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)
J. Ganor et al., 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), European journal of mineralogy, 8(4), 1996, pp. 719-732
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.