The Velay dome (French Massif Central): melt generation and granite emplacement during orogenic evolution

Abstract

This paper is a synthesis of available data on the Velay dome that include both small- and large-scale lithologic and structural mapping, strain analysis, isotope geochemistry, geochronology and pressure–temperature estimates. The Velay dome, one of the largest granite–migmatite domes of the Variscan Belt, formed during orogenic collapse at around 300 Ma. Its study allows an assessment of the thermal and geodynamic context leading to voluminous crustal anatexis of the Variscan orogenic crust. A first melting stage developed in connection with south-verging thrust zones during the Early Carboniferous, leading to a crustal thickening estimated at 20 km minimum. The involvement of fertile lithologies and the intrusion of plutons of deep origin contributed to the development of water-saturated melts. The volume of biotite granite extracted from melt during this period was limited. The second phase of melting, corresponded to generalized melting of gneiss achieved by biotite-dehydration melting reactions and accompanied by the generation of cordierite-bearing granites. At this stage, crustal-scale detachment faults were active and partially obliterated the earlier structures. The new structures were progressively tilted to the vertical at the margin of the Velay dome due to the southward and lateral ballooning of the granitic dome. The reconstructed P, T path indicate that the large volume of melt produced was a consequence of a significant increase in temperature at the onset of biotite dehydration melting. At the base of the crust, this melting event is coeval with granulite facies metamorphism associated to underplating of mantle-derived magmas as suggested by the geochemical signature of Late Paleozoic lower crustal xenoliths sampled by Cenozoic volcanoes and with the isotopic signature of the late granitic intrusions. Accordingly, it is proposed that asthenospheric upwelling was responsible for the temperature increase favoring melting of hydrous minerals.