Preservation of Permo–Triassic low‐pressure assemblages in the Cretaceous high‐pressure metamorphic Saualpe crystalline basement (Eastern Alps, Austria)

Abstract

AbstractMetapelites and intercalated metapegmatites of the Saualpe crystalline basement, which forms part of the Austroalpine nappe complex in the Eastern Alps, display a polyphase tectonometamorphic history. Here, we focus on the evolution that these rocks underwent prior to Cretaceous (eo‐Alpine) high‐pressure metamorphism and related penetrative deformation. Geothermobarometry on coarse‐grained porphyroclastic parageneses (garnet–biotite–muscovite–plagioclase–sillimanite–quartz), which occur as relics in kyanite–garnet, two‐mica gneiss, yielded 600 °C/0.4 GPa. Results from a corundum‐bearing lithology suggest that higher temperatures may have been reached in very restricted areas. The matrix of these rocks displays intense recrystallization during a pressure‐dominated metamorphic overprint. Microstructures and mineral chemistry indicate that this low‐pressure metamorphism was the first significant metamorphic imprint in these rocks. Mineral relics in all metapelitic rock types reflect low‐pressure conditions for this interkinematic crystallization phase.The distribution, macroscopic and microscopic observations and the mineralogical composition of intercalated metapegmatites point to regionally elevated temperature conditions during their emplacement. Therefore, pegmatite formation is correlated with mineral formation in metapelites. Sm–Nd‐dating of magmatic garnet from the pegmatite gneiss yielded 249 ± 3 Ma, which is interpreted to represent the age of pegmatite‐emplacement and low‐pressure metamorphism in the metapelites. Since the pegmatites are overprinted by mylonitisation and high‐pressure metamorphism, this Permo–Triassic age also sets an upper age‐limit to the eclogite facies metamorphic event, which affected considerable parts of the Saualpe crystalline basement.