PRECISE U-PB CHRONOMETRY OF 345-340 MA OLD MAGMATISM RELATED TO SYN-CONVERGENCE EXTENSION IN THE SOUTHERN VOSGES (CENTRAL VARISCAN BELT)

The Southern Vosges host a large volcano-sedimentary basin mhich conta ins elastic sediments of Famennian (Late Devonian; ca. 365 Ma) to Late Visran (ca. 330-335 Ma) age. The basin is bordered by an exhumed high -grade basement complex and intruded by several granitoid plutons. A c hronology for basin evolution and magmatism was established dating vol canic rocks of me basin as well as cross-cutting granitoid rocks by th e U-Pb technique applied to zircon and titanite: The basin evolution s tarted with olistolithic deposits and overlying marine shales that hav e a Famennian fauna (ca. 365 Ma) and contain detrital zircons with an age of 386 Ma. The lowermost rhyolite flows interlayered with the basi n sediments and tholeiitic basalts yielded an age of 345 +/- 2 Ma. Fol lowing a tectonic event at the bottom of the Upper Visean, the rates o f sedimentation and volcanism were strongly enhanced; leading to basin emersion, and were concluded by the extrusion of the uppermost ignimb rite (Molkenrain rhyolite) emplaced at 340 +/- 2 Ma. The whole volcano -sedimentary association was subsequently intruded by melts of the Bal lons and Crete granites as well as by small monzodioritic and diodtic satellite intrusions along the border of the Ballons granite within 3 +/- 2 myr (342 +/- 1 to 339.5 +/- 2.5 Ma). The zircons of most of the volcanic and plutonic rocks investigated contain inherited lead of 0.6 and 2.1 Ga age. The differences in initial Nd isotopic composition of the Ballons and Cretes granits can be explained by differential uptak e of crustal material during magma genesis. The precise age data prove that magmatism and basin formation in the Southern Vosges were extrem ely short-lived and coeval to the exhumation of adjacent high-grade gn eiss terrains, revealing an episode of extension at the end of the Low er Carboniferous between 345 and 340 Ma (Visean). The tectonic regime in the internal part of the Variscan orogen is that of extension and s trike-slip, while convergent tectonics were still ongoing in the exter nal pan of the Variscan orogen.