Geochemistry of spinel peridotite inclusions in basalts from Hoggar, Algeria

Abstract

Ultramafic xenoliths enclosed in Cenozoic alkali basaltic rocks from Hoggar (central Sahara, Algeria) are composed mainly of spinel peridotite. Some spinel lherzolites and harzburgites contain amphibole or its breakdown products or were cut by amphibolite veinlets. The spinel peridotites are upper mantle residue left after the removal of basaltic melt and were later affected by metasomatic processes which led to the formation of amphibole and the enrichment of several lithophile elements particularly in amphibole-bearing varieties. The model calculations using the total inverse method of Tarantola and Valette ( Rev. Phys. Space Phys. 20, 219–232, 1982) indicate that the original upper mantle beneath the Hoggar had a composition similar to the estimated mantle of Jagoutz et al. ( Proc. 10th Lunar Planet. Sci. Conf., pp. 2031–2050, 1979) and Taylor ( Proc. 11th Lunar Sci. Conf., pp. 333–348, 1980) and that the host alkali basalts are not genetically related to the spinel peridotite xenoliths. The basalts were derived from a deeper, garnet-bearing source and scavenged the spinel peridotite xenoliths during their ascent.