U–Pb SHRIMP geochronology of zircon from the Catoca kimberlite, Angola: Implications for diamond exploration

Abstract

We present the first age determinations of zircon from the diamondiferous Catoca kimberlite in northeastern Angola, the fourth largest kimberlite body in the world. The U–Pb ages were obtained using a Sensitive High Resolution Ion Microprobe II (SHRIMP II) on zircon crystals derived from tuffisitic kimberlite rocks and heavy-mineral concentrates from the Catoca kimberlite. The SHRIMP results define a single weighted mean age of 117.9±0.7Ma (Mean square weighted deviation MSWD=1.3). More than 90% of the results indicate a single age population. There is no evidence for variable ages within single crystals, and no diffusional profiles are preserved. These data are interpreted as the maximum age of the kimberlite eruption at Catoca. The U/Th values suggest at least two different sources of zircon crystals. These different populations may reflect different sources of kimberlitic magma, with some of the grains produced in U- and Th-enriched metasomatized mantle units. This idea is consistent with the two populations of zircon identified in this study. One population originated from a depleted mantle source with low total REE (less than 25ppm), and the other was derived from an enriched source, likely from the mantle or a carbonatite-like melt with high total REE (up to 123ppm).The tectonic setting of northeastern Angola is influenced by the opening of the south Atlantic, which reactivated deep NE–SW-trending faults during the early Cretaceous. The eruption of the Catoca kimberlite can be correlated with these regional tectonic events. The Calonda Formation (Albian–Cenomanian age) is the earliest sedimentary unit that incorporates eroded material derived from the diamondiferous kimberlites. Thus, the age of the Catoca kimberlite eruption is restricted to the time between the middle of the Aptian and the Albian. The new interpretation will be an important guide in future exploration for diamonds because it provides precise data on the age of a diamond-bearing kimberlite pulse in Angola.