Evidence of a distinct Permian thermal event by EMP-Th-Pb-monazite ages in metapelites of the polymetamorphic Austroalpine basement

During the Alpine orogeny, the Austroalpine basement complex of the Eastern Alps was thrusted upon the Penninic ophiolites and the European basement, now exposed in the Tauern and Engadine tectonic windows. The Austroalpine basement underwent a polymetamor- phic evolution since the Early Paleozoic. An Ordovi- cian-Silurian event, the Devonian-Carboniferous Var- iscan orogeny, as well as the Cretaceous and Tertiary Alpine orogenic periods have been recognised in many parts. Although a wealth of radiometric data on these events exist, a distinct Permian metamorphic episode has not yet been established. Corresponding mica cool- ing ages were considered as Variscan-to-Alpine “mixed ages” in this polymetamorphic frame. However, the intrusion of Permian pegmatites between 270-250 Ma is an important marker of the tectonic and magmatic activity (Schuster et al. 2001).

The Permian pegmatites can be traced from the Ortler- Campo basement through the basement to the south of the Tauern Window toward the East into the Saualpe and Koralpe units. The electron microprobe (EMP) Th-U-Pb monazite dating method (Montel et al. 1996) has been applied to the garnet-bearing metapelitic host rocks of these Permian pegmatites. In the Saualpe, the Permian pegmatites were strongly deformed during the eclogite-facies Cretaceous event. In the metapelitic host rocks, the Permian monazites have the largest grain sizes and abundance. They are often character- ised by spectacular coronas of apatite and allanite of a partial decomposition.

In the Schobergruppe and the Defereggen Alps to the south of the Tauern Window, the Permian monazites are a less prominent population, but clearly distinct from the Carboniferous monazites (Krenn et al. 2012). The characteristic coronas around the Permian mona- zites are lacking. Permian monazites occur mainly in a zone with fibrolitic sillimanite and andalusite in the vicinity of the pegmatites.

The Oetztal-Stubai basement to the W of the Tauern Window is characterised by a Cretaceous metamor- phic overprint. Permian pegmatites have not yet been reported. The monazite Th-U-Pb EMP ages in the Sellrain area and in the central Oetztal valley (Umhau- sen, Sölden) in the northern vicinity of the Cretaceous metamorphic zone are mostly Carboniferous. They are interpreted to be related to a Variscan amphibolite- to-eclogite-facies garnet crystallisation (Rode et al. 2012). Recent investigations in the Stubai valley re- vealed garnet metapelites with exclusively Permian monazites. These monazites are often surrounded and partly replaced by coronas of apatite and allanite. The mineral-chemical properties and the special character of the dating method allow the conclusion that the Permian monazites represent a distinct crystallisation event at low pressures, apparently in an occasional association to the pegmatites.

References

Krenn, E., Schulz, B. & Finger, F. (2012): Three generations of monazite in Austroalpine basement rocks to the south of the Tauern Window – evidences for Variscan, Permian and Alpine metamorphism. – Swiss Journal of Geosci- ences, 105, DOI 10.1007/s00015-012-0104-6.

Montel, J.-M., Foret, S., Veschambre, M., Nicollet, C. & Provost, A. (1996): A fast, reliable, inexpensive in-situ dating technique: Electron microprobe ages on monazite. – Chem. Geol., 131: 37-53.

Rode, S., Rösel, D. & Schulz, B. (2012): Constraints on the Variscan P-T evolution by EMP Th-U-Pb monazite dat- ing in the polymetamorphic Austroalpine Oetztal-Stubai basement (Eastern Alps). – Z. Dt. Ges. Geowiss. 163: 43- 67; Stuttgart.

Schuster, R., Scharbert, S., Abart, R. & Frank, W. (2001): Permo-Triassic extension and related HT/LP metamor- phism in the Austroalpine - Southalpine realm. – Mitt. Ges. Geol. Bergbaustud. Österr., 45: 111-141; Wien.