1. Introduction

The Nico Pérez Terrane, located in the central portion of Uruguay (Fig. 1), records the oldest basement ages of the region. Bossi and Campal (1992) identified this terrane from the main outcropping areas of the Río de La Plata Craton (sensu Almeida et al., 1973) and based on lithological and structural data they defined it as a Paleoproterozoic block affected by tectono-magmatic events related to the Brasiliano-Panafrican orogenic cycle. Aditionally, these authors postulated that the Nico Pérez Terrane represents the foreland of the Neoproterozoic orogenic belt known as Dom Feliciano Belt (Fragoso-César, 1980). Later works by Hartmann et al. (2001), Santos et al. (2003), Mallmann et al. (2007), Gaucher et al. (2010a), Oyhantçabal et al. (2012, 2018) and Oriolo et al. (2016a), among others, allowed the recognition of Archean and Paleoproterozoic basement rocks and the intense impact of Neoproterozoic granitic intrusions and deformation in the Nico Pérez terrane. Additionally, several authors (e.g., Bossi et al., 1998; Hartmann et al., 2001; Bossi and Gaucher, 2004; Mallmann et al., 2007; Gaucher et al., 2010 (a, b); Oriolo et al., 2016a; Oyhantçabal et al., 2018) have extended its eastern border to the Sierra Ballena shear zone (Fig. 2). Oyhantçabal et al. (2011) challenged its affinity with the Río de la Plata Craton based on lithologic, structural, geochronologic, and isotopic data. Recently, Oriolo et al. (2016a, 2017) interpreted this terrane as a basement inlier of the Dom Feliciano Belt, and they suggested its affinity with the West Africa Congo Craton based on tectonostratigraphy, U-Pb ages, and Hf isotopic data.

 

Fig. 1. Geologic map from Uruguay showing the basement most conspicuous units. SYSZ: Sarandí del Yi shear zone; FMSZ: Fraile Muerto shear zone; MASZ: María Albina shear zone; CTSZ: Cueva del Tigre shear zone; RT: Retamosa thrust; TSZ: Tupambaé shear zone; RSZ: Rivera shear zone; SBSZ: Sierra Ballena shear zone; ACSZ: Alférez-Cordillera shear zone. (Modified from Loureiro et al., 2019).

After almost 30 years since its original definition, the Nico Pérez Terrane is considered an Archean-Paleoproterozoic crustal block with Neoproterozoic reworking related to the Brasiliano Orogeny, even though its affinity with the Río de la Plata Craton and its limits are still being subjects of controversy (e.g., Sánchez Bettucci et al., 2010a; Santos et al., 2019; Oyhantçabal et al., 2018; Bologna et al., 2019; Loureiro et al., 2019). In this paper, new SHRIMP U-Pb zircon ages from rocks of nine samples from six sites (see Fig. 2) are presented. These ages are integrated with geological, geophysical, and geochronological evidence to provide constraints for the understanding of the geological evolution of the Uruguayan basement. Also, an appraisal about the limits and the affinity of the Nico Pérez Terrane with the Congo or Río de La Plata cratons are presented based on available information.

 

Fig. 2. Geologic map of the Valentines and Pavas Blocks (Nico Pérez Terrane) showing de dated points. SYSZ: Sarandí del Yí shear zone; FMSZ: Fraile Muerto shear zone; MASZ: María Albina shear zone; CTSZ: Cueva del Tigre shear zone; RT: Retamosa thrust; TSZ: Tupambaé shear zone; SBSZ: Sierra Ballena shear zone. Modified from Loureiro et al. (2019).

2. Geological framework of the Nico Pérez Terrane

The Nico Pérez terrane is located between the Sarandí del Yí shear zone to the west and the Retamosa thrust (RT) to the east and south (Fig. 1). Its northeastern limit is along the Fraile Muerto shear zone. Both, the Retamosa thrust and Fraile Muerto shear zone separate it from the supracrustal sequence of the Dom Feliciano Belt (e.g., Bossi and Campal, 1992; Sánchez Bettucci et al., 2010b). Meanwhile, the Sarandí del Yí shear zone separates the Nico Pérez Terrane from the Piedra Alta Terrane developed to the west. The Piedra Alta Terrane (Bossi et al., 1993) includes low to medium metamorphic orogenic belts (ca. 2.1 Ga), a layered mafic complex, a gneissic-migmatitic basement, pre- to syn- orogenic granitoids, mafic and felsic late to post orogenic magmatism (1.9-2.3 Ga), A-type rapakivi granites (2.078 Ga), and extensional magmatism (1.7 Ga) represented by a mafic dike swarm (Sánchez Bettucci et al., 2010a; Franceschinis et al., 2019 and references therein). All these lithostratigraphic units were assigned to the Transamazonian orogenic cycle (Hurley et al., 1967; Choubert and Faure-Muret, 1969; Almeida et al., 1973) or Transplatense orogenic cycle ).

From Almeida et al. (1973) the current areas of the Nico Pérez Terrane and Piedra Alta Terrane have been considered a part of the Río de la Plata Craton. The Río de La Plata Craton (sensu Almeida et al., 1973) occupies approximately the third part of the south-western region of Uruguay (Fig. 1). It extends towards the north of the country in Rivera and Aceguá (near the Brazilian border), and crops out in Brazil, at the eastern part of Rio Grande do Sul State (Taquarembó and Encruzilhada terranes). Also, it crops out in the Tandil mountain ranges in Argentina. Oyhantçabal et al. (2011) questioned this concept in a review of the Río de La Plata Craton. These authors based on features like Sm/Nd T_DM model ages (Piedra Alta Terrane T_DM 2.8-2.3 Ga; Nico Pérez Terrane T_DM 3.0-2.6 and 2.3-1.6 Ga), crystallization ages (Piedra Alta Terrane 2.2-2.1 Ga; Nico Pérez Terrane 3.1-0.57 Ga), and differences in gravity signature for both terranes, proposed that the Nico Pérez Terrane and Taquarembo block are allocthonous to the Río de La Plata Craton, comprising only the Piedra Alta and Tandilia terranes. However, Santos et al. (2017) reported for the first time Archean inheritance (T_DM Hf: 2.52 Ga; average epsilón Hf: 3.62) and Brasiliano reworking in the Piedra Alta Terrane, modifying the established idea of only Rhyacian juvenile crust. Besides, Bologna et al. (2019) based on a magnetotelluric study of the Piedra Alta and Nico Pérez terranes mentioned that their results showed no lithospheric-scale contrast in the electrical resistivity across the Sarandí del Yí shear zone. In addition, they suggested the Nico Pérez Terrane is the remobilized metacratonic portion (sensu Liégeois et al., 2013) of the Río de La Plata Craton, as previously suggested by Santos et al. (2019). Regional syntheses of the tectonic evolution of the Río de La Plata Craton in Uruguay are available in Rapela et al. (2007); Sánchez Bettucci et al. (2010a); Oyhantçabal et al. (2011, 2018); Cingolani et al. (2012); Santos et al. (2019); Bologna et al. (2019) and references therein.

As stated above, the Nico Pérez Terrane is made up of Archean and Paleoproterozoic high-grade metamorphic sequences and granitoid rocks, as well as post-orogenic magmatic rocks. It comprises the Pavas, Valentines, Rivera and Aceguá structural blocks, as shown in figures 1 and 2. The main features of each of these structural blocks are presented below.

2.1. Pavas Block

The Pavas Block (sensu Preciozzi et al., 1985) is composed by La China and Las Tetas Archean complexes (Hartmann et al., 2001), covering approximately 2,025 km² and 635 km², respectively (Loureiro et al., 2019), both with an NE-SW regional structural trend. To the north and west, La China Complex is in tectonic contact with the Valentines Block through Cueva del Tigre shear zone. To the south-east this complex thrusts Las Tetas Complex by the María Albina shear zone, and to the north-east, it is in tectonic contact with the supracrustal rocks of the Dom Feliciano Belt by the Fraile Muerto shear zone (see Fig. 2).

La China Complex is described as a set of orthogneisses, granitoids, and a greenstone belt consisting of igneous mafic and ultramafic rocks with subordinate cherts, that underwent significant deformation and metamorphism under amphibolite facies conditions; also it contains a few Neoproterozoic intrusions. The complex presents a general composition of tonalite-trondhjemite-granodiorite (TTG) according to Hartmann et al. (2001). The protoliths of the metaultramafic rocks have been interpreted as hazburgites, dunites and komatiites. Serpentinites, talc-chlorite-tremolite schists and amphibolites are common (Preciozzi et al., 1979, 1985; Hartmann et al., 2001; Gaucher et al., 2014a, b). Metatonalites are the rocks where most of the Archean ages have been reported. They present irregular compositional bands, with amphibole and quartz-feldspathic rich bands (Fig. 3). The Zapicán diorite intrudes the La China Complex, and crops out as NNE-SSW body (Lossada et al., 2014). It comprises diorites to granodiorites, often containing hornblende and gabbroic enclaves (Fig. 3).

Las Tetas Complex is defined as a succession of metasedimentary rocks affected by amphibolite facies metamorphism, consisting of meta-quartzites with fuchsite, metaconglomerates, micaschists with staurolite and garnet, gneisses with muscovite and tourmaline, BIFs and dolomitic marbles (Hartmann et al., 2001; Masquelin et al., 2017; and references therein). This complex has a decreasing metamorphic grade from north to south (Gaucher et al., 2014a; Oyhantçabal et al., 2018) from quartzites with sillimanite in the north, to micaceous shales with staurolite and garnet in the south (Hartmann et al., 2001). According to Oyhantçabal et al. (2018) and Loureiro et al. (2019), along its southeastern limit, represented by the Retamosa thrust (this work), the unit presents small flakes intercalated with supracrustal rocks of the Dom Feliciano Belt.

 

Fig. 3. A. General view of Illescas rapakivi granite (sample NP-02); B. General view of foliated granitoid (sample NP-07) at the Cerro de las Cuentas; C. Cerro Chato granite (sample NP-08); D. Mylonitic granite inside the Cueva del Tigre shear zone (NP-09A): recrystallized granite; E. Foliated granitoid (NP-09B); F. Deformed Zapicán diorite (NP-10); G. Foliated leucogranite of the La China Complex (NP-11) that shows irregular compositional bands, with amphibole and quartz-feldspathic rich bands.

2.2. Valentines Block

The Valentines Block (Preciozzi et al., 1979) is located between the Sarandí del Yí shear zone and the Cueva del Tigre shear zone, next to the Pavas Block (Fig. 2). It is characterized by metatrondhjemites, metagranites and metatonalites. Ferriferous conglomerates, magnetite-rich quartzites, banded magneto-augite quartzites, BIFs, also cherts have been described, as well as amphibolites, pyroxenites, garnet-pyroxene quartzites and forsterite- diopside marbles (e.g., Bossi and Umpierre, 1969; Masquelin, 2006; Oyhantçabal et al., 2011; Oriolo et al., 2016a). The Valentines Block is constituted by the Valentines-Rivera Granulitic Complex (Oyhantçabal et al., 2011), and a few Proterozoic igneous intrusions. Lithologically, the Valentines-Rivera Granulitic Complex is composed mainly by granulitic gneisses, para-gneisses, meta-pyroxenites, magnetite-augite quartzites, meta-tonalites, meta-trondjemites, migmatites, among others (Mallmann et al., 2007; Oyhantçabal et al., 2011 and references therein). The supracrustal rocks (paragneisses, quartzites, BIF) of this complex are included in the Valentines Formation (Bossi et al., 1965). The granulites are correlated and grouped with similar rocks from Brazil in the Taquarembó Block (Hartmann et al., 1999; Oyhantçabal et al., 2011; and references therein) and with the Santa María Chico Complex in Rio Grande do Sul (Massonne et al., 2001; and references therein).

Within this block, Campal and Schipilov (1995) described the occurrence of a large A-type rapakivi granite (Illescas Batholith, Fig. 3). It has a coarse- to medium-grain texture, with K-feldspar, plagioclase, quartz, amphibole, and biotite. It presents ductile deformation in its borders and it is cut by Sarandí del Yí shear zone (Campal and Schipilov, 1995). Gaucher and Blanco (2014) based on few and partial geochemical data suggested a within plate tectonic environment. On the contrary, Oriolo et al. (2019) conducted a geochemical study concluding that the Illescas granite exhibits meta- to peraluminous nature with dominant shoshonitic and ferroan composition, suggesting a post-collisional/post-orogenic setting.

2.3. Rivera Block

The Rivera Block (Preciozzi et al., 1979) is in the northeastern portion of Uruguay (Figs. 1 and 4) and is an isolated block surrounded by Palaeozoic sedimentary rocks from the Paraná Basin. The basement is a bimodal felsic-mafic association that underwent high-grade metamorphism constituted by granulitic othogneisses and subordinate orthopyroxene and garnet-bearing mafic granulites, grouped by Oyhantçabal et al. (2012) in the Valentines-Rivera Granulitic Complex. The regional structural trend is EW, affected by ductile shear zones (Fig. 4), attributed originally to the Brasiliano orogeny by Cordani and Soliani (1990).

In addition, the rocks of the Vichadero Formation are interleyed with the Valentines-Rivera Granulitic Complex (Oyhantçabal et al., 2012). This formation is composed of pyroxene fels, sillimanite gneisses, quartzites, forsterite marbles, metabasites, BIFs, calc-silicate rocks, and manganese-formations (Ellis De Luca, 1998). Important ore deposits (gold) occur related to the regional structural trend.

 

Fig. 4. Rivera Block geological map, modified from Loureiro (2008).

2.4. Neoproterozoic cover rocks, granitic plutons, and shear zones

In the Valentines Block, the Neoproterozoic cover is represented by the Cerro San Francisco and Cerros Victoria Formations, made up by meta-sandstones and meta-subarkoses, and by the oolitic and stromatolitic meta-limestone with low metamorphic grade, respectively, deposited in restricted sedimentary basin (Montaña and Sprechmann, 1993). In the Rivera Block, some outcrops of low-grade metamorphic rocks are described (e.g., Preciozzi et al., 1985; Gaucher, 2000).

An important Neoproterozoic magmatism, related to the Brasiliano orogenic cycle, crops out all over the Nico Pérez Terrane (Preciozzi et al., 1979; Cordani and Soliani, 1990; Oyhtançabal et al., 2012; Oriolo et al., 2016a; among others). Foliated granitic plutons occupy about 30% of the area covered by the Nico Pérez Terrane (Fig. 2). The granitoids are syn-, late-, to post-orogenic and they are coupled with the regional structures with NE-SW trends related to the Brasiliano Orogeny. Two of them, the Cerro de las Cuentas and the Cerro Chato granites (Fig. 3), are studied in this work (see Section 4).

The most important shear zone that affects the Nico Pérez Terrane is the Paleoproterozoic Sarandí del Yí shear zone (Fig. 1). It comprises up to 13 km wide and more than 250 km long in the north-south direction (Gómez Rifas, 1989). Preciozzi et al. (1979) were the first to recognize this feature, with a N10º structural trend developed between the Sarandí of Yí town (Durazno department) and the Sierra de Las Ánimas Complex (Fig. 1). Bossi and Campal (1992) described it as a dextral megashear zone, based on features like the Florida dike swarm flexure, which are clearly well defined in the aerogeophysical survey (Dinamige, 2016). Oyhantçabal et al. (1993) suggested that this shear zone was reactivated with a sinistral sense during the Brasiliano orogenic cycle. Furthermore, Oriolo et al. (2016b) proposed this shear zone is due to the collision of the Nico Pérez Terrane and the Río de La Plata Craton, suggesting the onset of the deformation at 630-625 Ma, with dextral shearing up to 596 Ma and subsequent sinistral shearing at 594-584 Ma.

Another important shear zone occurring in the Nico Pérez Terrane is the Cueva del Tigre shear zone (Preciozzi et al., 1979) developed at the boundary between the Valentines and Pavas blocks (Fig. 2). It is represented by a group of granitic mylonites, schists and phyllonites. This shear zone affected both, the Archean basement and the Neoproterozoic intrusions (Oriolo et al., 2016b).

3. Previous radiometric data of the Nico Pérez Terrane

The available ages for the three structural blocks of Nico Pérez Terrane are displayed in the table 1. The pioneer works of Soliani (1986) and Cordani and Soliani (1990) reported Paleoproterozoic and Neoproterozoic ages for the Rivera Block. These authors obtained a Rb-Sr (WR) isochron of 2250±60 Ma for the quartz-feldspathic gneissic basement; also, Cordani and Soliani (1990) obtained several K-Ar ages in biotite, plagioclase, and whole rock around 500-600 Ma. Recently, Santos et al. (2003), Oyhantçabal et al. (2012) and Oriolo et al. (2016a) obtained several U-Pb single crystal zircon ages within the interval 2200-2000 Ma, for the high grade gneisses, which were interpreted as crystallization ages of the magmatic protoliths, confirming the paleoproterozoic age of the block. Oriolo et al. (2016a) and Oyhantçabal et al. (2012) also reported a few ages around 580 Ma for Ediacaran granitic plutons.

In Valentines Block only a few U-Pb zircon ages have been obtained. Hartmann et al. (2001) reported Archean detrital zircon ages between 3000 and 2700 Ma in a metaconglomerate (Valentines BIF’s). For metagranites of this unit Santos et al. (2003), obtained ages between 2619 and 2058 Ma. More recently, Oriolo et al. (2016b) obtained two Paleoproterozoic ages for Valentines-Rivera Granulitic Complex of 2016 and 2048 Ma.

The Pavas Block also has a few geochronological data available. As mentioned above, Hartmann et al. (2001) obtained U-Pb zircon SHRIMP ages of 3400 Ma for zircon cores and 2700 Ma for zircon rims in a tonalitic orthogneiss of the La China Complex. Gaucher et al. (2011, 2014b) obtained a zircon U-Pb LA-ICP-MS age of 3096 Ma for the same lithology, and detrital zircon ages of 3000, 2970, 2760 and 2600 Ma in a metaconglomerate of the Las Tetas Complex.

All the magmatic crystallization ages (e.g., zircon nuclei with Th/U ratios greater than 0.1) reported by the different authors (Hartmann et al., 2001; Gaucher et al., 2011, 2014a and references therein) for the metatonalites from La China Complex can be grouped into Mesoarchean (3.03 Ga) and Neoarchean (2.79-2.72 Ga). On the other hand, metamorphic events (e.g., zircon rims with Th/U ratios less than 0.1) show Mesoarchean (3.1 Ga) and Neoarchean (2.69 Ga) ages. Detrital zircons found in the Las Tetas Complex suggest Archean source areas and a maximum Mesoarchean deposition age (Loureiro et al., 2019 and references therein).

The intrusive Illescas granite was dated by Bossi and Campal (1992) by whole-rock Rb-Sr method, yielding 1760±32 Ma; later Campal and Schipilov (1995) reported a Pb-Pb age of 1784±5 Ma. More recently Oriolo et al. (2019) obtained a zircon U-Pb LA-ICP-MS age of 1768±11 Ma for this granite.

Younger ages reported in the Nico Pérez Terrane are 739±20 Ma (Rb-Sr WR) for the Cerro Chato granite (Bossi and Campal, 1987), and 610 Ma for the Zapicán diorite that intrudes La China Complex (Oriolo et al., 2016a).

4. Results of U-Pb zircon geochronology

New U-Pb SHRIMP zircon results on eight samples of granitoid rocks are presented in this work with the goal of constraining the tectonic evolution of the Nico Pérez Terrane. The analytical data are included in table 2. The zircon crystals were extracted from the samples by conventional methods, mounted in epoxy mounts, together with a few fragments of the TEMORA-2 standard, and polished. Then, cathodoluminescence images were obtained to reveal their internal structure to target the best sites for the U-Pb isotopic analyses. All measurements were made at Centro de Pesquisas Geocronológicas (CPGeo) of the University of São Paulo (USP), Brazil.

The eight samples were taken from six outcrops and were analyzed using the SHRIMP-II instrument of the CPGeo. Details of the analytical procedures, including the calibration methods, were presented by Williams (1998) and the work at the São Paulo laboratory was described by Sato et al. (2014). U abundance and U/Pb values of the studied zircon crystals were calibrated against the Z6266 (903 ppm) and the TEMORA-2 (416.78 Ma) standards, respectively. Individual ages were determined from five successive scans of the mass spectrum, and the ages reported in the text are with 95% confidence limits. Correction for common Pb was made using the measured ²⁰⁴Pb, and the typical error component for the ²⁰⁶Pb/²³⁸U ratios is less than 2 percent. The data were reduced by using a SQUID software and the Concordia diagrams were prepared using Isoplot /Ex (Ludwig, 2009).

Zircons are typically prismatic to equant, where several present oscillatory zoning. Some crystals present homogeneous cores, either bright or dark. In many cases the crystals are fragmented. The cathodoluminescence images of representative analyzed zircons are shown in figure 5. The concordia diagrams for each analyzed sample are shown in figures 6 to 13.

 

Fig. 5. The cathodoluminescence images of representative analyzed zircons.

4.1. Sample NP-07-Cerro de las Cuentas foliated granitoid

This sample was collected at Cerro de las Cuentas granite, in the northernmost part of the Valentines Block (Fig. 2). This granite intrudes the Valentines-Rivera Granulitic Complex, and it corresponds to an equigranular fine-grained foliated granite, composed of quartz+K-feldspar+plagioclase+biotite. The deformation is not observed in the outcrop scale, but undulose extinction and bulging recrystallization in quartz crystals occur. Zircon crystals are mostly prismatic showing oscillatory zoning and some recrystallization phenomena (Fig. 5b). A total of nine spots out of six zircon crystals were analyzed (Table 2). Three zircon grains (#1, #2 and #7) were analyzed in their core and rim (Fig. 5b). Both, core and rim, yielded similar ages. Figure 6 shows the concordant age of 584±8 Ma obtained from three out of the six grains (highly discordant data was discarded). Based on zircon morphology and Th/U ratios (>0.34), this result is considered a good estimate for its crystallization age. Zircon #7 is an inherited xenocrystal, with a rather imprecise Rhyacian age of about 2100 Ma.

 

Fig. 6. U-Pb SHRIMP Concordia Diagram for the Cerro de las Cuentas granite (NP-07); a) ellipses indicate the analyses used for the age calculation. Error ellipses are at 95% confidence level.

4.2. Sample NP-08-Cerro Chato granite

This sample was collected at the western margin of the Cerro Chato granite, in the central part of the Nico Pérez Terrane; also, it intrudes the Valentines-Rivera Granulitic Complex (Fig. 2). It is a fine to coarse-grained granite, sometimes porphyric, with large zoned feldspars, plagioclase, quartz, muscovite and biotite. Zircon, apatite, and epidote are also present as accessory minerals. Locally, this granite presents mylonitic foliation.

A total of ten spots were analysed out of seven zircon crystals (Table 2 and Fig. 5c). Zircon grains are prismatic, rounded, or fragmented, and show cores that are either light or dark homogeneous or zoned. Homogeneous dark rims are observed. Most of the analytical results show high discordance, and Pb loss is observed (Fig. 7). A concordant age of 591±7 Ma is obtained using two zircon grains (highly discordant data was discarded), probably it represents the crystallization age of this pluton. Two inherited crystals yielded Paleoproterozoic (ca. 2.1 Ga) and Mesoproterozoic (ca. 1.5 Ga) ages, respectively.

 

Fig. 7. U-Pb SHRIMP Concordia Diagram for the Cerro Chato granite (NP-08); a) ellipses indicate the analyses used for the age calculation. Error ellipses are at 95% confidence level.

4.3. Samples NP-09A, NP-09B1 and NP-09B2-Mylonitic granites within the Cueva del Tigre shear zone

These samples were taken at an outcrop located within the Cueva de la Tigre shear zone in the contact of two deformed granitoids, affected by the shear zone (Fig. 2). Sample NP-09A is a granitoid specimen, classified in the field as protomylonitc granite, presenting quartz ribbons and quartz crystals with subgrain rotation and bulging recrystallization. Sample NP-09B is a foliated granitoid, showing a banded fabric, splitted into a lighter specimen (NP-09B1) and a darker one (NP-09B2) with some more biotite and less plagioclase due to mineral segregation. Petrographically, it is composed of quartz+K-feldspar+plagioclase+biotite and chlorite. K-feldspar crystals also show bulging recrystallization and undulose extinction.

Zircon grains in sample NP-09A are prismatic with oscillatory zoning, and some of them show xenocrystal cores. Twelve zircon crystals were analyzed in 17 spots. Figure 10 shows a concordant age of 588.0±6.1 Ma obtained from 10 spots out of 17 (discordant data was discarded), interpreted as the crystallization age. Xenocrystal core yielded Mesoarchean ages (#2.1 and #2.3 in Table 2). Besides, all the analyzed crystals of this sample exhibited Th/U ratios (0.40 to 1.32) typical of magmatic rocks.

Zircon grains in samples NP-09B1 and NP-09B2 are also prismatic with oscillatory zoning cut off by areas of re-homogenization and with Th/U ratios between 0.17 to 0.70. Eight grains of the NP-09B1 sample yielded a concordant age of 3089±11 Ma (10 spots) (Fig. 8). In the case of sample NP-09B2, 16 spots were measured in 12 zircon crystals. A concordant age of 3092±5 Ma (Fig. 9) were obtained from 11 out of 16 spots (highly discordant data was discarded). Both samples show almost identical ages interpreted here as the crystallization age.

 

Fig. 8. U-Pb SHRIMP Concordia Diagram for Mylonitic granite form Cueva del Tigre shear zone (NP-09B1); ellipses indicate the analyses used for the age calculation. Error ellipses are at 95% confidence level.

 

Fig. 9. U-Pb SHRIMP Concordia Diagram for Mylonitic granite form Cueva del Tigre shear zone (NP-09B2); ellipses indicate the analyses used for the age calculation. Error ellipses are at 95% confidence level.

 

Fig. 10. U-Pb SHRIMP Concordia Diagram for Mylonitic granite form Cueva del Tigre shear zone (NP-09A); ellipses indicate the analyses used for the age calculation. Error ellipses are at 95% confidence level.

 

Fig. 11. U-Pb SHRIMP Concordia Diagram for La China Complex (NP-10); ellipses indicate the analyses used for the age calculation. Error ellipses are at 95% confidence level.

 

Fig. 12. U-Pb SHRIMP Concordia Diagram for La China Complex (NP-11); ellipses indicate the analyses used for the age calculation. Error ellipses are at 95% confidence level.

 

Fig. 13. U-Pb SHRIMP Concordia Diagram for Illescas rapakivi granite (NP-02); ellipses indicate the analyses used for the age calcula-tion. Error ellipses are at 95% confidence level.

4.4. Sample NP-10-Deformed Zapicán diorite

This sample was also collected close to the Cueva del Tigre shear zone, not far from outcrop NP-09 (Fig. 2). The rock corresponds to the Zapicán diorite, composed by plagioclase+amphibole (+quartz+K-feldspar+biotite). It is locally foliated, presenting some bulging recrystallization and undulose extinction in quartz crystals.

Eleven zircon crystals were analyzed (Table 2). Zircon grains are generally prismatic with oscillatory zoning; also, some grains contain areas of re-homogenization and local recrystallization (Fig. 5). Th/U ratios of the analyzed zircon grains range from 0.71 to 1.61. Nine out of twelve spots were concordant and produced a very robust age of 585.8±8.7 Ma (Fig. 11) interpreted as the time of rock crystallization.

4.5. Sample NP-11-Foliated leucogranite of the La China Complex

This sample was collected in the southern exposure of the La China Complex and corresponds to a foliated biotitic leucogranite (Fig. 2). Petrographically, it is made up of plagioclase, quartz and perthitic microcline as the predominant minerals, muscovite and biotite are arranged parallel to the foliation.

Zircon grains are generally prismatic, stubby, with areas of re-homogenization (Fig. 5) and contain Th/U ratios between 0.07 and 0.62. Twelve spots from eleven zircon crystals were analyzed yielding a concordant age of 2707±7 Ma (Fig. 12) interpreted as the crystallization age.

4.6. Sample NP-02-Illescas rapakivi granite

The analyzed sample was collected in the central part of the Illescas rapakivi granite (Fig. 2) and corresponds to an inequigranular, medium grained granitoid, composed of quartz+K-feldspar±plagioclase±amphibole±biotite±titanite±zircon±apatite. Deformation (undulose extinction and bulging recrystallization) is observed in quartz and K-feldspars, and perthites are present. Profuse sericitic alteration is observed.

Twelve zircon crystals were analyzed, being prismatic, either dark or light, with oscillatory zoning, and Th/U ratios from 1.10 to 2.40. Eleven concordant spots yielded a robust age of 1734±11 Ma, shown in figure 13, and interpreted as the crystallization age.

5. Discussion

5.1. Archean ages in the Nico Pérez Terrane

When considering the whole geochronologic data, both old and new data presented here, relevant improvements for the understanding of the tectonic evolution of the Nico Pérez Terrane are achieved. Archean age of metaigneous rocks from the Pavas Block, formerly reported by Hartmann et al. (2001), was confirmed by three samples of La China Complex. Two of them are from a mylonitic granite (NP-09B) affected by the Cueva del Tigre shear zone. These samples show Mesoarchean ages whose results are identical within experimental error (NP-09B1=3,089±11 Ma and NP-09B2=3092±5 Ma), suggesting a magmatic event at that time. The third sample from Pavas Block is a foliated leucogranite yielding an age of 2707±7 Ma assumed as magmatic. Thus, the ages of magmatism (ca. 3-2.7 Ga; Table 1) and metamorphism (ca. 2.7 Ga; Table 1) reported by different authors (Hartmann et al., 2001; Gaucher et al., 2014a; and references therein), as well as those carried out in this study, can be gathered into two groups: Mesoarchean and Neoarchean.

Moreover, the detrital zircon grain ages reported in the literature for Las Tetas Complex and Valentines-Rivera Granulitic Complex (Valentines Formation), suggest the existence of Paleo to Mesoarchean source areas. This is also supported by Lu-Hf data obtained by Oriolo et al. (2016a) for this area. The age of 3.4 Ga reported by Hartmann et al. (2001) could correspond to a Paleoarchaean crust. Then, in tectonomagmatic events around 3.1 Ga formed the tonalitic rocks and around 2.7 Ga occured magmatism and metamorphism at medium-high grade (Hartmann et al., 2001; Gaucher et al., 2011).

5.2. The Illescas Batholith: tectonic significance

The robust U-Pb Statherian zircon age of 1734±11 Ma obtained here for the Illescas Batholith, which is intrusive into the western part of the Valentines Block, is very similar to the ages indicated previously by Bossi and Campal (1992), Campal and Schipilov (1995) and Oriolo et al. (2019). This batholith has been considered related to an extensional episode within the Nico Pérez Terrane, possibly genetically coeval with the extensional event that originated the important Florida dike swarm in the Piedra Alta Terrane located immediately west of the Sarandí del Yí shear zone (Gaucher and Blanco, 2014). In this framework, the Illescas Batholith and the Florida dike swarm could represent the break-up or widespread extension of the Atlantica supercontinent or the Statherian taphrogenic episode (Brito Neves et al., 1995; Gaucher and Blanco, 2014). The Sarandí del Yí shear zone, is located between these two units, along which some horizontal displacement may have taken place (Bossi and Campal, 1992).

Scarce and incomplete set of geochemical data (only two samples) for the Illescas Batholith presented by Gaucher and Blanco (2014) indicate a within plate anorogenic environment. However, Oriolo et al. (2019) based on a complete set of geochemical data (seven samples), suggested a post-collisional/post-orogenic setting. According to the available ages for the Nico Pérez Terrane, this suggestion is debatable, since the previous recorded orogenic event happened ca. 200 Ma before. Therefore, the most realistic hypotesis is an anorogenic setting. In addition, the alkali-calcic signature mentioned by Oriolo et al. (2019) could be related to typical crustal contamination.

5.3. Neoproterozoic magmatism in the Nico Pérez Terrane

>Neoproterozoic crystallization ages are recorded in four intrusive granitoids in Valentines-Rivera Granulitic Complex and in La China Complex. The mylonitic granite (sample NP-09A) affected by Cueva del Tigre shear zone, yielded an Ediacaran age of 588±6.1 Ma. This sample contains an inherited zircon crystal with a xenocrystal core of 3089±13 Ma, identical to the neighbor sample NP-09B. This feature also suggests a Mesoarchean age for the protolith of this unit. In addition, it is confirmed the Ediacaran age for the Zapicán diorite (585.8±5.7), even though it is younger than the previous report made by Oriolo et al. (2016a) of 610±2.5 Ma.

In the Valentines Block, a few Rhyacian inherited zircon xenocrystals with ages between 2200 and 2100 Ma were obtained in Cerro de Las Cuentas and Cerro Chato Ediacaran granitic plutons (NP-07 and NP-08, respectively). This evidence confirms previous ages obtained by Santos et al. (2003, 2017) and Oriolo et al. (2016a) for the widespread Paleoproterozoic basement rocks. Hence, the obtained results of our work confirm the post-orogenic Ediacaran magmatism that affected both the Valentines and Pavas blocks. These Ediacaran ages are comparable with several data reported in the literature for the Nico Pérez Terrane and the Dom Feliciano Belt (Mallmann et al., 2007; Sánchez Bettucci et al., 2010b; Oriolo et al., 2016a).

The widespread granitoid magmatism of Neoproterozoic age (see Fig. 2) was coupled with the marked structural NE-SW shearing trend (Preciozzi et al., 1979; Sánchez Bettucci et al., 2010b). This structural feature affected the entire terrane and caused important disturbances, as shown by multiple shear zones, mylonitic features and regional foliation (Sánchez Bettucci et al., 2010b). A good example is the Cueva del Tigre shear zone, represented by a group of granitic mylonites, schists and phyllonites at the boundary between the Valentines Block and La China Complex (Hartmann et al., 2001). In addition, extensional tectonics may have been the cause of the formation of restricted sedimentary basins affected by low grade metamorphism over the Valentines Block, such as the meta-sandstones and meta-subarkoses of the Cerro San Francisco Formation and the oolitic and stromatolitic meta-limestones of the Cerros Victoria Formation (Montaña and Sprechmann, 1993), during Neoproterozoic times.

The intrusion of the granitic plutons can be attributed to the influence of the long-term tectono-magmatic episode related to the Brasiliano orogenic cycle. The field work carried out during sampling, allows the recognition of the Retamosa thrust as the eastern limit of the Nico Pérez Terrane against the Dom Feliciano supracrustal sequences (Figs. 1 and 2). This thrust is characterized by NNE general orientation with ESE vergence, being cut in the southern portion by a NS normal fault. In the hanging wall Neoproterozoic supracrustal rocks outcrop.

5.4. Cratonic affinity of the Nico Pérez Terrane

The relative interaction between the Piedra Alta Terrane and the Nico Pérez Terrane (Fig. 1) has been interpreted in two different ways. According to Mallmann et al. (2007), Sánchez Bettucci et al. (2010a) and Santos et al. (2017, 2019) these terranes formed a coherent block, before the collision with the Kalahari Craton, when the Dom Feliciano Belt was formed. Therefore, the Nico Pérez Terrane would correspond to a reworked margin of the Río de la Plata Craton that was intensely deformed during the Brasiliano-Pan-African orogeny and adopted the NNE regional structural trend of the Dom Feliciano Belt. On the other hand, Oyhantçabal et al. (2011) challenged the Nico Pérez Terane affinity with the Río de la Plata Craton based on lithologic, structural, geochronologic, and isotopic data. Oriolo et al. (2016a) and Oyhantçabal et al. (2018) suggested that the Nico Pérez Terrane would be an allochthonous crustal segment, originally part of the Congo Craton, that was accreted to the east (currently coordinates) of Piedra Alta Terrane along the Sarandí del Yi shear zone in Ediacaran times. Although interesting, this proposal is still an unfinished debate.

According to Dragone et al. (2017), the Congo Craton presents negative gravity bouguer anomalies whereas the Río de La Plata Craton, including Nico Pérez Terrane, have positive bouguer anomalies suggesting distinct lithosphere. Also, these authors mentioned that Congo Craton has crustal thicknesses above 40 km, while the Río de la Plata has a thinner and denser crustal thickness around 35 km. Furthermore, Bologna et al. (2019) based on magnetotelluric data concluded that no lithospheric-scale contrast in the electrical resistivity between Valentines Block and Piedra Alta Terrane occurs across the Sarandí del Yí shear zone. In fact, Bologna et al. (2019) mentioned that the overall resistivity structures of the Valentines Block and Piedra Alta Terrane are similar. These authors also suggest these two units were already a single and stable tectonic unit probably since the Paleoproterozoic, inferring that the Sarandí del Yi shear zone should be relatively shallow. In addition, Bologna et al. (2019) suggested that Neoproterozoic magmatism might not affected the mantle root under the Valentines Block, concluding that this block is an extension of the Río de La Plata Craton.

The Piedra Alta Terrane, at the core of the Río de La Plata Craton, presents partially different tectonic evolution, even though Archean ages were recently identified in some of its lithological units (Santos et al., 2017). The influence of the Brasiliano orogenic cycle, which is widespread in the Nico Pérez Terrane, is also recorded in some units of the Piedra Alta Terrane (Río de La Plata Craton) according to Santos et al. (2017). Up to now, the only Ediacaran magmatic body in the Piedra Alta Terrane is the anorogenic La Paz granite, with an age of 587 Ma (Cingolani et al., 2012).

In a general way, the mainly Rhyacian units of the Piedra Alta Terrane, located within Arroyo Grande and San José belts, were originated in a magmatic arc setting, where tonalites, trondhjemites and granites predominate (Sánchez Bettucci et al., 2010a and references therein). Nevertheles, in the Valentines Block (Nico Pérez Terrane) Rhyacian ages are present being the different units strongly deformed and metamorphosed by the Brasiliano orogenic cycle. A similar relation is encountered to the north, within the Rivera Block.

The exposed crustal level at the present surface in the Piedra Alta Terrane and the Nico Pérez Terrane seems to be different, considering the presence of a great amount of granulitic rocks in the Nico Pérez Terrane. In contrast, high grade metamorphic rocks are restricted to the Arroyo los Alamos Unit in the Piedra alta Terrane (Oyhantçabal et al., 2007), indicating a higher crustal level. A possibility to explain this difference could be to envisage local displacements along the Sarandí del Yí shear zone, which was certainly one of the main tectonic elements in the Ediacaran/Cambrian times (Oriolo et al., 2016b). It could also correspond to a pre-collision feature. The Rhyacian development of the bulk of the Piedra Alta Terrane possibly sets an upper time limit for the beginning of its behavior as a fault zone. From that time, the Sarandí del Yí shear zone was probably acting at successive pulses. Another speculative scenario could have been a thrust displacement of the Nico Pérez Terrane over the Piedra Alta Terrane, in which the Nico Pérez Terrane was uplifted and a deeper cortical level is now exposed at the present surface. This is supported by the recent work of Bologna et al. (2019) where the crust and upper mantle high-resistivity show no lithospheric-scale contrast across the Sarandí del Yí shear zone. This suggests local displacement, not affecting the upper mantle. Also, the preliminary results of Moho depth and Vp/Vs ratio presented by Rodríguez et al. (2019) using one station near Tacuarembó (Nico Pérez Terrane) and the other near Salto city (Piedra Alta Terrane) show similar depth for the Moho.

Another important correlation between the Piedra Alta Terrane and the Nico Pérez Terrane could be the Statherian extensional tectonic event suggested by the emplacement of the Illescas rapakivi granite intruding the Valentines Block and the widespread Florida dike swarm occupying a great part of the Piedra Alta Terrane. Dyke swarms are distinctive representatives of mafic igneous activity associated with major continental rifting/plume events (e.g., Ernst and Buchan, 1997; Salminen et al., 2019) and are frequently related with rapakivi granites, particularly in Precambrian times. These dyke swarms associated with rapakivi granites, of similar ages, are an important tool for performing paleogeographic reconstructions of supercontinents. In Fact, rapakivi magmatism presents a characteristic temporal distribution in the world that could be contemporaneous with the supercontinent cycles (Larin, 2009; Salminen et al., 2019, and references therein).

The above mentioned evidence of reworking in the Nico Pérez Terrane could be indicative of a metacratonization in the sense of Abdelsalam et al. (2011), Liégeois et al. (2013), Santos et al. (2017, 2019) and Girelli et al. (2018), during the Neoproterozoic Brasiliano-Pan African orogenic cycle (Mallmann et al., 2007; Sánchez Bettucci et al., 2010b; Oriolo et al., 2017; among others).

6. Conclusions

The Nico Pérez Terrane is made up of a mosaic of tectonic blocks with different sizes, comprising a variety of rocks with different age and geological history. Archean granitoids are present in the Pavas Block, limited to the west by Cueva del Tigre shear zone. This block is affected by greenschist to amphibolite facies metamorphism, recording multiple metamorphic episodes and deformation. For the Valentines and Rivera blocks, a similar structural situation occurs, but the predominant rocks are Rhyacian granulites.

The Brasiliano-Pan African orogenic cycle, responsible for the amalgamation of Gondwana, affected the Nico Pérez Terrane and produced tectono-thermal activity and strong reworking of its rocks.

Acknowledgements
We are grateful to F. Santos Fernandes, V. Camara Maurer, E. Abelenda for the support in the laboratory and fieldwork and A. Latorres for the english review. Financial support was received from the Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) through grant 2013/12754-0 to UGC. We would like to thank S. Oriolo and the anonymous reviewer for the rigorous and constructive comments and suggestions.

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