Mineralogy and petrology of lava of the Goldberg volcano, West Eifel volcanic field, Germany

Abstract

The Goldberg is the northwesternmost volcano of the West Eifel basanitic and nephelinitic volcanic field in Germany. It was built in the EARLY to MIDDLE PLEISTOCENE, during a monogenic eruption with successive phreatomagmatic and Strombolian activities. The lava is an olivine-, pyroxene-, and phlogopite-phyric leucite nephelinite. Olivine types comprise phenocrysts (Fo% 90–84) and some xenocrysts (Fo% 92). Different pyroxenes crystallised with compositions ranging from Mg-diopside in megacrysts to diopside in the green core of phenocrysts and fassaitic diopside in the rim of phenocrysts. These chemical variations are discussed in terms of correlation of the pyroxene solid solutions. They reveal complex processes with polybaric crystallisation under decreasing pressure for the megacrysts and the green cores of phenocrysts in an initial batch of magma. Then, due to a new supply of magma and its ascent though the crust, the green cores were coated with a different composition and new phenocrysts developed. Amphibole is scarce and consists of Ti-rich pargasite. Mica consists of Al-rich and Ti-rich phlogopite. K-rich nepheline and leucite are common in the groundmass. Oxides, haüyne, and apatite complete the mineral composition. Petrological data indicate the successive emplacement of several batches of magma. The bulk geochemical composition of the lava and scoria is mafic, potassic and highly undersaturated. The volcano belongs to the Eifel F-suite (Foidite). The normalized trace element patterns show a lithophile element enrichment except for Th and U, and a high field strength element depletion. Compared with the composition of the nephelinites and basanites of the western Germany volcanic fields, the Goldberg’s magma shares similar mantle source and no crustal contamination. Geochemical data are consistent with a low degree of melting of an enriched mantle at the garnet-spinel transition region. Low Nb/Ta suggests a potential carbonatitic input into the melting region.