Understanding the occurrence, extent, and lithological variability of UHP metamorphic domains is fundamental to plate tectonics and its evolution through geologic time. In the central Saxonian Erzgebirge (Germany), the highest metamorphic conditions are recorded in the “Gneiss-Eclogite Unit”, a heterogeneous nappe comprising MP to UHP rocks. Although extensively investigated, UHP rocks were only locally confirmed by the occurrence of coesite in eclogite (e.g., Massonne 2001, Eur. J. Mineral.; Gose & Schmädicke 2018, J. Petrol.) and diamond in paragneiss (e.g., Nasdala & Massonne 2000, Eur. J. Mineral). This leads to the major question whether the metamorphic rocks of the area are a product of different peak metamorphic conditions or whether they share a joint P-T path but equilibrated and/or re-equilibrated at a different metamorphic stage.
In order to screen the variety of poorly exposed rocks in the area, we present a comprehensive dataset of mineral inclusions occurring in 2100 detrital garnet grains. Based on the findings of diamond, coesite, and nanogranitoid inclusions (including cristobalite, kokchetavite, kumdykolite), and by comparing the chemical composition of detrital garnet with that of garnet from crystalline rocks, we show that (i) UHP rocks occur in a wider area than previously expected, (ii) UHP rocks comprise not only eclogite and paragneiss lenses but also their surrounding felsic country rocks, and (iii) the country rocks extensively re-equilibrated at high-temperature conditions during exhumation (Schönig et al. 2019, Geology; Schönig et al. 2020, Gondwana Res.). Thus, we conclude that the metamorphic rocks of the area were subducted to UHP conditions as a largely coherent slab. The results highlight the potential of detrital garnet to tackle major questions arising in UHP research, even in well-studied regions like the Erzgebirge.
Jan Schönig (1), Hilmar von Eynatten (1), Guido Meinhold (1,2) & Keno Lünsdorf (1)
Georg-August-University Göttingen, Geosciences Center, Germany (1);Keele University, School of Geography, Geology and the Environment (2)