1. Introduction In the Cyclades, two main groups of tectonic units with contrasting metamorphic histories can be distinguished (for overviews and references see Dürr et al. 1978a; Dürr, 1986; Schliestedt, Altherr & Matthews, 1987; Okrusch & Bröcker, 1990). The lower group of units was affected by at least two metamorphic events: (1) a period of regional eclogite to blueschist metamorphism as a result of conti- nent–continent collision caused by the subduction of the Apulian microplate beneath the Eurasian continent; and (2) subsequent greenschist- to amphibolite-facies overprints which are interpreted to result from isothermal decompres- sion during uplift and/or an independent prograde meta- morphic event. The physical conditions inferred for the high-pressure episode are pressures at about 15 ± 3 kbar and temperatures of approximately 450–500 °C (Schliestedt, Altherr & Matthews, 1987; Okrusch & Bröcker, 1990). Pressures attained during the greenschist- facies overprint are estimated at 4 to 7 kbar and inferred temperatures are between 450 and 500 °C (Schliestedt, Altherr & Matthews, 1987; Okrusch & Bröcker, 1990). As a consequence of re-equilibration during the exhumation- related overprint, high-pressure mineral assemblages are widely replaced by medium-pressure parageneses, but small occurrences of eclogites and blueschists can be found on many islands closely associated with their green- schist-facies counterparts. K–Ar, 40 Ar– 39 Ar and Rb–Sr dat- ing of white mica from blueschists and eclogites yielded Eocene ages between about 55 and 40 Ma; dates between 25 and 18 Ma were obtained using white mica of green- schist and amphibolite-facies rocks (e.g. Altherr et al. 1979; Andriessen et al. 1979; Wijbrans & McDougall, 1986, 1988; Maluski, Bonneau & Kienast, 1987; Wijbrans et al. 1990; Bröcker et al. 1993). Geol. Mag. 135 (3), 1998, pp. 369–382. Printed in the United Kingdom © 1998 Cambridge University Press 369 Rb–Sr isotope studies on Tinos Island (Cyclades, Greece): additional time constraints for metamorphism, extent of infiltration-controlled overprinting and deformational activity MICHAEL BRÖCKER*& LEANDER FRANZ† * Institut für Mineralogie, Zentrallaboratorium für Geochronologie, Corrensstr. 24, D-48149 Münster, Germany † GeoForschungsZentrum Potsdam, Telegrafenberg, D-14473 Potsdam, Germany (Received 7 April 1997; accepted 6 November 1997) Abstract – This study presents new Rb–Sr age data concerning the metamorphic evolution of the Attic- Cycladic Crystalline Belt which represents a complex polymetamorphic terrane within the Alpidic orogenic belt of the Hellenides. Two major groups of tectonic units can be distinguished. Metamorphism in parts of the upper units is commonly considered as a Cretaceous event. In contrast, the group of lower units experi- enced Tertiary high-pressure metamorphism which was followed by a medium-pressure overprint. We focus on the island of Tinos where a representative spectrum of the rock units found in the Cyclades is exposed in three tectonic units: the Upper Unit, the Intermediate Unit and the Basal Unit. The complete range of tectono-metamorphic and magmatic events affecting the Attic-Cycladic Crystalline Belt is documented by numerous petrological and tectonic studies. Phyllites and phyllonites from the ophiolitic Upper Unit yielded Rb–Sr apparent ages (phengite–whole-rock) between c. 92 and 21 Ma. The older age differs from the Cretaceous dates reported for upper unit rocks elsewhere in the Cyclades. It is suggested that the sequence studied belongs to the Jurassic ophiolites of the Hellenides rather than to Cretaceous occurrences. The spread to younger ages is related to non-pervasive rejuvenation and resetting of the Rb–Sr system during tectonic juxtaposition of the Upper Unit over the Intermediate Unit. The youngest age obtained so far for a sample from the Upper Unit (21 Ma) is believed to approximate the timing of tectonic juxtaposition which probably occurred during a regional greenschist-facies episode producing a pervasive overprint in the struc- turally lower tectonic unit. The major phyllite/meta-gabbro/serpentinite sequence of the Upper Unit is inter- preted as an emplacement-related ductile shear zone which experienced reworking under brittle conditions. In the Intermediate Unit, a gradient in Rb–Sr ages from top (c. 40 Ma) to the bottom (c. 22 Ma) was recog- nized, which is interpreted to represent greater effects of fluid infiltration and overprinting in the lower parts of this unit, possibly controlled by variable intensity of deformation which might be related to tectonic jux- taposition onto the Basal Unit. We suggest that synmetamorphic stacking of all three tectonic units took place during an Oligocene–Miocene greenschist event. Extensional deformation continued after tectonic stacking and after intrusion of the main granite, as is indicated by a Rb–Sr whole-rock isochron (15.1 ± 0.6 Ma) for a ductilely deformed garnet-bearing leucogranite from the marginal parts of the main undeformed pluton. Application of the Rb–Sr dating technique provided no unequivocal evidence that previously pub- lished Eocene K–Ar and 40 Ar– 39 Ar dates for high-pressure phengites from the lower units are significantly contaminated with excess argon. * Author for correspondence: brocker@nwz.uni-muenster.de