Abstract

The Ntungamo gneiss dome has a quartzo-feldspathic core that has an abrupt and generally concordant margin with the overlying Pre-Cambrian Karagwe–Ankolean metasediments. The sediments, which dip off the dome on all sides, lie in synclinal belts around it and between the Ntungamo dome and adjacent domes. There is no good evidence of the intrusive character of the granitic rocks of the dome, and it is suggested that they are deformed Karagwe-Ankolean basement. The deformation history of the metasediments is divisible into two phases separated by a time-interval during which porphyroblasts of andalusite, kyanite, and staurolite developed. The schistosity of the sediments was formed in the first phase and was deformed in the second; the dome and synclines were produced in the second phase. This interpretation differs from earlier ones in which all the Karagwe-Ankolean structures were described as forming in one deformational episode, the schistosity being produced during dome-forming movements that were themselves the result of granite intrusion.