White mica 40Ar/39Ar geochronology and geochemistry of basement shear zones on Folegandros, Cyclades, Greece
Olivia Dale, David Schneider
Department of Earth and Environmental Sciences, University of Ottawa
Olivia Dale, David Schneider
Department of Earth and Environmental Sciences, University of Ottawa
The current understanding of the tectonic history and timing of deformational events that shaped the island of Folegandros, within the Cyclades, is incomplete. Within the region there has been an Eocene compressional event during subduction of the African plate under the Eurasian plate followed by a Miocene extensional event, which gave rise to a network of fault systems throughout the Cyclades as well as three distinct shear zones on the island itself. To investigate the relationship of these shear zones to the regional tectonic history, a combination of major element geochemical and 40Ar/39Ar geochronological methods on white mica was utilized. Structures seen in the field were foliations with distinct mineral lineations oriented at approximately north-south as well as winged inclusions and shear bands which indicate shear sense. The dominant metamorphic mineral assemblage from the island is quartz, calcite, white mica, chlorite, epidote with rare glaucophane. The white mica in the shear zone is very fine grained with some samples showing protomylonites whereas the mica in the less deformed rocks is in larger grain clusters interstitially throughout the samples. The white mica geochemistry of the samples shows a very low pyrophyllitic content in all samples and Tschermak substitution producing micas that vary in composition between ideal muscovite and (theoretical) Al-celadonite, a higher-pressure phase. The 40Ar/39Ar ages range from 38 Ma to 20 Ma, with muscovite from the shear zones yielding late Miocene (23-20 Ma) ages, and the older ages are from samples that have a higher celadonite proportion. The early Eocene ages are likely preserved assemblages from the high-pressure metamorphism. Notably, the Angali shear zone divides the island in half and denotes a marked jump in metamorphic grade: peak temperatures are >400°C north of the shear zone and <350°C to the south, suggesting this shear zone is a major nappe boundary. The shear zones present on the island were likely active during Miocene extension under greenschist facies conditions, juxtaposing contrasting Eocene metamorphic packages. The results highlight the need for geochemical analyses in conjunction with geochronology to properly interpret ages from tectonites.