Reaction overstepping and re-evaluation of peak PT conditions of the blueschist unit Sifnos, Greece: implications for the Cyclades subduction zone

Equilibrium thermodynamic modelling, quartz in garnet (QuiG) Raman geobarometry, and modelling of garnet nucleation at overstepped conditions were applied to three garnet-bearing blueschists from a 1.5km-long transect across the eclogite-blueschist unit in Sifnos, Greece, in order to evaluate the accuracy of PT conditions calculated via equilibrium thermodynamics. QuiG barometry uses the Raman shift of quartz inclusions in garnet to estimate the pressure of garnet nucleation and is independent of chemical equilibrium. Garnet nucleation temperatures were estimated by determining the stability field of the palaeo-assemblage inferred from garnet inclusion suites on mineral assemblage diagrams calculated in the MnNCKFMASH system and on temperatures obtained from Zr in rutile thermometry. These conditions were then compared to PT conditions calculated at the equilibrium garnet isograd, and the method of intersecting isopleths. The PT conditions calculated with intersecting garnet isopleths over- and underestimated the temperature of nucleation in samples SPH99-1a and SPH99-7, respectively, whereas they significantly underestimated nucleation pressure in SPH99-5. Nucleation of garnet in SPH99-1a at 12 kbar and similar to 484 degrees C requires overstepping of similar to 6 kbar and a reaction affinity of 2.2kJ mol(1) O. SPH99-5 requires overstepping of similar to 8 kbar with garnet reaction affinities of at least 2.0kJ mol(1) O at 15 kbar and similar to 520 degrees C. SPH99-7 requires overstepping of approximately 15 kbar and affinities of about 2.0-2.4kJ mol(1) O at similar to 23 kbar and similar to 530 degrees C. The geotherms calculated from SPH99-7 (similar to 6.7 degrees C km(1)) and SPH99-5 (9.8 degrees C km(1)) are in accordance with previous studies. The geotherm calculated from SPH99-1a, however, is warmer (11.3 degrees C km(1)), and could 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 thrusting and accretion of thin slices of underplated wedge material facilitated by slab rollback and gravitational collapse.