Alkaline Rocks and Carbonatites of the World

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Piratini

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Occurrence number: 
025-00-115
Country: 
Brazil
Region: 
Rio Grande do Sul
Location: 
Longitude: -53, Latitude: -31.08

This is a province of over 30 pipes, up to 500 metres in diameter, which extends over a northwest-trending area of 20x5 km along the Arroio Moirao Graben. Most of the pipes intrude red beds of the Caneleiras Formation, are related to fractures and occur in clusters of two or more. Dykes and apophyses are also present, and there appear to be some related lavas and pyroclastics. Rock types include trachyte, phonolite and tinguaite, and some zonation in the pipes to tinguaitic margins is reflected in an increase in alkalis, etc. (Ribeiro, personal communication, 1981). Sodic pyroxene, arfvedsonite, melanite, nepheline, cancrinite, sodalite, pseudoleucite and aenigmatite occur. Spectrochemical data for a few trace elements are available (Sanchez et al. 1979).

Age: 
79 Ma by K-Ar on trachyte (Cordani et al. 1974).
References: 
CORDANI, U., HALPERN, M. and BERENHOLE, M. 1974. Comentarois sobre as determinacoes geocronologicas do Folha Porto Alegre SH.22. In. Carta Geologica do Brasil ao miliuonesimo: texto explicativo. 70-88. Ministerio das Minas e Energia, Departamento Nacional da Producao Mineral, Brasilia. RIBEIRO, M. 1971. Uma provincia alcalina no Rio Grande do Sul - I Estudos Preliminares. Iheringa, Museu Rio-Grandense de Ciencias Naturais, Porto Alegre, Geologia, 4: 59-71. RIBEIRO, M. 1978. Structural setting of some alkaline pipes in the Rio Grande do Sul Shield. Proceedings of the First International Symposium on Carbonatites, Pocos de Caldas: 171-5. M. Ribeiro personal communication, 1981. SANCHEZ, J.C.D., BIDONE, E.D., PINTAUDE, D.A. and PEREIRA, S.R. 1979. Estudo da distribuicao geoquimica de bario, estroncio, titanio, niobio, lantanio, zirconio e itrio, em rochas alcalinas da area Cangucu-Piratini, R.S. Pesquisas, Porto Alegre, 12: 79-117. ULBRICH, H.H.G.J. and GOMES, C.B. 1981. Alkaline rocks from continental Brazil. Earth Science Reviews, 17: 135-54
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