Igneous and sedimentary ‘limestones’: the puzzling challenge of a converging classification

Abstract

The vast majority of extrusive carbonatites are calcitic rocks which may be confused with sedimentary limestones, thus requiring a disambiguation criterion. Extrusive carbonatites are classified based on quantitative criteria that tend to avoid genetic mechanisms. Carbonatite nomenclature is in progress but regulated by the International Union of Geological Sciences norm for igneous rocks. Carbonate sedimentary rock nomenclature is mainly regulated by the Dunham, Embry and Klovan, and Sibley and Gregg classification systems. These limit the description of rock types from various depositional mechanisms and makes comparison with sedimentary rocks difficult. Igneous and sedimentary carbonate rocks display no apparent differences in the field and at meso–micro-scale. They may be layered, massive crystalline or show discrete clasts in a matrix, which make both rock types resemble one another. The study analyses the situations in which classification inconsistencies are most common. Adopting these guidelines may increase confidence, reliability and value in the petrographic classification of igneous and sedimentary lithologies. This study poses a challenging target. Can igneous carbonate rocks be classified using the same approach that is used for sedimentary carbonate rocks and vice versa? So far, the scheme chosen has been arbitrary or limited to the aim of the study being undertaken. The authors start an unexperienced dialogue for the first time between volcanologists and sedimentologists by examining a range of sedimentary and volcaniclastic rock textures which may resemble each other.