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 Cean Schists from the NW Iberian Massif show that the paragenesis chloritoid + glacophane, 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 similar to 500 degrees 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) Prp(0.03) Grs(0.38) Sps(0.09)) +/- glaucophane + phengite (3.5-3.4Si p f.u) + paragonite + chlorite + epidote + rutile + ilmienite + quartz, preserved as micro-inclusions in garnet, chloritoid and albite porphyroblasts defining an S, internal fabric The matrix foliation (SA additionally contains a high-pressure association formed by garnet (Alm(0.68) Prp(0.04) Grs(0.25) Sps(0.03))+ phengite (34-3 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 Cean Schists (C) 2009 International Association for Gondwana Research Published by Elsevier B V All rights reserved