Jorge Hachiro

Absolute Dating of Permian Ash-Fall in the Rio Bonito Formation, Paraná Basin, Brazil

Abstract The Rio Bonito Formation in southern Parana basin contains a set of tonsteins interbedded with coal-seams. These tonsteins are composed mainly of kaolinite with zircon, apatite and beta-quartz paramorphs as accessory minerals, and were interpreted as volcanic ashes deposited by ash falls over pits protected by barrier islands in a barrier-lagoon system. A U-Pb dating of zircons in the tonstein A, which furnished an age of 267.1 ± 3.4 Ma (Early Permian) confirming previous age-dates based on palynology and correlating them with one of the main periods of volcanic activity in the Gondwana. The source of the pyroclastic material was attributed to the early Permian Choiyoi magmatic arc in Argentina, developed during the Sanrafaelic orogeny, and with a main peak of volcanic activity between 260 and 272 Ma.

Distribution, mineralogy, petrography, provenance and significance of Permian ash-carrying deposits in the Paraná Basin

Seccoes delgadas de testemunhos de sondagem e afloramentos no sul do Brasil demonstraram a ocorrencia de frequentes quedas de cinzas vulcânicas durante o Permiano entre 280 e 245 Ma. Minusculas (ca. 0,1mm) lascas de vidro vulcânico (glass shards) alterado, encontram-se dispersas ou concentradas nos sedimentos permianos das formacoes Rio Bonito e Tatui, na Bacia do Parana. O sedimento portador de glass shards e geralmente um lamito siltoso contendo em geral variavel quantidade de calcita calcrete. Restos vitreos soterrados foram substituidos por analcita ou mais raramente calcita, silica, zeolitas ou montmorilonita. Explosoes vulcânicas sao responsaveis por queda de cinzas em extensas areas, e por isso, um possivel vulcanismo permiano gerador foi procurado na Africa do Sul, na Cordilheira Andina e margens da Bacia do Parana. Foi proposta como fonte mais adequada deste vulcanismo um enxame de centros rioliticos descritos na Cordilheira Frontal e na Provincia Argentina de La Pampa (Patagonia). Nesta area os vulcoes devem ter expelido cinzas que caminharam milhares de quilometros antes de se depositarem e se preservarem em ambientes deltaicos ou marinhos rasos. Vinte e tres ocorrencias conhecidas de sedimentos permianos contendo glass shards foram locados em mapa da Bacia do Parana. Os relatos de ocorrencia deste tipo de sedimento sao gradualmente decrescentes para NE. Por esse motivo, os autores imaginam densas nuvens provindas da Patagonia e alcancando a Australia, depositando quantidades cada vez menores de material a medida que se deslocavam. Sedimentos com shards devem ser diferenciados dos tonsteins, ash falls e ash flows que tem sido tambem registradas no Gondwana permiano e que seriam indicadoras de fontes mais proximas.

A bone fragment of a probable aquatic vertebrate in the Serra Alta Formation (upper permian of Paraná Basin)

much darker than the cores, variations due to varying Fe and Mg contents. A second generation of light green tourmalines also occurs in quartz veins of Quartzite. These are alkalideficient, Crand V-bearing tourmalines with higher Mg# than those of the schorl-dravite series. The occupancy of the X-site, according to X-ray fluorescence data, is δ0,51Ca0.33Na0.15, thus corresponding to foitite, considered as an alkali-deficient schorl. Raman studies also discriminated two groups of tourmalines, one belonging to the buerguerite-schorl series and the other to the dravite-buerguerite-uvite series. Stable isotope data allowed to define sediment and hydrothermal waters as fluid sources, ruling out the association of the tourmalines with e.g. the Brasiliano granitoid bodies found in the area. δ18O compositions for tourmalines (+12 per mil) and host metachert and quartz veins (+13 per mil) are very similar, showing fluid equilibration during (re)crystallization of quartz and tourmaline. The presence of at least two distinct groups of tourmalines indicates distinct environments and timing for tourmaline generation. In Tapera Grande, tourmalines were formed in a submarine exhalative-sedimentary environment. Their composition was not strongly affected by medium-grade metamorphism. In Quartzite, tourmaline compositions reflect that of the country rock, once fluid percolation along Sertaozinho fault and associated fractures caused leaching of Cr (and V) and the crystallization of alkali-deficient, Cr(V-)bearing tourmalines in veins, together with quartz. The heat source for mineralizing fluids must have been a granitoid body (Pau Pedra) south of Tapera Grande and intermediate to acid pipes in the Quartzito area. These fluids were also responsible for distinct types of mineralization, characterized in Tapera Grande by the assemblage gold-pyrite-pyrrhotite-chalcopyrite, and in Quartzite by electrum-pyrite-chalcopyrite-sphaleritegalena-scheelite-molibdenite. — ( December 14, 2001 ) .