Abstract The thorough redox alteration of a lava flow is an undescribed feature in intraplate basaltic provinces. The Early Cretaceous (134.5 Ma) Paraná Province displays that alteration in the major Muralha Flow. This oxidized and reduced flow from the southern part of the province was studied with satellite images, field surveying, petrography, and published whole rock geochemistry. The 100 x 100 km flow from the Cuesta de Haedo presents two hydrothermal tiers – lower Tier 1 is gray to white, upper Tier 2 is red. Iron oxyhydroxides characterize Tier 2. Tier 1 contains clay minerals, zeolites, pyrite and calcite, and agate (possibly amethyst) geodes. In a first event, the upper Tier 2 was oxidized by hot water from the underlying Guarani Paleoaquifer. The high water/rock ratio decreased due to porosity clogging by precipitation of secondary minerals, and the fluid became reducing. Lowering of Eh and pH was caused by reaction of water with reducing particles (calcite, organic molecules) present in the paleoerg sandstones and with fresh rock surfaces. A lower Tier 1 was then formed during slow, hot water percolation. Reduction was interrupted below 30 °C (calcite formation). Large scale, similar alteration occurred in all studied oceanic ridges and only rarely in continental environments. provinces 134.5 1345 134 5 (134. Ma Flow images surveying petrography geochemistry 10 white red minerals zeolites calcite possibly amethyst geodes event Paleoaquifer waterrock molecules surfaces slow percolation 3 C formation. formation . formation) scale environments 134. 13 (134 (13 (1 (

Resumo: No presente estudo, dados detalhados de microanálise (elementos maiores e traços) em minerais de sills da Formação Serra Geral representam a primeira tentativa de avaliar o coeficiente de partição de Sc, Ti, V, Cr, Co, Ni, Cu, Zn, Rb, Sr, Y, Zr, Nb, Mo, Cs, Ba, REE, Hf, Ta, Pb, Th e U entre plagioclásio, augita e pigeonita coexistentes em 12 sills basálticos toleíticos ao longo da borda leste da Bacia do Paraná, Brasil. Estas 12 amostras são dos estados do Rio Grande do Sul, Paraná, São Paulo e Goiás. Coeficientes de partição em plagioclásio foram comparados em zonas de núcleo, intermédio e borda, usando LA-ICP-MS, e os resultados foram comparados com as variações das áreas de distribuição dos elementos Ca, Al, Na e Si. O coeficiente de partição do Sr e Eu diminui com o aumento da concentração de Ca no plagioclásio e aumenta para a borda, relacionado com o aumento da concentração de Na e Si e diminuição da temperatura. O KD do Ni e V nos clinopiroxênios analisados possui correlações positivas para Mg2+, Ca2+ e Al3+ devido a altas concentrações destes elementos no núcleo, diminuindo para a borda, e correlações negativas de KD Sc, Cr, Co, Pb e Lu, devido a substituição preferencial pelo Fe2+ em coordenações octaédricas. O uso de LA-ICP-MS e EPMA possibilitam a caracterização química de elementos maiores e traços e a determinação do coeficiente de partição em minerais coexistentes e o líquido.

Abstract: In the present study, detailed micro-analytical data (major and trace elements) in minerals from sills of Serra Geral Formation represents the first attempt to evaluate the partition coefficient of Sc, Ti, V, Cr, Co, Ni, Cu, Zn, Rb, Sr, Y, Zr, Nb, Mo, Cs, Ba, REE, Hf, Ta, Pb, Th and U among coexisting plagioclase, augite and pigeonite from 12 tholeiitic basaltic sills along the eastern border of the Paraná Basin, Brazil. These 12 samples come from Rio Grande do Sul, Paraná, São Paulo and Goiás states. Plagioclase/melt partition coefficients KD were determined in core, intermediate and rim zones using LA-ICP-MS and the results were compared with the variations of the elements Ca, Al, Na and Si in those areas. Partition coefficients of Sr and Eu decrease with the increase of Ca concentration in plagioclase. A reduction in temperature and consequently a rise in Na and Si levels increase the partition coefficients of Sr and Eu at the rim of mineral The KD of Ni and V in clinopyroxene has positive correlations with Mg2+, Ca2+ and Al3+ due to higher concentrations of those elements at the core and lower concentration towards the rim and negative correlations of KD of Sc, Cr, Co, Pb and Lu, due to preferential substitutions by Fe2+ in sixfold coordination. The use of LA-ICP-MS and EPMA made possible the chemical quantification of major and trace elements and the determination of partition coefficients in coexisting plagioclase and clinopyroxene in basaltic melts.