Two orogenic cycles recorded in eclogite-facies gneiss from the southern Armorican Massif (France)
European Journal of Mineralogy Volume 21 Number 6 (2009), p. 1173 - 1190
published: Dec 1, 2009
ArtNo. ESP147052106007, Price: 29.00 €
The Les Essarts Unit (Armorican Massif, Vendée, western France) contains eclogites that are considered as remnants of an old oceanic crust eclogitized during an eo-Variscan subduction and subsequently incorporated into the Variscan orogenic belt. They form km-scale boudins within orthogneiss and paragneiss. This study is devoted to some of these gneisses that show evidence of two distinct high-grade episodes. The first episode was characterised by intrusion of granite and migmatisation of cordierite-bearing metapelites (T ≈ 670 °C, P ≈ 0.32 GPa). The second episode is an eclogite-facies overprint (T ≈ 700 °C, P > 1.6 GPa), which gave rise to many pseudomorphic and coronitic reactions and caused high-pressure minerals to grow (garnet, phengite, kyanite, rutile, and probably jadeite) at the expense of the previous high-temperature parageneses. These two high-grade stages were separated by a first retrogression (T < 350 °c), attested to by the following facts: (1) the eclogite-facies phe + qtz symplectite, which partly replaced the high-temperature k-feldspar, grew preferentially parallel to perthitic lamellae, indicating that the latter had already been exsolved when the high-pressure metamorphism occurred. (2) the eclogite-facies pseudomorphs after cordierite, composed of grt + ky + qtz + micas, are rich in k2O and deficient in MgO + FeO relative to a true cordierite; their compositions are similar to that of an altered cordierite (so-called “pinite”), indicating that cordierite had been altered to Chl + Ms before being replaced by the high-pressure metamorphic pseudomorphs. The compositions of various reaction microdomains, estimated by mass-balance studies, have been used to model P-T isochemical diagrams (i.e., P-T pseudosections), which, when taken together, reveal a complex P-T path with two distinct cycles. Therefore, the ortho- and para-gneisses probably belong to a pre-Variscan continental crust (first cycle) that was involved in the same eo-Variscan subduction (second cycle) as the neighbouring eclogitized oceanic mafic rocks. Similar rocks are known elsewhere, particularly in the Variscides, but they were not generally described as polycyclic. Many metamorphic rocks, with an apparent anticlockwise P-T path that is difficult to explain in terms of geodynamic models, could well be polycyclic rocks, whose two cycles are difficult to deconvolve. As for the Vendée eclogite-facies gneisses, they attest that subduction of the pre-Variscan continental crust occurred during the Variscan orogeny.