ZIRCON GRAINS AS RECORDERS OF SUBDUCTION ZONE METAMORPHISM IN THE SOUTHERN CYCLADES, GREECE AND THE BETIC CORDILLERA, S. SPAIN

Zircon grains record rock-fluid interactions and can be used to unravel the timing and metamorphic conditions of tectonic events. In this study, we analyze multiple generations of zircon rims to constrain metamorphic processes of high-pressure rocks during subduction. We investigate these processes by integrating LA-split-stream-ICP-MS U-Pb and Trace Element (TE) depth profiling, 2D geochemical mapping, and oxygen isotope SIMS analyses. We apply these techniques to zircon grains from metamorphic rocks of the Cycladic Blueschist Unit (CBU) and Cycladic Basement (CB) in the southern Cyclades, Greece and from the Veleta and Mulhacen units in the Nevado-Filabride Complex in S. Spain. In both studies, non-magmatic zircon rims fall into two age modes in the Cenozoic that are characterized by diagnostic TE signatures, indicative of zircon growth during two distinct metamorphic events.

In the Cyclades, Eocene metamorphic rims associated with subduction are observed in both units, suggesting that the units were in proximal contact prior to or during subduction and HP metamorphism. In contrast, Oligocene zircon rims appear to be restricted to the CB-CBU contact, characterized by garnet break-down, and show enriched HREE and higher ẟ18O values relative to their magmatic cores, suggesting the involvement of fluids derived from underlying sedimentary sources during progressive subduction rollback synchronous with Oligo-Miocene extension in the overriding plate. In S. Spain, Eocene and Miocene metamorphic rims are only observed in structurally higher domains of the units and they are absent from the lower Veleta unit. Eocene rims show REE similar to their cores, whereas Miocene rims show depletion of HREE relative to their cores. These patterns indicate that subduction in the western Mediterranean was active since at least the Eocene, which is ~20 Myr earlier than previous findings, and subsequent metamorphism facilitated garnet growth during the Miocene.

Combined CL images with elemental and age 2D maps from multiple zircon zones reveal the complicated morphology of these grains and set a spatial context on their anatomy. These isotopic and elemental microanalyses with dense outcrop sampling can provide high-resolution record of distinct metamorphic events with variations in temperature, rheology, fluids, and metamorphic reactions.