Permian mylonites in the footwall of a Miocene Cycladic core complex (Ios, Greece): Insights from (micro)structurally integrated apatite U-Pb petrochronology

Abstract

Oligo-Miocene large-magnitude extension and the formation of metamorphic core complexes accommodated exhumation of High Pressure/Low Temperature (HP/LT) rocks in the eastern Mediterranean. Previous studies on Ios Island in the southern Cyclades have associated mylonitic fabrics exposed within the crystalline Cycladic Basement and along its contact with the overlying Cycladic Blueschist Unit, termed the South Cycladic Shear Zone, with Oligo-Miocene extension in the backarc of the retreating Hellenic subduction zone. We utilized apatite U-Pb geo-thermochronology, combined with Ti-in-Quartz and quartz c-axis opening-angle thermometry and microstructural characterization of mylonitic fabrics, to constrain the timing of mylonitization preserved in the footwall of the South Cycladic Shear Zone. Apatite U-Pb ages of the high-temperature (> ca. 500°C) mylonites in the Basement Core are Permo-Carboniferous (ca. 302—270 Ma), with two samples yielding apparent Mesozoic apatite U-Pb ages and whose U-Pb and REE systematics may have been perturbed by Cenozoic tectonics. Despite lower-temperature (300—400°C) mylonitization within the South Cycladic Shear Zone, rocks there preserve exclusively Permo-Carboniferous (ca. 307–297 Ma) apatite U-Pb ages, corresponding to either relict high-temperature quartz domains (ca. >500°C) or a detrital age signature sourced from the Cycladic Basement, but not to Cenozoic movement along the interface. The dominance of Permian cooling ages suggests that significant exhumation of the Cycladic Basement occurred prior to and concurrent with deposition of the earliest Cycladic Blueschist Unit in the late Permian to early Triassic, and that Cycladic core complexes preserve high-temperature fabrics related to pre-Miocene extension. Importantly, this work advises caution when assuming the age of mylonitic fabrics in the footwalls of metamorphic core complexes across the globe without integrating deformation conditions within thermochronometric and geochemical context.