The contact aureole on Tinos (Cyclades, Greece): tourmaline-biotite geothermometry and Rb-Sr geochronology

In order to shed some light on the timing of metamorphic and defommational processes operating during the late stages of exhumation in the Attic-Cycla dic Crystalline Belt, the contact metamorphism around Miocene granitoids wa s studied on Tinos. Based on the occurrence of biotite, plagioclase, diopsi de and scapolite, four mineral zones were mapped in the thermally overprint ed area. An attempt was made to constrain relative temperature variations a cross the contact aureole by application of tourmaline-biotite geothermomet ry. While part of the calculated temperatures are broadly in accordance wit h the mapped mineral isograds of the aureole, temperatures far too low were obtained for certain areas. The latter may be explained by retrograde cati on exchange in biotite and refractory behaviour of tourmaline but may also be attributed to systematic shortcomings of this method. New petrological a nd isotopic data suggest that samples from the outer aureole mostly consist of regional metamorphic phengite with ages unaffected by the thermal overp rint. This renders phengite dating unsuitable to determine timing of the co ntact metamorphism. The hypothesis of cryptic contact metamorphic effects o n the radiogenic isotope systems of white mica outside the mapped contact a ureole could not be substantiated. Reliable age information for this event is provided by biotite, because most of this phase formed during contact me tamorphism and inherited components only cause minimal error on the ages. T n the eastern aureole, the thermal overprint is dated at about 14 Ma, corre sponding to previously published geochronological results for the granitoid s. Younger biotite ages of samples from the western aureole (8-10 Ma) possi bly date deformation that affected the marginal parts of the main intrusion . Alternatively, the younger ages may indicate a different cooling history in this part of the contact aureole, caused by an undated magmatic pulse wi thin a composite pluton. The Sr isotope characteristics of regional and con tact metamorphic metabasic rocks cannot be distinguished. Thus, the intrusi on-related infiltration of large amounts of fluids having a Sr-87/Sr-86 rat io different from the country rocks can be ruled out.