Zircon U-Pb and geochemical signatures in high-pressure, low-temperature metamorphic rocks as recorders of subduction zone processes, Sikinos and Ios islands, Greece

Abstract

Zircon U-Pb dating is a powerful and widely used geochronologic technique to constrain the timing and rates of magmatic and high and lower-grade metamorphic processes, as well as sediment provenance. Zircon trace element (TE) compositions also record magmatic and metamorphic processes during zircon growth. In this study, zircon laser ablation split-stream (LA-SS)-ICP-MS U-Pb and TE depth-profiling and novel two-dimensional zircon mapping techniques are used in combination with oxygen isotope analyses (secondary ion mass spectrometry, SIMS) to reconstruct the timing and metamorphic conditions recorded by recrystallization and growth of zircon rims, which provide valuable insight into the petro-tectonic evolution of high-pressure/low-temperature (HP/LT) metamorphic rocks formed in subduction zones. These techniques are applied to zircon grains from HP/LT metamorphic rocks of the Cycladic Blueschist Unit (CBU) and Cycladic Basement (CB) on Sikinos and Ios islands, Greece, which experienced metamorphism and deformation associated with subduction and subsequent back-arc exhumation. Zircon records multiple episodes of non-magmatic zircon rim growth at ~50 Ma and ~ 26 Ma. Eocene metamorphic rims are associated with HP/LT metamorphism and are observed in both units, suggesting likely juxtaposition prior to or during subduction and associated HP metamorphism. The similarity between TE concentrations and δ18O values of the Eocene rims and their corresponding cores is an indicator for re-crystallization and precipitation as a mechanism of zircon growth. In contrast, Oligocene 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 HREE enrichment and higher ẟ18O values in the rims compared to the cores, consistent with a model suggesting metasomatic infiltration of fluids derived from dehydrating sedimentary rocks during progressive subduction and underplating prior to back-arc extension. This metamorphism 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.