Original paper

Garnet zonation in metapelitic schists from the Eclogite Zone, Tauern Window, Austria: comparison of observed and calculated profiles

Hoschek, Gert

Abstract

Three quartzitic metapelites with garnet-chloritoid-kyanite assemblage from the Eclogite Zone, Tauern Window, Austria have been studied based on pseudosections in the KNCFMMnASTH and partly KNCFMMnASTHO system compared with measured and calculated garnet zonations along three P-T paths. Deduced garnet core growth is in the range 502?521 °C, 20.2?21.4 kbar, corresponding to overstepping the garnet-in reaction by ?25?100 °C for P-T paths 1, 2 and ?72?133 °C for P-T path 3. The peak assemblage garnet-chloritoid-kyanite-phengite-paragonite-rutile-quartz in sample A is stable in a narrow maximum ?10 °C band from ?598 °C/14 kbar to ?570 °C/24.9 kbar. In sample B and C this stability is much more restricted with ?2 °C width from ?581 °C/22.3 kbar to ?572 °C/24.5 kbar and ?4 °C/0.7 kbar width at ?570 °C/24.8 kbar. Temperature differences between measured and calculated pyrope and grossular isopleths are generally in the range from 0 to 20 °C, however larger values are sometimes obtained. The proposed P-T paths reach peak values around 570?575 °C/24.6 kbar. Comparison of measured and calculated garnet zonation suggests a better accordance for unfractionated than for fractionated models. Several factors could be responsible for this and the partly large overstepping values, e.g. insufficient thermodynamic data, or nonequilibrium reactions due to sluggish nucleation or breakdown reactions and later garnet re-equilibration.

Keywords

eclogite zonegarnet zonationoversteppingp-t path calculationp-t pseudosectiontauern window