Resumen de Kimberlites associated with the Lucapa structure, Angola
Six kimberlite pipes within the Lucapa structure in northeastern Angola have been investigated using major and trace element geochemistry of mantle xenoliths, macro- and megacrysts. Geothermobarometric calculations were carried out using xenoliths and well-calibrated single crystals of clinopyroxene. Geochronological and isotopic studies were also performed where there were samples available of sufficient quality. Results indicate that the underlying mantle experienced variable conditions of equilibration among the six cites. Subsequent metasomatic enrichment events also support a hypothesis of different sources for these kimberlites. The U/Th values suggest at least two different sources of zircon crystals from the Catoca suite. These different populations may reflect different sources of kimberlitic magma, with some of the grains produced in U- and Th-enriched metasomatized mantle units, an idea consistent with the two populations of zircon identified on the basis of their trace element compositions. Calculated temperature and pressure from xenoliths are less scattered than T-P data calculated from single crystals. The calculated northeastern Angola paleogeotherm is consistent with a single value for the CA and the CU79 kimberlites. The differences in T-P values between these kimberlites may reflect the different way each kimberlite sampled the lithosphere. The lithospheric thickness calculated from the northeastern Angola paleogeotherm yielded 192 km. This research shows that the absence of fresh Mg-rich ilmenite in the Catoca kimberlite (one of the largest bodies of kimberlite in the world), as well as the occurrence of Fe3+-rich ilmenite, do not exclude the presence of diamond in the kimberlite. This is a new insight into the concept of ilmenite and diamond exploration, and leads to the conclusion that compositional attributes must be evaluated in light of textural attributes. The tectonic setting of northeastern Angola was influenced by the opening of the South Atlantic Ocean, which reactivated deep NE–SW-trending faults during the early Cretaceous. The new interpretation of a kimberlitic pulse during the middle of the Aptian and the Albian, which provides precise data on the age of a significant diamond-bearing kimberlite pulse in Angola, will be an important guide in future exploration for diamonds. These findings contribute to a better understanding of the petrogenetic evolution of the kimberlites in northeastern Angola and have important implications for diamond exploration.
