P-T-t-EVOLUTION OF VARISCAN REMNANTS IN THE EASTERN ALPS: THE KAINTALECK METAMORPHIC COMPLEX

The Eastern Greywacke Zone is composed of three Alpine nappes. From bottom to top these are (1) the Veitsch nappe (Early Carboniferous to Permian molasse), (2) the Silbersberg nappe with intercalated slivers of the Kaintaleck Metamorphic Complex and Permian phyllites and conglomerates as cover, and (3) the Noric nappe (mainly Ordovician to Devonian shelf sediments and Permian cover). All units experienced Eo-Alpine lower greenschist facies metamorphism. Due to the development of ductile shear zones during Alpine nappe stacking, the Kaintaleck Complex was dismembered and emplaced as lens-shaped bodies of 10-100m thickness that stretch from West (Kalwang, Upper Styria) to East (Gloggnitz, Lower Austria) below the Noric nappe of the Eastern Greywacke Zone. Lithologically, the Kaintaleck Complex is represented by a mafic suite, comprising amphibolite, garnet-amphibolite, greenschist and serpentinite, and a felsic suite that consists mostly of gneiss and mica-schist (some of them garnet-bearing). The felsic suite corresponds to metamorphosed clastic sediments and granitoids, whereas the mafic suite represents most likely a former oceanic crust. This work tries to constrain the P-T-t path of the Kaintaleck Metamorphic Complex by applying U-Th/Pb monazite and zircon dating and geothermobarometry. Based on whole rock geochemistry, amphibolites from the locality of Frauenberg represent tholeiitic basalts with an E-MORB affinity, whereas garnet-amphibolites, amphibolites and greenschists from the localities of Prieselbauer, Oberdorf, Unteraich, Kalwang, Arzbach and Schlöglmühl show a T-MORB signature. Samples from the localities of Stübminggraben and Utschgraben have a N-MORB affinity. Garnet-amphibolite samples show distinct plagioclase-epidote-rich symplectitic coronae, which are indicative of decompression from former eclogite-facies conditions. P-T estimations based on Zr-in-rutile thermometry and phengite barometry yield up to 720°C and 19 kbar for the felsic suite, and 700°C and 21 kbar for the mafic suite, both interpreted as peak metamorphic conditions. Monazite dating by EPMA in garnet-mica-schist from the localities of Prieselbauer, Arzbach, Schlöglmühl and Oberdorf, revealed weighted average U-Th-total Pb dates of 351 ± 4 Ma, 358 ± 16 Ma, 349 ± 3 Ma and 362 ± 6 Ma, which are interpreted as reflecting peak Variscan metamorphism. Monazite in these samples is partly replaced by an apatite-allanite-corona, related to monazite-breakdown due to Alpine lower grade metamorphic overprint. Preliminary LA-MC-ICP-MS U/Pb age dating results of zircon grains from a garnet-amphibolite from the Prieselbauer locality yield a Devonian mean date of 400 ± 4 Ma ascribed to the protolith formation.