Compositional affinities of volcanic fragments in sedimentary rocks using electron microprobe analysis

The identification of clasts of volcanic origin in terrigenous detrital rocks is generally based on the observation of textural criteria. In the absence of quantitative analytical data on the chemical composition of such fragments, it is often quite difficult to identify their specific origin. However, it is now possible to use a defocused microprobe beam to obtain high-precision analyses of microcrystalline elements. The application of this technique to the groundmass of various lavas shows that the distribution of certain elements varies as a function of whole-rock composition. Three ternary diagrams, using the most discriminant parameters (K20/Na20/CaO, Fe20 3/Na20 + K20/MgO and SiO2/CaO × 10/MgO x 10), are proposed for the identification of different groundmass compositions (ultrabasic, basic, intermediate, and acid). The study of reworked clastic material from the Upper Ordovician sediments of Sardinia shows that fragments of volcanic groundmass can preserve their initial geochemical characteristics. The application of this method to microcrystalline fragments contained in Proterozoic sandstones of the Armorican Massif shows that it can be an additional approach for the identification of siliciclastic source-areas.