OVERSTEPPING OF THE GARNET ISOGRAD AND REEVALUATING THE PEAK P-T CONDITIONS OF THE BLUESCHIST UNIT SIFNOS, GREECE: IMPLICATIONS FOR CYCLADIC SUBDUCTION

We applied thermodynamic modeling and quartz in garnet (QuiG) Raman geobarometry to three blueschists from a 1.5 km long transect across the eclogite-blueschist unit in Sifnos, Greece to evaluate the accuracy of P-T conditions calculated via equilibrium thermodynamic methods and the extent of overstepping required for garnet nucleation. QuiG barometry uses the Raman shift of quartz inclusions in garnet to estimate the pressure of garnet nucleation. The paleoassemblage from which garnet nucleated was inferred from the suites of minerals included within garnet and the stability of this paleoassemblage determined from mineral assemblage diagrams calculated in the MnNCKFMASH system. Additional constraints were imposed by trace element thermometry. These conditions were then compared to P-T conditions calculated at the equilibrium garnet isograd and with the method of intersecting isopleths.

Samples SPH99-1a and SPH99-5 are metapelites with the matrix assemblage Grt+Phn+Pg+Qtz+Rt+Zrn+Gln+Ep+(Ab+Mt)±Ilm. Ab+Mt clots are interpreted to be pseudomorphs after jadeite. SPH99-7 is a metabasite with the matrix assemblage Grt+Jd+Gln+Ep+Rt+Sph+Pg+Phn+Qtz+Zrn. The paleoassemblage in SPH99-1a and SPH99-5 is Chl+Ms+Cam+Pg+Cpx, while the paleoassemblage in SPH99-7 is Cam+Chl+Cpx+Ms+Cld. Major element maps of garnet in SPH99-1a reveal complex zoning possibly indicative of two phases of growth, and a discernible increase in X_sps at the rim is likely a function of back-diffusion as a result of retrograde net-transfer reactions. Garnet in SPH99-7 displays similar rim zoning.

Garnet in samples SPH99-1a, SPH99-5 and SPH99-7 nucleated at 12 kbar and ~484 °C, 15 kbar and ~520 °C, and 23 kbar and 530 °C, respectively, which requires overstepping of ~6 kbar, ~8 kbar and ~15 kbar and reaction affinities of 2.2, 2.0 and 2.4 kJ/mol O. Geotherms calculated from SPH99-7 (~6.7 °C/km) and SPH99-5 (9.8 °C/km) are consistent with previous studies, while the geotherm calculated from SPH99-1a is warmer (11.3 °C/km), and may reflect changes in the rate of subduction or differences in structural position within the down-going slab. The 10 kbar pressure difference between SPH99-7 and SPH99-1a can be explained by faulting and accretion of thin slices of underplated wedge material facilitated by slab rollback and gravitational collapse.