THE TCHIVIRA-BONGA ALKALINE-CARBONATITE COMPLEX (ANGOLA) - PETROLOGICAL STUDY AND COMPARISON WITH SOME BRAZILIAN ANALOGS

The Tchivira alkaline-carbonatite complex, of Lower Cretaceous age (130-138 Ma), crops out in the Precambrian crystalline basement of southwestern Angola as a circular intrusion covering an area of about 100 km(2). The 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. Bonga, an isolated plug a few kilometres northeast of Tchivira, consists exclusively of carbonatites with an area of about 12 km(2). A number of dykes, ranging in composition from tephrite to alkali basalt, hawaiite, mugearite, phonolite and trachy-phonolite, 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. Mass balance calculations account for the generation of the various rock-types in both suites, starting from basanitic and alkali basaltic parental magmas, respectively. Ca-carbonatite melts may be genetically linked to nepheline syenite by liquid immiscibility processes, and may themselves undergo fractional crystallization -mostly of calcite- leading to differentiated Mg-carbonatites. Initial Sr-87/Sr-86 isotopic ratios mostly range from 0.7038 to 0.7050 in the nepheline syenite-ijolite suite, and from 0.7046 to 0.7059 in the alkali gabbro-syenite suite. Significantly higher isotopic ratios (0.7073-0.7255), which are observed in some dykes and in the border facies of the intrusion, may be related to contamination by the granitic basement. 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.