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 NWIberian 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 (3.5–3.4Sip.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 (3.4–3.3Si 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.