Alkaline Rocks and Carbonatites of the World

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Rondonianas Granites

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Occurrence number: 
025-00-015
Country: 
Brazil
Region: 
Amazonas and Rondonia
Location: 
Longitude: -60.33, Latitude: -7.83

The Rondonianas granites comprise over 30, usually circular, anorogenic intrusions emplaced in the Xingu complex of granulites, migmatites and gneisses. Most of the granites are biotite-bearing rapakivi types, but a number are peralkaline (see also Teotonio, No. 16). These are represented by aegirine and riebeckite granites, granophyres and syenites with dominantly potassic feldspars. The most significant accessory minerals are zircon and opaques with lesser sphene, fluorite and carbonate (Araujo et al. 1978).

Economic: 
Tin mineralization is widespread and many of the intrusions have been, and are being, worked.
Age: 
As a whole the granites give ages, by Rb-Sr, lying between 750 and 1100 Ma (Araujo et al. 1978). A Rb-Sr isochron on whole rocks and separated biotite (11 samples) gave 977±20 Ma (Priem et al. 1971) and K-Ar ages on biotite averaged 993±40 Ma.
References: 
ARAUJO, H.J.T., RODARTE, J.B.M., DEL'ARCO, J.O., SANTOS, D.B. dos, BARROS, A.M., TASSINARI, C.C.G., LIMA, M.I.C. de, ABREU, A.S. and FERNANDES, C.A.C. 1978. DNPM. Projeto Radambrasil. Folha SB.20 Purus. Levantamento de Recursos Naturais, Geologia, 17: 17-128. LEAL, J.W.L., SILVA, G.H., SANTOS, D.B. dos, TEIXEIRA, W. LIMA, M.I.C. de, FERNANDES, C.A.C. and PINTO, A.do C. 1978. DNPM. Projeto Radambrasil. Folha SC.20 Porto Velho. Levantamento de Recursos Naturais, Geologia, 16: 19-184. PRIEM, H.N.A., BOELRIJK, N.A.I.M., HEBEDA, E.H., VERDURMEN, E.A.T., VERSCHURE, R.N. and BON, E.H. 1971. Granitic complexes and associated tin mineralizations of 'Grenville' age in Rondonia, western Brazil. Bulletin of the Geological Society of America, 82: 1095-102. VERSCHURE, R.H. and BON, E.H. 1972. Geology and geochronology of tin-bearing volcano-plutonic complexes in Rondonia (western Brazil); reflections on the genesis of this type of magmatism. ZWO Laboratorium voor Isotopen-Geologie, Amsterdam, Progress Report 1970/72: 177-98
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