Continental gabbros in the eastern Austroalpine basement of Eastern Alps: Middle Permian to earliest Triassic back-arc rifting?

Small metagabbro bodies associated with the voluminous Middle Permian porphyric granite (“Grobgneiss”) batholith in the Lower Austroalpine Raabalpen Complex were studied by detailed geochronological and geochemical investigations. Petrographically, their composition ranges from gabbro to gabbrodiorite. LA–ICP–MS zircon U Pb dating of four metagabbros provides evidence for two stages of plutonism, at ~265 Ma (Middle Permian) slightly younger than the Middle Permian granitic Grobgneiss intrusion (272–267 Ma) and at ~252 Ma (Permian-Triassic boundary). Whole-rock geochemistry indicates that the metagabbros have calc-alkaline and tholeiitic characteristics. Zircon Hf isotopic compositions from the Raabalpen gabbros show ε Hf( t ) values of −5.1 to 4.4, with t DM model ages of 0.74–1.15 Ga, indicating that crustal components had been incorporated in the mantle-derived mafic magmas. Together with recent data from the literature, both groups of metagabbros are interpreted being part of a hitherto unknown province of bimodal mafic (and felsic) magmatic province in Alps and Carpathians. We interpret the Middle Permian to earliest Triassic metagabbros as mantle-derived magmatism formed during back-arc extension in a continental rift setting triggered by northward Paleotethys subduction. • Austroalpine metagabbros provide evidence for two stages of mafic plutonism, at ~265 Ma and ~ 252 Ma. • Two constrasting magma groups, calc-alkaline and alkaline, are found. • Both groups of gabbros are interpreted being part of a hitherto unknown mafic magmatic province. • Metagabbros suggest back-arc extension due to northward subduction of the Paleo-Tethys Ocean.