The Pien complex is characterized by a series of NE-SW trending basic and ultrabasic lenses. The country rock is the Agudos granitic body and, predominantly, extensively developed migmatites with some minor occurrences of granulite. Metaperidotites, metapiroxenites, metanorites, serpentinites and magnesian schists form the uitramafic bodies; hornblende metagabbro is the main constituent of the basic lenses. On the basis of petrographical, chemical and geochronological evidences, the geological history is reconstructed as follows: a sequence of basic and ultrabasic intrusions, probably comagmatic, was followed, during the Transamazonic cycle, by an essentially isochemical high grade (granulitic) metamorphic event. A second metamorphism, at lower temperatures and with water influx, overprinted, the first event, during the Brazilian cycle, and controlled the extensive formation of amphiboles, and perhaps of serpentines and tale, at the expense of the pre-existing mineralogy of the basic and ultrabasic rocks. Regional migmatization and intrusion of the Agudos granite are also related to this Brazilian event.The Pien complex is characterized by a series of NE-SW trending basic and ultrabasic lenses. The country rock is the Agudos granitic body and, predominantly, extensively developed migmatites with some minor occurrences of granulite. Metaperidotites, metapiroxenites, metanorites, serpentinites and magnesian schists form the uitramafic bodies; hornblende metagabbro is the main constituent of the basic lenses. On the basis of petrographical, chemical and geochronological evidences, the geological history is reconstructed as follows: a sequence of basic and ultrabasic intrusions, probably comagmatic, was followed, during the Transamazonic cycle, by an essentially isochemical high grade (granulitic) metamorphic event. A second metamorphism, at lower temperatures and with water influx, overprinted, the first event, during the Brazilian cycle, and controlled the extensive formation of amphiboles, and perhaps of serpentines and tale, at the expense of the pre-existing mineralogy of the basic and ultrabasic rocks. Regional migmatization and intrusion of the Agudos granite are also related to this Brazilian event.