Petrologic modeling of chloritoid-glaucophane schists from the NW Iberian Massif

Two P-T pseudosections computed with Thermocalc and Perplex software for high-pressure pelitic Ceán Schists from the NW Iberian Massif show that the paragenesis chloritoid + glaucophane, preserved as relict micro-inclusions in garnet porphyroblasts, is stable above 17-18 kbar, indicating a first stage of subduction with 65-70 km of burial. The subsequent growth of biotite and albite porphyroblasts, according to their stability fields in a MnNCKFMASH P-T phase diagram, indicates strong decompression accompanied by slight heating to reach a metamorphic peak at ∼ 500 °C. This mineral association, described in many subduction-related terranes around the world as one of the high-pressure indicators for metapelites, has not been reported previously in the NW Iberian Massif. The schists contains an initial high-pressure assemblage formed by chloritoid + garnet (Alm0.58 Prp00.03 Grs0.38 Sps0.09) ± glaucophane + phengite (3.5-3.4 Si p.f.u.) + paragonite + chlorite + epidote + rutile + ilmenite + quartz, preserved as micro-inclusions in garnet, chloritoid and albite porphyroblasts defining an S1 internal fabric. The matrix foliation (S2) additionally contains a high-pressure association formed by garnet (Alm0.68 Prp0.04 Grs0.25 Sps0.03) + phengite (3.4-3.3 Si p.f.u.) + paragonite + winchite + barroisite + hornblende + chloritoid + chlorite + epidote + rutile + ilmenite + albite + quartz ± biotite. An initial pseudosection calculated in the KFMASH system with Thermocalc 3.26 gives pressure estimates through Si-content in phengite barometry that are in agreement with conventional thermobarometry and Thermocalc average P-T calculations, but is unable to describe the full complexity of the mineral assemblages of the schists. For this reason, a more complete P-T pseudosection in the model system MnNCKFMASH was calculated with Perplex 07. This provides a reliable succession of mineral assemblages that are consistent with the petrographic observations and allows a mineralogical sequence to be set for each metamorphic event in the P-T evolution of the high-P pelitic Ceán Schists. © 2009 International Association for Gondwana Research.