The role of viscosity in the emplacement of high-temperature acidic flows of Serra Geral Formation in Torres Syncline ( Rio Grande do Sul State , Brazil )

The acidic flows from Serra Geral Formation in Torres Syncline, Rio Grande do Sul, Brazil, are on the top of a volcanic sequence composed by a complex facies association of compound, simple and rubbly pahoehoebasic flows, acidic lava domes, and tabular acidic lava flows. The origin and emplacement conditions of the acidic volcanic rocks are discussed in this paper based on petrology, on calculated apatite saturation thermometry temperatures, and on estimated viscosity data. The liquidus temperatures for metaluminous rhyodacite to rhyolite samples are about 1,067.5 ± 25oC in average. The viscosity (η) values vary from 105 to 106 Pas for anhydrous conditions, suggesting the emplacement of high-temperature - low-viscosity lava flows and domes. The occurrence of acidic lava domes above simple pahoehoe flows as flow-banded vitrophyres was 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 eruption of rubbly pahoehoe flows and the geometry of these deposits rugged the relief. Presence of tabular acidic lava flows covering the landscape indicates that it was under high effusion rates conditions and such flows had well-insulated cooled 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 emplacement.

The role of viscosity in the emplacement of high-temperature acidic flows of Serra Geral Formation in Torres Syncline (Rio Grande do Sul State, Brazil)

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 1994).The CFBs are individual volcanic constructs that are laterally extensive (several hundred km 2 ) 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 1992;Sheth 2007).
Paraná-Etendeka CFB occupies, approximately, an 1,3 x 10 6 km 2 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. 1988).Waichel et al. (2012) 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. 2014).
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. 1984;1986).
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. 1989;Whittingham 1989;Roisenberg 1989;Milner et al. 1992;1995;Bryan et al. 2010).The effusive origin for the same rock association was assumed (Bellieni et al. 1986;Comin-Chiaramonti et al. 1988;Henry & Wolff 1992;Umann et al. 2001;Waichel et al. 2012;Polo & Janasi 2014) and the volcanic conduits of lava flows were described in the Northeastern portion of Rio Grande do Sul State (Lima et al. 2012).
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. 1992).In the Paraná Basin, temperatures obtained by the coexisting pyroxenes method are 1,030 ± 38ºC (Bellieni et al. 1984) and 1,117 ± 17ºC (Bellieni et al. 1986) 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 10 6 Pa s for quartz latites (Milner et al. 1992).
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 1972) are insufficient to describe the viscosity of melts over the entire temperature interval, which are now accessible using new techniques (Vetere 2006).Hence, the evolution of viscometers for silicate liquids has been traced by changing from Arrhenian temperature dependence (e.g.Shaw 1972;Bottinga & Weill 1972) to non-Arrhenian models, considering magma composition and volatile effects (e.g.Baker 1996;Hess & Dingwell 1996;Russell et al. 2002;Giordano & Dingwell 2003;Russell & Giordano 2005;Giordano et al. 2006;Vetere et al. 2006;Hui & Zhang 2007;Giordano et al. 2008).
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).Lima et al. (2012) described the SGF volcanic succession in São Marcos-Antônio Prado region and identified, above the low-TiO 2 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-TiO 2 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. 2014).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 SiO 2 , Al 2 O 3 , Fe 2 O 3 , CaO, MgO, Na 2 O, K 2 O, MnO, TiO 2 and P 2 O 5 .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) 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 1981;Prowatke & Klemme 2006) and used wt.% of P 2 O 5 as apatite partition in the magma, considering phosphorus as an essential structural constituent (Sun & Hanson 1975).
The model used in this work to estimate viscosity (Giordano et al. 2008) 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 H 2 O content is an estimated value.Results of geochemical analysis, calculated temperatures and viscosities are presented in Tabs. 1 and 2.

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. 2012).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) 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.
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).
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 SiO 2 contents between 66.05 and 68.96 wt.%, with Na 2 O+K 2 O usually lower than 7.0 wt.%.In the R 1 -R 2 (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 TiO 2 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).
To classify the samples regarding magma-types, the use of total alkali-silica (TAS) diagram, with SiO 2 against Na 2 O+K 2 O, 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).
The apatite saturation model of Harrison & Watson (1984) suggests that apatite dissolution in felsic magmas is   limited by diffusion of phosphorous in the melt.In the studied samples, P 2 O 5 contents range from 0.25 to 0.28 wt.% and the lnDapatite/melt varies between 9.6 and 9.7.These results within the SiO 2 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. 1984;1986;Milner et al. 1992;Janasi et al. 2007).Figure 7. Diagram of estimated viscosity (η) values for 0, 2, 4, and 6 wt.% of H 2 O 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 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 H 2 O 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.

Sample BR029A BR-29B BR-41B BR-41C Average SD LR-28-A LR-29-A LR-32-A LR-33-B LR-35-A Average SD
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. 1985).For the analyzed samples, the values of NBO/T are between 0.05 and 0.1, which is common for rhyodacite compositions (Hellwig 2006).

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. 2001;Waichel et al. 2006;2012;Simões & Lima 2011;Rossetti 2011;Lima et al. 2012;Polo & Janasi 2014).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) 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 1993;Lima et al. 2012).
Presence of apatite as phenocryst and microphenocryst is a characteristic of Palmas-type acidic rocks (Comin-Chiaramonti et al. 1988), and the very high temperatures between 1,000ºC and 1,100ºC obtained in this study reflect the amounts of P 2 O 5 and SiO 2 in the studied samples.It is well explained because the level of dissolved P 2 O 5 required for apatite saturation is positively dependent upon temperature, and negatively dependent upon SiO 2 content (Harrison & Watson 1984).
As a consequence of high-temperature values, viscosities calculated for anhydrous conditions are below the expected for rhyolites and rhyodacites (Giordano et al. 2008;Whittington et al. 2009;Pierosan et al. 2011;Sommer et al. 2013;Simões et al. 2014).Although low-viscosity lava flows have been described in other Large Igneous Provinces as Gawler Ranges, Australia (Pankhurst et al. 2011) and Snake River Plain (Branney et al. 2008).The decrease of viscosity with addition of water content (Fig. 6) is explained by the H 2 O 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 1991;Giordano et al. 2008).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), 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) 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 1973).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 2009).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. 2012).
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.

Figure 3 .
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) Slightly domeshaped vitrophyre with different color alteration; (F) Lava dome with spheroidal exfoliation.

Table 2 .
Geochemical analyses from RS-486 road (BR samples) and Caxias do Sul -Feliz (LR samples) with major elements, apatite temperatures, and viscosities results.