OXYGEN ISOTOPIC HETEROGENEITIES OF METAMORPHIC ROCKS - AN ORIGINAL TECTONOSTRATIGRAPHIC SIGNATURE, OR AN IMPRINT OF EXOTIC FLUIDS - A CASE-STUDY OF SIFNOS AND TINOS ISLANDS (GREECE)

Oxygen isotope compositional heterogeneities among Eocene high P/T roc ks and retrograde overprinting assemblages on the Cycladic islands of Sifnos and Tinos are studied with the aim of determining the extent to which they reflect an original tectonostratigraphic signature or the effects of fluid infiltration. Plots of whole-rock delta(18)O values a gainst the chemical index of Garlick show that it is possible to inter pret regional isotopic compositional variation in terms of either exch ange with an O-18-enriched fluid during retrograde eclogite to greensc hist-facies transformations or an originally heterogeneous pre-metamor phic sequence of rock units. However, a detailed analysis of isotopic compositional variations on an outcrop scale shows that there are no i dentifiable delta(18)O changes accompanying the retrograde metamorphic transformations, thus favouring the latter alternative. The isotopic data support the view that outcrop- and layer-scale variations in the degree of retrograde metamorphic transformation were controlled by sel ective infiltration (possibly involving local deformation-enhanced per meability creation) of small amounts of fluids. Simple mass-balance ca lculations based on the stoichiometries of model eclogite-blueschist a nd blueschist-greenschist reactions suggest that these amounts of flui d were of the order of several weight percent. The isotopic compositio nal heterogeneity of regional metamorphic sequences must be taken into account when applying models of fluid infiltration.