Differences in decompression of the high-pressure Cycladic Blueschist Unit (Naxos Island, Greece): what can inclusions tell us?

Determining the tectonic evolution and thermal structure of a tectonic unit that experiences a subduction-related pressure temperature (P-T) loop is challenging. Within a single unit, P-T conditions can vary from top to bottom which can be only revealed by detailed petrological work. We present micropetrological data of the middle section of the Cycladic Blueschist Unit (CBU) in Naxos, Greece, which indicate a different P-T loop than the top of the section. In the middle section, strong deformation associated with high-T metamorphism erases most of the earlier tectonometamorphic imprints preventing to apply "traditional" geothermobarometry methods. Using Zr-in-rutile and Ti-in-biotite thermometry coupled with quartz-in-garnet elastic barometry and phase equilibrium thermodynamic modeling, we identify a prograde path from ~15.4 kbar to ~19.9 kbar and from ~496 °C to ~572 °C, equilibration during decompression at ~8.3 kbar and ~519 °C followed by near-isobaric heating to ~9.2 kbar and ~550 °C (or even ~584 °C), and a final greenschist-facies equilibration stage at ~3.8 kbar and ~520 °C. We compare these P-T estimates with published data from the top of the CBU section and find that the bottom half of the CBU on Naxos records higher peak high-pressure (HP) of about 4 kbar than the top, defining the thickness of the CBU to about 15 km in the Eocene. We determine that crustal thickening of up to ~15% occurs in the upper half of the CBU section during exhumation of the HP rocks in an extrusion wedge during convergence.