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A succession of trachydacitic lava flows over 150 m thick constitutes the lowermost portion of the Serra Geral Fm. along the Paranapanema River valley in the Piraju-Ourinhos region, State of São Paulo. The flows have a well-defined zonality: the central portion is made up of grey porphyritic dacites with low glass and vesicle proportion which grade towards the contacts to strongly vesiculated varieties with glassy groundmass which, when in contact with the Botucatu Fm. basal sandstones or with larger bodies of inter-flow sandstone, show a typical chocolate-brown color. The observed compositional variation is small, and in part a reflection of secondary alteration. Independent of texture, mode of occurrence and stratigraphic position, the rocks show similar contents of SiO 2 (65-67 wt%) and all other major oxides, and a ferrous (mg#= 20-27) and metaluminous A/CNK= 0.85-0.95) character; alteration associated to vesicle infilling and color change to chocolate-brown results in lower mg# and higher A/CNK. Mineral and whole rock chemical data indicate high crystallization temperatures around 1000ºC (pyroxene thermometry; apatite saturation), consistent with the predominant mode of occurrence as flows.Keywords: trachydacite, Serra Geral Fm., Paraná Magmatic Province, mineral chemistry, magma temperatures.Resumo Geologia, petrografia e geoquímica do vulcanismo ácido da Província Magmática Paraná, na região de Piraju-Ourinhos, SE Brasil. Uma sucessão de derrames de composição traquidacítica com espessura superior a 150 m constitui a base da Fm. Serra Geral em uma faixa de ca. 60 km ao longo da calha do rio Paranapanema, entre as cidades de Piraju e Ourinhos (SP). Os derrames mostram zonalidade bem marcada: a porção central é formada por dacitos porfiríticos cinza com baixa proporção de vidro e vesículas, que gradam, em direção aos contatos, para variedades vítreas fortemente vesiculadas que, em contato com arenitos basais da Fm. Botucatu ou com corpos maiores de arenitos inter-derrame, apresentam cor marrom-chocolate característica. A variação composicional observada é muito pequena, e em parte devida a efeitos secundários. De modo independente das texturas, forma de ocorrência e posição estratigráfica, as rochas apresentam teores similares de SiO 2 (65-67%) e todos os demais óxidos maiores, caráter ferroso (mg#= 20-27) e metaluminoso (A/CNK= 0,85-0,95); a alteração associada ao preenchimento de vesículas e geração da cor chocolate se reflete na diminuição de mg# e aumento de A/CNK. Dados de química de minerais e de rocha indicam temperaturas liquidus elevadas, próximas a 1000ºC (termometria de piroxênios; saturação em apatita), consistente com modo de ocorrência predominante na forma de derrames.Palavras-Chave: dacito, Fm. Serra Geral, Província Magmática Paraná, química mineral, temperaturas de magmas.

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The Serra Geral Fm. in the Piraju-Ourinhos region (SW of the State of São Paulo) is made up of initial flows of acidic composition which were deposited over the sandstones of the Botucatu Fm., and were covered by basalt flows which crop out north of the Paranapanema river. Geochemical analyses of a suite of extrusive and intrusive basic rocks reveal significant variations that can be used as tools for stratigraphic correlation. All the rocks from flows and a significant proportion of the intrusive rocks show affinity with the Pitanga basalt type. A set of dike rocks, however, shows lower Ti/Y ratios and weakly fractionated primitive mantle-normalized incompatible element patterns, and are interpreted as conduits of younger basalt flows, now eroded, and equivalent to the Paranapanema type. Samples from one sill, on the other hand, are characterized by more fractionated incompatible element patterns and high Sr contents, indicative of the presence of Urubicitype magmas, previously unknown in this region. Resumo Geoquímica do magmatismo basáltico eocretácico na região de Piraju-Ourinhos, SE doBrasil, e implicações para a estratigrafia da Formação Serra Geral. A Formação Serra Geral na região de Piraju-Ourinhos (SW de São Paulo) é consituída por derrames iniciais de composição ácida, que recobriram os arenitos da Fm. Botucatu e foram cobertos por derrames basálticos, que afloram a norte do rio Paranapanema. Análises geoquímicas de uma suite de rochas básicas extrusivas e intrusivas revelam variações significativas, que podem ser usadas como ferramentas de correlação estratigráfica. Todas as amostras de rochas de derrame e uma proporção significativa das amostras de corpos intrusivos têm afinidade com os basaltos do tipo Pitanga. Um conjunto de rochas de dique, contudo, mostra menores razões Ti/Y e padrões de elementos incompatíveis pouco fracionados, e é interpretado como condutos de derrames basálticos mais jovens, equivalentes ao tipo Paranapanema, já erodidos na região. Amostras de um sill, por outro lado, se caracterizam por padrões de elementos incompatíveis mais fracionados, e elevados teores de Sr, indicando a presença de magmas tipo Urubici, previamente desconhecida na região.Palavras-chave: basalto, dacito, Fm. Serra Geral, Província Magmática Paraná, estratigrafia química.

A geração de magmas ácidos na Província Magmática Paraná, região de Piraju-Ourinhos (SP): uma contribuição da geoquímica isotópica e de elementos traço em rochas e minerais iii II. ABSTRACT Dacitic rocks from Piraju-Ourinhos, State of São Paulo, outcrop for ca. 60 km along the Paranapanema River valley, and constitute the northernmost expositions of the Parana Magmatic Province acid magmatism. They rest directly over the Botucatu Formation eolic sandstones and are recovered by Pitanga-type high Ti basalt flows. Basalt dykes and sills that occur in the region show different chemical relationship and can be compared to the Paranapanema, Urubici and Pitanga basalt types. Chemically classified as trachydacites, these rocks are porphyritic with 5 to 15% plagioclase, clinopyroxene, opaque minerals and apatite phenocrysts. Aphanitic to phaneritic groundmass is composed of glass, plagioclase, clinopyroxene, quartz and alkali feldspar. Glass can make up 10 to 90% of the original groundmass and is usually devitrified, being recognized by relict quenching textures. Vesicles and amygdalas are abundants in such trachydacites varieties and can achieve to 40% of rock. In this work, the age of the acid magmatism was obtained by U-Pb TIMS in baddeleyite and zircon concentrates. The value [134.4 0,9 (2σ)], is more accurate and precise compared with ages previously obtained in the trachydacites from region [133 -134 ± 6 Ma (K-Ar); 128.7 ± 1 Ma ( 40 Ar/ 39 Ar)], and within the short age interval currently admitted for the Paraná volcanism climax.Initial 87 Sr/ 86 Sr ratios of the trachydacites (0.7078 to 0.7080) are slightly more radiogenic than those of associated basalts (0.7056 to 0.7068), whereas εNd 134 are more negative (~ -5 versus -4). These differences suggest that, although the basalts must have a genetic link with the acid magmatism of region, some crustal contribution may exist in the acid magmas. Initial 87 Sr/ 86 Sr obtained by LA-ICPMS show identical values for the groundmass and plagioclase and apatite phenocrysts (~0.7077), consistent with equilibrium crystallization. Most of the clinopyroxene phenocrysts have initial 87 Sr/ 86 Sr different from the groundmass (usually smaller; 0.7045-0.7071; only one crystal is more radiogenic, 0.7084). Together with two the plagioclase phenocrysts (with initial 87 Sr/ 86 Sr= 0.7083 and 0.7074), they did not crystallize in equilibrium to the groundmass, and are likely antecrysts. Geochemical modelling using major elements and both compatible and incompatible trace-elements show that it is possible to obtain the acid magmas after 60 to 80% fractional crystalization of a Pitanga-type basalt. The main obstacle for such model would be the wide compositional silic gap between acid and basic magmas.; however, this gap could result from Dissertação de Mestrado Vivian Azor de Freitas iv physical limitations to crystal-liquid separation at intermediate compositions and to the extraction by filter pressing of more evolved residual liquids. The small differences in Sr-Nd isotopic signature be...