PROGRADE MICA GROWTH RECORDED BY 40AR/39AR AGES IN HIGH-PRESSURE ROCKS (SYROS, CYCLADES, GREECE)

We investigated a massive phengite-rich eclogite and an omphacitic metagabbro from the well known high-pressure sequences of the Cycladic island of Syros (Greece). The age of high-pressure metamorphism in the Cyclades is broadly constrained, however, published K-Ar, Rb-Sr, and ⁴⁰Ar/³⁹Ar ages in the Cyclades scatter between 38-53 Ma (e.g. Altherr et al. 1979; Bröcker et al. 1993). The relatively low temperatures of metamorphism and the preservation of pre-metamorphic stable isotope signatures (Putlitz et al. 2000) suggest that the phengite in these HP rocks may preserve relics of their prograde P-T history. We have used the in situ UV-laser ⁴⁰Ar/³⁹Ar laser probe in an attempt to date a discrete segment of their prograde P-T path. In both samples phengite is in textural equilibrium with the other eclogite-facies minerals, and microprobe analyses and X-ray mapping indicate that they are compositionally homogeneous. The variation in apparent ages for both of these samples is ca. 10 My with ages ranging from 40-50 Ma for the eclogite and 41-52 Ma for the metagabbro. This spread of ages is visible at all scales – within individual grains as well as in domains of several 1000 µm in size. Traditionally such variations in ⁴⁰Ar/³⁹Ar mica ages have been interpreted to record metamorphic cooling. However, textural equilibrium between the phengites and other high-pressure phases, and the chemical homogeneity of the phengite is inconsistent with chemical adjustment due to cooling. Rather, the observed age variation is interpreted to reflect variation of radiogenic Ar acquired during phengite growth. This implies that the oldest phengite ⁴⁰Ar/³⁹Ar ages represent the best estimate of a minimum crystallization age, consistent with recently published Lu-Hf garnet ages of 47 and 51 Ma (Lagos et al. 2003). The ⁴⁰Ar/³⁹Ar results are consistent with available stable isotope data and suggest that, under certain conditions, both stable and radiogenic isotopic systems can survive without significant isotopic exchange during subduction and exhumation from eclogite-facies P-T- conditions.