representing Precambrian bimodal volcanism. The dolerites suffered hydrothermal alteration and low-grade metamorphism. Electron-microprobe analyses were carried out of plagioclase, pyroxene, amphibole, chlorite, epidote, opaque oxides and calcite. Petrography and compositional variations of mineral phases are discussed in terms of metamorphic and hydrothermal alteration, igneous differentiation and temperature - pressure estimations.