Retrogressed lawsonite blueschists from the NW Iberian Massif: P-T-t constraints from thermodynamic modelling and 40Ar/39Ar geochronology

Abstract

Blueschist facies terranes in the Variscan Ibero-Armorican Arc are restricted to scarce and relatively small areas. One of these examples is the Ceán Unit, which is the westernmost exposure of the middle allochthonous sheet of the Variscan belt in the Malpica-Tui Complex (NW Iberian Massif). The Ceán Unit is a highly condensed metamorphic succession with a lower part in the blueschist facies and an upper part without HP relicts. It comprises variable proportions of glaucophane-chloritoid-bearing metapelites and mafic rocks with abundant well-preserved pseudomorphs after euhedral lawsonite. Both lithologies show systematic changes in texture and mineral composition that are spatially related depending on deformation. The metamorphic evolution of the metabasic rocks has been constrained in the P-T space through pseudosection approach and is characterised by H2O-undersaturated prograde evolution induced by the crystallisation of lawsonite. Peak conditions in the blueschist/LT-eclogite facies have been constrained at ca. 2.2 GPa and 560 °C. Exhumation-related metamorphism is characterised by a nearly isothermal decompression from the lawsonite-bearing fields to fields with stable albite at P ≈ 1 GPa. This lead to the pseudomorphism of lawsonite in the early-decompression stages, and a subsequent amphibolite-greenschist facies overprint at P < 0.8 GPa and T ≈ 440-480 °C. The preservation of the lawsonite crystal shape despite complete retrogression indicates that pseudomorphism occurred as a static process and that particular levels of the blueschist host rock were not affected by penetrative deformation during exhumation. 40Ar/39Ar step heating of phengitic muscovite from the pelitic schists interbedded with the lawsonite pseudomorph-bearing metabasic rocks yield plateau ages of ca. 363 ± 2 and 354 ± 1 Ma. The older age is interpreted as the age of the peak blueschist facies metamorphism. The age of 355 Ma is interpreted as a cooling age and is inferred to represent some point relatively close to peak conditions at the onset of the isothermal decompression. 40Ar/39Ar dating of muscovite from the quartzo-feldspathic mylonites of the Bembibre-Ceán detachment, at the base of the Ceán Unit, yields an age of ca. 337 ± 3 Ma, interpreted as the age of the post-nappe extensional tectonics. Similar data obtained from the blueschists of Ile de Groix (Armorican Massif; Bosse et al. in Chem Geol 220:21-45, 2005) support the equivalence of the Ceán Unit and the Upper Unit of Ile de Groix along the Ibero-Armorican Arc and suggest that these units share a blueschist facies event constrained at ca. 360-370 Ma, that is inferred to represent the Late Devonian-Early Carboniferous subduction of the northern margin of Gonwana beneath Laurussia. © 2014 Springer-Verlag Berlin Heidelberg.