Abstracts

INTRODUCTION

Large Igneous Provinces (LIPs) are a continuum of voluminous Fe and Mg rich rock emplacements, which include continental flood basalts (CFB) and associated intrusive rocks, volcanic passive margins, oceanic plateaus, submarine ridges, seamount groups, and ocean basin flood basalts (Coffin & Eldholm 1994Coffin M.F., Eldholm O. 1994. Large Igneous Provinces: crustal structure, dimensions and external consequences. Reviews of Geophysics, 32:1-36.). The CFBs are individual volcanic constructs that are laterally extensive (several hundred km²⁾ and thick (about 1 km average), representing the eruption of enormous volumes of mantle-derived magma in relatively short-time periods (few million years) generally of tholeiitic composition (Coffin & Eldholm 1992Coffin M.F. & Eldholm O. 1992. Volcanism and continental break-up: a global compilation of large igneous provinces. In: Storey B.C., Alabaster T., Pankhurst R.J. (eds.) Magmatism and the Causes of Continental Breakup. London, Special Publication of the Geological Society of London, p. 17-30.; Sheth 2007Sheth H. 2007. Large Igneous Provinces (LIPs): definition, recom-mended terminology, and a hierarchical classification. Earth Science Reviews, 85:117-124.).

Paraná-Etendeka CFB occupies, approximately, an 1,3 x 10⁶ km² area, in which 90% are located in South America, above the aeolian sedimentary rocks of Botucatu Formation in the Paraná Basin. The stratigraphic reference to this part of the Paraná-Etendeka CFB is defined as Serra Geral Formation (SGF), which is a tholeiitic volcanic rock association, 1,700 m thick, with a great proportion of basic and less representative intermediate to acidic volcanic rocks (Melfi et al. 1988Melfi A.J., Nardy A.J.R., Piccirillo E.M. 1988. Geological and magmatic aspects of the Paraná Basin: An introduction. In: Piccirillo E.M. & Melfi A.J. (eds.) The Mesozoic flood volcanism of the Paraná Basin: Petrogenetic and geophysical aspects. IAG-USP, p. 1-13.).

Waichel et al. (2012)Waichel B.L., Lima E.F., Viana A.R., Scherer M.S., Bueno G.V., Dutra G.T. 2012. Stratigraphy and volcanic facies architecture of the Torres Syncline, Southern Brazil, and its role in understanding the Paraná-Etendeka Continental Flood Basalt Province. Journal of Volcanology and Geothermal Research, 215:74-82. described the facies architecture of SGF in the Torres Syncline region, identifying five distinct volcanic episodes: Basic Volcanic Episode I (BVE I) that covers Botucatu paleoerg with compound pahoehoe flows; BVE II representing the volcanic climax and composed of ~ 500 m tabular-classic simple pahoehoe flows; Acidic Volcanic Episode I (AVE I), which is composed of acidic lava dome-field facies association; BVE III that comprises 'a'a flows with tabular/lobate escoriaceous facies association; and AVE II represented by acidic tabular facies. Recently, detailed studies of basic lavas from the BVE III identified a four-part structure with internal characteristics typical of rubbly pahoehoe lavas (Rossetti et al. 2014Rossetti L.M.M., Lima E.F., Waichel B.L., Scherer C.M.S., Barreto C.J. 2014. Stratigraphical framework of basaltic lavas in Torres Syncline main valley, Southern Paraná-Etendeka Volcanic Province. Journal of South American Earth Sciences, 56:409-421.).

The acidic volcanism of Paraná-Etendeka CFB has been largely studied regarding petrographic and geochemical aspects. It is divided into two main groups: Palmas and Chapecó types (Bellieni et al. 1986Bellieni G., Comin-Chiaramonti P., Marques L.S., Melfi A.J., Nardy A.J.R., Papatrechas C., Piccirilo E.M., Roisenberg A. 1986. Petrogenetic aspects of acid and basaltic lavas from Paraná Basin (Brazil): geological, mineralogical and petrochemical relationships. Journal of Petrology, 27:915-944; Peate 1997Peate D.W. 1997. The Paraná-Etendeka Province. In: Mahoney J.J., Coffin M.F. (eds.) Large igneous provinces: continental, oceanic and planetary flood volcanism. Geophysics Monography Series, 100, p. 217-245.). Palmas type rocks are aphyric low-Ti dacites and rhyolites. The dacites are subdivided into Caxias do Sul (0.91 wt.%<TiO₂<1.03 wt.% and 0.25%<P₂O₅<0.28 wt.%), Anita Garibaldi (1.06 wt.%<TiO₂<1.25 wt.% and 0.32 wt.%<P₂O₅<0.36 wt.%), and Jacuí (1.05 wt.%<TiO₂<1.16 wt.% and 0.28 wt.%<P₂O₅<0.31 wt.%) sub-types (Nardy et al. 2008Nardy A.J.R., Machado F.B., Oliveira M.A.F. 2008. As rochas vulcânicas mesozoicas ácidas da Bacia do Paraná: litoestratigrafia e considerações geoquímicas-estratigráficas. Revista Brasileira de Geociências, 38(1):178-195.).The rhyolites are subdivided into Santa Maria (P₂O₅≤0.21%) and Clevelândia (0.21 wt.%<P₂O₅≤0.23 wt.%) sub-types (Peate et al. 1992; Nardy et al. 2008Nardy A.J.R., Machado F.B., Oliveira M.A.F. 2008. As rochas vulcânicas mesozoicas ácidas da Bacia do Paraná: litoestratigrafia e considerações geoquímicas-estratigráficas. Revista Brasileira de Geociências, 38(1):178-195.). The Chapecó type are high-Ti porphyritic trachytes with higher Ba, Nb, La, Ce, Zr, P, Nd, Y, Yb, Lu and K and lower Rb, Th and U contents than Palmas type. This magma-type is also divided into Ourinhos, Guarapuava (Peate 1997Peate D.W. 1997. The Paraná-Etendeka Province. In: Mahoney J.J., Coffin M.F. (eds.) Large igneous provinces: continental, oceanic and planetary flood volcanism. Geophysics Monography Series, 100, p. 217-245.), and Tamarana (Nardy et al. 2008Nardy A.J.R., Machado F.B., Oliveira M.A.F. 2008. As rochas vulcânicas mesozoicas ácidas da Bacia do Paraná: litoestratigrafia e considerações geoquímicas-estratigráficas. Revista Brasileira de Geociências, 38(1):178-195.) sub-types.

In the African counterpart of Paraná-Etendeka CFB, a series of acidic magma-types has been described, and the low-Ti quartz latites are represented by Goboboseb, Springbok, Wereldsend, Grootberg, Beacon, Hoas, and Fria types. The high-Ti magma-types are represented by Nil Despeandum, Nadas, Sechomib and Hoarusib latites and also by Sarusas, Ventura, Khoraseb, Naudé and Elliott quartz latites (Marsh et al. 2001Marsh J.S., Ewart A., Milner S.C., Duncan A.R., Miller R. 2001. The Etendeka Igneous Province: magma types and their stratigraphic distribution with implications for the evolution of the Paraná-Etendeka flood basalt province. Bulletin of Volcanology, 62:464-486.). The geochemical equivalences of major and trace elements between the two counterparts of Paraná-Etendeka CFB were recognized as Santa Maria→Fria, Ourinhos→Khoraseb and Guarapuava→Sarusas (Marsh et al. 2001Marsh J.S., Ewart A., Milner S.C., Duncan A.R., Miller R. 2001. The Etendeka Igneous Province: magma types and their stratigraphic distribution with implications for the evolution of the Paraná-Etendeka flood basalt province. Bulletin of Volcanology, 62:464-486.) or Guarapuava→Ventura and Tamarana→Sarusas (Bryan et al. 2010Bryan S.E., Peate I.U., Peate D.W., Self S., Jerram D.A., Mawby M.R., Marsh J.S., Miller J.A. 2010. The largest volcanic eruptions on Earth. Earth-Science Reviews, 102(3-4):207-229.).

The mineral chemistry studies available present anorthite contents of An_63-38 for the plagioclase phenocrysts and micro-phenocrysts of Palmas-type rhyodacites and An_68-63 for resorbed plagioclase grains. Pyroxenes in the acidic units are generally pigeonite with Wo_12-7 with initial Ca increase accompanied by Fe decrease in the late pyroxenes (Bellieni et al. 1984Bellieni G., Brotzu P., Comin-Chiaramonti P., Ernesto M., Melfi A.J., Pacca I.G., Piccirillo E.M., Stolfa D. 1984. Flood basalt to rhyolite suites in the southern Paraná plateau (Brazil): paleomagnetism, petrogenesis and geodynamic implications. Journal of Petrology, 25:579-618.; 1986Bellieni G., Comin-Chiaramonti P., Marques L.S., Melfi A.J., Nardy A.J.R., Papatrechas C., Piccirilo E.M., Roisenberg A. 1986. Petrogenetic aspects of acid and basaltic lavas from Paraná Basin (Brazil): geological, mineralogical and petrochemical relationships. Journal of Petrology, 27:915-944).

Identification of acidic rock strata with lateral extension >> thickness conducted many authors to think of a pyroclastic genesis hypothesis for the silicic volcanic rocks of Parana-Etendeka Province CFB, which could be explained by high-grade welding and formation of rheoignimbrites (Petrini et al. 1989Petrini R., Civetta L., Iacumin P., Longinelli A., Belliene G., Comin-Chiaramonti P., Ernesto N., Marques L.S., Melfi A., Pacca I., Piccirillo E.M. 1989. High temperature flood silicic lavas (?) from the Paraná Basin (Brasil). New Mexico Bureau of Mines & Mineral Resources Bulletin, 131:213.; Whittingham 1989Whittingham A.M. 1989. Geological features and geochemistry of the acid units of the Serra Geral Formation, south Brazil. In: Continental Magmatism, Santa Fé IAVCEI Abstracts: Santa Fé, New Mexico, p. 293.; Roisenberg 1989Roisenberg A. 1989. Petrologia e geoquímica do vulcanismo ácido mesozoico da Província Meridional da Bacia do Paraná. PhD Thesis, Instituto de Geociências, Universidade Federal do Rio Grande do Sul, Porto Alegre, 285 p.; Milner et al. 1992Milner S.C., Duncan A.R., Ewart A. 1992. Quartz latite rheoignimbrite flow of the Etendeka Formation, North-Western Namibia. Bulletin of Volcanology, 54:200-219.; 1995Milner S.C., Duncan A.R., Whittingham A.M., Ewart A. 1995. Trans-Atlantic correlation of eruptive sequences and individual silic volcanic units within Paraná- Etendeka Igneous Province. Journal of Volcanology and Geothermal Research, 69:137-157.; Bryan et al. 2010Bryan S.E., Peate I.U., Peate D.W., Self S., Jerram D.A., Mawby M.R., Marsh J.S., Miller J.A. 2010. The largest volcanic eruptions on Earth. Earth-Science Reviews, 102(3-4):207-229.). The effusive origin for the same rock association was assumed (Bellieni et al. 1986; Comin-Chiaramonti et al. 1988Comin-Chiaramonti P., Bellieni G., Piccirillo E.M., Melfi A.J. 1988. Classification and petrography of continental stratoid volcanic and related intrusive from the Paraná Basin (Brasil). In: Piccirillo E.M., Melfi A.J. (eds) The Mesozoic flood volcanism of the Paraná Basin: petrogenetic and geophysical aspects. São Paulo, Instituto Astronômico e Geofísico, 600 p.; Henry & Wolff 1992Henry C.D. & Wolff J.A. 1992. Distinguishing strongly rheomorphic tuffs from extensive silicic lavas. Bulletin of Volcanology, 54:171-186.; Umann et al. 2001Umann L.V., Lima E.F., Sommer C.A., De Liz J.D. 2001. Vulcanismo ácido da região de Cambará do Sul-RS: litoquímica e discussão sobre a origem dos depósitos. Revista Brasileira de Geociências, 31(3):357-364.; Waichel et al. 2012Waichel B.L., Lima E.F., Viana A.R., Scherer M.S., Bueno G.V., Dutra G.T. 2012. Stratigraphy and volcanic facies architecture of the Torres Syncline, Southern Brazil, and its role in understanding the Paraná-Etendeka Continental Flood Basalt Province. Journal of Volcanology and Geothermal Research, 215:74-82.; Polo & Janasi 2014Polo L.A. & Janasi V.A. 2014. Volcanic stratigraphy of intermediate to acidic rocks in Southern Paraná Magmatic Province, Brazil. Geologia USP Série Científica, 14:83-100.) and the volcanic conduits of lava flows were described in the Northeastern portion of Rio Grande do Sul State (Lima et al. 2012Lima E.F., Philipp R.P., Rizzon G.C., Waichel B.L., Rossetti L.M.M. 2012. Sucessões Vulcânicas e Modelo de Alimentação e Geração de Domos de Lava Ácidos da Formação Serra Geral na Região de São Marcos-Antonio Prado (RS). Geologia USP Série Científica, 12:49-64.).

Paraná-Etendeka CFB acidic rocks are characterized by high crystallization temperatures. In the Etendeka Group quartz-latites, the obtained temperatures range between 995 and 1,025ºC by apatite saturation method (Milner et al. 1992Milner S.C., Duncan A.R., Ewart A. 1992. Quartz latite rheoignimbrite flow of the Etendeka Formation, North-Western Namibia. Bulletin of Volcanology, 54:200-219.). In the Paraná Basin, temperatures obtained by the coexisting pyroxenes method are 1,030 ± 38ºC (Bellieni et al. 1984Bellieni G., Brotzu P., Comin-Chiaramonti P., Ernesto M., Melfi A.J., Pacca I.G., Piccirillo E.M., Stolfa D. 1984. Flood basalt to rhyolite suites in the southern Paraná plateau (Brazil): paleomagnetism, petrogenesis and geodynamic implications. Journal of Petrology, 25:579-618.) and 1,117 ± 17ºC (Bellieni et al. 1986Bellieni G., Comin-Chiaramonti P., Marques L.S., Melfi A.J., Nardy A.J.R., Papatrechas C., Piccirilo E.M., Roisenberg A. 1986. Petrogenetic aspects of acid and basaltic lavas from Paraná Basin (Brazil): geological, mineralogical and petrochemical relationships. Journal of Petrology, 27:915-944) for earlier crystals. In Piraju-Ourinhos region, central part of the Paraná Basin, the application of apatite saturation method yielded temperatures between 980 and 1,010ºC (Janasi et al. 2007). Viscosity studies are available for the Etendeka Group and yielded relatively low viscosity of 10⁶ Pa s for quartz latites (Milner et al. 1992Milner S.C., Duncan A.R., Ewart A. 1992. Quartz latite rheoignimbrite flow of the Etendeka Formation, North-Western Namibia. Bulletin of Volcanology, 54:200-219.).

Viscosity is, consensually, one of the most important properties of the magma rheology. The Arrhenius relation and the conclusion derived from it (e.g. Shaw 1972Shaw H.R. 1972. Viscosities of magmatic silicate liquids: An empirical method of prediction. American Journal of Science, 272:870-893.) are insufficient to describe the viscosity of melts over the entire temperature interval, which are now accessible using new techniques (Vetere 2006Vetere F., Behrens H., Holtz F., Neuville D.R. 2006. Viscosity of andesitic melts - New experimental data and a revised calculation model. Chemical Geology, 228:233-245.). Hence, the evolution of viscometers for silicate liquids has been traced by changing from Arrhenian temperature dependence (e.g. Shaw 1972Shaw H.R. 1972. Viscosities of magmatic silicate liquids: An empirical method of prediction. American Journal of Science, 272:870-893.; Bottinga & Weill 1972Bottinga Y., Weill D. 1972. The viscosity of magmatic silicate liquids: a model for calculation. American Journal of Science, 272:438-475.) to non-Arrhenian models, considering magma composition and volatile effects (e.g. Baker 1996Baker D.R. 1996. Granitic melt viscosities: Empirical and configurational entropy models for their calculation. American Mineralogist, 81:126-134.; Hess & Dingwell 1996Hess K.U. & Dingwell D.B. 1996. Viscosities of hydrous leucogranitic melts: A non-Arrhenian model. American Mineralogist, 81:1297-1300.; Russell et al. 2002Russell J.K., Giordano D., Dingwell D.B., Hess K.U. 2002. Modelling the non-Arrhenian rheology of silicate melts: Numerical considerations. European Journal of Mineralogy, 14:417-427.; Giordano & Dingwell 2003Giordano D. & Dingwell D.B. 2003. Non-Arrhenian multicomponent melt viscosity: A model. Earth and Planetary Science Letters, 208:337-349.; Russell & Giordano 2005Russell J.K. & Giordano D. 2005. A model for silicate melt viscosity in the System CaMgSi2O6-CaAl2Si2O8-NaAlSi3O8. Geochimica et Cosmochimica Acta, 69:5333-5349.; Giordano et al. 2006Giordano D., Mangicapra A., Potuzak M., Russell J.K., Romano C., Dingwell D.B., Di Muro A. 2006. An expanded non-Arrhenian model for silicate melt viscosity: A treatment for metaluminous, peraluminous and peralkaline melts. Chemical Geology, 229:42-56.; Vetere et al. 2006Vetere F., Behrens H., Holtz F., Neuville D.R. 2006. Viscosity of andesitic melts - New experimental data and a revised calculation model. Chemical Geology, 228:233-245.; Hui & Zhang 2007Hui H. & Zhang Y. 2007. Toward a general viscosity equation for natural anhydrous and hydrous silicate melts. Geochimca et Cosmochimica Acta, 71:403-416.; Giordano et al. 2008Giordano D., Russell J.K., Dingwell D.B. 2008. Viscosity of magmatic liquids: A model. Earth and Planetary Science Letters, 271:123-134.).

This work presents a specific study of geochemistry based on thermometry and viscosity estimations together with field features to evaluate the conditions of emplacement and arrangement of facies associations for the Southern portion of SGF in Torres Syncline.

GEOLOGICAL SETTING

The study areas are located in three different sites in the Northeastern part of Rio Grande do Sul State along Torres Syncline (Fig. 1): São Marcos-Antônio Prado Region, RS - 122 road, and RS - 486 road (Rota do Sol).

Figure 1.
Simplified geologic map of Paraná Basin (after Renner 2010).

Lima et al. (2012)Lima E.F., Philipp R.P., Rizzon G.C., Waichel B.L., Rossetti L.M.M. 2012. Sucessões Vulcânicas e Modelo de Alimentação e Geração de Domos de Lava Ácidos da Formação Serra Geral na Região de São Marcos-Antonio Prado (RS). Geologia USP Série Científica, 12:49-64. described the SGF volcanic succession in São Marcos-Antônio Prado region and identified, above the low-TiO₂ basic tholeiitic volcanic flows of BVE II and III, the root of acidic lava dome feeder conduits. These volcanic rocks are rhyodacites of Palmas magma-type and Caxias do Sul sub-type and are characterized by a vertical magmatic flow foliation in positive flower structure, which becomes a flat-lying foliation away from the conduits. In RS - 122 road, between Caxias do Sul and Feliz cities, the stratigraphic framework is composed by pahoehoe flow fields (Unit I) and simple rubbly flows (Unit II). Geochemically, both units are low-TiO₂ tholeiitic basalts and andesi-basalts of Gramado magma-type. Unit I is build up by pahoehoe lava flow fields that cover the sandstones of Botucatu Formation, and the first pahoehoe lavas are primitive olivine basalts with high contents of MgO. These flows occur as sheet and compound pahoehoe, and as ponded lavas in the interdune settings. Unit II is formed by thick simple rubbly lavas, characterized by a massive core and a rubbly top (Rossetti et al. 2014Rossetti L.M.M., Lima E.F., Waichel B.L., Scherer C.M.S., Barreto C.J. 2014. Stratigraphical framework of basaltic lavas in Torres Syncline main valley, Southern Paraná-Etendeka Volcanic Province. Journal of South American Earth Sciences, 56:409-421.). The acidic rocks are lava domes and flows with tabular horizontal jointing classified as Palmas magma-type and Caxias do Sul sub-type and occur above the basaltic rocks in the area.

In the RS-486 road (Rota do Sol) outcrops, the acidic flows happen after BVE I and II compound and simple pahoehoe basic flows from the base to the top of the pile. This part of the volcanic succession is represented by 4 to 15 m thick lava domes with conspicuous spheroidal exfoliation, alternating with resinous aspect vitrophyres (pitchstones) that show several devitrification textures, such as spherulites. The BVE III rubbly pahoehoe basic flows occur superimposed with variable thickness (10 to 20 m). The last volcanic manifestation is represented by acidic tabular flows from AVE II with 15 m thickness, which is characterized by flat-lying jointing (5 to 15 cm spacing) and flow foliation structures associated with different degrees of devitrification (5 to 10 mm).

METHODS

The fieldwork was made in road outcrops and pits and 18 rock samples were selected for the confection of thin sections and geochemical analyses. The whole rock chemical analyses were made at Acme Analytical Laboratories Ltd., in Vancouver, Canada, using analysis routines 4A and 4B. In the first one, total abundance of main oxides and several minor elements was obtained from 0.2 g of the analyzed sample by inductively coupled plasma - emission spectrometry (ICP-ES) with the detection limit of 0.01% for SiO₂, Al₂O₃, Fe₂O₃, CaO, MgO, Na₂O, K₂O, MnO, TiO₂ and P₂O₅. In the second one, the results of rare earth and refractory element were obtained from 0.2 g of analyzed sample by ICP - MS (mass spectrometry) with detection limit of 0.1 ppm for Rb and Zr.

The apatite saturation model of Harrison & Watson (1984)Harrison T.M. & Watson E.B. 1984. The behaviour of apatite during crustal anatexis: equilibrium and kinetic considerations. Geochimica et Cosmochimica Acta, 48(7):1467-1477. was used to estimate liquidus temperatures for the analyzed samples, since apatite is the main accessory phase as microphenocrysts. The model is based on experimental data of apatite solubility as function of magma temperature and composition. We assumed, by definition, the phosphorus partition coefficient as 42 (Watson & Green 1981Watson E.B. & Green T.H. 1981. Apatite/liquid partition coefficients for rare earth elements and strontium. Earth and Planetary Science Letters, 56:405-421.; Prowatke & Klemme 2006Prowatke S. & Klemme S. 2006. Trace element partitioning between apatite and silicate melts. Geochimica et Cosmochimica Acta, 70:4513-4527.) and used wt.% of P₂O₅ as apatite partition in the magma, considering phosphorus as an essential structural constituent (Sun & Hanson 1975Sun S.S. & Hanson G.N. 1975. Origin of Ross Island Basanitoids and limitations upon the heterogeneity of mantle sources for alkali basalts and nephelinites. Contributions to Mineralogy and Petrology, 52:77-106.).

The model used in this work to estimate viscosity (Giordano et al. 2008Giordano D., Russell J.K., Dingwell D.B. 2008. Viscosity of magmatic liquids: A model. Earth and Planetary Science Letters, 271:123-134.) is based on whole-rock chemical compositions and provides Newtonian non-Arrhenian temperature dependence of silicate melts. It was calibrated from more than 1,770 samples of an experimental analysis in a wide range of temperatures and magma compositions. The loss on ignition (LOI) values up to 2.9 wt.% do not substantially affect the final result of the estimated viscosities, and the effective H₂O content is an estimated value. Results of geochemical analysis, calculated temperatures and viscosities are presented in Tabs. 1 and 2.

RESULTS

Field and petrographic background

In a wide scale, the acidic flows in Torres Syncline are described as occurrences of lava dome fields and tabular flows representing two main events (AVE I and AVE II, Waichel et al. 2012Waichel B.L., Lima E.F., Viana A.R., Scherer M.S., Bueno G.V., Dutra G.T. 2012. Stratigraphy and volcanic facies architecture of the Torres Syncline, Southern Brazil, and its role in understanding the Paraná-Etendeka Continental Flood Basalt Province. Journal of Volcanology and Geothermal Research, 215:74-82.). In a more detailed scale of work, there is a more complex arrangement of facies architecture, therefore lava domes and flows can be individually divided into massive pitchstones, flow-banded vitrophyres, tabular-foliated flows and auto-clastic breccia.

The acidic lava dome feeder conduits in São Marcos - Antônio Prado described by Lima et al. (2012)Lima E.F., Philipp R.P., Rizzon G.C., Waichel B.L., Rossetti L.M.M. 2012. Sucessões Vulcânicas e Modelo de Alimentação e Geração de Domos de Lava Ácidos da Formação Serra Geral na Região de São Marcos-Antonio Prado (RS). Geologia USP Série Científica, 12:49-64. show a composed magmatic foliation revealed by alternation of different crystallization and oxidation grades, and presence of autoliths and xenoliths from the lower volcanic sequences. This area is characterized by the occurrence of tabular flows, massive pitchstones, lava domes, and feeder conduits.

The Caxias do Sul - Feliz profile is characterized by an association of tabular facies with sub-horizontal jointing (Fig. 2) and flow-banding with lava dome facies. Lava domes are characterized by an autobrecciated external portion and a coherent core with increase of crystallinity from external portions towards the core.

Figure 2.
Lava flow with tabular jointing in Caxias do Sul - Feliz region. 29°13'40.28"S/51°19'53.13"W/elevation: 733 m.

In RS-486 (Rota do Sol) profile, the first occurrence of acidic rocks is represented by flow-banded vitrophyres with millimetric to centimetric flat-lying flow-banding, which is symmetrically folded in some outcrops (Figs. 3A and 3B). The petrographic features of these flows are represented by sparse plagioclase and pyroxene phenocrysts, some with swallow-tail textures, rotated by flow foliation and a microlith-rich groundmass with hyalopilitic texture (Fig. 4A). The lateral variations of the flow-banded vitrophyres are autobreccias with centimetric clasts of red glass with perlitic cracks and external irregular shape, cemented by zeolite (Fig. 3C).

Figure 3.
Field and textural characteristics of RS - 486 (Rota do Sol) road acidic flows. (A) Tight recumbent fold in flow-banded vitrophyre; (B) Schematic sketch of the fold in Fig. A; (C) Auto-clastic facies of the flow-banded vitrophyre with millimeter to centimeter size volcanic rock fragments cemented by zeolite; (D) Pitchstone with centimeter size spherulite presenting different colors of alteration; (E)

Figure 4.
Petrographic features of Rota do Sol profile. (A) Plagioclase phenocryst in a groundmass with hyalopilitic texture (crossed polarizers); (B) Autobreccia with perlitic rock auto-clasts (indicated by arrows; uncrossed polarizers); (C) Plagioclase + Clinopyroxene in glomeroporphyritic texture set in an intersetal groundmass (crossed polarizers); (D) Sieve texture in plagioclase phenocryst set in an intersetal groundmass (crossed polarizers).

Towards the top of the sequence, there are spherulite-bearing pitchstones (Fig. 3D) with porphyritic and glomeroporphyritic textures of plagioclase, pyroxene, and Fe-Ti oxides. Plagioclase commonly presents sieve texture and groundmass is composed of microliths in intersetal texture (Figs. 4C and 4D). The pitchstones are intercalated with 4 to 6 m lava domes with sparse plagioclase and pyroxenes within a glassy groundmass and rubbly pahoehoe basic lava flows. This sequence is superimposed by tabular acidic flows with sub-horizontal centimetric jointing.

Geochemistry, temperatures, and viscosities

The studied rocks are characterized by SiO₂ contents between 66.05 and 68.96 wt.%, with Na₂O+K₂O usually lower than 7.0 wt.%. In the R₁-R₂ (De La Roche et al. 1980) and A/NK versus A/CNK plots (Shand 1943), samples can be classified as metaluminous rhyodacites, with only one occurrence of rhyolite (Fig. 5). The TiO₂ contents between 0.85 and 0.97 wt.% are near the lower limit established for high-Ti acid rocks of SGF (Nardy et al. 2008).

Figure 5.
R1 - R2 diagram (De La Roche et al. 1980) and alumina saturation index (A/CNK-A/NK) diagram (Shand 1943) for selected samples. A/CNK=Al2O3/(CaO+Na2O+K2O) (mol.%) and A/NK=Al2O3/(Na2O+K2O) (mol.%). Empty circles are from RS-486 profile, filled triangles are from Caxias do Sul - Feliz profile, and crosses are samples compiled from Lima et al. (2012).

To classify the samples regarding magma-types, the use of total alkali-silica (TAS) diagram, with SiO₂ against Na₂O+K₂O, shows that the studied samples occupy the Palmas-type field (except for BR-41B). In the Zr versus Rb diagram, the samples plot within or near the Caxias magma sub-type (Fig. 6).

Figure 6.
Geochemical discrimination for the studied samples. (A) Total Alkali-Silica (TAS) diagram (Le Bas et al. 1986) for lithological classification with magma-type fields; (B) Zr versus Rb diagram with magma sub-type fields, from Nardy et al. (2008). Same legend as in Fig. 4.

The apatite saturation model of Harrison & Watson (1984)Harrison T.M. & Watson E.B. 1984. The behaviour of apatite during crustal anatexis: equilibrium and kinetic considerations. Geochimica et Cosmochimica Acta, 48(7):1467-1477. suggests that apatite dissolution in felsic magmas is limited by diffusion of phosphorous in the melt. In the studied samples, P₂O₅ contents range from 0.25 to 0.28 wt.% and the lnDapatite/melt varies between 9.6 and 9.7. These results within the SiO₂ contents provided a calculated temperature of 1,077.3 ± 19ºC in São Marcos-Antônio Prado profile; 1,047.2 ± 27ºC in Caxias do Sul - Feliz profile, and 1,070.8 ± 16ºC in RS-486 (Rota do Sol) profile. These results are in agreement with previous data for Parana-Etendeka CFB (Bellieni et al. 1984Bellieni G., Brotzu P., Comin-Chiaramonti P., Ernesto M., Melfi A.J., Pacca I.G., Piccirillo E.M., Stolfa D. 1984. Flood basalt to rhyolite suites in the southern Paraná plateau (Brazil): paleomagnetism, petrogenesis and geodynamic implications. Journal of Petrology, 25:579-618.; 1986Bellieni G., Comin-Chiaramonti P., Marques L.S., Melfi A.J., Nardy A.J.R., Papatrechas C., Piccirilo E.M., Roisenberg A. 1986. Petrogenetic aspects of acid and basaltic lavas from Paraná Basin (Brazil): geological, mineralogical and petrochemical relationships. Journal of Petrology, 27:915-944; Milner et al. 1992Milner S.C., Duncan A.R., Ewart A. 1992. Quartz latite rheoignimbrite flow of the Etendeka Formation, North-Western Namibia. Bulletin of Volcanology, 54:200-219.; Janasi et al. 2007Janasi V.A., Montanheiro T.J., Freitas V.A., Reis P.M., Negri F.A., Dantas F.A. 2007. Geology, petrography and geochemistry of the acid volcanism of the Paraná Magmatic Province in the Piraju-Ourinhos region, SE Brazil. Revista Brasileira de Geociências, 37:745-759.).

The viscosity (η) data obtained for the studied samples were firstly calculated for an anhydrous basis. In São Marcos - Antônio Prado profile, η values are about 5.79 ± 0.14 log Pas, whereas in Caxias do Sul - Feliz profile, they are about 5.92 ± 0.20 log Pas and in RS-486 (Rota do Sol), about 5.78 ± 0.16 log Pas. To compare these quantities with an aqueous basis, we selected one sample from each profile and estimated the η values for 0, 2, 4, and 6 wt.% of H₂O and for 1,000 and 1,100ºC. This information show a drastic decrease of viscosity from up to 6.7 to 2.6 - 2.8 log Pas for 1,000ºC and from 5.3 - 5.5 to 2.0 - 2.2 log Pas for 1,100ºC (Fig. 7) with water addition.

Figure 7.
Diagram of estimated viscosity (?) values for 0, 2, 4, and 6 wt.% of H2O over 1,000 and 1,100ºC. The viscosity was estimated for the samples GA-03P, BR-29A, LR-28A using the model of Giordano et al. (2008).

The number of non-bridging oxygen per tetrahedrally coordinated cations (NBO/T) is a way to quantify the polymerization degree of the melt. If NBO/T=0, the melt is fully polymerized (Mysen et al. 1985Mysen B.O., Virgo D., Seifert F.A. 1985. Relationship between properties and structure of aluminosilicate melts. American Mineralogist, 70:88-105.). For the analyzed samples, the values of NBO/T are between 0.05 and 0.1, which is common for rhyodacite compositions (Hellwig 2006Hellwig B.M. 2006 The viscosity of dacitic liquids measured at conditions relevant to explosive arc volcanism: determining the influence of temperature, silicate composition, and dissolved volatile content. MS dissertation, University of Missouri-Columbia, 158 p.).

DISCUSSION AND CONCLUSIONS

The previous studies concerning facies architecture and geochemistry characterization of the volcanic rocks in the Southern portion of SGF have assumed an effusive nature for the acidic units (Umann et al. 2001Umann L.V., Lima E.F., Sommer C.A., De Liz J.D. 2001. Vulcanismo ácido da região de Cambará do Sul-RS: litoquímica e discussão sobre a origem dos depósitos. Revista Brasileira de Geociências, 31(3):357-364.; Waichel et al. 2006Waichel B.L., Lima E.F., Lubachesky R., Sommer C.A. 2006. Pahoehoe flows from the central Paraná Continental Flood Basalts. Bulletin of Volcanology, 68:599-610.; 2012Waichel B.L., Lima E.F., Viana A.R., Scherer M.S., Bueno G.V., Dutra G.T. 2012. Stratigraphy and volcanic facies architecture of the Torres Syncline, Southern Brazil, and its role in understanding the Paraná-Etendeka Continental Flood Basalt Province. Journal of Volcanology and Geothermal Research, 215:74-82.; Simões & Lima 2011Simões M.S., Lima E.F. 2011. Geoquímica, geotermometria e viscosidades dos fluxos ácidos da Formação Serra Geral nas regiões de São Marcos e da Rota do Sol. In: XIII Congresso Brasileiro de Geoquímica. RS. Anais do XIII Congresso Brasileiro de Geoquímica, Gramado.; Rossetti 2011Rossetti L.M.M. 2011. Arquitetura de fácies vulcânicas da Formação Serra Geral na região de Feliz - Caxias do Sul. Graduation Conclusion, Curso de Graduação em Geologia, Instituto de Geociências, Universidade Federal do Rio Grande do Sul, Porto Alegre, 65 p.; Lima et al. 2012Lima E.F., Philipp R.P., Rizzon G.C., Waichel B.L., Rossetti L.M.M. 2012. Sucessões Vulcânicas e Modelo de Alimentação e Geração de Domos de Lava Ácidos da Formação Serra Geral na Região de São Marcos-Antonio Prado (RS). Geologia USP Série Científica, 12:49-64.; Polo & Janasi 2014Polo L.A. & Janasi V.A. 2014. Volcanic stratigraphy of intermediate to acidic rocks in Southern Paraná Magmatic Province, Brazil. Geologia USP Série Científica, 14:83-100.). In the present work, three profiles of volcanic rocks exposure were investigated. The field features, such as tabular and dome geometries associated with auto-clastic facies, presence of magmatic flow foliation and absence of pyroclastic textures, accidental fragments and fall, surge or pyroclastic flow deposits, agree with an effusive origin for the acidic rocks in Torres Syncline. The complex arrangement of the effusive acidic volcanism of SGF was also identified by Polo and Janasi (2014)Polo L.A. & Janasi V.A. 2014. Volcanic stratigraphy of intermediate to acidic rocks in Southern Paraná Magmatic Province, Brazil. Geologia USP Série Científica, 14:83-100. in the South of Soledade city.

The concordance of very high temperatures for acidic rocks of Parana-Etendeka CFB (liquidus>1,100ºC and equilibration temperature>1,000ºC, Bellieni et al. 1986) is confirmed and has an important role due to the resemblance with super-liquidus temperatures (Green & Fitz III 1993Green J.C. & Fitz III T.J. 1993. Extensive felsic lavas and rheoignimbrites in the Keweenawan Midcontinent Rift plateau volcanics, Minnesota: petrographic and field recognition: Journal of Volcanology and Geothermal Research, 54:177-196.; Lima et al. 2012Lima E.F., Philipp R.P., Rizzon G.C., Waichel B.L., Rossetti L.M.M. 2012. Sucessões Vulcânicas e Modelo de Alimentação e Geração de Domos de Lava Ácidos da Formação Serra Geral na Região de São Marcos-Antonio Prado (RS). Geologia USP Série Científica, 12:49-64.).

Presence of apatite as phenocryst and microphenocryst is a characteristic of Palmas-type acidic rocks (Comin-Chiaramonti et al. 1988Comin-Chiaramonti P., Bellieni G., Piccirillo E.M., Melfi A.J. 1988. Classification and petrography of continental stratoid volcanic and related intrusive from the Paraná Basin (Brasil). In: Piccirillo E.M., Melfi A.J. (eds) The Mesozoic flood volcanism of the Paraná Basin: petrogenetic and geophysical aspects. São Paulo, Instituto Astronômico e Geofísico, 600 p.), and the very high temperatures between 1,000ºC and 1,100ºC obtained in this study reflect the amounts of P₂O₅ and SiO₂ in the studied samples. It is well explained because the level of dissolved P₂O₅ required for apatite saturation is positively dependent upon temperature, and negatively dependent upon SiO₂ content (Harrison & Watson 1984Harrison T.M. & Watson E.B. 1984. The behaviour of apatite during crustal anatexis: equilibrium and kinetic considerations. Geochimica et Cosmochimica Acta, 48(7):1467-1477.).

The composition of SGF acidic rocks in Torres Syncline, with SiO₂ contents between 66.05 and 68.96 wt.% and P₂O₅ contents between 0.25 and 0.28 wt.%, is different from other acidic volcanic rocks associations and has relative higher phosphorous content than rhyolites of shoshonitic associations (P₂O₅~0.11 - 0.20 wt.%; Lima & Nardi 1998Lima E.F. & Nardi L.V.S. 1998. The Lavras do Sul Shoshonitic Association: implications for the origin and evolution of Neoproterozoic shoshonitic magmatism in the southernmost Brazil. Journal of South American Earth Sciences, 11:67-77.); rhyolites associated with tholeiitic basalts in bimodal associations (P₂O₅~0.02 - 0.07 wt.%; Streck & Grunder 2008Streck M.J. & Grunder A.L. 2008. Phenocryst-poor rhyolites of bimodal, tholeiitic provinces: the Rattlesnake Tuff and implications for mush extraction models. Bulletin of Volcanology, 70:385-401.); A-type metaluminous to weakly peraluminous rhyolites (P₂O₅~0.14 wt.%; Pierosan et al. 2011Pierosan R., Lima E.F., Nardi L.V.S., Bastos Neto A.C., Campos C.P., Jarvis K., Ferron J.M.T.M., Prado M. 2011. Geochemistry of Palaeoproterozoic volcanic rocks of the Iricoumé Group, Pitinga Mining District, Amazonian Craton, Brazil. International Geology Review, 53:946-979.; Simões et al. 2014Simões M.S., Almeida M.E., Souza A.G.H., Silva D.P.B., Rocha P.G. 2014. Characterization of the volcanic and hypabyssal rocks of the Paleoproterozoic Iricoumé Group in the Pitinga Region and Balbina Lake area, Amazonian Craton, Brazil: Petrographic distinguishing features and emplacement conditions. Journal of Volcanology and Geothermal Research, 286:138-147.); and peralkaline rhyolites (P₂O₅~0.01 - 0.04 wt.%; Sommer et al. 2013Sommer C.A., Lima E.F., Machado A., Rossetti L.M.R., Pierosan R. 2013. Recognition and characterisation of high-grade ignimbrites from the neoproterozoic rhyolitic volcanism in southernmost Brazil. Journal of South American Earth Sciences, 47:152-165.).

As a consequence of high-temperature values, viscosities calculated for anhydrous conditions are below the expected for rhyolites and rhyodacites (Giordano et al. 2008Giordano D., Russell J.K., Dingwell D.B. 2008. Viscosity of magmatic liquids: A model. Earth and Planetary Science Letters, 271:123-134.; Whittington et al. 2009Whittington A.G., Hellwig B.M., Behrens H., Joachim B., Stechern A., Vetere F. 2009. The viscosity of hydrous dacitic liquids: Implications for the rheology of evolving silicic magmas, Bulletin of Volcanology, 71:185-199.; Pierosan et al. 2011Pierosan R., Lima E.F., Nardi L.V.S., Bastos Neto A.C., Campos C.P., Jarvis K., Ferron J.M.T.M., Prado M. 2011. Geochemistry of Palaeoproterozoic volcanic rocks of the Iricoumé Group, Pitinga Mining District, Amazonian Craton, Brazil. International Geology Review, 53:946-979.; Sommer et al. 2013Sommer C.A., Lima E.F., Machado A., Rossetti L.M.R., Pierosan R. 2013. Recognition and characterisation of high-grade ignimbrites from the neoproterozoic rhyolitic volcanism in southernmost Brazil. Journal of South American Earth Sciences, 47:152-165.; Simões et al. 2014Simões M.S., Almeida M.E., Souza A.G.H., Silva D.P.B., Rocha P.G. 2014. Characterization of the volcanic and hypabyssal rocks of the Paleoproterozoic Iricoumé Group in the Pitinga Region and Balbina Lake area, Amazonian Craton, Brazil: Petrographic distinguishing features and emplacement conditions. Journal of Volcanology and Geothermal Research, 286:138-147.). Although low-viscosity lava flows have been described in other Large Igneous Provinces as Gawler Ranges, Australia (Pankhurst et al. 2011Pankhurst M.J., Schaefer B.F., Betts P.G., Phillips N., Hand M. 2011. A Mesoproterozoic continental flood rhyolite province, the Gawler Ranges, Australia: the end member example of the Large Igneous Province clan. Solid Earth, 2:25-33.) and Snake River Plain (Branney et al. 2008Branney M.J., Bonnichsen B., Andrews G.D.M., Ellis B., Barry T.L., McCurry M. 2008. "Snake River (SR)-type" volcanism at the Yellowstone hotspot track: distinctive products from unusual, high-temperature silicic super-eruptions. Bulletin of Volcanology, 70(3):293-314.). The decrease of viscosity with addition of water content (Fig. 6) is explained by the H₂O effect in the values B and C of the used model, a consequence of its role in the structure of silicate melts relative to other network-modifying cations (Dingwell 1991Dingwell D.B. 1991. Redox viscometry of some Fe-bearing silicate liquids. American Mineralogist, 76:1560-1562.; Giordano et al. 2008Giordano D., Russell J.K., Dingwell D.B. 2008. Viscosity of magmatic liquids: A model. Earth and Planetary Science Letters, 271:123-134.). If the magma of the studied rocks were not anhydrous, the viscosities would have been significantly lower.

The emplacement of erupted lava is controlled mainly by viscosity and effusion rate. Secondarily, the vent geometry, characteristic of the shallow ground water and the shape of landscape drive the manner of emplacement. Lateral confinement of the lava flow will depend on the shape of the relief. According to Bardintzeff and McBirney (1998)Bardintzeff J.M. & McBirney A. 1998. Volcanology. Jones Bartlett Pub., Sudbury, Mass., p. 268., there are six main types of domes based on their morphological aspects: cryptodomes, plug domes, Peléan domes, spines, lava domes, and couleés. The classification of lava domes based on viscosity measurements of Yokoyama (2005)Yokoyama I. 2005. Growth rates of lava domes with respect to viscosity of magmas. Annals of Geophysics, 48:957-971 explains that squeeze-type lava domes are the result of magma squeezes through conduits, as Bingham bodies pilling up to the vent with growth rates governed by Hagen-Pouiseuille Law (involving viscosity and eruption parameters). On the other hand, spyne-type lava domes are the result of displacements of solidified lava driven up by fluid magma beneath with growth rates smaller than squeeze types because of larger resistance between solidified spines and conduit walls.

Moreover, lava will emplace as lava dome or flow depending on heat loss per volume unit (Walker 1973Walker G.P.L. 1973. Lengths of lava flows. Philosophical Transactions of the Royal Society of London, 274:107-118.). The distance traveled by the flow, after the initial cooling, will depend on the effect of thermal insulation of cooled surface crusts that allows the magma to flow in the core of the flow unit (Harris & Rowland 2009Harris A. & Rowland S. 2009. Effusion rate controls on lava flowlength and the role of heat loss: a review. In: Thordarson T., Self S., Larsen G., Rowland S.K., Hoskuldsson A. (eds.) Studies in Volcanology: The Legacy of George Walker. Special Publications of IAVCEI, p. 33-51.). These concepts explain why flows erupted at high effusion rates have more potential to attain greater distances than those at low ones.

In Torres Syncline, the first volcanic episode (BVE I) covered Botucatu paleoerg with simple pahoehoe flows, associated with low effusion rates, near the dunes and with ponded pahoehoe flows (~40 m thick) in interdune areas, flattening the rouged relief. The BVE II is composed of tabular-classic simple pahoehoe flows and covers the BVE I under high effusion rates conditions (Waichel et al. 2012Waichel B.L., Lima E.F., Viana A.R., Scherer M.S., Bueno G.V., Dutra G.T. 2012. Stratigraphy and volcanic facies architecture of the Torres Syncline, Southern Brazil, and its role in understanding the Paraná-Etendeka Continental Flood Basalt Province. Journal of Volcanology and Geothermal Research, 215:74-82.).

The occurrence of acidic lava domes above the BVE II as flow-banded vitrophyres was probably controlled by squeezes of lava under low effusion rates, in spite of their high temperature and low viscosities, which are reflected in their small height. The emplacement of lava domes has continued until the BVE III, characterized by the eruption of rubbly pahoehoe flows, and the relief probably became rugged. Presence of tabular acidic lava flows covering the landscape indicates that it was under high effusion rates conditions and that these flows had well-insulated surface crusts. The capacity to attain greater distances and overpass relief obstacles is explained not only by high effusion rates, but also by very low viscosities at the time of the emplacement.

ACKNOWLEDGMENTS

We thank Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) for the financial support (Project 479784/2012-4), Programa de Pós-graduação em Geociências of UFRGS and the reviewers Doctors Zorano de Souza and Umberto Cordani for the contributions.

REFERENCES

Publication Dates

History