Flow of the partially molten crust in the Variscan foreland revealed by U–Th–Pb dating of metamorphism, magmatism and deformation (Agly Massif, Eastern Pyrenees)

Abstract

In this contribution, we investigate the spatial and temporal evolution of mid-crustal flow in the Agly Massif (North Pyrenean Zone) that represents the southern foreland of the Variscan orogenic plateau. In the Agly Massif, the middle crust is represented by an Ediacarian–Devonian series of metasedimentary rocks that recorded high-grade metamorphism synchronously with crustal thinning (D2) and dextral wrenching (D3) during Carboniferous. D2 crustal thinning formed a penetrative S2 flat-lying foliation and localized C2 extensional shear zones with a top-to-the-North kinematics. D2 planar fabrics are deformed by a D3 dextral transpression localized into a two-kilometer wide high-strain zone. We performed LA-ICPMS U–Th–Pb dating on zircon and monazite from small magmatic bodies and from metamorphic rocks showing strain features relative to D2 and/or D3. Our results, compiled with published data, argue that the middle crust of the Agly Massif reached high-temperature and suprasolidus conditions at ca. 325–320 Ma and was partially molten until ca. 300 Ma. They also indicate that the D2 thinning and top-to-the north shearing was active from ca. 325–290 Ma. D2 extension and D3 transpression were synchronous from ca. 308–290 Ma. Making a comparison with the Pyrenean Axial Zone, the Montagne Noire and the French Central Massif, we propose a two-step tectonic model for the mid-crustal flow with a horizontal flow towards South in both the orogenic plateau and the southern forelands between ca. 325 and 310 Ma and locally reoriented into an E–W longitudinal flow between 310 and 300 Ma in high-strain dextral strike-slip shearing domains.