The Tchivira-Bonga alkaline-carbonatite complex (Angola): petrological study and comparison with some Brazilian analogues

The Cretaceous Tchivira alkaline-carbonatite plutonic complex forms a roughly circular concentric pattern and comprises, from the outside inward, nepheline syenites, ijolites/urtites and carbonatites. The western side of the complex is cut by a second intrusion of alkali gabbros and syenites. A number of dykes of varying composition are widespread in the complex and in the basement country rocks. Field relationships and petrological data can be used to define two magmatic suites: 1) nepheline syenite-ijolite-carbonatite, and 2) alkali gabbro-syenite, both of which evolved essentially by fractional crystallization under nearly closed system conditions. The age relations, petrology, Sr isotopes, rock associations and spatial arrangements of the Tchivira-Bonga complex are comparable with those of the south Brazilian alkaline-carbonatite complexes, particularly Juquia and Jacupiranga. Such a comparison suggests a common geodynamic and petrogenetic setting for the South Brazilian and Angolan alkaline magmatic suites just before the opening of the South Atlantic Ocean. -from Authors