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

Abstract

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 (Alm 0.58 Prp0 0.03 Grs 0.38 Sps 0.09) ± 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 S 1 internal fabric. The matrix foliation (S 2) additionally contains a high-pressure association formed by garnet (Alm 0.68 Prp 0.04 Grs 0.25 Sps 0.03) + 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.