METASOMATISM IN MANTLE XENOLITHS FROM GEES, WEST EIFEL, GERMANY - EVIDENCE FOR THE GENESIS OF CALC-ALKALINE GLASSES AND METASOMATIC CA-ENRICHMENT

We have investigated new samples from the Gees mantle xenolith suite ( West Eifel), for which metasomatism by carbonatite melt has been sugge sted. The major metasomatic change is transformation of harzburgites i nto phlogopite-rich wehrlites. Silicate glasses are associated with al l stages of transformation, and can be resolved into two major groups: a strongly undersaturated alkaline basanite similar to the host magma which infiltrated the xenoliths during ascent, and Si-Al-enriched, va riably alkaline glass present exclusively within the xenoliths. Si-Al- rich glasses (up to 72 wt% SiO2 when associated with orthopyroxene (Op x) are usually interpreted in mantle xenoliths as products of decompre ssional breakdown of hydrous phases like amphibole. In the Gees suite, however, amphibole is not present, nor can the glass be related to ph logopite breakdown. The Si-Al-rich glass is compositionally similar to glasses occurring in many other xenolith suites including those relat ed to carbonatite metasomatism. Petrographically the silicate glass is intimately associated with the metasomatic reactions in Gees, mainly conversion of harzburgite orthopyroxene to olivine + clinopyroxene. Bo th phases crystallize as microlites from the glass. The chemical compo sition of the Si-Al-enriched glass shows that it cannot be derived fro m decompressional melting of the Gees xenoliths, but must have been pr esent prior to their entrainment in the host magma. Simple mass-balanc e calculations, based on modal analyses, yield a possible composition of the melt prior to ascent of the xenoliths, during which glass + mic rolite patches were modified by dissolution of olivine, orthopyroxene and spinel. This parental melt is a calc-alkaline andesite (55-60 wt% SiO2), characterized by high Al2O3 (ca. 18 wt%). The obtained composit ion is very similar to high-alumina, calc-alkaline melts that should f orm by AFC-type reactions between basalt and harzburgite wall rock acc ording to the model of Kelemen (1990). Thus, we suggest that the Si-Al -enriched glasses of Gees, and possibly of other suites as well, are r emnants of upper mantle hybrid melts, and that the Gees suite was meta somatized by silicate and not carbonatite melts. High-Mg, high-Ca comp osition of metasomatic olivine and clinopyroxene in mantle xenoliths h ave been explained by carbonatite metasomatism. As these features are also present in the Gees suite, we have calculated the equilibrium Ca contents of olivine and clinopyroxene using the QUI1F thermodynamical model, to show that they are a simple Function of silica activity. Hig h-Ca compositions are attained at low a SiO2 and can thus be produced during metasomatism by any melt that is Opx-undersaturated, irrespecti ve of whether it is a carbonatite or a silicate melt. Such low a SiO2 is recorded by the microlites in the Gees Si-Al-rich glasses. Our resu lts imply that xenolith suites cannot confidently be related to carbon atite metasomatism if the significance of silicate glasses, when prese nt, is not investigated.