Precise U-Pb chronometry of 345-340 Ma old magmatism related to syn-convergence extension in the Southern Vosges (Central Variscan Belt)

Details Precise U-Pb chronometry of 345-340 Ma old magmatism related to syn-convergence extension in the Southern Vosges (Central Variscan Belt) Precise U-Pb chronometry of 345-340 Ma old magmatism related to syn-convergence extension in the Southern Vosges (Central Variscan Belt)

Nov 1996

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996e%26psl.144..403s&link_type=abstract

Earth and Planetary Science Letters, Volume 144, Issue 3-4, p. 403-419.

Computer Science

14

Scientific paper

The Southern Vosges host a large volcano-sedimentary basin which contains clastic sediments of Famennian (Late Devonian; ca. 365 Ma) to Late Visean (ca. 340-335 Ma) age. The basin is bordered by an exhumed high-grade basement complex and intruded by several granitoid plutons. A chronology for basin evolution and magmatism was established dating volcanic rocks of the basin as well as cross-cutting granitoid rocks by the U-Pb technique applied to zircon and titanite: The basin evolution started with olistolithic deposits and overlying marine shales that have a Famennian fauna (ca. 365 Ma) and contain detrital zircons with an age of 386 Ma. The lowermost rhyolite flows interlayered with the basin sediments and tholeiitic basalts yielded an age of 345 +/- 2 Ma. Following a tectonic event at the bottom of the Upper Visean, the rates of sedimentation and volcanism were strongly enhanced, leading to basin emersion, and were concluded by the extrusion of the uppermost ignimbrite (Molkenrain rhyolite) emplaced at 340 +/- 2 Ma. The whole volcano-sedimentary association was subsequently intruded by melts of the Ballons and Crêtes granites as well as by small monzodioritic and dioritic 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 Crêtes granites can be explained by differential uptake of crustal material during magma genesis. The precise age data prove that magmatism and basin formation in the Southern Vosges were extremely short-lived and coeval to the exhumation of adjacent high-grade gneiss terrains, revealing an episode of extension at the end of the Lower Carboniferous between 345 and 340 Ma (Visean). The tectonic regime in the internal part of the Variscan orogen is that of extension and strike-slip, while convergent tectonics were still ongoing in the external part of the Variscan orogen.

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