Phengites from the eclogite and blueschist-facies sequences of the
Cycladic island of Syros (Greece) have been dated by the in situ
UV-laser ablation Ar-40/Ar-39 method. A massive, phengite-rich eclogite
and an omphacite-rich metagabbro were investigated. The phengites are
eubedral and coarse-grained (several 100 mum), strain-free and exhibit
no evidence for late brittle deformation or recrystallization. Apparent
ages in these samples range from 43 to 50 Ma for the phengite-rich
eclogite and 42 to 52 Ma for the ompbacitic metagabbro. This large
spread of ages is visible at all scales-within individual grains as well
as in domains of several 100 mum and across the entire sample (ca. 2
cm). Such variations have been traditionally attributed to metamorphic
cooling or the incorporation of excess argon. However, the textural
equilibrium between the phengites and other high pressure phases and the
subtle compositional variations within the phengites, especially the
preservation of growth textures, alternatively suggest that the observed
range in ages may reflect variations of radiogenic argon acquired during
phengite formation and subsequent growth, thus dating a discrete event
on the prograde path. This implies that the oldest phengite 40Ar/39Ar
ages provide the best estimate of a minimum crystallization age, which
is in agreement with recently reported U-Pb and Lu-Hf geochronological
data. Our results are consistent with available stable isotope data and
further suggest that, under fluid-restricted conditions, both stable and
radiogenic isotopic systems can survive without significant isotopic
exchange during subduction and exhumation from eclogite-facies P-T
conditions. (C) 2004 Elsevier B.V. All rights reserved.