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

Abstract

In order to shed some light on the timing of metamorphic and deformational processes operating during the late stages of exhumation in the Attic-Cycladic Crystalline Belt, the contact metamorphism around Miocene granitoids was studied on Tinos. Based on the occurrence of biotite, plagioclase, diopside and scapolite, four mineral zones were mapped in the thermally overprinted area. An attempt was made to constrain relative temperature variations across the contact aureole by application of tourmaline-biotite geothermometry. While part of the calculated temperatures are broadly in accordance with the mapped mineral isograds of the aureole, temperatures far too low were obtained for certain areas. The latter may be explained by retrograde cation exchange in biotite and refractory behaviour of tourmaline but may also be attributed to systematic shortcomings of this method. New petrological and isotopic data suggest that samples from the outer aureole mostly consist of regional metamorphic phengite with ages unaffected by the thermal overprint. This renders phengite dating unsuitable to determine timing of the contact metamorphism. The hypothesis of cryptic contact metamorphic effects on the radiogenic isotope systems of white mica outside the mapped contact aureole could not be substantiated. Reliable age information for this event is provided by biotite, because most of this phase formed during contact metamorphism and inherited components only cause minimal error on the ages. In the eastern aureole, the thermal overprint is dated at about 14 Ma, corresponding to previously published geochronological results for the granitoids. Younger biotite ages of samples from the western aureole (8–10 Ma) possibly 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 within a composite pluton. The Sr isotope characteristics of regional and contact metamorphic metabasic rocks cannot be distinguished. Thus, the intrusion-related infiltration of large amounts of fluids having a 87Sr/86Sr ratio different from the country rocks can be ruled out.