The Variscan belt ofWestern Europe exposed in the French Massif Central
is a perfect example of a collision zone characterized by protracted
syntectonic magmatism and partial melting (from 380 to 280 Ma) with
a wide range of petrologic and geochemical signatures (calc-alkaline,
high-K, Mg-K, peraluminous) that have been inferred to fingerprint lithospheric
subduction, mantle upwelling and/or partial melting of the
orogenic wedge.
The nappe pile encompasses an upper gneiss unit (UGU) and a lower
gneiss unit (LGU) that are separated by an association of maficultramafic
rocks designated as the Leptynite-Amphibolite Group (LAG)
and has been interpreted as representing remnants of former small immature
oceanic basins. Both the UGU and the LGU are made of migmatites
but are distinguished on the basis of their structural position with
respect to the LAG and of their metamorphic record. The UGU has preserved
relics of high-pressure metamorphism whereas the LGU has only
recorded a high-temperature metamorphism.
We present a synthesis of structural, petrologic, geochemical and geochronological
data from the various lithologic-tectonic units exposed
along a transect across the Variscan belt of Western Europe from the
French Massif Central to the Pyrenees. In particular, the new geochronological
and geochemical dataset presented by Couzinie et al. (this conference)
suggests the contribution of mantle and crustal derived magmas
with a southward younging of syntectonic emplacement. These data
provide a basis to elaborate a model for the structure of the Laurussia-
Gondwana plate boundary at the onset of convergence and for the generation
and flow of migmatites during orogenic evolution from the early
stage of subduction of the continental crust to gravitational collapse of
the orogenic belt in a context of a convergent plate boundary marked by
southward slab retreat.
The identification of oceanic sutures is key for understanding the evolution of the Paleozoic Variscan belt and the structure of the West European lithosphere. In the French Massif Central, the suture of the “Mid-Variscan” ocean would be stamped by a distinctive lithological formation known as the “Leptynite–Amphibolite Complex”. This formation comprises a Cambrian–Ordovician bimodal (meta-)igneous association interpreted as rifted-margin magmatism and preserves evidence for Devonian high-pressure metamorphism. Our study provides new geochronological and geochemical data on mafic–felsic rocks from the Riverie belt, part of the Monts-du-Lyonnais Leptynite–Amphibolite Complex. There, metaluminous, amphibole-bearing felsic gneisses represent former tonalites originally intrusive within mafic rocks (now amphibolites). LA–ICP–MS zircon U–Pb dating reveals a latest Ediacaran (c. 545 Ma) crystallization age for the tonalitic magmas, and a latest Devonian (c. 360 Ma) very limited metamorphic overprint related to the Variscan orogeny. Whole-rock geochemistry (notably pronounced Nb negative anomalies) and the highly radiogenic zircon Hf isotope compositions with εHf(545Ma) of c. +11 (in the range expected for the Depleted Mantle reservoir) indicate that the parent tonalitic melt originated from a mafic precursor sourced in a mantle metasomatized by oceanic slab-derived fluids. The (meta-)mafic rocks share a similar “arc” signature and were possibly generated from the same mantle source. The mafic–felsic association of the Riverie belt bears no relation to the rifting that led to the opening of the Mid-Variscan ocean, in marked contrast with what is observed in other Leptynite–Amphibolite Complexes. Instead, they can be correlated to a discrete juvenile magmatic event identified in the southern Variscan realm (Maures and Iberian massifs, Corsica) and interpreted as reflecting Cadomian back-arc magmatism.
