Late to Post-Orogenic Brasiliano-Pan-African Volcano-Sedimentary Basins in the Dom Feliciano Belt, Southernmost Brazil

The Neoproterozoic/Early Paleozoic transition in Southern Brazil was marked by the last phases of the Brasiliano/Pan-African Cycle (900-540 Ma, Chemale Jr., 2000). These include the development of a trough series related to the different Brasiliano Orogenesis phases, which occupied one preferential subsiding locus called Camaquã Basin (Paim et al., 2000). This basin is located in the Sul-Riograndense Shield (Rio Grande do Sul State, Southernmost Brazil, Fig. 2). The Camaquã Basin is a northeast-southwest elongated basin positioned along the Dom Feliciano Belt limits (to the east) and the Rio de la Plata Craton (to the west). The basin was filled with a thick (ca. 11 km) sedimentary rocks package deposited in marine (at the base) and continental (at the top) environments, and a volcanic rocks series interbedded with the sediments. At least three volcanic events were recognized and related to the Hilário and Acampamento Velho Formations, and Rodeio Velho Member, from the base to the top (Fig. 1). Paim et al. (2000) divided the Camaquã Basin into four sub-basins named as TaquarembóRamada, Santa Bárbara, Guaritas and Piquiri-Arroio Boici, from West to East (Fig.1). Each subbasin showed a partial independent evolution, and consequently, the stratigraphic framework shows some differences in a regional scale. Santa Bárbara and Guaritas sub-basins show the most complete stratigraphic section. Paim et al. (2000) defined five major depositional cycles from tectonic origin separated by unconformities. Every cycle corresponds to a unit ranking as group or allogroup. From the base to the top, there are the following groups: Maricá, Bom Jardim, Cerro do Bugio, Santa Bárbara and Guaritas (Table 1). This chapter aims to make the characterization of the two last volcanic events associated to the filling of Camaquã Basin, named as Acampamento Velho Formation and Rodeio Velho Member in Rio Grande do Sul, Brazil. The book chapter will be composed of an introduction. After the introduction, it will be presented the Camaquã Basin stratigraphy and evolution compilation, aiming to introduce the reader to the volcanic events


Fig. 1. Geological map showing the main Camaquã Basin stratigraphic units (modified from
and the localization of the main regions.
According to Basei et al. (2000), the magmatic, metamorphic structural and geotectonic features in Southeastern South America recorded the superposition of two Neoproterozoic-Eopaleozoic orogenies: Brasiliano (700-620 Ma) and Rio Doce (620-530 Ma) orogenies. The end of the extensional regime related to the Brasiliano cycle is marked by late magmatism (600±10 Ma), conspicuous in Southern Brazil. The foreland basins, with sedimentation around 560±20 Ma and deformation around 530 Ma, represent the main volcanicsedimentary record of Rio Doce Orogeny in Southern Brazil. The Camaquã Basin is considered to represent this type of basin. Differently from the current thoughts, Fragoso-Cesar et al. (2001) suggested an intraplate evolution to this basin, detached from the evolution of the Brasiliano Cycle. These authors related the development of the basin to the reactivation of the Brasiliano structures formed in the Neoproterozoic III to Eocambrian. They stated that the knowledge about the tectonic events and deformation of the sedimentary and volcanic rocks is still incipient, and there are no radiometric ages to position the deformation phases in time. They also ratify the connection between the Guaritas sedimentation and volcanics rocks from Rodeio Velho, and Paraná Basin. These units would be related to the development of SW-NE rifts during the initial stages of the evolution from Paraná Basin. Table 1. Stratigraphic concepts for Camaquã Basin. Silva et al. (2005) as cited in Borba et al. (2008) focusing on the tectonic evolution of Brasiliano events in the Mantiqueira Province (South and Central Brazil and Uruguay, Fig. 3) revealed three main orogenic systems: (a) Brasiliano I, with juvenile island-arc accretion and collisional climaxes at ca. 790 Ma (Embu Domain, Southeastern Brazil) and 730-700Ma (São Gabriel Orogeny, Southern Brazil); (b) Brasiliano II, with dominance of crustal 79 volcanogenic beds interlayred with sedimentary rocks of the lower succession. Circular structures resembling degassing pipes also suggest the activity of volcanic hot springs (Borba et al., 2004). According to Paim et al (2000), in terms of deformation at the Taquarembó/Ramada sub-basin, are recognizes reverse ruptile faults and weak folds, while the Piquiri/Boici sub-basin is dominated by structures associated to the transcurrent processes in ruptile to ductile-ruptile structural level. The Bom Jardim Group (Paim et al, 2000), is a volcano-sedimentary sequence that shows a lithostratigraphic complexity in its area-type and similarity lithological of some its units with others from Camaquã Supergroup units (Table 1). Bom Jardim Group, in its area-type (Janikian et al., 2003), shows approximately 4,000 thicknesses and constituted of andesitic volcanic rocks, pyroclastic rocks (lapilli tuffs, lithic tuffs and thick vitreous tuffs -Hilário Formation) and sedimentary rocks. These rocks were generated in an active tectonic lacustrine basin. Robertson (1966) made the first attempted correlation among the volcanosedimentary units from Bom Jardim area and others expositions from Camaquã Supergroup. Ribeiro et al. (1966) and Tessari & Picada (1966) correlated the outcropping units from Bom Jardim and Vale Piquiri (Piquiri Sub-basin) areas, covering the Arroio dos Nobres Formation. The Hilário term was first used by Robertson (1966) to designate a variety of rock types composed of mainly andesitic to dacitic composition (flows, tuffs, volcanic conglomerates, mudflows, dykes and intrusive mafic rocks). Ribeiro et al. (1966) formalized this unit as a member of Crespos Formation -Bom Jardim Group. Santos et al. (1978) used the Hilário term to refer a member of Cerro dos Martins Formation, while Horbach et al. (1986) upgraded these rocks to the formation level. Several studies have been carried out Hilário Formation rocks. Nardi & Lima (1985), Porcher et al. (1995), Lima & Nardi (1998) works are especially relevant. Thus, Lima & Nardi (1998) defined the Lavras do Sul Shoshonitic Association, which comprises a wide compositional range of intrusive and extrusive basic to acid rocks. The extrusive rock includes the Hilário Formation and the spessartitic lamprophyres. In the Bom Jardim Group, the volcanic rocks are mostly in the lower stratigraphic levels, as lava flows or small shallow intrusion. It the Santa Bárbara Sub-basin (Paim et al., 2000) toward the Guaritas Sub-basin, the volcanic rocks of this group are replaced gradational by alluvial conglomerates rich in volcanic clasts from basic to intermediate composition, and more distally, by sandy and silty related to the bottom flows (turbidites) , which are bounded to underwater deltaic systems. Next to the high topographic that today subdivides the Santa Bárbara and Guaritas sub-basins (e.g. Alto Caçapava), is common the occurrence of the erratic conglomerates containing local derivation clasts (shales and marble), indicating that the partitioning of the Camaquã Basin in several sub-basins occurred prior to the deposition of this unit. Cerro do Bugio Allogroup (or Group) is composed of volcanic rocks at the base (Acampamento Velho Alloformation or Formation) and alluvial conglomerate deposits from Santa Fé Alloformation (ou Formation). According Paim (2000), Cerro do Bugio is limited by two angular unconformity that delineate its contact with the lower (Bom Jardim Allogroup) and upper (Santa Bárbara Allogroup) units. Cerro do Bugio Unit shows approximately 500 m thick and is composed of, from the base to the top, Acampamento Velho (volcanic) and Santa Fé (alluvial conglomerates, and secondarily, by sand-pelitic rhytmits and pelites), separated by an erosive unconformity. The Santa Fé Formation consists mainly of alluvial conglomerates rich in volcanic and plutonic clasts of the acid composition, which vary vertically to sand-pelitic rhytmits. These lithologies represent the alluvial systems interlaced, considering the transversal (deltaic fans, conglomeratic with paleocurrents for SE) and longitudinal (fans of the interlaced plain) characters. This allogroup was affected by dip-slip and transcurrent faults generated in crustal to ductile-ruptile level. Considering the field work observations, the Acampamento Velho Formation volcanic rocks are often interdigitated with rocks from Santa Fé Formation, suggesting the contemporaneity between both units. Acampamento Velho Formation (sensu Cordani et al., 1974) is the basal unit of the Santa Bárbara Group and crops out mainly in the Santa Bárbara and Taquarembó-Ramada subbasins. The basal contact of the Acampamento Velho Formation is unconformable with the Maricá and Bom Jardim Groups. According to Almeida et al. (2002), in the Cerro do Bugio , Perau and Serra de Santa Bárbara (Fig. 1), the volcanic rocks (basaltic and rhyolitic compositions) upper contact is a disconformity by the alluvial conglomerates, and locally, by deltaic-lacustrine sandstones and mudstones from Santa Fé Formation. The Santa Bárbara Group, defined by Robertson (1966), Ribeiro et al., (1966) and Ribeiro & Fantinel (1978) as Santa Bárbara Formation, comprises red-colored, sandstones and siltstones, and includes the Acampamento Velho and Santa Bárbara formations. It was named as Santa Bárbara Allogroup by Paim et al. (2000). This group is composed of Acampamento Velho, Santa Fé, Lanceiros and Segredo Formations, from the base to the top (Table 1). According to the same authors, the Lanceiros Formation is related to the progradation of an interlaced and sandy deltaic system of the longitudinal character, and the Pedra do Segredo Formation can be bounded to the progradation of an interlaced and sandy-conglomeratic of transversal nature. Borba & Mizusaki (2003) organized the Santa Bárbara Formation rocks in three depositional sequences named as, from the base to the top: Sequence I, Sequence II and Sequence III. The two basal sequences (I and II) represent a coherent depositional pattern, with axial fluvial and deltaic fan systems, which deposit North-Eastward with lateral contribution from alluvial fans. Such coarse deposits are composed mainly of metamorphic clasts derived from Alto Caçapava and Eastern steep Santa Bárbara Basin. The Sequence I shows a dip ranging from 32 o to 40 o NE in the Northern outcrops and 10 o to 16 o NE in the South, and the Sequence II from 28 o to 30 o SE. The Sequence III lies unconformable over basal subunits and reflects the axial system inversion, in that the gravel bed deposits paleocurrents systematically point South/South-Westward. The alluvial fan deposits of the Sequence III also suggest a tectonic rearrangement basin, with partial erosion from basal sequences, and the presence of granitic composition fragments, which reflects the deeper denudation stage of the Alto Caçapava and possibly a significant hiatus at the Sequence III base. The third dip sequence is between 23 o to 26 o ENE. The Acampamento Velho Formation is not included in the Santa Bárbara Formation (Borba & Misuzaki, 2003). The Guaritas was defined by Goñi et al. (1962), Ribeiro et al (1966), Robertson (1966) and Ribeiro & Fantinel (1978) and is lithologically similar to the Santa Bárbara Group. This unit represents the last great depositional event preserved in Camaquã Basin and is above unconformity of the previous units. It was denominated Guaritas Allogroup (Paim et al., 2000) and is composed of two alloformations or formations, from the base to the top: Pedra Pintada and Varzinha (Table 1). These units are bounded by an erosive unconformity. The Guaritas Group shows around 800 m thick and represents the last depositional episode preserved within of the Camaquã Basin. The Pedra Pintada Formation is temporal and spatially related to the volcanic rocks named as Rodeio Velho Member. It was originated in a desert environment with crescent eolian dunes, in an inter-dune area that describes The common 204 Pb, after Hg correction based on 202 Hg simultaneously measured, is insignificant in most situations. For instance, typical signal intensity due to 204 Hg during a laser ablation on standard Zircon is 600-1000 cps range, while the calculated count rate for 204 Pb is less than statistical error of ca. 25-33 cps. We assume that the 204  For common lead isotope composition, we assumed an isotope composition evolution proposed by Stacey and Kramers (1975), which is required to attribute an initial estimated age. Soliani Jr. et al. (1984) presented a synthesis of all the radio-chronological determinations obtained for the crystalline and sedimentary rocks from Sul-Riograndense Shield meridional portion, including part of the Santa Catarina, the whole Rio Grande do Sul and Uruguay, thus preserving the Cordani et al (1974) and Teixeira (1982) proposal, which says that the West Sul-Riograndense Shield region would have had an Transamazonic evolution, and the meridional portion would have been generated in the Brasiliano Cycle. Sartori & Kawashita (1985) obtained the age of 550 Ma for Caçapava do Sul Granitic Batholith using 21 Rb-Sr isotope analyses on total-rock. According to Soliani et al. (2000), the first work of the regional integration and geochronological synthesis was made by Cordani et al (1974). The authors relate the K-Ar and Rb-Sr radiometric data to the Eopaleozoic rocks, which fill the Camaquã Basin. Thus, the Rb-Sr data of the Eastern portion with syntectonic characteristics (granites and migmatites) and post-tectonic (isotropic granites) show respectively, values from 650-600 Ma. K-Ar ages of the same region are in the same coherent interval characteristics of the Brasiliano Cycle. For the Midwest region of the shield were found radiometric values that suggest an older age, although a significant data percentage show Brasiliano ages for the granitic massive such as the Caçapava do Sul and Lavras do Sul among others. In the same way was confirmed that the volcanic rocks (Hilário and Acampamento Velho formations) were generated at the end of the Pre-Cambrian or at the beginning of the Phanerozoic, exactly as has been commented by Minioli & Kawashite (1971). Leite et al. (1995Leite et al. ( , 1998 and Leite (1997) mentioned for the Rio Grande do Sul region, the first U-Pb data obtained in zircons using the sensitive high mass-resolution ion microprobe method, being the isotope data for the Caçapava do Sul granitic massives of the 561± 6 Ma and 541± 11 Ma. The first age is considered as corresponding to the igneous protolith that generated the Caçapava Granite. The second age is related to a magmatic event that generated the granite. Remus et al. (1996Remus et al. ( , 1997 used sensitive high mass-resolution ion microprobe method to obtain the U-Pb ages in zircons for the Lavras do Sul Granite core and edge, respectively: 592±5 and 580 ±5 Ma. The age of 565± 14 Ma was obtained for the Caçapava do Sul Granite.

Maricá Formation
The first geochronological investigation in Maricá Formation was performed by Borba et al. (2008). These authors made analysis of zircons from pyroclastic cobbles, which yielded an age of 630.2±3.4 Ma (2σ), interpreted as the age of syn-sedimentary volcanism, and thus of the deposition itself. This result indicates, according to Borba et al. (2008), that Maricá Formation was deposited during the main collisional phase (640-620 Ma) of the Brasiliano II Orogenic system from Silva classification (2005). Paim et al (2000) suggested a depositional age between 620 and 600 Ma in a foreland basin.  Fig. 4). The 601 ±13 Ma age (based on eight zircons - Table 2, Fig. 4 -Early Ediacaran) was considered as the age older from Maricá Formation (Fig. 6). All the zircons that showed older ages were considered xenocrystal zircons. Thus, the magma would have assimilated Neoarchean (NA), Paleoproterozoic (Siderian, Rhyacian and Statherian) material during its ascension. The selected zircons that allowed the Maricá age setting (Fig. 5), in general are colorless and dirty, all grains are between 175 µm and 344 µm in size and show diffuse zonation and subrounded contours. The zircons do not have impurities and inclusions, but show few fractures. In the A-I-02 zircon (Fig. 5) is observed an incomplete concentric fracturing at the edge of the crystal.   Ribeiro & Teixeira (1970) mentioned values between 510-535 Ma for Hilário Andesite. These values were obtained through the K-Ar method. Cordani et al. (1974) obtained ages of 510 and 535 Ma using the Rb/Sr method. 206 Pb206/ 238 U data (sensitive high mass resolution ion microprobe) were obtained for Hilário Andesite by Remus et al. (1999) and gave an age of 580 Ma. Janikian et al (2008) mentioned ages from 590 to 585 Ma, which are interpreted for us as representative of Hilário Formation extrusion, being U-Pb age of 590 ± 5.7 Ma obtained in the Hilário Andesite as the most representative. The ages obtained by Remus et al. (2000) using the U-Pb (sensitive high mass-resolution ion microprobe) for Lavras Granite (592±5 Ma, 597±5 Ma, 580±7 Ma) and Caçapava do Sul (589±5 Ma) are considered as comagmatic to the Bom Jardim volcanism. Chemale  reported an age of 592 Ma for Andesite Hilário. The LV-243 sample (alkaline basalt) dated using the U-Pb method by us was collected 15 km away from the Lavras do Sul, following by RS 357 road toward Caçapava do Sul (Fig.1). Lima & Nardi (1998) said that the out crop is a dome with size around 0.5 km 2 and a circular shape. It shows a marked columnar disjunction with hexagons of variable diameter, but no exceeding 15 cm diameter. Nine zircon crystals (sample LV-243) were dated ( Table 3). The obtained age of 591.8 ±3.0 Ma was calculated based on individual zircon ages from Table 3. This age defines the lamprophyre age (Fig. 6), which belong to the Hilário volcanism manifestation. The selected zircons that allowed the lamprophyres ages setting, in general, are colorless and clean, all grains are between 150 µm and 333 µm in size and the most show very clear zoning. The grains also show prismatic and pyramidal shapes, incomplete and partially serrulated contours and few fractures (Fig. 7).

Acampamento Velho Formation
The first geochronological investigation of the rhyolites was performed by Cordani et al. (1974) followed by Sartori (1978) and Teixeira (1982). Soliani Jr. (2000) compiled their data and obtained an age of 529 ± 4 Ma and R 0 = 0,706 (Rb-Sr whole rock). Another Rb-Sr dating was performed by Almeida et al. (2002), who studied the rhyolitic flows from Cerro do Bugio area and the dykes intruding the Maricá Formation. These authors obtained two whole rock isochrons: 545.1 ± 12.7 Ma (R 0 = 0.709) and 546 ± 12.9 Ma (R 0 = 0.714). However, Sommer et al. (2005) presented U-Pb data (sensitive high mass-resolution ion microprobe) using eleven zircon crystals from rhyolites of Vila Nova do Sul area (Ramada Plateau), obtaining an age of 549.3 ± 5 Ma, which is more reliable. Therefore, all the obtained ages indicate that the rocks from Acampamento Velho Formation upper felsic association belong to the Early Neoproterozoic III (NP 3 ).
The MH-13 sample (andesitic basalt) was selected to obtain the absolute age from lower mafic association of Acampamento Velho Formation base. Nineteen zircons crystals were dated (Table 4). Based on the dating results, we detected the presence of four age group: from anchored age, that corresponds to intersection at 2442 ± 54 Ma (three zircons) ( Table 4, Fig. 8a). The 553 ± 5.4 Ma age (based on six zircons - Table 4, Fig. 8b) was considered as the lower mafic association age. All the zircons that showed older ages were considered xenocrystal zircons. Thus, the magma would have assimilated Neoarchean (NA), Paleoproterozoic (Siderian and Rhyacian) and older Neoproterozoic material during its ascension.
The selected zircons that allowed the Acampamento Velho Formation base age setting, in general are colorless and dirty, all grains are between 87 µm and 158 µm in size and show diffuse zonation, prismatic and pyramidal shapes, incomplete and partially serrulated contours, few fractures and inclusions (Fig. 9). The fain and broad zoning suggest that it may have been formed by very slow and complex crystallization of a magma body with prolonged residence time in the lower crust. The crystallization velocity appears to be the major controlling factor of the elongation ratio to the zircon. Skeletal zircon crystals are the most extreme form of rapid growth. It is relevant to point out that the Acampamento Velho Formation mafic flow dated zircons show morphologic characteristics of a slow generation in the magmatic chamber.     Hartmann et al. (1998) determined the U-Pb age of 470 ± 19 Ma (sensitive high massresolution ion microprobe method) for this event (Middle-Ordovician), which suggest that Rodeio Velho Member is related to the beginning oh the Paraná Basin formation. On the other hand, Chemale Jr. (2000) obtained Sm-Nd model ages (T DM ) from 1.6 to 1.9 Ga, suggesting a modified mantle origin for these rocks. Rodeio Velho Member was probably the source of heat and hydrothermal solutions for Cu (Ag, Au), Pb and Zn (Cu, Ag) mineralizations in the Camaquã Mines (Lima et al., 2001). Hydrothermal illites collected in those mines also yielded K-Ar ages around 465 Ma (Bonhomme & Ribeiro, 1983). We used the sample RLP-20 (alkaline basalt) to date Rodeio Velho event using U-Pb method.

Rodeio Velho Member
The sample was collected between Rodeio Velho and Pedra de Arara area ( Fig. 1). At this area, the volcanic rocks show two lava flows, being the lower flow more vesiculated (flow top) than the upper, which is massive. This is related to the sandstones with great crossstratification, suggesting the eolian character of this rock and it would correspond to the Pedra Pintada Formation from Guaritas Group. We dated sixteen zircon crystals from sample RLP-20. Based on these dating results, we detected the presence of 4 age groups: 2190± 18 Ma (one zircon); 1079 ± 12 Ma (six zircons); 658 ± 8.3 Ma (two zircons) and 547 ± 6.3 Ma (based on five zircons), (Table 5, figs. 10a and 10b). The younger obtained age, that was calculated based on individual zircon ages from Table 5, defined the Rodeio Velho Member age (Fig. 10a). All the zircons that showed older ages were considered xenocrystal inherited zircons. Thus, the magma would have assimilated Paleoproterozoic (PP 2 ), Mesoproterozoic (MP 3 ) and Neoproterozoic (NP 2 ) rocks during its ascension. The selected zircons used to obtain the Rodeio Velho Member age are in general (Fig. 11) colorless and transparent, the  (Table 5). The A-I-02 zircon sample shows high percentage of discordance and was not included in the calculating data (black ellipse). (b) -U-Pb concordia data diagram with tracing considering only the older zircon ages of the 547 Ma (table 5).

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Late to Post-Orogenic Brasiliano-Pan-African Volcano-Sedimentary Basins in the Dom Feliciano Belt, Southernmost Brazil 91 size varies between 202 µm and 321 µm, the crystals show zonation, euhedral to subhedral shape, contours partially serrulated, few fractures, impurities and inclusions. They are very small and occupy the crystal rims. The A-I-19 zircon ( Fig. 11d) is constituted by an older core with age between 983 Ma and 1000 Ma (spot a, figure 11d) and 545 to 554 Ma rims (spot b, figure 11d).

Acampamento Velho Formation: geological context, petrography and geochemistry
The earliest references to acid volcanic rocks in Southern Brazil are made by Leinz et al. (1941), who described tuffs, vitrophyres, feldsites and quartz-porphyries, which unconformably overlying the Maricá Formation. This unit was named as Ramada Rhyolite by Robertson (1966) and as Acampamento Velho Formation by Cordani et al. (1974). However, Leites et al. (1990) considered the Acampamento Velho Group as Volcanogenic Sequence III. Paim et al. (2000) named the unit as Acampamento Velho Formation (from Cerro do Bugio Allogroup), and this set of volcanic rocks were considered as an unit limited on the top and base by discontinuities, which marked a special period in the evolution of the basin. It has been traditionally considered as exclusively acid in composition, but detailed geological mapping in the Cerro do Bugio, Cerro do Perau and Serra de Santa Bárbara area (West of Caçapava do Sul town) revealed the existence of a basaltic/andesitic unit at the base and a felsic unit at the top (Zerfass & Almeida 1997, Zerfass et al. 2000, Almeida et al. 2002. This observation leads to the existence of a bimodal alkaline volcanism, mafic at the base and a felsic at the top. Sommer (2000) (1999) verified that the rocks are alkaline, satured in silica, and have post-collisional characteristics. Sommer et al. (2005) recognized the existence of a bimodal mildly alkaline magmatism related to post-collisional events at Ramada Plateau. The four main areas of occurrence of the volcanism Acampamento Velho are Santa Bárbara Sub-Basin, Guaritas Sub-Basin and Taquarembó-Ramada Sub-Basin from Camaquã Basin (Paim et al. terminology, 2000 - Fig. 1). At Santa Bárbara and Guaritas sub-basins, the volcanic package is limited at the top and base by sedimentary rocks from own Camaquã Basin, while at Taquarembó-Ramada Basin, the volcanic rocks maintains the relief, sat down on rocks from Sul-Riograndense Shield.

Geological setting
In the Santa Bárbara area Sub-basin, the area is a long narrow N20•E ridge formed by Acampamento Velho Formation volcanic rocks, where the main elevations are Cerro do Bugio (419 m), Cerro do Perau (331 m) and Serra de Santa Bárbara (440 m), from North to South (Fig. 12). In this area, an unconformity marks the lower contact of the Acampamento Velho Formation over the sedimentary rocks from Maricá or Bom Jardim allogroups (sensu Paim et al. 2000). The upper contact with Santa Fé or Lanceiros alloformations is delineated by a disconformity. The Acampamento Velho Formation is composed of a Lower Mafic Association and an Upper Felsic Association (Fig. 12). The Lower Mafic Association is composed of basaltic and andesitic basalt flows, as well as subordinate andesitic breccias that occur as a continuous bed, with thickness between 10 m and 350 m. It is usually massive, with rare stratification, dipping about 20• to the E or SE (Fig. 12). These rocks show porphyritic texture with plagioclase phenocrysts (Almeida et al. 2002(Almeida et al. , 2003a. The Upper Felsic Association is composed of rhyolitic rocks and comprises alternating pyroclastic rocks (lapilli-tuffs, tuffs, welded tuffs) at the base and flows at the top (Fig. 12). Its stratification is tilted, dipping about 20• to the E or SE. The lapilli-tuffs are preserved as discontinuous strata of thicknesses up to 40 m. The tuffs occur as lenses of variable thickness (up to 30 m) and internally consist of parallel layers, poorly sorted in general terms. The welded tuffs are also poorly sorted and present predominantly ash fraction, occurring as lenticular layers up to 350 m thick. The rhyolitic flows form a continuous layer of variable thickness from 20 m to 600 m. They display internally of flow foliation, which is frequently folded (Zerfass & Almeida, 1997. The lapilli-tuffs, tuffs and welded tuffs are interfingered and associated of pyroclastic flows generated during the rhyolitic eruptive phase, as a product of the eruptive column collapse. Pyroclastic fall processes are predominant in distal regions, as it is suggested by the well sorting of the finer tuff members. The rhyolitic flows overlie all of the previous facies, suggesting that the UFA is related to plinian volcanism (Zerfass et al., 2000).

Mineralogy and petrography
The basalts and andesitic basalts from lower mafic association show relict pilotaxitic texture and zoned plagioclase phenocrysts with diffuse appearance. Sericite, kaolinite, carbonate, chlorite and opaque minerals replace totally the pyroxene phenocrysts and sometimes, partially the plagioclase. Quartz, plagioclase and sanidine grains present on the top layers incipient "kidney-shaped" texture. The matrix is formed by plagioclase microliths, chloritecarbonate, a ferro-magnesian pseudomorph (pyroxene?) and a large quantity of opaque minerals (Almeida et al., 2002(Almeida et al., , 2003a. Electron microprobe analyses show that plagioclase phenocrysts and microlites are sometimes totally albitized, with compositions between Ab 99.6 and Ab 98 (Table 6), which should be related to the interaction of late fluids on the plagioclase resulting in the albite formation. The smaller grains in the matrix are strongly altered to kaolinite. The pyroxene is altered to chlorite and generated opaque minerals. The late magmatic fluids rich in CO 2 used part of the Ca from plagioclase and/or pyroxene to form calcite. Albite and clay minerals are therefore product of late fluids relatively enriched in Na. Zircons show prismatic-pyramidal shapes and diffuse zonation (Fig. 9). They occur as inclusions in Ti-rich magnetite and pyroxene (Almeida et al., 2007). According to Almeida et al. (2007), for the lower mafic association, the crystallization sequence is: (1) zircon, (2) Ti-rich magnetite, (3) pyroxene, (4) plagioclase, (5) albite resulting from the late introduction of Na-and CO 2 -rich fluids, which affected mainly the plagioclase and pyroxene (subordinate), (6) sericite, chlorite and calcite from late solutions that altered the pyroxene, and sericite-kaolinite that altered the plagioclase.
In the Upper Felsic Association, the lapilli-tuffs contain poorly sorted lithoclasts (3 to 40 mm in diameter); vitroclast pseudomorphs (cuspate and platy shapes) substituted by silica and phyllosilicates, quartz crystalloclasts with corrosion gulfs, sanidine and heterogeneous alkali-feldspar (Fig. 13a). The latter is produced by the sodic metassomatic alteration of sanidine, forming heterogeneous pseudomorphs, where part of sanidine is transformed to albite. The lapilli-tuffs matrix is tuffaceous and microcrystalline. The tuffs and welded tuffs differ from each other on the welding degree. They contain crystalloclasts of euhedral quartz or with corrosion gulfs, heterogeneous alkali-feldspar, sanidine (altered to phyllosilicates) and magnetite. Eutaxitic flow structures, conchoidal fractures and perlitic textures are common (Fig. 13b). The tuffaceous matrix is composed of cuspate and platy-shaped fragments (pseudomorphs of volcanic glass shards) and pumice shard-shaped fragments in the welded tuffs ( fig. 13c), suggesting pumice pseudomorphs. Original glass is strongly devitrified. Spherical spherulitic structures occur subordinately. Table 6. Albitized plagioclase analysis (electron microprobe) of the Lower Mafic Association from basaltic and andesitic basalt flows from Acampamento Velho Formation. n.d. = not detected. (Almeida et al., 2007).
Tuff analyses by electron microprobe (Table 7) show that the alkali-feldspar crystalloclasts are totally albitized (Ab 99.6 to Ab 98.9 ). Scanning electron microscope analysis show that alkali-feldspar is heterogeneous, contain albite and sanidine in the same crystals. Albite is the product of sanidine alteration. These tuffs display shard pseudomorphs devitrified to illite (Fig. 14a) and crystalloclasts of sanidine with illite pseudomorphs (Fig. 14b).
Magnetite is Ti-rich (Table 8) and has inclusions of zircon grains, which are also located around the grain edges. Ti-rich magnetite is pseudomorphically replaced by sanidine and ilmenite along cleavages planes, and their edges are partially corroded by reaction with the matrix (Fig. 14c).   (Almeida et al., 2007).
The analyses of welded tuffs by electron microprobe (Table 9) show that the alkalifeldspar crystalloclasts are composed predominantly by sanidine with variable amounts of albite and K-sanidine (Fig. 15a). The matrix contains predominantly K-sanidine and also sanidine Na-rich (Ab=32). Plagioclase crystalloclasts (andesine-labradorite) are present in some samples. The welded tuffs present pseudomorph pumices in fiammes, heterogeneous alkali-feldspar and sanidine crystalloclasts that are altered to illite and sometimes corroded by matrix (Fig. 15b). The Ti-rich magnetite crystalloclasts are altered to ilmenite and rutile (Table 8), which are disposed according to twinning and/or cleavage planes (Fig. 15c), and sometimes replaced by sanidine. Homogeneous and zoned zircons usually occur as inclusions, similar to those observed in tuffs, in heterogeneous alkali-feldspar and quartz. Table 8. Oxides analysis (scanning electron microscope) of the Upper Felsic Association -Acampamento Velho Formation tuffs, welded tuffs and flows. Mt = magnetite; Il = ilmenite; Ru = rutile. n.d. = not detected. The Si and Al detected is due probably the sanidine contamination that is inside the Ti-enriched magnetite. (Almeida et al, 2007).
The rhyolitic flows are homogeneous or banded. Relict structures of perlitic devitrification and conchoidal fractures are common in the microfelsitic matrix. Sanidine, heterogeneous alkali-feldspar and quartz phenocrysts display corrosion gulfs and conchoidal fractures (Figs. 16a). Iron oxide/hydroxide and sericite are also present as alteration products. Rhyolitic flows, when banded, show an intercalation of thick spherical spherulites, product of devitrification, and microcrystalline bands of quartz and feldspar. Electron microprobe analyses of spherulites show heterogeneous composition with fine aggregates of anorthoclase and albite grains (Fig. 16b). Analyses of rhyolitic flows by electron microprobe ( indicate that Ti-rich magnetite (Table 8) and zircon are similar to those described before. The Ti-rich magnetite crystalloclasts are pseudomorphically replaced by sanidine, rutile and ilmenite, which upon interaction with a late fluid altered to TiO 2 , probably rutile and hematite, according to the reaction: 4FeTiO 3 + O 2 → 4TiO 2 + 2Fe 2 O 3 . The interaction with this late fluid also promoted the migration of Ti into cleavage planes and the crystallization of ilmenite and rutile.   Table 9. Sanidine and heterogeneous alkaline feldspar analysis from Upper felsic association -Acampamento Velho Formation welded tuffs. Data obtained through the electron microprobe and scanning electron microscope: inter = intermediary; In Mt = analyses made in magnetite that is substituted by sanidine of the matrix. n.d. = not detected. (Almeida et al., 2007).  (4) quartz. The introduction of late Na-rich fluids generated the formation of (5) heterogeneous alkali-feldspar, (6) ilmenite and rutile from the Ti-rich magnetite, (7) albite in the spherulites and finally generated, the alteration of sanidine, vitroclasts and pumice to (8) illite. Almeida et al. (2002Almeida et al. ( , 2003a already published the Acampamento Velho Formation geochemical characteristics and the text above is a synthesis of mentioned papers. Twenty samples were analyzed for major, trace and Rare-Earth elements using Argonium Plasma Spectrometry (ICP) at Activation Laboratories LTD (ACTLABS, Canada). The results shown in Table 11 correspond to four samples of the basalts and andesitic basalts, two of the tuffs, four of the welded tuffs and ten of the rhyolitic flows.

Geochemistry of major, trace and Rare-Earth elements
For the Lower Mafic Association group (  Fig. 17a).  For the Upper Felsic Association, the tuff samples show a SiO 2 average of 73 wt%, with a low alkalinity (average of Na 2 O = 1.5 wt% and K 2 O = 3.9 wt%), and high CaO content (2.9 wt%). The light Rare Earth element pattern shows slight fractionation (1.6 < La/Sm N < 7.9, average of 4.7), with a variable Eu negative anomaly (Eu N /Eu* = 0.1 to 0.3, average of 0.2). The welded tuffs are also highly siliceous with SiO 2 average of 78 wt%, low CaO content (average of 0.1 wt%) and the alkalinity higher than tuffs (Na 2 O average = 1.7 wt% and K 2 O = 5.7 wt%). The Rare Earth element behaviour is similar to the tuffs, although they present much more pronounced light Rare Earth element fractionation (4.1< La/Sm N < 21.2, average of 7.2) and an important Eu negative anomaly (0.1 < Eu N /Eu* < 0.2, average of 0.1) ( Table 11, Fig. 17b). Likewise, the rhyolitic flow samples are also siliceous, with SiO 2 average of 77 wt%, low CaO content (average of 0.2 wt%) and normal alkalinity, although these rocks are more sodic than the pyroclastic ones (Na 2 O average = 2.2 wt% and K 2 O = 5.3 wt%). The REE pattern is similar to the tuffs and welded tuffs, with LREE fractionation (1.7 < La/Sm N < 12.2, average of 4.5) and Eu negative anomaly (0.1 < Eu N /Eu* < 0.3, average 0.1) (Table 11, Fig. 17b).  The light Rare Earth element fractionation in the rhyolitic flows is slighter than that observed at welded tuffs and very similar to the tuffs (1.7 < La/Sm N < 12.2, average of 4.5). The Eu anomaly is similar to that of the welded tuffs (Fig. 17b) Almeida et al. (2002Almeida et al. ( , 2003a, the Acampamento Velho Formation bimodal volcanism is characterized by the presence of dominant acid and subordinated mafic rocks, with overall absence of rocks with SiO 2 content between 54 wt% and 67 wt%. The Nb x Zr and Y x Zr diagrams (Fig. 18) show different evolutionary trends for the mafic and felsic successions, reinforcing the bimodal character of this magmatism.

Rodeio Velho Member: Geological context, petrography and geochemistry
The Rodeio Velho Formation (sensu Ribeiro et al., 1966) has been described as bearing at least three vesicular andesite flows, with thickness estimated at 100 m and no evidence of explosive activity. However, it was observed that the Rodeio Velho Member manifested itself as flows, pyroclastic deposits and shallow intrusions. There are few petrographic and geochemical studies on the Rodeio Velho Member. Silva Filho (1996) showed the intrusive character of the magmatism, rejecting former ideas of an exclusively volcanic event. Fragoso Cesar et al. (2000) named these rocks as Rodeio Velho Intrusive Suite, which are represented by tabular intrusions within the sub-horizontal continental deposits of the Guaritas Group. Lopes et al. (1999) mentioned its occurrence in subsurface at the CQP-1-RS probing, North Camaquã Mines, where reach 119.50 thickness. Almeida et al. (2000Almeida et al. ( , 2003 studied this event at the following areas within Camaquã Basin (Fig. 1)

Geological setting
The studied regions are localized in Southeast, South and Southwest Caçapava do Sul city (RS), in the Camaquã Basin (Fig.1). The text is a synthesis of Almeida et al. (2000Almeida et al. ( , 2003aAlmeida et al. ( and 2003b.

Santa Bárbara sub-basin
The Rodeio Velho Member volcanic rocks are localized South-centre and North-centre portions from Arroio Santa Bárbara and Arroio Carajás (topographic map, respectively). This event presents four volcanic cones opens to SE and lined up NNE-SSW direction, parallels to the regional flow directions, evincing the structural control in the positing of these cones. Ejected bombs and blocks of vesicular basaltic andesitic rocks, local concentrations of jasper, chalcedony and geodes are present throughout the area. They probably represent the end of a strombolian-type event associated with the evolution of the cones. The preservation of this structure is regarded as due to the presence of younger sedimentary rocks across the region. These sedimentary rocks include arkosic sandstones, conglomeratic sandstones, and conglomerates. They correspond to the sedimentation named as Varzinha Alloformation (or Formation) from the Guaritas Allogroup (Table 1). An elliptical caldera that shows its longest axis about 7.2 km NNE-SSW and it is found in the surrounding of the cones. (Fig. 19).
To the West and South of the cones, Rodeio Velho Member basaltic dykes intrude the sediments building the Lanceiros Alloformation when they are solidified. These layers of the basaltic rocks, sub-vertical and preferentially aligned to N26 o E, are massive in the centre and vesicular along the borders. They have been interpreted as a Rodeio Velho Member intrusive manifestation. However, considering that a dyke-sandstone contact was found in the sandstones vesicles, which was probably generated by gases coming from the magma and suggests that the sediment was still unconsolidated, so, the Rodeio Velho Member volcanism would be contemporary with the formation of the sandstone from Pedra Pintada Formation. 2 km to the South of the cones, in the Carajás Creek, the Rodeio Velho Member also crops out as lava flows, showing aa and pahoehoe structures with centimetric to decimetric hollow tubes, as well as amygdales and/or vesicles that are up to 5 mm long at the base and top of each flow (Fig. 20a). The sedimentary rocks layers are mainly composed of middle granulometry to fine sandstones and show crossed stratification of very low angle. Levels of pelites associated to this sandstone show feature of subaerial exposure with contraction cracks (Fig. 20b). Rodeio Velho Member is positioned as a flow that went through the dunes, when the solidification occurs, the structure looks like an intrusion of the volcanic in the sedimentary rocks, but they correspond to a volcano-sedimentary interaction of the Rodeio Velho Member and Pedra Pintada Formation, and rhythmites from Guaritas Group (Fig. 21a). The volcano-sedimentary interaction features include also flow striation, xenoliths, clastic dykes, marks in increasing and peperites (Figs. 21 b, 21c and 21d). Based on field characteristics, it seems that the emitting centers would be exactly at the cones, which are 2 km North and open towards the South .

Guaritas sub-basin
The Rodeio Velho Member are located at Minas do Camaquã, Rodeio Velho, Passo do Moinho and Pedra da Arara regions, and Arroio dos Neves drainage line (Fig. 1). These volcanic rocks show flows interdigitated in Pedra Pintada Alloformation rhythmites and the volcano-sedimentary interaction. Field observations indicate a contemporaneity between sedimentation and volcanism. The peperites, flux channels, clastic dykes and other features development suggest interaction between hot lava and wet poorly consolidated sediments. In Pedra de Arara area (Fig. 22a), it is observed the same structure found at Arroio Carajás region with volcanic rock positioned as the flow that went through the dunes, when the solidification occurred. The structure looks like an intrusion of the volcanic in the sedimentary rocks, but they correspond to an volcano-sedimentary interaction in this area with peperites in the sandstone-volcanic rock contact. Immediately above, at Passo do Moinho (South Pedra da Arara, Fig. 1), the sandstones are restricted to the enclaves and clastic dykes (Fig. 22b). www.intechopen.com

Mineralogy and petrography
According to Almeida et al. (2000), the RVM is characterized by mafic and intermediate lava flows, pyroclastic deposits and intrusions. Petrographically, these rocks are andesites, subalkaline basalts and trachyandesites. The rocks show pylotaxitic, vitrophyric or ophitic textures, with glomeroporphyritic euhedric to subedric plagioclase phenocrysts. The plagioclase shows size around 0.5 mm, and is generally altered to carbonate and can be oriented within de matrix. Relics of euhedric pyroxene, olivine and opaques (Ti-magnetite and other Fe-Mn oxides) are present and show sizes that vary from 0.3 to 0.1 mm. Accessory minerals such as apatite and zircon are observed. Intersertal glass is recrystallized (forming spherulites) or altered. The presence of vesicles and amygdales is common and the sizes vary from 0.5 to 10 mm. The amygdales are filled with quartz and carbonate or both of them. The quartz precedes the carbonate. Pyroclastic rocks are stratified and vary from ashflow tuffs to lapillites, brecciated and poorly sorted. Shards and fiammes attest to the pyroclastic character. Plagioclase and euhedral quartz crystalloclasts (<2%) are dispersed in the tuffaceous, partially glassy matrix. Almeida et al. (2000Almeida et al. ( , 2003aAlmeida et al. ( and 2003b already published the complete Rodeio Velho Member geochemical characteristics. The text above is a synthesis of mentioned papers. The chemical analysis data are shown in  Table 12. Geochemical analysis of the major, trace and Rare Earth elements . These analysis were performed at Activation Laboratories (ACTLAB), Canada, using the Argonium Plasma Spectrometry -ICP. The Rodeio Velho Member rocks were plotted in the Nb/Y versus Zr/TiO 2 classificatory diagram (Winchester & Floyd, 1977 -Fig. 23a), which allowed classify the flows as subalkaline basalts, andesites, trachyandesites, alkaline basalts and basaltic andesites. The pyroclastic rocks were classified as trachyandesites, and the epizonal as trachyandesites and alkaline basalts. The REE pattern normalized by the chondrite (Nakamura, 1977-Fig. 23b) shows a similar behavior to the alkaline basalts and the values correspond to the evolved rocks (Culler & Graf, 1984), displaying a marked fractionation and parallelism between all the rocks. According to the data and the REE distribution, the light Rare Earth elements (LREE) enrichment occurs under conditions of low partial melting, especially from a source that contains garnet, suggesting a great deep for genesis of these rocks.   (Wood, 1980). A = MORB type N; B = MORB type E, tholeiitic WPB; C = alkaline WPB and differentiated; D = destructive continental margin basalts and differentiated. Legend for figure 23 (b) Multi-elements spiderdiagram normalized by Sun and McDonough (1989) values .

Geochemistry
Considering the chemical data, all the Rodeio Velho Member rocks are similar. Thus, the samples plotted on Hf/3 x Th x Nb/16 diagram (Wood, 1980 -Fig. 24a) show that the rocks were generated in a domain of intraplate continental basalts (alkaline and/or E-MORB type). This behavior is confirmed in the Zr/Y versus Zr diagram (Pearce & Norry, 1979) and in the spidergram normalized by N-MORB (Sun & McDonough, 1989 -Fig. 24b). These rocks probably were formed by fractionated crystallization, as indicated by the good correlation between the incompatible elements pairs, such as Ce versus Sm-La, and Zr versus Nb and Y. Almeida et al. (2005) already published the complete Acampamento Velho Formation and Rodeio Velho Member isotopic characteristics. The text above is a synthesis of the mentioned papers.

Rb, Sr, Sm and Nd isotope data for Acampamento Velho Formation and Rodeio Velho Member
The isotopic results obtained in this study are shown in Table 13 and (Fig. 25a). The opposite occurs with the Acampamento Velho Formation mafic lavas. The data in figure  25b show a negative correlation between radiogenic Sr and εNd for the Acampamento Velho Formation samples.  figure 25. .       figure 25. .

Acampamento Velho Formation -Upper Felsic Association
Regarding the Nd model ages (Table 13), we can see Acampamento Velho Formation and Rodeio Velho Member mafic associations with values raging from 1.1 to 1.8 Ga and 1.5 to 2.0 Ga, respectively. These model ages point to also for a modified mantle at end of Brasiliano Cycle, in post-orogenic tectonic setting (during the Upper Neoproterozoic to Eopaleozoic). However, Acampamento Velho Formation has some juvenile component with already modified mantle since it displays somewhat younger model ages and lesser negative εNd values. On other side, the Acampamento Velho Formation felsic association Nd model age is from 1.3 to 2.8 Ga (Table 13). This information coupled with Acampamento Velho Formation felsic association lower εNd negative values, if compared to those of mafic association, suggest at least two alternatives, namely, either (1) the felsic magma is a melting of a lower crust with a dominant Paleoproterozoic age or mixed Brasiliano-older crust, or (2) the felsic magma may be derived from the mixture of mafic magma and a crustal enrichment component as described below.
In this case, the existence of the different sources in the mantle during the Neoproterozoic magmatism in the Camaquã Basin could be connected with a sedimentary component. The subduction process could be responsible for 20-30% of contamination by sediments, probably Neoproterozoic arkosic quartzites, and the establishment of a reservoir. The generated magma at some point initiated its ascension under conditions of high oxygen fugacity. At a certain stage, this magma separated into two fractions: a mafic portion, from which Acampamento Velho Formation mafic lavas were originated, and another one, enriched in crustal component, which gave rise to the AVF felsic rocks. The rocks belonging to the Rodeio Velho Member seem to have been originated from a different reservoir, much more enriched in incompatible elements and depleted in radiogenic Sr with EMI I characteristics.

Camaquã Basin Volcanism-tectonic environment -Conclusions
The Camaquã Basin developed during the final stages of the Brazilian-Pan-African Cycle (700 Ma -500 Ma.) in the Dom Feliciano Belt. This basin was formed in a retroarc setting and was filled essentially by clastic sediments and volcanogenic rocks . The Maricá Formation (Leinz et al., 1941) is the lowest sedimentary sequence for Camaquã Basin formed as retroarc foreland basin, and comprises a fluvial to marine sedimentation formed due to the load of the adjacent granitic-gneissic Pelotas Belt and volcano sedimentary Porongos Belt (Gresse et al., 1996;Paim et al., 2000), which contains depositional sequences formed in fluvial and marine sediments. The deposition interval of the Maricá Formation is between 601 and 592 Ma based on the age obtained of detrital zircon from Maricá Formation sandstone, at 601±13 Ma, and the age of the Hilário Member, which overlies in unconformity the Maricá Formation units, at 590 ± 5.7 Ma (Janikian et al., 2008) and 591±3.0 Ma (Fig. 29). The geochemical and isotope signature of the Maricá Formation sediments point to reworked crustal material based on the Sm-Nd data (Borba et al., 2008). The detrital zircons of the sample CK 239D have contribution of the Brasiliano magmatic arc material and also some Paleoproterozoic crustal material (Siderian, Rhyacian and Statherian) and Neoarchean. The overlying Bom Jardim units consist of a typical continental sedimentation with fluvial, delta and lacustrine sediments with a strong presence of intermediate shoshonitic magmatism at the base, represented by Hilário Andesites and associated to lamprophyres (Lima et al., 1995). These magmatic rocks are very well constrained by U-Pb ages obtained for Hilario Formation at 590 ± 5.7 Ma (Janikian et al., 2008) and 591.8 ±3.0 Ma (this work) (Fig. 29). The last age was obtained by us using a lamprophyric sample (alkaline basalt).
Considering that the lamprophyres correspond to the last manifestation of the Hilário volcanism, we can suggest that this event is positioned roughly between 592 and 590 Ma. The volcanic rocks that compose the Bom Jardim Group base are related to the calc-alkaline magmatism from shoshonitic affinity (Nardi & Lima, 1985;Lima et al., 1995) and also associated with a stabilization phase of the plate subduction process (Lima & Nardi, 1992;Chemale et al., 1999;Almeida et al., 1999 Sommer (2005) for Ramada Plateau acid volcanic rocks of the is 549 ± 5 Ma and by Janikian et al. (2008) for acid volcanic rocks in the Caçapava region is 574±7 Ma. However, we obtained the U-Pb zircon age of 553 ± 5 Ma for Acampamento Velho Formation using an andesitic basalt sample of the lower mafic association, which is close to that age of Ramada Plateau. In the Santa Bárbara (Fig.29, west column) and Guaritas sub-basins (Fig. 29, east column), the basic flows from basal contact, pyroclastic manifestations and rhyolitic flows (from the base to the top) of the Acampamento Velho are in unconformity with the Bom Jardim Group, and the upper contact is made by unconformity with the Sequence I (or Santa Fé and Lanceiros Formation base from Santa Bárbara Group). The observed stratigraphic position in field -basalts and andesitic basalts on the base and felsic sequence on the top (Figs. 12 and 29), as well as the geochemical and isotopic behavior, show that the first phase formed the basalts and andesitic basalts, and later, the rhyolitic rocks, which confirm that the Acampamento Velho Formation is bimodal volcanism. The best estimated ages for the Acampamento Velho Formation and Santa Barbara Group are 553 and 549 Ma, during which, an extensional events controlled the deposition and magmatism. The Santa Barbara Group units are overlied in angular unconformity by the sediments of the Guaritas Group. At the base of this group, occurs the alkaline basalt lavas called as Rodeio Member. We obtained the age of 547 ± 6.3 Ma for the Rodeio Velho Member alkaline basalt, which can be interpreted the maximum age for the magmatism, therefore, the deposition of the Guaritas Formation. On other side, detrital zircon collected from the upper portion of the Guaritas Group is dated at 535±10 Ma ). The present data suggest a Cambrian age for the Rodeio Velho Formation, instead the U-Pb age of 470 ± 19 Ma (sensitive high mass-resolution ion microprobe -SHRIMP) obtained by Hartmann et al. (1998) for the Rodeio Velho Member. Santa Bárbara Sub-basin ( Fig. 29 -west column), which is above the Rodeio Velho Member (Pedra Pintada Formation), is in contact by disconformity with the Sequence III (or Segredo Formation). Guaritas Sub-basin ( Fig. 29 east column), that is above the Rodeio Velho (and Pedra Pintada Formation), is in contact by disconformity and we can see the Santa Bárbara Group sedimentary rocks (Fambrini, 2005). Thus, beyond the U-Pb ages, the stratigraphic position showing the Rodeio Velho Member above Acampamento Velho Formation, allow us to postulate that first was generated the Acampamento Velho volcanism, and then, the Rodeio Velho volcanism. The Guaritas Group sedimentation occurred under more stable conditions in the retroarc region, and it is deposited under a transtensional tectonics. Fig. 29. Santa Bárbara and Guaritas sub-basin: stratigraphic columns including the main unconformity and disconformity, which bounds the Camaquã Basin unit. West column is modified from Paim et al (1995Paim et al ( , 2000, Almeida et al (2002) and Borba & Misuzaki (2003). East column is modified from Paim et al. (1995), Wildner (2002), Fambrini et al. (2005) and Janikian et al. (2003Janikian et al. ( , 2008. The Rodeio Velho Member is interbedded or arranged in volcano-sedimentary features of the interaction (flow striation, xenoliths, clastic dykes, marks in increasing and peperites) with the Pedra Pintada Formation sedimentary rocks from Guaritas and Santa Bárbara subbasins. We would like to point out that, between these two sub-basins, there is the Caçapava structural hill, the Caçapava Granite (average U-Pb age of 550 Ma) and the Caçapava do Sul Suture ( Fig. 1 and Fig. 29). According to Nardi and Bitencour (1989), the Caçapava Granite was positioned in mesozone level, with microgranitiques and aplitiques rocks positioned at summit of the body and mainly associated to leucogranitiques. Considering these information, we may suggest that the granite together with the rocks above (more than 6 km thick) may have suffered uplift and denudation processes during a long time before the granite emerge. These same authors mentioned that the Caçapava do Sul Granite Complex is a body composed of two contrasting blocks bounded by NW-trending structures. Borba et al. (2002) from the study about apatite fission-track (FT) thermochronology for samples of the Caçapava do Sul granites determined to the Northern block, an apparent FT ages of 293.5 and 274.8 Ma, and an estimated for the initial track recording an age of 366 Ma. These results are interpreted in the frame of the uplift and denudation history of the region during the Paleozoic-Mesozoic intraplate Parana´ Basin evolution, as influenced by the convergent tectonic setting of Southwestern Gondwana. In the Southern block, where dioritic and gneissic portions crop out and apparent FT ages range from 252.1 to 245.5 Ma. A possible subsequent Upper Cretaceous uplift event can be inferred from the modeled thermal history curves and the apparent FT age of the Northernmost collected sample is 73.7 Ma. If it is considered the presence of aplites and microgranites at the summit of the body, than it can be said that it has suffered little denudation after have emerged. Costa (1997), based on geophysical data interpretations, mentioned that Caçapava do Sul Suture corresponds to a deep geophysical anomaly that separates two magnetic domains. Therefore, it is important to consider an independent basin evolution for each suture side and beyond that this subbasin may have shown different subsidence degree. If we consider that Guaritas Sub-basin the Santa Bárbara Group rocks show marked dips, and the Guaritas Group rocks do not show this feature, while that in Santa Bárbara sub-basin, the rocks Guaritas Group´s base (Pedra Pintada Formation) according to Almeida et al. (2003a), the layers exhibit latitudes 188 o /40 o NW, 194 o /23 o NW e 195 o /26 o NW, then, we could suggest that tilting would be produced by biggest Caçapava Suture influence (when reactivated), which is in direct contact with these rocks. Paim et al. (2000) attributed tilting of these rocks as a reflex of normal falls, and subordinately strike slip movements. Thus, these declivities can be formed due to basin different subsidences. Almeida et al. (2002Almeida et al. ( , 2003aAlmeida et al. ( , 2003bAlmeida et al. ( and 2005 based on Brazilian Orogeny evolution pattern from Chemale Jr. (2000); considered that the Acampamento Velho Formation was generated in an extensional regime preceding the Rio de la Plata and Kalahari continental plate collisions. The Rodeio Velho Member may correspond to the late magmatic manifestation and may have occurred after the collisional phase between Rio de la Plata (with Encantadas Microcontinent attached) and Kalahari Continental Plate. In the context of adopted pattern, it would correspond to the Phase IV of the same author ( Fig. 2 and Fig. 30). Thus, we propose that between 560-550 Ma, considering only the Camaquã Basin central portion, it may exist two extensional basins depocentre, such as Santa Bárbara and Guaritas sub-basins (Fig. 30b), being them limited by Caçapava Hill. These sub-basins were already partially filled by coastal deposits at their base, including fluvial and shallow marine sediments, with scarce volcanic contribution and U-Pb age of 601 Ma (Maricá Group). On these rocks, is an unconformity named as Bom Jardim Group, composed of volcanic rocks at the base (Hilário Formation) and sediments at the top. On these rocks, in disconformity, are volcanic rocks of the basaltic and rhyolitic composition (Acampamento Velho). According to Almeida et al (2005Almeida et al ( , 2007, the generated magma (of Acampamento Velho) at some point initiated its ascension. At a certain stage, this magma separated into two fractions: a mafic portion and another one, enriched in crustal component. Between 560-550 Ma, after the magmatic chamber formation as well as the separation of the magma into two fractions, both fraction magmas rise and mafic flow manifestations (lower mafic association) took place around 553 Ma. The other magma fraction underwent through a significant enrichment in crustal component. This viscous magma slowly rise around 549 Ma revealing itself in a first phase as pyroclastic rhyolitic rock (tuff, lapillitic tuff and welded tuff) and afterwards as a rhyolitic flow (the upper felsic association).
In the range of 547-535 Ma (Fig. 30c), an alkaline mantelic magma may ascend in the last magmatism stages by the deep faults during an intra-plate mechanic extensional event, generating the Rodeio Velho Member event (Fig.30c). The Rodeio Velho Member volcanic manifestations took place as flows interbedded which sometimes show interactions of the lava with sediments poorly consolidated, and correspond to the Pedra Pintada Formation eolian deposits from Guaritas Group. We can observe these features at Santa Bárbara and Guaritas sub-basins.