ZIRCON U-PB AND TRACE-ELEMENT SIGNATURES IN HIGH-PRESSURE METAMORPHIC ROCKS AS RECORDERS OF SUBDUCTION AND EXHUMATION PROCESSES, SIKINOS AND IOS ISLANDS (CYCLADES, GREECE)

Zircon U-Pb dating is widely used to determine crystallization ages of igneous rocks and detrital provenance signatures of sedimentary strata. In-situ methods and SEM-CL imaging have allowed the investigation and interpretation of complex internal magmatic zircon growth zones in a petrologic and geologic context. More recently, it has been recognized that zircon can also record metamorphic reactions or rock-fluid interactions. While such zircon overgrowths are especially well-suited for unraveling the timing and tectono-metamorphic conditions of growth, rims are often thin and difficult to analyze with traditional methods. In this study, we dissected multiple generations of zircon growth to investigate metamorphic and metasomatic processes of high-pressure rocks during subduction and exhumation by applying LA-split-stream-ICP-MS U-Pb and trace element (TE) depth profiling and 2-D geochemical mapping, coupled with oxygen isotope ion probe spot analyses.

We applied these techniques to zircon from metamorphic rocks of the Cycladic Blueschist Unit (CBU) and Cycladic Basement (CB) in the southern Cycladic islands Sikinos and Ios, Greece. Non-magmatic zircon rims fall into distinct Eocene and Oligo-Miocene age modes that are characterized by diagnostic TE signatures, indicative of zircon growth under different metamorphic conditions. Eocene metamorphic rims, associated with subduction, are observed in both units, suggesting likely juxtaposition prior to or during subduction and high-pressure metamorphism. In contrast, Oligo-Miocene zircon rims appear to be restricted to a <0.5 km thick zone along the CB-CBU contact, characterized by garnet break-down, and show TE signatures (enriched HREE) and bimodal ẟ¹⁸O values (~4 and ~8‰) consistent with metasomatic infiltration of fluids derived from sedimentary and altered basalt sources that dehydrated during progressive subduction and underplating prior to Oligo-Miocene back-arc extension. This metasomatism appears to be static in nature and does not support major late Cenozoic reactivation of the contact as an extensional shear zone during back-arc extension. The combination of isotopic and elemental microanalyses provides critical constraints on timing and metamorphism/metasomatism during prograde and retrograde subduction metamorphism.