Ilha Dos Buzios

This is an island of 7.5 km2, 90% of which is occupied by syenite and quartz syenite, the northwestern part being of Precambrian gneisses and hypersthene charnockites. The syenite is a coarse, grey rock of mesoperthite, K-feldspar, aegirine-augite, hornblende, biotite and opaque minerals and grades in places into quartz syenites. Near contacts with Precambrian gneisses and charnockites, the quartz syenites pass into a fine to medium grained, green variety of the same mineralogy but with the addition of remnant orthopyroxene; these rocks are rich in xenoliths.