In situ ultraviolet (UV) laser-ablation 40Ar/39Ar dating, microstructural analysis, and stable O, H, and C isotope analyses were performed on white mica–bearing calcite– and quartz–mica schists of the West Cycladic detachment system footwall in order to resolve outstanding uncertainties about the timing of deformation and the role of rock rheology on 40Ar/39Ar dating systematics. In both quartz-rich and calcite-rich samples, deformed and chemically zoned white micas form two chemical populations: (1) a high component of Al-celadonite in undeformed portions of grains (high-pressure remnants), and (2) enrichment in muscovite in deformed portions (low-pressure neocrystallization). Micas in the quartz-rich rocks record higher internal strain, illustrated by elongated, sheared grains and boudinaged mica-fish structures. In this lithology, quartz formed a load-bearing framework that transferred strain to the muscovite packets and facilitated the formation of mica-fish structures. Recrystallization was promoted by coeval fluid infiltration, supported by stable isotope analyses and indented boundaries on bulging quartz grains. In rocks containing calcite-muscovite aggregates, the calcite formed an interconnected weak layer, with strain being accommodated by dislocation creep. In these rocks, micas were only partially neocrystallized. Prismatic white micas, largely unaffected by boudinage or kinking, yielded 40Ar/39Ar ages that are up to 10 m.y. older than deformed (kinked or sheared) portions of the same grains. Overall, the ages attest to strong lithological control on deformation- and fluid-controlled white mica neocrystallization. The oldest, undeformed grain ages in the calcite-rich rocks are consistent with the timing of Eocene metamorphism, with the deformed grain ages interpreted as representing the transition to lower-pressure conditions during nascent extension. Completely neocrystallized grains in the quartz-rich rocks are interpreted as defining the minimum age of Miocene ductile extension along the detachment system. The new data show the power of combining in situ laser-ablation 40Ar/39Ar dating, microstructural analysis, mineral chemistry, and stable isotope data for unraveling the timing and time scales of complex deformation histories.

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