Petrochronological constraints on the tectono-metamorphic evolution of the Pelion blueschist unit, Greece

In the Aegean region, the Cycladic Blueschist Unit (CBU) and Ambelakia Blueschist Unit (ABU) represent early Cenozoic subduction-related HP-LT metamorphic complexes exhumed in the back-arc of the Hellenic subduction zone. One of the largest coherent packages of blueschist-facies metamorphic rocks is exposed in the Pelion peninsula of eastern Thessaly. While this large tectonic slice has been linked to both the ABU of Ossa-Olympos further north and the CBU of the Cycladic Islands further southwest, little is known about the tectonic affinity, structural position, or timing of metamorphism of these rocks in Pelion. Structurally, they are located below Pelagonian marbles and above Carboniferous orthogneisses and a marble of debated tectonic affinity. The internal tectono-stratigraphic sequence of the package consists of limestones and calc-schists, transitioning upward to more quartzose, micaceous schists, with consistent intercalations of meta -carbonates and -basic lenses throughout. Glaucophane-bearing HP-LT metamorphic assemblages are best preserved in western Pelion likely due to the lesser degree of overprinting by the North Cycladic Detachment Fault. This study provides new insights into both the sedimentary provenance and tectonic affinity on the basis of detrital zircon (DZ) U-Pb data and the timing and nature of the HP-LT metamorphic history of the Pelion unit deduced from metamorphic zircon rim and apatite U-Pb geochronology. Preliminary DZ results reveal two metasedimentary packages (1) Triassic metavolcanics and (2) metasediments with Cretaceous maximum depositional ages (MDA) characterized by mixed provenance signature of Permo-Triassic, Carboniferous, Ordovician, and Proterozoic DZ ages. New zircon U-Pb results from the underlying orthogneisses corroborate Carboniferous protolith ages. The DZ provenance and MDA data strengthen the pre-subduction tectonic relationship between Pelion and the ABU at Ossa, as do field observations of structural relationships and metamorphic grade. Detailed zircon and apatite U-Pb geochronology is needed to elucidate the tectonic affinity of this basement. The tectono-metamorphic evolution of Pelion HP-LT rocks is critical for the understanding of early Aegean subduction models.