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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.

código de financiamento 001, processo 1425945) pelo financiamento das análises, cursos, auxílios e bolsas referentes a esta pesquisa. I also thank the Argon Geochronology Laboratory of The University of Queensland (UQ-AGES) for all financial support during my research period in Australia.Agradeço profundamente aos meus orientadores pela imensa dedicação e generosidade.À Professora Doutora Leila Marques, pela incansável busca. Ao Professor Doutor Paulo Vasconcelos, pela incansável pergunta.Agradeço aos colaboradores com os quais tive a sorte de contar ao longo deste trajeto. À Professora Doutora Marly Babinski, agradeço o incentivo criterioso desde os meus tempos de graduação. À Doutora Isabela Carmo, agradeço a frequente troca de boas ideias, a confiança, belas fotografias de campo e amostras adicionais. I am also thankful to Professor Teresa Ubide for reawakening my curiosity whenever we worked together, and for all valuable discussions. Agradeço a todo o corpo docente e técnico-administrativo do Instituto de Astronomia, Geofísica e Ciências Atmosféricas da USP (IAG-USP) pela ajuda e prestatividade em diversos momentos. Sou especialmente grata aos Professores Doutores Marcia Ernesto, Naomi Ussami, Marcelo Bianchi, Carlos Mendonça e Ricardo Trindade pela orientação e suporte constantes. Durante o desenvolvimento deste trabalho no Brasil, fui auxiliada por inúmeras pessoas de diferentes laboratórios da Universidade de São Paulo (USP) e da Universidade Estadual Paulista (UNESP). Agradeço ao Mestre (em breve Doutor) Caio Vicentini (IAG-USP), e aoProfessor Doutor Fábio Machado (UFPR) pela ajuda em meu primeiro campo de amostragem, o qual serviria de base para os seguintes. Agradeço à coordenadora técnica do Laboratório de Preparação de amostras e da Litoteca do IAG-USP, Sônia Alonso, pela prestatividade, pela sabedoria generosa e pelos bolos. Sou grata a toda a equipe do Centro de Pesquisas Geocronológicas do Instituto de Geociências da USP (CPGeo -IGc -USP), sobretudo à especialista técnica em química isotópica e espectrometria de massa Liliane Petronilho.

The timing and mechanisms controlling the transition from large igneous provinces (LIPs) into oceanic crust during supercontinent breakup are poorly constrained. In the case of Pangea rifting and opening of the South Atlantic, the oldest seafloor magnetic anomalies usually postdate flood basalt extrusion by ∼20-30 Ma; however, geochronological evidence shows that minor mid-ocean ridge basalt (MORB)-like intrusions were almost coeval with late-stage flood basalt volcanism (e.g., Jagoutz et al., 2007;Jourdan et al., 2007). Little is known about the role and significance of this early MORB-like intrusive magmatism during rifting, and whether it marks a stepwise transition from a LIP into a passive margin, or whether it represents isolated asthenospheric upwellings still controlled by the LIP system.

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