Pre-Alpine Metamorphism in Alpine low-grade metamorphic units in the Eastern Alps

Pre-Alpine basement units, derived from the northeastern Gondwana margin, are incorporated within the Eastern Alps and were overprinted during Alpine nappe stacking. However, some of these units were only slightly affected by Alpine metamorphism and could therefore provide significant information about the pre-Alpine history of these basement units. The Kaintaleck Metamorphic Complex as part of the Eastern Greywacke Zone and the Silvretta-Seckau Nappe System experienced greenschist facies metamorphic conditions during Eo-Alpine times and are nowadays affiliated to the Upper Austroalpine Subunit.

The Kaintaleck Metamorphic Complex comprises a mafic suite of amphibolite, garnet-amphibolite, greenschist and serpentinite, and a felsic suite, mainly composed of gneiss and mica-schist, some of them garnet-bearing. Geochemical results of metabasites indicate a tholeiitic basalt source with MORB affinity. U-Pb zircon dating of a garnet-bearing amphibolite yields an Early Devonian age of 414 ± 5.6 Ma, interpreted as age of protolith formation. Chemical U-Th-Pb dating of monazites from the felsic suite revealed Late Devonian to Early Carboniferous ages of 362 ± 6 Ma, 358 ± 15 Ma, 351 ± 4 Ma and 349 ± 3 Ma, reflecting peak metamorphic conditions during Variscan orogeny. A two-stage metamorphic history of a HT/LP and a subsequent LT/HP metamorphic event, indicated by Zr-in-rutile thermometry and thermodynamic modeling, relates the Kaintaleck Metamorphic Complex to the opening and closure of the short-lived Balkan-Carpathian Ocean and implies a correlation to other ophiolitic relicts of Devonian age, exposed in the North-Gemeric Klatov and Rakovec Complexes in the Western Carpathians. The Seckau Complex, a part of the Silvretta-Seckau Nappe System is characterized by various metagranitoids, which have been extensively analyzed in recent studies. Based on these studies, the metagranitoids of the Seckau Nappe are subdivided into the Late Cambrian to Early Ordovican Hochreichart Plutonic Suite and the Late Devonian to Early Carboniferous Hintertal Plutonic Suite. The host rock for these large intrusions is the so-called Glaneck Metamorphic Suite, which is mainly composed of fine-grained paragneiss and mica-schist, some of them garnet-bearing. U-Pb zircon ages of the paragneisses indicate a detrital origin and ages of the cores cluster in the Neoarchean, Paleoproterozoic and Ediacaran, between 2.7 Ga and 559 Ma. A migmatized paragneiss yields an age of 505 Ma, which indicates, that migmatization was probably triggered by the intrusion of the Hochreichart Plutonic Suite. The timing of pre-Alpine metamorphism can therefore be constrained to have happened between 559 Ma and 505 Ma. Some samples do show a distinct two-phase garnet growth, suggesting an additional metamorphic event possibly during Variscan times. The Schladming Crystalline Complex, also part of the Silvretta-Seckau Nappe System, again comprises paragneisses, that were intruded by various metagranitoids. In contrast to the Seckau Complex, these metagranitoids do not only show Cambrian and Late Devonian to Early Carboniferous ages, but also Permian ages.

In order to complement the knowledge of the pre-Alpine metamorphic history of the Eastern Alps, new geochronological, geochemical and geothermobarometric data from various metapelitic and metabasic rocks within the Silvretta-Seckau Nappe system are being examined to reconstruct the tectonic development of these units in pre-Alpine times.