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 eclogit e-facies rocks from the Champtoceaux Complex (Armorican Massif, France), Sm -Nd, Rb-Sr and Ar-40/Ar-39 methods are combined with a petrological study t o 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 Sou th Armorican Domain. The lower unit, the Cellier Unit, consists of leucocra tic gneisses, mica-schists and well-preserved eclogites. Petrological data on selected samples from different lithologies show (1) preservation of gro wth zoning in garnet, (2) no amphibolite- or greenschist-facies overprint i n the eclogite and (3) variations in the Si content of phengite as a functi on of bulk-rock chemistry, P-T conditions and partial reequilibration durin g 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) obta ined 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 i nterpreted as the age of the high-pressure event. Eight Ar-40/Ar-39 plateau ages, ranging from 352.0 +/- 1.6 to 340.5 +/- 1.4 Ma, are obtained from phengite single grains from six samples. The existen ce of Ar inheritance is unlikely, because (1) Ar-40/Ar-39 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 case s, and (3) a concordant Rb-Sr age is obtained on one common sample. These p lateau ages probably represent closure temperatures (possibly on the order of 450-500 degreesC) 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 retrogressio n. Copyright (C) 2000 John Wiley & Sons, Ltd.