PERMIAN MYLONITES IN A MIOCENE METAMORPHIC CORE COMPLEX: INTEGRATED EBSD AND APATITE U-PB THERMOCHRONOLOGY ON IOS ISLAND (SOUTHERN CYCLADES, GREECE)

The South Cycladic islands in the Aegean Sea are comprised of Miocene metamorphic core complexes that expose Gondwanan rocks previously subducted below the Eurasian plate in Paleocene-Eocene time: the Permian-Cretaceous marbles metabasites, and schists of the Cycladic Blueschist Unit and the underlying Cycladic Basement, composed of Carboniferous meta-granitoids intruded into older metasedimentary rocks. The nature of the contact between these two units is debated: one model proposes that the contact represents a large-scale thrust associated with early Cenozoic subduction, while another suggests that the contact was utilized as a major extensional shear zone, dubbed the South Cycladic Shear Zone (SCSZ), and later cut by the Ios detachment fault during the formation of the Ios metamorphic core complex. In this study, we utilize apatite U-Pb high-temperature thermochronometry to constrain the cooling of mylonitic rocks within the SCSZ, as well as Basement rocks in the footwall of the SCSZ, below the apatite U-Pb closure temperature (~450°C). We couple these ages with EBSD analyses and microstructural observation to constrain the timing of formation of high-temperature mylonitic fabric formation in the Basement and the SCSZ. Apatite U-Pb ages are consistently Permian in age across Ios island, suggesting these rocks have been cooler than temperatures below 450°C since the late Paleozoic. Apatite trace element concentrations are consistent with a magmatic source and show little variation. EBSD analyses from metasedimentary Basement rocks within the SCSZ show orientations and quartz opening angles consistent with low- to medium-temperature (~350°C) quartz recrystallization conditions. Footwall granitoids preserve evidence of a higher-T fabric with similar kinematics that show feldspar with bulging and subgrain rotation and quartz with rapid grain boundary migration; coupled with their Permian apatite U-Pb cooling ages, these data suggest that the some of the high-temperature mylonitic fabrics exposed on Ios island may have formed during extension and exhumation of the Cycladic Basement associated with the development of the Permo-Triassic rifted margin of the Pindos Basin rather than during Miocene core complex formation and exhumation.