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

Two NW-SE trending profiles across the Pleiing 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 Pleiing 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 Pleiing 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 Pleiing 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 Pleiing 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 tpsk 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 Pleiing, 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 Bite gneiss. The sample from the Pleiing nappe contained in addition a second age group with 620 8 Ma.Based on our geothermobarometric results the Pleiing 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.tpsk, P., Hacker, B. R., Racek, M., Holder, R., Kylander-Clark, A. R. C., Schulmann, K., Hasalov, P. (2015). Monazite dating of prograde and retrograde PTd paths in the Barrovian terrane of the Thaya window, Bohemian Massif. Journal of Petrology, 56(5), 1007-1035.