Olivine melilitites of the SW German Tertiary volcanic province: Mineralogy and petrogenesis

The mineralogy and mineral chemistry of olivine melilitites from the Late Tertiary Urach and Hegau volcanic provinces of SW Germany, and from Mahlberg Castle, north of the Kaiserstuhl carbonatite complex within the Upper Rhinegraben, provide important constraints on the petrogenesis of these these rather rare magma types The principal features of the mineralogy, are controlled by the high degree of silica undersaturation, relatively low total alkalis (<5 wt %, Na(2)O > K(2)O) and the high Ca and Mg contents of the magmas. Olivine (Fo(79-89), frequently reverse-zoned), Ca-saturated clinopyroxene exhibiting strong disequilibrium zonation, and akermanite-rich melilite are the dominant crystallizing phases, along with Cr-spinel and magnetite. Rare Ba-Ti phlogopite, containing up to 18 wt % BaO and 9 wt % TiO(2), occurs in the groundmass of most samples, along with Nb-rich perovskite, fluorapatite, nepheline and devitrified glass. Strongly resorbed Cr-diopside cores to clinopyroxene phenocrysts are inferred to be xenocrystic, representing disaggregated fragments of mantle wall-locks exhumed by the melilitite magmas. Similar xenocrystic cores also occur in the olivine and spinel phenocryst populations. These xenocrystic cores acted as nuclei during subsequent crystallization of the the melilitite host magmas during magma ascent. The combination of previously published geochemical data with the results of this petrographic study suggest that initial melt generation occurred within the dolomite-garnet stability field in the asthenosphere, the main episode of magma generation occurred at the base of the lithosphere, and additional wall-rock assimilation occurred as the magmas rose through the lithosphere, including the incorporation of spinal peridotite xenoliths and sheared olivine xenocrysts which last equilibrated close to the base of the crust. Thus, the bulk-rock composition of the magmas records a composite product of a complex multi-stage and multi-source history.