Flow of the partiallymolten Variscan orogenic crust fromsyn-orogenic exhumation of subductedcontinental crust to gravitational collapsealong a convergent plate boundary markedby slab retreat

International audienceThe Variscan belt ofWestern Europe exposed in the French Massif Centralis a perfect example of a collision zone characterized by protractedsyntectonic magmatism and partial melting (from 380 to 280 Ma) witha wide range of petrologic and geochemical signatures (calc-alkaline,high-K, Mg-K, peraluminous) that have been inferred to fingerprint lithosphericsubduction, mantle upwelling and/or partial melting of theorogenic wedge.The nappe pile encompasses an upper gneiss unit (UGU) and a lowergneiss unit (LGU) that are separated by an association of maficultramaficrocks designated as the Leptynite-Amphibolite Group (LAG)and has been interpreted as representing remnants of former small immatureoceanic basins. Both the UGU and the LGU are made of migmatitesbut are distinguished on the basis of their structural position withrespect to the LAG and of their metamorphic record. The UGU has preservedrelics of high-pressure metamorphism whereas the LGU has onlyrecorded a high-temperature metamorphism.We present a synthesis of structural, petrologic, geochemical and geochronologicaldata from the various lithologic-tectonic units exposedalong a transect across the Variscan belt of Western Europe from theFrench Massif Central to the Pyrenees. In particular, the new geochronologicaland geochemical dataset presented by Couzinié et al. (this conference)suggests the contribution of mantle and crustal derived magmaswith a southward younging of syntectonic emplacement. These dataprovide a basis to elaborate a model for the structure of the Laurussia-Gondwana plate boundary at the onset of convergence and for the generationand flow of migmatites during orogenic evolution from the earlystage of subduction of the continental crust to gravitational collapse ofthe orogenic belt in a context of a convergent plate boundary marked bysouthward slab retreat