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The onset and origin of differentiated Rhine Graben volcanism based on U-Pb ages and oxygen isotopic composition of zircon

¹ Dept. of Earth and Space Sciences, UCLA, Los Angeles CA 90095, USA
² Institut für Geowissenschaften, Eberhard-Karls-Universität, 72074 Tübingen, Germany
³ Hessisches Landesamt für Umwelt und Geologie, 65203 Wiesbaden, Germany

^* Corresponding author, e-mail: axel{at}argon.ess.ucla.edu

Ion microprobe U-Pb zircon ages for late (phase II) syenite dikes of the Katzenbuckel (SW Germany) average 69.6 ± 1.9 Ma (mean square of weighted deviates MSWD =0.36; number of spot analyses n =9; 2 error) and represent a minimum age for the emplacement of earlier, volumetrically dominant phonolites and nepheline syenites (phase I). This age is ~ 13 and 6 Ma older than previously published K-Ar whole-rock and bulk biotite ages for phase I rocks, respectively. Because of close chemical affinities in magma composition and the small volume of the Katzenbuckel subvolcanic intrusion, zircon crystallization in phase II syenite dikes presumably only shortly postdates phase I magmatism. Trachytes from the Northern Upper Rhine Graben (Sprendlinger Horst, Wetterau) yielded similar U-Pb zircon ages of 68.1 ± 1.4 Ma (MSWD =2.2) and 70.3 ± 1.6 Ma (MSWD =0.4), respectively. Zircon oxygen isotopic ratios of Katzenbuckel indicate primitive mantle-like compositions, whereas Sprendlinger Horst and Wetterau trachytes zircons show minor crustal contamination. The investigated differentiates rank among the oldest known post-Permian igneous rocks in the Northern foreland of the Alpine collisional belt, and are contemporaneous with recently dated lamprophyres in the Northern Rhine Graben area. This suggests that a short (<1–2 Ma) but regionally widespread magmatic pulse occurred along the E and NE flanks of the nascent Rhine Graben rift as early as Late Maastrichtian.

Key-words: Rhine Graben, rift zones, syenite, trachyte, zircon, U-Pb Isotopes, oxygen isotopes, ion-probe.