P-T-t evolution of the Pulkau and Pleißing Nappes, Thaya Window, Lower Austria

Two NW-SE trending profiles across the Pleißing and Pulkau nappes were investigated in detail and 56 samples from the Pernegger trench, 31 samples from the Pulkau and 32 samples from the Thaya valleys recovered. The Pleißing nappe exhibits amphibole-gneiss, metapelite, orthogneiss / metatuffite and marble with the corresponding mineral assemblages of hornblende - mica - feldspar - quartz - titanite - ilmenite; staurolite - garnet - mica ± graphite ± turmaline ± monazite; mica - biotite -epidote -kalifeldspar – quartz - ilmenite ± garnet ± chlorite; and calcite - biotite -mica -quartz -epidote - titanite ± amphibole. Grt-amphibolites, partly retrograded to greenschists, metapelite / phyllonite, and orthogneiss / metatuffite are recognized in the Pulkau nappe with the mineral assemblages amphibole -epidote - chlorite - rutile - ilmenite -quartz - titanite; mica - biotite - garnet - hornblende -quartz -plagioclase -ilmenite - monazite ± staurolite; mica - biotite - chlorite -Kfeldspar -quartz -ilmenite -magnetite ± turmaline.

P-T estimates from the Thaya valley profile are 580-620 °C / 0.8-0.9 GPa for the Pleißing nappe and 600-640 °C / ca. 0.5-0.8 GPa for the Pulkau nappe. Two phase garnets occur in some grt-metapelite from the Pleißing nappe. Garnet cores indicate P-T conditions of ca. 550 °C / 0.5 GPa and garnet rims in equilibrium with matrix minerals give ca. 590 °C / 0.9 GPa. P-T estimates along the Pulkau valley are 590-630 °C and 0.8-0.9 GPa for the Pleißing nappe and 540-580 °C / 0.5-0.8 GPa for the Pulkau nappe.

U-Pb dating of monazite by LA-ICPMS yielded a metamorphic age of 331 ± 2Ma , similar to the younger monazite age group of Štípská et al. (2015). The Th-U-Pb ages obtained by electron microprobe are slightly younger at 317 ± 6 Ma, which may be related to Pb loss during monazite recrystallization. Zircons from two meta-tuffite samples, one from the Pleißing, the other from the Pulkau nappe yielded concordant ages of 587 ± 8 Ma and 592 ± 8 Ma, respectively, similar to the magmatic age of the Biteš gneiss. The sample from the Pleißing nappe contained in addition a second age group with 620 ± 8 Ma.

Based on our geothermobarometric results the Pleißing and Pulkau nappe experienced a nearly identical metamorphic overprint in the northern part of the Thaya window. A slight decrease in metamorphic grade is seen in the southern part of the Pulkau nappe, suggesting a decreasing metamorphic gradient from NE to SW.

Štípská, P., Hacker, B. R., Racek, M., Holder, R., Kylander-Clark, A. R. C., Schulmann, K., & Hasalová, P. (2015). Monazite dating of prograde and retrograde P–T–d paths in the Barrovian terrane of the Thaya window, Bohemian Massif. Journal of Petrology, 56(5), 1007-1035.