Devonian geodynamic evolution of the Variscan Belt, insights from the French Massif Central and Massif Armoricain

Abstract

International audienceThe Paleozoic French Variscan Belt in Massif Central and Massif Armoricain is a collision belt that provides a good example of a suture zone where ophiolites are rare, and the frontal (i.e., the magmatic arc) part of the upper plate is not present. In the lower plate (or Gondwana), the continental rocks are subdivided into an Upper Gneiss Unit (UGU) and a Lower Gneiss Unit (LGU). The UGU experienced a high-pressure (and likely ultra-high-pressure) metamorphism followed by crustal melting during their exhumation. New chemical U-Th-Pb monazite ages and ion-probe U-Pb zircon ages on migmatites allow us to constrain the P-T-t paths followed by the UGU and LGU. By comparison with thermomechanical experiments, a possible geodynamic evolution scenario can be proposed for the Variscan convergence. The high-compression regime of continental subduction developed during the initial subduction of the northern margin of Gondwana under Armorica in Silurian times. This induced the formation of a new subduction zone in the back-arc basin, which is the youngest, hottest, and thus mechanically the weakest part of the overriding plate. As a result, the arc-back-arc basin domain has been almost totally subducted below Armorica. Only a limited part of the back-arc basin rocks remains exposed in the Devonian St-Georges-sur-Loire Unit. Subsequently, the continental subduction of Gondwana resumed with a steeper dip associated with low-compression regime that in turn allowed the high-pressure rocks to be exhumed and partly melted in Late Devonian times. Such a scheme depicts quite well the complexity of the Variscan Belt

A review of the pre-Permian geology of the Variscan French Massif Central

“…Among the unsolved points, the "basement problem" is still questioned. In contrast to [30] and [28]. …”

“…Devonian crustal melting observed in the UGU and LGU and the top-to-the-SW ductile shearing (D 1 event) are coeval with the exhumation of the HP rocks [29].…”

“…The LGU underwent Middle Devonian metamorphism leading to crustal melting coeval with a ductile shearing dated in the Limousin at 375-370 Ma and attributed to the D 1 event (cf below, 21,30,38). Moreover, the…”

“…The Devonian migmatite (Migmatite I) that crops out in the upper part of the UGU and in the LGU are dated between 385 Ma and 375 Ma [21,29]. They are developed orthogneiss and paragneiss during the D1 event, and are related to the exhumation of the UGU rocks.…”

“…The UGU contains high pressure rocks, such as eclogites or HP granulitic orthogneiss, that locally may reach the coesite-eclogite facies [ 35,39,61] however, blueschists are very rare in the FMC. The upper part of the UGU consists of Devonian migmatites, with ages around 385-380 Ma, [21,29,38] formed by the partial melting of pelitic and quartzo-feldspathic rocks. These rocks contain amphibolite blocks derived from retrogressed eclogites that did not experience partial melting.…”