Carlos Augusto Sommer

Volcanic cycles and setting in the Neoproterozoic III to Ordovician Camaquã Basin succession in southern Brazil: characteristics of post-collisional magmatism

The Camaqua Basin comprises a volcano-sedimentary succession, located in southernmost Brazil, and represents a molasse basin formed at the post-collisional stage of the Brasiliano/Pan-African orogenic cycle in the Neoproterozoic III to Ordovician period. This basin is one of the most well-preserved ancient volcano-sedimentary sequences undeformed and unmetamorphic in the world, dominantly developed on a continental setting under subaerial conditions. It is composed of five major stratigraphic units, four of them with a distinct volcanic character from the bottom to the top, as: (1) Marica; (2) Bom Jardim; (3) Acampamento Velho; (4) Santa Barbara; and (5) Guaritas Allogroups. A concise sight of geochemical and isotopic rock data is presented, as well as stratigraphic correlation and description of rock structures and textures that lead to the identification of their genetic processes, the aim of this paper, indicating a relation with a coeval plutonism, and volcanism that evolved from high-K calc-alkaline to shoshonitic and ended with a silica-saturated sodic alkaline magmatism, with a crustal component represented by peraluminous granites. Volcanic deposits from bottom to top are made mostly of volcanogenic sedimentary deposits, succeeded by basic to intermediate lava and pyroclastic flows of shoshonitic affinity, followed by intermediate and acid lava flows and ignimbrites of sodic alkaline affinity. The last volcanic event is represented by basalt pahoehoe flows, probably of mildly alkaline sodic affinity.

The evolution of Neoproterozoic magmatism in Southernmost Brazil: shoshonitic, high-K tholeiitic and silica-saturated, sodic alkaline volcanism in post-collisional basins

The Neoproterozoic shoshonitic and mildly alkaline bimodal volcanism of Southernmost Brazil is represented by rock assemblages associated to sedimentary successions, deposited in strike-slip basins formed at the post-collisional stages of the Brasilian/Pan-African orogenic cycle. The best-preserved volcano sedimentary associations occur in the Camaquã and Campo Alegre Basins, respectively in the Sul-riograndense and Catarinense Shields and are outside the main shear belts or overlying the unaffected basement areas. These basins are characterized by alternation of volcanic cycles and siliciclastic sedimentation developed dominantly on a continental setting under subaerial conditions. This volcanism and the coeval plutonism evolved from high-K tholeiitic and calc-alkaline to shoshonitic and ended with a silica-saturated sodic alkaline magmatism, and its evolution were developed during at least 60 Ma. The compositional variation and evolution of post-collisional magmatism in southern Brazil are interpreted as the result mainly of melting of a heterogeneous mantle source, which includes garnet-phlogopite-bearing peridotites, veined-peridotites with abundant hydrated phases, such as amphibole, apatite and phlogopite, and eventually with the addition of an asthenospheric component. The subduction-related metasomatic character of post-collisional magmatism mantle sources in southern Brazil is put in evidence by Nb-negative anomalies and isotope features typical of EM1 sources.

REINTERPRETAÇÃO DO COMPLEXO INTRUSIVO LAVRAS DO SUL, RS, DE ACORDO COM OS SISTEMAS VULCANO-PLUTÔNICOS DE SUBSIDÊNCIA. PARTE 1: GEOLOGIA, GEOFÍSICA E GEOCRONOLOGIA (207PB/206PB/238U)

The Lavras do Sul intrusive complex (LSIC) and coeval volcanics, formed during the Late Neoproterozoic in the Sul-Riograndense Shield, are important because they contain historic Au-Cu occurrences. Field relationships, and gravity and magnetic data are consistent with the LSIC as nested intrusions of a volcano-plutonic subsidence system. These data also show the LSIC evolved from north to south, so we divided it into two sectors: the Tapera monzonite (TM) in the north, and the granitic body in the south. The Arroio do Jacques monzodiorite (AJM) occurs between the two sectors, surrounding the north-northeast border of the latter. TM is an intrusion emplaced during the formation and stabilization of an upper-crustal magma chamber linked to the coeval volcanics. Granites with distinct geochemical affinities forma concentric body that probably represents a resurgent pluton emplaced after some caldera collapse. Alkali-calcic granites are in the core above the main root zones, whereas alkaline granites occur as semicircular and peripheral bodies. 207 Pb/ 206 Pb and 206 Pb/ 238 U ages for several LSIC-units are comparable, and confirm that the intrusive complex is a multicyclic body, probably formed over a period of ca. 23 m.y. (606-583 Ma). Nonetheless, most of LSIC-units (TM, AJM and the core granites) have similar crystallization ages, of ca. 601 and 599 Ma, suggesting that this is the major period of igneous activity forming the volcano-plutonic system. Two distinct ages were obtained for alkaline granites, 207 Pb/ 206 Pb age of 598± 3 Ma (2σ) and 206 Pb/ 238 U age of 586.0 ± 2.8 Ma (2σ) respectively. Such difference in ages maybe resolved if the alkaline granites were emplaced in increments. The 207 Pb/ 206 Pb age of 567 ± 4 Ma (2σ) for the Jaguari Granite, situated to the west of LSIC, confirms it represents another younger alkaline event.

Geochemical evidence concerning sources and petrologic evolution of Faial Island, Central Azores

Volcanic rocks that make up Faial Island, Central Azores, consist of four volcano-stratigraphic units, with ages between 730 ka and the present. Lavas range from alkali basalts to trachyandesites and belong to the alkaline-sodic series. The oldest unit is the Ribeirinha Volcanic Complex, generally characterized by low MgO contents. The Cedros Volcanic Complex is composed of basalts to benmoreites with low MgO contents. The Almoxarife Formation represents fissure flows, containing MgO contents similar to to slightly higher than those of the underlying Cedros Volcanic Complex. The youngest unit, the Capelo Formation, consists of mafic rocks with MgO values higher than those of the other units. Bulk-rock major and trace element trends suggest that differentiation of the three earliest units were dominated by fractional crystallization of plagioclase ± clinopyroxene ± olivine ± titanomagnetite. Capelo bulk-rock compositions are the most primitive, and are related to a period when volcanic activity was fed by deep magmatic chambers, and melts ascended more rapidly. Comparison among geochemical patterns of the trace elements suggests a strong similarity between the lavas from Faial and Pico islands. Corvo Island volcanism contrasts with the geochemistry of Faial and Pico lavas, reflecting its strong K and Rb depletion, and Th, U, Ta, Nb, La, and Ce enrichment. Absence of the Daly gap in the Faial volcanics is attributed to early crystallization of Ti-Fe oxides. The probable source of the Faial magma coincides with the MORB-FOZO array, which implies the presence of ancient recycled oceanic crust in the mantle source. Ratios of incompatible trace elements suggest the similarity of Corvo volcanic rocks with magmas derived from HIMU sources, whereas the Faial and Pico volcanic rocks could have been produced from sources very close to EMII-type OIB.

Mineral Chemistry of Volcanic Rocks of South Shetland Archipelago, Antarctica

This study presents the results of mineral chemistry data of Meso-Cenozoic volcanic rocks of Livingston, Robert and Ardley islands (South Shetland Arc, Antarctica). These rocks include basalts and basaltic andesites that exhibit pilotaxitic, intergranular, and intersertal textures. Glomeroporphyritic clusters consist of phenocrysts of plagioclase, augite, and Ti-magnetite or only plagioclase. The rocks are composed of plagioclase, augite, olivine, Ti-magnetite, and ilmenite phenocrysts. Geochemistry studies indicate that these rocks are rich in Al2O3, Rb, Ba, Sr, and show enrichment in LREE relative to HREE. The Ni, Cr, Co, and MgO contents are lower than primary compositions, and demonstrate that these magmas are evolved. Mineral chemistry indicates that the volcanic rocks belong to the calc-alkaline series typical of island arcs. Plagioclase crystals are mostly bytownite and labradorite in Livingston and Robert islands. Nevertheless, some Robert Island phenocrysts are of anorthite composition. The plagioclase of volcanic rocks of Ardley Island is only labradorite. Augite is the main mafic phase (Wo23-48, En40-58, Fs9-21) in all studied islands. Olivine shows a chrysolite composition. Oxide phases are represented by Ti-magnetite and ilmenite. The evaluation of whole-rock geochemistry and mineral chemistry supports the hypothesis of fractional crystallization for the evolution of the rocks of Livingston, Robert, and Ardley islands. Geothermometry for olivine-clinopyroxene pairs suggests a crystallization temperature of 962°C ± 10°C (1 bar) and of 965°C ± 10°C (500 bars).

Neoproterozoic, Mildly Alkaline, Bimodal Volcanism in Southern Brazil: Geological and Geochemical Aspects

Neoproterozoic magmatism in southern Brazil is associated with translithospheric shear belts and strike-slip basins in a post-collisional setting related to the last stages of the Brasiliano—Pan African orogenic cycle. It evolved from an association of high-K calc-alkaline, leucocratic peraluminous and continental tholeiitic magmas, to shoshonitic, and eventually to sodic mildly alkaline series. Bimodal volcanism in the last group was coeval with subaerial siliciclastic sedimentation in all post-collisional basins preserved in the region. Three magmatic associations were identified in the bimodal volcanism: (1) low-Ti basalts and rhyolites; (2) high-Ti basalts and rhyolites; and (3) high-Nb rhyolites and basalts. Basic parental magmas were produced from sources related to EM1-type mantle previously modified by Brasiliano subduction, whereas silicic high- and low-Ti rocks were probably produced through fractional crystallization. Significant crustal contamination was recognized only in some slightly peraluminous rhyolites with low contents of HFS elements. High-Nb rhyolites, the youngest volcanic rocks, probably reflect the participation of asthenospheric components in the source, and mark the last magmatic event related to the Brasiliano-Pan African orogenic cycle in southern Brazil.

Morfologia e química de cinzas do vulcão Puyehue depositadas na região metropolitana de Porto Alegre em junho de 2011

The National Geological Survey of Chile reported a volcanic explosion of order 5 in the Los Rios area (Volcanic Complex in Puyehue - Cordon Caulle) on June, 2011. This complex comprises Pleistocene volcanoes built in six large evolutionary phases and preserves 131 km³ of the vulcanites. The Puyehue stratovolcano (69 ky) had its last records of the explosive and effusive activity in 1921, 1922, and 1960. The explosion on June, 2011 generated a tephra and gas column of 10 km high under high-temperature conditions. The surrounding air was heated and incorporated reducing the density of the the column, which rises into the atmosphere. The pyroclastic material stops in a balance zone between the densities of the medium and head of the plume, and the wind action determines the dispersion of the ashes. On June 9, 10 and 14, 2011, the Fundacao Estadual de Protecao Ambiental (FEPAM-RS) collected Puyehue ashes in its filters at the eighth District of the Meteorology (Porto Alegre). The concentrations remained below the Conselho Nacional do Meio Ambiente - CONAMA standards (150 mg/m³) and Guide Value (WHO-VG = 25 mg/m³). The research by using scanning electron microscope (SEM) indicated that the ashes are thin (2 - 30 microns), with elongated angular and placoid habits. The curved ends, corresponding to the boundaries between vesicles and melt, are vitreous (shards), rich in SiO2 and Al2O3, and show low alkali and FeOt contents. The diffractometer obtained in reworked tephra indicates plagioclase and cristobalite presence, and these minerals were also observed in the fall deposits studied in Argentina.

O vulcanismo alta-sílica da região do Tupanci, NW do Escudo Sul-Rio-Grandense: faciologia, petrografia e litoquímica

Volcanic and hypabyssal acid rocks occur in the Tupanci area, NW portion of the Sul-Rio-Grandense Shield. These rocks are stratigraphically correlated to the Acampamento Velho Formation (~550 Ma), at the Camaqua Basin. This region has the northernmost exposure of this volcanic episode, which comprises effusive/ hypabyssal and pyroclastic rocks, with dominantly acid composition and sodic-alkaline affinity, whose genetic processes are linked to the post-collisional stages of the Brasiliano/Pan-Africano orogenic cycle. Acid volcanic rocks occur mainly as effusive deposits and also as pyroclastic deposits, in two Cerros (Hills): Tupanci and Picados. Cerro Tupanci defines an elongated (N-S) sub-volcanic intrusion of porphyritic rhyolites with phenocrysts of alkali feldspar and quartz surrounded by equigranular-fine-grained to aphanitic quartz-feldspar matrix, with a strong flow foliation on border regions. Cerro dos Picados shows texturally similar rhyolites, but with aphanitic to glassy matrix and presence of biotite; and pyroclastic deposits, characterized by rhyolitic ignimbrites. Ignimbrites occur in two facies: lithic-rich, with few devitrified and poorly elongated pumice and crystal fragments; and rheomorphic, with abundance of devitrified pumices with detachable eutaxitic texture, crystal fragments and rarely lithic fragments. Geochemical behavior allows to classify the magmatism as silica oversaturated, similar to the systems with high-silica, alkaline affinity and a metaluminous to slightly peralkaline trend with similar characteristics to “A” type granites. Petrographic and lithochemical data indicate a genetic linkage with the Acampamento Velho Formation magmatism

Corpos hipabissais correlacionados à formação Serra Geral na região do cerro do Coronel, RS : geologia e petrologia

A Formacao Serra Geral e caracterizada por um expressivo volume de derrames basicos de composicao toleitica e pela presenca muito subordinada de vulcanitos acidos. Inumeros corpos intrusivos (soleiras e diques) sao correlacionados com esta unidade e, juntamente com os depositos vulcânicos, constituem a Provincia Magmatica do Parana. Este trabalho trata da investigacao geologica e petrologica das intrusoes basicas/intermediarias da regiao do Cerro do Coronel, a sudeste da cidade de Pantano Grande, RS. Esses corpos hipabissais estao estratigraficamente vinculados a Formacao Serra Geral e formam um alinhamento com orientacao NW-SE. Apresentam contatos concordantes com rochas sedimentares das Formacoes Rio Bonito e Irati. Disjuncoes colunares sao comuns em todas as ocorrencias estudadas e sao afetadas por forte fraturamento NE e NW. A pequena variacao faciologica e caracterizada por termos equigranulares finos a muito finos e, raramente, porfiriticos. Texturalmente, essas rochas apresentam o predominio da textura intergranular e localmente subofitica. Sao constituidos essencialmente por plagioclasio, augita, minerais opacos e, raramente, olivina. Apatita e quartzo ocorrem como minerais acessorios. Mesostase felsica ocorre como etapa final da cristalizacao. Os dados geoquimicos de elementos maiores e elementos-traco permitem classificar as rochas desses corpos hipabissais como andesitos basalticos de afinidade toleitica, cuja evolucao deu-se por mecanismos de cristalizacao fracionada, envolvendo principalmente o fracionamento de plagioclasio e augita. As caracteristicas dos elementos maiores, elementos-traco e elementos terras raras sao compativeis com as apresentadas para magmatismo vinculado a grandes provincias basalticas continentais, como Provincias do Deccan Traps, Parana-Etendeka, Columbia River, Siberian Traps e Karoo. Estas rochas apresentam concentracoes de TiO2 inferiores a 2%, mostrando uma tendencia para o magma-tipo Esmeralda da Provincia Magmatica do Parana.

Pb isotope geochemistry and reappraisal of Sr-Nd isotopes of the Cerro Morado basic magmatism (Ischigualasto-Villa Union Triassic basin, NW Argentina): Implications for the mantle sources

Manuscript ID: 20170106. Received in: 09/08/2017. Approved in: 12/10/2017. ABSTRACT: Sr-Nd-Pb isotopic signatures were determined for silica-undersaturated hypabyssal alkaline basic rocks of the Los Baldecitos Formation in the Cerro Morado area, situated in the Ischigualasto — Vale de la Luna Provincial Park (northernmost San Juan Province, Argentina). The basic rocks show slight variations of the Sr-Nd-Pb isotopes and similar behavior of whole-rock geochemistry, which suggest a single source for the Cerro Morado volcanic rocks. The present-day Pb isotopic data show moderately radiogenic Pb compositions (206Pb/204Pb = 18.31 – 18.35), in addition to low values of 87Sr/86Sr initial ratios of 0.70314 – 0.70386 and highly radiogenic Nd initial isotopic compositions (+7.1 < εNd (228 Ma) < +9.3), which point to a PREMA mantle reservoir signature for the alkaline basalts with little evidence of crustal contamination. The involvement of a HIMU component in the source, previously suggested for the Cerro Morado volcanics, should be disregarded in the light of the Pb isotopic signature. The Sr-Nd signature together with Nd-TDM model ages not older than 295 Ma does not support the involvement of an ancient crust.