Geochemical constraints on the origin of xenolith-bearing alkali basaltic rocks and megacrysts from the Hoggar, central Sahara.

Abstract

Some xenolith-bearing Hoggar basanites and nephelinites possibly represent primary or near-primary magmas on account of their mg and Ni values. Simple model calculations show that they could be derived by 5–8 percent partial melting of a garnet-bearing pyrolite source. They are characterized by LREE enriched abundance patterns and by highly variable abundances of compatible and incompatible trace elements. These compositional features are compatible with derivation by small degrees of melting of an inhomogeneous mantle source recently enriched in LREE and other incompatible elements. Model calculations involving transition metal and REE contents do not support a cogenetic origin of the ultramafic xenoliths as source or residual material. Megacrysts of kaersutite, clinopyroxene, orthopyroxene, oligoclase, ilmenite and zircon are considered to represent high-pressure near-liquidus to near-solidus phases precipitated from basanitic magmas. Inconsistencies in trace element contents of some kaersutites and clinopyroxenes and host basalts suggest that these megacrysts are not strictly cognate with respect to their present hosts.