UNIVERSIDADE ESTADUAL PAULISTA Instituto de Geociências e Ciências Exatas Câmpus de Rio Claro ANA CAROLINA FRANCIOSI LUCHETTI Vulcanologia e Petrologia das Rochas Vulcânicas Ácidas da Província Magmática do Paraná. Tese de Doutorado apresentada ao Instituto de Geociências e Ciências Exatas do Câmpus de Rio Claro, da Universidade Estadual Paulista “Júlio de Mesquita Filho”, para obtenção do título de Doutor em Geologia Regional. Orientador: Prof. Dr. Antonio José Ranalli Nardy RIO CLARO – SP 2015 AGRADECIMENTOS Muito especialmente, desejo agradecer ao meu orientador Prof. Antonio José Nardy, pela disponibilidade, atenção dispensada, paciência, dedicação e profissionalismo ... um Muito Obrigada. A todos os Professores pesquisadores colaboradores com esse projeto, Fábio Machado, José Madeira, Marcelo Arnósio, Guilherme Gualda, Darren Gravley e Francisco Negri, obrigada pelo grande conhecimento passado. À minha família, pelo incentivo, compreensão e encorajamento, durante todo este período. Ao meu namorado Douglas, pela paciência e ajuda em muitos momentos. Ao colega Nuno, pela grande ajuda com questões da tese. Aos alunos da graduação, Juliane, Vítor, Juan, Taís, Rômulo, Letícia e Isabela, pelos trabalhos desenvolvidos e que fizeram parte desta tese. A todos os técnicos do Depto de Petrologia e Metalogenia, sem os quais o trabalho ficaria inviável. Aos meus amigos de sempre e aos que conquistei durante a pós-graduação, obrigada pela força e momentos de descontração. Resumo Os traquitos e dacitos do tipo Chapecó (ATC) e dacitos e riolitos do tipo Palmas (ATP), de idade cretácica, compõem 2,5% dos ~ 800.000 km3 de lavas da Província Magmática do Paraná (PMP) geradas anteriormente à quebra do Gondwana. Seções colunares estudadas ao longo da PMP mostram que as rochas ácidas encontram-se na parte superior das sequências, sobrepostas aos basaltos toleíticos e, em alguns perfis da região central da PMP, entre os municípios de Guarapuava (PR) e Campina da Alegria (SC), as rochas ATC se sobrepõem às ATP. Na região sul, entre os municípios de Sobradinho e Soledade (RS), e ao longo da rodovia Rota do Sol (BR-453) (RS), assim como na região central, os depósitos revelam ignimbritos de alto a extremamente alto grau de soldadura (reoignimbritos). Esses reoignimbritos (sheet-like) formam extensos platôs (dezenas a centenas de km) de depósitos maciços, monótonos, variavelmente desplacados (juntas) e com brechas peperíticas locais. Bandamento por blobs de vidro (lentes escuras microcristalinas) dispostos em material esbranquiçado é característico dos depósitos ATC e ATP. A matriz está intensamente devitrificada, localmente com raros fiammes e glass shards extremamente deformados. Na região sul, subjacentes aos reoignimbritos, também ocorrem lava-domos e/ou coulées, em cujos afloramentos há aglomerados de pitchstones com base e exterior autobrechados, separados localmente por peperitos, e/ou corpos devitrificados com juntas cerradas (sheeting joints) concêntricas. Ainda na região sul, depósitos bandados a complexamente dobrados de rochas ATP ocorrem na base da sequência ácida. Esses depósitos podem representar lava-domos, ignimbritos lava-like ou mesmo fluxos de lava fountain-feed. Além disso, esses alforamentos, assim como os dos lava-domos ocorrem alinhados em uma mesma direção, podendo indicar fissuras que alimentaram o vulcanismo ácido na PMP, cuja direção é paralela à linha de costa, possivelmente refletindo a tectônica extensional que levou à quebra do Gondwana. Temperaturas de cristalização estimadas para plagioclásio e piroxênios indicam valores elevados (ATC: 1030 ±4°C – plagioclásio, 969 ±18°C – piroxênio; ATP: 976 ±2°C a 1040 ±2°C – plagioclásio), estando os piroxênios das rochas ATP em equilíbrio com magma básico a andesítico, com T de cristalização de 1134 ±18°C e 1100 ±11°C respectivamente. Além disso, as profundidades de cristalização dos mesmos revelam um sistema plumbing raso abaixo da Bacia do Paraná (ATC: 4.3 ±0.9 kbar – plagioclásio, 3.3 ±2.4 kbar – piroxênio; ATP: 2.6 ±0.7 a 1.1 ±1.5 – plagioclásio, 2.8 ±1.1 a 1.0 ±1 kbar – piroxênio). Assim, os dados obtidos mostram que os lava-domos representam fluxos degaseificados de pequeno volume, intermitentes, limitados por corpos magmáticos iniciais, de pequeno volume. Os reoignimbritos extensos e volumosos expressam erupções que drenaram corpos magmáticos maiores, regidas por processos rasos resultantes de descompressão rápida. As altas temperaturas foram responsáveis pelas baixas viscosidades e consequente aglutinação/coalescência dos piroclastos (viscosidades magmáticas estimadas em 104.31-6.39 Pa s para ATC, e 104.27-7.22 Pa s para ATP) e, aliado ao baixo conteúdo de água (médias de 0.6 ±0.2 e 1.2 ±0.3 %H2O), originaram erupções do tipo boil-over (baixas colunas de colapso e eficiente retenção de calor) e correntes piroclásitcas quentes e progressivas durante a deposição. Palavras-chave: Formação Serra Geral, Vulcanismo Ácido, Membro Chapecó, Membro Palmas Abstract The Cretaceous Chapecó trachydacites-dacites (ATC) and Palmas dacites- rhyolites (ATP) make up 2.5% of the ~ 800.000 km3 of lava of the Paraná Magmatic Province (PMP), prior to Gondwana breakup. Columnar sections along PMP show the silicic rocks lying on top of the volcanic sequences, overlapping tholeiitic basalts. Furthermore, in some sections of central PMP, between the Guarapuava (PR) and Campina da Alegria (SC) towns, the ATC rocks overlap those of ATP. In the southern region, between the Sobradinho and Soledad (RS) towns, and along the Rota do Sol highway (BR-453) (RS), as well as in the central region, the deposits comprise high to extremely high grade ignimbrites (rheoignimbrites). They form extensive plateaus (tens to hundreds of km) of sheet-like, monotonous, variably horizontally jointed and with local peperitic breccia deposits. Glass blobs (microcrystalline dark lenses) arranged in whitish material make up an intrinsic eutaxitic texture (or banding). The groundmass is through devitrified, with local rare flattened and/or extremely stretched fiamme and shards. In the southern area lava domes and/or coulées also appear underlying rheoignimbrites. Pitchstones clusters with brecciated base and exterior, locally separated by peperites, and/or concentrically sheeting jointed, devitrified outcrops. Flow banded to complexly folded ATP rocks occur at the base of the silicic sequence. These rocks are ambiguous and may represent lava domes, lava-like ignimbrites or, even, fountain-feed lava flows. Moreover, its outcrops, as well as those of lava domes follow a northeast-trend, parallel to the coastline, which may indicate feeder fissures and also reflect the extensional setting that led to the Gondwana breakup. Crystallization temperatures estimated for plagioclase and pyroxene indicate high values (ATC: 1030 ±4°C – plagioclase, 969 ±18°C – pyroxene; ATP: 976 ±2°C a 1040 ±2°C – plagioclase), with the ATP pyroxenes in equilibrium with basic-to-intermediate magma (1134 ±18°C e 1100 ±11°C, respectively). Furthermore, its crystallization depths reveal a shallow plumbing system beneath the Paraná Basin (ATC: 4.3 ±0.9 kbar – plagioclase, 3.3 ±2.4 kbar – pyroxene; ATP: 2.6 ±0.7 a 1.1 ±1.5 – plagioclase, 2.8 ±1.1 a 1.0 ±1 kbar – pyroxene). Thus, in a proposed model, the lava domes represent small volume, intermittent, degassed flows, limited by small initial magmatic bodies. The extensive and voluminous rheoignimbrites express eruptions driven by shallow processes resulting from rapid decompression, draining largest magmatic bodies. High temperatures were responsible for the low viscosities and consequent agglutination/coalescence of the pyroclasts (magmatic viscosities estimated in 104.31-6.39 Pa s for ATC, and 104.27-7.22 Pa s for ATP). This, coupled with low water contents (average 0.6 ±0.2 and 1.2 ±0.3% H2O), originated boil- over eruptions (low collapse columns and little heat loss) and hot pyroclastic density currents. Keywords: Serra Geral Formation, Silicic Volcanism, Chapecó Member, Palmas Member. Ana Carolina Franciosi Luchetti Vulcanologia e Petrologia das Rochas Vulcânicas Ácidas da Província Magmática do Paraná. Tese de Doutorado apresentada ao Instituto de Geociências e Ciências Exatas do Câmpus de Rio Claro, da Universidade Estadual Paulista “Júlio de Mesquita Filho”, como parte dos requisitos para obtenção do título de Doutor em Geologia Regional. Comissão Examinadora Prof. Dr. Antonio J. R. Nardy Prof. Dr. Marcos Aurélio F. De Oliveira Prof. Dr. Valdecir de Assis Janasi Profa. Dra. Cristina Maria Pinheiros de Campos Prof. Dr. José Eduardo de Oliveira Madeira Rio Claro, SP, 30 de Outubro de 2015 SUMÁRIO Chapter 1: Peperites and sedimentary deposits within the silicic volcanic sequences of the Paraná Magmatic Province, Brazil ................................................. 1 1 INTRODUCTION ...................................................................................................... 2 2 PETROGRAPHIC AND GEOCHEMICAL ASPECTS OF SILICIC VOLCANIC ROCKS ........................................................................................................................ 4 3 STRATIGRAPHY ...................................................................................................... 7 3.1 Peperites and sedimentary deposit .................................................................... 8 4 DISCUSSION AND CONCLUDING REMARKS .................................................... 18 REFERENCES .......................................................................................................... 20 Chapter 2: Physical Volcanology of Silicic Volcanic Rocks from Paraná Magmatic Province ....................................................................................................................... 24 1 INTRODUCTION .................................................................................................... 24 2 GEOLOGICAL SETTING ....................................................................................... 26 3 SILICIC CHEMICAL GROUPS AND SUB-GROUPS ............................................. 28 4 VOLCANIC DEPOSITS .......................................................................................... 30 4.1 Stratigraphic Aspects ........................................................................................ 30 4.2 Silicic Lava Domes ........................................................................................... 36 4.3 Extensive Sheets - Rheoignimbrites ................................................................ 37 4.4 ‘Enigmatic Flow Banded’ Rocks ....................................................................... 43 5 PETROGRAPHY .................................................................................................... 46 6 DISCUSSION ......................................................................................................... 48 6.1 ‘Enigmatic Flow Banded’ Rocks ....................................................................... 52 6.2 Tectonic Alignments and Source Areas ............................................................ 53 9 CONCLUSIONS ..................................................................................................... 55 REFERENCES .......................................................................................................... 57 Chapter 3: Physical Parameters and Crystal Size Distribution of Silicic Volcanism of the Paraná Magmatic Province .............................................................................. 65 1 INTRODUCTION .................................................................................................... 65 2 GEOLOGICAL SETTING ....................................................................................... 67 3 ANALYTICAL METHODS ...................................................................................... 69 3.1 EBSD (Electron Backscatter Diffraction) .......................................................... 70 4 GEOCHEMISTRY ................................................................................................... 73 5 MINERALOGY – PETROGRAPHY ........................................................................ 77 6 THERMOBAROMETRY ......................................................................................... 84 6.1 Crystal-liquid equilibrium tests .......................................................................... 85 7 RESULTS ............................................................................................................... 87 7.1 Temperature, pressure and H2O estimates ...................................................... 87 7.2 Crystal Size Distribution (CSD) ........................................................................ 92 8 DISCUSSION ......................................................................................................... 99 8.1 Crystallization temperatures and depths .......................................................... 99 8.2 Water Content ................................................................................................ 101 8.3 Rheological and eruptive aspects .................................................................. 101 8.4 Crystal size distributions (CSDs) .................................................................... 103 8.4.1 Timescales of crystallization .................................................................... 106 9 CONCLUSIONS ................................................................................................... 108 REFERENCES ........................................................................................................ 109 1 Chapter 1: Peperites and sedimentary deposits within the silicic volcanic sequences of the Paraná Magmatic Province, Brazil (published in Solid Earth, 5, 121-130, 2014) A. C. F. Luchetti1, A. J. R. Nardy1, F. B. Machado2, J. E. O. Madeira3, J. M. Arnosio4 1 Instituto de Geociências e Ciências Exatas, Universidade Estadual Paulista, Rio Claro, SP, 13506-900, Brazil, caroluch@rc.unesp.br, nardy@rc.unesp.br 2 Departamento de Ciências Exatas e da Terra, Universidade Federal de São Paulo, São Paulo, SP, 09920-540, Brazil, fabio.machado@unifesp.br 3 Departamento de Geologia da Faculdade de Ciências (GeoFCUL), Instituto Dom Luiz, Universidade de Lisboa, Portugal, jmadeira@fc.ul.pt 4 Instituto Geonorte, Facultad de Ciencias Naturales, Universidad Nacional de Salta, Salta, Argentina, marnosio@unsa.edu.ar Abstract The PMP (Paraná Magmatic Province) is characterized by lava flows of the Early Cretaceous Serra Geral Formation which covers about 75 % of the Paraná Basin (southern and southeastern Brazil), composed of a thick (up to 1600 m) volcanic sequence formed by a succession of petrographically and geochemically distinct units of basic and silicic composition. The whole package must have been emplaced during approximately 3 million years of nearly uninterrupted activity. A few aeolian sandstone layers, indicating arid environmental conditions (the Botucatu Formation), are interlayered in the lower basalts. Above the basalts, the Palmas and Chapecó Members are composed of silicic volcanic rocks (quartz latites, dacites, rhyodacites and rhyolites) and basalts. This paper presents new evidence of sedimentation episodes separating silicic volcanic events, expressed by the occurrence of sedimentary deposits. Interaction between the volcanic bodies and the coeval unconsolidated sediments formed peperites. The sediments were observed between basaltic lava flows and silicic rocks or interlayered in the Palmas-type rocks, between the Chapecó-type rocks and overlying basaltic flows, between silicic bodies of the Palmas and Chapecó types, and interlayered within Palmas-type units. The observed structures indicate that the sediments were still wet and unconsolidated, or weakly consolidated, at the time of volcanism, which, coupled with the sediment features, reflect environmental conditions that are different from those characterizing the Botucatu arid conditions. 2 1 INTRODUCTION The Early Cretaceous Serra Geral Formation is the result of a major volcanic phase that covered about 917,000 km2, about 60% of the surface of the Paraná Basin (FRANK et al., 2009; Fig. 1). Three main petrographic types can be distinguished from macroscopic observation of these rocks. The most common corresponds to basalt presenting predominant intergranular texture and its variations (subofitic, intersertal, hialofitic, etc). The other two types, corresponding to silicic rocks, present massive and aphiric textures (Palmas type - ATP) and porphyritic textures (Chapecó type - ATC). Macroscopic characteristics allowed easy separation in the field of these two members of the Serra Geral Formation and their geological mapping (BELLIENI et al., 1983; PICCIRILLO et al., 1988). The Palmas and Chapecó rock-types occur in association with basaltic flows that are more frequent near the top and bottom of these two lithostratigraphic units. Geological mapping also shown that Palmas and Chapecó Members cover 63,000 km2, in the states of Paraná, Santa Catarina, and Rio Grande do Sul. The volume of the two members amounts to approximately 14,500 km3, which corresponds to 2.5% of the total volume of the Serra Geral Formation (NARDY et al., 2002, 2008). Geochronological dating by 40Ar/39Ar shows that the age of volcanic rocks of the Serra Geral Formation ranges from 133.6 to 131.5 Ma in its northern sector, and from 134.6 to 134.1 Ma in the south (RENNE et al., 1992, 1996a, b; TURNER et al., 1994; ERNEST et al., 1999, 2002; MINCATO et al., 2003; THIEDE & VASCONCELOS, 2010; PINTO et al., 2010). More recently, Janasi et al. (2011), using U/Pb ratios from baddeleyite/zircon crystals determined by ID-TIMS from rocks of the Chapecó Member, obtained an age of 134.3 ± 0.8 Ma, compatible with the previous age determinations. However, ages obtained in the dominant basaltic flows indicate duration of the volcanism of around 3 Ma, which is consistent with paleomagnetic data presented by Ernesto & Marques (2004). Up to now, the presence of sediments (sandstones of the Botucatu Formation) intercalated in the volcanic sequence was only reported in the lower basaltic pile. These consist of sand bodies presenting aeolian structures such as bypass surfaces, single- dunes, sand-filled cracks, multi-dune ergs (JERRAM & STOLLHOFEN, 2002; PETRY et al., 2007; WAICHEL et al., 2008), representing a desert environment that persisted 3 during the voluminous initial phase of basaltic volcanism. In this work, the occurrence of sedimentation and development of associated peperites in the final stage of the PMP volcanic event, is presented. The sediments, dominantly sandy-silty, and unrelated to the Botucatu Formation, indicate a change in the environmental conditions in the Paraná Basin, and attest to the occurrence of significant periods of quiescence during the final stages of the magmatic activity. Fig. 1 - Map of the Paraná Basin with the location of the acidic members of the Serra Geral Formation according to Nardy et al. (2008). Legend: 1- Embasement; 2- Pre-volcanic sedimentary rocks; 3- Basalts (Serra Geral Formation); 4– acidic (a) Chapecó and (b) Palmas Members (Serra Geral Formation); 5- Sedimentary post-volcanic sequences (Bauru Basin); 6- Anticline structures; 7- Syncline structures; 8- Oceanic lineaments; 9- Continental lineaments. 4 2 PETROGRAPHIC AND GEOCHEMICAL ASPECTS OF SILICIC VOLCANIC ROCKS The Palmas type silicic volcanic rocks (ATP) are characterized by light-gray to brownish red color, hypohyaline-holohyaline aphyric textures with a striking salt-and- pepper aspect. The mineralogy is composed of dominant micro-phenocrysts (granularity smaller than 0.2 mm) of plagioclase (labradorite) comprising up to 16% of the total volume of the rock, 11% of augite, 3% of pigeonite, 5% of magnetite, and less than 1% of apatite. These crystals may exhibit rapid cooling structures (quenching), developing skeletal, lath, and hollow shapes, or swallowtail terminations. The matrix reaches 63% of the rock volume on average, and is composed of dark-brown slightly birefringent glass, characterized by a granophyric texture of abundant microlites, and alkali feldspar and quartz intergrowth, that surrounds the crystal phases. When holohyaline (pichstone), these rocks show black color and prominent conchoidal fractures. However, due to its amorphous nature, the glass alters easily and thus in most outcrops the rock is completely weathered, presenting a brownish color and (often resembling sedimentary deposits) dotted with abundant vesicles and quartz-filled amygdales up to 10 mm in length. The Chapecó type silicic volcanic rocks (ATC) are porphyritic, with an average of 24% of plagioclase phenocrysts up to 2 cm long, in a light gray (when fresh) to brown (when weathered) aphanitic matrix. The mineralogy consists of euhedral andesine phenocrysts in a matrix composed of 4.5% of augite, 2.2% of pigeonite, 3.7% of magnetite, and 1.7% of apatite (average composition) surrounded by quartz and alkali feldspar fabric in felsitic, locally granophyric, arrangement (vitrophyric texture). Silicic volcanic rocks chemical compositions of the Paraná Magmatic Province, according to Nardy et al. (2008), show two main groups which may be observed in a R1xR2 diagram (Figure 2, De La Roche et al., 1980). The first one, Low-Ti suite, belongs to tholeiitic field (tholeiitic basalts, andesi-basalts and andesites) associated to Palmas type silicic volcanic rocks, the latter plot in the rhyodicite and rhyolite field. The second group, High-Ti suite is displaced towards the transitional field (transitional basalt, lati-basalt and latites). The Chapecó type silicic volcanic rocks belong to this 5 group in the rhyodacite and quartz latite fields (Fig. 2). The bulk-rock representative compositions for both Palmas and Chapecó types are listed in table 1. According to Bellieni et al. (1984a) the chemistry of the volcanic rocks and their spacial distribution allow the Paraná basin to be schematically subdivided into three main regions: (1) southern, encompassing the tholeiitic suite in the southern Uruguay River alignment; (2) northern, where tholeiitic-transitional rocks occur in the northern Piquiri River alignment; and (3) central, located between Piquiri and Uruguay Rivers alignments, where both rock types are present, (Fig. 1). The spatial distribution of the tholeiitic – ATP and transitional-tholeiitic-ATC suites indicates that acidic volcanic rocks may have derived from the associated basalts, or ATC melts are derived from tholeiitic- transitional basalts and ATP from tholeiitic basalts as suggested by Bellieni et al. (1986) and Garland et al. (1995). Fig. 2- R1xR2 diagram (De La Roche et al., 1980). A- tholeiitic suite (Low-Ti basalts and ATP), B- Tholeiitic-transitional suite (High-Ti basalts and ATC). 6 Table 1- Representative analysis of Palmas and Chapecó silicic volcanics of the PMP. (major elements- in oxide wt%; trace elements- in ppm) Palmas Chapecó Sample KC KSE KSE KS KSU KC PU KNO # 505 406 419 319 237 482 1011 442 SiO2 65.48 66.44 67.58 68.70 70.46 64.38 65.44 66.93 TiO2 1.11 0.96 0.95 0.95 0.71 1.46 1.57 1.24 Al2O3 12.96 12.74 12.35 12.12 12.38 12.83 13.03 13.03 Fe2O3 6.78 6.15 6.15 5.69 5.22 7.01 7.57 6.54 MnO 0.08 0.11 0.20 0.10 0.09 0.16 0.12 0.10 MgO 1.08 1.70 1.22 1.28 0.61 1.36 1.28 0.75 CaO 2.78 2.93 3.09 2.68 2.20 2.91 2.94 2.07 Na2O 3.64 2.87 2.78 2.30 2.92 3.32 3.61 3.36 K2O 4.01 3.89 4.17 4.69 4.74 4.45 4.33 4.61 P2O5 0.33 0.27 0.26 0.26 0.20 0.48 0.46 0.33 LOI 1.93 1.91 1.01 0.99 0.57 1.05 0.61 1.63 SUM 100.17 99.98 99.75 99.76 100.09 99.41 100.96 100.60 Cu 128 75 78 63 18 7 9 14 Ni 7 8 7 8 3 4 5 5 Ba 610 706 694 588 613 1076 1003 1199 Rb 169 160 165 175 206 100 101 136 Sr 135 143 127 137 102 360 337 318 Zr 279 258 252 266 319 633 670 592 Y 63 41 57 42 55 65 66 60 Nb 22 20 20 21 23 48 51 44 U 3.51 4.07 4.04 4.17 3.25 1.74 1.91 2.68 Th 11.53 11.30 11.64 12.09 12.25 8.76 8.62 12.69 La 42.0 35.0 36.1 36.0 43.2 60.4 63.7 67.8 Ce 88.0 76.0 78.0 73.0 95.5 144.6 147.0 145.0 Nd 46.0 36.0 34.8 34.0 42.7 68.4 71.7 70.9 Sm 9.40 7.00 7.47 7.10 8.78 15.78 15.10 14.50 Eu 2.02 1.66 1.53 1.53 1.60 3.60 3.52 3.11 Gd 9.80 7.30 7.28 8.10 8.78 14.04 13.10 11.90 Dy 8.30 6.90 7.43 7.40 8.32 10.83 11.50 10.20 Ho 1.70 1.40 1.60 Er 4.70 4.00 4.54 4.60 5.25 5.97 6.21 5.42 Yb 4.50 3.40 3.92 3.70 4.61 4.79 5.03 4.39 Lu 0.60 0.50 0.61 0.60 0.71 0.65 0.76 0.65 7 3 STRATIGRAPHY The 1600 m thick Paraná Magmatic Province (PMP) volcanic sequence consists of up to 32 lava flows of predominant basic to intermediate compositions (tholeiitic basalts, andesi-basalts, and andesites), as well as felsic volcanic rocks (dacites, rhyodacites, and rhyolites; BELLIENI et al., 1984, 1986). The base of the stratigraphic column is composed of a thick sequence of basic to intermediate flows that overlap the aeolian sandstones of the Botucatu Formation. The sandstones may also occur interlayered in the first hundred meters of the basaltic pile. The Palmas and Chapecó Members overly the basalt flows. The Palmas Member is characterized by silicic volcanic bodies (ATP type) associated with a few basaltic lava flows, crops out from the central region of the basin southwards, where it may reach 270 m thickness. The Chapecó Member, composed of silicic volcanic rocks (ATC type), occurs in the northern and central regions of the Paraná Basin; the largest thickness, reaching 250 m, is present in the central region. This Member overlaps the basalts, but in the northern portion of the basin (Paranapanema River region - SP) it is found directly on the sandstones of the Botucatu Formation. In the center of the basin the two silicic members overlap indicating that the Palmas Member is older than Chapecó, although ATP type rocks may be found interlayered in the Chapecó Member. The last pulses of Paraná volcanism emplaced basalt flows that cover both the Palmas and Chapecó type rocks and become thicker towards the north of the basin. 8 Fig. 3 - Map with the location of sediment and peperite outcrops (lozenges) within the Palmas and Chapecó Members; Yellow – Palmas units, red – Chapecó units. 3.1 Peperites and sedimentary deposit The Literature (MARQUES & ERNESTO, 2004; THIEDE & VASCONCELLOS, 2011) indicates that the PMP magmatism occurred quickly, during a time interval that did not exceed 3 Ma, and in a rather continuous way, based on scarce observations of sedimentary intercalations or paleosoils within the volcanic sequence. However, recent 9 field work revealed the presence of frequent sediment lenses and peperites in various stratigraphic levels within the silicic volcanic sequence. Peperite is a genetic term used for rocks formed in situ by the interaction between hot magma (intrusive bodies, lava or pyroclastic flows) and coeval wet sediments (FISHER, 1960; WILLIAMS & MCBIRNEY, 1979; WHITE et al., 2000; SKILLING et al., 2002). Nonetheless, interaction with dry sediments has also been described by some authors (JERRAM et al., 1999; JERRAM & STOLLHOFEN, 2002; PETRY et al., 2007). Peperite is classified into two basic types according to the shape of its elements (BUSBY-SPERA & WHITE, 1987): blocky, in which the volcanic clasts present angular shapes and show jigsaw-fit texture reflecting in situ quench fragmentation in a brittle state; and fluidal in which volcanic clasts present irregular, fluid (amoeboid), globular to undefined shapes, reflecting a ductile state during fragmentation, with the sediment often filling vesicles and being injected into cracks in the volcanic clasts/rocks, although more complex shapes may also be found (MCPHIE et al., 1993; SKILLING et al., 2002). Sedimentary deposits and peperites found in the Chapecó and Palmas Members present a wide distribution in the Paraná Basin, as shown in Fig. 3, are described below. In the São Jerônimo da Serra (Paraná State) region, in the northern sector of the Paraná Basin, ATC type rocks overly a sandstone forming blocky peperite (Fig. 4). It comprises few centimeters to just over 1 m thick clastic dikes (Fig. 4a) and breccias composed of matrix-supported angular to rounded volcanic blocks of variable size (Fig. 4b). The sandstone is poorly sorted with angular to rounded quartz grains (Fig. 4c and d) and it was silicified by thermal metamorphism by the overlying volcanic material. In the region of Mangueirinha and Palmas (Paraná State), in the center of the basin, both ATC and ATP type rocks crop out, either overlying basalts or overlapping each other. A sandstone layer was observed intercalated between ATC type silicic body and an overlying basaltic lava flow. The base of the sediment is a breccia formed by vesicular ATC clasts set in a sandy matrix, implying some erosion degrees of the top of the volcanic body during sedimentation. In another location, an ATC type silicic unit overlies a vesicular ATP type silicic body, with a red clayey-silty sediment intercalated between the two units. The sediment was injected upwards into fractures in the 10 overlying ATC unit forming peperite with both fluidal and blocky features (Fig. 5a and b), while in the underlying ATP unit, the sediment filled cooling cracks without any peperitic interaction (Fig. 5c and d). In a third exposure, a reddish brown silty sediment, intercalated between two ATP volcanic units, formed peperite with blocky jointing morphology (Skilling et al., 2002), characterized by the injection of sediment into centimeter to millimeter spaced joints in the base of the overlying volcanic unit (Fig. 6). Fig. 4 – Peperite from São Jerônimo da Serra: (a) clastic dike more than 1 m thick; (b) angular to sub-angular shaped clasts set in a sandstone matrix. S –sandstone; V - volcanic rock; (c, d) photomicrographs of the immature and poorly sorted sediment (c = // pol.; d = X pol.). 11 Fig. 5 – Silicic ATC and ATP type rocks (V) in contact with red clayey-siltstone (s): (a, b) peperites in the base of ATC type rocks; (c and d) fractures in the top of ATP type rock filled with sediment, in the central region of the Paraná Basin. In the southern region of the Paraná Basin (Rio Grande do Sul State), sediments and peperites were observed between some basaltic lower units and the overlying Palmas silicic sequence. In Santa Maria region, blocky peperite was observed in the base of the lowermost silicic volcanic unit. The peperite displays a well-developed jigsaw-fit texture, and closely packed, blocky to cuneiform juvenile clast shapes of several sizes, separated by an orange colored sedimentary material (Fig. 7). The sediment exhibits a high degree of baking (thermal metamorphism) and the juvenile clasts show intense devitrification, indicating very high temperature of the volcanic material interacting with the sediment. Near the city of Venâncio Aires, a 10 m thick sedimentary layer underlies an ATP silicic volcanic unit. The sediment is a reddish colored sandstone composed of sub- angular to rounded quartz grains (Fig. 8). The volcanic unit presents well developed horizontal jointed base and the contact with the sandstone is sharp, apparently lacking peperitic interaction (Fig. 8). 12 A road-cut on the Soledade to Lajeado highway exposes a peperite in the base of a 25 m thick basaltic lava flow interlayered within the ATP silicic volcanic sequence. Volcanic clasts in the peperite are vesicular and display a variety of morphologies, from blocky to fluidal. The sediment is a red poorly sorted sandstone, which partially fills vesicles in the volcanic clasts. Fig. 6 - Blocky jointed peperite in the base of an ATP type unit (Domingos Soares city - Paraná State, central region of the Paraná Basin); V- ATP rock, S – sediment. 13 Fig. 7 –Blocky peperite formed by interaction of an ATP type volcanic body with sediment in Santa Maria region (Rio Grande do Sul State): well-developed jigsaw-fit texture, closely packed, gray blocky to cuneiform clasts of variable sizes, separated by orange sedimentary material, displaying intense baking. 14 Fig. 8 - Outcrop in the Venâncio Aires area (Rio Grande do Sul State), where a silicic volcanic unit of the ATP type (V) rests on a reddish sedimentary layer (s) lacking peperitic interaction. On the right lower corner a photomicrograph of the sandstone (X pol.). Fig. 9 – Aspects of a peperite produced by interaction of a thin basalt flow (within the silicic Palmas Member) and fine grained sediment near Nova Petrópolis (Rio Grande do Sul State), south part of the basin: (a-c) irregular blocky volcanic clasts (V) in a reddish sandstone matrix (S); (d) photomicrograph showing the amygdaloidal basalt (V) in the upper part and a poorly sorted fine-grained sandstone in the bottom (S) (// pol.). 15 Eastward, in the area between Bento Gonçalves and Cambará do Sul (Rio Grande do Sul), all the observed peperites are interlayered within the silicic volcanic sequence and the sedimentary material becomes finer, dominantly silty. A peperite was observed in a quarry floor in Nova Petrópolis, near Gramado city, resulting from the interaction of sediment with a thin (~ 1 m thick) amygdaloidal basalt interlayered in the silicic volcanic sequence. It displays blocky juvenile clasts with irregular shapes separated by orange to reddish, poorly-sorted fine-grained sandstone. The sediment presents vesicles that probably resulted from volatilization of sediment water by heating (Fig. 9). On the Rota do Sol highway, connecting Caxias do Sul to the coast, three peperite levels were observed interlayered in a sequence of Palmas Member black glassy volcanic units. The stratigraphically lower two are similar and characterized by vesicular, pale to greenish glassy volcanic clasts, presenting angular to rounded irregular shapes (Fig. 10). The sedimentary material is a brown moderate to poorly sorted siltstone that also fills vesicles in volcanic clasts. The uppermost peperite presents juvenile volcanic clasts with a wide variety of morphologies, being the fluidal shapes more frequent (Fig. 11). The green and vesicular volcanic clasts are surrounded by a reddish brown siltstone, which also fills vesicles and fractures in volcanic clasts (Fig. 11d). It is slightly coarser and more poorly sorted than the previous two sediments (Fig. 12), and many clasts of the stratigraphically lower peperite display perlitic fractures (Fig. 12c). 16 Fig. 10 - Aspects of the two lowermost peperites in the Palmas volcanic sequence, along the Rota do Sol highway (Rio Grande do Sul State): (a, b, and c) images of the lowermost peperite; and (d) photo of the middle peperite. The volcanic clasts are pale and light or dark green, while the siltstone is dark brown. Fig. 11 - Peperite from the stratigraphically higher level on the section exposed in the Rota do Sol highway: green to pale volcanic clasts, displaying sub-angular to irregular morphologies, set in a reddish brown siltstone matrix, which also fills vesicles in volcanic clasts (yellow arrows in d). 17 Fig. 12 - Photomicrographs of peperites from the Palmas volcanic sequence, along Rota do Sol highway: (a, b and d) volcanic glassy clasts (V) set in a brown siltstone matrix (S); (c) volcanic clast displaying concentric fractures - perlitic texture - common in the two lowermost peperites; (e and f) detail of the poorly sorted, slightly coarser siltstone from d (a, b, c, d and e - // pol.; f – X pol.; P = plagioclase). 18 4 DISCUSSION AND CONCLUDING REMARKS The PEMP (Paraná-Etendeka Magmatic Province) is considered one of the largest LIPs (Large Igneous Province) in continental crust in the world, with nearly 1 million km3 (Bryan et al, 2010). 95% of the total volume of the volcanic products is preserved in the South American continent, the Paraná Magmatic Province (PMP). All volcanic material was erupted in a short period of time (~ 3 million years) without significant interruption, as deduced from the scarcity of sediments interlayered within the volcanic sequence. In fact, and up to now, the only references to the presence of sediments interbedded within the volcanics corresponded to layers or lenses of sandstones (intertraps), a few centimeters to several meters thick, from the Botucatu Formation. These occur only in the base of the lava flow pile in both the African (JERRAM et al., 1999, 2000; JERRAM & STOLLHOFEN, 2002) and South American continents (PETRY et al., 2007; WAICHEL et al., 2008). Sandstone intertraps in rhyodacites of the Piraju – Ourinhos region (São Paulo State; JANASI et al., 2007; LUCHETTI, 2010), where the thick basaltic sequence is missing, were also known. However, until now, there were no reports of features indicating significant time breaks in the upper part of the PMP stratigraphic sequence. New observations of the occurrence of sedimentary lenses and peperites, resulting from volcano-sedimentary interaction, at the base and within the upper silicic sequences of the PMP are presented in this paper. These were observed throughout the Paraná Basin associated with silicic and basaltic units of the Palmas and Chapecó Members. Some works on the PMP report peperites derived from the basalt flow-sediment interaction (PETRY et al., 2007; WAICHEL et al., 2007, 2008), the most common types found in worldwide volcanic sequences. Nevertheless, structures observed in this study reveal that this interaction also occurred between silicic extrusive bodies and wet sediment. These “silicic peperites” are mostly blocky, highlighting a more viscous magma (DADD & WAGONER, 2002). However, they may also display fluidal features, which indicate less viscous melts, likely due to higher temperatures (> 1000 oC, PICCIRILLO & MELFI, 1988). 19 The sediments display immature features suggesting limited transport and, depending on the location, range from moderately to poorly-sorted sandstones to siltstones. Often, there was lava/sediment interaction producing peperites or sediment deformation by the weight of overlying volcanic units. Evidences for the presence of wet and unconsolidated or poorly consolidated sediments are the variety of morphologies found, injections of sediment into fractures, vesicles in the juvenile clast filled by sediment and vesiculated sediment (Skilling et al., 2002). Paleoenvironment climatic conditions at the time of the PMP volcanism were quite dry, since the Serra Geral Formation overlies the Botucatu Formation, described as a dry aeolian system (SHERER, 2000). Sandstone lenses up to 20 m thick interlayered in the first basalt lava flows (JERRAM et al., 1999, 2000; JERRAM & STOLLHOFEN, 2002; PETRY et al., 2007; WAICHEL et al., 2008) reveal that this system remained active during the magmatic event. However, the new evidences of wet and locally silty sediment between basalt and silicic units and interlayered in the silicic units (end of the volcanic sequence) presented here reflect a paleoenvironmental change, from a dry climate in the beginning of the PMP volcanic activity to a more humid environment (fluvial-lacustrine) during the latest phases of the magmatism, as also evidenced by Waichel et al. (2007) in peperites found in some basalt units of the PMP in contact with lacustrine sediments. At São Jerônimo da Serra region, northern area of the basin, the change must have started before the volcanism. Furthermore, this seems to be common in this type of setting in which humid and dry aeolian systems may alternate over time due to climate changes (ASSINE et al., 2004). The deposition of sediments must have taken place in depressed portions of the paleo-relief (small valleys or depressions of the original volcanic morphology), and also suggests a decrease in eruptive frequency towards the end of the volcanic activity, allowing time for the deposition of sedimentary material between individual volcanic events. In conclusion, the PMP volcanism was not totally continuous, but presented significant pauses, mainly in the initial and terminal phases. On the other hand, the occurrence of sediments separating the top of the lower basaltic sequence and the beginning of the silicic extrusions may represent a pause in the volcanic activity that 20 coincides with the compositional change in the magmatism of the Paraná Magmatic Province. Acknowledgements. This study was supported by São Paulo Research Foundation (FAPESP) and National Counsel of Technological and Scientific Development (CNPq). We thank Francisco Negri for help in field work in the central basin and Dr Scarlato, Piergiorgio, Dr Giacomoni, Pier Paolo and anonymous reviewer for valuable suggestions and review. REFERENCES ASSINE, M. L., PIRANHA, J. M., CARNEIRO, C. D. R.: Os paleodesertos Pirambóia e Botucatu. In: Mantesso-Neto, V., Bartorelli, A., Carneiro, C. D. R., Brito-Neves, B. B. (Eds), Geologia do Continente Sul-Americano: evolução da obra de Fernando Flávio Marques de Almeida. SP, Brasil: Beca Prod Cult Ltda, p. 77-92, 2004. BELLIENI, G., BROTZU, P., COMIN-CHIARAMONTI, P., ERNESTO, M., ERNESTO, M., MELFI, A. J., PACCA, I. G., PICCIRILLO, E. M.: Flood basalt to rhyolites suites in the southern Paraná plateau (Brazil): paleomagnetism, petrogenis and geodynamic implications. J. Petrol., 25, 579- 618, 1984. BELLIENI, G., COMIN-CHIARAMONTI, P., MARQUES, L. S., MELFI, A. 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F.: As rochas vulcânicas mesozóicas ácidas da Bacia do Paraná: litoestratigrafia e considerações geoquímico-estratigráficas. Brazilian Journal of Geosciences, 38 (1), 178-195, 2008. PEATE, D., HAWKESWORTH, C. J., MANTOVANI, M. S. M.: Chemical Stratigraphy of the Paraná Lavas (South America): Classification of Magma Types and their Spatial Distribution. B. Volcanol., 55, 119-139, 1992. PETRY, K., JERRAM, D. A., DE ALMEIDA, D. P. M., ZERFASS, H.: Volcanic-sedimentary features in the Serra Geral Fm., Paraná Basin, southern Brazil: Examples of dynamic lava- sediment interactions in an arid setting. J. Volcanol. Geoth. Res., 159, 313-325, 2007. PICCIRILLO, E. M. & MELFI, A. J. (eds.): The Mesozoic Flood Volcanism of the Paraná Basin: Petrogenetic and Geophysical Aspects. Instituto Geofísico, Astronômico e Ciências Atmosféricas, Universidade de São Paulo, São Paulo, Brazil, 1988. RENNE, P. R., ERNESTO, M., PACCA, I. G., COE, R. S., GLEN, J. M., PRÉVOT, M., PERRIN, M.: The age of Parana flood volcanism, rifting of Gondwanaland, and the Jurassic–Cretaceous boundary. Science 258, 975–979, 1992. RENNE, P. R., DECKART, K., ERNESTO, M., FÉRAUD, G., PICCIRILLO, E. M.: Age of the Ponta Grossa dyke swarm (Brazil), and implications to Paraná flood volcanism. Earth. Planet. Sc. Lett. 144, 199–211, 1996a. RENNE, P. R., GLEN, J. M., MILNER, S. C., DUNCAN, A. R.: Age of Etendeka flood volcanism and associated intrusions in southwestern Africa. Geology 24, 659–662, 1996b. SANTOS, R. N., MARQUES, L. S., BRENHA-RIBEIRO, F., NICOLA, S. H. A.: Determination of uranium concentrations and activity ratios in silicate rocks by alpha spectrometry: application to volcanic rocks from Trindade and Martin Vaz Islands. Appl. Radiation and Isotopes, 56, 5, 741- 750, 2002. SANTOS, R. N., MARQUES, L. S., BRENHA-RIBEIRO, F., NICOLA, S. H. A.: Determination of thorium concentrations and activity ratios in silicate rocks by alpha spectrometry. Appl. 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Volcanol., 62, 65-66, 2000. 24 Chapter 2: Physical Volcanology of Silicic Volcanic Rocks from Paraná Magmatic Province 1 INTRODUCTION Chapecó and Palmas silicic volcanic rocks are included in the Serra Geral Formation, in the Paraná Basin, and make up part of the Lower Cretaceous age Paraná Magmatic Province (PMP), a giant bimodal tholeiitic volcanism, with ~900,000 km3, which preceded the Gondwana breakup and the subsequent opening of the Atlantic Ocean. Geochronological dating by 40Ar/39Ar shows that the age of volcanic rocks of the Serra Geral Formation ranges from 133.6 to 131.5 Ma in its northern sector, and from 134.6 to 134.1 Ma in the south (RENNE et al., 1992, 1996a, b; TURNER et al., 1994; ERNESTO et al., 1999; MINCATO et al., 2003; THIEDE & VASCONCELOS, 2010; PINTO et al., 2010). More recently, Janasi et al. (2011), using U/Pb ratios from baddeleyite/zircon crystals determined by ID-TIMS from rocks of the Chapecó Member, obtained an age of 134.3 ± 0.8 Ma, compatible with the previous age determinations. These ages obtained in the dominant basaltic flows indicate duration of the volcanism of around 3 Ma, also consistent with paleomagnetic data presented by Marques & Ernesto (2004). Chapecó type rocks (ATC) appear more northerly and include porphyritic, crystal- rich, high-Ti dacites and trachydacites, whereas Palmas type (ATP) includes fine- grained, crystal-poor, low-Ti dacites and rhyolites. Even comprising a small proportion of the total erupted volume (2.5%), these rocks correspond to an extensive and voluminous (~14,500 km3) (NARDY et al., 2008), significant flare-up of silicic volcanism over a short period of time. Nardy (1995) noted a chemical lateral homogeneity along 45 km of a single cooling unit in the central region of the Paraná Basin, and paleomagnetic data show that a single cooling unit may be extended up to 30 km away (MARQUES & ERNESTO, 2004). The mode of emplacement (lavas vs. pyroclastic flows) of the volcanic units has been the subject of much controversy (e.g. WHITTINGHAN, 1989; MILNER et al., 1992; GARLAND et al., 1995; WAICHEL et al., 2012). In this way, the aim of this project was 25 to better understand the origin and evolution of the PMP silicic rocks, in some key regions in particular, combining field information from stratigraphy, structures and textures to the thin section scale to characterize eruption dynamics and magma condictions prior to eruption. Nevertheless the silicic eruption products seem to differ from the general pattern of worldwide registered large-volume silicic eruptions (tens to thousands km3 of magma) which typically involve fallout layers of pumice lapilli with subordinate lithic clasts produced by Plinian explosive eruptions, accompanied by emplacement of voluminous pumice lapilli-bearing ignimbrites, ending with extrusion of relatively small-volume domes and coulées of degassed silicic lava (e.g. WHITE et al., 2009; BRYAN et al., 2010). In contrast, the silicic volcanism evolved from lava domes to extensive high to extremely high grade ignimbrites showing a lack of lithic fragments and broken phenocrysts and non preservation of vitroclastic textures and fiamme. 26 2 GEOLOGICAL SETTING The Paraná Magmatic Province (PMP) comprises volcanic rocks of the Serra Geral Formation covering 917,000 km2, with average 650 m thick sequences and corresponding to more than 600,000 km3 of volcanic products (FRANK et al., 2009). Basalt flows make up about 90%, whereas intermediate (basaltic-andesites and andesites) and silicic compositions make up 7 and 2.5% respectively (BELLIENI et al., 1984; PICCIRILLO & MELFI, 1988). However much larger volumes must be considered, since significant erosion affected the South American Platform (GALLAGHER et al., 1994). Also part of this volcanism is preserved in the African Continent, in the Etendeka (Namibia), Kwanza and Namibe (Angola) basins (ALBERTI et al., 1992), giving rise the Paraná-Etendeka LIP. In Brazil the PMP volcanics were emplaced on aeolian sandstones of the Botucatu Formation in the Paraná Basin. The silicic rocks lie on top of the volcanic sequence, representing only 2.5% of total volume, however by no means they are negligible. Based on volcanics chemistry and spacial distribution data Bellieni et al. (1984) proposed the Paraná basin to be schematically subdivided into three main regions: (1) southern, encompassing tholeiitic suite lying south of the Uruguay River alignment; (2) northern, with tholeiitic-transitional rocks lying north of the Piquiri River alignment; and (3) central, between Piquiri River and Uruguay River alignments (Fig. 1). Main outcrop remnants of Chapecó type silicic volcanic rocks (ATC) occur mainly north of the Uruguay River alignment, with some outcrops south of the Uruguay River alignment. They currently cover an area of ~6,000 km2 with a total volume of ~1,000 km3. Palmas type silicic volcanic rocks (ATP) are spatially and volumetrically more significant than ATC rocks. They lie mainly in the southern region covering a current area of ~35,000 km2 with a total volume of ~12,000 km3, whereas smaller units (~5,000 km2 and ~700 km3 total) crop out in the central region (Fig. 1). 27 Fig. 1 - Geological and location map for the ATC and ATP units in the PMP, modified from Nardy et al. (2002). Dashed lines mark the Piquiri-Uruguay rivers lineaments which divide the Paraná Basin in northern, central and southern regions (BELLIENI et al.,1984). 28 3 SILICIC CHEMICAL GROUPS AND SUB-GROUPS Geochemical data presented in this study were obtained from a set of 250 samples of fresh acidic rocks. Major and trace elements were carried out at Unesp laboratories, using X-ray fluorescence spectrometry. Major elements were analysed using fusion beads (1:10 lithium tetraborate) while trace elements were obtained using pressed (30 ton/cm2) powder discs (mixed with 25 wt% of micropowder wax). All methodology (including errors) is described in Nardy et al (1997). REE were analysed by ICP-OES, using chromatographic concentration of elements, and the analytical approach is presented in Malagutti et al. (1998). The silicic volcanics of the PMP are chemically quite different. The Chapecó type silicic volcanic rocks (ATC) are trachytes to dacites (Fig. 2), enriched in TiO2, P2O5, Al2O5 and Fe2O3, and impoverished in CaO and MgO when compared to Palmas dacites to rhyolites (ATP). ATC rocks are also enriched in some incompatible elements (Ba, Nb, La, Ce, Zr, P, Nd, Y, Yb, Lu and K) and impoverished in Rb, Th and U relative to ATP rocks. Fig. 2 – TAS diagram (LE BAS et al., 1986) for classification and nomenclature of ATC and ATP silicic volcanic rocks. 29 These two main silicic groups have been divided in subgroups or magma subtypes based on TiO2 and P2O5 contents (PEATE et al., 1992; GARLAND et al., 1995; NARDY et al., 2008). Thus the Guarapuava (TiO2 ≥ 1,47%), Ourinhos (TiO2 ≤ 1,29%) and Tamarana (1.38 ≤ TiO2 ≤ 1.5%) subgroups comprise the ATC rocks. ATP rocks are formed by two subgroups: Low-Ti (TiO2 ≤ 0,87%), composed by the Santa Maria (P2O5 ≤ 0,21%) and Clevelândia (0,21%< P2O5 ≤0,23%) subtypes; and High-Ti (TiO2 ≥ 0,90%), composed by the Caxias do Sul (0,91%< TiO2 <1,03% and 0,25%< P2O5 <0,28%), Jacuí (1,05%< TiO2 <1,16% and 0,28%< P2O5 <0,31%) and Anita Garibaldi (1,06%< TiO2 <1,25% and 0,32%< P2O5 <0,36%) magma subtypes. Furthermore, it can be noted from the TAS diagram (Fig. 2) that Jacuí and Anita samples fall in the dacites field, Caxias do Sul in the dacites-rhyolites fields, and Santa Maria and Clevelândia in the rhyolites field. 30 4 VOLCANIC DEPOSITS 4.1 Stratigraphic Aspects The silicic volcanic rocks of the PMP overlap tholeiitic basalt-to-andesite flows, lying in the top of the volcanic sucession. Interlayered basalt and/or andesite flows and silicic rocks also occur in places in the southern region. The central region, between Guarapuava (PR) and Campina da Alegria (SC) towns, is the only place occuring both silicic rock types, ATC (Guarapuava subtype trachydacites-dacites) and ATP (Clevelândia subtype rhyolites). Geomorphologically, it is characterized by remaining flattened surfaces (plateaus) with gently undulating tabular tops and variably thick alteration mantle. ‘Fresh’ outcrops are rare, scattered, and the contact between units ranging up to 375 m altitude with both ATC and ATP rocks occurring in the same topographic levels in places. This reflects tectonic processes, most likely due to the resulting uplift from the Gondwana breakup, which also led to the emergence of the basement and pre-volcanic sedimentary deposits in the eastern portion. Despite this, representative columnar sections of the area (Fig. 3a-a’) show ATP sheets (at least two units) underlie ATC rocks (sections: MA5 and MA6 in Fig. 3a, CA and MA4 in Fig. 3a’), in aggrement with northwest Etendeka sequences, where the Fria ‘quartz latite’/Clevelândia underlies the Sarusas ‘quartz latite’/Guarapuava (MARSH et al., 2001; EWART et al., 2004). Both silicic units are overlapped by basalts in places. In the southern region, ATP rocks make up the broader silicic volcanic rock outcrops. The landscape is dominantly characterized by extensive flattened surfaces with gently undulating tabular tops, as the central region, and up-and-down surfaces toward to the base of the silicic volcanic sequence. Columnar sections (Fig. 3b), especially those in the eastern part (TA, PC, RA, SJ and GB), show the contact with the Botucatu sandstone varying up to 980 m altitude, indicating this region was also affected by the tectonic event resulting from the separation of the South American and African plates. Nevertheless, one can observe the ATP chemical subtypes are also arranged in a stratigraphic sequence. The Santa Maria subtype magma was the latter erupted, overlapping all other ATP subtypes 31 (columnar sections SO2 – LS, Fig. 3b). In turn, despite the Jacuí subtype does not appear underlying Caxias do Sul rocks in this region, it seems to have been the oldest magma erupted. Furthermore, this subtype underlies Caxias do Sul subtype rocks in the eastern part (columnar sections BM and RA, Fig. 3b), as in the Southern Etendeka, Namibia, where the Goboboseb ‘quartz latite’/Jacuí underlies the Springbok ‘quartz latite’/Caxias do Sul (MILNER et al., 1995; MARSH et al., 2001). ATC outcrops liyng further north (Ourinhos subtype), which are not part of the scope of this study, were, hitherto, the only silicic units observed laying directly on the Botucatu sandstones (e.g. NARDY et al., 2002; JANASI et al., 2007). However, ATP units (Jacuí subtype) in the southern region, near the Sobradinho town, were found overlapping a 40 m thick Botucatu sandstone (columnar sections SO2 and SO3, Fig. 3c), which in turn was intruded at the base by an ATP body (Jacuí subtype?), forming peperite. Peperites were also broadly found in the base and interlayered in the silicic volcanic succession in both areas (LUCHETTI et al., 2014), highlighting periods of quiescence between the basic and silicic volcanism and the silicic eruptions. 32 33 Fig. 3 continued. 34 Fig. 3 continued. 35 Fig. 3 continued. Fig. 3 - Columnar sections showing silicic stratigraphic subdivisions and major characteristics of the deposits. a-a’ = columnar sections of rheoignimbrite deposits from northern region; b = chemical-stratigraphic sections from southern region, of which some ‘key sections’ were chosen to represent the major features of the deposits (c-c’). Dashed lines (in gray) are probable stratigraphic correlations. Location of sections in Fig. 1. 36 4.2 Silicic Lava Domes Silicic lava domes are the early products of the Caxias do Sul, Jacuí and Santa Maria silicic sequences and the Anita Garibaldi unit in the southern region. (Fig. 3c-c’). They form clusters of staked outcrops with each thicknesses ranging from 90 to 10 m of pitchstones with vesicular outer layer and/or wrapped by matrix-supported autobreccias (Fig. 4a-b). Autobreccias comprise mainly angular to rounded, vesicular to dense clasts of the same composition with sizes ranging from large blocks (centimetric) to smaller clasts (millimetric). They are altered to beige and light-dark green colors and show perlitic fractures. Vesicles in clasts are spherical to elongated and stretched, many filled by quartz or silica minerals. Basal breccias can also have matrix made up of unsorted fine-grained sediment in places representing peperites (LUCHETTI et al., 2014). Joints are poorly developed in the pitchstones which show a massive structure and conchoidal fracture. However, devitrified domains display conspicuous horizontal to platy joints, with spacing between them increasing inward (Fig. 4c). They can be tightly curved (concentric joints) in places, mainly in the outer parts of small lobes. Thicker units consist of massive pitchstone interior surrounded by horizontal-to-curved sheeting jointed devitrified rock with usually altered, brecciated in places, glassy external parts. Small lobate shapes are also common with massive/glassy interior and altered or oxidized, vesicular to devitrified exterior. At outcrops further west, near the Sobradinho town, lava domes directly overlap ~35 m thick aeolian sandstones (paleo-dune) of the Botucatu Formation, which in turn is intruded at its base by a silicic body forming peperite in places (Fig. 4d). The vesicles-amigdales in the volcanic clasts forming the peperite are upward oriented and the sandstone matrix contain vesicles, indicating it was wet. The coarser and thoroughly crystalline groundmass, compared to upper glassy lava domes, also point to a slower cooling due to shallow intrusion setting. 37 Fig. 4 – Lava dome features in ATP rocks. (a) Pitchstone (black interior) with brecciated base and external part in Caxias do Sul unit; (b) Santa Maria outcrop showing pitchstone with brecciated exterior (white dashed line) in contatct with overlying ignimbrite (yellow dashed line). (c) Curved sheeting to platy joints in a Jacuí lava dome; (d) peperite formed in the top of a Jacuí unit intruded in a sandstone from Botucatu Formation. Vesicles in the volcanic clasts (V) are upward oriented and the sandstone matrix also contain vesicles. 4.3 Extensive Sheets - Rheoignimbrites Extensive sheets occur in the central region (Fig. 3a-a’) and around and above lava domes and basalts in the southern region (Fig. 3c-c’). As previously mentioned, Santa Maria subtype sheets seems to be the youngest manifestations, overlying all other silicic units and basalts in the western region, with a package thickness of up to 220 m, whereas Caxias do Sul and Jacuí subtype sheets reach 160 m and 130 m thickness, respectively. In the central region, between Guarapuava and Palmas towns, Clevelândia subtype sheets totalling 120 m thick, underly Guarapuava subtype sheets (ATC), which in turn reach 220 m thickness. These extensive sheets form flat-topped terraces with a constructional architecture comprised of multiple sub-parallel, tabular packages. Total package thicknesses for ATP sheets range from ~30 to 160 m for Caxias do Sul subtype, 70 to 220 m for Santa Maria subtype and 130 m for Jacuí subtype in the southern region, and 120 m for Clevelândia subtype in the central region. 38 However these silicic units must have been much thicker, since the top was eroded and the base is not exposed in central region. They can be traced laterally for tens to hundreds of kilometers, thickening locally in small valleys and thinning to less than 2 m across topographic highs. These make estimation of eruption volumes difficult and, as the sources areas are unknown, thicknesses and extents estimation of proximal-distal ignimbrite is almost impossible. A conspicuous feature in these sheets are the horizontal to subhorizontal jointing ranging in spacing from a few centimetres, in which the term ‘sheeting joints’ is appropriate, to a few metres and resulting in flaggy outcrops in the field (Fig. 5a). Vertical joints are also present in places with a few to several metres spacing. Sheets comprise largely massive, structureless rock or with a horizontally (to sub-horizontal) continuous diffuse layering marked by the alternation of microcrystalline quartz- feldspathic and spherulitic layers. Layer thicknesses ranges from millimeters to several centimeters with small-scale isoclinal folds in places (Fig. 5b), and parallel or sub- parallel jointing. Massive packets can occur interfingered with altered and/or weathered portions, indicating local variations in welding and multiple flow units (Fig. 5c). 39 Fig. 5 – Common aspect of the extensive sheets, in especially Santa Maria unit, displaying massive horizontal jointed outcrops (a), flow banding-folding in places (b), and massive packets interfingered with altered and/or weathered portions, indicating local variations in welding (c); In (b) joints are sub-horizontal and the flow banding horizontal. Basal autobreccias or lithic breccias are absent, with rare local breccias formed by the interaction with sediment, characterizing peperitic breccias or peperites. Localized black vitrophyre zones occur in the bottom and tops of Caxias do Sul and Santa Maria units, in most cases being completely homogeneous. An exception is an apparent vitrophyre in the bottom of Caxias do Sul sheet with a lamination marked by millimetre-scale vesicles filled by quartz and reddish lenses (Fig. 6a) which, in thin section, show a thoroughly devitrified-recrystallized groundmass. Glassy dark brown lenses-laminae show undulating margins and feathery terminations evidencing fiamme, however its outlines are diffuse and locally overprinted by the devitrification (Fig 7a-b). The sheets also show varying abundance of spherical to ellipsoidal vesicles and amigdales filled by silica minerals, with sizes ranging from millimeter to centimeter. 40 Fig. 6 – (a) Extremely flattened-stretched glassy reddish lenses and vesicles filled by quartz in apparent vitrophyre in the bottom of Caxias do Sul sheet; Estwing hammer cable as scale. (b-d) Oxidized Santa Maria outcrop with flattened-stretched juvenile clasts similar to fiamme (b); just above a lava-like lithofacies with flow banding and folding (c); and a granitic lithic clast (d) (dashed outline). Vitroclastic texture was observed in a Santa Maria outcrop. The rock is reddened by oxidation, flat jointed in places and consists of flattened to extremely flattened juvenile clasts resembling collapsed pumices (fiamme) (Fig 6b). In thin section the matrix comprises plagioclase and Fe-Ti oxide phenocrysts, numerous small plagioclase- pyroxene laths and stretched to extremely stretched glass shards (Fig. 7c-e). The latter are deformed around the crystals and the Y-shape is still exhibited by some shards (Fig. 7d). The rock becomes strongly flow banding-laminated and folded in places (Fig. 6c), with millimeter-to-centimeter scale band thicknesses, glass shards are no longer recognizable and the crystals laths are orientend parallel to flow layering (Fig. 7f). A homogeneous vitrophyre, ~10 m thick, lies beneath this sheet and may represent the basal vitrophyre. For the rest, the sheet is flat jointed and massive. Joint spacing commonly coincide with the banding, but it can be sub-parallel in places. 41 Fig. 7 – Photomicrographs of textures in the Caxias do Sul apparent vitrophyre (a-b) and Santa Maria outcrop (c-f) shown in Fig. 6. (a-b) extremely flattened, deformed by phenocrysts dark lenses in a quartz-feldspathic mosaic matrix. Lens outlines become quite diffuse in places overprinted by the devitrification; (a) = // pol. and (b) = X pol.; (c) oxidized groundmass overall aspect showing plagioclase-opaque microphenocrysts to microlites and vitroclastic texture with Y-shaped (d) (arrows) to extremely flattened-stretched, deflated by plagioclase laths glass shards (e) (arrows); (f) extremely flattened-stretched juvenile clasts ranging in devitrification (note the spherulite formed from the clast rim) in an entirely homogenized groundmass, with crystal laths oriented parallel to flow, characterizing a lava-like lithofacies; // pol. Lithic fragments are rare, but where found they are mostly basalt-andesite or own silicic volcanics and more rarely granite (Fig. 6d), with angular to rounded shapes. The Santa Maria subtype seems to be composed of multiple flows and shows a remarkable vertical and lateral zoning of phenocryst abundance in the whole package. Lower parts 42 can consist of up to 3% of phenocrysts, with the top and westward being virtually phenocrysts-free. An upward decrease in phenocrysts abundance was also seen in a single outcrop. Although some phenocrysts show some breakage, it is not widespread, even in the ATC rocks bearing large plagioclase phenocrysts, and it can be said that broken phenocrysts are rare compared with typical ignimbrites. Groundmass is thoroughly devitrified to recrystallized showing spherulites into crypto-to-microcrystalline patches, giving a mottled or salt and pepper texture to the rock, to an entirely quartz- feldspathic microcrystalline texture. ATC and Caxias do Sul sheets (ATP) show a characteristic eutaxitic texture marked by dark lens-shaped blobs set in a light matrix (Fig. 8). In basal sections of thick outcrops blobs are extremely flattened and/or stretched into the millimetre-scale laminations. Spherical to elongated vesicles are scattered throughout the sheet with some blobs deflated around them. Differential alteration can form ‘blocky’ outcrops highlighting lensoidal blocks and lumps (Fig. 8c). Where extremely flat, joint (sheeting) spacing can coincide with the layering. Some vertical fractures found in outcrops displaying this banding may indicate pipe-like gas escape structures. Fig. 8 – Flattened and stretched glass blobs in Caxias do Sul (a-c) and Guarapuava (d-e) sheets. Note the large detached lens-shape block and lumps in (c). 43 Microscopically the contact between blobs and matrix is diffuse, almost indistinguishable, except for differences in groundmass textures, zonal alteration and crystal size distribution. In hand sample scale the matrix is made up of white dots (or spots) set in dark areas. Under optical microscope, dots are dark brown clouds and interstices comprise crypto-to-microcrystalline patches. SEM analysis show the dark clouds are minute fibres to aggregates forming a feldspathic composition mass and, in samples from outcrops showing less flattened, larger blobs, these dark brown clouds encompasses several small glass globules. White dots are mostly smaller and sparsely distributed in the blobs with a groundmass consisting mainly of crypto-to- microcrystalline patches of quartz-feldspathic composition. Furthermore, these two domains show different distribution patterns for plagioclase-pyroxene-magnetite microlites (≤ 100 µm), for which the blobs display a noticeably larger amount in SEM images. For phenocrysts, subtle differences in distribution pattern is better noted in the ATC porphyritic rocks in which blobs show larger amount of crystals, including larger crystal clusters. The Jacuí unit is the least extensive, outcropping for ~25 km long from a likely source, where an intrusive body was found (columnar sections SO2 and 3). It comprises a massive and devitrified rock and the main feature is a ubiquitous horizontal to subhorizontal jointing. 4.4 ‘Enigmatic Flow Banded’ Rocks In the eastern region, where Caxias do Sul subgroup rocks dominate, some pitchstones from the base of the lava domes sequence or underlying extensive silicic sheets show sudden flow banding-lamination. It is characterized by millimeter width and several centimeters long, flattened, stretched oxidized reddish lenses and vesicles filled by quartz in a dark fine-grained microcrystalline rock. Upward the banding is like that in lava flows, with alternation between reddish spherulitic flow bands, with varying thicknesses (Fig. 9a) and internally brecciated in places, and microcrystalline dark rock. It becomes steeply dipping with ramp structures, folding, magma mingling-like 44 structures (Fig. 9b) and red vesicular packets and/or clasts. In a quarry nearby Caxias do Sul town this steeply dipping and contorted banding is quite evident (Fig. 9e) and Lima et al. (2012) have proposed it being a feeder dike-like source area. Another interesting feature observed in the quarry is another type of flow banding-lamination comprising dark thin layers in a light matrix (Fig. 9c-d), including basalt-andesite clasts or enclaves with up to ~3.5 cm long, embayed outlines and rotated (Fig. 9d). SEM observations coupled to EDS analyzes show three different domains characterized by: light layers = (1) coarser plagioclase microlites; (2) fine-grained alkali-feldspar microlites; and dark layers = (3) quartz+alkali-feldspar microcrystalline intergowth, with all domains containing pyroxene and Fe-Ti oxides microlites. This may reflect a compositional layering between basic-intermediate and silicic magmas. Both flow banding-laminations comprise, folding, refolding, swirls, and magma mingling-like texture. The dark microcrystalline rock also occurs as massive and internally structureless domains or as centimeter size-scale welded lens-shaped clasts with tapered extremities interlayered or interfingered in the flow banded rock (Fig. 9c), and similar to the glass blobs previously described. Scattered spherulites and spherical vesicles filled by quartz of up to ~5 mm in diameter give a mottled appearance to the rock. Furthermore some giant vesicles of up to half a meter long also occur through the quarry and they can be elongated and/or deformed parallel to the flow in places (Fig. 9e). 45 Fig. 9 – Aspects of the flow banded ‘fluidal’ unit, displaying oxidized red bands and stretched lenses filled by quartz (a) in dark devitrified rock; mingling-like structure (b); coins have 2.5 cm; portion formed by glass blobs (spatters?) (dashed outline) interlayerd in compositionally- granulometrically flow banded material (c); (d) polished slab showing flow lamination between dark crystalline material and light material comprising plagioclase and k-feldspar microlites domains, and (rotated) basalt clasts; (e) giant vesicle (ballon shape) in complex steeply dipping flow banded rock in the quarry wall; scale card is 8.5 cm long. 46 5 PETROGRAPHY ATC type rocks from this study are porphyritic to sparsely porphyritic with up to ~20% of plagioclase (An30-49), pyroxene (augite and pigeonite), Fe-Ti oxides (Ti- magnetite and ilmenite) and apatite phenocrysts (Figures 8 and 9). Euhedral to sub- rounded plagioclase crystals can reach up to 1 cm in length (major axis) and compose up to 15% of the rock. Augite and pigeonite crystals are subhedral to rounded and make up ~5%, rarely exceeding 2 mm long. Ti-magnetite and ilmenite comprise Fe-Ti oxides, not exceeding 2% of the rock. Apatite amounts to less than 1% and long crystals may reach 0.5 mm in length. The groundmass contains plagioclase, pyroxene and Fe-Ti oxide microlites with tabular habit to hollow, dendritic and swallowtail shapes and is thoroughly devitrified-recrystallized. It shows brown spherulitic patches are surrounded by microcrystalline material (quartz-feldspar intergrows) or entirely quartz-feldspar intergrown acquiring a microgranophyric texture. ATP type rocks are sparsely porphyritic to aphyric with up to ~4% of plagioclase (An41-74) (prevailing), pyroxene (orthopyroxene, augite and pigeonite) and Fe-Ti oxide pheno-microphenocrysts. Plagioclase crystals make up to 3% of the rock usually does not exceeding 1.5 mm of length, except some reaching 3 mm long. They are euhedral- subhedral to rounded and can show embayment, corroded edges and coarse sieve. Few crystals in a Caxias do Sul pitchstone are shattered and rotated, similar to “phenoclast” textures described by Best and Christiansen (1997) originated by bursting melt inclusions decompressed during explosive eruption. Pyroxene crystals represent up to ~1% of the rock, occurring only in the groundmass in the Santa Maria unit. They rarely exceed 1 mm in length, except for some long orthopyroxenes reaching more than 4 mm in length, and show subhedral to extremely rounded shapes, reacted or corroded edges and partial to complete alteration to brown or green material. Fe-Ti oxides make up less than 1% and are primarily Ti-magnetite with rare ilmenite thin lamellae. Apatite occurs as minute quantities in the groundmass. The Clevelândia and Anita units and some Santa Maria sheets are virtually phenocryst-free. Groundmass is glassy to thoroughly devitrified groundmass with a broad range in microlite densities of plagioclase, pyroxene and Fe-Ti oxides. Devitrified samples groundmass show brown spherulitic patches surrounded by microcrystalline quartz- 47 feldspar intergrows to microgranophyric texture throughout. Microlite shapes comprise tabular (laths) to hollow, dendritic, acicular and swallowtail ends for plagioclase and irregular to acicular (up to 4 mm long) for pyroxene. Furthermore crystal clusters (or “glomerocrysts”) are a common feature in both ATC and ATP rocks consisting of plagioclase, pyroxene, plagioclase+pyroxene and plagioclase+pyroxene+Fe-Ti oxides assemblages. 48 6 DISCUSSION In recent years it has been discussed more often the emplacement mode of the Paraná Magmatic Province silicic volcanics. Works by Whittingham (1989), in the South Brazil, and Milner et al. (1992) and Ewart et al. (1998), in the north-western Namibia, agree with a pyroclastic origin for the ATP rocks and related rocks in Namibia (Etendeka Formation) respectively. According to these authors, as such rocks show features typical of lava (e.g. flow banding, unbroken crystals, local breccias) and, at the same time, remaining glass shards and fiamme from a non welded unit, as well globular texture (fused shards) (MILNER et al., 1992), they have been emplaced as rheoignimbrites. Siviero et al. (2005) also interpreted some ATP units from South Brazil as ignimbrites, and for Arioli et al. (2008) ATC rocks from Guarapuava region are rheomorphic ignimbrites. On the other hand other researchers argue for an effusive emplacement mode for the silicic volcanics. Despite Garland et al. (1995) report on pyroclastic textures preserved in a thin layer and crystal segregation in ATP flows, owing the lack of caldera structures and sparsity of pyroclastic textures, the authors did not rule out an effusive way for ATP rocks. For Chapecó type magma, they proposed a generation in mid-lower crustal depths (5-15 Km), rapid ascent via dykes and the spreading on the surface as lava flows. Umann et al. (2001), Waichel et al. (2012) and Lima et al. (2012) suggest the Caxias do Sul units from the eastern border of PMP comprise lava domes and tabular lavas with similar interpretation for the silicic units in the western region by Liza & Janasi (2014). Extensive ATC and ATP units are interpreted here as high to extremely high grade ignimbrites originated from high mass-flux pyroclastic fountaining eruptions (BRANNEY & KOKELAAR, 1992, 2002), possibly via fissures. The problem in distinguishing between lava and ignimbrite is the lack or non- preservation of primary pyroclastic textures such as pumice lapilli, fiamme, glass shards, broken phenocrysts and lithic fragments. According to Henry and Wolf (1992) base and/or top of units are the best parts for this recognition, since lavas contain thick upper and basal autobreccias (BONNICHSEN & KAUFFMAN, 1987) which are uncommon in ignimbrites. Likewise pyroclastic features would be preserved in these 49 parts due to quenching (HENRY WOLF, 1992). However these are often hidden, weathered or have been eroded. Furthermore pyroclastic deposits can be intensely welded from its bottom to top (ROSS & SMITH, 1961) or even have been undergone to rheomorphism, wherein explosive features can be totally destroyed and the terms ‘high to extremely high grade’ or ‘lava-like’ are applied (BRANNEY et al., 1992; BRANNEY AND KOKELAAR, 1992; 2002). Therefore, the lack of primary pyroclastic textures does not always means a lava flow origin. Many other works exist in the literature on the subtle differences and complexities between lavas and lava-like ignimbrites, in terms of their eruption, deposition, welding and rheomorphism, and field characteristics (e.g. SCHMINCKE & SWANSON, 1967; WOLFF AND WRIGHT, 1981; Walker 1983; HENRY et al., 1988; BRANNEY et al., 2008; FREUNDT & SCHMINCKE, 1995; FREUNDT, 1998; KOBBERGER & SCHMINCKE, 1999; SUMNER & BRANNEY, 2002). Unit extents are also a striking feature, with ignimbrites reaching hundreds of km in lenght and extensive silicic lavas little more than a few tens of km (BONNICHSEN & KAUFFMAN, 1987; BRANNEY et al., 2008). The units show widespread sheet-like geometry, with lateral extents > 40 km, and problably ≥ 100 km for some sheets, areal extents up to ~16,000 km2 (not considering individual units in the Etendeka reaching up to 8,800 km2, Milner et al., 1992). In the southern region the topography where lava domes crop out is characterized by rounded-convex hills (Fig. 10a), whereas extensive plateaus dominate wide areas in the same region and in the central region (Fig. 10b), where extensive sheet deposits occur. Fig. 10 - Different landscapes from areas where silicic lava domes crop out (a) vs flatten relief (b) of extensive sheet-like deposits. Surface irregularities (hillocks) in (b) would be due to differences in welding. 50 The absence of both basal and top autobreccias is also a strong indication for an emplacement by pyroclastic density currents rather than viscous lava flows. Main examples of extensive silicic lava flows described in the literature show widespread basal autobreccias (Snake River Plain – BONNICHSEN & KAUFFMAN, 1987; Bracks Rhyolite, Trans-Pecos Texas – HENRY et al., 1990; Gawler Range Province - ALLEN et al., 2008). Furthermore the vertical and/or horizontal zoning of phenocryst abundance in the Santa Maria sheets is an indication of transport and deposition regimes from granular fluid-based pyroclastic density currents (BRANNEY & KOKELAAR, 2002; BRANNEY et al., 2008). Glass shards, fiamme, lithic fragments or broken phenocrysts are rare or not observed, except for some localized layers with fiamme texture and/or flattened to stretched shards described in this work and other pyroclastic features reported by Milner et al. (1992) and Garland et al. (1995). However all or nearly all remaining vestiges of shard and fiamme texture may have been pervasively transposed and obliterated by the welding, rheomorphism and subsequent devitrification- recrystallization. Less welded to non welded lithofacies are wheathered or have been eroded, since the exhumation is estimated in more than 1 km by apatite fission track (GALLAGHER et al., 1994). The lack of fiamme or pumice lapilli and lithic lapilli is a common characteristic of many rheoignimbrites from Snake River Plain, in which lava- like lithofacies comprise most part of the sheets (BONNICHSEN & CITRON, 1982; BRANNEY et al., 2008), revealing originally true tuffs (ANDREWS et al., 2008; BRANNEY et al., 2008). However, another feature shown by these rheoignimbrites is the presence of extensive, parallel-laminated, medium to coarse-grained ashfall deposits with large cuspate shards, reflecting high explosive eruptions (VEI 6-8) and therefore pointing to a peculiarity in the fragmentation process, not understood, with possible influence of meteoric water (BRANNEY et al., 2008). Unfortunately ashfall deposits have not been found or recognized as such in the PMP volcanism. Further the scarcity of lithic fragments and unbroken phenocrysts in extremely high-grade ignimbrites elsewhere indicate low explosivity eruptions from relatively hot, fluid and gas poor magmas (HENRY & WOLF, 1992). The flow layering-lamination may have been established from healed fractures, 51 former vesicular zones and/or other textural heterogeneities originated in the surface in the coalesced flows after the end of primary flowage. Such structures developed parallel planes along which the ductile or ruptile shear can occur (BONNICHSEN & CITRON, 1982; PIOLI & ROSI, 2005; BRANNEY et al., 2008). Therefore these layers become planes of weakness along which the joints would form parallel during or after the devitrification. Low angle joints also occur sub-parallel to the flow banding probably developed during static devitrification after the anisotropy of the flow (BRANNEY et al., 2008). Completely homogeneous vitrophyres suggest extreme conditions of welding (ROSS & SMITH, 1961) or coalescence and ponding of glass particles to a viscous mass with nowhere to flow (BONNICHSEN & CITRON, 1982; BRANNEY & KOKELAAR, 1992), consistent with a likely flat surface at the time the sheets were emplaced, consisting of small valleys or basins set by previous basalt flows and silicic lava domes. Moreover the flat topography seems to have played a key role in the bulk deposition, since pervasive secondary flowage structures, commonly seen in many rheomorphic ignimbrites and near absent in the present rocks, are more easily originated in adownslope mass flowage moved downslope (BONNICHSEN & CITRON, 1982; PIOLI & ROSSI, 2005). The eutaxitic texture defined by glass blobs in ATC and Caxias do Sul rocks also suggest these units are ‘hybrids’ and not exactly similar to conventional ignimbrites. As the fragmentation process is not as intense, the phenocrysts size distribution in blobs and matrix may indeed be similar to subtly different, as in ATP specimens. On the other hand a certain difference in the phenocryst size distribution in porphyritic ATC samples and a remarkable difference in the small crystals abundance in both ATC and ATP samples may be explained by the transport in a pyroclastic density current where they would be more easily transported, occurring in smaller quantities in the matrix. Numerous glass blobs are also described in the Pagosa Peak Dacite, interpreted as a voluminous lava-like ignimbrite which preceded a major ash-flow tuff, the Fish Canyon Tuff (BACHMANN et al., 2000). Moreover, despite the advanced devitrification coupled with alteration, the globule-shaped glass may express relict globular shards similar to those found in other rheomorphic ignimbrites (e.g. JOHNSON 1968; SUMNER & BRANNEY, 2002; BRANNEY et al., 2008) or glassy particles coalescence texture 52 reported for high grade Nuraxi Tuff by Gimeno et al., 2003, or even similar to those obtained from welding experiments for the rhyolitic Rattlesnake Tuff by Grunder et al., 2005. The final appearance of the sheet, with horizontally fairly continuous banding downward, resulted from an end-stage compaction followed by sheeting-joint formation, as the pattern typical of some is to outline dark lenses. Temperatures estimated from phencrysts for these magmas are too high, ~ 950°C to > 1000°C (e.g. BELLIENI et al., 1986; GARLAND et al., 1995; NARDY et al., 2011; this study), even the primarily aphyric character of some units indicates liquidus temperature. Such a high temperature would be responsible for the decrease in viscosity and the resulting particle agglutination (BRANNEY et al., 1992; BRANNEY & KOKELAAR, 1992; 2002). The water content is also low, up to 2wt % maximum (e.g. NARDY et al., 2011; this study), consistent with the anhydrous mineralogy. Such combination of high T and low gas content and, therefore low gas velocity, led to low collapse heights and little heat loss during collapse, prerequisites for producing highly welded tuffs (e.g. SPARKS et al., 1978). Nevertheless, some sheets are still ambiguous, such as the least extensive Jacuí unit. It may represent an extensive silicic lava flow, but more detailed studies in this unit are required in order to find pumiceous and/or brecciated carapaces. In the central region, the base of the lower Clevelândia sheet is buried and its top is extremely altered, but a vesicular-amigdaloidal carapace with several geodes in places is recognizable and it may indicate a lava flow origin for this sheet. However this pumiceous carapace crop outs along at least 30 km of a highway. 6.1 ‘Enigmatic Flow Banded’ Rocks The flow banding observed in some Caxias do Sul outcrops and in a quarry are typical of lava flows. However such banding can also form in lava-like rheomorphic ignimbrites (e.g. SUMMER & BRANNEY, 2002) and fountain-fed lava flows (e.g. STEVENSON et a., 1993). Furthermore lensoidal glass fragments or blobs occurring above and interdigited in the flow banded rock may represent spatter-like structures. Spatter-rich deposits of non-basaltic composition are reported in the literature, 53 evidencing its transport in pyroclastic flow deposits (e.g. MELLORS & SPARKS, 1991; FURUKAWA & KAMATA, 2004), fallout deposits (e.g. SORIANO et al., 2002) and fountain-fed lava flows (e.g. STEVENSON et al., 1993; STEVENSON & WILSON, 97; FURUKAWA & KAMATA, 2004). Giant vesicles seem to be concentrated in the quarry area, since they were not observed elsewhere, and such structures to much larger ones are also described in a rheomorphic vent fill from Canary Islands (SORIANO et al., 2009). Nevertheless, distinguishing between lava-like rheomorphic ignimbrites and fountain-fed lava flows is almost impossible, since clasts (spatters) outlines can be completely erased resulting in a massive, homogenized rock (STEVENSON et a., 1993), and fountain-fed lava flows can be an end-member of ignimbrite-grade continuum (BRANNEY & KOKELAAR, 1992). Thus, an emplacement mode as lava-like rheomorphic or fountain-fed lava flows must be considered for these banded units, and therefore, more detailed studies are required in order to better define the contact relationships, reconstruct source areas and establish its emplacement mechanisms. 6.2 Tectonic Alignments and Source Areas Another problematic factor in volcanological studies on the PMP silicic volcanics is its unknown sources locations. However silicic lava domes typically do not flow more than a few kilometers, and even coulées do not travel more than 10 km (CAS & WRIGHT, 1988; FINK AND ANDERSON, 2000; FRANCIS AND OPPENHEIMER, 2004). For this reason, lava domes are good indicators of feeder sources location or at least of its proximity and aligned outcrops can indicate fissure feeder. Therefore, in the southern region, lava dome outcrops can indicate the approximate location of source areas, and they seem to follow an alignment pattern parallel to the coastline, including ‘enigmatic flow banded’ outcrops (Fig. 11). This alignment is also similar to the northeast-trending linear feature defined by the Damaraland intrusive complexes in the South Etendeka, Namibia (e.g. MILNER & LE ROEX, 1996), of which the Messum Igneous Complex is regarded being the eruptive centre for the Goboboseb and Springbok quartz latites (MILNER et al., 1995; EWART et al., 1998). This reflects the extensional tectonic setting which resulted in the Gondwana continent breakup 10 Ma later. 54 Fig. 11 –Pre-drift sketch of the Paraná Magmatic Province (PMP), modified from Milner el al. (1995b), Stewart el al. (1996), Peate (1997) e Nardy et al. (2002), showing ATP lava dome and ‘flow banded-fluidal’ outcrops in the southern region indicating possible sources areas. These lava domes also show a distribution parallel to the coastline, similar to the alignment of ATC outcrops. 55 9 CONCLUSIONS Silicic volcanism in the Paraná Magmatic Province (PMP) is characterized by lava domes and high to extremely high grade ignimbrites. Lava domes are found in the southern region of PMP, where dacites, rhyodacites and rhyolites of Palmas type (ATP) crop out, and are characterized mainly by massive pitchstones with an usually altered, vesicular to brecciated carapace, or massive pichstone interior wrapped by concentrically jointed, devitrified rock. A silicic unit intruding in sandstone was also observed in the same area where lava domes appear. Its outcrops indicate nearby source areas and they form a trend parallel to the coast line, similar to those of ATC outcrops and the Damaraland intrusive complexes in the South Etendeka, reflecting the extensional tectonic setting which resulted in the Gondwana continent breakup 10 Ma later. High to extremely high grade large volume ignimbrites comprise trachydacites to dacites of Chapecó (ATC) type and ATP rhyolites in the central region of PMP, where they occur interlayered in the basalt sequence with ATC sheets overlying ATP rocks in places; and ATP dacites to rhyolites overlying lava domes and basalts in the southern region. Such a scenario provides the silicic volcanism in the southern region evolved from effusive small volumes to explosive large volume flows. The characteristic features of these high to extremely high grade large volume ignimbrites are: (1) sheet-like geometry, with lateral extents > 40 km, and problably ≥ 100 km for some sheets, areal extents up to ~16,000 km2. However if we consider sheets from Etendeka, Namibia, these areal extents are even much higher (individual units in the Etendeka reach up to 8,800 km2, Milner et al., 1992); (2) Landscapes dominated by extensive flat-topped terraces emphasizing the sheet-like geometry; (3) Absence of both basal and top autobreccias, as well as basal pumiceous layers; (4) Ubiquitous horizontal to subhorizontal joints ranging in thickness from a few centimetres, (sheeting joints) to a few metres, resulting in flaggy outcrops in the field. Vertical joints are also present; (5) Massive to horizontally banded-laminated rock with flow folding in places; 56 (5) Completely homogeneous vitrophyres with some flow banding-lamination in places; (6) Vertical and horizontal zoning of phenocryst abundance, with upward and lateral decrease in abundance; (7) welded glass blobs characterizing eutaxitic texture;