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

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López Carmona, Alicia and Pitra, Pavel and Abati Gómez, Jacobo (2013) Blueschist-facies metapelites from the Malpica–Tui Unit (NW Iberian Massif): phase equilibria modelling and H2O and Fe2O3 influence in high-pressure assemblages. Journal of metamprphic geology, 31 . pp. 263-280. ISSN 0263-4929

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Official URL: http://onlinelibrary.wiley.com/journal/10.1111/(ISSN)1525-1314



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.


Item Type:Article
Uncontrolled Keywords:Blueschist-facies metapelites, H2O content, Iberian Massif, MnNCKFMASHTO, Variscan subduction
Subjects:Sciences > Geology > Petrology
ID Code:25499
Deposited On:20 May 2014 11:53
Last Modified:20 May 2014 12:20

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