Geology of part of the Altkristallin sheet in the Eastern Alps
The study area essentially lies within a suture zone between the African and European plates and represents the lower part of the African plate. The African plate is made up of crystalline basement rocks of Palaeozoic and older age,referred to as the Altkristallin, and their Mesozoic cover-rock sequences. At present the Altkristallin forms an enormous thrust sheet resting on Mesozoic pelagic and epicontinental metasediments, trapped between two continental masses. In this study phases of deformation and metamorphism, constrained by K/Ar and Rb/Sr ages, are related to the major phase of emplacement of the Altkristallin. The Altkristallin in the study area is comprised of two distinct units separated by a major mylonite zone (MMZ) which trends west-east and is deformed by upright SE-plunging folds (Dq). The mylonite zone was originally a low-angle fault formed near the brittle / ductile transition zone. Microstructures suggest that after removing the effects of Dq the original upper block to the mylonite shear zone moved NW (transport direction 300 ± 40^o). The lower tectonic unit within the Altkristallin lies north of the mylonite zone, is about 2km thick and has been named the Polinik Unit. This unit is characterised by ortho and para-gneisses, metapelites and pods of eclogite-amphibolites. Previous workers have established that the micas within this unit yield uniform K/Ar mica cooling-ages clustering around 80 Ma. In the aluminous metapelites 80 Ma micas form equilibrium assemblages with kyanite, staurolite, garnet, plagioclase and quartz. The peak metamorphic conditions deduced from Polinik Unit metapelites are estimated with 620±60^oc and 6.25 ±1.25 kbars and occurred at c. 105 Ma. The temperature estimates obtained from garnet - biotite pairs tentatively suggest an inverted thermal gradient in the upper part of the Polinik Unit along the Polinik hut - Mt. Polinik section. The high-temperature mineral assemblages are deformed (D_3). D_3 minor fold asymmetries suggest a large flat-lying recumbent fold trending approximately east-west and closing towards the south. The inverse geothermal gradient is located on the inverted upper limb of this fold structure. It is concluded that the Polinik Unit was emplaced as a fold nappe shortly after 80ma. The Polinik Unit eclogite pods show a distinct zonation and mineral textures which demonstrate the gradual replacement of the primary eclogite assemblage (garnet-omphacite-quartz-rutile) by hydrous amphibolite facies mineral assemblages. P/T estimates deduced from the least affected primary assemblage suggest pressures in excess of 1lkbars and temperatures of 600±50oC. Late stage amphibolite facies hydrous alterations formed under similar P/T conditions to those determined from the Polinik Unit metapelites. The age of the high-pressure (eclogite facies) metamorphism is not known. It is postulated that the eclogite amphibolites and metapelites have suffered a common history of Cretaceous high pressure metamorphism; the eclogite amphibolites retained the highest pressure assemblages, while the metapelites retained their most dehydrated assemblage, which formed at intermediate pressures and peak temperatures and provided the necessary fluids to facilitate the hydrous alterations in the eclogite amphibolites. The rocks above the mylonite zone (MMZ) and the mylonites themselves are called the Strieden Unit. This unit is comprised of augengneisses, pegmatites, amphibolites, schists and marbles in the MMZ and mainly metapelites, a few marble- and amphibolite-bands and pegmatites (restricted to the sillimanite-zone) in the higher part.