|Title:||The onset and origin of differentiated Rhine Graben volcanism based on U‐Pb ages and oxygen isotopic composition of zircon|
|Authors:||A.K. Schmitt, M.A.W. Marks, H.D. Nesbor, and G. Markl|
|Publication:||Europ. Jour. of Min., v. 19, p. 849‐857.|
|Publish Date:||Dec 2007|
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; 2σ 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 (<‐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.