Blueschist-facies metapelites from the Malpica–Tui Unit (NW Iberian Massif): phase equilibria modelling and H2O and Fe2O3 influence in high-pressure assemblages

Abstract

The Malpica–Tui Unit (Galicia, NW Spain) records eclogite- and blueschist-facies metamorphism during the onset of the Variscan orogeny in Europe. Petrological analysis involving pseudosections calculated using THERMOCALC shows that the Upper Sheet of this unit, the Ceán Schists, recorded a threestage metamorphic evolution involving (i) Early subduction-related medium-pressure ⁄ low-temperature metamorphism (M1) constrained at ~º350–380 ºC, 12–14 kbar, which is only recorded in the basal part (lower metapelites, LM) of the Cea´n Schists. (ii) Subduction-related blueschist facies prograde metamorphism (M2) going from 19 kbar, 420 ºC to 21 kbar, 460 ºC in the LM, and from 16 kbar 430 ºC to 21–22 kbar, 520 ºC in the structurally upper metapelites (UM). (iii) Exhumation-related metamorphism (M3) is characterized by a decompression to 8–10 kbar, 470–490 ºC in the LM. This decompression is also recorded in the UM, but it was not possible to estimate precise P–T conditions. The calculations indicate that (i) the prograde evolution in subduction zones may occur in fluidundersaturated conditions due to the crystallization of lawsonite, even in metapelitic rocks. This significantly influences phase equilibria and hence the P–T estimates. (ii) The proportion of ferric iron also has a strong influence on phase equilibria, even in metapelites. However, the analysed values of Fe2O3 may not reflect the oxidation state during the main metamorphic evolution and are probably easily modified by superficial alteration even in apparently fresh samples. The use of P–T–X(Fe2O3) pseudosections together with petrographic observations is then necessary to estimate the real oxidation state of the rocks and correctly evaluate the P–T conditions.