Produção Nacional
PROVENIÊNCIA DE ARENITOS DAS FORMAÇÕES UBERABA E MARÍLIA (GRUPO BAURU) E DO GARIMPO DO BANDEIRA: IMPLICAÇÕES PARA A CONTROVÉRSIA SOBRE A FONTE DO DIAMANTE DO TRIÂNGULO MINEIRO
2002; Sociedade Brasileira de Geologia; Volume: 32; Issue: 4 Linguagem: Inglês
10.5327/rbg.v32i4.1006
ISSN2317-4889
AutoresÉrica Gonçalves Gravina, Camilla Vasconcelos Kafino, José Affonso Brod, Géraldo Resende Boaventura, Roberto Ventura Santos, Edi Mendes Guimarães, Hardy Jost,
Tópico(s)Geochemistry and Geologic Mapping
ResumoClay minerals and REE provide important evidence on weathering, transport, and deposition of sediments, since they tend to reflect the mineralogical and chemical signatures of the source areas. Some Bauru Group sandstones in the Triângulo Mineiro region, particularly those of the Uberaba Formation, have a peculiar composition, given by large amounts of clastic material derived from alkaline igneous rocks. In this paper we combine chemical and mineralogical evidence from sandstones of the Uberaba and Marilia Formations, and from the sandy matrix of a diamond-bearing conglomerate (Garimpo do Bandeira), of unknown stratigraphic position, to investigate the provenance of their clastic load. Integrated analysis of clay mineralogy, and major, trace and REE, allowed the inference of three distinct sources for the studied sedimentary units. Clay-minerals of the Uberaba Formation are dominated by smectite and illite, indicating strong contribution of mafic rocks and incipient chemical weathering of the source area. Semi-quantitative modeling based on the absolute REE abundances indicates that at least 30% of a component similar to the alkaline rocks from the nearby Alto Paranaiba Igneous Province (APIP) must be added to the average continental upper crust, in order to generate the observed normalised REE ratio modeling indicates a contribution of at least 50-60 % of clasts derived from kamafugite and bebedourites of the APIP. In the Marilia Formation, palygorskite is the dominant clay-mineral, but it is probably associated with diagenetic carbonate. Semi-quantitative modeling indicated less than 5% of alkaline rocks contribution.The normalised REE ratios poit to the addition of 10-15% of APIP kamafugites or bebedourites to an average continental upper crust. Kaolinite is the dominant clay mineral in the matrix of the Garimpo do Bandeira conglomerate, suggesting an intense chemical weathering in the source area. These rocks do not fit consistently in the numerical models applied. The evidence presented in this paper indicates that the contribution of alkaline-rock clastic material is strong for the Uberaba Formation, minor for the Marilia Formation, and negligible for the Garimpo do Bandeira conglomerate, with important implications for the controversy on the origin of the Triângulo Mineiro diamond deposits. We suggest that a similar approach may be used in the study of other sedimentary sequences, where the compositional distinctions are less obvious.
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