Post-collisional basalts of the Acampamento Velho Formation , Camaquã Basin , São Gabriel Terrane , southernmost Brazil

Manuscript ID: 20170019. Received in: 13/02/2017. Approved in: 07/18/2017. ABSTRACT: The basic volcanic rocks in the Palma region, southern portion of the São Gabriel Terrane, have always been interpreted as generated during the active subduction period of the São Gabriel orogeny (Cryogenian). This terrane was built as the result of the Charrua Ocean closure between 900–680 Ma. The basalts show a subhorizontal igneous flow foliation and porphyritic texture, with plagioclase phenocrysts in a thin matrix composed of plagioclase, augite and magnetite, commonly altered to actinolite, chlorite and epidote. They have amygdales and veinlets reflecting a pervasive hydrothermal phase and are affected by thermal metamorphism related to Jaguari granite intrusion. Two samples were dated by the U-Pb zircon geochronology and yielded crystallization ages of 563±2 Ma and 573±6 Ma. The basalts have transitional composition from tholeiitic to calc-alkaline, metaluminous character, trace elements patterns rich in large-ion lithophile element (LILE) with negative anomalies of Nb, P and Ti, slight enrichment in light rare-earth elements (LREE) and horizontal pattern of heavy rare earth elements (HREE). The data allow interpreting the basalts as belonging to the Acampamento Velho Formation of the Camaquã Basin, and related to the basalts of the Ramada and Taquarembó plateaus. These associations represent the final evolution of the volcanism generated in the post-collisional period of the Dom Feliciano Belt.


INTRODUCTION
Recognizing the composition and meaning of the volcanic rocks is important in the characterization of sedimentary basins and oceanic crust subduction processes.The difficulties in rock classification are associated to the low degree of crystallinity of the matrix, whose characterization depends on obtaining their chemical composition.Similarly, the presence of volcanic rocks is associated with the occurrence of partial crust and/or mantle melting processes, and with the existence of structures such as faults, that allow the rise and outpouring of lava.The chemical composition of the volcanic rocks is essential to characterize the magmatic series and to assess the fractionation processes, the occurrence of assimilation and the sources of magmas.These factors, associated with the regional geology and geochronology features, define their tectonic significance.
The Brasiliano Orogeny Cycle in Rio Grande do Sul is represented by the Dom Feliciano Belt (DFB), which is a unit consisting of rock associations developed during three orogenic events.These orogenies were called Passinho (900-860 Ma), São Gabriel (770-680 Ma) and Dom Feliciano (650-540 Ma), and are characterized by a wide generation of igneous, metamorphic and sedimentary rocks (Chemale Jr. 2000, Hartmann et al. 2007, Saalmann et al. 2010, Philipp et al. 2016).The units that represent the subduction period and juvenile accretion are restricted to the São Gabriel Terrane (SGT).This terrane is the western portion of the DFB and features an elongated shape in the N20-30ºE direction, about 110 km long and 60 km wide (Fig. 1).The composition of the SGT is defined by the tectonic intercalation of ophiolite complexes with metavolcano-sedimentary associations and granite-gneiss complexes (Fig. 2).This interleaving is characterized by the thrusting of elongated bodies in the NE-SW direction, through ductile shear zones of medium angle and eastward vergence.
The Palma Complex was proposed by Garcia and Hartmann (1981) to designate a flysch-type sequence arranged as an elongated structure, in the N30ºE direction, exposed between São Gabriel and Lavras do Sul (RS) regions, extending from 5 km, south of Palma village, up to about 20 km, northeast of it.The complex is subdivided into two major associations composed of metasedimentary and meta-igneous rocks (granites, metadacites, meta-andesites, metabasalts, metagabbros, meta-ultrabasic rock and lamprophyres).Other authors have adopted similar proposals, recognizing these groups of rocks and associating them to subduction processes and orogenic regional metamorphism (Chemale Jr. 1982, Santos et al. 1990, UFRGS 1996, Laux et al. 2012).This article proposes the separation of the unit originally characterized as metavolcanic in this context, since the obtained petrological and geochronological data associate this unit to volcanism of a more recent age, correlated to the Acampamento Velho Formation, Camaquã Basin (CB).
The main goal of this article was to characterize the mafic volcanic rocks occurring in the Palma region, through the stratigraphic relationships obtained with field data, combined with petrographic, geochemical and geochronological data.The described basalts were compared with other mafic volcanic rocks belonging to the Acampamento Velho Formation, 20 km to the northeast, on the Ramada Plateau, and about 20 km to the southeast, on the Taquarembó Plateau.

MATERIALS AND METHODS
This survey was based on the integration of the contact relationships obtained in geological mapping in 1:25.000scale with structural, petrographic and geochemical data.Ten samples of mafic volcanic rocks were analyzed, with U-Pb zircon geochronological analyses of two basalt samples.
The thin sections were prepared in the Institute of Geosciences of Universidade Federal do Rio Grande do Sul (UFRGS).The steps of sample preparation are: ■ Cutting a slab: a slide is cut from a piece of rock collected in field; ■ Initial lapping on the slab: on side of the sample is lapped flat and smooth on a cast iron lap with 400 and 600 grit carborundum; ■ Glass slide: glass slide is glue with epoxy on the lapped face of the slab; ■ Section: using a saw, the slab is cut-off for reduce the thickness; ■ Final lapping: using a hand on a glass plate the thickness of 30 microns are achieved with 600 grit carborundum; ■ Polishing: the polishing is made with a spun on a polishing machine using nylon and diamond paste.
The geochemical analyses were obtained at Acme Laboratories LTDA. by Inductively Coupled Plasma (ICP), for the major elements with detection limit of 0.01% and for V, Ba, Sr, Y and Zr, with a detection limit of 1 to 5 ppm.The analyses of trace elements and rare earth elements were obtained by Inductive Coupled Plasma -Mass Spectrometry (ICP-MS), with detection limit of 0.005 to 2 ppm.The analytical protocol at the ACME Laboratories included the analysis of Standard SO-18 and Blank (STD SO-18 and BLK) and of four sample duplicates.The samples were milled in an agate mill.Representative samples were selected and were not affected by weathering.
In order to carry out Laser Ablation-Multi Collector-Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) U-Pb zircon analysis, two samples of basalts of the Arroio da Palma stream (LV-53 and LV-70) were crushed and milled using a jaw crusher, and then reduced to powder in a disk mill.
Zircon grains were concentrated by conventional magnetic and heavy liquid procedures, the final concentration was carried out by hand picking.To avoid bias introduced during handpicking, no visual morphological or color differentiation was made.These steps were made at UFRGS at the sample preparation laboratories.
The grains used for zircon dating were mounted in epoxy resin, polished using diamond paste to expose their inner parts.Imaging was made by backscattered electrons (BSE) and cathodoluminescence (CL) to determine their internal structures and crystallization phases.Only zircon grains free of imperfections, fractures, and mineral inclusions were selected for isotopic analysis.The most clear and inclusion-free minerals were selected for LA-ICP-MS analyses.CL images of zircon were obtained using a Quanta 250 FEG electron microscope equipped with Mono CL3+ CL spectroscope (Centaurus) at the Geochronological Research Center in Universidade de São Paulo (USP), Brazil.
Isotopic data were obtained using a NEPTUNE ICP-MS coupled with an excimer laser ablation system.The cup configurations optimized for U-Pb data acquisition were IC3 = 202 Hg, IC4 = 204 (Hg+Pb), L4 = 206 Pb, IC6 = 207 Pb, L3 = 208 Pb, H2 = 232 Th and H4 = 238 U, in which L and H were low, with a high mass to faraday cup position, and ICs are ion counting (continuous dynode system).The ICP configurations were: radio frequency power = 1,100 W; cool gas flow rate = 15 L/min (Ar); auxiliary gas flow rate = 0.7 L/min (Ar); sample gas flow rate = 0.6 L/min.Laser Setup: energy = 6 mJ, repetition rate = 5 Hz, spot size = 25-38 μm, helium carrier gas = 0.35 + 0.5 L/min.The routine U-Pb analysis consists of two blanks, two National Institute of Standards (NIST), three external standard (GJ1 standard), 13 unknown samples, two external standards and two blank measurements.Each run consisted of 40 cycles, with 1 second per cycle.The 204 Hg interference on 204 Pb was corrected by 202 Hg, and the value of 204 Hg/ 202 Hg ratio is 4.2. 207Pb/ 206 Pb ratio normalization was achieved by combined NIST and external standards.

206
Pb/ 238 U ratio normalization was achieved by external standards.The GJ1 standard (602±4.4Ma, Elhlou et al. 2006) was used for mass bias correction.Zircon typically contains low concentrations of common Pb.Thus, the reliability of the measured 207 Pb/ 206 Pb and 206 Pb/ 238 U ratios is critically dependent on accurate assessment of the common Pb component.The residual common Pb was corrected according to the measured 204 Pb concentration using the known terrestrial composition (Stacey and Kramer 1975).
The uncertainty introduced by laser-induced fractionation of elements and mass instrumental discrimination were corrected using a reference standard of zircon (GJ-1) (Jackson et al. 2004).The isotope ratios and inter-element fractionation of data obtained by the ICP-MS were evaluated by interspersing the GJ-1 zircon standard in every set of 13 zircon samples (spots).The GJ-1 standard meets the requirements for the methods used in our laboratory, and the ratios of 206

The Dom Feliciano Belt and São Gabriel Terrane
The Sul-Rio-Grandense Shield comprises Paleo and Neoproterozoic tectonostratigraphic units, including fragments of the Rio de La Plata and Luiz Alves cratons (Chemale Jr. 2000, Hartmann et al. 2007, Saalmann et al. 2010, Philipp et al. 2016).These fragments are composed of Paleoproterozoic and Mesoproterozoic metamorphic and granitic rocks, surrounded the Dom Feliciano Belt, a Neoproterozoic orogen derived from the collision of the Rio de La Plata and Kalahari cratons (Fernandes et al. 1992, Saalmann et al. 2010, Philipp et al. 2016).
The units of the SGT represent the subduction period of the Brasiliano orogenesis in the Sul-Rio-Grandense Shield.The units that constitute this terrane have accretionary features and correspond to a juvenile Neoproterozoic crust segment, consisting of the tectonic intercalation of metavolcano-sedimentary associations, paragneisses, orthogneisses and meta-granites, cutting by undeformed granitoids (Babinski et al. 1996, Philipp et al. 2008, Saalmann et al. 2010, Hartmann et al. 2011, Lena et al. 2014, Gubert et al. 2016) (Fig. 2).These units characterize the upper and lower portions of the São Gabriel magmatic arc (Chemale Jr. 2000, Saalmann et al. 2005, Hartmann et al. 2007) and are also intercalated by metamafic-ultramafic sequences that represent ophiolitic associations.
The complex was subdivided by Chemale Jr. (1982) recognized the Cerro da Cruz and Pontas do Salso sequences.The first one consisting of metaultramafites, metabasites, metapelites and quartzites, marbles and calc-silicate rocks with restricted interbedding of metarhyolites and metandesites.The second sequence consisted of calc-silicate rocks, pelitic and arkosean metasediments, metamorphosed to amphibolite facies.Santos et al. (1990) have redefined the Pontas do Salso Sequence as Pontas do Salso Formation.
More recently, the geological mapping conducted by UFRGS (1996) redefined the Pontas do Salso Group as the Monumento and Arroio da Palma formations.The first one consists of epiclastic metasedimentary rocks, metatuffites, metapelites, metasandstones and metaconglomerates, while the latter is represented by metabasalts, metandesites and metadacites.
In mapping conducted by the Geological Survey of Brazil (Laux et al. 2012), the Metamorphic Palma Complex was subdivided into three units: 1. Cerro do Ouro Formation, consisting of serpentinites, magnesian schists, basic schists, amphibolites and metagabbros; 2. Pelitic calc-silicate unit, composed of marbles, quartzites, chlorite schists and BIF's; 3. Metavolcanic unit, consisting of metadacites and metandesites.Laux et al. (2012) interpreted the Palma Complex as an ophiolite complex related to the subduction process that occurred in the Cryogenian period.

The Camaquã Basin
The CB was deposited on the central portion of the Sul-Rio-Grandense Shield, during the tardi-to post-collisional period of the Dom Feliciano Belt covering the igneous and metamorphic units of the Taquarembó, São Gabriel and Tijucas terranes, and over the Pelotas Batholith the three ones included in the belt.
The CB is a depositional locus consisting of four distinct cycles of deposition defined by different sedimentary and volcano-sedimentary units, associated with plutonic igneous rocks, separated from each other by angular or erosive regional unconformities (Chemale Jr. 2000, Paim et al. 2000, Almeida et al. 2012, Janikian et al. 2012).
The sedimentary and volcanic rocks of the CB have deposition ages between 600 and 540 Ma.The volcanic rocks of the Hilário Formation, Bom Jardim Group, have Ar-Ar and U-Pb SHRIMP and LA-ICP-MS zircon ages between 600 and 590 Ma.The acidic volcanic rocks of the Acampamento Velho Formation, Santa Barbara Group, have two U-Pb zircon age groups, between 580-570 and 550 Ma (Wildner et al. 1999, Sommer et al. 2005, Almeida et al. 2010, Almeida et al. 2012, Janikian 2004, Janikian et al. 2012, Oliveira et al. 2014, Matté et al. 2016).The andesites of the Rodeio Velho Formation, Guaritas Group, have U-Pb zircon age around 547±6.5 Ma (Almeida et al. 2012).
According to Sommer et al. (2011) and Matté et al. (2016), there is a predominance of juvenile magmatic components in the pyroclastic deposits, such as crystaloclasts, pumices and glass shards, while lithoclasts are more numerous on the basal portions, and mainly connate.The geometry of the deposits and the degree of welding is variable, from stratified and partially welded deposits, to highly welded massive ignimbrites.With respect to the lavas, the autobrecciated, foliated and massive structures are the most common ones.The authors also stress that the preservation of typical features of primary pyroclastic processes, typical of high temperature flows and large amount of gases, associated with the occurrence of lavas and hypabyssal bodies, indicate subaerial volcanism, possibly related with volcanic calderas.
Petrographic and geochemical studies of the basic rocks in the Palma region were conducted by several authors (Garcia 1980, UFRGS 1996, Strieder et al. 2000, Lopes & Hartmann 2003, Laux et al. 2012).However, geochronological studies (U-Pb) had not been performed in the area.

Field and petrography descriptions
The basalts of the Acampamento Velho Formation occurring in the Palma region form a continuous body, elongated in the N50-60ºE direction, approximately 10 km long and 5 km wide.Volcanic rocks cover the metasediments of the Pontas do Salso Complex in the NE portion, and the metaultramafites of the Palma Ophiolite, in the SW portion.They show tectonic contacts by transcurrent faults with the metaultramafic rocks and host orthogneisses of the Cambaí Complex in the NW portion of the body, and by normal fault with the Jaguari Granite, in its southernmost portion.The main outcrops are along the Palma stream, where were collected the LV-70 sample, which are in a small body (< 5 m 2 ) in the right margin of the Vacacaí River (Fig. 3).The contact with metagabbro shown in the map of Fig. 3 was not apparent.Near the Granite Jaguari contact, the basalts are cut by tabular bodies of pink leucocratic aplites with low thickness (< 3 m).
The basalts are grayish green in color and show massive apparent structure, with thin flow lamination in altered terms (Fig. 4A and 4B).The average foliation orientation is N40ºE, with soft NW dipping.The main texture is porphyritic, characterized by low concentrations of plagioclase phenocrysts, 1 to 5 mm in size, immersed into a fine equigranular to aphanitic matrix composed of clinopyroxene, plagioclase and magnetite (Fig. 4C and 4D).The top portions feature a large amount of ellipsoidal-shaped amygdales and vesicles, with sizes between 5 and 15 cm.They are filled by quartz, epidote, chlorite, zeolite and carbonate (Fig. 4E and 4F).
They feature three patterns of rectilinear and subvertical fractures, oriented in the NW-SE, NE-SW and E-W directions.The fractures are rectilinear, with spacing between 10 and 40 cm, and often filled with veins of quartz and carbonate, accompanied by the disseminated occurrence of pyrite, chalcopyrite and magnetite.
Under the microscope, the flow foliation is defined by the alignment of plagioclase phenocrysts.The porphyritic texture is characterized by with up to 15% of plagioclase phenocrysts of elongated euhedral prismatic shape, highly sericitized, surrounded by very fine to fine equigranular matrix composed of clinopyroxene and plagioclase, with magnetite, titanite and apatite and opaque minerals as accessories.The matrix is commonly transformed by hydrothermalism and thermal metamorphism, with intense replacement of clinopyroxene by actinolite, chlorite and epidote, and of the plagioclase by aggregates of sericite and epidote, with variable occurrence of carbonate (Fig. 4D).Actinolite has elongated prismatic shape and acicular habit, forming disoriented porphyroblasts of up to 1 mm in size.These transformations are recorded throughout the length of the body of mafic volcanic rocks.

Geochemistry
The obtained geochemical analyses (Tab. 1) added the analyses of Lopes and Hartmann (2003) were compared with the available analyses of basalts and andesites of the Acampamento Velho Formation outcropping in the Taquarembó (Wildner et al. 2002) and the Ramada plateaus (Sommer et al. 2005, Matté et al. 2016).
The mafic volcanic rocks of the Palma region feature subalkaline affinity and are classified as basalts and andesitic basalts (Fig. 5A).The ratios between alumina and alkalis define a metaluminous character (Fig. 5B).In AFM (Irvine & Baragar 1971) and FeOt/MgO vs. SiO 2 diagrams (Miyashiro 1974), the samples are arranged on the boundary between the fields of calc-alkaline and tholeiitic rocks (Fig. 5C and 5D).The calc-alkaline composition is reinforced in the Jensen diagram (1976) and characterized as a medium-K type (Le Maitre et al. 1989) (Fig. 5E and 5F).The subalkaline character is also seen in trace elements, as seen in the Winchester and Floyd (1977) diagrams (Fig. 6A and 6B).
The correlation between the composition of trace elements and rare earth elements of the Palma region mafic volcanic rocks, with similar occurrences found in the Acampamento Velho Formation in the Taquarembó and Ramada plateaus, was conducted in multi-element diagrams (spidergrams) normalized to primitive-mantle values.The three sets of samples show a similar pattern, characterized by slight enrichment in Rb, Ba and Th, negative Nb, P and Ti anomalies, and horizontal heavy rare earth elements (HREE) pattern.This affinity is even more evident with the basic low-Ti type rocks of both plateaus based in the works by Wildner et al. (2002), Sommer et al. (2005) and Matté et al. (2016).High-Ti and low-Ti are evolutionary trends and vary with the SiO 2 content.In the case of basalts, above 2.0% of TiO 2 will be high-Ti, and below, low-Ti.Considering the intermediate and acidic values will be less than 2.0%.However, the general contents of elements are lower for the Palma basalts, which have a very similar composition with the medium-K calc-alkaline basalts of the Honshu arc, in Japan, and different from the high-K basalts of Stromboli volcano (Fig. 7A and 7B).
The mafic rocks feature similar chondrite-normalized rare earth element concentrations (Nakamura 1974), with a   general enriched light rare-earth elements (LREE) pattern, incipient Eu anomalies and horizontal pattern of heavy rare earth elements (REEs) (Fig. 8A).The comparison of the ocean-island basalt (OIB)-normalized lithotypes (Sun & McDonough, 1989) shows the correlation of the basic volcanic rocks of Palma region with the low-Ti basalts of the Taquarembó and Ramada plateaus, highlighting the general higher levels of the latter.There is an enrichment in large-ion lithophile element (LILE) type elements, with a slight depletion in high field strength (HFS) elements and enrichment in heavy REE's of these lithotypes, compared to OIB (Fig. 8B).
The tectonic discrimination of the mafic volcanic rocks was investigated through the concentration of trace elements in Pearce and Cann (1973) (Fig. 9A) and Meschede (1986) (Fig. 9B) diagrams, in which the analyzed samples are concentrated mainly in the field of calc-alkaline arc basalts.The tectonic analysis by Shervais (1982) diagram shows a predominance of the Taquarembó and the Ramada plateau rocks in the context of alkaline basalts and oceanic island basalts, whereas the Palma basic rocks are in the field of retroarc basin and mid-ocean ridge basalt (MORB) basalts (Fig. 10A).In the Pearce (2008) diagrams, the Palma basalts suggest an enriched mid-ocean ridge basalts (E-MORB) source and the participation of crustal melting during the rise of magma (Fig. 10B and 10C).

Geochronology
In this study, a basalt and andesitic basalt were analyzed for U-Pb zircon geochronology.The location of the samples is indicated in Figure 3.
CL images of the analyzed crystals internally reveal concentric oscillatory zoning, characteristic of igneous rocks (Corfu et al. 2003).The Th/U ratios of the zircon crystals range from 0.261 to 1.286, characterizing the igneous origin (Belousova et al. 2002).Thirty analyses were carried out in 19 spots (Tab.2), to obtain the 238 U/ 260 Pb and 235 U/ 207 Pb ratios, and the results of eight concordant zircons were used, whose 238 U/ 206 Pb ages ranged between 558 and 576 Ma.The selected crystals showed a 238 U/ 206 Pb and 207 Pb/ 206 Pb age of 563.3±2.1 Ma, with a mean square of weighted deviates (MSDW) of 0.0061 and concordance probability of 94% (Fig. 12).
1:3 and length between 100 and 200 µm.CL images of the zircon crystals also reveal concentric oscillatory zoning, characteristic of magmatic rocks (Corfu et al. 2003).This feature is confirmed by the variation of Th/U ratios of the zircon crystals, between 0.280 and 3.516.To obtain the U-Pb ages, 26 zircon crystals were analyzed, and the results of 10 concordant zircons were used (Tab.3), whose 238 U/ 206 Pb ages ranged between 548 and 586 Ma.The selected concordant crystals showed a 238 U/ 206 Pb and 207 Pb/ 206 Pb age of 572±5.5 Ma, with MSDW of 0.052 and concordance probability of 82% (Fig. 11B).Four inherited zircon crystals were also analyzed, showing ages between 2.017 and 2.076 Ma, in which the U 238 /Pb 206 and Pb 207 /Pb 206 concordant age of 2.016±15 Ma was obtained (Fig. 13).

DISCUSSION
The Palma Complex was interpreted as an association of metamorphic rocks, generated during the orogeny that    created the SGT units and part of the Dom Feliciano Belt itself (Chemale Jr. 2000, Laux et al. 2012).Initially, the first chemical results presented by Garcia (1980) and later by Strieder et al. (2000) indicated the tholeiitic character and the high levels of Al of the basalts, suggesting that they represent high aluminum basalts associated with subduction zones.This hypothesis of lithosphere subduction in an environment of active or post-collisional continental margins was reinforced by Wildner et al. (1999), by the presence of a comenditic trend, with Zr levels about 15 times higher than the Nb levels.Subsequently, Lopes and Hartmann ( 2003) recognized the alkaline/tholeiitic transitional behavior of metabasalts and interpreted it as a possible oceanic plateau.The data presented herein, however, indicate that the basic volcanic rocks previously included in the Palma Complex were not affected by orogenic metamorphism.Field and petrographic surveys carried out in the basalts show the presence of flow structures with a sub-horizontal arrangement, indicating that the body represents a subaerial lava flow.The elongated shape of the body, in the N45ºE direction, and the lava flow disposition suggest that the rise and positioning of the body were controlled by a tectonic structure, representing a younger reactivation stage of the Palma-Vila Nova Shear Zone (Fig. 2 and 3).The basalts exhibit porphyritic texture with high content of plagioclase phenocrysts and trachytic texture.The presence of zones rich in amygdales and vesicles indicates the preservation of the upper portion of the flow, along with intense hydrothermal alteration.
Petrographic analyses indicate that metamorphism that affected the basalts is of thermal origin and is associated with the intrusion of the Jaguari Granite.The development of actinolite porphyroblasts with radiating fiber texture and micas with decussate texture attest to the thermal metamorphism.The transformation associated with intense hydrothermal alteration gives the rocks a greenish color, which is associated with the growth of actinolite, epidote, chlorite and sericite in the basalt matrix.The low temperature metamorphism and its effects on vulcanites led the researchers to suggest that these rocks were metamorphosed.However, the decussate and radial textures and the absence of deformation features discards its transformation by orogenic metamorphism, as previously mentioned by Garcia andHartmann (1981), Chemale Jr. (1982), Santos et al. (1990), UFRGS (1996), Lopes and Hartmann (2003) and Laux et al. (2012).The Ramada Plateau basalts have very fine-grained groundmass (40%), labradorite phenocrysts (20%) up to 1 mm, hematite (20%), and chlorite (15%) as a mafic alteration.Remnants of altered diopside are visible (5%), and apatite and rare zircon crystals represent the accessory minerals (Matté et al. 2016).The Taquarembó Plateau basalts have plagioclase (labradorite) in flow-oriented grains, are the dominant phase, followed by augite ranging and ilmenite microphenocrysts in a glomeroporphyritic to subophitic texture with a groundmass composed of intersertal glass, plagioclase microlites and K-feldspar (Wildner et al. 2002).
The basalts of the Palma region have a K/Ar age of 559±17 Ma obtained by Teixeira (1982).The U-Pb zircon analyses of basalts yielded ages of 563±2 Ma and 572±6 Ma, indicating that the rocks are Ediacaran and can be correlated with the volcanic rocks of the Acampamento Velho Formation, CB.
The CB fill was interpreted by Paim et al. (2000) as having occurred during the last stages of the Brasiliano Cycle orogenies.The deposition of the Maricá Group occurred in marine environment, transitioning to deep-sea conditions with associated border fans of the Bom Jardim Group.Later, the Santa Barbara Group was deposited in shallow lacustrine environment with deltas and fans, ending with the shallow lacustrine, alluvial and eolic facies of the Guaritas Group.The two first depositional cycles were controlled by transcurrent shear zones of NE-SW direction and in brittle-ductile conditions.The last depositional episodes began with the eruption of the andesites of the Hilário Formation, Bom Jardim Group, of shoshonitic affinity, going to the eruption of bimodal transitional volcanic rocks (basalts and subordinate  rhyolites), of tholeiitic to sodium alkaline nature, of the Acampamento Velho Formation, Santa Barbara Group.The volcanic activities end with the alkaline basalts of the Rodeio Velho Formation, Guaritas Group (Wildner et al. 2002, Almeida et al. 2012, Janikian et al. 2012).The associated granitic plutons with high-K calc-alkaline composition have U-Pb SHRIMP and LA-ICP-MS zircon ages between 598 and 570 Ma, while the alkaline plutons have crystallized between 570 and 560 Ma.Recently, Paim et al. (2014) proposed a review on the four evolutionary stages of the CB evolved between 630 and 510 Ma.The authors considered the CB evolution as related to the final stages of the Dom Feliciano orogeny, from tardi-orogenic basins (Maricá retroarc foreland basins and East and West Bom Jardim transcurrent basins) to post-orogenic basins (West and East Santa Barbara rifts and Guaritas Rift).Table 4      characteristics different from the effusive and high and low-Ti hypabyssal deposits of Acampamento Velho Formation found in the Ramada and the Taquarembó plateaus.The geochemical compositions show differences, such as the alkaline character of the basalts of the Ramada and Taquarembó plateaus and the subalkaline character of the Palma basalts.The apparent alkalinity of some samples of Palma basalts can be associated with the transformation imposed by thermal metamorphism, characterized by intense sericitization of the plagioclase phenocrysts and the matrix of the basalts.Similarly, these basalts are arranged on the boundary between the calc-alkaline and tholeiitic rock fields, and can also reflect the intense oxidation of the mafic minerals present in the matrix of vulcanites.
The high and low-Ti compositional differences of basic magmas of the Taquarembó and Ramada plateaus can be related to different melt fractions of the asthenosphere or the lithospheric mantle (Baker et al. 1977), derived from an intraplate rift environment or the mantle wedge affected by a subduction event (Wilson 1989).The geochemical similarity of the Palma basalts with the active subduction environment basalts may suggest that the melting conditions that generated the latter were different from the alkaline basalts of the Taquarembó and Ramada plateaus.The basalts of the plateaus show transitional characteristics between volcanic arc basalts and intraplate basalts, a typical behavior of the basalts of moderate alkaline affinity, usually related to extensional environments of continental rifts or associated with the post-collisional period of orogenic systems (Leat et al. 1986).
The basalts of the Palma area have compositions similar to calc-alkaline arc basalts.These characteristics may be associated with the source rocks that constitute the SGT.The units of this terrane were generated by subduction processes and present geochemical composition of juvenile rocks.In the same way, the underlying mantle presents the effects of metassomatism associated with the fluids involved during the subduction processes.

CONCLUSIONS
Data integration shows that the basalts of the Palma village area belong to the Acampamento Velho Formation, of the Santa Barbara Group and CB.
The calc-alkaline characteristics of this magmatism differ with respect to the basic volcanic rocks occurring in the Ramada and Taquarembó plateaus, which have alkaline character.Geochemical data suggest that the basic volcanic rocks studied were generated from the partial melting of a lithospheric mantle affected by previous subduction events.
The generation of this magmatism is associated with the post-collisional evolution period of the Dom Feliciano Belt. Fig.2

Figure 3 .
Figure 3. Geological map of the Palma region, central portion of the São Gabriel Terrane.Modified from Laux et al. (2012).

Figure 11 .
Figure 11.Cathodoluminescence image of analyzed zircons with indications of U-Pb ages in millions of years of the: (A) LV-53 sample and (B) LV-70 sample.

Figure 12 .
Figure 12.Concordia plot of U-Pb zircon data for the sample LV-53 of andesitic basalt from the Acampamento Velho Formation.

Table 3 .Figure 13
Figure 13.(A) Concordia plot of U-Pb zircon data for the sample LV-70 of basalt from the Acampamento Velho Formation.(B) Concordia of U-Pb zircon data for the sample LV-70 with inherited xenocrystals.

Table 1 .
Geochemical data from five of representative samples of the Acampamento Velho Formation located in Palma region.

Table 2 .
LA-ICP-MS U-Pb results for igneous zircons from the sample LV-53 of the Acampamento Velho Formation.
presents a review of the geochronological data of the volcanic rocks of CB.The correlation results indicate that the basic rocks occurring in Palma village area have, however, compositional

Table 4 .
Geochronological data of the volcanic rocks of Camaquã Basin. Continue...