Alkaline Rocks and Carbonatites of the World

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Itabuna

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Occurrence number: 
025-00-036
Country: 
Brazil
Region: 
Bahia
Location: 
Longitude: -39.28, Latitude: -14.75

Itabuna comprises a complex of syenite, diorite, monzodiorite and granite, with associated dykes, of approximately 18x40 km, intruding a broad range of Precambrian granulite facies rocks, but is overlapped on its eastern margin by Mesozoic sediments. Syenites, the most abundant rock type, include a range of pyroxene-, amphibole- and biotite-bearing types, the feldspars varying between perthite, microcline and sodic plagioclase. The pyroxenes vary from Ti-augite to aegirine-augite. The amphiboles are principally arfvedsonite but hornblende and katophorite also occur; the biotite is Ti-rich. The diorites and monzodiorites consist of oligoclase, perthite, Ti-biotite, hornblende and accessories. Granites are relatively minor and it is not clear from descriptions if they are peralkaline. A varied suite of dykes occur within the complex and cutting the surrounding granulites and include alkali basalts, (?)nephelinites, phonolites, trachytes and syenitic aplites. The trachytes contain biotite and a sodic amphibole. The so-called nephelinites comprise poikilitic kaersutite, nepheline, plagioclase, carbonate and opaques. The chemistry of the rocks, most of which are nepheline-normative and miaskitic, is described and discussed by Oliveira et al. (1980).

Age: 
K-Ar on biotite from syenite gave 556±20 Ma (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. OLIVEIRA, E.P., LIMA, M.I.C. and MARQUES, N.M.G. 1980. Macico sienitico de Itabuna, Bahia. Anais Congresso Brasileiro de Geologia, 31(4): 2149-62
Fig. 195 The Bahia province of alkaline rocks (based principally on the Carta Geologica do Brasil ao milionesimo: Folha Salvador SD. 24 accompanying Pedreira et al. 1976).
Location: 
Scratchpads developed and conceived by (alphabetical): Ed Baker, Katherine Bouton Alice Heaton Dimitris Koureas, Laurence Livermore, Dave Roberts, Simon Rycroft, Ben Scott, Vince Smith