Mingling of Immiscible Dolomite Carbonatite and Trachyte in Tuffs from the Massif Central, France

Abstract

A pyroclastic tuff from the Velay volcanic province in the French Massif Central contains blocks up to 30cm long of local basement rocks, lava clasts, coarse-grained cumulates and pyroclastic fragments, with more or less diffuse boundaries with the host tuff, which probably represent more consolidated parts of the tuff. All of the pyroclastics examined and approximately 10% of the cumulate xenoliths contain carbonates in variable amounts, textures and mineralogy. In some of the tuff samples, dolomite occurs in large amounts (up to 57%), principally as immiscible globules in trachytic melt (now glass), and represents the first occurrence of carbonatite reported from the Massif Central. The other carbonates, magnesiosiderite in the mafic cumulates or occasionally in some tuffs, and calcite in the felsic cumulates, are always associated with a silicate glass of trachytic composition. Coexisting feldspars and carbonates in the various types of sample are approximately in Sr isotopic equilibrium with an initial ratio of about 07042. C- and O-isotopic compositions of the carbonates covary and cover a very wide range of composition fromˇ2 9t o 3 9 %(PDB) and from 87 to 245% (SMOW), respectively; the globular dolomites have primary igneous isotopic signatures and plot within the primary igneous carbonatite field. The combined C---O ---Sr isotopic data indicate that both the dolomitic carbonatitic and silicate magmas came from a mantle source with very similar isotopic characteristics. On compositional arguments, injection of a non-cogenetic carbonatite magma into a differentiating body of felsic magma within the crust is preferred to the unmixing of a relatively late, fractionated melt to carbonatitic and trachytic magmas. The textures, high d 13 Ca nd d 18 O values of Mg-siderite and calcite and their trend are consistent with post-magmatic precipitation of these carbonates, probably by interaction between a CO2---H 2O-bearing fluid (520 mol % CO2) undergoing Rayleigh distillation processes and the minerals and glass in the cumulates. Although minimum isotopic temperatures are 4100C for the calcites and 4275C for the magnesiosiderites, the isotopic data are compatible with the chemical, solvus temperatures of 4500C for the calcites and 4500---700C for the magnesiosiderites, if the CO2 content of the fluid decreased during the distillation processes. The highly variable K2O/ Na2O ratios of some of the glasses on the scale of a few microns may be another consequence of H2O---CO2 metasomatic processes. Explosive eruption of the two immiscible magmas that entrained the xenoliths probably occurred as a result of CO2---H 2O degassing processes soon after the arrival of the carbonatitic magma in the trachytic system.