Petrological, oxygen isotope and *Ar/"Ar studies were used to constrain the Tertiary metamorphic evolution of the lower tectonic unit of the Cyclades on Tinos. Polyphase high-pressure metamorphism reached pressures in excess of 15 kbar, based on measurements of the Si content in potassic white mica. Temperatures of 450-500" C at the thermal peak of high-pressure metamorphism were estimated from critical metamorphic assemblages, the validity of which is confirmed by a quartz-magnetite oxygen isotope temperature of 470" C. Some 40Ar/'yAr spectra of white mica give plateau ages of 44-40 Ma that are considered to represent dynamic recrystallization under peak or slightly post-peak high-pressure metamorphic conditions. Early stages in the prograde high-pressure evolution may be documented by older apparent ages in the high-temperature steps of some spectra.Eclogite to epidote blueschist facies mineralogies were partially or totally replaced by retrograde greenschist facies assemblages during exhumation. Oxygen isotope thermometry of four quartz-magnetite pairs from greenschist samples gives temperatures of 440-470" C which cannot be distinguished from those deduced for the high-pressure event. The exhumation and overprint is documented by decreasing ages of 32-28 Ma in some greenschists and late-stage blueschist rocks, and ages of 30-20 Ma in the lower temperature steps of the Ar release patterns of blueschist micas. Almost flat parts of Ar-Ar release spectra of some greenschist micas gave ages of 23-21 Ma which are assumed to represent incomplete resetting caused by a renewed prograde phase of greenschist metamorphism.Oxygen isotope compositions of blueschist and greenschist facies minerals show no evidence for the infiltration of a 6''O-enriched fluid. Rather, the compositions indicate that fluid to rock ratios were very low, the isotopic compositions being primarily controlled by those of the protolith rocks. We assume that the fundamental control catalysing the transformation of blueschists into greenschists and the associated resetting of their isotopic systems was the selective infiltration of metamorphic fluid. A quartz-magnetite sample from a contact metamorphic skam, taken near the Miocene monzogranite of Tinos, gave an oxygen isotope temperature of 555°C and calculated water composition of 9.1%. The value of ~3~~0 obtained from this water is consistent with a primary magmatic fluid, but is lower than that of fluids associated with the greenschist overprint, which indicates that the latter event cannot be directly related to the monozogranite intrusion.

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Structural, metamorphic, and geochronologic work shows that the Ampelos/Dilek nappe of the Cycladic blueschist unit in the eastern Aegean constitutes a wedge of high-pressure rocks extruded during early stages of orogeny. The extrusion wedge formed during the incipient collision of the Anatolian microcontinent with Eurasia when subduction and deep underthrusting ceased and the Ampelos/Dilek nappe was thrust southward over the greenschist-facies Menderes nappes along its lower tectonic contact, the Cycladic-Menderes thrust, effectively cutting out a ∼30- to 40-km-thick section of crust. The upper contact of the Ampelos/Dilek extrusion wedge is the top-to-the-NE Selçuk normal shear zone, along which the Ampelos/Dilek nappe was exhumed by ∼30–40 km. Detailed Rb-Sr and 40Ar/39Ar dating of mylonites demonstrates that both shear zones operated between 42 and 32 Ma. There is no evidence for episodic motion during the ∼10 Myr life span of the shear zones, suggesting that both shear zones operated in a steady, nonepisodic fashion. Our data provide supporting evidence that simultaneous thrust-type and normal sense shearing can accomplish the early exhumation of deep-seated rock

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