Precambrian geotectonic units of the Río de La Plata craton

This article was downloaded by: [Instituto de Geociencias - USP] On: 20 November 2009 Access details: Access Details: [subscription number 906065936] Publisher Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House, 3741 Mortimer Street, London W1T 3JH, UK International Geology Review Publication details, including instructions for authors and subscription information: http://www.informaworld.com/smpp/title~content=t902953900 Precambrian geotectonic units of the Río de La Plata craton Leda Sánchez Bettucci a; Elena Peel ab; Pedro Oyhantçabal a Instituto de Ciencias Geológicas, Facultad de Ciencias, Iguá, Montevideo, Uruguay b Instituto de Geociências da Universidade de São Paulo, Cidade Universitaria, São Paulo-SP, Brazil a Online publication date: 16 November 2009 To cite this Article Sánchez Bettucci, Leda, Peel, Elena and Oyhantçabal, Pedro(2009) 'Precambrian geotectonic units of the Río de La Plata craton', International Geology Review, 52: 1, 32 — 50 To link to this Article: DOI: 10.1080/00206810903211104 URL: http://dx.doi.org/10.1080/00206810903211104 PLEASE SCROLL DOWN FOR ARTICLE Full terms and conditions of use: http://www.informaworld.com/terms-and-conditions-of-access.pdf This article may be used for research, teaching and private study purposes. Any substantial or systematic reproduction, re-distribution, re-selling, loan or sub-licensing, systematic supply or distribution in any form to anyone is expressly forbidden. The publisher does not give any warranty express or implied or make any representation that the contents will be complete or accurate or up to date. The accuracy of any instructions, formulae and drug doses should be independently verified with primary sources. The publisher shall not be liable for any loss, actions, claims, proceedings, demand or costs or damages whatsoever or howsoever caused arising directly or indirectly in connection with or arising out of the use of this material. International Geology Review Vol. 52, No. 1, January 2010, 32–50 Precambrian geotectonic units of the Río de La Plata craton 1938-2839 0020-6814 Geology Review, TIGR International Review Vol. 1, No. 1, Oct 2009: pp. 0–0 Leda Sánchez Bettuccia*, Elena Peela,b and Pedro Oyhantçabala International L.S. Bettucci et Geology al. Review a Instituto de Ciencias Geológicas, Facultad de Ciencias, Iguá, Montevideo, Uruguay; b Instituto de Geociências da Universidade de São Paulo, Cidade Universitaria, São Paulo-SP, Brazil Downloaded By: [Instituto de Geociencias - USP] At: 19:43 20 November 2009 (Accepted 13 June 2009) The main Precambrian tectonic units of Uruguay include the Piedra Alta tectonostratigraphic terrane (PATT) and Nico Pérez tectonostratigraphic terrane (NPTT), separated by the Sarandí del Yí high-strain zone. Both terranes are well exposed in the Río de La Plata craton (RPC). Although these tectonic units are geographically small, they record a wide span of geologic time. Therefore improved geological knowledge of this area provides a fuller understanding of the evolution of the core of South America. The PATT is constituted by low- to medium-grade metamorphic belts (ca. 2.1 Ga); its petrotectonic associations such as metavolcanic units, conglomerates, banded iron formations, and turbiditic deposits suggest a back-arc or a trench-basin setting. Also in the PATT, a late to post-orogenic, arc-related layered mafic complex (2.3–1.9 Ga), followed by A-type granites (2.08 Ga), and finally a taphrogenic mafic dike swarm (1.78 Ga) occur. The less thoroughly studied NPTT consists of Palaeoproterozoic high-grade metamorphic sequences (ca. 2.2 Ga), mylonites and postorogenic and rapakivi granites (1.75 Ga). The Brasiliano-Pan African orogeny affected this terrane. Neoproterozoic cover occurs in both tectonostratigraphic terranes, but is more developed in the NPTT. Over the past 15 years, new isotopic studies have improved our recognition of different tectonic events and associated processes, such as reactivation of shear zones and fluids circulation. Transamazonian and Statherian tectonic events were recognized in the RPC. Based on magmatism, deformation, basin development and metamorphism, we propose a scheme for the Precambrian tectonic evolution of Uruguay, which is summarized in the first Palaeoproterozoic tectonic map of the Río de La Plata craton. Keywords: Palaeoproterozoic; Río de La Plata craton; tectonic map; metamorphic belts; granitic intrusions; Uruguay Introduction The geology of Uruguay has been described in several articles, mostly published in Uruguayan and Brazilian journals and/or congresses. During the last decade, several articles published in international journals have reported the geologic complexity but also noted scarcity of geochemical and geochronological data. The South American platform, which is characterized by two lithospheric components: cratonic areas and orogenic belts partially covered by sedimentary basins (Almeida et al. 1981, 2000), is represented by five Palaeoproterozoic cratons: Río de La Plata, Amazonian, São Francisco, São Luiz and Luis Alves. These cratons were involved in different *Corresponding author. Email: [email protected] ISSN 0020-6814 print/ISSN 1938-2839 online © 2010 Taylor & Francis DOI: 10.1080/00206810903211104 http://www.informaworld.com Downloaded By: [Instituto de Geociencias - USP] At: 19:43 20 November 2009 International Geology Review 33 amalgamation and break-up events during consecutive orogenic cycles (Cordani et al. 2000). During Palaeoproterozoic time these cratons were assembled as the Atlantica supercontinent (sensu Rogers 1996; Rogers and Santosh 2002, 2003; Meert 2002, Hou et al. 2008, among others); in Mesoproterozoic time, they belonged to the supercontinent Rodinia or Palaeopangea (see Piper 1982, 2000); in the Neoproterozoic, they were part of western Gondwana; finally, during the Carboniferous, they were part of Pangaea. Rogers and Santosh (2002, 2004) suggest that Atlantica formed at 2.1–2.0 Ga attending a period of rapid crustal growth and mantle reorganization (Condie 2000, 2002). In Uruguay, mafic dike swarm and anorogenic rapakivi granites formed at ca. 1.8 Ga may represent the breakup or widespread extension of this supercontinent or the Staterian taphrogenic episode (Brito Neves et al. 1995). Also the location of the Río de La Plata craton (RPC) in the palaeogeographic reconstruction of Rodinia proposed by Zhao et al. (2002), together with the synchronic distribution of rapakivi granites (Figure 1) with ages around 1.7 Ga showed by Vigneresse (2005), seems to be consistent with the available data. Rodinia probably formed at ca. 1.1 Ga and was dismembered into three blocks between 0.8 and 0.6 Ma (Rogers and Santosh 2004). Paleomagnetic data and paleogeographic models propose the break-up of Rodinia at ca. 750 Ma (Powell et al. 1993; Dalziel 1997; Thover et al. 2006; among others). The orogenic episodes and crustal growth events developed between 2.5 and 2.0 Ga, particularly during the Transamazonian Cycle form a large belt from Venezuela to Amapá in northern Brazil and discontinuous belts in the São Francisco and Río de La Plata cratons. Those events and their consequential belts have been characterized in Guyana, French Guiana, Venezuela, Suriname, Brazil, Argentina, and Uruguay. The orogenic cycles registered in Uruguay are the Transamazonic cycle in the SW and the Brasiliano-Pan African cycle in the SE Phanaerozoic sedimentary basins include the Figure 1. Main tectonic units of Uruguay – Piedra Alta and Nico Pérez tectonostratigraphic terranes, Dom Feliciano belt (including basement inliers). L. Sánchez Bettucci et al. Downloaded By: [Instituto de Geociencias - USP] At: 19:43 20 November 2009 34 Figure 2. Tectonic map of Uruguay. Paraná foreland basin, related to the Gondwana supercontinent, exposed in the NE and extensional magmatism related to rifting and the break-up of Gondwana in the NW region. The Uruguayan Precambrian basement is divided into two major tectonic units (Figure 2), the Piedra Alta tectonostratigraphic terrane (PATT) and the Nico Pérez tectonostratigraphic terrane (NPTT). These terranes are separated by the Sarandí del Yí Shear Zone. The PATT crops out to the west of the shear zone and it includes low to medium metamorphic orogenic belts (ca. 2.1 Ga), layered mafic complex, late to post-orogenic magmatism (1.9–2.3 Ga), A-type-rapakivi granites (2.078 Ga) and finally extensional magmatism (1.7 Ga) represented by a mafic dike swarm. These units were assigned to Transamazonic orogenic cycle (Choubert 1964; Choubert and Faure-Muret 1969; Almeida et al. 1973). In Uruguay, the Transamazonic orogenic cycle was historically used as a chronostratigraphic 35 Downloaded By: [Instituto de Geociencias - USP] At: 19:43 20 November 2009 International Geology Review term without tectonic connotation. The NPTT is constituted by medium- to high-grade metamorphic orogenic belts (Pavas, Valentines and Rivera blocks) and anorogenic granite (rapakivi) with an age of 1.78 Ga. The NPTT was affected by Neoproterozoic events of the Brasiliano-Pan African orogenic cycle. The aim of this work is to present the tectonic evolution of the Palaeoproterozoic units of the Río de La Plata Craton summarized in a tectonic map. Background The RPC (sensu Almeida et al. 1973) occupies approximately the third part of the southwestern region of Uruguay (Figure 2). The first descriptions of western part of the RPC Downloaded By: [Instituto de Geociencias - USP] At: 19:43 20 November 2009 36 L. Sánchez Bettucci et al. were presented by d’Orbigny (1844) and Darwin (1846) and in more detail by Walther (1911, 1919, 1920, 1924 and 1927). The first regional approach was made by Mc Millan (1933) who established the first great division separating, on the one hand, the Archaic Complex (PATT and NPTT) and, on the other hand, the Minas Series (Dom Feliciano Belt). The first geologic map (1:500.000) was elaborated by Caorsi and Goñi (1958). They suggested that the basement of Uruguay was part of Guyana–Brasilia– Patagonia shield, with similar ages of South African, Laurentian–Siberian and Scandinavian shields. Later, Ferrando and Fernández (1971) made the first chronostratigraphic interpretation based on isotopic data presented by Bossi (1966), Hart (1966), and Umpierre and Halpern (1971). Two major units were recognized by Ferrando and Fernández (1971), one of them correlated to Baikalian cycle (850–650 Ma) and the other one correlated to the older LimpopoKibali (Africa) event (ca. 2000 Ma). From the first isotopic data made by Hart (1966) systematic studies began, and areas affected by different orogenic cycles were recognized. The first geo-structural map of Uruguay (Preciozzi et al. 1979) established a new stratigraphic column where the basement units were defined following chrono-lithostratigraphic criteria. This map resulted in a better comprehension of the principal mechanism involving tectonic processes. Río de La Plata craton Lithologies belonging to the RPC crop out in the southwest of Uruguay and in the neighbourhood of Tandil mountain ranges in Argentina (Almeida et al. 1973). According to Fragoso Cesar and Soliani (1984), it extends towards the north of Uruguay cropping out in Rivera and Aceguá (Uruguay–Brazil border). This craton is also exposed in Brazil, in the eastern part of Rio Grande do Sul State (Taquarembo and Encruzilhada blocks); and in the eastern part of Santa Catarina State (Luiz Alves block sensu Fragoso Cesar and Soliani 1984). Rb-Sr isochron data in Aceguá and Rivera regions show ages of 2.272 ± 33 Ma (gneisses and granitoids). These rocks are intruded by Brasiliano granites with Rb/Sr ages around 690 and 580 Ma (Soliani 1986). Juvenile Palaeoproterozoic Piedra Alta tectonostratigraphic terrane The PATT is composed almost entirely (Hasui et al. 1975) of plutonic, granite-gneissic terranes and of low to medium volcano-sedimentary metamorphic belts with E-W structural trend. These volcano-sedimentary orogenic belts were originally named, from north to south, as Arroyo Grande (Ferrando and Fernández 1971), Paso Severino, and Montevideo Formations (Bossi et al. 1965). These sequences were metamorphosed under low- to medium-grade conditions and folded forming synclinoria with vertical foliations, symptomatic of important horizontal shortening. These sequences represent a Palaeoproterozoic thrust belt. The sedimentary sequence may represent turbiditic deposits with volcanic intercalations. In addition, features similar to greenstone belts were indicated in Paso Severino and Arroyo Grande Formations (Fragoso Cesar 1984; Fragoso Cesar et al. 1987; Bossi et al. 1996). Bossi et al. (1993) defined the Piedra Alta terrane as a part of the RPC located to the west of Sarandí del Yí shear zone (Figure 3). Peel and Preciozzi (2006) suggested that the PATT represents a juvenile Palaeoproterozoic unit stable from 1.7 Ga without record of the Neoproterozoic orogenies. It is considered as the best exposed Palaeoproterozoic area of the RPC. This terrane is represented by low- and medium-grade metamorphic belts (Arroyo Grande, San José – including the Paso Severino Formation – and Montevideo), Downloaded By: [Instituto de Geociencias - USP] At: 19:43 20 November 2009 International Geology Review 37 Figure 3. Palaeogeographic reconstruction modified from Zhao et al. (2002). Ages of Korosten, Pitinga, Tapajos, Shachang rapakivi granites taken from Vigneresse (2005). AUS, Australia; B, Baltica and eastern Europe; C, Congo; SF, San Francisco; EA, East Antartica; G, Greenland; IND, India; K, Kalahari; M, Madagascar; NA, North America; RP, Río de La Plata craton; S, Siberia; NC, North China. separated by important granitic-gneissic areas (Bossi et al. 1993; Oyhantçabal et al. 2007). These metamorphic belts were formerly considered greenstone belts and correlated with granitic-greenstone belts of Rio Grande do Sul by Fragoso Cesar (1984); Soliani (1986) and Bossi et al. (1996). Anatectic granitoids and migmatites are developed within the basement of the supracrustal belts. These rocks show two phases of ductile deformation and a brittle one recognized by Bossi et al. (1998). The first ductile deformational phase generated tight folds and metamorphism in the volcano-sedimentary sequences. The second one consists of a thrusting phase associated to peraluminous muscovitic granites and it is related to the development of open folds in mylonites (Garat 1990). The anatectic processes occurred contemporaneously with an important bimodal magmatism. The anatectic granitoids are related with basic magmas (dioritic). Also a typical example of heterogeneous migmatites with phlebitic to stromatic structures occurs ranging laterally to granites and gneissic rocks (e.g. ‘AFE’ quarry in Suárez town; Coronel and Oyhantçabal 1988). The structure of the migmatites varies considerably at the scale of the outcrops. These morphological types of migmatites may be the result of the structure and composition of the parent rocks, segregation and/or migration of partial melts and deformation during and after the process of the migmatite formation (Ashworth and McLellan 1985). Mixing and mingling phenomena are frequently observed (outcrops of Piedra Alta and Cerro Colorado Colorado localities). An age of 2100 ± 3.3 Ma was obtained for the Piedra Alta outcrop (Preciozzi et al. 2005). Amphibolites and migmatites surround these granitic bodies. The Piedra Alta outcrop, near Florida city, consists of a graniticgranodioritic intrusion, cut by micro-granodioritic dikes. The existence of continuous interpenetration and diverse flow structures allow inference that the leucocratic and melanocratic components are involved as two magmas without significant thermal contrast between them. 38 L. Sánchez Bettucci et al. Downloaded By: [Instituto de Geociencias - USP] At: 19:43 20 November 2009 Low- and medium-grade orogenic belts (Palaeoproterozoic volcano-sedimentary belts, ca. 2.1 Ga) Three metamorphic belts were originally defined by Bossi et al. (1993): Arroyo Grande, San José, and Montevideo, with regional trend near N80° E (Figure 3). The metamorphic grade varies from green-schist to amphibolite facies. Oyhantçabal et al. (2003) postulated the continuity between San José and Montevideo belts and consider both as one tectonic unit: the San José belt. These authors established that the outcrop areas of Precambrian rocks were interrupted by the development of a Mesozoic rift (named Santa Lucía Basin). The Arroyo Grande belt (sensu Bossi et al. 1993) is delimited by faults, being in tectonic contact with a granitic-gneissic area. Its general trend is E–W and it comprises igneous, sedimentary, and volcanic rocks. The sedimentary sequence is represented by quartzite, metaconglomerate, metapelite, chlorite-schist, and amphibolites. The volcanism is bimodal (basalt – rhyolite). The igneous rocks are represented by peridotites, piroxenites, gabbros, and horblendite (Preciozzi 1989; Bossi et al. 1998). This belt is cut by a late to postorogenic intrusion (Marincho Complex, Paso del Puerto Granite). The San José belt is constituted by the Paso Severino, San José, and Montevideo Formations (Oyhantçabal et al. 2003). Paso Severino Formation is represented by volcanic rocks (rhyolite and basalt), metapelite, some carbonatic rocks (dolomitic marble), and some level of banded iron formations (BIFs) (Algoma type). The BIF deposits have lateral extents (∼1 km), with thicknesses in the range of 1–3 m. This formation hosts several mineralizations: Cu associated with phyllites and interstratified rhyolites, Au in quartz veins, talc related to metabasic rocks, manganese, and iron formation. The San José belt was affected by green-schist and amphibolite metamorphic conditions. U-Pb (SHRIMP, zircon) isotopic analysis made in acidic metavolcanic rocks of Paso Severino Formation yielded an age of 2146 ± 7 Ma (Santos et al. 2003). Preciozzi (1993) recognized two deformation phases, the first one syn-metamorphic generating folds with vertical axes, and the second one characterized by fold superposition with horizontal axes. The latter phase affected latetectonic granites and it could be related to gabbro-granitic magmatism at 2.0 Ga (Mahoma – Guaycurú Complex). The San Juan Unit belonging to the San José belt is constituted by volcanic rocks (acidic metatuff), which yielded a U-Pb conventional age of 1.753 ± 5.7 Ga (Preciozzi et al. 2005). Xenocrystals in these rocks yielded ages of 2358 Ma. The San José Formation is represented by metavolcanic rocks (basic to acidic) and a metasedimentary succession. Based on geochemical data of San José Formation, Mutti et al. (1995) and Bossi et al. (1996) suggested that volcanic rocks (rhyolites, rhyodacites, andesites and basalts) varied from tholeiitic to calk-alkaline magmas. These authors suggested an evolution from an extensional (mantelic plume) to subductional (compressional) regimen related to an immature volcanic arc. The Montevideo Formation (sensu Oyhantçabal et al. 2003) is constituted by amphibolites, micaschists, and gneisses. This formation reaches the amphibolite facies and is consistent with island arc/back-arc basins (sensu Veizer 1983). The Montevideo Formation presents an approximately E–W trend and it is situated along the southern margin of the PATT (Figure 3). Walther (1948) has provided the most important and exhaustive petrography of the Montevideo basement. The distinctive lithologies are oligoclase and biotite gneisses, amphibolite, and micaschist (Bossi et al. 1975; Preciozzi et al. 1985, 1991; Coronel and Oyhantçabal 1988) cut by aplites and pegmatites. The available isotopic data for this formation are scarce; however, an age (U-Pb, SHRIMP) on a gneiss yields 2165 ± 38 Ma (Santos et al. 2003) and U-Pb (zircon, conventional) yields an age of 2158 +24/–23 Ma upper intercept interpreted as magmatic crystallization (Preciozzi et al. 2005). International Geology Review 39 The San José belt is cut by late to post-orogenic magmatism (Albornoz Complex) with ages between 2.3 and 1.9 Ga by ultramafic layered rocks (The Cerros Negros Complex) and by a dike swarm (1.7 Ga). The late to post-orogenic rocks are represented by granites, granodiorites, migmatites, aplites, and pegmatites. The aplites and pegmatites also cut amphibolites and gneisses of the granitic-gneissic area (Bossi and Navarro 1991). The relationship between sediments, volcanic rocks, and granites may represent intra-arc or a back-arc tectonic setting; both possibilities are plausible and both suppose subductionrelated processes. Downloaded By: [Instituto de Geociencias - USP] At: 19:43 20 November 2009 Late to post-orogenic magmatism (Albornoz Complex): calcalkaline, peraluminous, and alkaline granites and gabbros (2.3–1.9 Ga) This unit comprises acidic and basic batholith intrusions emplaced into supracrustal rocks of Arroyo Grande and San José belts (Figure 3). These complexes are known as Isla Mala, Cufré, Marincho, and Mahoma Plutons (Figure 4a). These granites are unfoliated or have only a brittle and non-persistent foliation. Traditionally, they are considered as ‘post-tectonic’ or ‘postkinematic’ granites. The first isotopic ages obtained in some plutons of the postorogenic magmatism varied from 2.5 to 1.8 Ga (Hart 1966; Umpierre and Halpern 1971; Preciozzi and Bourne 1992, 1993; Cingolani et al. 1997). More recent studies help us to locate this magmatism between 2053 and 2086 Ma (Hartmann et al. 2000; Peel and Preciozzi 2006). The ages published by theses authors correspond to numerous bodies of granites, aplites, pegmatites, as well as xenoliths of mica-schist inside granodiorites. U-Pb (conventional) isotopic ages of 2053 ± 14 Ma and 2086 ± 11 Ma were obtained for Cufré and Isla Mala granitic Complex, respectively (Peel and Preciozzi 2006). U-Pb (zircon, SHRIMP) ages in the last complex show values between 2065 ± 9 Ma and 2074 ± 6 Ma (Hartmann et al. 2000). Granitic intrusions in this continental crust are important indicators of tectonic regimes (ancient arc) in the past, as well as potential sources of information on the composition and history of deep crustal protoliths. Mafic to ultramafic layered complexes The Cerros Negros complex, intruding the San José belt, is represented by pyroxenite, gabbro, leuco-gabbro and anorthosite. It displays a conspicuous banding and is affected by deformation and low-grade metamorphism. Also, it is cut in the southern part by the Cufré shear zone, and to the north and west is intruded by granitic intrusions (Oyhantçabal et al. 2007). The Mahoma Gabbro commonly contains cummulate layers consistent with fractional crystallization in a magma chamber. Petrographically, this unit presents magmatic fluidal and cummular textures, and is composed of plagioclase (An55–68), pyroxene, olivine, subordinate amphibole, apatite, and biotite as accessory minerals. Oyhantçabal et al. (1990) indicated the presence of clinopyroxene (En50–70) and inverted pigeonite as cummular minerals. Oyhantçabal et al. (1990) and Villar and Segal (1990) suggested that the Mahoma cummulate presents low differentiation and was emplaced at upper crustal levels during Transamazonic cycle. Oyhantçabal et al. (1990) obtained in the gabbro a K-Ar (Pl) age of 2033 ± 44 Ma. Related to the Rospide Gabbro, an important level of Ti-magnetite deposit occurs. Cingolani et al. (1997) obtained a Rb-Sr (WR) age of 2016 ± 108 (R0 = 0.7002) for the Carreta Quemada Gabbro. 40 L. Sánchez Bettucci et al. The geological setting of Mahoma and Rospide gabbros and their ages (2033 Ma) is reliable with an emplacement in a postorogenic extensional tectonic setting or crustal rifting (sensu Naldrett 2004). Downloaded By: [Instituto de Geociencias - USP] At: 19:43 20 November 2009 A-type Soca granite The Soca granite was first identified by Jones (1956) and named Tío Diego granodiorite and renamed by Bossi et al. (1965) as Soca granite. It is located 65 km to the east of Montevideo city outcropping in the eastern portion of Piedra Alta Terrane (Figure 4a and b). This pluton is emplaced into graphite mica-schist, quartzites, gneisses, amphibolites, and deformed granitoids belonging to the Montevideo Formation of the San José belt (Oyhantçabal et al. 1998). The geometry of this pluton is approximately elliptical; the eastern border is cut by Sarandí del Yí shear zone. This granite was defined as subalkaline, metaluminous or slightly peraluminous A-type rapakivi granite based on chemical and petrographic analysis made by Oyhantçabal et al. (1998). These authors suggest that this granite presents similar features of typical Scandinavian and central Amazonian rapakivi granites (sensu Rämö and Haapala 1995; Haapala and Rämö 1999; Dall’Agnol et al. 1999, 2005). The Soca granite was emplaced after the Transamazonic orogenic cycle and before the dextral shearing of Sarandí de Yí shear zone. Chemically this rapakivi granite (sensu Oyhantçabal et al. 1998) presents an important enrichment in L-REE and high field strength elements. This body has a U-Pb (zircon, conventional) isotopic age of 2078 ± 8 Ma (Peel and Preciozzi 2006) and a U–Pb (SHRIMP) age of 2056 ± 6 Ma (Santos et al. 2003). Although this age is similar to the late and post-orogenic magmatism mentioned above, the available data are inconsistent with the time span necessary for the formation of rapakivi magma in the lower crust and its emplacement in the upper crust. In other parts of the world where rapakivi granites occur, they are associated with extensive sub-parallel tholeiitic dike swarms (Haapala and Rämö 1992). Thus, one possibility is that the ages obtained for Soca granite could represent zircon heritage. Another possibility is that the continental crust was very thick and probably involved reworking of an older crust; however, it is not supported by evidences of juvenile crust of PATT. Moreover, some rapakivi granitoids occur emplaced in post-collisional/post-orogenic extensional tectonic environment (Zhang et al. 2007). The Soca granite is also 200– 400 Ma older than the oldest rapakivi granite of Amazonia, Laurentia and Fennoscandia. The question is whether this A-type granite represents a post-tectonic alkaline intrusion regarding the short time span after orogenic event, or it is a rapakivi granite. Consequently, the Soca Granite seems not to be correlated with other anorogenic events around the world. Extensional magmatism (1.7 Ga late Palaeoproterozoic–early Statherian): mafic dike swarm The Florida mafic dike swarm, nearly 100 km wide, crops out along more than 300 km (Figure 3), cutting the RPC. Generally, these dikes outcrop vertically. However, some of them dip 70 S (Bossi and Campal 1991; Teixeira et al. 1999). They have thicknesses between 2 and 50 metres and lengths of 1 to 2 kilometres. Bossi and Campal (1992) suggested that the dike swarm is deflected by Sarandí del Yí shear zone, revealed by the curvature in its eastern part making up a ‘drag fold’ (Figure 3). In fact, no dikes occur to the east of the Sarandí del Yí shear zone. Petrologically and geochemically, the dikes are divided into two groups, one of high TiO2 andesites and the other of low TiO2 andesite-basalts (Bossi and Campal 1991; Bossi et al. 1993; Halls et al. 2001). The mafic dikes have an Ar-Ar isotopic age of 1725 ± 10 Ma (Teixeira et al. 1999). Halls et al. (2001) established that the minimum International Geology Review 41 (A) PLSZ Marincho Complex TRINIDAD N FLORIDA Río d 34° CSZ el aP ta la SANTA LUCIA RIFT Dikes Complex Downloaded By: [Instituto de Geociencias - USP] At: 19:43 20 November 2009 Complex Rosario MSZ Intrusives B 0 PIRIAPOLIS 100 km 56° 58° (B) N Empalme Olmos 8 Río la de ta Pla Soca 34º41´ Atlántida Figure 4. (A) Geographic distribution of main Palaeoproterozoic granitic plutons. (B) Geological sketch of Soca rapakivi granite. MSZ, Mosquitos shear zone; CSZ, Cufré shear zone; PLSZ, Paso Lugo shear zone. 42 L. Sánchez Bettucci et al. Downloaded By: [Instituto de Geociencias - USP] At: 19:43 20 November 2009 age of the PATT is constrained by this dike swarm, based on U/Pb (baddeleyite) age of 1790 ± 5 Ma. The generation of the dike swarm is associated with an extensional aborted event occurred around ca. 1790 Ma based on Halls et al. (2001) data. Palaeomagnetic studies performed by Pacca (1999) and Pacca and D´Agrella (1999) on the mafic dikes yielded a palaeomagnetic pole in 337.0°E; 74.4°N. These authors also suggested a primary thermoremanent magnetization. Whereas, Halls et al. (2001) provided a palaeomagnetic pole suggesting that the remanence did not yield a demonstrable primary origin. The emplacement ca. 1.7 Ga of the east-northeast regional trend mafic dike swarm implies pervasive extension during this period in this part of the RPC. The assembly of this region – after 1.7 Ga – (PATT and NPTT – see below) probably involved transcurrent movements juxtaposing different Palaeoproterozoic histories side by side of the Sarandí del Yí shear zone. The Nico Pérez tectonostratigraphic terrane The NPTT is located between the Sarandí del Yí and the Fraile Muerto-María Albina shear zone (Figure 2). Formerly, it was separated by Preciozzi et al. (1979) as Valentines block. Later, Bossi and Campal (1992) redefined it as Terreno Nico Pérez. We adopt here the term NPTT but with the original limits established by Preciozzi et al. (1979). This terrane is constituted by Palaeoproterozoic high-grade metamorphic sequences, granites, mylonites, and postectonic Brasiliano magmatism. In spite of the scarcity of data and following the original scheme of Preciozzi et al. (1979), we describe three Palaeoproterozoic units affected by medium and high metamorphic grade: Pavas, Valentines, and Rivera blocks (Figure 5). These blocks were reworked by the Brasiliano orogenic cycle. In addition, within this terrane an A-type rapakivi granite was reported. Medium- and high-grade orogenic blocks These orogenic blocks are Palaeoproterozoic and/or Mesoproterozoic, reworked by the Brasiliano event (Pavas; Valentines and Rivera Blocks, see Figure 5). Late to postorogenic granites Undivided granitoids Low-grade metamorphic rocks Undivided metamorphic rocks Pyroxenites, deformed granites, granulitic gneisses,lherzolites Faults Rivera block 31°38´ Rive ra s hear zone Vichadero N 0 10 20 Km 55° Figure 5. Geological sketch of the Nico Pérez tectonostratigraphic terrane showing its main units. 1: Valentines block, 2: Pavas block and 3: Rivera block. In the upper right border of the figure a detailed geology of Rivera block is presented (After Ellis 1998). CTSZ, Cueva del Tigre shear zone; FM-MASZ, Fraile Muerto-María albina shear zone; RSZ, Rivera shear zone. International Geology Review 43 Downloaded By: [Instituto de Geociencias - USP] At: 19:43 20 November 2009 Pavas block The Pavas block forms an elongated unit with NNE trend limited by de Fraile MuertoMaría Albina shear zone to the east and south and the Cueva del Tigre shear zone to the west and north (see Figure 5). It is represented by amphibolic gneisses, amphibolites, quartzites with fucsite, and meta-ultramafics – tremolite and actinolite schists – (Preciozzi et al. 1979; Oyhantçabal and Vaz 1990). U-Pb (SHRIMP) in zircons of tonalitic orthogneisses analysed by Hartmann et al. (2001) yielded ages of 3.41 Ga (core) and from 3.1 to 2.7 Ga (rim). We can interpret these ages as Palaeoproterozoic inheritance of older crust. Mesoproterozoic ages of 1252 Ma (K-Ar and Rb-Sr, Campal et al. 1995) were obtained in mylonites of Cueva del Tigre shear zone (Figure 5) developed between this block and Valentines block. These authors interpret these data as Grenvillian ages. Due to the lack of detailed studies, this cooling age could not be reliably interpreted (Preciozzi et al. (1979) ~ La China Complex (sensu Hartmann et al. 2001). Valentines block The Valentines block (or Valentines granulitic belt sensu Fragoso Cesar 1984) is located between Sarandí del Yí shear zone and Cueva del Tigre shear zone, next to the Pavas block (see Figure 5). It is represented by granulitic gneisses, metapiroxenites and magnetiteaugite quartzites. Santos et al. (2003) suggested, based on U-Pb (SHRIMP) isotopic data, an age of 2058 ± 3 Ma for the metamorphism and an age of 2163 ± 8 for the protholith of the Valentines block granulites. This block was affected by Neoproterozoic granitic intrusions (0.9–0.5 Ga, Preciozzi et al. 2001) and by the Tupambaé shear zone (see Figure 2) (Preciozzi et al. 1979). Rivera block This block, located in the NE part of Uruguay (Figure 5), is isolated from the previous blocks by a Palaeozoic sedimentary sequence. Piroxenites, deformed granites, sillimanite gneisses, granulitic gneisses, lherzolites, quartzites, meta-anorthosites, forsterite marbles, meta-basites, and coarse charnokitic ortho-gneisses with intercalated ironstones constitute the basement. The regional trends are EW to NW and NE associated to ductile shear zones (Figure 5), which can be traced for about 110 km. The supracrustal rocks are a low-grade metasedimentary sequence intruded by granites with ages around 600 Ma (Cordani and Soliani 1990). The granulitic gneisses present a Rb-Sr (WR) age of 2250 ± 60 Ma, which was interpreted as the main metamorphic event (Cordani and Soliani 1990). More recently, Santos et al. (2003) obtained ages (U-Pb) on meta-trondhjemite of 2140 ± 6 Ma (crystallization age) and 2077 ± 6 Ma for the metamorphic event. Important ore deposits (gold) occur related to the regional trends. The main alteration assemblage related with gold mineralization comprises chlorite + epidote + carbonate + sericite + silica + pyrite (Preciozzi et al. 1979). Anorogenic rapakivi granites (1.7 Ga) Even though regional geological literature mentioned some plutons as rapakivi granites, there is a lack of petrologic, geochemical and geological descriptions to support a serious tectonic evolution. A-type rapakivi granite was described by Soliani (1986) nearby Minas de Corrales locality – Rivera block. This author reported an age of 1.75 Ga (Rb-Sr) for the body. Another granite is located in Valentines block, known as Illescas Batholith (Campal 44 L. Sánchez Bettucci et al. and Schipilov 1995). The ages determined there are Rb/Sr (WR) 1760 ± 32 Ma (Bossi and Campal 1992) and Pb-Pb 1784 ± 5 Ma (Campal and Schipilov 1995). This granite presents ductile deformation in the borders and is cut by Sarandí del Yí shear zone. Downloaded By: [Instituto de Geociencias - USP] At: 19:43 20 November 2009 Neoproterozoic cover The Neoproterozoic cover in the PATT is represented by Piedras de Afilar Formation (Coronel et al. 1982) and is located in its western portion (see Figure 2). It is constituted by silicoclastic and carbonatic rocks developed in unconformity with Palaeoproterozoic basement (A-type Soca Granite). In NPTT (Valentines block) the Neoproterozoic cover is represented by the Cerro San Francisco Formation (Montaña and Sprechmann 1993) constituted by meta-sandstones and meta-subarkoses and by Cerros Victoria Formation represented by oolitic and stromatolitic limestone with low metamorphic grade. In the Rivera block outcrops, low-grade metamorphic rocks are defined by Preciozzi et al. (1985) as Minas de Corrales Formation, which is correlated with the low-grade orogenic belts of the PATT. On the other hand, this formation was considered as Neoproterozoic cover by Gaucher (2000). Major shear zones The most important high-strain zone affecting the Palaeoproterozoic units reviewed above is the Sarandí del Yí shear zone (SYSZ). It cuts the Río de la Plata craton separating the PATT from NPTT. The SYSZ is up to 13 kilometres wide and more than 250 kilometres long in the N–S direction (Gomez Rifas 1989). The Sarandí del Yí shear zone was formerly recognized and described by Preciozzi et al. (1979). This shear has N10° structural trend, and it is developed between the Sarandí of Yí town (Durazno department) and the Sierra de Las Animas Complex (see Figure 3). Oyhantçabal et al. (1993) suggested that this shear was reactivated with a sinistral sense during the Brasiliano orogenic event. Towards the northwest this tectonic lineament determines blocks with different thickness of Arapey Formation (Paraná flood basalts) and the basement horsts of the Litoral Oeste Basin. Diverse minor shear zones have been recognized in the RPC. The available information on kinematics, conditions and timing of deformation is still scarce. A summary of the main data is presented in Table 1. Table 1. Summary of the main shear zones recognized in the Río de La Plata craton. Name Mosquitos (MSZ) Cufré (CSZ) Paso de Lugo (PLSZ) Sarandí del Yí (SYSZ) Tupambaé (TSZ) Cueva del Tigre (CTSZ) Fraile Muerto-Maria Albina (SSSZ) Age Orientation Kinematic Location Palaeoproterozoic Palaeoproterozoic Palaeoproterozoic ∼260° ∼260° ∼270 Sinistral Sinistral Sinistral PATT PATT PAT Palaeoproterozoic? Reactivated in Meso (?) – Neoproterozoic Neoproterozoic? Neoproterozoic? ∼340° Dextral (sinestral reactivation) Border PATT – NPTT-DFB ∼250° ∼30° Dextral Dextral (sinestral reactivation) Sinestral NPTT NPTT Neoproterozoic? ∼25° Border NPTT-DFB Downloaded By: [Instituto de Geociencias - USP] At: 19:43 20 November 2009 International Geology Review 45 Other important shears affecting the PATT are Mosquitos, Paso Lugo, and Cufré shear zones (Figure 3). In the NPTT, the most important are Rivera, Fraile MuertoMaría Albina, and Cueva del Tigre shear zones (Figure 5). These shear zones are well recognized in the field but no structural, geochemical, and isotopic data were obtained. In the northern part of the PATT, Preciozzi (1993) described the Paso Lugo Shear Zone as a mylonitic granite shear zone with an age (Rb/Sr) of 2544 ± 38 Ma; R0 = 0.7073. The initial ratio is relatively high, suggesting contamination by older crustal material incorporated in the magma. Bossi et al. (1993) proposed that the Arroyo Grande belt was deformed in Archaean times. However, Preciozzi (1993) reported another Archaean age for an alkali granite (Rb-Sr (WR): 2501 ± 112 Ma; R0 = 0.7003), where the low initial ratio suggests that the primary magma was derived by partial melting of previously depleted mantle. In the central portion of the PATT the Cufré shear zone and its conjugates affect the San José belt. Preciozzi (1993) suggested for the major shear zone an age of ca. 2263 Ma (Soliani 1986). Recently reported U-Pb (conventional and SHRIMP) isotopic data (Santos et al. 2003; Peel and Preciozzi 2006) do not confirm events older than 2.2 Ga in the PATT. The Mosquitos shear zone, developed in the southern portion of San José belt (Montevideo Formation), was defined as sinistral shear with N60° regional trend by Oyhantçabal et al. (2006). This shear zone is related to the emplacement and deformation of granitic bodies. These authors present K-Ar (Ms) isotopic data with ages between 1900 and 2050 Ma. This shear zone acted as a weak zone where in Mesozoic times, the Santa Lucía Rift was developed (see Figure 2). Final remarks The Palaeoproterozoic fold-thrust belts of the PATT are represented by supracrustal rocks constituted by turbiditic deposits, volcanic rocks (rhyolite and basalt), and syntectonic granites with ages ca. 2.2–2.0 Ga. These units were affected by magmatism, metamorphism, and deformation related to the Transamazonic orogenic event (ca. 2.1–1.9 Ga), which corresponds to a fast period of crustal growth and mantelic reorganization (Condie 2000, 2002). Preciozzi et al. (1999), based on Rb-Sr and Sm-Nd (WR), suggested that the metamorphic event occurred ca. 2.0 Ga. Based on the presence of petrotectonic associations, like large volumes of metavolcanic units, conglomerates, BIFs, and turbiditic deposits, a back-arc or trench basin setting is proposed for the supracrustal sequences. The important volumes of granitic intrusions are interpreted as arc-related plutons. Available data allow us to define two generations of granites, the first event at ca. 2053–2086 Ma and the second one related to mafic intrusions at ca. 2016–2033 Ma, both intruding the volcano-sedimentary belts. The 2.0 Ga A-type granite, located in the PATT (Soca granite), cannot be correlated with any similar rapakivi magma generation event analysed by Ernst et al. (2008). Thus, it is not possible to link with another craton. Probably, the Rodinia break-up was initialized prior in some areas and the event of amalgamation–disaggregation was longer. At 1750 Ma Archaean cores were accreted to Palaeoproterozoic units (Proto-Kalahari craton in the sense of Jacobs et al. 2008), but perhaps in other areas, extensional phenomena would have occurred. The late Palaeoproterozoic (ca. 1.8 Ga) is marked by intracontinental rifting that implies the Statherian cratonic stabilization. In Uruguay, the record of extensional tectonic events shows Statherian ages (1.8–1.6 Ga), while in the Tandilia system (RPC – Argentina), the tholeiitic dike swarm yields ages of 1588 ± 11 (Iacumin et al. 2001; Teixeira et al. 2002). The basic dike swarm in the PATT forms parallel to subparallel patterns over Downloaded By: [Instituto de Geociencias - USP] At: 19:43 20 November 2009 46 L. Sánchez Bettucci et al. hundreds of kilometres, intruding granitic gneisses, granodiorites, and supracrustal rocks. This dike swarm reflects an important crustal extension and may point to a mantelic paleo-plume (Ernst and Buchan 1997, 2001). It is generally accepted that mantelic magmas suffer some degree of crustal contamination during the ascent and/or residence in magmatic chambers (Mohr 1987). The rigorous geologic information and the available isotopic data do not support the hypothesis of Bossi et al. (2005) suggesting another terrane named Tandilia for the outcrops of the PATT located at the south of Santa Lucía rift (Figure 2). The NPTT affected by Palaeoproterozoic high-grade orogenic metamorphic events may rework Archaean crust. The Valentines block is in tectonic contact with Pavas block through the Cueva del Tigre thrust belt (shear zone). This thrust was reactivated during the Brasiliano orogenic event. In contrast, the Rivera block shows other trends and possibly it was rotated during the orogenic event. During the Brasiliano orogenic cycle the NPTT was partly reworked and intruded by Neoproterozoic plutons. In addition, a Neoproterozoic cover occurs in the Río de La Plata Craton, being more developed in the NPTT. It is important to remark that the RPC lacks Mesoproterozoic rocks. References Almeida, F.F.M., Hasui, Y., Brito Neves, B.B., and Fuck, R.A., 1981, Brazilian structural provinces: An introduction: Earth-Science Reviews, v. 17, p. 1–29. 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