Zircon U-Pb geochronology, Sm-Nd and Pb-Pb isotope systematics of Ediacaran post-collisional high-silica Acampamento Velho volcanism at the Tupanci area, NW of the Sul-Rio-Grandense Shield, Brazil

Geocronologia U-Pb em zircão e isótopos Sm-Nd e Pb-Pb no vulcanismo Ediacarano alta-sílica da Formação Acampamento Velho na região do Tupanci, NW do Escudo Sul-Rio-Grandense, Brasil

ABSTRACT:

We present new U-Pb zircon ages and Sm-Nd-Pb isotopic data for volcanic and hypabyssal acid rocks from the northernmost exposure of the Acampamento Velho Formation in the NW portion of the Sul-Rio-Grandense Shield, Brazil. The first volcanic episode, grouped in the high-Ti rhyolites from the Tupanci hill, shows age of 579 ± 5.6 Ma, which is in agreement with the post-collisional Acampamento Velho Formation volcanism in the Bom Jardim Group of the Camaquã Basin. A poorly constrained age of 558 +/- 39 Ma was obtained for rhyolites from the low-Ti group at the Picados Hill, which may indicate a younger acid volcanism, or a greater time span for the volcanism of the Acampamento Velho Formation in southernmost Brazil. Regarding magmatic sources, Sm/Nd isotopic data coupled to Pb isotopes and a review of trace element geochemistry indicate different amounts of Paleoproterozoic (Dom Feliciano, Pinheiro Machado Suite) to Neoproterozoic (Rio Vacacaí terrane) lower crust melting. Our data, coupled with literature data, contribute to a better understanding of the stratigraphic evolution for the Neoproterozoic post-collisional volcanic successions of the Camaquã Basin in the Sul-Rio-Grandense Shield.

KEYWORDS:
Volcanism; Acampamento Velho Formation; Camaquã Basin; Tupanci; Geochronology

RESUMO:

Novas idades obtidas por meio do método U-Pb em zircão e dados isotópicos Sm-Nd-Pb são apresentados para as rochas vulcânicas e hipabssais ácidas da exposição mais setentrional da Formação Acampamento Velho na porção NW do Escudo Sul-Rio-Grandense, Brasil. O primeiro episódio vulcânico, agrupado nos riolitos alto-Ti do Cerro Tupanci, possui idades de 579 ± 5,6 Ma, o que está de acordo com dados da literatura para o vulcanismo pós-colisional da Formação Acampamento Velho na Bacia do Camaquã. Idades de 558 +/- 39 Ma foram obtidas para o grupo de riolitos baixo-Ti no Cerro dos Picados, o que pode indicar um episódio vulcânico mais recente ou um período de tempo maior para o vulcanismo da Formação Acampamento Velho no Sul do Brasil, embora o grau de incerteza nos dados seja alto. Em relação às fontes magmáticas, os dados isotópicos de Sm-Nd acoplados a isótopos de Pb e uma revisão da geoquímica de elementos-traço indicam diferentes graus de fusão de crosta inferior paleoproterozoica (Suíte Dom Feliciano, Pinheiro Machado) a neoproterozoica (Terreno Rio Vacacaí). Os dados apresentados neste estudo, quando analisados em conjunto com dados da literatura, contribuem para a melhor compreensão da evolução estratigráfica das sucessões vulcânicas neoproterozoicas pós-colisionais da Bacia do Camaquã no Escudo Sul-Rio-Grandense.

PALAVRAS-CHAVE:
Vulcanismo; Formação Acampamento Velho; Bacia do Camaquã; Tupanci; Geocronologia

INTRODUCTION

The Sul-Rio-Grandense Shield (SRGS) in southernmost Brazil is constituted by rocks related to the Brasiliano/Pan-African orogenic cycle emplaced in a Paleoproterozoic metamorphic basement (Hartmann et al. 1999, Soliani et al. 2000, Nardi et al. 2008). The magmatism in this era was developed in two main phases: arc-related environment, with ages from 850-700 Ma; and post-collisional environment (Liégeois 1998), with ages from 650-535 Ma, marked by magmatism along transcurrent shear zones (Fernandes et al. 1995, Babinski et al. 1997, Bitencourt & Nardi 2000, Chemale Jr. 2000, Hartmann et al. 2000, 2007, Nardi & Bitencourt 2009).

The post-collisional stage in the eastern portion of the SRGS is marked by a shear belt with voluminous granitic magmatism in the Pelotas Batholith (Philipp et al. 2000). General aspects of this extensive granite production episode in the Pelotas Batholith include syn-transcurrent, high-K, calc-alkaline and peraluminous granites that evolve to shoshonitic affinity and late to post-transcurrent, dominantly metaluminous alkaline granites (Philipp & Machado 2005). Excluding the leucocratic peraluminous granites, all granitoid types have basic magmatic rocks associated, normally represented by mafic microgranular enclaves and dykes in commingling systems (Bitencourt & Nardi 2000). Western and northwestern portions of the SRGS represent less deformed areas, with volcano-sedimentary sequences associated with the Camaquã Basin (Fig. 1). Magmatism in the W and NW portion of the SRGS developed mainly between 650-530 Ma and shares a similar geochemical evolution as observed in the eastern part of the SRGS, with the presence of high-K, calc-alkaline granitoids, shoshonitic and tholeitic plutono-volcanic sequences (Nardi et al. 2008). The sedimentary sequences in the Camaquã Basin evolve from shallow marine in the Maricá Formation to lacustrine and alluvial in the Bom Jardim and Camaquã groups (Paim et al. 2000).

Figure 1:
Simplified geological map of the Sul-Rio-Grandense Shield and Uruguay Shield (cf. Oyhantçabal et al. 2011), with location of the northernmost exposure of the Acampamento Velho volcanism in the Tupanci region (modified from Philipp et al. 2016: Ductile Shear Zones: 1) Itajai-Perimbó, 2) Major Gercino, 3) Caçapava do Sul, 4) Dorsal de Canguçu, 5) Passo do Marinheiro, 6) Ibaré, 7) Sarandí del Yí, 8) Sierra Ballena, 9) Cerro Amaro, 10) Arroio Grande).

Magmatism in the Bom Jardim Group is related to the Lavras do Sul Shoshonitic Association, which is plutonic and volcanic basic to acid rocks mainly with ages between 610-590 Ma (Lima & Nardi 1985). Volcanism in the Bom Jardim Group (Fig. 1) was established predominantly under subaerial conditions and has been described in detail by Wildner et al. (2002) and Lima et al. (2007). The first volcanic episode was between 585-590 Ma (Janikian et al. 2008) and is dominantly characterized by intermediate rocks with shoshonitic affinity (Bom Jardim Group, Hilario Formation). This volcanic episode is followed by a bimodal volcanism with sodic mildly alkaline volcanism (Bom Jardim Group, Acampamento Velho Formation) and ages between 560-550 Ma at the Ramada Plateau (Matté et al. 2016, Sommer et al. 2005a) and 574 Ma at the Passo do Salsinho Region (Chemale Jr. 2000, Janikian et al. 2008). The Acampamento Velho Formation (AVF) is the extrusive counterpart of the voluminous granitic magmatism, mostly metaluminous with minor peralkaline components, generated in the Brasiliano-Pan African cycle (Nardi & Bonin 1991). This episode is mainly exposed in volcanic plateaus and ridges composed mostly of acid effusive and pyroclastic rocks with minor intermediate and basic components (Sommer et al. 2005a, Wildner et al. 2002, Almeida et al. 2002). Eruptive periods were initiated with explosive episodes and closed by effusive events (Sommer et al. 2006). The best exposures of this volcanic sequence are in the Ramada Plateau (Vila Nova do Sul City), Taquarembó Plateau (Dom Pedrito City), Bugio and Perau Hills (Caçapava do Sul area, Santa Bárbara Hill), besides the Tupanci region, the northernmost exposure of this volcanism in the SRGS. Although some research has been developed in the deposits of the Tupanci region (Menegotto & Medeiros 1976, Roisenberg et al. 1986, Leitzke et al. 2015), integrated and detailed geochemistry, geochronology and isotopic data are scarce for this area.

Therefore, this paper focuses on dating and isotope systematics of acid volcanic rocks from the AVF in the Tupanci region, Brazil. We present new zircon U-Pb geochronological data for the high-silica volcanic successions, which coupled with Sm-Nd-Pb isotope systematics, and whole-rock geochemistry from literature data, enabled to better constrain the magmatic source and genesis of acid volcanic associations that occur in neoproterozoic post-collisional tectonic settings.

GEOLOGICAL SETTING

The SRGS (Fig. 1) is the southern portion of the Mantiqueira Province, located in the Rio Grande do Sul state, Brazil (Almeida et al. 1981). This shield includes rocks related to Brasilian-Pan African neoproterozoic orogenic cycle and a metamorphic basement of paleoproterozoic age, constituted by remaining fragments of the Rio de la Plata Craton (Teixeira et al. 2004). Well-preserved volcano-sedimentary sequences linked to magmatism in the collisional and post-collisional stages of the Brasilian orogenic cycle (Sommer et al. 2005a) are grouped in the Camaquã Basin (CB).

The CB is a retroarc basin that includes a set of preserved sedimentary and volcanogenic units, which were deposited between 620 and 535 Ma (Brito Neves & Cordani 1991, Gresse et al. 1996, Brito Neves et al. 1999, Chemale Jr. 2000, Fragoso-César et al. 2000, 2003, Paim et al. 2000, Sommer et al. 2006, Janikian et al. 2008, 2012). The basement of the CB is heterogeneous and ranges from paleoproterozoic granulitic complex to associations of metamorphic and deformed igneous rocks related to the Brasiliano-Pan-African orogeny (Paim et al. 2000, Lima et al. 2007). The evolution of the CB includes an alternation between accumulation periods, with the predominance of volcano-sedimentary sequences and erosional periods. The sedimentary sequence of CB is represented (Borba et al. 2007) from base to top by:

• Maricá Formation, with marine and coast deposits;

• Bom Jardim Group, with alluvial deposits intercalated with andesites, acid effusive and pyroclastic rocks (Hilário and Acampamento Velho formations);

• Camaquã Group characterized by fluvial, lacustrine and aeolian continental deposits (Santa Barbara and Guaritas formations).

These stratigraphic units evolved mainly in continental environments and the volcanism under subaerial conditions (Wildner et al. 2002).

Volcanic cycles in the CB occurred during a transition between a collisional and post-collisional period developed from the Neoproterozoic III to the Early Cambrian during the Brasiliano-Pan-African orogeny. The magmatic source of these rocks is probably related to partial melting of a lithospheric mantle previously chemically affected by a subduction process (Nardi & Bonin 1991, Wildner et al. 1999, 2002, Nardi & Lima 2000, Almeida et al. 2005, Sommer et al. 2006). Sommer et al. (2005a, 2005b, 2006) suggest that these volcanic cycles represent preserved portions of a typical evolutionary sequence from a post-collisional magmatism (from Liégeois 1998), with shoshonitic affinity rocks in the early stages, chemically affected by subduction-related processes, followed by silica-saturated bimodal volcanic sequences, with sodic-alkaline affinity and finally by tholeitic rocks.

STUDY AREA

Samples analyzed in this study were collected at the Tupanci region, along the Tupanci and Picados hills, northwest of the SRGS (Fig. 2). Acid igneous rocks occur as shallow intrusions (hypabyssal rocks) at the Tupanci Hill and as lava flows (porphyritic rhyolites) and pyroclastic deposits (ignimbrites) at the Picados Hill. In this section, we will summarize the main petrographic and geochemical features of both occurrences, which were studied in detail by Leitzke et al. (2015) and are necessary for the interpretation of new isotopic data reported in this study.

Figure 2:
Simplified geological map of the study area with the location of the samples selected for this study (modified from Menegotto & Medeiros 1976 and Leitzke et al. 2015).

Summary of facies analysis, petrography and whole-rock geochemistry

Picados Hill area

Volcanic rocks in the Picados Hill area (Fig. 2) have effusive and explosive origin. The rhyolitic rocks are porphyritic, and the best outcrops are dominantly in the north-central portion of the hill. Pyroclastic rocks have a high degree of welding and predominate in the southern portion of the area. In this region lapilli-size fragments, with abundant lithic fragments, predominate. Predominantly, reomorphic portions are less abundant, being mostly ash-size crystal-rich fragments.

The effusive regime is characterized by porphyritic rhyolites with phenocrysts of quartz and sanidine set in an aphanitic glassy matrix (Figs. 3A-3D). Igneous flow foliation is mainly sub-horizontal and shows some autobrecciated portions (Fig. 3E). Microphenocrysts of opaque minerals, zircon and apatite occur as accessory minerals. Mafic minerals are predominantly biotite and chlorite, possibly originated by alteration of amphiboles. The biotite grains are subhedral, with diameter from 0.5-1 mm, ranging from brown to green and poikilitic texture. Biotite aggregates associated with chlorite has almost entirely replace amphibole crystals. The very fine matrix consists of a micro to cryptocrystalline quartz-feldsphatic arrange. Devitrification processes are common and several stages are observed, with spherulitic, axiolithic texture and granophyric intergrowths (Fig. 3F).

Figure 3:
Summary of macro and microscopic features of representative effusive rocks at the Tupanci and Picados Hill (cf. Leitzke et al. 2015): (A) porphyritic rhyolite with glassy matrix; (B) fractured sanidine phenocryst in devitrified matrix; (C) glomeroporphyritic texture with euhedral quartz and k-feldspar phenocrysts; (D) reabsorbed quartz phenocryst with surrounding igneous flow foliation; (E) quartz phenocryst with microbreccia features; (F) spherulites showing evidence for high-temperature devitrification of the matrix.

Explosive volcanism in the area is characterized by poorly selected ignimbrites with fragments ranging from ash to lapilli size, consisting of rock, crystal and glass fragments (Fig. 4). The ignimbrites are strongly welded and have rhyolitic composition. These rocks may be divided into two facies: lithic-rich ignimbrites (IgL); and rheomorphic crystal - rich ignimbrites (IgnReo).

Figure 4:
Summary of macro and microscopic features of representative pyroclastic rocks at the Picados Hill (cf. Leitzke et al. 2015): (A) lapilli-size pyroclastic fragments in hand specimen; (B) pumice fragments with distinct orientations in a shard-based matrix; (C) highly-welded ignimbrite with eutaxitic texture of the pumice fragments; (D) stretched pumice fragments with eutaxitic texture and inclusions of quartz crystals under the microscope.

The IgL corresponds to poorly selected brownish to pink color rocks with predominance of lapilli size fragments (Fig. 4A) immersed in a tuffaceous matrix. Lithic fragments are common and constituted by fragments of rhyolites and ignimbrites. Fragments of pumice (Fig. 4) occur in various sizes in the matrix and as lapilli fragments (Fig. 4). They are devitrified to a quartz-feldpathic granophyric arrangement with a low degree of flatenning, even though in some portions of the rock it is possible to identify an incipient eutaxithic texture. Alteration processes leave a reddish-brown clay-rich layer over the fragments. Quartz and alkali-feldspar crystal fragments are few. These mineral phases also occur as phenocrysts, with similar sizes, subhedral to euhedral and some of them preserved within the pumices. The matrix is extremely fine and supported by shards with spike geometry, with lengths smaller than 0.5 mm (Fig. 4B).

Rheomorphic ignimbrites in the area are characterized by high degrees of welding (Fig. 4C) and a great amount of crystal fragments (phenocrysts and fragments) in a very fine-grained eutaxithic and often parataxithic matrix. These factors can be used to characterize them as “lava-like” ignimbrites (Ekren et al. 1984). Pyroclastic fragments are predominantly represented by quartz and alkali feldspar and stretched/deformed pumices (Fig. 4D). The lithic fragments are rare and, when observed, have rhyolitic composition. Pumice fragments are flattened (fiamme) and devitrified to quartz-feldspar aggregates.

Tupanci Hill area

The Tupanci Hill area comprises a N-S elongated subvolcanic body with a dominant NE-SW fault pattern. The main lithology is represented by porphyritic rhyolites with phenocrysts of sanidine and quartz surrounded by an equigranular-fine-grained to aphanitic quartz-feldspar matrix, with a strong flow foliation on border regions, where phenocrysts are substantially smaller in size. Spherulites occur in the matrix or around phenocrysts, showing evidence for high-temperature devitrification processes (Logfren 1971). Microphenocrysts of opaque minerals (pyrite), zircon, apatite and altered mafic minerals occur as accessories. Mafic minerals are rare and occur altered to chlorite. Small (≈ 0.05 mm) acicular crystals of late crystallization sodic amphibole (arfvedsonite) occur in the matrix, suggesting a peralkaline affinity. Fragments of sedimentary wall rocks of the Maricá Formation occur as xenoliths. Microfolded portions, rotated and fractured crystals (microbreccias) are also observed on the border of the intrusion, generating “tuff-like” textures similar to eutaxitic and parataxitic textures in pyroclastic rocks (Manley 1995), common in rhyolitic systems due to the high viscosity of the flow (Figs. 4E and 4G).

Whole-rock geochemistry

The geochemical features of both groups of rocks were described in detail by Leitzke et al. (2015) and are similar for major elements, but show distinct aspects regarding trace elements. The rocks have high contents of SiO₂ (73 to 77 wt.%), alkalis (Na₂O+K₂O > 8.4 wt.%) and FeO_t/(FeO_t+MgO) (> 0.9); with low contents of Al₂O₃, CaO and MgO; and agpaitic index close to the unity (> 0.9). At the Nb/Y versus Zr/TiO₂ classification diagram, these rocks are classified as rhyolites, which are confirmed by the Zr/TiO₂ versus SiO₂ diagram, in which some samples are classified as alkali rhyolites (Fig. 5). Major and trace elements data allowed the identification of two different evolutionary trends: low and high-Ti, related respectively to the effusive/pyroclastic rocks of the Picados Hill and subvolcanic rocks of Tupanci Hill (Leitzke et al. 2015). The same behavior is observed for trace and rare earth elements (REE) in multi-element variation diagrams. Zr contents are higher than 500 ppm for the high-Ti group that indicates, according to Leat et al. (1986), a peralkaline affinity for these rocks. On the other hand, the low-Ti group has Zr contents not higher than 420 ppm. Ba and Sr contents are > 70 and 8 ppm in the high-Ti group and < 16 and 6 ppm, respectively. Rb displays high contents in both groups, but concentrations are much higher in the low-Ti group (> 170 ppm) than in the high-Ti group (< 100 ppm). Nb, Y and Ga behave in a similar way, with higher contents for the low-Ti group (> 50, 130 and 25 ppm) compared to the high-Ti group (< 30, 70 and 26 ppm). High contents of Zr, Nb, Y, Ga and Rb and low contents of Ba and Sr for rocks of both groups (cf. Leitzke et al. 2015) are characteristic of acid magmatism with alkaline affinity (Pearce et al. 1984, Whalen et al. 1987, Nardi 1991).

Figure 5:
Classification diagrams for the high and low-Ti groups of silicic rocks from the Acampamento Velho Formation with comparisons between both groups at the Tupanci region and other occurrences of the post-collisional volcanism in southern Brazil. Data taken from Leitzke et al. (2015), and Sommer et al. (2005a): (A) TAS diagram (Le Bas et al. 1986); (B) SiO2 vs. Zr/TiO2 classification diagram (Winchester and Floyd 1977).

ANALYTICAL PROCEDURES

This work presents a new isotopic dataset (zircon U-Pb, Sm-Nd and Pb-Pb) obtained on volcanic and hypabyssal rocks from the Tupanci area, which are coupled to whole-rock geochemistry data (Leitzke et al. 2015) to provide a better understanding of the evolution of the Acampamento Velho volcanism in the SRGS. Two samples of the Tupanci Hill (TH), which belong to the high-Ti group and consist of porphyritic rhyolites, were selected for isotope analysis and one for zircon U-Pb dating (CT-38). In the Picados Hill (PH), three samples of the low-Ti group were selected for isotope determination. Additionally, a porphyritic rhyolite (sample CT-66) was selected for zircon U-Pb geochronology. Location of selected samples for the U-Pb zircon separation and isotopic measurement are given in Figure 2. Isotopic analyses were conducted at the University of São Paulo (USP) and at the Federal University of Rio Grande do Sul (UFRGS), and are described in detail in the following sections.

Zircon U-Pb geochronology (U-Pb LA-ICP-MS)

Approximately 0.5 kg of sample material was crushed, pulverized and sieved for zircon separation. The zircon grains were concentrated using conventional magnetic and heavy liquid techniques, after which more than 300 grains were handpicked and mounted in epoxy resin. From this population, 13 grains were analyzed from the high-Ti group (sample CT-38) and five grains from the low-Ti (sample CT-66).

The mount was polished using different granulometric pastes of diamond to expose the internal surfaces of the zircon. For the dating, the zircon grains were imaged using backscattered electrons and cathodoluminescence to determine their internal structures and crystallization phases. Only zircon grains without imperfections, fractures, and mineral inclusions were selected for isotopic analyses. All U-Pb isotopic analyses were performed at the Geochronological Research Center of the Geoscience Institute, USP, using a Thermo Fischer Scientific Neptune inductively coupled plasma-mass spectrometer (ICP-MS) and a 193 nm excimer laser ablation (LA) system (Photon Machines - Redmond, Washington, USA).

The details of the ICP-MS configurations and the laser parameters used during the analyses are available in Oliveira et al. (2015). The analytical procedure of the U-Pb method included the analysis of the materials in the following sequence: two blanks, two synthetic glass NIST standards , three external reference materials, 13 samples, two external reference materials, and two blanks. Each experiment consisted of 40 cycles of 1 s/cycle. The ²⁰⁴Hg interference for ²⁰⁴Pb was corrected using ²⁰²Hg, in which ²⁰⁴Hg/²⁰²Hg = 4.2. Both NIST glass and external reference material were used to normalize the ²⁰⁷Pb/²⁰⁶Pb ratio, whereas the external reference material was used to normalize the ²³⁸U/²⁰⁶Pb ratio. The standard GJ-1 (602 ± 4.4 Ma; Elhlou et al. 2006) was used as the external reference material. Zircon typically contains low concentrations of common Pb. Thus, the reliability of the measured ²⁰⁷Pb/²⁰⁶Pb and ²³⁸U/²⁰⁶Pb ratios critically depends on accurately assessing the common Pb component. The residual common Pb was corrected based on the measured ²⁰⁴Pb concentration using the known terrestrial composition (Stacey & Kramers 1975).

External errors were calculated using error propagation for the individual measurements of the standard GJ-1 and the individual zircon sample measurements (spots). The ages were based mainly on ²³⁸U/²⁰⁶Pb and ²⁰⁷Pb/²⁰⁶Pb ratios and calculated through Isoplot software version 4.0 (Ludwig 2008). More details of the analytical methods and data handling are available in Chemale Jr. et al. (2011).

Sm-Nd and Pb isotopic measurements

Isotopic analyses of Sm, Nd and Pb were carried out at the Isotope Geology laboratory of the UFRGS by thermal ionization mass spectrometry using a VG Sector 54 multicollector mass spectrometer. Whole rock samples were powered in agate mortar in order to get fraction lower than 200 mesh. Then, the samples were digested together with a ¹⁴⁹Sm/¹⁵⁰Nd spike through concentrated HF-HNO₃ and HCl in 7 mL teflon vials in a hot plate for seven days. After complete digestion, the samples were dried down and redissolved in 2.5 N HCl, and the REE were separated by using standard cation exchange chromatography with a DOWEX AG 50X8 resin (200-400 mesh) using 6 N HCl. Nd and Sm were separated from the other REE by using HDEHP LN resin (50-100 mm) and collected after elution with 0.18 N HCl for Nd and 0.5 N HCl for Sm. Pb was separated using DOWEX AG 1X8 resin (200-400 mesh) with 0.6N HBr and 6 N HCl. The isotopic analyses were carried out in static mode. Sm and Pb were run on Re single filaments, while Nd isotopes were loaded on Ta-Re-Ta triple filaments. Sm, Nd and Pb were deposited with H₃PO₄, and the latter was also deposited with silica gel. Nd isotopic ratios were normalized to ¹⁴⁶Nd/¹⁴⁴Nd = 0.7219 and repeated measurements of La Jolla Nd reference material furnished a mean ¹⁴³Nd/¹⁴⁴Nd value of 0.511848 ± 0.000021 (1s; n = 100). Pb was corrected from a mass discrimination effect of 0.1% amu^-1 based on 38 analyses of NBS-981 reference material. The total procedure blanks were lower than 500 pg for Sm, 150 pg for Nd, and 100 pg for Pb. Typical analytical errors for ¹⁴⁷Sm/¹⁴⁴Nd and ^206,207,208Pb/²⁰⁴Pb ratio were equal or better than 0.1%. Nd-T_DM model ages were calculated according to De Paolo (1981). The decay constants recommended by Steiger and Jäger (1977) and Wasserburg et al. (1981) were used. The error for all isotopic data were lower than 25 ppm for Sm/Nd; 20 ppm for ¹⁴³Nd/¹⁴⁴Nd; and lower than 0.0020 (SD absolute) for Pb isotopic analyses.

RESULTS

Zircon morphology and U-Pb results

The zircon populations comprise elongate, prismatic, subrounded grains, rarely showing well-preserved bipyramidal faces. These grains show low length-to-width ratios from 2:1 to 14:1 (with the average of 5:1), which are characteristics for deep-seated and slowly cooled intrusions (Corfu et al. 2003). Such morphological features suggest that zircon grains selected for the analysis are magmatic. Cathodoluminescence (CL) images of selected zircon grains (Fig. 6) from each sample allowed the recognition of inclusions and fractures and metamictic domains, which were avoided during the analyses, whenever possible. Some zircon grains display darker cores developed during magmatic crystallization. Zircon crystals from sample CT-66 show clear signs of alteration (Fig. 6) and high contents of common lead (14 to 59%), which affected negatively on the ages obtained for this group of analysis.

Figure 6:
Cathodoluminescence images of zircons from the (A) Tupanci Hill, with internal zonation due to crystal growth during magma cooling, and (B) Picados Hill, which show features of alteration. White circles show where LA-ICP-MS analysis were performed.

High-Ti rhyolite (sample CT-38)

Zircons from this sample are bright when analyzed through CL, although a few zircons exhibit either dark rims or dark cores (Fig. 6A). The U-Pb analytical data are presented in Table 1 and plotted in Concordia diagram (Tera & Wasserburg 1972), based on ²⁰⁷Pb/²⁰⁶Pb versus ²³⁸U/²⁰⁶Pb (Fig. 7A). All the zircon crystals have very high contents of U (1,679-4,296 ppm) and Th (1,089-4,086 ppm), but common Pb is still very low. Thirteen analyzed zircon yield a range in the ²⁰⁶Pb/²³⁸U ages from 559 to 605 Ma and all the analytical points are concordant. Therefore a Concordia age of 579 ± 5.6 Ma (mean square weighted deviation - MSWD = 0.16) has been calculated using these zircons (Fig. 7A), which is interpreted as the crystallization age of this sample.

Table 1:
U-Pb LA-ICP-MS data for zircon grains from the silicic volcanic and hypabyssal rocks in the northernmost exposures (low and high-Ti) of the Acampamento Velho Formation in the NW portion of the Sul-Rio-Grandense Shield.

Sample (Group)	Grain spot	206 Pbc (%)	U (ppm)	Th (ppm)	206 Pb (ppm)	232 Th/238 U	206 Pb/238 U (age-Ga)	1σ (%)	207 Pb/206 Pb (age-Ga)	1σ (%)	207 Pb/206 Pb	1σ (%)	207 Pb/235 U	1σ (%)	206 Pb/238 U	1σ (%)	Err. Corr.
CT-38 (High-Ti)	1.1	0.21	3114	2168	385	0.696	0.585	1.20	0.570	9.47	0.0591	2.54	0.7741	2.62	0.0950	1.26	0.97
	2.1	0.11	2173	1505	278	0.693	0.586	1.19	0.582	10.65	0.0594	2.86	0.7801	2.92	0.0952	1.26	0.99
	3.1	0.22	2562	1627	304	0.635	0.579	1.21	0.559	10.38	0.0588	2.72	0.7612	2.82	0.0939	1.28	0.92
	4.1	0.09	3025	2615	401	0.864	0.559	1.25	0.562	9.79	0.0589	2.55	0.7351	2.76	0.0906	1.32	0.98
	5.1	0.22	2648	1601	335	0.605	0.573	1.32	0.568	9.86	0.0590	2.54	0.7569	2.72	0.0930	1.29	0.98
	6.1	0.04	2973	2296	397	0.772	0.575	1.25	0.574	9.76	0.0592	2.53	0.7606	2.70	0.0932	1.29	0.97
	7.1	0.18	2307	1463	277	0.634	0.561	1.16	0.581	10.15	0.0594	2.69	0.7439	2.93	0.0909	1.32	0.97
	8.1	0.10	2348	1440	301	0.613	0.605	1.20	0.585	9.57	0.0595	2.52	0.8075	2.65	0.0984	1.22	0.95
	9.1	0.58	1679	1089	209	0.648	0.581	1.22	0.561	11.05	0.0588	2.89	0.7644	2.90	0.0942	1.27	0.96
	10.1	0.33	2377	1633	285	0.687	0.576	1.25	0.576	9.20	0.0593	2.53	0.7630	2.63	0.0934	1.28	0.91
	11.1	0.03	4296	4086	549	0.951	0.560	1.19	0.570	8.95	0.0591	2.37	0.7389	2.61	0.0907	1.21	0.98
	12.1	0.09	2634	1483	334	0.563	0.590	1.18	0.569	9.67	0.0591	2.54	0.7806	2.65	0.0959	1.25	0.95
	13.1	0.03	2219	1536	298	0.692	0.594	1.16	0.583	9.95	0.0594	2.69	0.7914	2.77	0.0966	1.24	0.98
*CT-66 (Low-Ti)	1.1	14.39	2780	2237	333	0.800	0.692	1.15	0.588	5.78	0.0596	1.51	0.9311	1.94	0.1134	1.23	0.99
	2.1	23.69	3510	2811	448	0.800	0.607	1.11	0.580	5.52	0.0594	1.52	0.8082	1.78	0.0987	1.22	0.88
	3.1	30.65	3844	4018	588	1.050	0.632	1.03	0.572	5.42	0.0591	1.35	0.8394	1.82	0.1029	1.17	0.92
	5.1	58.71	2180	1773	317	0.810	0.677	1.05	0.558	6.09	0.0588	1.53	0.8967	1.77	0.1107	1.08	0.92
	4.1	18.20	2396	1704	268	0.710	0.572	1.13	0.553	6.51	0.0586	1.71	0.7496	1.87	0.0927	1.19	0.97

• *Sample with high content of common ²⁰⁶Pb_c. 1σ expressed in %.

Figure 7:
U-Pb Concordia diagrams displaying obtained data from zircons of the AVF high (A) and low (B) -Ti rhyolites at the Tupanci region, NW of the Sul-Rio-Grandense Shield.

Low-Ti rhyolite (sample CT-66)

Zircon grains from this sample are invariably dark with many inclusions and no discernible zoning. When compared to the sample CT-38, these zircons are not as clear, which reflect hydrothermal alteration (Fig. 6B). A few zircons exhibit locally either dark rims or dark cores (Fig. 6B). The U-Pb analytical data are presented in Table 1, and plotted in the conventional Concordia diagram (Whetherill 1956) based on ²³⁸U/²⁰⁶Pb versus ²⁰⁷Pb/²³⁵U (Fig. 7B). The five analyzed fractions yield a range in the ²⁰⁷Pb/²⁰⁶Pb ages from 553 to 588 Ma and are discordant. A Discordia (Fig. 7B) using the five fractions yields an upper intercept at 558 ± 39 Ma with a low MSWD (0.70). This high error is due to the small number of crystals used for dating associated with the high content of uranium, which may have been lost due to alteration and deterioration of the crystalline reticulum.

Sm-Nd isotopic results

For Sm-Nd whole-rock determination and T_DM age calculation, five samples were selected:

• Two from the high-Ti rhyolites of the Tupanci Hill (CT-37A, CT-37B);

• Three samples from the low-Ti rhyolites of the Picados Hill (CT-47A, CT-49A, CT-50A).

Table 2 shows the Sm-Nd isotopic results as well as the initial e_Nd values calculated at the time of emplacement, which is yielded by the zircon U-Pb ages obtained in this study. For the high-Ti samples, a crystallization age of 579 Ma is assumed, while the age of 558 Ma is assumed for the emplacement of the low-Ti rhyolites.

Table 2:
Isotopic results for the silicic volcanic and hypabyssal rocks in the northernmost exposure of the Acampamento Velho Formation in the NW portion of the Sul-Rio-Grandense Shield.

Group	Sample	206 Pb/204 Pb	2σ	207 Pb/204 Pb	2σ	208 Pb/204 Pb	2σ	Sm (ppm)	Nd (ppm)	147 Sm/144 Nd	143 Nd/144 Nd	Error	εNd (0)	εNd (t)	TDM (Ga)
High-Ti	CT-37A	16.140	0.002	15.128	0.001	37.089	0.002	10.19	49.41	0.1247	0.511743	0.000015	-17.47	-12.15	2.22
	CT-37B	15.971	0.002	15.113	0.001	37.009	0.001	14.73	85.71	0.1039	0.511715	0.000007	-18.00	-11.15	1.83
Low-Ti	CT-47A	15.923	0.001	15.062	0.001	36.590	0.001	7.22	21.20	0.2058	0.512146	0.000009	-9.6	-10.09	-
	CT-49A	16.007	0.001	15.097	0.001	36.862	0.001	11.81	34.49	0.2070	0.512103	0.000015	-10.43	-11.21	-
	CT-50A	15.823	0.002	15.066	0.001	36.567	0.002	11.37	33.50	0.2052	0.512131	0.000011	-9.88	-10.53	-

• εNd (t) was calculated on basis in the crystallization age of 579 Ma.

The set of results provided T_DM model ages of 1.83 and 2.22 Ga and more negative present day e_Nd(0) values of -17.47 and -18.00 for the high-Ti rhyolites, while the low-Ti rhyolites exhibit negative e_Nd(0) values from -10.43 to -9.60. The e_Nd(t) of the high-Ti samples are of -12.15 and -11.15, while low-Ti samples show e_Nd(t) ranging from -11.21 to -10.09. The high-Ti rhyolites displayed homogeneous ¹⁴⁷Sm/¹⁴⁴Nd ratios of 0.125 and 0.104, which are within the 0.080-0.120 range, usually considerable acceptable for T_DM age calculation. On the other side, the samples of the low-Ti rhyolites have unusually high ¹⁴⁷Sm/¹⁴⁴Nd ratios of 0.205-0.207, which is close to the value of depleted mantle (¹⁴⁷Sm/¹⁴⁴Nd = 0.225) and normally found in mafic rocks, which led to difficulties in interpreting model ages. The uncertainty in the age increases dramatically as the ¹⁴⁷Sm/¹⁴⁴Nd ratio of the samples approaches that of the depleted mantle. In this study, therefore, following the work of Harris et al. (1994) and DePaolo (1981), model ages obtained from samples with high ¹⁴⁷Sm/¹⁴⁴Nd ratios (> 0.15) were not considered because they are unrealistic.

Pb isotope systematics results

The high-Ti rhyolites show present-day Pb isotope compositions of 15.971‒16.141 for ²⁰⁶Pb/²⁰⁴Pb; 15.113‒15.128 for ²⁰⁷Pb/²⁰⁴Pb, and 37.009‒37.089 for ²⁰⁸Pb/²⁰⁴Pb. The low-Ti rhyolites display slightly lower Pb isotope ratios compared to the high-Ti rocks, although the ²⁰⁶Pb/²⁰⁴Pb ratios are within the same range. The ratios are 15.830‒16.006 for ²⁰⁶Pb/²⁰⁴Pb; 15.062‒15.097 for ²⁰⁷Pb/²⁰⁴Pb; and 36.567‒36.862 for ²⁰⁸Pb/²⁰⁴Pb (Table 2).

DISCUSSION

Timing and emplacement of volcanic episodes of the Acampamento Velho Formation in the NW of the SRGS

Recent geochronological literature data provided ages between 549 and 573 Ma (Tab. 3) for the volcanic rocks of the AVF. At the Passo do Salsinho region, U-Pb SHRIMP zircon analysis in rhyolitic lavas revealed ages of 573 ± 18 Ma (Chemale Jr. 2000). Similar and more precise results of 572 ± 3 Ma were obtained through Pb/Pb isotopic ratio in zircon grains from granitoids of the Leões Ring Complex (Gastal & Lafon 2001). These granitoids are genetically associated with the volcanic rocks of the Taquarembó Plateau. U-Pb zircon analysis in volcanic rocks of the Ramada Plateau provided the age between 549 and 560 Ma to the AVF volcanism, which suggests a greater time span for this volcanic episode in the SRGS (Sommer et al. 2005b, Matté et al. 2016). Analysis of two rhyolitic pebbles from the conglomeratic fluvial deposits of the basal portion of the AVF provided zircon U-Pb ages of 579 ± 13 Ma and 569 ± 2.4 Ma (Janikian et al. 2012). However, these authors proposed the individualization of a new younger acid volcanic unit related to reactivation of extensional structures identified in Taquarembó Plateau, for which the crystallization age, obtained by U-Pb LA-ICP-MS method in lapilli-tuffs previously considered as part of the AVF, is 544.2 ± 5.5 Ma (Wildner et al. 1999, 2002, Sommer et al. 2006). This was correlated with the U-Pb zircon SHRIMP age of 549.3 ± 5 Ma obtained by Sommer et al. (2005a) in hypabyssal rhyolites of the Ramada Plateau. The new age of 579 ± 5 Ma of the high-Ti rhyolites from the AVF obtained in this study is in agreement with those ones previously obtained in the literature (Chemale Jr. 2000, Janikian et al. 2005, 2008, Gastal & Lafon 2001, Sommer et al. 2005a, 2006). For the age of 558 ± 39 Ma obtained in the low-Ti group, the high uncertainty associated with the data may indicate an overlap with the ca. 570 Ma event of the AVF, or suggest a younger volcanic episode at the Camaquã Basin (cf., Janikian et al. 2012, Matté et al. 2016).

Table 3:
Summary of geochronological data available in the literature for the Acampamento Velho volcanism and comparison with results from this study.

Outcrop	Type of rock	Age (Ma)	Error (Ma)	Method	Reference
Ramada Plateau	Subvolcanic rhyolite	549.0	5.0	U-Pb SHRIMP	Sommer et al. (2005)
Ramada Plateau	Diorite	550.2	1.5	U-Pb LA-ICP-MS	Matté et al. (2011)
Taquarembo Plateau	Acid tuff	553.0	17.0	U-Pb LA-ICP-MS	Janikian et al. (2012)
Bugio Hill	Basalt	553.0	5.0	U-Pb LA-ICP-MS	Almeida et al. (2012)
Eastern Ramada Plateau	Rhyolitic Ignimbrite	560.0	2.0	U-Pb LA-ICP-MS	Matté et al. (2016)
Eastern Ramada Plateau	Subvolcanic trachyte	560.0	14.0	U-Pb LA-ICP-MS	Matté et al. (2016)
Ramada Plateau	Rhyolitic Ignimbrite	560.7	2.1	U-Pb LA-ICP-MS	Matté et al. (2016)
Eastern Ramada Plateau	Subvolcanic rhyolite	561.7	1.8	U-Pb LA-ICP-MS	Matté et al. (2016)
Taquarembo Plateau	Rhyolite	570.7	4.1	U-Pb LA-ICP-MS	Sommer et al. (in prep.)
Bom Jardim	Subvolcanic rhyolite	572.2	6.5	U-Pb LA-ICP-MS	Janikian et al. (2012)
Perau Hill	Rhyolite	573.0	18.0	U-Pb SHRIMP	Chemale Jr. (2000)
Ramada Plateau	Rhyolite	574.0	7.0	U-Pb TIMS	Janikian et al. (2008)
Tupanci Hill	Rhyolite	579.1	5.6	U-Pb LA-ICP-MS	This study
Picados Hill	Rhyolite	558.0	39.0	U-Pb LA-ICP-MS	This study

• LA-ICP-MS: laser ablation inductively coupled plasma mass spectrometry; SHRIMP: sensitive high-resolution ion microprobe; TIMS: thermal ionization mass spectrometry.

Trace element geochemistry indicates that rocks from the high-Ti group were likely generated in a post-collisional setting, while those ones of the low-Ti group were generated in a progressive post-collision to intraplate setting (cf. Leitzke et al. 2015). Assuming this, it is expected that our geochronological results are coherent with the tectonic environment, and, therefore, show distinct ages between both groups. Nevertheless, the poorly constrained age for the low-Ti group still does not allow to confirm the tectonic environment distinction between both groups. Regarding whole-rock trace element patterns, both high and low-Ti groups have high contents of HFSE (Zr, Nb, Y), Ga and Rb, together with lower contents of Ba and Sr (Fig. 8A), characteristic of acid magmatism with an alkaline affinity, typical of final stages of orogenic cycles, with sources previously affected by subduction-related metasomatism (Pearce et al. 1984, Whalen et al. 1987, Nardi 1991, Sommer et al. 2005a). The magmatism of the AVF has been recognized to be produced from subduction-modified mantle sources, likely of EM1 type, during the final stages of Brasiliano-Pan-African cycle (Wildner et al. 2002). Moreover, the origin of the rhyolitic liquids has been modelled to be fractional crystallization of plagioclase+alkali feldspar+pyroxene+magnetite from trachytic magmas, which, in turn, are produced by differentiation of mildly alkaline basaltic magmas (Sommer et al. 1999, Matté et al. 2012). Shellnutt et al. (2009) use the Th/Ta ratio to indicate to which extent the interaction crust-mantle was present during the petrogenesis of an igneous association of rocks. This ratio should be around 2.0 produced exclusively by mantle partial melting, while it should be greater than 6.9 to the ones originated by crustal partial melting. In our high-Ti group (CT) Th/Ta > 6.9 predominates, while at our low-Ti group (CP) the values range between 6.9 and 4.2, indicating crustal contribution to their origin. According to Eby (1990, 1992), the fractionation of “A” type magmas has little effect on the Y/Nb ratio, which could also be used to indicate the source of the magmatism. Mantle-derived magmas would have Y/Nb < 1.2, while those of crustal origin should have it > 1.2. These geochemical criteria would indicate crustal origin for both groups of acid volcanic rocks in this study, in which Y/Nb ranges from 1.6 to 4.1 (cf. Leitzke et al. 2015).

Figure 8:
Multi-element diagram for the low and high-Ti groups of the Acampamento Velho Formation at the Tupanci region, NW of the Sul-Rio-Grandense Shield (cf. Leitzke et al. 2015) plotted together with literature data: (A) ORG-normalized (Pearce et al. 1984) trace element (+K2O) pattern; (B) chondrite-normalized rare earth elements (REE) pattern (Nakamura 1974).

In both groups, REE are present in moderate to high concentrations (ΣREE = 198-694 ppm). The fractionation between light rare-earth elements (LREE) and heavy rare earth elements (HREE) is low for the low-Ti (CP) group (La_N/Yb_N = 1.7-6.9) and higher for the high-Ti (CT) group (La_N/Yb_N = 3.48-17.64). In general terms, the fractionation of LREE is more pronounced than HREE, with LaN/SmN (1.6-5.1) higher than TbN/LuN (1.2-1.5). In all samples, there is a strong negative Eu anomaly (Eu/Eu* = 0.01-0.19), related to its bivalent character and consequently previous fractionation into feldspars. As already observed for the AVF at the Ramada Plateau, the high-Ti rhyolites are enriched in REE, mainly LREE, when compared to the low-Ti group (Fig. 8B). These characteristics were also identified in other occurrences of the AVF at the SRGS (Sommer et al. 2005b, 2006), where low and high-Ti groups of basalts and rhyolites were identified. The magmatic source of these groups and their petrogenetic correlation are still not well constrained.

When compared to tectonic environment (Fig. 9) trace element patterns (cf. Pearce et al. 1984), the samples are divided accordingly to their group, with high-Ti rhyolites showing a post-collisional settings pattern, while the low-Ti are more likely related to intraplate granites, which is coherent with the increase of peralkalinity. In the granitoids classification diagram (Whalen et al. 1987), all the samples are classified as “A” type granites, which are by definition alkaline and anorogenic. This classification is confirmed by the (10⁴*Ga)/Al > 2.6, Ce+Y+Nb+Zr > 500 ppm and FeOt/FeOt+MgO > 0.9, as proposed by Nardi and Bitencourt (2009) for the post-collisional granitic magmatism at the southern portion of Brazil. Nevertheless, when compared to other occurrences of the AVF, the rocks from the high-Ti group (CT) of the Tupanci area have lower contents of Rb, Nb, Y and higher of Zr and Ce when compared to the low-Ti group (CP), suggesting a different evolution for this group.

Figure 9:
Tectonic discrimination diagram for the studied rocks (cf. Leitzke et al. 2015): (A) Rb vs. Y+Nb according to Pearce et al. (1984); (B) Zr vs. 10,000*Ga/Al (Whalen et al. 1987).

The new ages for the AVF at the Tupanci region, coupled with trace element content of the two volcanic sequences (Leitzke et al. 2015) are coherent with the tectonic environment expected for the different groups, suggesting that the high-Ti group was generated in a post-collisional setting, while the low-Ti group was generated in a progressive post-collision to intraplate setting. Our new ages for the high-Ti group at the Tupanci area agrees well with previous studies, while the age obtained to the low-Ti group, although with high uncertainty, may be related to different episodes of the Acampamento Velho volcanism in its northernmost exposure SRGS (cf. Janikian et al. 2012).

Constraints for the magmatic sources of the AVF volcanism based on Sm-Nd and Pb-Pb isotope systematics

The significance of negative ε_Nd and differences in the ¹⁴⁷Sm/¹⁴⁴Nd observed in acid lavas can be addressed to crust-derived lavas, or to crustal contamination of basaltic lavas during differentiation or metasomatic rocks (Almeida et al. 2005). According to the isotopic results, the high-Ti group would also represent a fraction of these volcanic episodes that had a greater component of crustal contamination, since it presents a slightly more negative εNd.

T_DM model ages (DePaolo 1981, 1988) ranging from 1.83 to 2.22 Ga for the high-Ti rocks are in agreement with the ones previously obtained (Chemale Jr. 2000, Almeida et al. 2005, Janikian et al. 2012), suggesting a contribution of paleoproterozoic basement in the source of these magmas (Fig. 10). Isotopic data obtained from rocks of the neoproterozoic plutono-volcanic alkaline association at the Taquarembó Plateau, that also belongs to the AVF (Wildner et al. 1999), suggest an EM1-type mantle source (Hart 1988). Chemale Jr. 2000) reported εNd (t = 570 Ma) values between -15 and -17 and ⁸⁷Sr/⁸⁶Sr_i ratios close to 0.7045, which are similar to the results obtained by Gastal and Lafon (1998) for intrusive rocks of alkaline associations in the SRGS, correlated to the Taquarembó Plateau magmatism. When coupled with major and trace elements geochemistry, our data suggest that the source of the Tupanci magmatism may also be a lithospheric mantle previously affected by subduction metasomatic processes. The e_Nd versus time diagram (Fig. 10) compares the Nd isotopic evolution of the investigated rocks with distinct crusts (Rio Vacacaí Terrane, Santa Maria Chico Complex and Dom Feliciano Belt) from the SRGS (cf. Janikian et al. 2012). In this context, it is clearly seen that studied samples of the AVF show paleoproterozoic T_DM ages from Rhyacian to Statherian and plot in an intermediate position between the neoproterozoic (Rio Vacacaí Terrane) and the paleoproterozoic crusts (Dom Feliciano, Pinheiro Machado Suite and Encantada Gneisses).

Figure 10:
Diagram showing εNd vs. T (Ga) for the low and high-Ti rhyolitic rocks from the Acampamento Velho Formation at the Tupanci region, as well as their possible sources (modified from the compilation by Janikian et al. 2012).

The Pb isotope data (Fig. 11) of the studied samples suggest a significant crustal contribution with lower paleoproterozoic crust melting related to the Dom Feliciano Complex. The high ¹⁴⁷Sm/¹⁴⁴Nd ratios of the low-Ti rocks coupled to the REE-chondrite normalized pattern observed is a feature produced in ultra-evolved silicic magmas attributed to magmatic fractionation, which may lead to an erroneous estimate of the Nd model age of the low-Ti rocks. Another possible explanation for the high ¹⁴⁷Sm/¹⁴⁴Nd in the low-Ti group could be related to a local source control, mineral sorting, with fractionation of minerals such as monazite, or hydrothermal alteration (Verma et al. 2005). Nevertheless, the similarity between εNd(t) for both rhyolite groups suggest a similar source, which may be a paleoproterozoic crust, as evidenced by the ¹⁴³Nd/¹⁴⁴Nd vs. time diagram.

Figure 11:
(A) ²⁰⁸Pb/²⁰⁴Pb vs. ²⁰⁶Pb/²⁰⁴Pb; (B) ²⁰⁷Pb/²⁰⁴Pb vs. ²⁰⁶Pb/²⁰⁴Pb isotopic signatures for the low and high-Ti rhyolitic rocks from the AVF at the Tupanci region plotted together with possible end-member sources evolution with time (cf. Stacey & Kramers 1975).

CONCLUSIONS

New U-Pb ages obtained in zircon grains from high-Ti rhyolites from the AVF at the NW portion of the SRGS indicate an event at 579.1 +/- 5.6 Ma and are in good agreement with reported ages for the time span of the AVF volcanism in the Bom Jardim Group of the CB (cf. Chemale Jr. 2000, Janikian et al. 2005, 2008, Gastal & Lafon 2001). A second U-Pb age of 558 Ma was obtained for rhyolites from the low-Ti group at the Picados Hill, although with a high uncertainty associated (+/- 39 Ma) probably due to the high degree of alteration of the zircon grains. Nevertheless, more precise geochronological data reported recently in the literature may be used to indicate a younger acid volcanism, or a greater time span for the AVF volcanism in southernmost Brazil (Janikian et al. 2012, Matté et al. 2016), although the high uncertainty obtained in this study does not allow this issue to be conclusive. Regarding magmatic sources, Sm/Nd isotopic data coupled to Pb isotopes and trace element geochemistry of these rocks indicate variable amounts of paleoproterozoic lower crust melting, in a progressive post-collisional do intraplate environment. The high values for ¹⁴⁷Sm/¹⁴⁴Nd ratios of rocks from the low-Ti group are probably related to the fractionation of minerals or hydrothermal alteration of the samples. Our new data allows to better understand the stratigraphic evolution of the neoproterozoic post-collisional volcanic successions of the CB in the SRGS and the evolution of magmatic cycles during the late stages of the Brasiliano-Pan-African orogeny in southern Brazil.

ACKNOWLEDGMENTS

The authors acknowledge CNPq (303015/2015-2; 400724/2014-6, 441766/2014-5, 302213/2012-0, 303584/2009-2, 471402/2012-5, and 470505/2010-9), for research funding. We also acknowledge the referees for their constructive inputs. IGEO/UFRGS and CPGEO/USP are thanked for support during the field and laboratory work.

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Publication Dates

• Publication in this collection
  Oct-Dec 2017

History

• Received
  03 May 2017
• Accepted
  29 Sept 2017