QUIG BAROMETRY AND ZIR THERMOMETRY FROM TWO TECTONIC UNITS, IOS, GREECE

Nine samples from the metamorphic core complex of Ios in the Greek Cyclades were examined to determine the pressure-temperature (P–T) conditions of metamorphism and especially the P–T conditions of garnet nucleation. P–T conditions were constrained by application of Raman spectroscopy of quartz inclusions in garnet (QuiG barometry) coupled with Zr-in-rutile (ZiR) thermometry.

Samples were examined from two tectonic units — an upper plate and a lower plate — separated by a mapped thrust fault. Upper plate samples include one eclogite and two blue amphibole-white mica-quartz schists. Lower plate samples include five garnet-white mica-quartz schists and one garnet-white mica-actinolite amphibolite. The combined QuiG barometry and ZiR thermometry for the two upper plate suites yielded P–T conditions of about 15.3 Kbar and 550 ˚C and 11-13 Kbar and 450-500 ˚C respectively. The lower plate schists yielded P-T conditions of 12-15 Kbar and 500-550 ˚C. The lower plate amphibolite provided P–T conditions of approximately 15.5 Kbar and 525 ˚C. Although collected from two distinct tectonic units separated by a mapped thrust fault, the similarity in P–T conditions from each tectonic unit suggests that the thrust faulting must have been pre-metamorphic.

These results will be compared with the P–T conditions inferred from calculations of Mineral Assemblage Diagrams (MADs) in which the theoretical stability fields of various metamorphic assemblages are calculated from internally consistent thermodynamic data assuming that all assemblages crystallized in equilibrium. Bulk rock compositions of each sample will be determined from scanning electron microscope EDS analysis and used as input for the calculation of MADs. Specifically, the P–T conditions of garnet nucleation as well as the peak metamorphic P–T conditions inferred from QuiG barometry and ZiR thermometry will be compared to the calculated stability fields to determine whether garnet nucleated and grew in near-equilibrium with the matrix minerals or whether garnet nucleation involved significant overstepping of the garnet isograd. Inasmuch as garnet growth from precursor chlorite involves considerable release of H₂O, these results will place important constraints on the depth and temperature of a major devolatilization reaction in a subduction complex.