Abstract The Campo Alegre Basin is a volcano-sedimentary sequence covering an area of about 500 km 2 , located at the northeast portion of Santa Catarina state (Brazil), and formed during the late stages of the Neoproterozoic era. Three main stratigraphic units compose this basin, the lowermost of which is the Bateias Formation, corresponding to the pre-volcanic stage. It is characterized by the fanglomeratic sediments at the basin’s northern boundary, which were deposited in a fluvial environment and progressively replaced by fluvial sandstones towards the south. Poorly sorted conglomerates and breccias compound the fanglomeratic facies, which comprises angular to subrounded fragments in a matrix ranging from sand fractions to fragments larger than gravel. The ubiquitous presence of volcanogenic and metamorphic fragments in the fanglomeratic facies strongly suggests that the Piên Magmatic Arc and a volcanic manifestation coeval with the earlier stages of the basin formation were important source areas, as well as the basin basement (the Luís Alves Terrane). In this sense, geological, petrographic, and detrital zircon geochronology data are combined in order to interpret the mechanisms that had acted during the deposition of these sedimentary rocks and to constraint the maximum depositional ages of Bateias Formation at ca. ~606 Ma.

RESUMO: Novas idades obtidas por meio do método U-Pb em zircão e dados isotópicos Sm-Nd-Pb são apresentados para as rochas vulcânicas e hipabssais ácidas da exposição mais setentrional da Formação Acampamento Velho na porção NW do Escudo Sul-Rio-Grandense, Brasil. O primeiro episódio vulcânico, agrupado nos riolitos alto-Ti do Cerro Tupanci, possui idades de 579 ± 5,6 Ma, o que está de acordo com dados da literatura para o vulcanismo pós-colisional da Formação Acampamento Velho na Bacia do Camaquã. Idades de 558 +/- 39 Ma foram obtidas para o grupo de riolitos baixo-Ti no Cerro dos Picados, o que pode indicar um episódio vulcânico mais recente ou um período de tempo maior para o vulcanismo da Formação Acampamento Velho no Sul do Brasil, embora o grau de incerteza nos dados seja alto. Em relação às fontes magmáticas, os dados isotópicos de Sm-Nd acoplados a isótopos de Pb e uma revisão da geoquímica de elementos-traço indicam diferentes graus de fusão de crosta inferior paleoproterozoica (Suíte Dom Feliciano, Pinheiro Machado) a neoproterozoica (Terreno Rio Vacacaí). Os dados apresentados neste estudo, quando analisados em conjunto com dados da literatura, contribuem para a melhor compreensão da evolução estratigráfica das sucessões vulcânicas neoproterozoicas pós-colisionais da Bacia do Camaquã no Escudo Sul-Rio-Grandense.

ABSTRACT: We present new U-Pb zircon ages and Sm-Nd-Pb isotopic data for volcanic and hypabyssal acid rocks from the northernmost exposure of the Acampamento Velho Formation in the NW portion of the Sul-Rio-Grandense Shield, Brazil. The first volcanic episode, grouped in the high-Ti rhyolites from the Tupanci hill, shows age of 579 ± 5.6 Ma, which is in agreement with the post-collisional Acampamento Velho Formation volcanism in the Bom Jardim Group of the Camaquã Basin. A poorly constrained age of 558 +/- 39 Ma was obtained for rhyolites from the low-Ti group at the Picados Hill, which may indicate a younger acid volcanism, or a greater time span for the volcanism of the Acampamento Velho Formation in southernmost Brazil. Regarding magmatic sources, Sm/Nd isotopic data coupled to Pb isotopes and a review of trace element geochemistry indicate different amounts of Paleoproterozoic (Dom Feliciano, Pinheiro Machado Suite) to Neoproterozoic (Rio Vacacaí terrane) lower crust melting. Our data, coupled with literature data, contribute to a better understanding of the stratigraphic evolution for the Neoproterozoic post-collisional volcanic successions of the Camaquã Basin in the Sul-Rio-Grandense Shield.

This paper reports the frst occurrence of Ordovician volcanic - plutonic rocks in the Nor-thwestern Sierras Pampeanas of Argentina located at the southeast of the Sierra de Vinquis and in the northern tip of the Sierra de Velasco. They form an exogenous lava dome of medium to coarse - grained dacite, andesite and rhyolite porphyries and local breccias syn-genetically formed, affected by post-emplacement deformation. The Ordovician porphyritic Cerro Negro rocks, with tectonic deformation, show intrusive contacts with banded schists that contain quartz-tourmaline veins with scheelite mineralization, and are intruded by Carboniferous granite in the meta-vulcanites. The Ordovician meta-vulcanites of the Punta Negra, are intruded at the east by a Carboniferous granite, and to the west show tectonic contacts with deformed Ordovician granites. The porphyritic rocks are intruded by late aplitic tourmaline-bearing dikes without ore-bearing mineralization. The meta-vulcanites are composed by quarte, plagioclase, biotite, chlonte, epidote and opaques. Metapelite xenoliths mcluded m the Punta Negra porphyry, show cordiente, biotite and potassium feldspar formed by thermal metamor-phism. Geochemically the both porphyries are calc-alkalme and peralummous. Trace elements geochemistry ís similar to the compositional patterns of Continental Islands Volcanic Are, m transition to Normal Are, that are similar to the composition of the Famatmian vulcanism elsewhere. The mitial valúes of isotopes of Sr 0.7117 - 0.7128 and D Nd of -5.0 - 5.9, indícate a crustal ongin. The Nd model ages obtamed of 1.5 and 1.6 Ga are concordant with those of other Famatmian granites. Both subvolcanic outerops are correlated on the basis of petrographic, geochemical and isotopic similanties and also by the geochronologic age of 481 + 9 Ma ín Cerro Negro, and 471 + 3 Ma for Punta Negra, that are the only known sub-volcanic manifestations ín the Northwestern Sierras Pampeanas.

En este trabajo, se avanza en la interpretación de un intrusivo volcánico - plutónico ordovícico en las Sierras Pampeanas Noroccidentales integrado por tonalitas epizonales que gradan a pórfidos dacíticos, andesíticos y riolíticos que forman un domo lávico exógeno, estando el conjunto afectado por brechamiento magmático primario y deformación tectónica posterior. Los afloramientos del domo subvolcánico-plutónico Ordovícico, se desarrollan en el sureste de la Sierra de Vin-quis y en el extremo norte de la Sierra de Velasco. El complejo plutónico-volcánico de Cerro Negro, presenta contactos intrusivos con esquistos bandeados y desarrollan vetas de cuarzo con mineralización de turmalina y scheelita; estando a su vez intruido por un granito Carbonífero. Asimismo, en la Punta Negra, el complejo está intruido al este por granitos de edad carbonífera y al oeste está en contacto con granitos ordovícicos deformados. Diques tardíos de aplitas con turmalina, sin mineralización atraviesan los pórfidos. Los pórfidos son dominantemente dacíticos, localmente gradan a tonalitas y ambos están constituidos por cuarzo, plagioclasa, biotita, clorita, epidota y opacos. Geoquímicamente muestran patrones calco-alcalinos y peraluminosos. Los xenolitos pelíticos incluidos en las brechas volcánicas de Punta Negra muestran metamorfismo térmico con formación de cordierita, biotita y feldespato potásico. La geoquímica de los elementos trazas muestra afinidades con arcos de islas continentales en transición a arcos normales, semejantes a las del Sistema de Famatina. Los valores iniciales de isótopos de Sr 0,7117 a 0,7128 y εNd de -5,0 a -5,9 indicarían un origen cortical y las edades modelos obtenidas entre 1,5 y 1,6 Ga son coincidentes con las de otros granitoides famatinianos. Ambos afloramientos subvolcánicos son correlacionables entre si, en base a las similitudes petrográficas, geoquímicas e isotópicas, así como por los valores geocronológicos de 481 ± 9 Ma para Cerro Negro y 471 ± 3 Ma para Punta Negra. Estos afloramientos corresponden a las únicas manifestaciones epizonales-subvolcánicas Ordovícicas conocidas en las Sierras Pampeanas Noroccidentales.

The out-crops of the Espinal Formation in the western margin of the Toro Negro Range, north of the Bolsón de Jagüé, La Rioja, near the tectonic limit with the outcrop of the Precordillera of Jagüé are analyzed. This unit corresponds to the medium to high grade metamorphic basement of the Western Pampean Ranges. Petrography, geochemistry and a U-Pb TIMS age on detrital circóns of 905 +14/-13 Ma, (MSWD = 0,74; discordia)are presented. This age is slightly younger than others obtained for this unit but still comparable with the Grenvillian cicle. The dominant lithologies are amphibolic, granatiferous and biotitic gneises and schists with evidences of retrogradation. The composition and texture of the samples points to a sedimentary and igneous origin for these outcrops. These rocks distinguishes from the ones that forms the basement of the Precordillera-Cuyania, mainly because the higher metamorphic grade.

Se analizan los aforamientos de la Formación Espinal en el margen occidental de la Sierra de Toro Negro, al norte del Bolsón de Jagüé, La Rioja, cerca del límite tectónico con los aforamientos de la denominada Precordillera de Jagüé. Esta unidad corresponde al basamento de medio a alto grado de Sierras Pampeanas Occidentales. Se presentan datos de petrografía, geoquímica y una edad U-Pb TIMS en circones detríticos de 905 +14/-13 Ma, (MSWD = 0,74; discordia) ligeramente más joven que otras edades obtenidas para esta unidad pero aun comparables con las del ciclo Grenvilliano. Las litologías predominantes son gneises y esquistos anfbólicos, granatíferos y biotíticos con evidencias de retrogradación. La composición y textura de las muestras son coherentes con un origen sedimentario e ígneo para estos aforamientos. Estas rocas se distinguen de aquellas del basamento de la Precordillera-Cuyania principalmente por el mayor grado metamóro.