GEOLOGICA CARPATHICA, JUNE 2018, 69, 3, 237–253 doi: 10.1515/geoca-2018-0014 www.geologicacarpathica.com The Oligocene Reifnitz tonalite (Austria) and its host rocks: implications for Cretaceous and Oligocene–Neogene tectonics of the south-eastern Eastern Alps FRANZ NEUBAUER 1, , BIANCA HEBERER 1 , ISTVÁN DUNKL 2 , XIAOMING LIU 3 , MANFRED BERNROIDER 1 and YUNPENG DONG 3 1 Department of Geography and Geology, University of Salzburg, Hellbrunner Str. 34, A-5020 Salzburg, Austria;  Franz.Neubauer@sbg.ac.at 2 Sedimentology and Environmental Geology, Geoscience Centre, University of Göttingen, Goldschmidtstrasse 3, D-37077 Göttingen, Germany 3 State Key Laboratory of Continental Dynamics, Department of Geology, Northwest University, Xi’an 710069, China (Manuscript received August 5, 2017; accepted in revised form May 11, 2018) Abstract: In the south-eastern Eastern Alps, the Reifnitz tonalite intruded into the Austroalpine metamorphic basement of the Wörthersee half-window exposed north of the Sarmatian–Pliocene flexural Klagenfurt basin. The Reifnitz tonalite is dated for the first time, and yields a laser ICP-MS U–Pb zircon age of 30.72 ± 0.30 Ma. The (U–Th–Sm)/He apatite age of the tonalite is 27.6 ± 1.8 Ma implying rapid Late Oligocene cooling of the tonalite to ca. 60 °C. The Reifnitz tonalite intruded into a retrogressed amphibolite-grade metamorphic basement with a metamorphic overprint of Cretaceous age ( 40 Ar/ 39 Ar white mica plateau age of 90.7 ± 1.6 Ma). This fact indicates that pervasive Alpine metamorphism of Cretaceous age extends southwards almost up to the Periadriatic fault. Based on the exhumation and erosion history of the Reifnitz tonalite and the hosting Wörthersee half window formed by the Wörthersee anticline, the age of gentle folding of Austroalpine units in the south-eastern part of the Eastern Alps is likely of Oligocene age. North of the Wörthersee antiform, Upper Cretaceous–Eocene, Oligocene and Miocene sedimentary rocks of the Krappfeld basin are preserved in a gentle synform, suggesting that the top of the Krappfeld basin has always been near the Earth’s surface since the Late Cretaceous. The new data imply, therefore, that the Reifnitz tonalite is part of a post-30 Ma antiform, which was likely exhumed, uplifted and eroded in two steps. In the first step, which is dated to ca. 31–27 Ma, rapid cooling to ca. 60 °C and exhumation occurred in an E–W trending antiform, which formed as a result of a regional N–S compression. In the second step of the Sarmatian–Pliocene age a final exhumation occurred in the peripheral bulge in response to the lithospheric flexure in front of the overriding North Karawanken thrust sheet. The Klagenfurt basin developed as a flexural basin at the northern front of the North Karawanken, which represent a transpressive thrust sheet of a positive flower structure related to the final activity along the Periadriatic fault. In the Eastern Alps, on a large scale, the distribution of Periadriatic plutons and volcanics seems to monitor a northward or eastward shift of magmatic activity, with the main phase of intrusions ca. 30 Ma at the fault itself. Keywords: Periadriatic magmatism, peripheral bulge, exhumation, cooling history, shortening. Introduction Intrusion of plutons during the late-stage orogenic processes is of high importance for several reasons. Such plutons often provide evidence of plate tectonic processes such as sub- duction, break-off or delamination of the subducted oceanic slab (e.g., von Blanckenburg & Davies 1995; von Blancken- burg et al. 1998; Seghedi & Downes 2012) or simply decom- pressional melting of the exhuming, previously subducted crust (Brown 2013). Plutons are often aligned along crustal- scale faults like the Periadriatic fault (Schmid et al. 1987, 1989; Rosenberg 2004; Handy et al. 2015; Cao & Neubauer 2016 and references therein). This may lead to rheological decoupling of different portions of the orogenic crust, influencing there- fore, the large-scale structure of mountain belts (Fig. 1). Finally, deciphering exhumation paths of such plutons may add to the reconstruction of vertical motion of the intruded crust along these major fault zones (e.g., Rosenberg 2004 and references therein; Cao & Neubauer 2016). Here, we report new data (U–Pb zircon age, microprobe data of garnet) from the hitherto undated Reifnitz tonalite from the south-eastern part of the Eastern Alps. This tonalite was not considered in recent geodynamic models as its age and significance were unknown (e.g., Rosenberg 2004). The U–Pb zircon and (U–Th)/He ages in combination with the first 40 Ar/ 39 Ar white mica age from the host rock and available regional geological data from adjacent sedimentary basins allow us to propose a major event of gentle N–S shortening by folding and associated erosion for this sector of the Eastern Alps, which was not known before. We also discuss the large-scale distribution of Oligocene to Miocene Periadriatic plutons, which clearly shows a northward or east- ward shift of magmatic activity, and discuss the potential significance.