Lithostratigraphy in high-grade terrains – An approach for the classification of metamorphic rocks?

A lithostratigraphic unit is a stratum or body of strata that conforms to the law of superposition and is defined based on lithic characteristics and stratigraphic position. The stratigraphic concept relies on the assumption that younger rocks are deposited on pre-existing rocks during earth's history, and that individual sedimentary layers of rather constant thickness expand laterally. This working hypothesis has been successfully applied in many sediment studies and the results have been presented on numerous geological maps. A stratigraphic approach is problematic in regional metamorphic series, since the primary stratigraphy is converted into tectonic layering during metamorphic overprint. Using the Saxon Granulite Massif as a case study, this contribution aims to demonstrate the difficulty in applying lithostratigraphic methods to high-grade metamorphic rocks. We provide a critical discussion of the arguments that favour a lithostratigraphic model for the Saxon Granulite Massif in the light of the state of the art literature. High-grade metamorphic rocks should be primarily characterized based on their macroscopic appearance and further investigated using geochronological, petrological and structural methods. Absolute age constraints can be derived from radiometric dating methods, however, it should be critically evaluated if these results reflect a protolith age, a metamorphic crystallisation age or an inherited age. Many metamorphic terrains are dominated by rocks of different ages and thus conventional principles of stratigraphy cannot be applied.