Co-eruption of carbonate and silicate magmas during volcanism in the Limagne graben (French Massif Central)

Peperites in the Limagne graben (French Massif Central) have been interpreted up to now as phreatomagmatic eruptions involving fragmentation of sedimentary rocks during magma-water interaction. We report about the possible magmatic origin of the carbonates involved in the peperites from Gergovie and Pileyre localities. In these two volcanic deposits, carbonates are manly dolomitic in composition, while the Limagne sediments are almost exclusively detrital formations or limestones. In the studied rocks, the carbonates can be found in different habits, but are sometimes closely associated with Cr-diopsides and Cr-spinels, minerals never found in the Limagne Miocene volcanic rocks and related to deep mantle processes. Dykes crosscutting the layered deposits are mainly composed of euhedral dolomite grains. Trace elements and Sr isotopes confirm that the carbonates are chemically different compared to the sedimentary rocks from the Limagne and therefore that the studied volcanic rocks are not a mixing between silicate magma and the local sediments. Based on our new petrological and geochemical data, we interpret these carbonates as evidence for the eruption of carbonatitic magmas associated with the silicate magmas found in the deposits. The high Sr isotopic ratios of these rocks imply the melting of an enriched metasomatised mantle for the source of the carbonates, as already advocated for many ultrapotassic or carbonatitic volcanic rocks in Europe. These results call for a reappraisal of the origin of the volcanism in the Limagne volcanic province and more generally of the carbonatitic volcanic provinces in Western Europe. (C) 2012 Elsevier B.V. All rights reserved.