DATING DEFORMATION WITH IN-SITU 40AR/39AR ON MICAS: RHEOLOGICAL IMPLICATIONS FOR QUARTZ- AND CALCITE-RICH ROCKS (WEST CYCLADIC DETACHMENT SYSTEM, GREECE)

Conventional Ar-Ar dating of tectonites can result in equivocal ages due to mixed mineral populations, excess 40Ar, or partial resetting of the K-Ar systematics. High-precision in-situ Ar-Ar dating was performed on white micas from footwall marbles and schists of the Western Cycladic Detachment System (WCDS) of the Aegean. The rocks contain undeformed, kinked, and neocrystallized micas, with micas in the quartz-rich rocks recording higher strain (elongated grains and mica-fish structures) than those in the calcite-dominated assemblages. In both quartzitic and calcitic rocks, deformed and chemically zoned white micas form two chemical populations: (1) a high component of Al-celadonite in undeformed portions of grains that likely crystallized during high pressure metamorphism; (2) enrichment in muscovite in deformed portions of grains as a result of Tschermak substitution that took place during deformation-induced neocrystallization under shallow crustal conditions. Stable isotope (O, C, H) analyses of calcite, dolomite and white micas suggest a moderate fluid-rock interaction resulting in a coupled depletion of ¹⁸δO vs ¹³δC and ¹⁸δO vs δD. Recrystallized quartz exhibits indented boundaries on the bulging grains, also consistent with fluid-assisted deformation. Completely neocrystallized grains, in quartzitic rocks from the center of the WCDS yield Ar-Ar deformation ages of c. 18 Ma, which are interpreted as dating the timing of ductile extension along the detachment system. Undeformed (unkinked, prismatic) portions of white micas in calcitic rocks from the southern end of the WCDS yield much older Ar-Ar ages of c. 43 Ma, whereas deformed (kinked or strained) portions of the same grains yield younger ages of c. 33 Ma. In calcite-muscovite aggregates, the calcite forms an interconnected weak layer, where strain is accommodated by dislocation creep, whereas in quartz-muscovite aggregates, the quartz forms a load-bearing framework transferring strain to the muscovite packets, which deform into mica-fish structures. Hence, ductiley deformed and completely neocrystallized micas occur more often in quartz-rich rocks, whereas calcitic rocks contain micas that only partially neocrystallized due to a lack of deformation and fluid infiltration.