Re-assessing the magmatic and metamorphic evolution of the Aar Massif, Central Alpine basement

The pre-Alpine evolution of the Central Alpine basement is dominated by magmatic and metamorphic events that occurred during an Ordovician orogenic cycle (ca. 480-440 Ma) and the Variscan orogenic cycle (ca. 350-300 Ma). A detailed zircon U-Pb data and Hf-isotope study of a large set of magmatic and meta-magmatic rocks revealed four magmatic pulses (Ruiz et al. 2022): at 340-350 Ma (calc-alkaline diorite and tonalite from the Surselva Group), 330-335 Ma (shoshonitic diorites, monzonites, granites and syenites of the Rötifirn Group), 307-310 Ma (calc-alkaline diorites, ranging from cumulate-like hornblende gabbros to hornblende-diorites and hornblende- or biotite quartz- monzonite, granodiorites and metaluminous weakly peraluminous I-type granites of the Fruttstock Group), and 297-300 Ma (late-orogenic, calc-alkaline I-type granites of the Haslital Group). High precision U-Pb dates from meta-magmatic rocks indicate a minor, but variable impact of Alpine metamorphism on the U-Pb dates (Gaynor et al. 2022, Ruiz et al. 2022). However, given the poly-cyclic metamorphic record of the country rocks, the relative contributions of the Alpine, Variscan and an earlier Ordovician orogenic cycle are difficult to quantify. More specifically, the physical conditions of the Variscan metamorphic overprint are only weakly constrained, and available radio-isotopic ages are not reliable. However, monazite, rutile, titanite and zircon ages of 329-317 Ma in high-grade metapelites and calcsilicate gneiss indicate a major high grade Variscan metamorphism along the northern rim of the massif (Schaltegger et al., 2003). In addition, frequently found U-Pb dates between 478 and 445 Ma on gabbros, metapelitic to metapsammitic gneisses in the northern part of the Aar massif (Schaltegger et al., 2003) show relics of an older metamorphism in these polycyclic basement units

In order to understand better the temperature-time evolution of this poly-cyclic basement, we will apply detailed U-Pb geochronology on different minerals together with mineralogical, chemical and textural characterization. Combining the mineralogical data with microstructures and petrological data should give better insights in the link of metamorphism and magmatism for the Variscan orogenic cycle. These data will allow placing the Aar massif evolution in a wider framework of the European Variscan orogen. Moreover, they will reveal the existence of one or several pulses of earlier, Ordovician-age high-grade metamorphism, anatexis and magmatism.

References: Gaynor S.P., Ruiz M., & Schaltegger U. (2022) Chem. Geol., 603, 120913; Ruiz M., Schaltegger U., Gaynor S.P., Chiaradia M., Abrecht J., Gisler C., Giovanoli F. & Wiederkehr M. (2022) Swiss J. Geosci., 115, 20; Schaltegger. U., Abrecht J. & Corfu F. (2003) Schweiz. Mineral. Petrogr. Mitt. 83, 183-195