Evidence for magma mixing within the Laacher See magma chamber (East Eifel, Germany)

The final pyroclastic products of the late Quaternary phonolitic Laacher See volcano (East Eifel, W.-Germany) range from feldspar-rich gray phonolite to dark olivine-bearing rocks with variable amounts of feldspar and Al-augite megacrysts. Petrographically and chemically homogeneous clasts occur along with composite lapilli spanning the compositional range from phonolite (MgO 0.9%) to mafic hybrid rock (MgO 7.0%) for all major and trace elements. Both a basanitic and a phonolitic phenocryst paragenesis occur within individual clasts. The phonolite-derived phenocrysts are characterized by glass inclusions of evolved composition, rare inverse zoning and strong resorption indicating disequilibrium with the mafic hybrid matrix. Basanitic (magnesian) clinopyroxene and olivine, in contrast, show skeletal (normally zoned) overgrowths indicative of post-mixing crystallization. In accord with petrographical and other chemical evidence, mass balance calculations suggest mixing of an evolved Laacher See phonolite containing variable amounts of mineral cumulates and a megacryst-bearing basanite magma. Magma mixing occurred just prior to eruption (hours) of the lowermost magma layer of the Laacher See magma chamber but did not trigger the volcanic activity. ?? 1984.