Ilha De Sao Sebastiao

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.