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Brazilian Journal of Geology

Print version ISSN 2317-4889On-line version ISSN 2317-4692

Braz. J. Geol. vol.49 no.2 São Paulo  2019  Epub July 01, 2019

https://doi.org/10.1590/2317-4889201920190017 

ARTICLE

Provenance of the Ediacaran Salinas Formation (Araçuaí Orogen, Brazil): Clues from lithochemical data and zircon U-Pb (SHRIMP) ages of volcanic clasts

1Universidade Federal de Minas Gerais, Programa de Pós-Graduação em Geologia, IGC-CPMTC - Belo Horizonte (MG), Brazil. E-mails: cdelucam@gmail.com; pedrosa@cnpq.br

2Unidade de Operação da Bacia de Campos: Exploração, Sedimentologia e Estratigrafia, Petrobras - Rio de Janeiro (RJ), Brazil. E-mail: saalima@petrobras.com.br

3Universidade de São Paulo - São Paulo (SP), Brazil. E-mails: ucordani@usp.br, keisato@usp.br


Abstract

Salinas Formation occurs in a large region of the Northern Araçuaí orogen, Southeastern Brazil. It includes turbiditic wackes (> 10% matrix) to arenites (< 10% matrix), pelites and clast-supported conglomerates, metamorphosed from the biotite zone of the greenschist facies to the sillimanite zone of the amphibolite facies. Salinas Formation lies unconformably on the top of or in tectonic contact with the Macaúbas Group, and hosts Cambrian granitic intrusions dated between 540 and 500 Ma. Aiming to unravel sediment provenances for the Salinas basin, we present a detailed lithochemical (45 samples) study on low-grade rocks preserved from the regional deformation, which are found in the type area of the Salinas Formation. In addition, we compare them with deformed and more metamorphic rocks of similar composition but located in other basin sectors. The lithochemical data indicate limited chemical weathering in the sediment sources, good correlations with the mineralogical compositions in respect to the variable amounts of metamorphic minerals typical of pelitic (micas, garnet, and other peraluminous silicates) and psammitic (feldspars, quartz) fractions. The main provenances of sedimentary protoliths are clearly related to continental magmatic arc and active continental margin environments. U-Pb (SHRIMP) analyses performed on zircon grains from clasts of intermediate to felsic volcanic rocks, extracted from a clast-supported metaconglomerate, yield concordant zircon Pb 206 /U 238 ages from ca. 579 Ma to ca. 697 Ma, with most of the ages in the interval of 587 to 630 Ma. This indicates that the main primary sediment source is the Rio Doce magmatic arc (630-580 Ma), in very good agreement with the arc-related lithochemical signature. Eleven zircon crystals yield a Concordia age of 620 ± 10 Ma, representing an important Ediacaran volcanic episode in the tectonic evolution of the Araçuaí orogen.

KEYWORDS: Lithochemistry; sediment provenance; volcanic contribution; Salinas Formation; Araçuaí orogen

INTRODUCTION

There are useful lithochemical approaches to investigate genetic attributes and tectonic environments of metasedimentary rocks, although processes like weathering, diagenesis, and metamorphism may considerably change the composition of sedimentary materials (cf. Bhatia 1985, Bhatia & Crook 1986, Rosen 1992, McLennan et al. 1993, Slack & Höy, 2000, Augustsson & Bahlburg 2008, Verma & Armstrong-Altrin 2013). The lithochemical investigation can be especially effective if combined with petrographic and isotopic studies on the preserved rocks and their modified equivalents.

In the semi-arid Jequitinhonha river valley, Northern Araçuaí orogen (Fig. 1), the Salinas Formation provides several good exposures of non-weathered metasedimentary rocks. They vary from very low-grade metasiliciclastic rocks (wacke, arenite, and pelite), which were preserved from the regional orogenic deformation, to their deformed equivalents metamorphosed up to the sillimanite zone of the amphibolite facies (Pedrosa-Soares 1995, Pedrosa-Soares & Leonardos 1996, Pedrosa-Soares et al. 2001, 2008, Lima et al. 2002, Santos et al. 2009, Peixoto et al. 2017).

Figure 1. Location of the study region in the Araçuaí orogen shown within a paleotectonic fit including the African counterpart and a simplified geological map of the main occurrence region of the Salinas Formation (modified from Pedrosa-Soares & Oliveira 1997; Pedrosa-Soares 1997a, b; Oliveira et al. 1997; Pedrosa-Soares & Grossi-Sad 1997; Baars et al. 1997; Guimarães & Grossi-Sad 1997). 

We present a detailed lithochemical investigation based on 45 samples of metasiliciclastic rocks, including 36 ones from this paper and nine compiled from Grossi-Sad and Motta (1991). Fourteen samples are from wackes metamorphosed in the very low-grade biotite zone, but preserved from orogenic deformation, collected in the type area of Salinas Formation, and 31 samples are from deformed and more metamorphic lithotypes collected in the southern sector of the Salinas basin (Fig. 1). This study is assisted by detailed petrographic examination on the analyzed lithotypes and by U-Pb (SHRIMP) analysis of zircon grains from pebbles and cobbles of intermediate to felsic volcanic rocks extracted from a Salinas metaconglomerate. The results disclose correlations between non-deformed and deformed lithotypes and reinforce the useful application of the lithochemical approach to study similar rocks in other orogenic belts. They also corroborate the orogenic nature of the Salinas basin (Lima et al. 2002, Santos et al. 2009, Peixoto et al. 2015, Costa et al. 2018). This definitely links it with sediment sources in the Rio Doce magmatic arc (Tedeschi et al. 2016, Novo et al. 2018).

GEOLOGICAL SETTING

Salinas Formation is one of the most extensive units of the Northern Araçuaí orogen, which occur in large areas of the Jequitinhonha river valley (Fig. 1). Regionally, Salinas Formation covers different units of the Macaúbas Group (e.g., the Chapada Acauã and Ribeirão da Folha formations), defining a regional unconformity. It also hosts late orogenic granitic intrusions dated from ca. 540 Ma to ca. 500 Ma (Fig. 1).

Study of the metasedimentary rocks currently ascribed to the Salinas Formation started early in 20th century, when they were correlated with the Macaúbas Formation (Moraes 1932). Half a century later, they were included in a separate stratigraphic unit, the Salinas Group, which was considered a more metamorphic unit intruded by granites and pegmatites that would correspond to the post-diamictite formations of Macaúbas Group (synthesis in Pedrosa-Soares 1984, and Karfunkel et al. 1985). Owing to its apparent continuity with the regional lithofacies distribution from the proximal to distal Macaúbas Group, Salinas Formation was formerly defined as the most distal and youngest unit of Macaúbas Group (Pedrosa-Soares et al. 1990a, 1990b, 1992). These authors subdivided the Salinas Formation into two units: an exclusively sedimentary unit located in the proximal (Western) part of theformation, and the Ribeirão da Folha facies or member, a more distal (Eastern) metavolcanic-sedimentary succession with oceanic sediments and mafic volcanic rocks, hosting tectonic slabs of ophiolitic rocks (Pedrosa-Soares et al. 1992, 1998, 2001, 2011).

In the second half of the 1990s, the construction of the BR-251 highway linking Salinas city to the Rio-Bahia (BR-116) highway led to extensive roadcuts that presented exceptional sections of the Salinas Formation completely preserved from weathering and, locally, from orogenic deformation (Pedrosa-Soares & Oliveira 1997). These outcrops allowed detailed studies by Lima et al. (2002), which defined the main lithofacies of Salinas Formation and presented the first U-Pb geochronological data on detrital zircon grains. According to Lima et al. (2002), Salinas Formation in the type area includes laminated, banded, graded, convolute, brecciated and massive wackes, cross-bedded sandstones, metapelites (mica schists and quartz-mica schists), clast-supported conglomerates and calc-silicate rocks, which are metamorphosed from the biotite to the garnet zones of the greenschist facies. The youngest detrital zircon grains suggest a maximum sedimentation age around 568 Ma that imply in redefinition of the Salinas Formation as a stratigraphic unit younger than the Macaúbas Group (Lima et al. 2002). Santos et al. (2009) presented detailed tectonic studies on the Northern Salinas Formation, which showed its progressive deformation from completely non-deformed to tightly folded and transposed zones, and suggested a model of flysch-type basin evolving from the pre-collisional to collisional stages of the Araçuaí orogen. Costa et al. (2018), based on the lithofacies investigation in the type area of Salinas Formation, characterized the Salinas basin as a large and curved trough, open to South-Southwest. According to these authors, accumulation in the Salinas Basin occurred along its axis, with general Southwest-South progradation of a turbidite fan system fed from the North. The detailed metamorphic studies presented by Peixoto et al. (2018) demonstrate a double metamorphic regime along the Salinas synclinorium (Fig. 1), with a Barrovian-type event related to the collisional tectonics and a Buchan-type event related to the plutonic igneous activity associated with the gravitational collapse of Araçuaí orogen. Mineral assemblages of metapelites in the Salinas synclinorium indicate maximum metamorphic conditions around 640ºC at 5.5 kbar, with no piece of evidence of partial melting (Peixoto et al. 2018).

To the South of Salinas synclinorium, the typical rock assemblage of Salinas Formation continues to occur along the Minas Novas corridor (Pedrosa-Soares et al. 1993, Pedrosa-Soares 1995, Alkmim et al. 2006), a double-verging transpressive structure extending from Turmalina - Minas Novas to Coronel Murta - Araçuaí regions (Fig. 1). In contrast to the well-preserved metasedimentary rocks from orogenic deformation locally found in the type area of the Salinas Formation (e.g., outcrops SP-4 and SP-5, Fig. 1), no rock exposure free from the regional deformation has been found in the Minas Novas corridor yet. The Salinas lithotypes in Minas Novas corridor are tightly folded quartz-biotite schists, quartzose metawackes (also called “impure quartzites”), carbonate schists, muscovite schists, calc-silicate rocks and sparse clast-supported metaconglomerate lenses, which frequently show the sedimentary layering tectonically transposed by the regional foliation (Pedrosa-Soares 1995). Along the Minas Novas corridor, Al-rich schists of Salinas Formation display intermediate to low pressure regional metamorphism with increasing temperature from SW to NE, passing through the biotite, garnet, staurolite, andalusite, and cordierite zones (Pedrosa-Soares et al. 1993, Pedrosa-Soares & Leonardos 1996).

ANALYTICAL METHODS

Besides conventional field and petrographic studies, this paper presents data from lithochemical analysis and U-Pb (SHRIMP) dating of zircon grains.

Lithochemical analysis

The selected representative samples are free of weathering and hydrothermal alteration, with no veins or fractures filled by secondary minerals. The samples were firstly cleaned and prepared using the conventional methods (crushing and milling) in the laboratories of the Centro de Pesquisas Professor Manoel Teixeira da Costa (CPMTC), Universidade Federal de Minas Gerais, and Department of Geology, Universidade Federal de Ouro Preto. Major oxides and trace elements including rare-earth elements (REE) were determined through the Inductively Coupled Plasma Mass Spectrometry (ICP-MS) on 14 whole-rock samples of low-grade (biotite zone) metawackes from the type area of Salinas Formation (outcrops SP-04 and SP-05, Fig. 1). Analyses were performed by the Actlabs (Activation Laboratories), in Canada. The ICP analysis needs complete dissolution, which is possible by melting the powdered rock with lithium tetraborate (Li2B4O7) firstly with subsequent attack by multi-acid solution (HCl, HNO3, HF and HClO4). Determinations of FeO and Fe2O3 were done using colorimetric and volumetric methods at the CPMTC-UFMG.

Thirty-one whole-rock analysis (major and trace elements) correspond to samples of clastic metasedimentary rocks of Salinas Formation collected in the Minas Novas corridor (Fig. 1). Twenty-two analyses are unpublished and the other nine were compiled from Grossi-Sad and Motta (1991). These rocks include quartz-biotite schist, muscovite schist, carbonate schist and quartzose metawacke, displaying regional metamorphism from garnet to staurolite zones (Fig. 1). Those 31 samples were analyzed in the GEOSOL Laboratory in Brazil by atomic absorption spectrometry, X-ray fluorescence spectrometry, ICP atomic emission spectrometry (ICP-AES), colorimetric and volumetric methods. Samples were melted with lithium tetraborate to determine major elements through the X-ray fluorescence. In order to analyze trace elements, including REE, by ICP-AES, the samples were melted with lithium metaborate and decomposed using a multi-acid solution (HCl, HNO3, HF and HClO4).

Whole-rock classification diagrams and molar element ratios were obtained using the GCDkit 2.3 software (Janousek et al. 2006).

U-Pb (SHRIMP) analysis

The U-Pb analysis of OPU-1995 sample was performed on zircon grains using the Sensitive High-Resolution Ion Microprobe (SHRIMP II) of the Chinese Academy of Geological Sciences, by means of the procedure described by Williams (1998). The analyses were made by UGC, at the São Paulo laboratory, operating the Chinese facilities through remote access, via the internet. The zircon grains were concentrated from pebbles and cobbles of volcanic rocks extracted from a clast-supported metaconglomerate of Salinas Formation. The separated volcanic clasts were cleaned as much as possible, although it was not possible to completely remove all traces of the matrix. Zircon grains were separated using the conventional methods (crushing, grinding, gravimetric and magnetic-Frantz isodynamic separator) and handpicked under binocular microscope at the LOPAG laboratory of the Universidade de Ouro Preto, Brazil. After mounted in epoxy resin and polished to expose their centers, cathodoluminescence (CL) images were prepared for all grains to identify their morphological features and internal structures.

To better understand significant tectonic processes evolving the Salinas Basin and to track its sediment sources, we also compiled and re-calculated the zircon U-Pb (SHRIMP) raw data from the wacke sample (SP-54) of the Salinas Formation type area formerly presented by Lima et al. (2002). This procedure improved the presentation quality of the original data, incorporating more spots than those previously considered. The Concordia diagrams and probability density plots were prepared with Isoplot/Ex (Ludwig 2003).

RESULTS AND INTERPRETATION

We synthesize field, petrographic, geochronological and lithochemical studies, comparing the results from the samples collected in the type area of Salinas Formation, located in the surroundings of Salinas city, with the samples collected along the Minas Novas corridor (Fig. 1).

Petrography

The best well-preserved exposures of Salinas Formation are located in the type area (Fig. 1), where successions of metawackes (matrix > 10%) to meta-arenites (matrix < 10%), metapelites and clast-supported metaconglomerates (meta-orthoconglomerates) are found. Compositionally, metawackes and meta-arenites only differ by the amount of clay-related matrix represented by mica content, reflecting small differences in rock tints. Although metamorphosed to greenschist facies, those rocks can be found completely unaffected by the orogenic deformation (Lima et al. 2002, Costa et al. 2018), as well as showing tectonic structures imprinted by the progressive deformation related to the collisional stage of Araçuaí orogen (Santos et al. 2009, Peixoto et al. 2018). Successions preserved from the orogenic deformation include massive to graded metawackes, banded to laminated metawackes, convolute metawackes, and cataclastic to brecciated metawackes, with sedimentary structures and striking evidence of syn-sedimentary tectonic activity (Fig. 2).

Figure 2. The Salinas Formation in outcrops of the type area (UTM: 801065 - 8222433; BR-251 highway roadcuts, 24 km to the east from the junction with MG-404 road), showing rocks without orogenic deformation but metamorphosed in the biotite zone of the greenschist facies: A, succession of banded to laminated wacke (BLW), convolute wacke (CW) and brecciated wacke (BrW), between massive wacke (MW) layers; B, water-escape structure marked by biotite-rich (i.e., mud-rich) darker upright flames in massive wacke; C, detail of brecciated wacke; D, detail of convolute wacke; E, cataclastic laminated wacke filling space in a syn-sedimentary extensional (growth) fault cutting massive wacke; F, syn-sedimentary (growth) micro-faults and graben-horst micro-pattern in laminated to banded wacke with coarsening-up graded layering done by the gradual decreasing of biotite content from the base (darker) to top (lighter); G, syn-sedimentary fault with drag microfolds in laminated wacke; H, water-escape structures along syn-sedimentary faults, cutting across coarsening-up graded, banded to laminated wackes; I, flame structures outlined by biotite-rich (i.e., mud-rich) bands, covered by massive wacke. 

The massive metawackes are generally medium-to-fine grained light-grey rocks that show a granoblastic to ­grano-lepidoblastic texture with weak recrystallization. They contain quartz, biotite, plagioclase, white mica, calcite, K-feldspar, and lithic fragments in variable proportions. Tourmaline, garnet, apatite, zircon, titanite and opaque minerals are accessories. Graded bedding is materialized by variation in the biotite amount representing the mud fraction, or by textural gradation of quartz-feldspar-rich matrix (Fig. 2). Upwards-coarsening graded bedding is a common feature in metawackes. The banded to laminated metawackes consist of bands and laminae relatively rich in biotite alternated with those of graded to massive metawackes (Fig. 2). The biotite-rich bands and laminae vary in composition from pelite-rich metawacke (biotite-rich metasandstone) to metapelite (quartz-mica schist to mica schist). The pelitic schists mark the regional metamorphism and may reach the garnet zone in the Salinas Formation type area.

Metric to decametric lenses of polymictic clast-supported metaconglomerates (orthoconglomerates) occur near the top of Salinas Formation (Fig. 3); therefore, they characterize a general upwards-coarsening succession (Santos et al. 2009). The scarce matrix has wacke composition similar to the metawacke layers hosting the orthoconglomerate lenses (Fig. 3A). Clasts are generally well-rounded and can be stretched and rotated along the regional ductile foliation. They are pebbles and cobbles of volcanic and subvolcanic rocks together with clasts of quartz, quartzite, gneiss, granite, and carbonate rock. Most clasts of volcanic rocks vary in composition from andesite to rhyolite, being generally porphyritic with feldspar phenocrysts immersed in fine-grained matrix (Figs. 3B to 3D). The Salinas metaconglomerate lenses, sparsely occurring from the type area southwards, roughly follow the axial zone of the Salinas synclinorium and continue along the central zone of Minas Novas corridor (where they mainly occur in the Araçuaí River banks and subsidiary creeks).

Figure 3. Clast-supported metaconglomerate (meta-orthoconglomerate) lenses of the Salinas Formation: A, outcrop in a quarry for dimension stone, showing the contact of a conglomerate lens and massive wacke (UTM: 789473 - 8220347; road MG-404, 12 km to the north of the road junction with the BR-251 highway); B, detail of the same metaconglomerate, showing stretched and rotate clasts (v, volcanic rocks); C, porphyritic volcanic rocks (v) of intermediate to felsic composition; D, pebble of porphyritic dacite; E, lens of clast-supported metaconglomerate intercalated with quartz-mica schist in the Minas Novas corridor (left bank of the Araçuaí River in front of the mouth of the Fanado River). 

From the type area, located in the axial zone of Salinas synclinorium, to the South and East, the Salinas Formation tends to become more metamorphic and deformed by the orogenic events that formed the Araçuaí orogen. This is the case of rocks found in Minas Novas corridor, from Coronel Murta to Turmalina (Fig. 1), in which the formation comprises successions of quartz-mica schist rich in biotite, muscovite schist, carbonate schist and quartzose metawacke, with lenses of calc-silicate rocks and clast-supported metaconglomerates (Figs. 1 and 4). These rocks show, at least, the penetrative regional foliation and prominent mineral lineation (Fig. 4). Generally, they are tightly folded and may show the sedimentary bedding transposed along fold hinges (Fig. 4C). The alternating bands and laminae with variably amounts of quartz, biotite and feldspars resemble the banded to laminated metawackes of the type area, and locally graded bedding and water-escape structures are preserved. Besides some metamorphic minerals not found in the type area (e.g., staurolite, cordierite), the mineralogical compositions of the psammitic to pelitic rocks along the Minas Novas corridor are similar to the non-deformed and weakly deformed lithotypes of Salinas Formation.

Figure 4. Outcrops of deformed and metamorphosed rocks of the Salinas Formation in the Minas Novas corridor: A, despite the rock composition, outcrops generally are dark gray to black owing to coats rich in manganese oxide (the so-called desert varnish); photo A also shows quartz veins, locally rich in gold, host by quartz-mica schist and metawacke (see Pedrosa-Soares & Leonardos, 1996); B, quartz-mica schist hosting pegmatite veins (close to Coronel Murta); C, folded banded quartz-mica schist with concordant quartz veins (close to Virgem da Lapa); D, muscovite schist with biotite and garnet porphyroblasts (close to Minas Novas); E, banded to laminated metawacke; F, carbonate schist (dark grey upper part), quartzose metawacke (light gray middle part) and laminated metawacke (lower part). The outcrops A, E and F are located in the Funil area, Araçuaí River banks, south of Virgem da Lapa. 

These rocks are essentially composed of variably contents of quartz, biotite, muscovite, and plagioclase. Calcite, K-feldspar, garnet, apatite, zircon, titanite, tourmaline, monazite, and opaque minerals are accessory phases. The carbonate schist is rich in quartz and biotite, with a significant amount of calcite. The muscovite schist is poor in Fe-rich minerals, which probably reflects a non-oxidizing environment during the sedimentation. Therefore, the banded to laminated packages composed of quartzose metawacke, carbonate schist and quartz-mica schist correspond to the successions of massive to graded, banded to laminated metawackes with intercalations of more pelitic terms.

Regionally, field and petrographic features of the main lithofacies associations indicate that Salinas Formation mostly comprises a turbiditic deep-sea sand-mud sequence, deposited in a basin episodically affected by syn-sedimentary tectonic activity, characterizing a flysch-type orogenic basin, as already suggested by other authors (Lima et al. 2002, Santos et al. 2009, Peixoto et al. 2015).

Lithochemistry

Results of major and trace element analysis for the whole-rock samples of Salinas Formation are listed in Tables 1, 2 (major element oxides) and 3 (trace elements). We will firstly consider the potential influences of sedimentary processes on lithochemical data and, then, evaluate the data in terms of protolith and depositional environment interpretations.

Table 1. Major element (wt%) compositions of clastic metasedimentary rocks from the Salinas Formation. 

Sample Lithofacies SiO2 TiO2 Al2O3 Fe2O3 FeO FeOt MnO MgO CaO Na2O K2O P2O5 LOI
SM51

  • Carbonate

  • schist

46.9 1,00 20.4 3.3 5.1 8.07 0.1 3.3 4.9 1.3 6.9 0.19 6.19
T171C 58.7 0.79 14.9 1.6 4.8 6.24 0.29 3.5 6.4 1.6 2.7 0.14 4.28
T3720 65.2 0.92 12.9 1.6 2.8 4.24 0.55 3.7 4.4 2.9 1.9 0.29 2.57
SM20 66.7 0.73 11.1 0.72 3.4 4.04 0.09 1.8 4.9 2.8 2.3 0.2 5.12
TM1085 68.3 0.81 13.1 1,00 3.32 4.22 0.11 1.6 3,00 2.8 2.5 0.18 1.73
TM1160 68.6 0.74 12,00 1.5 2.89 4.24 0.11 1.6 3.8 2.7 2.2 0.17 1.92
T185 69.8 0.64 13,00 0.48 2.9 3.33 0.09 1.8 3.1 2.3 2.4 0.17 3.12
TM2418 72.1 0.56 11.4 1.5 2.16 3.51 0.13 1.5 3.2 1.4 2.6 0.15 1.14
SP4A

  • Salinas

  • Type area

  • (metawacke)

70.54 0.66 12.73 0.98 4.24 5.12 0.23 1.12 1.25 3.74 3.22 0.2 0.93
SP4B 65.12 0.75 15.93 0.94 4.41 5.26 0.1 2.19 0.85 3.73 4.67 0.2 0.86
SP4C 62.39 0.79 16.81 1.26 4.48 5.61 0.14 2.73 1.52 3.15 5.18 0.19 0.8
SP4D 76.81 0.49 11.93 1.10 1.73 2.72 0.12 0.7 0.67 3.7 1.55 0.13 0.91
SP4E 70.01 0.59 14.66 1.10 2.83 3.82 0.08 1.55 0.58 3.5 3.99 0.18 1,00
SP4F 74.43 0.59 12.08 1.56 2.20 3.60 0.07 0.77 0.57 3.43 3.27 0.14 0.8
SP4G 70.47 0.52 14.51 0.45 3.03 3,00 0.13 1.22 0.92 3.64 4.09 0.16 0.66
SP4H 72.26 0.48 12.77 0.73 3.34 4,00 0.2 1.05 0.87 4.25 3.3 0.14 0.33
SP4I 63.82 0.8 16.71 1.08 4.50 5.57 0.11 2.39 1.01 2.41 5.39 0.21 1.17
SP4J 73.8 0.5 11.74 0.31 3.73 4.01 0.27 0.8 1.89 3.68 1.8 0.13 1.03
SP5A 65.46 0.56 15.5 0.05 3.53 3.57 0.15 1.68 2.04 4.57 3.61 0.13 1.2
SP5B 64.91 0.66 14.74 1.09 4.68 5.66 0.13 2.36 1.56 2.3 5.9 0.22 0.96
SP5C 72.34 0.39 14.11 - - 2.96 0.05 1,00 1.21 5.68 1.47 0.16 0.65
SP5D 66.15 0.56 15.66 - - 3.94 0.09 1.67 2.07 4.66 3.68 0.17 1.14
GSM1T8*

  • Muscovite

  • schist

59.3 0.74 19,00 5.4 2.1 6.95 0.26 3,00 1,00 1.3 5.2 0.15 1.78
GSM4T8* 63.8 0.92 16.6 4.7 2.4 6.63 0.11 2.8 0.43 1.9 3.8 0.19 1.95
GSM5T8* 64.7 0.93 16.6 3.9 2.8 6.31 0.18 2.6 0.4 2,00 3.7 0.19 1.56
GSM6T8* 65,0 0.8 16.4 4.5 2.2 6.25 0.1 2.6 0.29 1.9 3.5 0.16 1.85
T171B 57.3 0.89 18.6 4.6 3.5 7.64 0.1 3.8 1.6 2.3 5,00 0.15 1.96
T168 58.1 0.81 18.8 4.2 3.5 6.81 0.26 3.8 1.8 1.4 5.1 0.15 1.9
TM1091 58.9 0.89 17.4 4.2 3.03 6.98 0.59 3.3 2.3 2.9 4.1 0.2 0.81
GSM3T11*

  • Quartzose

  • metawacke

77.8 0.65 9.8 0.82 2.5 3.24 0.09 1.3 1.3 3.2 1.3 0.21 0.79
GSM4T11* 77.8 0.62 9.8 1.2 2.2 3.28 0.06 1.1 1,00 3.7 1.5 0.21 0.43
GSM2T10*

  • Quartz-

  • biotite

  • schist

70.3 0.76 13.5 2.9 2.7 5.31 0.13 1.1 1.6 2.9 2.4 0.19 1.07
GSM4T5* 63.3 1.1 16.4 2.1 4.9 6.79 0.16 2.8 0.63 2.2 4.1 0.17 1.83
GSM5T5* 65.1 0.89 16.1 1.4 5,00 6.26 0.11 2.7 0.53 2.2 3.3 0.17 2.06
R13 58.7 0.78 18.3 2.2 5,00 6.76 0.35 3.2 2.4 2,00 4.6 0.15 2.04
TM1084 60,0 0.83 17.7 1.9 5.05 5.77 0.37 2.6 2.3 2.8 4.4 0.18 0.83
T3721 61.5 0.79 16.6 1.3 4.6 6.78 0.41 3.5 3.5 3.4 2.7 0.25 1.13
T192A 61.9 0.92 17.8 2.2 4.8 6.9 0.14 2.9 0.57 2.2 3.9 0.16 2.15
T210 62.0 0.86 16.9 3,00 4.2 6.25 0.15 3.6 1.4 2.3 3.7 0.17 1.5
T170C 62.0 0.86 16.9 2.5 4,00 6.67 0.05 2.9 2.6 1.8 4.7 0.18 1.28
TM1097 64.1 1,00 15.1 1.8 5.05 5.85 0.16 2.6 1.2 2.8 3.6 0.21 0.43
T3706 64.1 0.98 15.8 0.72 5.2 5.11 0.09 4.4 0.47 2.3 3.4 0.18 1.11
R8B 64.9 0.73 16.1 1.9 3.4 5.94 0.09 2.5 2.1 2.4 4.1 0.19 1.31
T3722 66.3 0.91 15.3 0.82 5.2 5.67 0.1 3.3 0.71 1.9 3.2 0.2 1.64
T3716 66.8 0.73 14.2 0.97 4.8 5.67 0.08 4.6 0.41 1.2 4,00 0.15 1.91
UCC 66.0 0.5 15.2 - - 4.5 0.1 2.2 4.2 3.9 3.4 0.2 -

*Grossi-Sad & Motta (1991); and UCC (Upper continental crust average, Taylor & McLennan 1985).

Table 2. Chemical values calculated from major elements composition of Salinas metasedimentary rocks. 

Sample Lithofacies log(SiO2 log(Na2O log(FeOt CIA Al2O3 CaO* + Na2O K2O SiO2 K2O MgO SiO2 K2O+ TiO2+ Al2O3 FeOt+ Al2O3 DF1(A) Disc. Disc.
Al2O3) K2O) K2O) molar molar molar Al2O3 Na2O CaO 20 Na2O FeOt+MgO (CaO+Na2O) MgO SiO2 DF(B) func.1(C) func.2(D)
SM51

  • Carbonate

  • schist

0.36 -0.72 0.07 63.46 0.2 0.04 0.07 2.3 5.31 0.67 2.35 8.20 12.37 3.29 11.37 0.43 7.43 0.50 0.53 -51.25
T171C 0.60 -0.23 0.36 46.62 0.15 0.14 0.03 3.94 1.69 0.55 2.94 4.30 10.53 1.86 9.74 0.25 5.86 -1.28 0.26 -45.66
T3720 0.70 0.18 0.35 52.66 0.13 0.09 0.02 5.05 0.66 0.84 3.26 4.80 8.86 1.77 7.94 0.2 2.09 2.72 0.34 -38.76
SM20 0.78 0.09 0.25 48.68 0.11 0.09 0.02 6.01 0.82 0.37 3.34 5.10 6.58 1.44 5.85 0.17 -0.43 1.07 1.29 -44.06
TM1085 0.72 0.05 0.23 52.36 0.13 0.09 0.03 5.21 0.89 0.53 3.42 5.30 6.63 2.26 5.82 0.19 -0.60 1.19 0.45 -44.57
TM1160 0.76 0.09 0.28 51.58 0.12 0.09 0.02 5.72 0.81 0.42 3.43 4.90 6.58 1.85 5.84 0.17 -0.54 1.29 0.29 -44.07
T185 0.73 -0.02 0.14 56.12 0.13 0.07 0.03 5.37 1.04 0.58 3.49 4.70 5.77 2.41 5.13 0.19 0.27 1.26 0.27 -44.32
TM2418 0.80 -0.27 0.13 60.57 0.11 0.05 0.03 6.32 1.86 0.47 3.61 4.00 5.57 2.48 5.01 0.16 0.68 0.65 -1.14 -44.63
SP4A

  • Salinas

  • Type-area

  • (metawacke)

0.74 0.07 0.25 52.05 0.12 0.08 0.03 5.54 0.86 0.90 3.53 6.96 7.48 2.55 6.82 0.18 -1.71 1.68 0.71 -46.88
SP4B 0.61 -0.10 0.10 55.93 0.16 0.07 0.05 4.09 1.25 2.58 3.26 8.4 8.78 3.48 8.03 0.24 -0.77 1.51 1.35 -48.07
SP4C 0.57 -0.22 0.11 55.69 0.16 0.08 0.05 3.71 1.64 1.80 3.12 8.33 10.21 3.60 9.42 0.27 6.06 0.27 0.95 -48.56
SP4D 0.81 0.38 0.29 57.38 0.12 0.07 0.02 6.44 0.42 1.04 3.84 5.25 4.22 2.73 3.73 0.16 -2.02 2.63 -0.30 -43.04
SP4E 0.68 -0.06 0.03 57.21 0.14 0.07 0.04 4.78 1.14 2.67 3.5 7.49 6.39 3.59 5.8 0.21 -1.05 1.58 0.53 -46.56
SP4F 0.79 0.02 0.09 54.49 0.12 0.06 0.03 6.16 0.95 1.35 3.72 6.70 5.37 3.02 4.78 0.16 -2.60 1.06 -0.35 -45.97
SP4G 0.69 -0.05 -0.03 54.86 0.14 0.07 0.04 4.86 1.12 1.33 3.52 7.73 5.56 3.18 5.04 0.21 -0.76 1.49 0.61 -47.59
SP4H 0.75 0.11 0.13 51.52 0.13 0.08 0.04 5.66 0.78 1.21 3.61 7.55 5.98 2.49 5.5 0.18 -1.54 2.15 0.61 -46.84
SP4I 0.58 -0.35 0.05 59.36 0.16 0.05 0.06 3.82 2.24 2.37 3.19 7.8 9.28 4.89 8.48 0.26 -0.01 0.86 0.48 -49.10
SP4J 0.80 0.31 0.39 50.91 0.12 0.09 0.02 6.29 0.49 0.42 3.69 5.48 5.76 2.11 5.26 0.16 -1.71 2.49 0.22 -45.50
SP5A 0.63 0.10 0.04 50.8 0.15 0.11 0.04 4.22 0.79 0.82 3.27 8.18 6.22 2.34 5.66 0.24 -0.36 2.01 1.69 -46.51
SP5B 0.64 -0.41 0.03 53.52 0.14 0.06 0.06 4.4 2.57 1.51 3.25 8.20 9.32 3.82 8.66 0.23 0.13 0.63 0.61 -50.42
SP5C 0.71 0.59 0.35 52.07 0.14 0.11 0.02 5.13 0.26 0.83 3.62 7.15 4.68 2.05 4.29 0.20 -2.34 3.29 - -
SP5D 0.63 0.10 0.08 50.65 0.15 0.11 0.04 4.22 0.79 0.81 3.31 8.34 6.61 2.33 6.05 0.24 -0.99 1.77 - -
GSM1T8*

  • Muscovite

  • schist

0.49 -0.60 0.13 66.79 0.19 0.04 0.06 3.12 4.00 3.00 2.97 6.50 10.7 8.26 9.96 0.32 6.77 -1.28 -2.51 -46.03
GSM4T8* 0.58 -0.30 0.24 67.91 0.16 0.04 0.04 3.84 2.00 6.51 3.19 5.70 10.35 7.12 9.43 0.26 0.19 1.58 -1.56 -43.68
GSM5T8* 0.59 -0.27 0.23 67.9 0.16 0.04 0.04 3.9 1.85 6.50 3.24 5.70 9.84 6.92 8.91 0.26 0.36 1.83 -1.49 -44.03
GSM6T8* 0.6 -0.27 0.25 69.22 0.16 0.03 0.04 3.96 1.84 8.97 3.25 5.40 9.65 7.49 8.85 0.25 0.10 1.93 -1.73 -43.63
T171B 0.49 -0.34 0.18 60.85 0.18 0.06 0.05 3.08 2.17 2.38 2.87 7.30 12.33 4.77 11.44 0.32 6.37 0.38 -0.62 -46.52
T168 0.49 -0.56 0.13 54.99 0.18 0.05 0.05 3.09 3.64 2.11 2.91 6.50 11.42 5.88 10.61 0.32 6.68 -1.43 -1.57 -46.73
TM1091 0.53 -0.15 0.23 56.86 0.17 0.09 0.04 3.39 1.41 1.43 2.95 7.00 11.17 3.35 10.28 0.3 4.86 -1.67 -1.30 -43.38
GSM3T11*

  • Quartzose

  • metawacke

0.9 0.39 0.40 52.57 0.1 0.07 0.01 7.94 0.41 1.00 3.89 4.50 5.19 2.18 4.54 0.13 -1.23 2.34 0.27 -42.13
GSM4T11* 0.9 0.39 0.34 51.22 0.1 0.08 0.02 7.94 0.41 1.10 3.89 5.20 5 2.09 4.38 0.13 -2.09 2.10 0.49 -42.33
GSM2T10*

  • Quartz-

  • biotite

  • schist

0.72 0.08 0.34 57 0.13 0.07 0.03 5.21 0.83 0.69 3.52 5.30 7.17 3.00 6.41 0.19 -1.82 1.47 -0.48 -44.64
GSM4T5* 0.59 -0.27 0.22 64.45 0.16 0.05 0.04 3.86 1.86 4.44 3.17 6.30 10.69 5.80 9.59 0.26 0.25 1.41 -0.56 -46.37
GSM5T5* 0.61 -0.18 0.28 66.81 0.16 0.04 0.04 4.04 1.50 5.09 3.26 5.50 9.85 5.90 8.96 0.25 0.07 1.86 -0.08 -45.83
R13 0.51 -0.36 0.17 61.29 0.18 0.06 0.05 3.21 2.30 1.33 2.94 6.60 10.74 4.16 9.96 0.31 5.91 -1.35 -0.53 -48.07
TM1084 0.53 -0.20 0.12 56.94 0.17 0.08 0.05 3.39 1.57 1.13 3 7.20 9.2 3.47 8.37 0.3 5.27 -1.15 0.09 -48.01
T3721 0.57 0.10 0.40 54.06 0.16 0.11 0.03 3.7 0.79 1.00 3.08 6.10 11.07 2.41 10.28 0.27 4.33 -0.94 1.37 -43.71
T192A 0.54 -0.25 0.25 67.1 0.17 0.04 0.04 3.48 1.77 5.09 3.1 6.10 10.72 6.43 9.8 0.29 0.31 1.82 -0.36 -46.39
T210 0.56 -0.21 0.23 62.42 0.17 0.06 0.04 3.67 1.61 2.57 3.1 6.00 10.71 4.57 9.85 0.27 1.04 1.79 -0.27 -45.23
T170C 0.56 -0.42 0.15 60.55 0.17 0.06 0.05 3.67 2.61 1.12 3.1 6.50 10.43 3.84 9.57 0.27 7.37 1.55 -0.05 -48.32
TM1097 0.63 -0.11 0.21 59.01 0.15 0.06 0.04 4.25 1.29 2.17 3.21 6.40 9.45 3.78 8.45 0.24 0.02 1.50 0.43 -45.86
T3706 0.61 -0.17 0.18 65.97 0.15 0.04 0.04 4.06 1.48 9.36 3.21 5.70 10.49 5.70 9.51 0.25 1.23 2.02 0.53 -43.64
R8B 0.61 -0.23 0.16 57.24 0.16 0.07 0.04 4.03 1.71 1.19 3.25 6.50 9.17 3.58 8.44 0.25 -0.03 1.07 0.22 -46.77
T3722 0.64 -0.23 0.25 66.54 0.15 0.04 0.03 4.33 1.68 4.65 3.32 5.10 9.88 5.86 8.97 0.23 0.76 1.63 0.20 -45.10
T3716 0.67 -0.52 0.15 67.27 0.14 0.03 0.04 4.7 3.33 11.22 3.34 5.20 11.00 8.82 10.27 0.21 1.98 1.40 -0.50 -44.92

*Grossi-Sad & Motta (1991) and UCC (Upper continental crust average, Taylor & McLennan 1985); Adiscriminant Function 1 (Verma & Armstrong-Altrin 2013); Bdiscriminant Function 2 (Verma & Armstrong-Altrin 2013); Cdiscriminant Function 1 (Bhatia 1985); Ddiscriminant Function 2 (Bhatia 1985).

Table 3. Trace element (in ppm) compositions of clastic metasedimentary rocks from the Salinas Formation. 

Sample Lithofacies La Ce Nd Sm Eu Gd Dy Ho Er Yb Lu U Th Zr Sc Co Eu/Eu* Zr/10 Ti/Zr La/Sc
SM51

  • Carbonate

  • schist

44.11 97.23 42.94 8.37 1.53 6.01 4.91 1 2.33 2.02 0.28 - 13 290 22 - 0.05 29 16.5 2.01
T171C 27.15 61.52 28.79 5.98 1.1 4.71 5.26 1.19 3.18 2.95 0.4 - - - - - 0.07 - - -
T3720 50.68 110.5 54.08 10.63 1.91 8.46 8.89 1.87 5.35 4.84 0.63 - 13 370 19 - 0.04 37 14.06 2.67
SM20 34.97 76.69 36.42 7.36 1.18 5.55 5.6 1.25 3.26 3.19 0.43 - 15 390 15 - 0.05 39 16.81 2.33
TM1085 43.1 83.1 37.6 7.1 1.4 6.3 4.4 0.93 2.3 1.9 0.25 - - 430 - - 0.05 43 13.74 -
TM1160 30.3 50.4 27 5.3 0.98 5.1 3.9 0.83 2.1 1.9 0.25 - - 400 - - 0.07 40 16.17 -
T185 30.8 65.93 30.85 6.09 1.2 4.51 4.54 0.99 2.67 2.54 0.33 - 14 280 17 - 0.06 28 26.71 1.81
TM2418 43.1 82 37.4 7.2 1.3 6.3 4.1 0.86 2.1 1.9 0.26 - - 460 - - 0.05 46 18.58 -
SP4A

  • Salinas

  • Type-area

  • (metawacke)

84.5 161.53 60.53 10.33 16.92 7.95 6.59 1.27 3.76 3.64 0.54 4.45 27.29 522.3 9 35.16 0.04 52 13.95 9.39
SP4B 41.43 88.04 36.66 74.23 1.32 63.1 5.99 1.2 3.6 3.23 0.49 3.45 19.86 192.51 13 24.35 0.05 19 32.97 3.19
SP4C 60.55 105.99 47.14 8.43 1.54 6.77 5.61 1.08 3.23 3.01 0.44 2.18 20.77 180.48 15 23.89 0.05 18 33.4 4.04
SP4D 43.95 80.63 32.74 5.59 1.11 4.24 3.75 0.71 2.07 2.03 0.31 3.27 15.83 267.23 7 28.60 0.07 27 36.55 6.28
SP4E 50 87.13 38.82 6.95 1.36 5.92 5.2 0.99 2.93 2.77 0.39 2.25 16.96 249.31 9 19.38 0.06 25 32.48 5.56
SP4F 71.9 136.02 50.79 8.4 1.43 6.68 5.66 1.1 3.1 2.91 0.44 3.00 23.49 393.36 8 44.81 0.05 39 20.73 8.99
SP4G 49.02 95.62 37.38 6.52 1.28 5.06 4.75 0.9 2.77 2.55 0.38 2.46 14.63 223.26 8 25.14 0.06 22 40.99 6.13
SP4H 38.94 77.25 30.45 5.54 1.13 4.47 4.0 0.79 2.3 2.13 0.31 2.10 13.20 189.2 8 18.38 0.07 19 52.92 4.87
SP4I 44.3 92.37 38.68 7.39 1.41 6.21 6.18 1.21 3.64 3.3 0.51 3.44 21.12 179.05 15 25.49 0.05 18 33.37 2.95
SP4J 56.56 107.6 39.91 6.79 1.2 5.21 4.49 0.87 2.61 2.53 0.38 3.18 18.78 320.99 7 35.94 0.06 32 29.88 8.08
SP5A 41.73 84.27 36.77 7.08 1.37 5.78 5.31 1.06 3.15 2.91 0.44 2.20 18.65 242.77 11 35.82 0.06 24 34.95 3.79
SP5B 37.82 77.98 32.94 6.66 1.41 5.56 5.03 1.02 3.09 2.9 0.44 3.13 15.6 203.59 13 26.11 0.06 20 35.4 2.91
SP5C 20.27 27.43 17.94 3.9 0.67 3.44 4.18 0.91 3.14 3.29 0.53 4.72 17.04 200.32 8 21.26 0.1 20 61.11 2.53
SP5D 42.74 86.8 37.57 7.22 1.4 6.06 5.51 1.08 3.2 2.96 0.44 2.25 18.99 243.47 11 44.10 0.05 24 34.98 3.89
GSM1T8*

  • Muscovite

  • schist

42.61 93.21 40.4 7.68 1.41 6.17 6.2 1.25 3.4 3.21 0.39 - - - - 53 0.05 - - -
GSM4T8* 11.63 24.05 9.795 1.97 0.50 2.14 3.13 0.69 2.17 2.63 0.37 - - - - 13 0.20 - - -
GSM5T8* 38.36 83.94 35.89 6.71 1.23 5.1 5.14 1.05 2.96 3.26 0.41 - - - - 13 0.06 - - -
GSM6T8* 32.43 71.29 31.29 5.94 1.13 4.62 4.8 1 2.89 2.96 0.37 - - - - 20 0.07 - - -
T171B 56.93 86.46 55.55 10.27 1.77 7.06 5.34 1.08 2.47 1.8 0.23 - 7 200 23 - 0.04 32.78 2.48
T168 26.81 61.37 26.66 5.4 0.99 3.85 3.75 0.83 2.26 2.42 0.34 - 12 200 22 - 0.07 20 29.54 1.22
TM1091 22.1 59.1 21.7 4.5 0.88 4.3 3.8 0.84 2.4 2.6 0.35 - - 310 - - 0.09 31 17.35
GSM3T11*

  • Quartzose

  • metawacke

24.99 60.18 28.61 5.28 1.01 3.73 3.84 0.79 2.2 2.15 0.26 - - - - 7 0.07 - - -
GSM4T11* 22.07 45.34 20.41 5.15 0.9 4.58 4.1 0.82 2.17 2.52 0.36 - - - - - 0.08 - - -
GSM2T10*

  • Quartz-

  • biotite

  • schist

35.69 79.9 34.69 6.46 1.09 4.72 4.16 0.84 2.29 2.53 0.32 - - - - 9 0.06 - - -
GSM4T5* 42.14 92.28 40.43 8.03 1.41 6.29 6.74 1.39 3.97 4.04 0.54 - - - - 18 0.05 - - -
GSM5T5* 29.65 68.12 33.03 6.24 1.19 4.6 4.89 0.99 2.72 2.66 0.34 - - - - 36 0.06 - - -
R13 29.03 68.5 32.01 6.37 1.22 4.6 4.29 0.88 2.31 2.32 0.34 - 13 200 18 - 0.06 20 31.49 1.61
TM1084 43.6 92.3 40.6 8 1.5 6.5 4.2 0.84 2.1 2.2 0.3 - - 500 - - 0.05 50 8.7
T3721 32.69 78.9 34.35 6.57 1.22 4.9 5.16 1.06 3.06 3.13 0.42 - - - - - 0.06 - - -
T192A 30.67 67.25 31.08 6.37 1.25 4.8 4.75 0.98 2.67 2.81 0.39 - 11 230 19 - 0.06 23 26.68 1.61
T210 35.8 71.05 33.21 6.88 1.24 5.16 5.19 1.21 2.98 3.13 0.43 - 18 210 18 - 0.06 21 27.46 1.99
T170C 44.96 89.69 43.06 8.33 1.56 6.17 6.54 1.4 3.51 3.01 0.37 - - - - - 0.05 - - -
T3706 25.13 52.38 24.62 4.91 1.05 3.66 4.29 0.90 2.72 2.89 0.39 - 13 240 22 - 0.08 24 21.68 1.14
R8B 34.99 71.9 33.13 6.23 1.18 4.46 4.08 0.84 2.16 1.88 0.23 - 13 220 17 - 0.06 22 25.3 2.06
T3722 40.65 76.31 38.36 7.71 1.48 5.58 5.017 0.98 2.74 2.61 0.35 - 8 230 21 - 0.05 23 24.2 1.94
T3716 29.11 63.85 28.05 5.1 0.97 3.52 2.96 0.54 1.65 1.89 0.28 - 7 170 20 - 0.08 17 28.15 1.46
UCC 30 64 26 4.5 0.88 3.8 3.5 0.8 2.3 2.2 0.32 2.8 10.7 190 13.6 17 0.09 19 0.09 2.21

*Grossi-Sad & Motta (1991) and UCC (Upper continental crust average, Taylor & McLennan 1985).

Data evaluation and protolith interpretation

The chemical composition of sediments is a complex system influenced by the petrological evolution of the source rocks, as well as sedimentary maturation, weathering and diagenesis. Although diagenesis can promote significant changes in the chemical composition of siliciclastic sediments, lithochemical data from sedimentary and metasedimentary rocks may be useful to interpret their origin. Even if clastic fractions (e.g., feldspars and lithoclasts) are selectively dissolved or replaced by authigenic minerals during diagenesis (Morton & Hallsworth 1999), the overall lithochemical signature can be preserved. Erosion and transport of weathered rocks may produce chemical differentiation, but not chemical changes, through the selection of transported sediments (Nesbitt & Young 1984). Despite the variable intensity of these processes, weathering is the main cause of compositional change in siliciclastic rocks (Nesbitt 2003). According to Nesbitt and Young (1984), the weathering degree of the source rock can be measured through the chemical index of alteration (CIA = [Al2O3/(Al2O3 + CaO* + Na2O + K2O)] x 100)). The SiO2/Al2O3 ratio can be useful to indicate the degree of sediment maturity, reflecting the increase of the quartz fraction in relation to clay fraction during transport and recycling of sediments (Roser & Korsch 1999). The K2O/Na2O ratio indicates the proportion of potassic phases (e.g., K-feldspar, common micas, illite) in relation to plagioclase, and the MgO/CaO ratio can represent the relative contribution of iron-magnesian minerals and plagioclase in carbonate-free rocks (Pedrosa-Soares 1995, and references therein).

The individual chemical index of alteration (CIA) values for Salinas samples are low to intermediate, between 47 and 69 (Fig. 5), with a general average of 58, which is very close to the average CIA (= 50) for the non-weathered upper crust (Taylor & McLennan 1985). The distinct lithotypes of Salinas Formation show the following CIA averages: metawackes of the type area, 54; carbonate schists, 54; quartzose metawacke, 52; quartz-biotite schist, 62; and muscovite schist, 64. The general CIA average for the samples from Minas Novas corridor is 59, a little bit greater than the average CIA for the type area (54). In terms of lithotype correlations, the CIA values of the metawackes from the type area (54) and carbonate schist (54) are virtually equal, and a little higher than the CIA (52) of the quartzose metawacke, indicating a solid correlation between these lithotypes relatively rich in immature sand fraction. Indeed, the higher the mica content the greater the CIA value, as shown by the quartz mica schist (average CIA = 62) and muscovite schist (average CIA = 64), which indicate an increase of clay fraction and, consequently, more weathered sources.

Figure 5. Al2O3-(CaO*+Na2O)-K2O (in molar proportion) diagram for the Salinas samples, together with the chemical index of alteration (CIA) scale. The black arrow shows the compositional trend of the studied samples. The light grey dotted arrows indicate weathering trends of some common igneous rocks. 

The Al2O3-(CaO* + Na2O)-K2O diagram (Fig. 5) confirms the feebly weathered nature for the sedimentary protoliths, showing a moderate tendency toward the Al2O3 vertex, which corresponds to the maximum CIA. The Salinas samples outline a well-defined trend with relatively low CIA values close to the plagioclase corner and increase of Al2O3, K2O and CIA values (Fig. 5). The lower segment of this trend, with CIA from 46 to 60, includes the metawackes of the type area and most of their deformed equivalents (the carbonate schist and quartzose metawacke). The upper segment of the trend (CIA from 60 to 70) mainly includes the quartz-biotite and muscovite schists, reflecting the more pelitic (Al2O3- and K-rich) fractions present in these rocks. This trend suggests sediment sources originally rich in intermediate to felsic igneous rocks. The upper part of the trend may reflect clay-rich fractions directly eroded and transported from more weathered sources and/or the conversion of feldspars into clay during diagenetic processes. In turn, metawackes from the type area, carbonate schist and quartzose metawacke retain detrital compositions relatively poor in clay fraction, as suggested by the presence of lithic fragments, as well as abundant quartz and plagioclase, instead of micas and other Al-rich silicates.

Salinas rocks have an average SiO2/Al2O3 ratio of 5, and values between 2 and 8, reflecting a high degree of immaturity, which is typical of deposition close to the source (cf. Zhang 2004), although it can also be associated with quick transport and/or cold climate conditions.

The sediment maturity index (SiO2/Al2O3) can be compared with the relative contributions of K-rich components versus plagioclase (K2O/Na2O) and iron-magnesium minerals versus plagioclase (MgO/CaO), using the triangular diagram proposed by Pedrosa-Soares (1995) (Fig. 6). Besides the correlation of metawackes from the type area with carbonate schist and quartzose metawacke (mostly of them plotting closer the SiO2/Al2O3 vertex in relation to quartz-biotite schist and muscovite schist), the sample set follows a roughly curved trend from the more mature rocks toward the MgO/CaO vertex. It suggests that detrital plagioclase and iron-magnesium minerals were important contributions even in the pelitic protoliths, and the K-rich contribution was mainly related to mica and/or clay fractions rather than detrital K-feldspar. The CaO component can be mainly ascribed to detrital plagioclase, even in the carbonate schist and metawackes, as they plot far from the MgO/CaO vertex and tend to have a relatively narrow range of K2O/Na2O ratios.

Figure 6. Chemical compositions of rocks of the Salinas Formation plotted in the SiO2/Al2O3 - K2O/Na2O - MgO/CaO diagram of Pedrosa-Soares (1995). The dashed black arrow follows the curved trend shown by the sample set. 

As expected from the field and petrographic studies, diagrams for lithological classification based on major element content (Pettijohn et al. 1972, Herron 1988) show that the studied rocks plot mostly in the graywacke and lithoarenite fields (Fig. 7). The relative high values of SiO2 (47-78%), Al2O3 (10-20%) and FeOt (8-22%) reflect a terrigenous composition with significant argillaceous contribution. The higher content of silica corresponds to relatively quartz-rich and biotite-poor rocks, mostly the massive metawacke and quartzose metawacke, but also to some quartz-biotite schist and carbonate schist. The higher content of Al2O3 and FeOt correlates with increasing modal values of biotite and other peraluminous Fe-rich silicates (e.g., garnet and staurolite), reflecting the importance of the pelitic (mostly clay) contribution. TiO2, Al2O3, MgO, P2O5, K2O, FeOt and MnO are negatively correlated with SiO2, reflecting the decrease of silica with increase of the matrix proportion together with increase of the maturity index (Fig. 8). In contrast, Na2O exhibits a positive correlation with SiO2 (Fig. 4), and the higher Na2O values correspond to samples of metawacke from the Salinas type area and quartzose metawacke found in the Minas Novas corridor, which are generally the lithotypes richer in plagioclase. As the plagioclase found in those lithotypes is generally andesine to oligoclase, the lithochemical signature also points to sources rich in intermediate to felsic granitic rocks.

Figure 7. Chemical classification diagrams for rocks from the Salinas Formation: (a) Log (Na2O/K2O) versus log (SiO2/Al2O3) of Pettijohn et al. (1972); (b) Log (FeOt/K2O) versus log (SiO2/Al2O3) of Herron (1988). 

Figure 8. Bivariant diagrams of SiO2 versus (a) TiO2, (b) Al2O3, (c) MgO, (d) CaO, (e)Na2O, (f) K2O, (g) P2O5, (h) FeOt; and (i) MnO. Upper continental crust (UCC) values are from Taylor & McLennan (1985). 

The large compositional spread of major elements as to the upper continental crust average suggests great variability in sediment maturity, involving a wide spectrum of mixtures from distinct contributions, possibly also reflecting distinct sources (cf. Taylor & McLennan 1985).

Provenance and tectonic environment

The low metamorphic grade of Salinas metawackes, in the type area and elsewhere (Figs. 1 and 2), suggests that the metamorphism has not strongly affected their primary chemical signatures, allowing the characterization of sedimentary protolith compositions and related tectonic environments. Furthermore, the high chemical immaturity (i.e., low SiO2/Al2O3) and limited weathering of the source rocks (i.e., low CIA) allow us to assess the provenance of these rocks using petrographic and lithochemical features. The main sources for the Salinas sediments can be inferred from the relative abundance of their grains in thin section, in which the high proportion of quartz and plagioclase in the metawackes may indicate tonalitic-granodioritic sources.

However, chemical modifications may occur due to the high mobility of some major elements, even at low-grade metamorphic conditions. Therefore, trace elements such as La, Sc, Th, Ti, Zr and Co are considered reliable indicators of tectonic environments (Bhatia 1985, Bhatia & Crook 1986), and when combined with major elements, they can be used to better identify them. The rare earth element (REE) distribution patterns (Fig. 9) show some overall enrichment in the REE content, specially of the light REE (LREE), and intermediate to weakly negative Eu anomalies (Eu/Eu* = 0.04-0.20), with the average upper to continental crust (UCC) curve roughly at the midpoint within the compositional range for Salinas Formation. The range of negative Eu anomalies (Eu/Eu* = 0.04-0.20) overlaps the UCC corresponding value (Eu/Eu* = 0.09), suggesting contribution from predominantly felsic protoliths. Although these features are not conclusive, the general REE pattern tends to be close to those of mature active margins with continental magmatic arcs (Bhatia 1985, Taylor & McLennan 1985, Sun et al. 2017). Despite some scattered distributions, discriminant diagrams for tectonic environments (Figs. 10 and 11) show that the Salinas rocks plot mainly in the fields of continental magmatic arc, active continental margin, collisional and oceanic island arc, but they never do it in the passive margin or rift fields.

Figure 9. Chondrite-normalized REE patterns for the Salinas metawackes. Chondrite values from Boynton (1984), UCC values from Taylor & McLennan (1985). 

Figure 10. Major-elements diagrams for provenance discrimination of protoliths of Salinas rocks: (a) Discriminant-function multi-dimensional diagram for high-silica sediments, DF1 versus DF2 of Verma & Armstrong-Altrin (2013); (b) Discriminant-function multi-dimensional diagram for low-silica sediments, DF1 versus DF2 of Verma & Armstrong-Altrin (2013); (c) Ternary diagram K2O+Na2O - SiO2/20 - TiO2+FeOt+MgO of Kroonenberg (1994); (d) Al2O3/(CaO+Na2O) versus FeOt+MgO of Bhatia (1985); (e) Al2O3/SiO2 versus FeOt+MgO of Bhatia (1985); (f) TiO2 versus FeOt+MgO of Bhatia (1985); (g) Discriminant function diagram from Bhatia (1985), Discriminant Function 2 versus Discriminant Function 1; (h) K2O/Na2O of Roser & Korsch (1986). Fields: Col- collisional; A, oceanic island arc; B, continental magmatic arc; C, active continental margin; D, passive continental margin. 

Figure 11. Trace element diagrams for provenance discrimination of protoliths of Salinas rocks: Bhatia & Crook (1986): (a) Th-La-Sc, Zr/10-Th-Co and Sc-Th-Zr/10 ternary diagrams; (b) Ti/Zr versus La/Sc. Fields: A, oceanic island arc; B, continental magmatic arc; C, active continental margin; D, passive continental margin. 

Diagrams of Verma and Armstrong-Altrin (2013) have been used to infer tectonic setting of Precambrian clastic rocks. The high-silica diagram corresponds to a (SiO2)adj value between 63 and 95%, and low-silica diagram to a (SiO2)adj value between 35 and 63%, in which (SiO2)adj refers to the SiO2 value obtained after volatile-free adjustment of the ten major-elements to 100 wt%. Most studied samples belong to the high-silica type that plotted in the arc field (Fig. 10A). For the ­low-silica diagram, the remaining samples plotted in the collisional field (Fig. 10B). Comparing the composition of low-silica and high-silica types, the source rocks for the low-silica type are expressively more aluminous than for the high-silica samples. Thus, the percentage of sources rich in pelitic rocks should be higher for the low-silica samples. This signature could be derived from secondary sources as Macaúbas Group, Jequitinhonha Complex, and metavolcanic and meta-volcaniclastic rocks of the Rio Doce arc, in response to tectonic activity associated with the exhumation and erosion of thrust fronts, in a scenario consistent with a collisional setting (Fig. 10B).

Together, the major and trace element signatures indicate provenance of protoliths from magmatic arcs and active continental margins, which can be, indeed, of any age older than the regional metamorphism shown by the Salinas rocks (i.e., older than c. 570 Ma, cf. Peixoto et al. 2018). In the regional scenario of the Araçuaí orogen, possible candidates to provide sediment sources are the Ediacaran Rio Doce arc (Gonçalves et al. 2016, Tedeschi et al. 2016) and the Rhyacian-Orosirian Mantiqueira arc (Noce et al. 2007), as well as related active margins. Sources in island arcs can be found not only in the juvenile basement of Rio Doce arc, like Rhyacian-Orosirian Juiz de Fora and Pocrane magmatic arcs (Noce et al. 2007, Heilbron et al. 2010, Degler et al. 2018), but also in the basement of the Southern São Francisco craton (Ávila et al. 2010, Teixeira et al. 2015). Although far from the studied region, other possible sources in island arc settings are the juvenile magmatic arcs Serra da Prata and Rio Negro of the Ribeira belt (Tupinambá et al. 2012, Peixoto et al. 2017).

U-Pb (SHRIMP) GEOCHRONOLOGY

Volcanic clasts from an orthoconglomerate (sample OPU-1995), Salinas Formation type area

U-Pb Concordia diagram for OPU-1995 sample includes 20 zircon grains (Figs. 12 and 13) recovered from a concentrate of pebbles and cobbles of volcanic rocks from a clast-supported metaconglomerate (an orthoconglomerate) cropping out to the South of Salinas town (Fig. 1). The zircons grains include prismatic, euhedral to subhedral crystals. Their size varies from 90 and 200 µm in length, and the length/width ratios range from 2:1 to 3:1. They show well-developed oscillatory zoning in CL images, suggesting that the grains were derived from intermediate to felsic igneous rocks (Fig. 12). All the spot analyses have Th/U ratios higher than 0.1, which also indicate igneous growth. However, 11 of the analyses fulfilled the requirements of Ludwig (2003) Isoplot program to constitute a Concordia age of 620 ± 10 Ma (Fig. 13). Even though the zircon crystals may derive from different rocks, we consider that they could well be part of the same regional volcanic episode.

Figure 12. Cathodoluminescence (CL) images showing U-Pb spots in representative zircon grains of sample OPU-1995. 

Figure 13. Corcordia diagram showing ages for the pebble-cobble and matrix of sample OPU-1995. Insets showing the Concordia age of 620 ± 10 Ma, yielded by eleven spots. 

Some other grains reported in Table 4, such as zircons 10.1 (207Pb/206Pb age of 2700 Ma), 7.1 (2100 Ma age) and 13.1 (720 Ma age), among others, may be considered possible inherited grains within the volcanic rocks, or part of the detrital component of the matrix. Their sources shall be sought either within the nearby continental basement (e.g., the Guanhães or Porteirinha blocks), or within magmatic rocks formed during the early tectonic evolution of Araçuaí orogen. The age of 620 Ma represents an important Ediacaran volcanic episode that contributed for the sedimentary filling of the Salinas basin.

Table 4. U-Pb SHRIMP zircon data from OPU-1995 sample (volcanic pebbles and cobbles from clast-supported conglomerate of the Salinas Formation). Spots that yielded the Concordia age of 620 ± 10 Ma (Fig. 13) are in bold. 

spot U Th Th/U Pbrad 204Pb 206Pbcom 238U / error 207Pb/ error 207Pb/ error 206Pb/ error error 207Pb/ error 206Pb/ error 207Pb/ error Conc.
206Pb 206Pb 235U 238U corr. 206Pb 238U 235U
ppm ppm ppm ppb % age Ma age Ma age Ma %
1.1 185 172 0.92 22 1 0.08 9.73 0.22 0.0627 0.002 0.895 0.035 0.1027 0.0024 0.58 698 65 630 13 645 19 90
2.1 305 297 0.97 35 9 0.47 10.28 0.3 0.0614 0.0027 0.83 0.044 0.0973 0.0028 0.55 654 91 598 16 610 25 91
3.1 151 206 1.36 19 3 0.29 10.2 0.5 0.0587 0.0032 0.799 0.058 0.098 0.0048 0.67 555 116 603 28 593 34 108
4.1 149 64 0.42 50 2 0.07 3.19 0.06 0.1064 0.0016 4.633 0.117 0.3136 0.0064 0.8 1738 27 1758 31 1749 22 101
5.1 874 307 0.35 302 4 0.02 3.02 0.06 0.113 0.0012 5.202 0.124 0.3315 0.0071 0.89 1847 18 1845 34 1846 21 100
6.1 560 73 0.13 100 nd nd 5.52 0.37 0.0925 0.0015 2.329 0.161 0.1813 0.0122 0.97 1483 30 1074 66 1218 49 73
7.1 119 94 0.78 54 1 0.03 2.57 0.04 0.1237 0.002 6.682 0.15 0.389 0.0061 0.69 2010 28 2118 28 2063 20 105
8.1 114 80 0.7 13 6 0.85 9.47 0.24 0.058 0.0033 0.85 0.053 0.1056 0.0027 0.41 530 118 648 15 622 29 122
9.1 442 110 0.24 35 50 2.62 12.26 0.31 0.0633 0.0039 0.717 0.048 0.0816 0.0021 0.38 718 126 505 12 545 29 70
10.1 267 106 0.39 130 3 0.04 2.32 0.04 0.194 0.0016 11.6 0.239 0.4304 0.0081 0.92 2776 13 2307 36 2565 19 83
11.1 116 130 1.12 15 3 0.37 9.5 0.43 0.0588 0.0034 0.859 0.063 0.1052 0.0047 0.61 557 122 645 27 626 35 116
12.1 84 67 0.79 9 4 0.82 10.18 0.29 0.0572 0.0035 0.78 0.053 0.0982 0.0028 0.42 498 129 604 16 582 30 121
13.1 470 376 0.8 60 5 0.15 8.75 0.16 0.0611 0.0014 0.97 0.028 0.1143 0.002 0.61 640 48 697 11 684 15 109
14.1 1374 240 0.17 140 4 0.05 9.38 0.27 0.0599 0.0006 0.887 0.027 0.1066 0.003 0.94 598 22 652 17 640 14 109
14.2 666 87 0.13 61 2 0.06 10.51 0.26 0.0614 0.0006 0.803 0.023 0.0948 0.0024 0.88 646 26 587 3 599 6 89
15.1 337 312 0.92 39 3 0.14 10.1 0.4 0.0597 0.0012 0.821 0.036 0.0991 0.0039 0.89 591 42 608 23 605 21 103
15.2 465 396 0.85 47 nd nd 11.64 0.42 0.0602 0.0008 0.713 0.029 0.0859 0.0031 0.9 609 40 533 3 548 8 85
16.1 155 149 0.96 19 4 0.39 9.71 0.23 0.0585 0.0029 0.837 0.046 0.103 0.0024 0.43 548 105 632 14 614 26 115
16.2 143 153 1.06 17 1 0.11 16.94 0.46 0.0591 0.0016 0.816 0.046 0.1001 0.0046 0.82 400 219 603 75 562 74 107
17.1 622 125 0.2 64 3 0.09 9.63 0.24 0.0714 0.0013 1.023 0.033 0.1039 0.0026 0.77 933 37 642 5 710 11 66
18.1 136 68 0.5 19 2 0.19 7.71 0.09 0.0673 0.0014 1.203 0.041 0.1297 0.0016 0.36 814 64 792 13 798 21 93
19.1 169 192 1.13 31 nd nd 6.91 0.14 0.0709 0.0015 1.413 0.044 0.1446 0.003 0.67 948 52 876 15 897 20 91
20.1 582 633 1.08 80 46 1.06 8.65 0.41 0.0709 0.0017 1.130 0.063 0.1156 0.0055 0.85 850 45 702 5 738 13 74

Metawacke (sample SP-54), Salinas Formation type area

Lima et al. (2002), reporting on the sedimentary deposition within the Salinas synclinorium, presented 15 U-Pb SHRIMP age measurements made on 13 detrital zircon grains, taken from sample SP-54, a wacke typical of the Salinas Formation type area located at about 25 km SW from outcrop, where OPU-1995 was collected. The measurements were made by the UGC (Australian National University at Canberra) in 1998, using its SHRIMP instrument. The analytical data are reproduced in Table 5 (Lima et al. 2002). The zircon crystals of SP-54 sample were rounded, exhibited oscillatory zoning in the CL images and presented normal U content, as well as normal Th/U content (0.24-1.22), which are typical of magmatic crystallization. By observing Table 5, only grain six (measured twice) yielded a Paleoproterozoic age. All the other zircons presented Cryogenian to Early Cambrian ages between 700 and 540 Ma. This age span is broadly consistent with the age measurement of 620 ± 10 Ma made on sample OPU-1995 in this work. Figure 14 includes, along the pertinent part of the Concordia diagram, the analytical points (207Pb/206Pb ages) of the dated SP-54 zircons, besides the position of the volcanic episode established from the dating of OPU-1995 sample.

Table 5. U-Pb SHRIMP zircon data from SP-54 sample (metawacke of the Salinas Formation). 

Spot U Th Th/U Pbrad 204 Pb 206 Pb com 238 U/ error 207 Pb/ error 207 Pb/ error 206 Pb/ error error 207 Pb// 206 Pb error 206 Pb/ 238 U error 207 Pb/ 235 U error Conc.
ppm ppm ppm ppb % 206 Pb 206 Pb 235 U 238 U corr. age (Ma) age (Ma) age (Ma) (%)
1.1 176 210 1.2 20 3 0.28 10.63 0.33 0.0605 0.002 0.785 0.038 0.094 0.003 0.64 623 74 579 17 588 22 93
2.1 317 299 0.94 41 1 0.04 9.04 0.29 0.0611 0.001 0.931 0.035 0.1106 0.0036 0.85 642 36 676 21 668 19 105
3.1 240 171 0.71 29 8 0.51 9 0.28 0.0616 0.0012 0.944 0.037 0.1111 0.0035 0.8 661 43 679 20 675 19 103
4.1 324 68 0.21 40 33 1.51 7.85 0.28 0.0582 0.002 1.023 0.054 0.1274 0.0045 0.67 537 77 773 26 715 27 144
5.1 195 117 0.6 21 7 0.61 9.22 0.28 0.0589 0.0013 0.881 0.035 0.1085 0.0033 0.77 564 49 664 19 641 19 118
6.1 238 119 0.5 89 2 0.04 2.95 0.09 0.1259 0.0009 5.891 0.192 0.3393 0.0106 0.95 2042 12 1883 51 1960 29 92
6.2 159 116 0.73 62 3 0.09 2.96 0.18 0.1416 0.0026 6.603 0.428 0.3383 0.0203 0.92 2246 32 1879 98 2060 59 84
7.1 235 136 0.58 28 2 0.13 8.95 0.36 0.0625 0.0009 0.963 0.043 0.1117 0.0045 0.91 685 22 668 29 683 26 99
8.1 840 203 0.24 89 4 0.08 9.24 0.27 0.0622 0.0003 0.928 0.028 0.1082 0.0031 0.96 667 15 650 33 662 18 97
9.1 437 272 0.62 53 n.d. n.d. 8.89 0.62 0.0634 0.0003 0.984 0.069 0.1125 0.0078 0.99 696 36 681 52 687 46 95
10.1 171 207 1.22 21 2 0.17 10.29 0.33 0.0601 0.0011 0.804 0.031 0.0971 0.0031 0.82 599 18 599 25 598 18 99
11.1 207 141 0.68 23 5 0.4 9.78 0.33 0.0585 0.0012 0.824 0.034 0.1022 0.0034 0.81 611 19 600 23 627 20 114
12.1 359 276 0.77 39 5 0.24 10.31 0.06 0.0578 0.0007 0.773 0.048 0.097 0.0058 0.96 581 28 568 37 597 34 115
12.2 316 221 0.7 34 1 0.05 10.24 0.4 0.0609 0.0006 0.82 0.034 0.0976 0.0038 0.95 608 19 593 25 601 23 94
13.1 137 177 1.29 17 4 0.43 9.69 0.85 0.0561 0.0029 0.798 0.085 0.1032 0.0091 0.82 596 49 592 62 633 53 139

Figure 14. Concordia diagram for the Salinas type area metawacke (sample SP-54; data from Lima et al. 2002). 

DISCUSSION AND CONCLUSION

The extraordinary preservation degree of many outcrops makes the Salinas Formation type area an exceptional locality for unraveling the relationships between clastic composition and source rocks information. The presented lithochemical data from Salinas regional rocks and type area ones show a low chemical index of alteration (CIA = 47 to 70), and low SiO2/Al2O3 ratio (2-8), indicating limited weathering of the sediment source rocks. The composition and immaturity of the metawackes may suggest relatively nearby sources, although this is strongly dependent on sediment transportation velocity and climate conditions. The petrographic characteristics of the studied rocks are consistent with their derivation from mixed sources, especially well recorded by the great variability of clasts contained in the Salinas orthoconglomerate.

The first U-Pb geochronological study on clasts of volcanic rocks from a Salinas orthoconglomerate yielded a Concordia age of 620 ± 10 Ma, which strongly suggests a primary provenance from Rio Doce magmatic arc (630-580 Ma; cf. Tedeschi et al. 2016). Furthermore, the U-Pb results given by detrital zircon grains from a Salinas metawacke also show preponderance of ages in the Early Ediacaran.

Robust detrital zircon U-Pb data of Peixoto et al. (2015) provided the youngest main peak at 600 ± 16 Ma (29% of the population, corresponding to 31 grains) and a concordant age of 579 ± 11 Ma, which constrain the maximum depositional age for Salinas Formation. This age is identical with the youngest apparent age (579 ± 17 Ma, conc. 93%) obtained from SP-54 sample. U-Pb data set indicates a maximum depositional age quite younger than the crystallization of the studied clasts of volcanic rocks and suggests that Salinas basin was mainly filled during the development of Rio Doce magmatic arc.

Overall, the presented ages for Salinas Formation compared with zircon data from other Neoproterozoic rock assemblages suggest sources of sediments relatively close to the Salinas basin, as well as sources located far from it, as the following:

Although chemical-based discrimination diagrams of tectonic settings for provenance studies have been criticized by some authors (Weltje 2006, 2012, Caja et al. 2007, Borges et al. 2008), they may be useful if coupled with detailed petrographic studies, confident geochemical scrutiny, and robust geochronological data. In those diagrams (Figs. 10 and 11), most studied samples plot in the continental arc field and extend into the continental active margin field, which are clearly in agreement with interpretations suggested in literature, based on other evidence (Lima et al. 2002, Pedrosa-Soares et al. 2008, Peixoto et al. 2015, 2018, Costa et al. 2018). Furthermore, the tectonic signature shown by the discrimination diagrams supports a series of correlations with the available U-Pb geochronological data.

According to sedimentological and stratigraphic studies published by Martins-Neto et al. (2001), Lima et al. (2002), Santos et al. (2009) and Costa et al. (2018), the Salinas Formation in its type area records upwards-coarsening turbiditic sedimentation (graded wacke deposits with clast-supported conglomerate lenses at the top), followed by upwards-finning deposits (from wackes to pelites). All these deposits filled that part of the basin with sedimentation coming from NNE, under intermittent but strong seismic activity during the orogenic stages of Araçuaí orogen (Martins-Neto et al. 2001, Lima et al. 2002, Santos et al. 2009, Costa et al. 2018). The presented field, petrographic and lithochemical data allow us to correlate the sedimentary succession of the Salinas type area with the rock assemblage (of quartzose metawacke, carbonate schist, quartz-biotite schist, muscovite schist and sparse lenses of clast-supported metaconglomerate) found in Minas Novas corridor, which is a prolongation of the Salinas synclinorium to the South (Figs. 1 and 4). The clast-supported metaconglomerate lenses found in Minas Novas corridor also contain pebbles and cobbles of felsic to intermediate volcanic rocks (Pedrosa-Soares 1995), which are similar to those found in the Salinas type area (Fig. 3). Although it lacks geochronological data for those clasts, the distribution of orthoconglomerate lenses and regional lithofacies (see map from Pedrosa-Soares 1995) suggests that the Salinas basin was also filled from East in relation to the Minas Novas corridor (Fig. 1), i.e. with sediments provided by Rio Doce arc and collisional granites. Therefore, filling of the Salinas basin seems to have started with pre-collisional flysch-type sediments (Santos et al. 2009, Peixoto et al. 2015), mainly provided by Rio Doce arc, but it would have continued during the collisional stage of Araçuaí orogen.

ACKNOWLEDGEMENTS

We are grateful for the financial support provided by Brazilian research and development agencies (CNPq, CAPES, CODEMIG, and PETROBRAS). Our gratitude to the scientific and technical staff of the laboratories that provided analytical data for this paper. The authors are very thankful to Claudio Riccomini, Editor-in-Chief of the Brazilian Journal of Geology, and the anonymous associate editor, and to Robert Pankhurst and an anonymous reviewer for their suggestions, comments, and corrections that greatly help us to improve this manuscript.

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ARTICLE INFORMATION

1Manuscript ID: 20190017.

Received: March 05, 2019; Accepted: May 03, 2019

*Corresponding author.

Author C. D. wrote the first draft of the manuscript and prepared Figures 1 to 12; A. P. S. wrote the abstract and petrography and provided 31 lithochemical analyses, besides improving the manuscript by making corrections and suggestions. Author S. L. provided 14 lithochemical analyses and geochronological data from OPU-1995 sample. Author U. C. got geochronological data and wrote the results of the geochronology section (4.3). Author K. S. provided Figures 13 and 14 and improved the geochronology section.

Competing interests: The authors declare no competing interests.

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