Major and rare earth element geochemistry of metasedimentary granulite xenoliths in Quaternary alkali basalts from the Middle Atlas (Morocco): evidence for a partly restitic lower crust

High-alumina granulite xenoliths equilibrated under lower crustal conditions (850-300 degrees C; 9 +/- 1 kbar) have been found in the Tafraout maars and analysed for major and rare earth elements. starting from a protolith of pelitic composition, a strong decrease of SiO(2) and alkalis and a concomitant increase of Al(2)O(3), Fe(2)O(3) and MgO are observed. These variations are interpreted to reflect partial melting and extraction of granitic magmas. Lanthanide elements show a progressive depletion of LREE combined with a marked increase of HREE. These variations result, in the most restitic samples, in V-shaped chondrite-normalized patterns. These peculiar patterns are interpreted in terms of incomplete separation bem een a garnet-sillimanite-rich refractory residue, highly depleted in LREE and enriched in HREE, and a small amount of trapped partial melt, containing most of the LREE. Negative Eu anomalies, inherited from the pelitic protolith, occur throughout. They are nor erased, but rather enhanced, by partial melting and granitic magma removal. A simple modeling, based on K(2)O and La concentrations, allows us to propose that most of the xenoliths represent refractory residues derived from high degrees (about 65 %) of partial melting of evolved sources similar to average post-Archean shales. In the most restitic samples, the amount of partial melt left behind in the source can be estimated to be only a few percent. ((C) Academie des sciences : Elsevier, Paris.).