Carbonates or carbonatites? Examples of alkaline silicate plutonic xenoliths from volcanoclastic rocks in the Eifel, Germany

Plutonic xenoliths in volcaniclastic rocks from the Laacher See (LS), East Eifel and the Rockeskyller Kopf Volcanic Complex (RKVC), West Eifel, were studied. They show different ways of magmatic carbonate formation at plutonic levels.

Well-preserved original igneous textures of carbonate-bearing syenites were observed in LS plutonic xenoliths. Calcite is often crystallised in “micropegmatitic texture” with sanidine and nosean. These structures argue for syngenetic crystallisation of calcite and main silicate minerals in alkali syenite melts in the case of the LS volcano. Different proportions of euhedral calcite and silicate minerals point to calcite cumulate formation.

Occurrence of calcite associated to apatite, magnetite, phlogopite and pyrochlore in nosean syenite from xenoliths of the RKVC is in line with the assumption of magmatic carbonate formation in a highly fractionated undersaturated silicate magma.

Next to carbonatite - nosean syenite xenoliths and sanidinite, a series of different mafic xenoliths were found around the RKVC, from amphibolite to pyroxenite and magnetite cumulates, that could indicate one or more magma chambers below the volcano.

A comprehensive data set for geochemistry, mineralogy and mineral chemistry of the carbonates of LS and RKVC volcanoes is provided. Characteristic carbonate and whole rock trace element patterns are presented. Different chondrite normalised REE signatures of RKVC and LS argue for different ways of fractionation in these volcanic systems.