Lithodemes in metamorphic high-grade terrains: breaking the paradigm of basement lithostratigraphy in Saxony.

High-grade metamorphic rocks in Saxony were historically interpreted as supracrustal rocks, implying that their protolith have been formed by direct deposition on pre-existing basement. This fixistic working hypothesis has been widely applied in many mapping campaigns and has led to a lithostratigraphic subdivision of these metamorphic suites, which is presented on numerous geological maps. State of the art geological evidence, however, suggests that lithostratigraphic models for mapping metamorphic high-grade terrains should be rejected, and, instead, metamorphic rock characterisation in the field should follow international guidelines recently established for lithodemic units. In Saxony, metamorphic high-grade rocks can be classified and mapped in a lithodemic sense as gneisses, micaschists, phyllites, eclogites, serpentinites, etc. based upon observable rock characteristics such as mineralogical, textural and structural features. In contrast to lithostratigraphic models, lithodemic nomenclature for metamorphic rocks should be independent of the inferred geologic history, and basic field criteria should be strictly separated from the laboratory data obtained from the mapped rocks. To establish a full hierarchical lithodemic classification, approved lithodemes may be ranked within metamorphic suites according to their tectono-metamorphic evolution. The interaction between sedimentary strata, deposited on a subaerial exposed basement, and lithodemic basement rocks is always characterised by the absence of stratigraphic coherency between the two geologic levels. Hence, metamorphic lithodemes should be treated separately from official stratigraphic tables and can be assigned to time intervals where tectonic emplacement within their present structural position took place, which typically coincides with the last major regional metamorphic event in the respective area. [ABSTRACT FROM AUTHOR]