Metasomatic Overprint by Carbonatite derived CaO in Kaiserstuhl Phonolites, Germany

Carbonatites and silica-undersaturated rocks are closely related and occur in rift-related volcanic provinces. The proposed driving mechanism linking both magmatic series is that they share a common source which is driven by fractionation into a two-liquid solvus (Brooker & Kjarsgaard, JPet 52:1281-1305). Carbonatites and phonolites are, in consequence, closely related to each other. However, the interaction of both magmatic series after their separation and the role fluids play in this process are not fully understood. One location characterized by these types of rocks is the Kaiserstuhl volcanic complex (KVC) in Germany.

For the KVC we use Sr-Nd-Hf isotope, major and trace element data from phonolites, wollastonite-phonolites, carbonaceous sedimentary rocks and carbonatites to disentangle the possible effect of crustal contamination from primary mantle signatures. We found that the magmatic products show an overall EM-like signature. Moreover, the phonolites are enriched in CaO reaching up to 9.5 wt.% in the wollastonite-bearing phonolite. Previously it was thought that wollastonite formed due to contamination by carbonaceous sedimentary rocks, but this would not lead to EM-like signatures. This suggests that phonolitic melt became contaminated by carbonatitic melt, leading to a decarbonization reaction that caused the CaO enrichment. We therefore show that metasomatic processes can lead to mineral assemblages that are otherwise typical for Skarns. Furthermore, the role of metasomatic overprint could have been overlooked in other carbonatite-related volcanic provinces as well. This emphasizes the influence of melt and fluid phases on the formation of carbonatitic-related volcanic provinces, providing a cogenetic link between both magmatic series.