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

Abstract

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...

Petrological and volcanological insights into acid lavas from the Paraná-Etendeka Magmatic Province on the surroundings of Guarapuava city, Paraná, Southern Brazil: a contribution of detailed textural characterization combined with in situ Sr isotopes in plagioclase phenocrysts

“…15 Peate, 1997;Garland et al, 1995;Nardy et al, 2008;Freitas, 2009;Rocha-Júnior et al, 2013;Polo & Janasi, 2014;Barreto et al, 2016 Irrespective of composition, high and low titanium compositions prevail roughly at the north and south of the province, respectively (Fig. 1).…”

“…Literature data indicate that GAV whole-rock major element composition are remarkably similar, being classified as trachydacites (Bellieni et al, 1986;Garland et al, 1995;Nardy et al, 2008Nardy et al, , 2011Pinto et al, 2011;Luchetti et al, 2014; Table 2). Peate, 1997;Garland et al, 1995;Nardy et al, 2008;Freitas, 2009;Rocha-Júnior et al, 2013;Barreto et al, 2016. …”

Aspectos vulcanológicos dos traquidacitos da região de Piraju - Ourinhos (SP)

“…Existem vários trabalhos sobre as ocorrências das vulcânicas ácidas nessa região, começando com Sartori & Maciel Filho (1978), pioneiros na identificação dessas rochas, Raposo (1987) e Freitas (2009) sobre aspectos geológicos, petrográficos e detalhamento petrogenético, assim como outros trabalhos direcionados também ao mapeamento e à geoquímica de rochas e minerais nessas vulcânicas ácidas da região (Nardy et al, 2002;Dantas et al, 2003;Reis, 2006;Negri, 2006;Janasi et al, 2007).…”

“…Figura 21 -As rochas vulcânicas ácidas (círculos vermelhos) e básicas (círculos verdes) da região de Piraju -Ourinhos no diagrama TAS (LeBas et al, 1986), segundo Freitas (2009) Trabalhos prévios (Reis, 2006;Janasi et al, 2007;Freitas, 2009) (Lofgren, 1971). As porções vítreas mostram textura quenching de resfriamento rápido, com cristais de plagioclásio ocos e com terminações em 'cauda de andorinha' (Lofgren, 1974).…”

“…Na região de Piraju -Ourinhos, estas rochas são a única ocorrência do vulcanismo ácido no Estado de São Paulo, formando uma faixa que se estende por pouco mais de 60 km ao longo da calha do rio Paranapanema. Embora tenham sido alvo de trabalhos prévios, como Raposo (1987), Montanheiro (1999), Dantas (2003), Reis (2006), Janasi et al (2007) e Freitas (2009), com ênfase em aspectos petrográficos e geoquímicos, além da prospecção de vidros vulcânicos para uso na…”