Origin and duration of late orogenic magmatism in the foreland of the Variscan belt (Lesponne — Chiroulet — Neouvielle area, French Pyrenees)

Abstract

During the late stage of the Variscan orogeny, the pyrenean segment underwent intense magmatism and regional high temperature – low pressure metamorphism. In the Lesponne – Chiroulet – Neouvielle area, a granodioritic pluton was emplaced in the upper crust while dioritic to granitic magmas were emplaced in metamorphic domes. Magmatism was contemporaneous with the regional crustal partial melting recorded in the core of the domes. The area is therefore a key target in the Pyrenees to discuss potential magmatic sources as well as the age and duration of the late Variscan magmatism. Geochemical data on representative magmatic rocks highlight two distinct sources of magma: a mantle source and a metasedimentary crustal source that produced respectively metaluminous and peraluminous magmas. Geochronological results show that magmatism took place over a period of about 10 My from ca. 303 to ca. 290 Ma. During this period, the middle to lower crust was composed of partially molten metasediments intruded by mantle and crustal magmas that crystallized in a final pulse at ca. 290 Ma. Late Variscan metamorphism and magmatism recorded in the Pyrenees have to be related to a significant and rapid heating from the underlying mantle rather than to crustal processes such as the maturation of a thickened continental crust. We propose that the initiation of metamorphism and bimodal magmatism at ca. 305 Ma in the Pyrenees is the expression of the delamination of the Gondwanan lithospheric mantle at a global scale in the Variscan belt.