HT–LP crustal syntectonic anatexis as a source of the Permian magmatism in the Eastern Southern Alps: evidence from xenoliths in the Euganean trachytes (NE Italy)

Abstract

Oligocene trachytes from the Euganean Hills include various regionally metamorphosed gneissic and granulitic xenoliths. These xenoliths provide the unique opportunity to investigate South Alpine intermediate to deep crustal levels that are not at present exposed in the Eastern Alps. The estimated P – T conditions are in the range of 780–850°C and 0.45–0.55 GPa for a migmatitic gneiss xenolith. Sensitive high-resolution ion microprobe (SHRIMP II) U–Pb analyses on zircon from this xenolith provide concordant ages around 259.7 ± 3.5 Ma, consistent with a proton-induced X-ray emission (PIXE) U–Th–Pb age on monazite of 262 ± 12 Ma. The Sr–Nd–Pb isotopic compositions, and major and trace element data show distinct origins for the different types of xenoliths. Mafic granulite xenoliths have an isotopic signature close to mantle-derived rocks and to Permian gabbroic rocks from the Western Southern Alps. Metapelite xenoliths have high Sr and low Nd initial ratios like those of acid crustal rocks and could possibly represent the source of the crustal component that is dominant in the acid Permian supervolcanoes. The migmatitic xenolith provides the first documented evidence for a Permian thermal event associated with crustal thinning in the Eastern Southern Alps. Here the South Alpine basement escaped most of the Alpine crustal shortening and still preserves most of the original Permian extension under thick Mesozoic cover. Supplementary material: Microprobe analyses of mineralogical phases and Ti-in-biotite geothermometric calculations are available at https://doi.org/10.6084/m9.figshare.c.5032337