011; Martil et al. 2017). Furthermore, the presence of high-K calc-alkaline compositions in the complex is also indicative of magmas that had either interaction with mica-rich metasedimentary rocks (e.g. Patiño-Douce 1999), or magmas related with partial melting of metagranitic and metasedimentary sources (e.g. Brown 2013; Chappell and White 1992; Chappell et al. 2012; Frost and Frost 2011; Sawyer et al. 2011) or even magmas with a metasomatized mantle source (e.g. Kelemen et al. 1993; Pearce et al. 1990; Qin et al. 2018; Turner et al. 1996).

The newly reported crystallization and metamorphism ages in this study for the orthogneisses fall within the age span of the published ages of the Cerro Olivo Complex (Hartmann et al. 2002; Oyhantçabal et al. 2009; Lenz et al. 2011; Masquelin et al. 2012; Peel et al. 2018; Will et al. 2019), namely, 800–760 Ma for the magmatic stage of the complex and 660–650 Ma for the high-grade metamorphism. On the other hand, the new Lu–Hf isotopic compositions of the orthogneisses in the present study point to an important contribution of an older crust in the generation of the magmas, with negative εHf values between − 4 and − 12, and Paleoproterozoic T_DM ages between 1.9 and 2.2 Ma. These new εHf results are supported by previously obtained Sm–Nd data, with εNd between − 1.8 and − 6.7 and T_DM ages between 1.3 and 2.1 Ga (Lenz et al. 2010; Konopasek et al. 2017, Figs. 12, 13). This crustal signature has been previously interpreted as related to the assimilation/recycling of a Paleoproterozoic–Archean basement (Koester et al. 2016; Martil et al. 2017; Petrolli 2017; Hueck et al. 2022), but also due to the assimilation of sediments/supracrustal rocks (Will et al. 2019). Since there is no clear pre-Tonian basement related to the Cerro Olivo Complex, the participation of a non-outcropping or completely reworked basement is hard to explain but cannot be ruled out. Thus, this crustal component seems to be strongly influenced by the assimilation of sedimentary/metasedimentary rocks, which also accounts for the general geochemical fingerprint, the isotopic fingerprint of ortho- and paragneisses and previously reported structural relationships (Masquelin 1990, 2001, 2005; Masquelin et al. 2002, 2006; Preciozzi et al. 1993, 1999, 2002).

It has been suggested that melting of the Mesoproterozoic metabasic lithologies of the Nico Pérez Terrane with T_DM up to 1.8 Ga (Mallmann et al. 2007; Oriolo et al. 2019) could be the source of the Cerro Olivo Complex protoliths (Konopasek et al. 2017). However, these units are gabbros which show geochemical signatures coherent with an intraplate setting (Oriolo et al. 2019). If they were indeed the source of the magmatic protoliths of the Cerro Olivo Complex, the idea of an inherited arc-related geochemical signature for the complex, as proposed in Konopasek et al. (2017), seems to be contradictory. On the other hand, these metabasic units show crystallization ages of ca. 1.5 Ma (Oyhantçabal et al. 2005; Oriolo et al. 2019), considerably older as the dominant ages of inherited zircon crystals within the orthogneisses, namely 1.0–1.3 Ga (Lenz et al. 2011; Masquelin et al. 2012). These Late Mesoproterozoic ages are uncommon in the Nico Pérez Terrane, and, therefore, this terrane is discarded as a source for the magmas of the orthogneisses of the Cerro Olivo Complex.

Paraderived rocks

Age distributions of detrital zircons from the paraderived rocks of the Cerro Olivo Complex, show main peaks at ca. 660–650 Ma and 800–770 Ma (Konopasek et al. 2017; Peel et al. 2018, this work, Figs. 10, 14), corresponding, respectively, to the metamorphic peak of the complex and the main magmatic stage recorded in the orthoderived rocks. Additional peaks at 1.0–1.3 Ga are younger than the known Mesoproterozoic ages of the Nico Pérez Terrane. Paleoproterozoic and particularly Archean populations, which correspond to the main signatures of the basement units of the Nico Pérez Terrane, are also proportionally much less common than in the metasedimentary cover sequences of the latter (Oyhantçabal et al. 2020 and references therein, Fig. 14).

Fig. 14

Compilation of the available U–Pb individual zircon ages and Hf TDM model ages for the units of the Punta del Este Terrane. The light orange band represents the 630–670 Ma age span, and the dark orange line the 760–800 Ma span. Data from: the present study, Oyhantçabal et al. (2009), Lenz et al. (2011), Masquelin et al. (2012), Gaucher et al. (2014), Koester et al. (2016), Konopasek et al. (2017), Peel et al. (2018), Will et al. (2019), Abre et al. (2020), Lara et al. (2020), Vieira et al. (2019), Silva Lara et al. (2021) and De Toni et al. (2021)

On the other hand, one of the analyzed samples shows a peak at 716 Ma, an age between the main magmatism of the complex and the metamorphic peak. Equivalent ages have not been found in previous studies implying that younger sediments are also involved to some extent in this complex, in spite of the reported structural relationships between most ortho- and paragneisses. This age is difficult to assign to a certain source, but it could be related to the late stages of the magmatism in the São Gabriel Terrane, namely at ca. 720 Ma (e.g. Phillip et al. 2018).

Furthermore, isotopic signatures of detrital zircons yielding Neo- to late Paleoproterozoic U–Pb ages show dominantly positive εHf values and Meso- to Paleoproterozoic T_DM model ages (Figs. 12, 14), contrasting with known data for the cover successions of the Nico Pérez Terrane, where the Archean component is always well-represented and the εHf values of zircon yielding Neo- to Paleoproterozoic U–Pb ages are dominantly negative (Oyhantçabal et al. 2020 and references therein). Consequently, the Nico Pérez Terrane basement probably did not act as an important source area for the analyzed paragneisses of the Cerro Olivo Complex, in contrast with previous proposals (Konopasek et al. 2017).

Relationship between ortho- and paragneisses and tectonic implications

In spite of the crustal signature of the Cerro Olivo Complex, there is no clear basement recognized for it, but some features point to metasedimentary rocks as an important source for this Tonian magmatism. The geochemical characteristics of the complex point to an arc-like setting, but with parameters suggesting that the sources of the magmatism were influenced by metasedimentary rocks, such as the high K₂O contents, and peraluminous compositions in the A/CNK vs. A/NK diagram (see i.e. Masquelin et al. 2012; Tambara et al. 2019). Similar inferences were postulated by Will et al. (2019), taking into account the Hf–O isotopic composition of zircons and whole-rock Nd fingerprint. Furthermore, the trend observed in the Lu–Hf values between ortho- and paragneisses, also suggest a process of assimilation of the metasedimentary rocks by the original magmas (Fig. 12).

On the other hand, the presence of a large Tonian peak in the detrital zircon spectra of the paragneisses, with a similar age to the main magmatic stage of the Cerro Olivo Complex is contradictory with a within plate scenario as suggested by previous authors (Basei et al. 2010; Konopasek et al. 2017; Will et al. 2019, 2021), in which zircon grains coeval to the extension are scarcely or not represented in the associated cover successions (Cawood et al. 2012). Furthermore, the mainly tonalitic–granodioritic nature of the complex is also unlikely in a rift setting, where a bimodal or mainly mafic magmatism would be expected.

Lu–Hf results for the ortho- and paragneisses are dissimilar, with mostly positive εHf values for the Neo- to Paleoproterozoic zircons in the paragneisses, whereas the Neoproterozoic zircons of the orthogneisses are homogeneously negative. Since the known signatures for the Mesoproterozoic units of the Dom Feliciano Belt have a mix of crustal and mantellic signals, but with a tendency to crustal signals, it is suggested that these zircons have a distal source.

On the African side, very similar Tonian magmatic ages can also be found in the Coastal Terrane of the Kaoko Belt (Fig. 15, e.g. Kónopasek et al. 2008, 2017; Will et al. 2019). This correlation between both crustal blocks has already been envisaged not only based on the age spectra (e.g. Oyhantcabal et al. 2009; Kónopasek et al. 2017; Will et al. 2019), but also on the geochemistry of both, and is further supported by high-T/low-P granulite facies metamorphism at ca. 650 Ma (e.g. Goscombe and Grey 2007; Gross et al. 2009). When compared, detrital zircon spectra of the paraderived rocks of both blocks also show striking similarities, strengthening the already envisaged correlation between both blocks (Fig. 16). Furthermore, they also share a coeval Ediacaran granitic magmatism (e.g. Konopasek et al. 2005; Goscombe et al. 2005).

Fig. 15

Comparison between magmatic and metamorphic ages of the Punta del Este, Coastal and São Gabriel terranes. The pink error bars represent the magmatic ages and the violet ones the metamorphic ages. The ages of the Camaquã basin are represented as a single error bar to simplify the diagram. The data used for the Coastal and the Punta del Este terranes just comprise U–Pb ages and the data plotted for the São Gabriel Terrane include Sm–Nd ages (ages in mafic–ultramafic rocks). Data from: the present study, Cordani et al. (1973), Teixeira (1982), Soliani (1986), Silva and Soliani (1987), Machado et al. (1990), Remus (1991), Babinski et al. (1996), Franz et al. (1998), Leite et al. (1998), Remus et al. (1999), Seth et al. (1998), Silva et al. (1999), Basei et al. (2000, 2010, 2013), Hartmann et al. (2009, 20112002), Goscombe et al. (2005), Philipp et al. (2008, 2014), Konopasek et al. (2008, 2016, 2017), Oyhantçabal et al. (2009), Bongiolo et al. (2010), Bossi and Gaucher (2010), Lenz et al. (2011, 2014), Masquelin et al. (2012), Pecoits et al. (2012), Gaucher et al. (2014), Koester et al. (2016), Gubert et al. (2016), Peel et al. (2018), Klein et al (2018), Chaves Ramos et al (2018), Will et al. (2019), Tambara et al. (2019), Vieira et al. (2019), Lara et al. (2020), Ballivan Justiniano et al. (2020), and Silva Lara et al. (2021)

Fig. 16

Compared detrital zircon spectra of the paraderived rocks from the Punta del Este and Coastal terranes. COCx—Cerro Olivo Complex, CT—Coastal Terrane. Data from: the present study, Konopasek et al. (2014, 2017)

A possible alternative to reconcile apparently contradictory signatures of both, arc and rift settings, is an extensional arc, in a retreating accretionary orogeny. The geochemical characteristics of the orthogneisses, mainly arc-like, but with some extension characteristics as pointed out by Konopásek et al. (2017), could be explained by this setting. Results also indicate an important component of contamination related to the sediment assimilation, which may result from the mixing of these older crustal sources, with Tonian juvenile inputs, as proposed by Hueck et al. (2022).

This subduction scenario with upper plate extension can not only account for the particular signatures of the Cerro Olivo Complex, but also explains the presence of the partially coeval but very dissimilar São Gabriel Terrane (Fig. 17), which is characterized by Tonian magmatism dominated by juvenile mantle-derived contributions in an arc setting (e.g. Philipp et al. 2018; Hueck et al. 2022). An early subduction over the border of the Congo Craton, represented by the Punta del Este and Coastal Terrane might have resulted in localized crustal extension in the arc and back-arc domains, recorded within minor crustal fragments represented by the basement inliers of the Dom Feliciano Belt (Figs. 1, 17, see Hueck et al. 2022 for further details). Subduction led to the accretion of the São Gabriel juvenile association to the continental margin. This accretion is suggested as having occurred between 770 and 760 Ma, when magmatism in both blocks overlap in age (Figs. 16, 17). Afterwards, further magmatism took place in the new active margin up to 720 Ma (e.g. Phillip et al. 2016, 2020; Hueck et al. 2022). Magmatism with post-collisional characteristics is found in both terranes from ca. 700 Ma up to ca. 680 Ma (e.g. Phillip et al. 2018; Vieira et al. 2019), but magmatic ages in felsic gneisses can be found in the Punta del Este Terrane up to the age of the main metamorphic peak, namely ca. 665 Ma (e.g. Gaucher et al. 2014), which probably represents the same magmatism.

Fig. 17

Schematic model for the evolution of the Punta del Este terrane. SABF´s: South American Basement fragments, PET-CT: Punta del Este—Coastal Terrane, SGT: São Gabriel Terrane, RPC: Rio de la Plata Craton, L-PCx.: Lavalleja—Porongos Complex, CC: Congo Craton, KC: Kalahari Craton, RGB: Rocha—Gariep Belt, SBSZ: Sierra Ballena Shear Zone, SYSZ: Sarandí del Yí Shear Zone

The high-grade metamorphic event of the Cerro Olivo at ca. 660–640 Ma corresponds to the earliest record of the Brasiliano Orogeny in the Dom Feliciano Belt. The main phase of crustal shortening and high- to medium-grade metamorphism of the latter is constrained up to ca. 600 Ma, and was succeeded by sinistral shearing along orogen-parallel shear zones (Oriolo et al. 2016; Hueck et al. 2019; De Toni et al. 2021). Though some authors suggested a main phase of the deformation and metamorphism at ca. 550 Ma for the Dom Feliciano Belt (Frimmel 2010; Frimmel et al. 1998, 2002, 2011; Frimmel and Fölling 2003), these events are mainly recorded at the southeasternmost region (e.g. Rocha Formation) and temporally overlap with the final post-collisional magmatic and sedimentary processes of the Dom Feliciano Belt (e.g. Janikian 2004; Janikian et al. 2012; Basei et al. 2013; Matté et al. 2016; Sommer et al. 2011, 2017).

The Rocha Formation: sources and age constrains

The scarcely preserved primary sedimentary structures (Fig. 5), together with the dominantly pelitic deposits, are not enough to suggest a concrete depositional environment, being compatible with different depositional scenarios, from transitional fluvial to tidal plain (Basei et al. 2010). Features interpreted as related to turbidite deposits (Abre et al. 2020), mostly the cyclical arrangement of the bedding, are here interpreted as the result of structural repetition of the sequence due to folding. Nevertheless, a turbidite origin is not discarded.

The detrital zircon ages of the Rocha Formation are consistent between the samples, showing a main Neoproterozoic peak at ca. 660 Ma, with subordinated Tonian (ca. 0.8 Ga), Mesoproterozoic (ca. 1.0–1.1 Ga) and Paleoproterozoic peaks (ca. 1.8–2.0 Ga). These results are also consistent with those presented in previous studies (Basei et al. 2005; Abre et al. 2020), which also identified limited Neoarchean zircon grains (Figs. 10, 14). These detrital zircon spectra show a good correlation to the spectra of the Gariep Belt, particularly with the Oranjemund Group of the Marmora Terrane, which also lacks a large Paleoproterozoic peak (Fig. 18).

Fig. 18

Comparison between the detrital zircon spectra of the Gariep Belt, the Rocha Formation and the Nico Pérez Terrane. Data from: the present study, Basei et al. (2005), Hofmann et al. (2014, 2015), Konopasek et al. (2014, 2017), Naidoo et al. (2017), Andersen et al. (2018), and Abre et al. (2020)

Most of the Neoproterozoic zircon grains show positive εHf values between + 1 and + 14, suggesting a juvenile source for them. This fact discards the Cerro Olivo Complex, particularly the orthogneisses characterized by negative εHf values, as a source for the Rocha Formation. On the other hand, the absence of grains recording the typical ages for the voluminous and widespread Ediacaran granitic magmatism of the Dom Feliciano Belt (ca. 630–550 Ma), suggest that it was not involved as a source for this basin, though they may be coeval. Sm–Nd data and geochemical analyses (Basei et al. 2011; Abre et al. 2020) point to a juvenile input in the source of the Rocha Formation (Basei et al. 2011; Abre et al. 2020). To summarize, all the listed characteristics point to a source allochthonous to the Dom Feliciano Belt and its basement (e.g. Cerro Olivo Complex, Nico Pérez Terrane).

On the other hand, the isotopic signature and ages of the Kalahari Craton fit well with the new results reported here, suggesting that the Rocha Formation has a Kalahari affinity (Fig. 19, Oriolo and Beker 2018 and references therein). Particularly, the isotopic signatures of the Rocha Formation are coincident with those of the Oranjemund Group of the Marmora Terrane of the Gariep Belt (Fig. 19), reinforcing the already proposed link between both units (i.e. Basei et al. 2010; Abre et al. 2020). Therefore, the Rocha Formation is envisaged as a part of the Marmora Terrane of the Gariep Belt that was accreted to the Dom Feliciano Belt in the Ediacaran.

Fig. 19

Synthesis of εHf versus U–Pb zircon data of the Kalahari Craton, the Rocha Formation, the Gariep Belt and its Marmora Terrane (n = 4395; Basei et al. 2005; Zeh et al. 2007, 2008, 2009, 2010, 2011, 2013a, b, 2014; Zeh and Gerdes 2012; Frimmel et al. 2013; Foster et al. 2014; Hofmann et al. 2014, 2015; van Schijndel et al. 2014; Zirakparvar et al. 2014; Colliston et al. 2015; Cornell et al. 2015; Milani et al. 2015; Andersen et al. 2018; Abre et al. 2020). Data were recalculated considering a constant decay λ ¹⁷⁶Lu = 1.867 × 10⁻¹¹ year⁻¹ (Söderlund et al. 2004) and CHUR values of ¹⁷⁶Hf/¹⁷⁷Hf = 0.282772 and ¹⁷⁶Lu/¹⁷⁷Hf = 0.0332 (Blichert-Toft and Albarède 1997)

Folding geometries F₁ and F₂ show an interference pattern, where the F₂ folds re-fold the F₁ fold, thus, representing a progressive deformation. The F₂ folds show axial planes and fold axes, respectively, sub-parallel to the mylonitic foliation and stretching lineation observed in the Laguna de Rocha Shear Zone (Fig. 9). Together with the tightening of the folds on the proximity of the former, suggests that F₂ is related to the activity of the shear zone, as was previously interpreted for the neighboring Sierra de Aguirre Formation on the other side of the shear zone (Silva Lara et al. 2021).

The above mentioned Sierra de Aguirre Formation also represents an important age constraint for the Rocha Formation. Its deposition is directly associated with a restricted basin developed in association with the Laguna de Rocha Shear Zone, thus implying that the accretion between the Cerro Olivo Complex and the Rocha Formation happened prior to 582 Ma, an age of the youngest deposits of the Sierra de Aguirre Formation. This relationship is also supported by the fold geometries, recording the Sierra de Aguirre Formation the second folding phase of the Rocha Formation, but lacking in the first folding phase of the former. Based on this connection, the age of F₁, and, therefore, the age of the basin closure, is constrained between the age of the youngest deposits of the Rocha Formation, namely < 660 Ma, and the age of the age of the youngest deposits of the Sierra de Aguirre Formation (ca. 582 Ma, Silva Lara et al. 2021).

To constrain the age of the F₂, we need to take into account the age of the shearing of the Laguna de Rocha Shear Zone. As mentioned previously, this shear zone, as with most of the other shear zones of the belt, had a protracted evolution (e.g. Oyhantcabal et al. 2010, 2018; Oriolo et al. 2016; Hueck et al. 2018, 2019) since it was already active prior to the deposition of the Sierra de Aguirre Formation (Silva Lara et al. 2021). The F₂ folds are sheared between the Rocha and the Sierra de Aguirre Formation, hence, the lower age constraint for the F₂ is given by the age of the youngest deposits of the volcanic succession, namely ca. 570 Ma. Afterwards a change in the dynamics of the shear zone led to the folding of both supracrustal sequences. The upper age constraint for the F₂ folding is given by four different results; the age of the syn-kinematic Rocha Granite, the age of the dacitic dykes crosscutting the Rocha Formation, the age of the white mica of the Laguna de Rocha Shear Zone, and the age of the Santa Teresa Granitic Complex. First, from a regional point of view, the Rocha Granite, syn-kinematic to the neighboring Álferez-Cordillera Shear Zone, indicates that shearing was taking place in the region at ca. 565–560 Ma (Will et al. 2019, this work). Second, the syn-kinematic muscovites of the Laguna de Rocha Shear Zone indicate that by the age of ca. 565 Ma, the shearing system was active and getting cooled beneath the temperature of 450 °C, but the age of the fine-grained white mica fractions indicate that further low-temperature shearing took place up to ca. 535–525 Ma. Third, the age of the dacitic dykes crosscutting the F₂ structures of the Rocha Formation gave an age of 565 Ma, also do not show major signs of deformation. Fourth, the Santa Teresa Granitic complex, with an age of ca. 543 Ma (Basei et al. 2013), intrusive to the Rocha Formation, also crosscut the F₂ structures. Therefore, based on the available ages, we can describe the following deformation pattern for the F₂: (a) folding begins at ca. 570 Ma, (b) progressive strain localization. At the age of ca. 565 Ma, the temperature conditions in the Laguna de Rocha Shear Zone cooled beneath 450 °C and contemporaneously, the already formed F₂ structures were intruded by non-deformed dacitic dykes with a measured age of ca. 565 Ma. Similarly, by this age, the Rocha granite intruded in the neighboring Cordillera Shear Zone, (c) finally, the Santa Teresa Granitic Complex intruded the Rocha Formation, also crosscutting the F₂ structures. Any further deformation was by this time already localized within the Laguna de Rocha Shear Zone, as indicated by the fine white mica fractions. On the African side, the Gariep Belt shows a series of structural similarities that suggest a similar evolution to the one recorded in the Rocha Formation, with a main transpressive event and transpressive re-folding (e.g. Jasper et al. 1992; Hälbich und Alchin 1995; Gray et al. 2008; Brayshaw and Watkeys 2018).

On the other hand, the Paso del Dragón Complex shows a complex stratigraphy, hindering the previously proposed correlations with the Rocha Formation (Peel et al. 2018). The complex is composed of two different tectonically intercalated units, probably with dissimilar origins, namely the metamafic and paraderived rocks, contrary to the Rocha Formation. While metamafic rocks show a roughly Tonian to Cryogenian crystallization age (704 ± 58 Ma, Chavez Ramos et al. 2020) and metamorphism at 628–625 Ma (Will et al. 2019), the paraderived sequence shows a maximum deposition age at about 660 Ma and an Ediacaran metamorphism at ca. 570 Ma. Both units, mafic and metasedimentary are probably tectonically intermixed since the Ediacaran, as shown by the relationships with the intrusive bodies, for example the Três Figueiras Granite, with an age of 585 Ma.

In spite of the proposed relationships between the Rocha Formation and the Paso del Dragón Complex (Peel et al. 2018; Vieira et al. 2019), the latter records a spectrum with large Meso- and Paleoproterozoic populations, more comparable with spectra of the paraderived rocks of the Cerro Olivo Complex (Fig. 14). Evidence thus suggests that the Paso del Dragon Complex is not directly related to the Rocha Formation. Further studies of this complex will allow to discard or confirm a correlation between both.

Correlations and general evolution

The Punta del Este Terrane is possibly one of the most controversial tectonostratigraphic blocks within the basement configuration of the Dom Feliciano Belt. It has been alternatively correlated with rocks of the Coastal Terrane in the Kaoko Belt (e.g. Oyhantçabal et al. 2009; Konopasek et al. 2017) and the western Kalahari Craton (e.g. Blanco et al. 2011; Frimmel et al. 2011, 2013), which implies contrasting origins related to the Congo or Kalahari Craton, respectively.

Tonian orthogneisses of the Punta del Este Terrane basement show clearly subchondritic Lu–Hf compositions (εHf_(t) from − 12 to − 4), suggesting a significant recycling of paragneisses dominated by Meso- to Paleoproterozoic detrital zircons with mainly suprachondritic compositions (Fig. 12). Though late Mesoproterozoic and Tonian detrital zircons are abundant in the Rocha Formation, they yield contrasting suprachondritic compositions, discarding a possible source in the basement of the Punta del Este Terrane.

The Lu–Hf zircon isotopic composition of ortho- and paragneisses of the Punta del Este Terrane basement are clearly different from that of coeval detrital zircons in the Saldania and Gariep Belts in the western Kalahari Craton (Frimmel et al. 2013; Hofmann et al. 2014; Oriolo and Becker, 2018), which is, however, comparable with detrital zircons of the Rocha Formation (Fig. 19). These similarities further reinforce correlations of the Rocha Formation with coeval sequences of the Gariep Belt, particularly the Oranjemund Group of the Marmora Terrane, as previously suggested (e.g. Basei et al. 2005; Blanco et al. 2011; Abre et al. 2020), but seem to rule out a derivation from the Punta del Este Terrane basement. On the other hand, the latter seems to have an affinity with the Coastal Terrane of the Kaoko Belt, as already indicated by Oyhantçabal et al. (2009) and Konopasek et al. (2017), among others. This correlation is well-documented in the similar magmatic and metamorphic ages (Fig. 15), as well as in the detrital zircon spectra of the paraderived rocks (Fig. 16) and the geochemistry of the orthogneisses.

In this context, previous proposals indicating a Kalahari affinity for the Punta del Este Terrane do not seem to be valid, since the Rocha Formation seems to be the only sequence correlatable with that crustal block. The lack of Ediacaran zircons in the latter suggests that derivation from the Dom Feliciano Belt granitoids, which are widespread at ca. 620–570 Ma in the Punta del Este Terrane, is unlikely. Therefore, it could be inferred that deposition of the Rocha Formation took place together with comparable equivalents of the Gariep Belt, closer to the Kalahari Craton, where Ediacaran magmatism at ca. 630–570 Ma is absent. Consequently, the Rocha Formation seems to be allochthonous to the Dom Feliciano Belt and was possibly accreted to the latter prior to ca. 580 Ma, as suggested by the structural relationships with the Sierra de Aguirre Formation.

The aforementioned model satisfactorily explains all the geological and geochronological data of the Punta del Este Terrane, and reconciles contrasting hypotheses about its African origins (e.g. Frimmel et al. 2011). This implies a “two-stage” evolution of the Dom Feliciano Belt, with an early Ediacaran collision of the Rio de la Plata and Congo cratons, succeeded by the late Ediacaran assembly of the Kalahari Craton to the former (e.g. Rapela et al. 2011; Oriolo et al. 2016). The Rio de la Plata-Congo collision triggered the accretion of Congo/Angola-derived blocks, including the Punta del Este Terrane, to the margin of the Rio de la Plata Craton. Subsequent convergence, possibly starting at ca. 600–590 Ma (Lehmann et al. 2016), finally lead to the juxtaposition of the Kalahari Craton and marginal Gariep sequences, including the Rocha Formation, to the southeastern most Dom Feliciano Belt.

Conclusions

New zircon U–Pb and Lu–Hf data from ortho- and paragneisses of the Cerro Olivo Complex indicate that the basement of the Punta del Este Terrane does not comprise a large Mesoproterozoic crustal block but, instead, is mainly constituted of Early Neoproterozoic metaigneous and metasedimentary rocks. A Tonian retreating-mode accretionary orogen seems to be the most likely tectonic setting for this complex, which reconciles previous models suggesting arc vs. rift magmatism. Isotopic data allow us to discard the Nico Pérez Terrane as an important source for the magmatism or the sediments of the Punta del Este Terrane. Instead, previous correlations with the Coastal Terrane of the Kaoko Belt are confirmed, based on similarities in magmatic and metamorphic ages, as well as in the detrital zircon spectra of paraderived rocks of both blocks.

The deposition age of the Rocha Formation is constrained between the youngest detrital zircon age peak, at ca. 660 Ma, and the beginning of the deposition of the Sierra de Aguirre Formation at ca. 580 Ma, based on the structural relationship between them. Detrital zircon data of the Rocha Formation also indicate that it was not sourced in the Dom Feliciano Belt, but shares the sources of the Marmora Terrane, further reinforcing previous correlations with the Gariep Belt. It is thus proposed that the Rocha Formation belongs to the former and was only juxtaposed by the late Ediacaran to the South American margin. Deposition of the Sierra de Aguirre Formation at ca. 580 Ma represents the maximum age for the accretion of the Gariep Belt, including the Rocha Formation, to the Punta del Este Terrane.

As a corollary, a two-stage evolution model is considered as the most satisfactory explanation for the tectonometamorphic and sedimentary evolution of the Dom Feliciano Belt, with an early Ediacaran collision of the Rio de la Plata and Congo cratons, succeeded by the late Ediacaran assembly of the Kalahari Craton to the former.