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.

Temporal relationships between Mg-K mafic magmatism and catastrophic melting of the Variscan crust in the southern part of Velay Complex (Massif Central, France)

“…von Raumer et al 2003). A large portion of the Eastern French Massif Central is occupied by the Velay Complex, a c. 100 by 100 km migmatite dome (Lagarde et al 1994;Ledru et al 2001). A large volume of peraluminous granite forms its core (Williamson et al 1992;Lagarde et al 1994;Downes et al 1997;Ledru et al 2001), and cuts across the Early Carboniferous nappe stack (Faure et al 2009 and references therein).…”

Modélisation géométrique 3D des granites stéphaniens du massif du Pelvoux (Alpes, France)

“…However this study highlights difference during the late stage of the Palaeozoic evolution. The French Massif Central granites are characterised by dome shape and laccoliths geometry emplaced during N-S extension direction in a context of crustal thinning [8,15] Ces terrains représentent des fragments de la chaîne Paléozoïque Européenne [18], réactivés lors des compressions alpines le long de grandes discontinuités structurales puis exhumés il y a environ 15 Ma [20].…”

Extensional Flow during Gravitational Collapse: A Tool for Setting Plate Convergence (Padrón Migmatitic Dome, Variscan Belt, NW Iberia)

“…The combination of horizontal shortening and extension with diapiric flow explains the absence of a radial or centrifuge flow pattern and also the fact that the dome bends the vectors of extensional flow. These two features are observed along the axial zones of the Variscan belt, both in the Iberian Massif (this article), the French Massif Central (Malavieille et al 1990;Lagarde et al 1994;Ledru et al 2001;Faure et al 2009), and the German Variscides (Krohe and Eisbacher 1988;Krohe 1998).…”