Abstract
The Grobgneis complex, located in the eastern Austroalpine unit of the Eastern Alps, exposes large volumes of pre-Alpine porphyric metagranites, sometimes associated with small gabbroic bodies. To better understand tectonic setting of the metagranites, we carried out detailed geochronological and geochemical investigations on the major part of the porphyric metagranites. LA–ICP–MS zircon U–Pb dating of three metagranites sampled from the Grobgneis complex provides the first reliable evidence for large volumes of Permian plutonism within the pre-Alpine basement of the Lower Austroalpine units. Concordant zircons from three samples yield ages at 272.2 ± 1.2 Ma, 268.6 ± 2.3 Ma and 267.6 ± 2.9 Ma interpreted to date the emplacement of the granite suite. In combination with published ages for other Permian Alpine magmatic bodies, the new U–Pb ages provide evidence of a temporally restricted period of plutonism (“Grobgneis”) in the Raabalpen basement Complex during the Middle Permian. Comparing the investigated basement with that of the West Carpathian basement, we argue that widespread Permian granite magmatism occurred in the Lower Austroalpine units. They belong to the high-K calc-alkaline to shoshonitic S-type series on the base of geochemical data. Zircon Hf isotopic compositions of the Grobgneis metagranites show εHf(t) values of − 4.37 to − 0.6, with TDM2 model ages of 1.31–1.55 Ga, indicating that their protoliths were derived by the recycling of older continental crust. We suggest that the Permian granitic and gabbroic rocks are considered as rifted-related rocks in the Lower Austroalpine units and are contemporaneous with cover sediments.
Highlights
As long discussed for the Eastern and Southern Alps and testified by deposition of thick Permian sedimentary succession, Permian extension occurred in the aftermath of the Variscan orogeny (Krainer 1993 and references therein; Decarlis et al 2013)
For the Austroalpine nappe stack in the Eastern Alps, this leaves the question open, how Permian plutonism and low-pressure metamorphism relates to sedimentary successions, whether there are some major Permian granite bodies, and which evidence argues for a coherent tectonic model
6.1 Age of the porphyric metagranites (Grobgneis) This LA-ICP-MS zircon study of porphyric metagranites from three widely spaced localities within the Raabalpen basement of the Lower Austroalpine unit has complemented an ambiguous question about the age and origin of these rocks
Summary
As long discussed for the Eastern and Southern Alps and testified by deposition of thick Permian sedimentary succession, Permian extension occurred in the aftermath of the Variscan orogeny (Krainer 1993 and references therein; Decarlis et al 2013). Ongoing geochronological work found increasing evidence for small gabbroic Permian plutonism in Austroalpine units (Thöni and Jagoutz 1992; Thöni and Miller 1996), for a small granitic orthogneiss body dated by the Rb–Sr whole rock method (Morauf 1980) and for Permian pegmatites (Schuster et al 2001a, b; Knoll et al 2018), but no large granite bodies. For the Austroalpine nappe stack in the Eastern Alps, this leaves the question open, how Permian plutonism and low-pressure metamorphism relates to sedimentary successions, whether there are some major Permian granite bodies, and which evidence argues for a coherent tectonic model. Several tectonic processes have been proposed for explanation of the Permian post-Variscan tectonic evolution of Alps and central European Variscides: (a) the onset of a rifting process that continued until the Middle Triassic crustal breakup and oceanic spreading of the Meliata Ocean (Kozur 1991; Neubauer et al 2000, 2018; Plašienka 2018; Putiš et al 2019); (b) post-Variscan orogenic collapse (Ménard and Molnar 1988); (c) magmatic underplating (Schuster and Stüwe 2008); (d) a back-arc extension induced by the oblique subduction of the Paleotethys ocean beneath the southern margin of the Pangea continent (Cassinis et al 2012; Finger and Steyrer 1990; Stampfli and Kozur 2006); (e) an initial strike-slip to transtensional shearing affecting the southern side of the Variscan orogen between Eurasia and Gondwana (Muttoni et al 2009; Schaltegger and Brack 2007), which was followed by a period of progressive extension until the Alpine rifting that began during Carnian–Norian times; (f ) superplume emplacement (Doblas et al 1998)
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