Late Devonian subduction and early‐orogenic exhumation of eclogite‐facies rocks from the Champtoceaux Complex (Variscan belt, France)

In order to define the mechanisms involved during exhumation of the eclogite‐facies rocks from the Champtoceaux Complex (Armorican Massif, France), Sm‐Nd, Rb‐Sr and 40Ar/39Ar methods are combined with a petrological study to construct a pressure–temperature–time (P–T–t) path for the Cellier Unit. The Champtoceaux Complex is a crustal‐scale thrust sheet located in the South Armorican Domain. The lower unit, the Cellier Unit, consists of leucocratic gneisses, mica‐schists and well‐preserved eclogites. Petrological data on selected samples from different lithologies show (1) preservation of growth zoning in garnet, (2) no amphibolite‐ or greenschist‐facies overprint in the eclogite and (3) variations in the Si content of phengite as a function of bulk‐rock chemistry, P–T conditions and partial re‐equilibration during decompression. Sm‐Nd analyses on the eclogite sample give a Grt–Cpx–whole‐rock age of 362 ± 25 Ma, consistent with the U‐Pb age of 356 ± 8 Ma (recalculated) obtained from the same sample by J. L. Paquette in 1987. Preservation of growth zoning in the garnet and the absence of late overprint show that resetting of both Sm‐Nd and U‐Pb systems is unlikely. The age of c. 360 Ma is thus interpreted as the age of the high‐pressure event. Eight 40Ar/39Ar plateau ages, ranging from 352.0 ± 1.6 to 340.5 ± 1.4 Ma, are obtained from phengite single grains from six samples. The existence of Ar inheritance is unlikely, because (1) 40Ar/39Ar ages are younger than the age of the high‐pressure event as deduced from U‐Pb and Sm‐Nd ages, (2) duplicates display a high reproducibility of plateau ages in all cases, and (3) a concordant Rb‐Sr age is obtained on one common sample. These plateau ages probably represent closure temperatures (possibly on the order of 450–500°C) for the best preserved and oldest samples, whereas the younger plateau ages may represent a later closure of the K/Ar system due to continuous deformation and chemical re‐equilibration during retrogression.