COHERENT UNDERPLATING AND METAMORPHISM IN A SUBDUCTION ZONE: PETROCHRONOLOGY OF THE AMBELAKIA BLUESCHIST UNIT OF OSSA REGION, GREECE

In the Aegean region, Cenozoic slab rollback along the Hellenic subduction zone has led to back-arc extension, metamorphic core complex formation, and exhumation of HP-LT subduction-related units, including the Cycladic Blueschist Unit (CBU) in the Cyclades Islands and the Ambelakia Blueschist Unit (ABU) at Olympos-Ossa-Pelion in eastern Thessaly. While the ABU has been proposed to be a lateral equivalent of the CBU, this correlation remains largely unvalidated. Their relationship is critical for understanding the early evolution of subduction dynamics, underplating, and metamorphism. Detailed insights into the age of protoliths, detrital provenance, and age and structural context of metamorphism will shed light on the timing of subduction and underplating of the ABU and its tectonic relationship to the CBU.

This study provides new zircon U-Pb data from the metasedimentary ABU, the underlying flysch in the tectonic window, and the overlying Pelagonian basement at Mt. Ossa to provide constraints on detrital zircon (DZ) maximum depositional age (MDA) and sedimentary protolith provenance. These preliminary results reveal two primary DZ provenance signatures for the Ossa ABU: a unimodal Carboniferous provenance with Triassic through Jurassic MDAs, and a mixed provenance signature of Carboniferous, Ordovician, and Proterozoic DZ ages with late Cretaceous MDAs. Results for the Eocene Ossa flysch show a lack of syn-depositional ages and a provenance signature which resembles the Pindos flysch. This flysch separates the ABU from the underlying Olympos-Ossa marble. The depth-profile analysis of metamorphic zircon U-Pb overgrowths and apatite U-Pb dating were employed to shed light on the metamorphic and tectonic history of Ossa HP-LT rocks. While previous work documented blueschist-facies metamorphism between 61-54 Ma using Ar/Ar data, new metamorphic rim overgrowths from this study reveal Eocene HP metamorphism at 56 Ma. Apatite U-Pb ages are unreset and record Carboniferous ages, suggesting relatively cold subduction – an observation supported by the lack of garnet. MDAs and timing of metamorphism support a correlation between ABU and CBU as well as coherent underplating in a relatively cold subduction zone. However, differences in provenance and metamorphic grade incite further investigation.