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Alicia López-Carmona, Universidad Complutense de Madrid, Facultad de C.C. Geológicas (Spain)
Jacobo Abati, Universidad Complutense de Madrid, Facultad de C.C. Geológicas (Spain)
Joan Reche, Universitat Autňnoma de Barcelona (Spain)
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A P-T pseudosection in the model system MnNC(K)FMASH has been calculated with THERMOCALC (v 3.30) software of Powell and Holland (1988) using the tc-ds55 thermodinamic dataset of Holland et al. (1998), for a bulk composition of the high-pressure metapelitic Ceán Schists [SiO2:71.42; Al2O3:13.42; MgO:4.78; FeO:7.04; K2O:3.34]. The schists contains an initial high-P assemblage formed by garnet (Alm60 Prp3 Grs30 Sps9), phengite (3.5-3.4 Si p.f.u.), glaucophane, chloritoid, chlorite, clinozoisite and rutile preserved as micro-inclusions in garnet, chloritoid and albite porphyroblasts defining a S1 internal fabric. The granolepidoblastic matrix contains also a high-P association formed by phengite(3.4-3.3 Si p.f.u.), paragonite, chlorite, epidote-clinozoisite, rutile-ilmenite and quartz along the dominant foliation planes (S2) plus complex zoned garnets (rim-core average: Alm73Prp5 Grs20 Sps2), chloritoid and albite porphyroblasts.
A preliminary pseudosection in the basic pelitic KFMASH system made for the same rocks (López-Carmona et al., in press) results excessively simplified when it is applied to these schists, mainly due to the absence of CaO and MnO components that remarkably influence the stability of garnet, and because Na-bearing minerals, such as glaucophane, paragonite and albite, relevant for these rocks, can not occur in such a simple system. Furthermore, the deduced P-T path from this simplified pseudosection does not fit with conventional thermobarometric calculations, overestimating the temperature. The new pseudosection in the MnNC(K)FMASH system fully describe the succession of mineral assemblages in the schists, and results more coherent with the petrographic observations. Thus, this pseudosection provides an accurate mineralogical frame that reflects with fidelity the metamorphic evolution of the high-P Ceán Schists.
The Malpica-Tui Unit is the westernmost exposure of Basal Units in the NW Iberian Massif. They are considered part of the most external margin of north Gondwana, and show the imprints of a late Devonian high-P metamorphism that probably reflects its subduction below the southern border of Laurussia at the onset of the Variscan collision (Arenas et al., 1997; Martínez Catalá n et al., 1996). Therefore, constraints in the P-T path of rocks from these terrains are essential to understand the characteristics and mechanisms of the subduction of this margin.
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