Zircon and Apatite U‐Pb Constraints on the Tectonic Affinity and Metamorphic History of the Blueschist‐Facies Ambelakia Unit, Mt. Ossa, Greece.

In the Aegean region, the Cycladic Blueschist Unit (CBU) and the Ambelakia unit (Mt. Ossa, Thessaly, Greece) represent early Cenozoic subduction‐related HP‐LT metamorphic complexes exhumed in the back‐arc of the Hellenic subduction zone. The Ambelakia unit has been linked to the classic CBU in the Cyclades; however, the tectonic affinity, structural position, nature of the protolith, or timing of metamorphism for these rocks in eastern Thessaly remains largely unresolved. This study provides detailed new insights into both the provenance, protolith age, and tectonic affinity of the Pelagonian, Ambelakia, and Olympos‐Ossa tectonic units by integrating U‐Pb detrital zircon (DZ) and detrital apatite (DA) data in a structural context. DZ results suggest the existence of distinct metasedimentary units, spanning in depositional age from Carboniferous to Late Cretaceous, with provenance signatures that support a correlation with the classic CBU and strengthen the argument for a similar pre‐subduction tectonic relationship. Depth‐profiling analysis reveals metamorphic rims that record HP‐LT metamorphism in the Ossa Ambelakia during the Paleocene‐Eocene. Apatite U‐Pb data from the metasedimentary units preserve a detrital signature similar to the DZ signatures, indicating that apatites were not reset (<450 C) nor recrystallized during subduction metamorphism. These new data suggest that the Ambelakia unit is a lateral equivalent of the CBU subduction complex; however, this unit experienced distinctly lower pressure and temperature conditions during metamorphism than the along‐strike classic CBU. Plain Language Summary: We investigated a potential correlation between two similar rock units in Greece. Both units were subducted to blueschist‐facies metamorphic conditions; however, such rocks at Mt. Ossa are very poorly understood in comparison to the rocks of the Cycladic Blueschist Unit (CBU) in the Cyclades. We measured radioactive and stable elements within zircon and apatite grains to calculate geologic ages from the rocks at Mt. Ossa, then compared these ages to a compiled data set of ages from the CBU to demonstrate a reasonable correlation between the two units. We found that the rocks at Mt. Ossa are still in stratigraphic order from oldest to youngest, despite undergoing subduction. We also observed that few zircons have younger ages, which we interpret as a record of the subduction event, but no apatite grains record this young subduction event. The Ambelakia rocks were not subducted as deep as some rocks from the Cycladic islands in the southern Aegean. Key Points: The Ambelakia unit represents a single, upright, stratigraphically coherent structural sliver consisting of Permian‐Late Cretaceous strataZircon provenance of the Pelagonian domain, Ambelakia unit, and the Ossa Flysch support a direct correlation between Cycladic Blueschist Unit and AmbelakiaThe Ambelakia unit was metamorphosed at blueschist‐facies conditions during the Paleocene‐Eocene [ABSTRACT FROM AUTHOR]