Mantle Sources and Origin of the Four Overlapping Continental LIPs Generated Throughout 2500 m.y. of Kaapvaal Craton History in Southern Africa

The Archean Kaapvaal Craton of southern Africa hosts at least four spatially overlapping Large Igneous Provinces (LIPs), each of which generated substantial volumes ( 1-3 x 106 km3) of mafic magmatic rocks, over short time intervals (5 m.y. or less), between 2.7 and 0.18 Ga: the Ventersdorp Supergroup (2714 Ma, 0.7 x 106 km3), the Bushveld layered intrusion (2056 Ma, 1.5 x 106 km3), the Umkondo Igneous Province (1105 Ma, 2 x 106 km3) and the Karoo LIP (182 Ma, 3 x 106 km3). Therefore, over a time interval of >2500 m.y., a minimum collective volume of 7.2 x 106 km3 of mantle-derived, mafic lavas, sills, dikes and derivative cumulate rocks, was periodically emplaced through, into and/or onto the same cratonic region of Archean lithosphere. This long-term spatial superposition of Kaapvaal LIPs can be used as input to the vigorous debate on the nature of LIP mantle sources, and the possible role of crustal contamination in their petrogenesis. Continental LIP magmas, including all four of the Kaapvaal examples, have commonly been interpreted as products of direct partial melting of sub-continental lithospheric mantle (SCLM) sources, with little or no contribution from upwelling, plume-related, asthenospheric materials that provided the heat for melting. The Kaapvaal SCLM was stabilized at 3 Ga by prior melt extraction events that rendered it chemically depleted, and hence buoyant; it seems unlikely that it might have been capable of generating 1-3 x 106 km3 of basaltic magmas four times during its history. This would require repeated, substantial refertilization to counteract the extensive chemical depletion caused by recurrent extraction of LIP magmas. Chemical enrichment events sufficient to yield such extensive volumes of basaltic magma would necessarily increase bulk SCLM density, compromising its long-term buoyancy and stability. It seems far more likely, therefore, that the Kaapvaal LIPs were generated from sub-lithospheric sources, and that their diverse geochemical and isotopic signatures represent variable assimilation of Archean (dominantly 3.0-3.6 Ga) granitoid crustal contaminants, as many have suggested. These arguments challenge the plausibility of SCLM melting as a viable general process for the origin of other continental LIPs.