Deciphering the geochemistry of two key Paleoproterozoic siliciclastic sequences of the Piedra Alta Terrane (Río de la Plata Craton, Uruguay)

Decifrando la geoquímica de dos secuencias paleoproterozoicas siliciclásticas claves del Terreno Piedra Alta (Cratón del Río de la Plata, Uruguay)

Decifrando a geoquímica de duas sequências siliciclásticas paleoproterozóicas chave da Piedra Alta Terrane (Cráton Río de la Plata, Uruguai)

Blanco, Gonzalo; Abre, Paulina; Lanfranchini, Mabel; Cingolani, Carlos A.; Ferrizo, Hector; Piñeyro, Daniel; Uriz, Norberto J.; Benítez, Manuela E.

Gonzalo Blanco

gblanco@cure.edu.uy

Universidad de la República Uruguay

Paulina Abre

Universidad de la República Uruguay

Mabel Lanfranchini

Universidad Nacional de La Plata Argentina

Carlos A. Cingolani

Universidad Nacional de La Plata Argentina

Hector Ferrizo

Universidad de la República Uruguay

Daniel Piñeyro

Universidad de la República Uruguay

Norberto J. Uriz

Universidad Nacional de La Plata Argentina

Manuela E. Benítez

Universidad Nacional de La Plata Argentina

Agrociencia Uruguay

Universidad de la República, Uruguay

ISSN-e: 2730-5066

Periodicity: Bianual

vol. 26, no. 1, Esp., e525, 2022

agrociencia@fagro.edu.uy

Received: 29 January 2021

Accepted: 27 July 2021

Published: 27 May 2022

URL: http://portal.amelica.org/ameli/journal/506/50640915064091005/

DOI: https://doi.org/10.31285/AGRO.26.525

Corresponding author: gblanco@cure.edu.uy

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Abstract: The geochemistry of two metavolcano-sedimentary sequences deposited in the Piedra Alta Terrane is compared, and their geotectonic evolution is discussed. The Ojosmín Unit (OU) comprises MORB-like basic rocks at the base and a fine-grained siliciclastic sequence interpreted as marine turbidites towards the top. The succession was later obducted during the Orosirian. Whole-rock geochemistry of the metasedimentary rocks of the OU indicates the lack of source rock alteration during deposition (Chemical Index of Alteration, CIA ca. 40-53), implying ice-house climatic conditions in correlation with the worldwide Rhyacian glaciations. Recycling of zircon fractionates the Rare Earth Elements (REE) increasing the amount of HREE, Y, and Hf. Variation ranges of Th/Sc (0.4-4.9), Zr/Sc (30-410), Th/U (2.5-4.3), and of the Eu/Eu * negative anomaly (0.4-0.7) approximate Upper Continental Crust (UCC) values. A new clastic metasedimentary, gently folded unit, the Cerro de la Figurita Formation (CFFm), is erected. The CFFm clastic sedimentation (3,000 m in thickness) represents a deepening upward sequence, starting with polymictic conglomerates deposited in an alluvial fan-dominated environment that evolves to marine turbidites. The CFFm is probably related to a foreland geotectonic setting developed during the Orosirian. The geochemistry of the CFFm reveals similarities to unrecycled UCC, and weathering of the source rocks increases up section (CIA 45-92). Low ratios of Th/Sc (0.3-1.5), Zr/Sc (6-20), Th/U (3-6), high Cr/V (1.1-12.2), and a less pronounced Eu/Eu* negative anomaly of certain samples (0.5-0.9) suggest a contribution from mafic source rocks (probably ophiolitic).

Keywords: geochemistry, provenance, Ojosmín, Cerro Figurita, paleoproterozoic.

Resumen: Se comparan las características geoquímicas de dos secuencias metavolcano-sedimentarias depositadas en el Terreno Piedra Alta y se discute la evolución geotectónica. La Unidad Ojosmín (OU) está compuesta por rocas básicas tipo MORB en la base y una secuencia siliciclástica de grano fino interpretada como turbiditas marinas hacia el tope de la unidad y posteriormente obductadas durante el Orosiriano. La geoquímica en roca total de las rocas metasedimentarias de la OU muestra la ausencia de alteración de la roca madre por meteorización (Índice de Alteración Química, CIA, 40-53) durante la depositación, lo que indica condiciones climáticas del tipo ice-house y se correlaciona con las glaciaciones Rhyácicas a nivel mundial. Debido al reciclaje de circón existe un fraccionamiento de las tierras raras (REE), aumentando la cantidad de HREE, Y y Hf. Los rangos de variación de Th/Sc (0,4-4,9), Zr/Sc (30-410), Th/U (2,5-4,3) y la anomalía negativa de Eu/Eu * (0,4-0,7) tienen valores similares a los de la corteza continental superior (UCC). Se define una nueva unidad clástica metasedimentaria que se encuentra plegada en un amplio sinclinal, denominada Formación Cerro de la Figurita (CFFm). La CFFm representa una sedimentación clástica (3.000 m de espesor) grano y estrato decreciente dominada por paquetes de conglomerados polimícticos depositados en un ambiente de abanicos aluviales que evolucionan a condiciones marinas, dominado por turbiditas y depositada en una cuenca de antepaís desarrollada durante el Orosiriano. La geoquímica de CFFm revela similitudes con la UCC no reciclada, y aumento de la alteración de la roca fuente hacia el tope de la secuencia (CIA 45-92). La contribución de rocas fuente máficas (probablemente ofiolíticas) se revela por las bajas relaciones de Th/Sc (0,3-1,5), Zr/Sc (6-20), Th/U (3-6), alto Cr/V (1,1 -12,2) y anomalía negativa de Eu/Eu * menos pronunciada en algunas muestras (0,5-0,9).

Palabras clave: geoquímica, proveniencia, Ojosmín, Cerro Figurita, paleoproterozoico.

Resumo: A geoquímica de duas sequências metavulcano-sedimentares depositadas no terreno Piedra Alta é comparada e a evolução geotectônica é discutida. A Unidade Ojosmín (OU) é composta por MORB como rochas básicas no fundo e sequência siliciclástica de granulação fina interpretada como turbiditos marinhos em direção ao topo e posteriormente obduzidos durante o Orosirian. Toda a geoquímica das rochas metassedimentares OU mostra a ausência da alteração da rocha geradora por intemperismo (Indice de Alteração Química, CIA, 40-53) durante a deposição, indicando as condições climáticas da Ice-house e correlacionadas com as glaciações Rhyacian em todo o mundo. A reciclagem do zircão fraciona as terras raras (REE) aumentando a quantidade de HREE, Y e Hf. A variação de Th/Sc (0.4-4.9), Zr/Sc (30-410), Th/U (2.5-4.3) e a anomalia Eu/Eu* (0,4-0,7) são semelhantes à Crosta Continental Superior (UCC). Uma nova unidade metassedimentar clástica dobrada em amplo sinclinal, chamada Formação Cerro de la Figurita (CFFm.) é definida. O CFFm. sedimentação clástica (3.000 m de espessura) representa uma sequência ascendente de aprofundamento de conglomerados polimíticos depositados em um ambiente dominado por fendas aluviais que evoluem para condições marinhas, dominadas por turbiditos e provavelmente relacionadas a um cenário geotectónico de antepais desenvolvido durante o Orosiriano. A geoquímica da CFFm. mostra um comportamento semelhante com a UCC não reciclada, aumentando o intemperismo da rocha fonte para o topo (CIA 45-92). A contribuição de uma fonte de rochas máficas (provavelmente ofiolítica) é revelada pelas baixas razões de Th/Sc (0.3-1.5), Zr/Sc (6-20), Th/U (3-6), Cr/V alto (1,1 -12,2) e a anomalia negativa Eu/Eu * de algumas amostras (0,5-0,9).

Palavras-chave: geoquímica, proveniencia, Ojosmín, Cerro Figurita, paleoproterozoico.

1. Introduction and geological background

Cratons are continental blocks that preserve the oldest and most varied rocks that have existed throughout the history of the Earth and, therefore, they are key pieces to understand the processes that have formed the current configuration of our planet. The Río de la Plata Craton covers part of southern Brazil, Paraguay, Uruguay, and the central-eastern sector of Argentina, and is subdivided into several terranes or blocks(1)(2)(3)(4)(5)(6)(7)(8)(9)(10) (Figure 1). To the north, the Río de la Plata Craton borders the southern Amazon Craton (Bolivia and Brazil); whereas to the east it borders the allochthonous Cuchilla Dionisio Terrane, which displays a similar age range to the western edge of the Kalahari Craton, and is suspected to have been separated from it before the Brasilian orogeny(11)(12)(13)(14). To the west, the Río de la Plata Craton is bounded by the Pampia Terrane. The southern limit is marked by the Sierra de la Ventana fold belt, which is not part of the Río de la Plata Craton according to several geochronological studies(15)(16)(17).

The objective of sedimentary provenance studies is to interpret the history of sedimentary contribution from the erosion of a source rock to the final detritus deposition, allowing us to make paleogeographic reconstructions recognizing the lithogeochemical characteristics of the source rocks(18)(19)(20). Source rocks and sedimentary basins)( do not remain stable for long periods, and under ideal circumstances certain characteristics such as changes in the supply areas, drainage network, climate, tectonic environment, and paleoland scape can be recognized and identified to understand the geochemical evolution of the upper continental crust⁽²¹⁾. Therefore, sedimentary provenance studies of these metasedimentary units of the Río de la Plata Craton based on geochemistry provide relevant data to gain insights into the understanding of the evolution of the continental crust and the sedimentary history, from source to sink, of Paleoproterozoic units of Uruguay. In this study, we compare geochemical data of metasedimentary rocks cropping out at two poorly studied sectors of the Piedra Alta Terrane: the Ojosmín area, and the easternmost outcrops of the San José Belt (Fig. 1)

Figure 1
A: Location of the Río de la Plata Craton. B: and C: Piedra Alta Terrane in Uruguay Bossi and Cingolani⁽³⁾. CSZ: Colonia Shear Zone, NPT: Nico Pérez Terrane, FDS: Florida Dike Swarm, SYSZ: Sarandí del Yí Shear Zone

1.1 Piedra Alta and Tandilia Terranes

The Piedra Alta Terrane is constituted by supracrustal belts striking approximately east-west, separated from each other by extensive areas of granites, gneisses, and migmatites grouped into the Florida Belt (Fig. 1)(6)(7)(22)(23)(24). The low-grade San José and Andresito Belts are included within the Piedra Alta Terrane, whereas according to Bossi and Cingolani⁽³⁾, the medium-grade Pando Belt is part of the Tandilia Terrane, which extends to the south of the Buenos Aires Province in Argentina and is separated from the Piedra Alta Terrane by the Colonia Shear Zone⁽¹⁰⁾⁽²⁵⁾. Siliciclastic rocks occur in the supracrustal belts, in volcano-sedimentary units known as the Arroyo Grande Formation (Andresito Belt), Paso Severino Formation (San José Belt), and Montevideo Formation (Pando Belt). While in the Piedra Alta Terrane they are intruded by calc-alkaline, late to post-orogenic granites between 2.05 and 2.09 Ga, in the Tandilia Terrane anorogenic granites show similar crystallization ages (Fig. 2)(26)(27)(28). Geochronological data presented by Santos and others⁽⁵⁾ suggest a complex geotectonic scenario for the Piedra Alta-Tandilia Terranes indicating different magmatic events spanning from the Rhyacian to the Stenian period.

A different tectonic evolution characterized the volcano-sedimentary records of the Paso Severino and Arroyo Grande formations⁽⁷⁾. The Paso Severino Formation is dominated by marine facies composed of metapelites with intercalated metabasalts and carbonates at the top. The Arroyo Grande Formation is dominated by a thick package of feldspathic metarenites with well-preserved sedimentary structures and interlayered mafic rocks towards the top⁽²⁹⁾.

The Ojosmín Unit is one of the less-studied areas of the Uruguayan Precambrian Shield, that probably represents a metamorphosed ophiolitic fragment composed of ultramafic, mafic, and metasedimentary rocks intruded by granophyre and trachyte⁽²⁹⁾. Geological proxies poorly constrain the depositional age at ca. 2.0-2.1 Ga and the tectonic thrusting at ca. 1.9 Ga⁽²⁹⁾.

In the easternmost area of the San José Belt, at the Cerro Figurita, the metasedimentary rocks comprise coarse polymictic conglomerates, interbedded with volcanoclastic conglomerate at the base, feldspathic arenites at the middle of the sequence, and pelites and wackes resembling turbidites at the top of the section⁽³⁰⁾. Thus, the lithostratigraphy of the Cerro Figurita is markedly different to that of the Paso Severino Formation, and the Nd isotope data presented here suggest deposition before 2.0 Ga⁽³⁰⁾, implying that the unit is younger than the Paso Severino Formation.

Therefore, a new stratigraphic unit called Cerro Figurita Formation (CFFm) is erected in this work and described in detail later (see below).

Figure 2
Comparison between key meta(volcanic)sedimentary sequence of the Piedra Alta Terrane and their main stratigraphic features from Bossi & Piñeyro⁽²⁹⁾ and Blanco and others⁽³⁰⁾

2. Materials and methods

In order to understand the stratigraphy and the structural geology of both areas, geological maps at 1:20.000 scale and stratigraphic columns were elaborated based on outcrop descriptions and photointerpretation. Standard thin sections of the samples were analyzed using a Leica DM-2500 petrographic microscope at CURE (Treinta y Tres, Uruguay). For geochemical analyses, the samples were pulverized using a jaw crusher and a Cr-steel mill at the Laboratory of Geology (CURE). Geochemical analyses were carried out for 27 samples at Bureau Veritas Minerals Laboratories (Canada). Following lithium borate fusion preparation, major elements and Ni, Zn, Cu, Cr, and Sc were determined by ICP-ES, whereas all other trace elements (including rare earth elements) were measured by ICP-MS. Lower limits of detection (lld) are 0.01% for all major elements except Fe₂O₃, which is 0.04; lld of 0.1 ppm for Nb, Rb, U, Ta, Hf, Y, Zr, Cs, La and Ce; lld of 1 ppm for Ba and Sc; lld of 0.5 ppm for Sr and Ga; lld of 0.2 ppm for Co and Th; 8 ppm for V and Cr and 20 ppm for Ni. Lld of 0.02 ppm for Pr, Eu, and Ho; of 0.3 ppm for Nd; 0.05 ppm for Sm, Gd, Dy, and Yb; 0.01 ppm for Tb, Tm, and Lu and 0.03 ppm for Er.

3. Results

3.1 Lithostratigraphy

3.1.1 Cerro Figurita Formation

The sedimentary sequence described as the CFFm crops out over an area of 100 km² close to the Sarandí del Yí Shear Zone in the Piedra Alta Terrane (Fig. 1). Northwards, the CFFm is in tectonic contact with Rhyacian granodiorites whereas in the southern area it is overlain by Cretaceous basalts (Fig. 3 and 4). The presence of a recrystallized illite and chlorite matrix in siliciclastic rocks of the CFFm indicates low-grade conditions during the metamorphism. The sequence is gently folded into a syncline structure. The fining-upward sequence is composed of four sedimentary facies, which from base to top are:

1) Fining upwards cycles of coarse polymictic conglomerates and subordinate intercalated lithic- and feldspathic arenites (750 meters thick).

Figure 3
Stratigraphic columns of the CFFm and the OU. Note the Th/Sc and CIA variations along the stratigraphy give information about the weathering, source rocks and sediment recycling. See figures 4 and 5 for sample location

Figure 4
Geological map of the CFFm study area

2) The overlying unit is compositionally very immature and comprises acid and basic volcanic clasts and subordinate granitic and pelitic clasts (Fig. 5A-E). Erosive surfaces and gradational structures between conglomerates and within conglomerates and arenites strata are reliable polarity indicators. Basement clasts are scarce. The thickness is 250 meters. Up section, arenites are more common. Stream deposits of unchannelized conglomerates represent alluvial fans.

3) Middle section, fine- to coarse-grained, feldspathic and quarzitic litharenites dominate and are up to 1200 meters in thickness. The arenites are poorly sorted, and the clayey matrix is due to alteration of labile lithoclasts. Polycrystalline quartz is common. Through cross-bedding and other sedimentary structures as well as thin layers of heavy mineral concentrates occur and suggest high energy of the fluvial system. Up section, volcanoclastic breccia and conglomerate, intercalated with wackes and litharenites, indicate a temporally and geographically closely related syn-sedimentary magmatism.

4) Up section two sedimentary facies are recognized: a) laminated green pelites, and b) wacke-pelite rhythmites, with a combined thickness of 500 meters. They are classified as proximal turbidites, probably related to a submarine fan.

Figure 5
Outcrops and thin sections of the CFFm A coarse polymictic conglomerate B volcanoclastic breccia C conglomeratic sandstones D detrital pelite P and volcanic clasts Vm in a lithic arenite E alternation of detrital quartzfeldspar and illitechlorite laminae turbiditic facies OU F sandstone outcrop G normal grading in turbiditic sandstones H albitized plagioclase in fine sandstone showing concaveconvex grain contacts Scale bar represents 200 µm

5) At the top of the sequence, laminated grey pelites reach a thickness of 300 meters. The petrography reveals millimeter-thick interlayers of siltstones and claystones composed of reoriented clays dominated by illite and chlorite and angular clasts of quartz and feldspar (Fig. 5E). The accumulation of organic matter is another distinctive feature. The facies association suggests deep-water basin conditions during deposition of the top of the unit, and indicates a turbiditic environment.

CFFm represents a deepening-upward sequence (Fig. 3) indicating the evolution from an alluvial fan and braided fluvial-dominated environment to marine turbiditic conditions. The abundance of unstable lithoclasts, immaturity of the sandstones, and the thickness of conglomerate deposits point to a steep paleorelief linked to active tectonism during the sedimentation.

3.1.2 Ojosmín Unit

Bossi and Piñeyro⁽²⁹⁾ present the first comprehensive study of the Cerro Ojosmín area. The OU crops out over an area of 80 km² and comprises from base to top:

1) Serpentinized metagabbros and basic metavolcanic rocks, including high-magnesium ultrabasic rocks; tremolitic rocks, and MORB-derived metabasalts⁽²⁸⁾ (Fig. 6).

2) Metavolcano-sedimentary sequence composed of alternations of fine-grained metarenites (Fig. 5F), subordinate metapelites, and cherts, showing meter-thick tabular strata and scarce interlayers of basic metavolcanic rocks at the bottom. Feldspathic lithic arenite dominates the sequence in fining upward cycles (Fig. 5G-H). These sedimentary characteristics suggest turbiditic processes for the unit.

Some plagioclase grains show evidence of recrystallization and well-preserved albite-law twinning (Fig. 5H). Subordinate microcline feldspar shows grid twinning in cross-polarized light. Metamorphic minerals are common, such as biotite nests, euhedral sphene, and amphibole crystals, indicative of low to medium metamorphic grade.

3) Acid magmatism post-dates and intrudes the previous rocks, comprising sub-vertical dikes of microgranite and trachyte, and granophyres cropping out at the topographic high in the Cerros de Ojosmín.

In the northern area, the OU is overthrust by the Paleoproterozoic Cardona granodiorite. In the southern sector, pegmatites intruded along the thrust plane. Mainly based on geological considerations, Bossi and co-workers indicate that the granodiorite thrust occurred at 1900 ± 50 Ma, and the crystallization age of the porphyritic rocks is probably correlated to the acidic magmatism of the San José Belt at 1730 ± 10 Ma⁽²⁹⁾.

Figure 6
Geological map of OU study area

Modified after Bossi and Piñeyro⁽²⁹⁾

3.2 Geochemistry

3.2.1 Major elements

The CFFm sedimentary rocks are characterized by low to high SiO₂/Al₂O₃ and K₂O/Na₂O ratios, whereas the Ojosmín turbidites show narrower ranges of variation (Fig. 7 and Table 1 of the supplementary material). The Ojosmín sample set shows Na₂O concentrations varying from 3.5 to 5.5%, indicating Na-plagioclase detrital contribution and/or Na redistribution during diagenesis or metamorphism. A few arenites of the CFFm display Na₂O values as high as 3.2%. Both units show low CaO concentrations (<2%), with few exceptions. The arenites interlayered with the conglomerates of the CFFm have CaO concentrations between 2 and 3.5%. Al₂O₃ concentration ranges from 12 to 18% and is related to the feldspar and clay content of the pelites and turbidites of the CFFm. Al₂O₃ abundances are between 11 to 12% for the metasedimentary rocks of the OU and indicate low maturity.

Figure 7
K₂O/Na₂O-SiO₂/Al₂O₃ after Roser and Korsh⁽³¹⁾ and Pettijohn and Potter⁽³²⁾. PM: passive Margin, AM: active margin, A1: arc setting, A2: evolved arc setting

3.2.2 REE and other trace elements (Tables 2 and 3 of the supplementary material)

High field strength elements, Th, Zr, Hf, Nb, and rare earth elements (REE) are insoluble and usually immobile under surface conditions preserving the source rock characteristics in the sedimentary record(19)(33)(34)(35)(36).

The diagram after Winchester and Floyd⁽³⁷⁾ is broadly used in sedimentary geochemistry to discriminate sedimentary source rock composition. Samples of the OU point to acid components (rhyolite field) whereas an intermediate to mafic composition (andesite and andesite/basalt fields) is related to the CFFm (Fig. 8), except for the quartzarenites occurring in the middle of the stratigraphic column (Fig. 3).

Figure 8
Nb/Y-Zr/TiO₂ diagram after Winchester and Floyd⁽³⁷⁾

Figure 9
REE pattern for the studied units compared with Post Archean Australian Shales (PAAS)⁽³⁸⁾. Note the relative depletion on LREE and the enrichment in HREE due to fractionation in OU when is compared with the PAAS. The CFFm shows a similar pattern compared to the PAAS

Chondrite-normalized REE patterns (Fig. 9 and Table 3) of the CFFm are parallel to the PAAS (Post Archaean Australian Shales)⁽³⁸⁾. The metasedimentary rocks of the OU display a slight depletion in LREE and enrichment in HREE compared to PAAS, showing a flat REE pattern. LaN/YbN ratios <2 on average are far below the UCC average of 9.3⁽³⁹⁾.

Eu/Eu* values between 0.5 and 0.7 are typical for the UCC⁽³⁹⁾; metasedimentary rocks of the OU show Eu/Eu* negative anomalies in the range of the UCC, spreading between 0.5 and 0.7, whereas for the CFFm the Eu/Eu* values are between 0.7 and 0.8, and indicate the Ca-plagioclase addition.

4. Discussion

Geochemical data presented here for the (meta)sedimentary rocks give relevant information to understand processes such as weathering, sorting, diagenesis, and metamorphism and give insights regarding the paleoclimate and tectonism(14)(36)(40)(41)(42)(43)(44) during deposition of both units.

4.1 Weathering and Post-Sediementary Alteration

4.1.1 Major and trace elements

Chemical weathering exerts a major control in the composition of siliciclastic detritus and is controlled by paleoclimatic and paleoweathering conditions⁽⁴¹⁾. To assess weathering the CIA (Chemical Index of Alteration)⁽¹⁸⁾, PIA (Plagioclase Index of Alteration)⁽⁴³⁾, and Th/U vs Th tests were conducted⁽³⁶⁾.

Mobilization of major elements as a result of weathering is assessed using the CIA(18) in conjunction with A-CN-K ternary diagram⁽¹⁸⁾⁽⁴¹⁾ and in a similar way the PIA used the (A-K)-C-N diagram⁽⁴³⁾. These indexes use molar proportions as follows: 1) CIA = (Al₂O₃ / (Al₂O₃ + CaO* + Na₂O + K₂O)) x 100, and 2) PIA = (Al₂O₃- K₂O) / (Al₂O₃ - K₂O) + CaO* + Na₂O).

Effectively, the CIA index mostly measures the degree of alteration of feldspars (since this group of minerals composes approximately 70% of the upper crust) and volcanic glass to clay minerals during weathering. The PIA index focuses on the alteration of plagioclase. The Th/U ratio of UCC is 3.8, and for most sedimentary rocks derived from the average upper crust the Th/U ratio is 3.5-4.0⁽³³⁾. As detritus is subjected to weathering and/or recycling under oxidizing conditions, the Th/U ratio typically increases, because U⁴⁺ is oxidized to the more soluble U⁶⁺ species, and the latter is removed from sediments⁽³⁶⁾.

Ojosmín Unit.- Low CIA and Th/U values, 48 to 53 and 2.5 to 4.0, respectively, indicate almost no chemical weathering for the arenites of OU (Figs. 3 and 10A-C). The plagioclase abundance of the arenites appears related to the rapid unroofing of a plagioclase-rich source and to low intensity of chemical weathering. Post sedimentary Na-metasomatism probably affected OU to a low degree. Albitized plagioclase is detected in the OU arenites (Fig. 5H) and sodium metasomatism probably deviates some samples from a normal trend in the A-CN-K diagram toward the plagioclase apex (Fig. 10A). Low PIA values and oligoclase composition for the plagioclase suggested in the AK-C-N diagram (Fig. 10B) support a provenance from a non-altered granodioritic basement as a primary source rock component.

Such low CIA, PIA, and Th/U values for the OU probably indicate cold and arid climatic conditions during deposition(41)(43)(44). These climatic conditions match the worldwide Rhyacian glaciations (Huronian), the GOE (Great Oxygenation Event), and the Lomagundi carbon isotope excursion documented at the top of the Paso Severino Formation⁽⁴⁵⁾ and in Tandilia⁽⁴⁶⁾.

4.1.2 REE

The REE patterns can be affected by: I the sorting of accessory minerals enriched in REEs, II mixing of upper crustal sources, or III fractionation during weathering, diagenesis or metamorphism⁽³⁹⁾.

The flat REE patterns observed for the OU sample set deviate from the PAAS, and rather than pointing to a mafic component of the source rocks indicate fractionation of the REE. The exceptionally high concentrations of Zr (295 to 560 ppm, average UCC=190 ppm), Hf, and Y are linked to the addition of the heavy mineral zircon. HREE like Erbium and Ytterbium are distinctly enriched, as shown by a deviation from UCC and a vertical trend (Fig. 12). In highly oxidizing conditions Ce³⁺ oxidizes to Ce⁴⁺ and is less readily removed from the system. Positive Ce/Ce* anomalies, observed in five samples with values from 1.6 to 2.3, suggest REE removal in an oxidizing environment and implies fractionation during diagenesis or metamorphism⁽⁴²⁾. REE patterns parallel to the PAAS indicate no remobilization during diagenesis or metamorphism for the CFFm, although K-metasomatism is detected in some samples (Fig. 10).

Figure 10
Chemical alteration of the analyzed rocks. A) Ternary A-CN-K (Al₂O₃-[CaO*+Na₂O]-K₂O) diagram; arrow-headed lines indicate normally predicted weathering trend⁽⁴¹⁾ of average post-Archaean Upper Continental Crust (empty square denotes UCC and B unaltered basalt). The spread of the data indicate composition range from granodiorite to basalt. B) Ternary AK-C-N (Al₂O₃+K₂O)-CaO*-Na₂O. CIA: Chemical Index of Alteration. Note that most samples of Ojosmín turbidites plot in the bulk oligoclase field. PIA: Plagioclase Index of Alteration. An=anorthite, By=bytowinite, La=labradorite, Ad=andesine, Og=oligoclase, Ab=albite

Figure 11
Th/U vs. U after McLennan and others⁽³⁶⁾; see text for detailed discussion

Figure 12
GdN/YbN vs. LaN/SmN after Bock and others⁽⁴⁷⁾ shows a vertical trend in the OU sample set, which is related to the effect of zircon addition

4.2 Age of Cerro Figurita Formation

The Piedra Alta Terrane rocks (including amphibolites, calc-alkaline granites, and the Paso Severino Formation) show a range of crustal residence between 2.1 to 2.4 Ga and εNd(t) values between -0.4 and 3.1. Metapelites of the Paso Severino Formation show TDM model ages of 2.2 Ga⁽²⁸⁾ and εNd(t) between 2.1 and 2.5 and a precise sedimentation age of 2146 Ma⁽²⁷⁾. The CFFm shows positive εNd(t) up to 3.5 and TDM ages between 2.0 to 2.3 Ga, suggesting a maximum sedimentation age of ca. 2.0 Ga, at least 150 million years younger than the Paso Severino Formation. Therefore, positive εNd(t) and the low Th/Sc values around 0.1 (see Table 4 of the supplementary material) of most of the non-recycled samples of the CFFm indicate a juvenile crustal source component or at least low crustal contamination during deposition⁽³⁰⁾. In any case, more precise geochronological evidence will help to better constrain the sedimentation age of the CFFm.

4.3 Provenance and Tectonic Setting

Ratios of certain immobile elements (e.g. Th/Sc, Zr/Sc, Cr/V, Ti/Nb, La/Sc) are robust provenance indicators when compared to average upper continental crust composition, revealing the compositional heterogeneity of the source rocks (Fig. 13)(14)(21)(39). Ratios between an incompatible (Th) and a compatible element (Sc) reflect either the mafic or felsic composition of the source bulk, whereas the Zr elemental concentration indicates the recycling that resulted in the addition of the heavy mineral zircon. The geochemical source components are evaluated together with the geological and sedimentological features of the Paleoproterozoic units.

Figure 13
Th/Sc vs. Zr/Sc diagram⁽³⁹⁾. Discussion in the text

A mantle source component for the CFFm is confirmed by low Th/Sc and Zr/Sc ratios compared to UCC composition and indicates scarce recycling. Moreover, high Cr/V (7-12) and low Y/Ni (1.4-1.8) of some samples suggest that such mafic component has an ophiolitic signature⁽⁴⁷⁾⁽⁴⁸⁾ (Fig. 14). Ti/Nb ratios range from 400 to 1400 in the CFFm and match MORB basalt composition, supporting a mafic source component in the provenance. The presence of volcanoclastic conglomerates and breccias reported in this work (Fig. 5. A-D) confirm the influence of a synsedimentary mafic source component probably during the Orosirian, as discussed above. A probable link between the MORB-like lavas of OU and the ophiolitic component of the CFFm provenance is suggested in this work, although a precise mineralogical analysis is needed to further support this.

Figure 14
Cr/V vs. Y/Ni diagram Hiscott⁽⁴⁸⁾ shows the ophiolitic source rock component of CFFm and Y enrichment of Ojosmín turbidites due to zircon addition

Figure 13 shows derivation from felsic source rocks for the OU turbidites and recycling that resulted in the enrichment of the heavy mineral zircon during deposition. High Cr/V values ranging from 6 to 20 point to the presence of chromite (Cr between 130 and 294 ppm), probably added during recycling of a sedimentary source rock. Probable source rocks of the OU turbidites should account for the geochemical signature here deduced involving both mafic/ultramafic (old recycled) and felsic components. The juvenile felsic magmatism described in the Florida Arc could account for the granodioritic characteristics of the source component, whereas the mafic/ultramafic sources remain unidentified.

Figure 15
La/Sc vs. Ti/Zr discriminant diagram plot tectonic setting, after Bhatia and Crook⁽³⁴⁾

Geochemical proxies such as low Ti/Nb <180 (UCC=300), high Y/Ni (15-66), and Eu/Eu* negative anomaly support the preponderance of the felsic over the mafic provenance component. The granodioritic source explains the significant accumulation of detrital Na-plagioclase preserved in the OU turbidites, as reflected in the petrography and the high Na₂O/K₂O ratio between 1.5 and 10. Figures 13 and 14 could be used to decipher the sedimentological recycling and source rock compositions during the deposition of the OU and the CFFm. Recycling clastic sources in more than one cycle of sedimentation and/or a rich zircon source component could explain the high Zr/Sc values between 30 and 400 of the OU turbidites. The zircon enrichment and the consequent Zr, Y, and Hf additions account for a certain degree of spread observed in the provenance diagram of the Figure 16. Some clues regarding the tectonic setting of the OU basin are provided by geochemical data of the metavolcanic rocks linked to MORB activity⁽²⁹⁾. Notwithstanding, the sample OJ020 is an ultramafic rock with 25% of MgO content but with an Eu/Eu* negative anomaly evidencing crustal contamination during crystallization. High Mg, Cr, Ni content, and low TiO₂ suggest differentiation processes during crystallization probably related to an early volcanic arc (e.g. boninites) which evolved from a suprasubduction zone or plume activity. Thus, an incipient, shallow volcanic arc that explains the mafic component of the source of OU turbidites could be a plausible hypothesis, but more evidence is needed to confirm this.

The diagrams of figures 15 and 16 indicate sediment supply derived from a continental island arc or andesitic arc for the CFFm involving an active tectonic setting during deposition.

A plausible tectonic scenario for the evolution of the Andresito and San José Belts involves the closure and deformation of their sedimentary basins before deposition of the CFFm in a foreland setting (Fig. 17). Subduction towards the south and generation of a magmatic calc-alkaline arc, namely Florida, is a well-documented event⁽⁴⁾ that occurred around 2.1 to 2.0 Ga. The Paso Severino Formation, according to different authors, was probably deposited in a back-arc basin.

Figure 16
La/Th vs Hf discriminant diagram after Floyd and Levereridge⁽³⁵⁾ illustrating the radical change in provenance from a relatively unrecycled CFFm to an extremely reworked sediment for OU turbidites

Figure 17
Sketch representing the paleogeographic evolution and the main source areas during the deposition of the OU and the CFFm during the Paleoproterozoic. T.T.: Tandilia Terrane, SSZ: Suprasubduction Zone, CSZ: Colonia Shear Zone

After the final closure of the Paso Severino basin at ca. 2.08 Ga⁽²⁶⁾ uplift of the OU occurs and the calc-alkaline granodiorites (e.g. Cardona Granodiorite) are thrusted over the OU in the Piedra Alta Terrane. In this scenario, the OU would act as a probable source for CFFm. Synsedimentary magmatism coeval to deposition of the CFFm was related to the post-tectonic tectonothermal regime detected elsewhere in the Piedra Alta Terrane⁽⁷⁾. Facies analysis and geochemical proxies indicate evolution from non-recycled clastic continental to deep-marine turbiditic conditions. Sedimentation of the CFFm related to an active margin (Figs. 15 and 16) probably took place in a foreland geotectonic setting. Based on petrographic and geochemical proxies the source rock components are:

1. calc-alkaline granodiorites and acid to basic volcanic rocks.

2. Paso Severino Fm: meta volcano-sedimentary rocks, mostly meta-pelites and meta-basalts.

Ojosmín Ophiolites: mainly metabasalts with MORB affinities. Metaperidotites to metafelsic rocks. Clastic rocks are dominated by fine meta-arenites and meta-siltstones. However, it is fair to mention that confirmation of this source will provide a more robust tectonic model.

Minimum depositional age for the CFFm is indicated by the Florida Dike Swarm that intruded the Piedra Alta Terrane between 1.73 and 1.79 Ga⁽⁴⁶⁾. The CFFm remained unaffected by the widespread extensional magmatism, although it was affected and bent by a Mesoproterozoic tectonothermal accretional event along the Sarandí del Yí Shear Zone⁽⁴⁹⁾⁽⁵⁰⁾. In this work, a probably syn-sedimentary 1.9-2.0 magmatic event is suggested during the CFFm sedimentation based on Nd-isotopes⁽³⁰⁾ and provides a probable maximum depositional age for the sedimentary sequence. Nevertheless, more geochronological evidence is needed to confirm the 1.9-2.0 event in the Piedra Alta-Tandilia Terranes and the depositional age of the CFFm. Evidence of a complex tectono-magmatic scenario for the evolution of the RPC emerge in a diverse Paleoproterozoic sedimentary record. Interestingly, detrital zircon dating presented by Blanco and others⁽¹²⁾ and Gaucher and others⁽⁵¹⁾, reveals a major 1.9 Ga felsic magmatic event that sourced the Ediacaran Arroyo del Soldado Basin in the Piedra Alta and Nico Pérez Terranes which are part of the Río de la Plata Craton⁽¹²⁾⁽⁵¹⁾.

5. Conclusions

Geochemistry of the OU turbidites and the CFFm give information about two poorly known, key Paleoproterozoic volcano-sedimentary sequences of the Piedra Alta Terrane.

The source area of OU turbidites underwent little weathering, probably related to the prevalent climate conditions during the worldwide Paleoproterozoic glaciations. Provenance based on geochemistry and sedimentological considerations indicates a source area composed of a recycled mafic/ultramafic component and a felsic non-recycled granodioritic unit.

Detritus was probably shed from the Florida Arc granodiorites and an incipient volcanic arc (in a supra subduction setting?), as revealed by geochemical data of mafic/ultramafic gabbros and MORB-like lavas.

The Cerro Figurita Formation is recognized as a new lithostratigraphic unit based on facies analysis, petrographic, geochemical, and geochronological data. The CFFm represents a deepening upward sequence, ranging from continental deposits in an alluvial fan-dominated environment at the base, to marine turbiditic conditions up section. The geochemistry indicates a provenance dominated by felsic and mafic source rock components and the detrital supply was probably derived from the Florida Arc, the Ojosmín Unit, and the Paso Severino Formation. The ongoing geochronological studies on the Paleoproterozoic meta-volcano-sedimentary units will shed light on the understanding of the geotectonic and crustal evolution of the Piedra Alta Terrane and the Río de la Plata Craton, testing the ideas here presented.

Acknowledgments

This work is dedicated to the memory of Professor Jorge Bossi, whom we consider the "father" of modern Uruguayan geology. Johana Castillo and Richards Villanueva assisted during the laboratory tasks performed at CURE. The Academic Cooperation Agreement between the PDU Geology and Mineral Resources from the Department of Geosciences of CURE (UdelaR) and the University of La Plata, Argentina (Museo de La Plata), helped to carry out fieldworks. We thank Thomas Will and an anonymous reviewer for the constructive comments on the manuscript. Claudio Gaucher and Antonella Celio are thanked for the efficient editorial handling.

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Supplementary material

Table 1

Major element data of the OU and CFFm expressed in %. L/F: Lithology/facies. Ar: arenites. T: turbidites, UB and G: metavolcanic rocks. Cg: conglomerates. VCg: volcaniclastic conglomerates. Pe: pelites. Va: wackestone

Table 2

Trace element concentration of OU and CFFm samples, expressed in ppm

Table 3

Rare Earth Element concentrations of OU and CFFm samples, expressed in ppm

Table 4

Selected element ratios of OU and CFFm samples

Author notes

gblanco@cure.edu.uy

Alternative link

https://agrocienciauruguay.uy/index.php/agrociencia/article/view/525/1146 (pdf)