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 rocks 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 δ¹⁸O values against the chemical index of Garlick show that it is possible to interpret regional isotopic compositional variation in terms of either exchange with an ¹⁸O-enriched fluid during retrograde eclogite to greenschistfacies transformations or an originally heterogeneous pre-metamorphic sequence of rock units. However, a detailed analysis of isotopic compositional variations on an outcrop scale shows that there are no identifiable δ¹⁸O 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 selective infiltration (possibly involving local deformation-enhanced permeability creation) of small amounts of fluids. Simple mass-balance calculations based on the stoichiometries of model eclogite-blueschist and blueschist-greenschist reactions suggest that these amounts of fluid were of the order of several weight percent. The isotopic compositional heterogeneity of regional metamorphic sequences must be taken into account when applying models of fluid infiltration.