Alkaline Rocks and Carbonatites of the World

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Ilha De Sao Sebastiao

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Occurrence number: 
025-00-094
Country: 
Brazil
Region: 
Sao Paulo
Location: 
Longitude: -45.33, Latitude: -23.83

Four poorly exposed, stock-like bodies occupy about half the island and cut Precambrian gneisses and granites and some large basaltic dykes. The three larger stocks show mainly syenitic rocks (Serraria stock, to the north; Sao Sebastiao, southwest; Mirante, southeast) while the smallest is gabbroic (das Canas northernmost). The principal rock types in the two larger stocks are biotite- and pyroxene-alkali feldspar syenites, with some nordmarkites; the main minerals are perthitic K-feldspar, biotite, sodic-augite, sometimes with barkevikite; minor quartz and albite are observed in nordmarkites. These two stocks also show a zonal pattern, with nordmarkites apparently only found along the borders; they are cut by late trachytic to phonolitic dykes. The Mirante stock, which is cut by phonolite dykes, contains some nepheline syenites, and these apparently grade into syenites and pulaskites towards the margin. The constituent minerals are perthitic K-feldspar, nepheline, sodalite, aegirine-augite, biotite and some alkali amphiboles (barkevikite and hastingsite). The gabbroic das Canas stock is apparently a stratified intrusion showing cumulate textures and mineralogical banding, with (?)lowermost melagabbros (to olivine pyroxenites), (?)followed by olivine gabbros, theralites and rare cumulate ijolites. The mineralogy, in varying proportions, is zoned titanaugite, olivine, labradorite, nepheline and minor interstitial alkali feldspar. The age relationships between gabbroic and syenitic rocks is unknown.

Age: 
79-96 Ma by K-Ar on biotite and alkali feldspar from syenite, tinguaite and essexite; a diabase dyke gave 119 Ma (Amaral et al. 1966); 54-86 Ma by K-Ar on trachyte, syenite, gabbro, nepheline syenite and tinguaite (Hennies and Hasui, 1968).
References: 
AMARAL, G., CORDANI, U.G., KAWASHITA, K. and REYNOLDS, J.H. 1966. Potassium-argon dates of basaltic rocks from southern Brazil. Geochimica et Cosmochimica Acta, 30: 159-89. AMARAL, G., BUSHEE, J., CORDANI, U.G., KAWASHITA, K. and REYNOLDS, J.H. 1967. Geochimica et Cosmochimica Acta, 31: 117-42. APS, V. and BORN, G.H. 1975. Pesquisa do minerio fosfatico de Ipanema. Geologia e Metalurgia, Sao Paulo, 37: 24-34. FREITAS, R.O. de, 1947a. Geologia e petrologia da Ilha de Sao Sebastiao. Boletim da Faculdade de Filosofia, Ciencias e Letras, Unoversidade de Sao Paulo, Geologia, 3: 1-244. FREITAS, R.O. de, 1976. Definicao petrologica, estrutural e geotectonica das cintas orogenicas antigas do litoral norte do Estado de Sao Paulo. Boletim Instituto Geociencias, Universidade de Sao Paulo, 1: 1-174. HENNIES, W.T. and HASUI, Y. 1968. Geocronologia das rochas eruptivas da Ilha de Sao Sebastiao, SP. Anais, Congresso Brasileiro de Geologia, 22: 145-8. HENNIES, W.T. and HASUI, Y. 1977. Contribuicao ao reconhecimento da geologia da Ilha de Sao Sebastiao. Sociedade Brasileira de Geologia, Simposio Geologia Regional, Sao Paulo, 1: 199-209. Penteado and H. Ulbrich, personal communication, 1985.
Fig. 214 Ilha de Sao Sebastiao (after Hennies and Hasui, 1977, Fig. 1).
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