Inverted metamorphic field gradient towards a Variscan suture zone (Champtoceaux Complex, Armorican Massif, France)
Authors: PITRA P. , BALLÈVRE M. , RUFFET G. 
Published: December 2009

AbstractWe describe, date and constrain the P–T conditions of a syntectonic inverted metamorphic sequence associated with continental collision and crustal‐scale thrusting in one of the key regions of the late Palaeozoic Variscan belt of Western Europe – the Champtoceaux Complex (Armorican Massif, France), interpreted as a trace of the Variscan suture zone between Laurussia and Gondwana. The Complex consists of several stacked units, some of them eclogite‐bearing, that are sandwiched between two main pieces of continental crust – the Parautochthon and the Upper Allochthon. Moderately to steeply dipping foliation parallels the main lithological boundaries. From the bottom to the top of the metamorphic rock pile, the following sequence testifies to the syntectonic temperature increase: chlorite–biotite‐bearing metagreywackes (Parautochthon); orthogneisses with eclogite lenses; micaschists with chloritoid–chlorite–garnet; orthogneisses; micaschists with staurolite–biotite–garnet with chloritoid inclusions (Lower Allochthon); and migmatites with boudins of eclogite and kyanite–biotite–garnet‐bearing metapelitic lenses (Upper Allochthon). Mylonitic amphibolites with lenses of serpentinized peridotite mark the boundary between the Lower Allochthon and the overlying Upper Allochthon, suggesting the presence of a major thrust. It is inferred that the latter is responsible for the development of the inverted metamorphic zoning. Multiequilibrium thermobarometry and pseudosections calculated with thermocalc indicate that equilibration temperatures of the syntectonic peak metamorphic assemblages increase upwards in the rock pile from <500 °C in the Parautochthon to >650 °C in the Upper Allochthon. All units equilibrated at similar pressures between 7 and 10 kbar. In the Upper Allochthon, chronological results on muscovite suggest initial cooling from c. 343 Ma (muscovite Rb–Sr) to c. 337 Ma (muscovite 40Ar–39Ar). A subsequent very rapid temperature decrease is suggested by the synchronous closure of the muscovite and biotite K–Ar and biotite Rb–Sr isotopic systems (c. 337–335 Ma). This cooling is also recorded in the Upper Micaschists of the Lower Allochthon and in the Parautochthon with muscovite 40Ar–39Ar ages of c. 336–334 and 332 Ma, respectively. Ages of c. 343 Ma inferred from disturbed muscovite spectra from the Parautochthon are possibly linked to a previous higher pressure metamorphic event in this unit. It is suggested that the development of the inverted metamorphic zoning in the Champtoceaux Complex is due to the emplacement of a hot nappe over colder units and is contemporaneous with major crustal thrusting and associated pervasive ductile deformation. The preservation of this inverted field gradient was possible because of fast cooling, tentatively associated with the syn‐compressional denudation of the tectonic pile, expressed by the detachment at the top of the nappe pile. The efficiency of cooling is best shown by the near‐coincidence of Rb–Sr and 40Ar–39Ar ages, obtained on both sides of the major thrust. Finally, we highlight similarities with other regions of the West‐European Variscan belt (Iberian massif, French Massif Central) and suggest that inverted metamorphic zoning is systematically associated with the contact between the Lower and Upper Allochthons.