ABSTRACT: The Paraná Continental Flood Basalt (PCFB) province is one of the largest igneous provinces on Earth, but little is known about the architecture and geochemical characteristics of the lava flows. In this context, a set of borehole samples arranged in an NEE-SWW profile - more than 600 km long - located in the northern PCFB was used to define its chemostratigraphy. In this region, the outcrops are rare, and the volcanic piles are covered by hundreds of meters of sedimentary rocks from the Bauru Sub-Basin. This study aimed to explore the chemostratigraphic column from northern PCFB to improve the understanding of its internal architecture and to characterize the flood basalt sequences by using the petrography and elemental geochemistry of surface and borehole samples. The results showed the presence of the main four magma types, which were essentially distinguished by their P 2 O 5 , TiO 2 and Sr concentrations, as well as Zr/Y ratios, with excellent chemostratigraphy relations.

Abstract: This study covers the region between the cities of Sacramento and Araguari/Uberlândia (Minas Gerais State, Brazil), where basalt flows from the Paraná Continental Magmatic Province outcrop. The investigated rocks present tholeiitic signature, with high titanium content, and are classified as Pitanga magma-type. The preserved basalt thickness is between 10 and 200 meters and individual flows do not exceed 15 meters thick. Flows were identified as sheet lobes, smaller and thinner flows units - stacked laterally and vertically forming compound lavas -, or frontal, centimetric lobes. The basalt flows show decimetric to metric intercalations of clastic sedimentary rock, with depositional characteristics that can vary from aeolian to lacustrine, and are important markers on prevailing environmental conditions. The plagioclases are dominantly labradorite and pyroxene is augite, whereas olivine can be hyalosiderite or hortonolite/ferrohortonolite. The behavior of the major, minor and trace elements is compatible with the presence of at least two parental magmas, which were subjected to fractional crystallization mainly of plagioclase, clinopyroxene, ilmenite and magnetite. There is a chemistry distinction between basalts from Sacramento to those from Araguari/Uberlândia region, the former one showing more evolved than the last one. The high (La/Lu)N values are indicative of partial melting of a garnet peridotite, while the Rare Earth Elements (REE) values are indicative of fractional crystallization.

RESUMO: Este trabalho apresenta os primeiros resultados de composições isotópicas de Pb dos diques mesozoicos com alto titânio do Espinhaço Meridional. As composições isotópicas de Pb dos diques investigados não mostram grandes variações, sendo significativamente mais radiogênicas do que aquelas dos basaltos com alto titânio da Província Basáltica Continental do Paraná. Esses resultados foram integrados a análises geoquímicas e isotópicas de Sr-Nd realizadas nas mesmas amostras, com o objetivo de obter informações adicionais sobre a gênese desses diques. As composições isotópicas de Sr-Nd-Pb não são compatíveis com a ocorrência de processos de contaminação crustal e requerem que a geração desses magmas tenha ocorrido em manto litosférico subcontinental metassomatizado, com a participação de componentes carbonatíticos e manto do tipo HIMU. Esses magmas podem também ter sido originados em fonte mantélica contendo ~4 - 5% de componentes piroxeníticos e ~1% de carbonatíticos, concordando também com as composições isotópicas de Os dos basaltos com alto titânio da Província Basáltica Continental do Paraná. Esses agentes metassomatizantes foram os responsáveis pela refertilização da fonte mantélica, conforme proposto na literatura para explicar as características dos xenólitos que ocorrem na Província Alcalina de Goiás, localizada na borda do Cráton do São Francisco. Com o objetivo de avaliar os riscos de contaminação por Pb durante o processo de preparação de amostras para análise, foram também realizados vários testes experimentais que indicam a necessidade de remoção de superfícies serradas e de uma lavagem cuidadosa da rocha finamente britada antes da pulverização, especialmente para aquelas com [Pb] < 7 µg /g.

ABSTRACT: The first results of Pb isotope compositions of the high-Ti Mesozoic dykes of the Southern Espinhaço are presented. The results do not show large variations and are significantly more radiogenic than the Pb isotope compositions of the high-Ti tholeiites from the Paraná Continental Flood Basalts. The data combined with published geochemical and Sr-Nd isotope results rule out crustal contamination processes in the genesis of the dykes, requiring magma generation in metasomatized subcontinental lithospheric mantle with the involvement of HIMU-type and carbonatite components. The magmas may have been also derived from a mantle source containing ~4 - 5% of pyroxenite and ~1% of carbonatite melts, agreeing with published Os isotope compositions of high-Ti rocks from the Paraná Continental Flood Basalts. These metasomatizing agents could be responsible for mantle source refertilization, as was also proposed in the literature to explain the characteristics of xenoliths of the Goiás Alkaline Province, which also occurs in the border of the São Francisco Craton. Additionally, to evaluate the risks of Pb contamination during sample preparation for analysis, several experimental tests were accomplished, which indicate the need of sawed surface removal and a careful washing of small-sized rock fragments before powdering, especially for rocks with [Pb] < 7 µg/g.