Detrital zircon geochronology of high-grade metasedimentary rocks: The eclogite type unit (Eastern Alps, Europe)

Determining the depositional age of high-grade metamorphic metasedimentary terrains remains a major challenge; without reasonable age assignments, these units cannot be accurately incorporated into geodynamic models. The U-Pb zircon system is used to date such units, but recrystallization during high-grade metamorphism often obscures primary age signals. Furthermore, the lithologic dependencies of neomorphic zircons are uncertain. Answering these questions is essential for revealing the protolith age and evolutionary history of the Saualpe-Koralpe Complex in the Eastern Alps of Europe—for which the pre-Permian history was previously unknown. This unit was affected by early Late Cretaceous high-pressure (high-P) metamorphism (95−90 Ma; up to 2.4 GPa and 650−730 °C), which overprinted a Permian low-pressure/high-temperature (low-P/high-T) metamorphic event. Our U-Pb dating of detrital zircon from three distinct metasedimentary lithologies reveals three textural and age types: (1) nearly entirely reset zircons in mica-rich gneiss and micaschist that date the Cretaceous high-P metamorphism and subordinately the Permian low-P/high-T metamorphism, with almost no pre-Permian ages preserved in the cores; (2) zircons in mica-poor gneisses with broad rims reflecting Permian metamorphism and cores containing detrital age populations between ca. 960 Ma and ca. 320 Ma; and (3) zircons from quartzites that appear unaffected by either high-P or low-P/high-T metamorphism, yielding scattered detrital ages between 995 Ma and 460 Ma. Together, the youngest single detrital grains provide a maximum depositional age of ca. 320 Ma for the Saualpe-Koralpe Complex metasedimentary rocks. The Cretaceous metamorphic overprint also resulted in a spread of apparent ages between Permian and Cretaceous, interpreted as a fluid-mediated dissolution-precipitation process. In conclusion, quartzite samples should be the preferred target for detrital studies in high-grade metasedimentary terrains, whereas mica-rich and gneissic samples yield a convincing record of polymetamorphic overprinting.