Produção Nacional
PROVENANCE OF THE SEDIMENTARY ROCKS OF THE BOM JARDIM GROUP (NEOPROTEROZOIC, SOUTHERN BRAZIL): EVIDENCE FROM PETROGRAPHY, GEOCHEMISTRY AND NEODYMIUM ISOTOPES
2007; Volume: 14; Issue: 1 Linguagem: Inglês
ISSN
1851-4979
AutoresAndré Weissheimer de Borba, Anderson José Maraschin, Fábio de Lima Noronha, Júnia Casagrande, Ana Maria Pimentel Mizusaki,
Tópico(s)Geochemistry and Geologic Mapping
ResumoProvenance studies are helpful in reconstructing the tectonic, climatic and geographic setting of terrigenous sedimentary basins. The primary controls on the composition of detrital sedimentary rocks are the composition, climate and relief conditions of the source areas, the transport mechanisms, and diagenesis. In order to constrain the provenance of the Neoproterozoic (ca. 600 - 580 Ma) sedimentary rocks of the Bom Jardim Group (Arroio dos Nobres Formation) in the Lavras do Sul (west), Bom Jardim (central) and Cerro da Arvore (east) areas, petrography, major and trace element geochemistry, and Nd isotopic data were analyzed together. The results indicate a clear lateral variation of provenance parameters. In the region of Lavras do Sul, near syn-sedimentary volcanic rocks of shoshonitic affinity (the Hilario Formation), the sandstones are epiclastic, plotting in the undissected and transitional arc fields of the QFL diagram, display relatively higher values of Na, Ba, Sr and Zr, and yield a 143 Nd/ 144 Nd ratio of 0.512263 and present-day e Nd of -7.32. In the Bom Jardim area, east of Cacapava do Sul, the sedimentary rocks are also dominated by volcanic fragments, but display significant values of metamorphic detritus. The relatively higher values of Cr, Co and Ni suggest some contribution of ophiolitic or lamprophyric source areas, while the 143 Nd/ 144 Nd ratio is 0.511906 and the e Nd(0) is around -14.28. In the easternmost outcropping area, the Cerro da Arvore (or Piquiri) region, the sedimentary rocks are rich in metasedimentary clasts, with significant amounts of polycrystalline quartz fragments, plotting in the orogenic recycling field. Comparatively, the value of the K 2 O/Na 2 O ratio is higher, the 143 Nd/ 144 Nd ratio is 0.511887 and the e Nd(0) is of -14.65, suggesting older and more acidic source rocks, probably constituents of the Porongos metamorphic complex. a granted by CNPq; b granted by ANP 2 Ministerio Publico do Estado do Rio Grande do Sul, Divisao de Assessoramento Tecnico (DAT/MP-RS) #
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