Synkinematic tourmaline and quartz growth on slickensides of brittle–ductile normal fault planes was studied by electron-microprobe analysis and X-ray crystallographic methods. The faults show top-to-the-southwest sense of shear and cross-cut mylonitic gneisses of the Cycladic Blueschist Unit on Despotiko Island, located southwest of Antiparos, in the Cyclades, Greece. The black tourmaline crystals, up to 180 μm in length and 50 μm in diameter, form slickenside fibers on these fault planes and therefore have grown during fault slip. They record a chemical variation from X(Na0.66Ca0.200.14) Y(Fe2+1.17Mg0.92 Al0.78Ti4+0.12Mn2+0.01) Z(Al5.00Mg1.00) (BO3)3T(Si5.95Al0.05)O18V(OH)3W[(OH)0.88F0.12] to X(□0.50Na0.48Ca0.02) Y(Fe2+1.75Al1.31 Mn2+0.02Ti4+0.01) Z(Al5.49Mg0.51) (BO3)3T(Si5.98Al0.02)O18V(OH)3W(OH). The tourmaline compositions belong to the foitite – schorl – dravite series. On the basis of X□−XMg, X□−Ca, Ca–XMg relationships and significant chemical differences between analogous and antilogous poles, these tourmalines most likely grew at low- to medium-grade metamorphic conditions, corresponding to crystallization temperatures between 300 and 400 ± 50°C. This is consistent with the low amounts of [4]Al and F in these tourmalines, the observation of almost completely chloritized biotite near the slickensides, and the deformation microstructures of quartz in the fault. Together with structural measurements in the field, these data suggest that the tourmaline slickenside fibers crystallized during a late stage of overall NE–SW-oriented extensional exhumation, bringing the rocks from lower-greenschist facies conditions to the brittle–ductile transition zone.

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