Reversal of Fe-Mg Partitioning Between Garnet and Staurolite in Eclogite-facies Metapelites from the Champtoceaux Nappe (Brittany, France)

This paper concentrates on the petrology of eclogite-facies metapelites and, particularly, the significance of staurolite in these rocks. A natural example of staurolite-bearing eclogitic micaschists from the Champtoceaux nappe (Brittany, France) is first described. The Champtoceaux metapelites present, in addition to quartz, phengite, and rutile, two successive parageneses: (1) chloritoid+staurolite+garnet cores, and (2) garnet rims+kyanite±chloritoid. Detailed microprobe analyses show that garnet and chloritoid evolve towards more magnesian compositions and that staurolite is more Fe-rich than coexisting garnet. A comparison of the studied rocks with other known occurrences of eclogitic metapelites shows that whereas staurolite is always more Fe-rich than garnet in high-pressure eclogites, the reverse is true in low- to medium-pressure micaschists. Phase relations between garnet, staurolite, chloritoid, biotite, and chlorite are analysed in the KFMASH system (with excess quartz, phengite, rutile, and H2O). The topology of univariant reactions is depicted for a normal and a reverse Fe-Mg partitioning between garnet and staurolite. Mineral compositional changes are also predicted for varying bulk-rock chemistries. In the studied micaschists, the zonal arrangement of garnet inclusions and the progressive compositional changes of ferromagnesian phases record part of the prograde P–T path, before the attainment of ‘peak’ metamorphic conditions (at about 65O–7OO°C, 18–20 kb). The retrograde path, which records the uplift of the Champtoceaux nappe, occurs under decreasing temperatures.