Publication: Petrologic modeling of chloritoid–glaucophane schists from the NW Iberian Massif
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Publication Date
2010
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Elsevier Science B. V., Amsterdam
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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.