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.
a b s t r a c tThe metamorphosed banded iron formation from the Nogolí Metamorphic Complex of western Sierra de San Luis, Eastern Sierras Pampeanas of Argentina (Nogolí area, 32°55'S-66°15'W) is classified as an oxide facies iron formation of Algoma Type, with a tectonic setting possibly associated with an island arc or back arc, on the basis of field mapping, mineral and textural arrangements and whole rock geochemical features. The origin of banded iron formation is mainly related to chemical precipitation of hydrogenous sediments from seawater in oceanic environments. The primary chemical precipitate is a result of solutions that represent mixtures of seawater and hydrothermal fluids, with significant dilution by maficultramafic volcanic and siliciclastic materials. Multi-stage T DM model ages of 1670, 1854 and 1939 Ma and positive, mantle-like n Nd (1502) values of +3.8, +1.5 and +0.5 from the banded iron formation are around the range of those mafic to ultramafic meta-volcanic rocks of Nogolí Metamorphic Complex, which are between 1679 and 1765 Ma and +2.64 and +3.68, respectively. This Sm and Nd isotopic connection suggests a close genetic relationship between ferruginous and mafic-ultramafic meta-volcanic rocks, as part of the same island arc or back arc setting. A previous Sm-Nd whole rock isochron of $1.5 Ga performed on mafic-ultramafic meta-volcanic rocks led to the interpretation that chemical sedimentation as old as Mesoproterozoic is possible for the banded iron formation. A clockwise P-T path can be inferred for the regional metamorphic evolution of the banded iron formation, with three distinctive trajectories: (1) Relict prograde M 1 -M 3 segment with gradual P and T increase from greenschist facies at M 1 to amphibolite facies at M 3 . (2) Peak P-T conditions at high amphibolite-low granulite facies during M 4 . (3) Retrograde counterpart of M 4 , that returns from amphibolite facies and stabilizes at greenschist facies during M 5 . Each trajectory may be regarded as produced by different tectonic events related to the Pampean? (1) and the Famatinian (2 and 3) orogenies, during the Early to Middle Paleozoic. The Nogolí Metamorphic Complex is interpreted as part of a greenstone belt within the large Meso-to Neoproterozoic Pampean Terrane of the Eastern Sierras Pampeanas of Argentina.
RESUMOUma zona tectônica linear, de forma sigmoidal, ocorre de Teixeira (Paraíba, ao norte-nordeste) a Terra Nova (Pernambuco, ao sul-sudoeste), com disposição diagonal ao domínio da Zona Transversal -ZT, que é a porção central da Província Borborema, de forma a subdividir a ZT em dois domínios lito-estruturais e geotectônicos distintos. Para norte e noroeste ficam os terrenos do Riacho Gravatá -RG (Eoneoproterozóico) e Piancó-Alto Brígida -PAB (Neoproterozóico), caracterizados por baixo grau de metamorfismo. Para o sul e sudeste fica o terreno Alto Pajeú (TAP, Eoneoproterozóico, ortognaisses e rochas supracrustais mesozonais) e seu embasamento, terreno Alto Moxotó -TAM (Paleoproterozóico), de grau médio a elevado de metamorfismo. Esta zona linear ou "alto" constituído de ortognaisses (do TAP e do embasamento paleoproterozóico que aflora esparsamente) é perlongado e marcado por uma série quase contínua de rochas intrusivas, plutões e stocks graníticos variados e sieníticos do tempo Ediacarano (idades entre 630 e 570 Ma, principalmente). Na história evolutiva desta porção linear particularmente soerguida do Planalto da Borborema há o registro (direto e/ou indireto) de eventos geológicos policíclicos, do Paleoproterozóico, do Eoneoproterozóico, do Ediacarano, do Eocambriano e do Cenozóico (sedimentação correlativa, elaboração de crostas e soerguimento regional), com implicações interessantes na geografia e sócio-economia regional. No presente trabalho será dada ênfase aos dados isotópicos (métodos Rb-Sr, Sm-Nd sobretudo) desta zona diferenciada, e nos contextos por ela separados, assim como no seu papel dentro do contexto tectônico regional, usufruindo para isto de expressivo acervo de dados geológicos e isotópicos, obtidos na última década (grande parte do qual ainda inédita). Além disso, hipóteses interpretativas serão postuladas para esta linha axial tectônica que subdivide a Zona Transversal em domínios geológicos distintos, considerando-a como conseqüência e subproduto de três processos paralelos de subducção no Neoproterozóico.Keywords: Borborema Province, Transversal Zone, Alto Pajeú Terrane, Riacho Gravatá Terrane, Brasiliano, Neoproterozoic granites. ABSTRACTA sigmoidal-shape linear tectonic zone occurs from Teixeira (Paraíba, to the north-northeast)-to Terra Nova (Pernambuco, to the south-southwest) as a line diagonal to the "Transversal Zone", which is the central tectonic portion of the Borborema Province, thus dividing this province into two different litho-structural and tectonic domains. To the north and northwest occur the Early Neoproterozoic Riacho Gravatá (RG) and the Neoproterozoic Piancó-Alto Brígida (PAB) terranes, both of them characterized by supracrustal rocks with low-grade of regional metamorphism. To the south-southwest occur the Alto Pajeú terrane (TAP, Early Neoproterozoic, orthognaisses and mesozonal supracrustal rocks) and the domain of its basement, the Alto Moxotó terrane (TAM, Paleoproterozoic in age), with high grade metamorphic rocks. This proposed "tectonic
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