Geochemistry of granitoids from the Austroalpine Seckau Complex: a key for revealing the pre-Alpine evolution of the Eastern Alps

Abstract

Recent studies revealed that the calc-alkaline metagranitoids of the Seckau Complex comprise both (1) a Late Cambrian to Early Ordovician and (2) a Late Devonian to Early Carboniferous (early Variscan) intrusive complex. The older rocks of the Hochreichart Plutonic Suite reflect I to S-type affinity and are peraluminous and characterized by a general decrease in TiO2, Al2O3, MgO, CaO, P2O5, FeOt and MnO with increasing SiO2. Chondrite-normalized rare earth element (REE) plots display a slight enrichment in light rare earth elements (LREE) relative to heavy rare earth elements (HREE) as well as negative Eu anomalies ((Eu/Eu*)N = 0.15- 0.77). The whole-rock initial 87Sr/86Sr ratios calculated back to the time of emplacement (~ 496 Ma) vary between 0.7056 to 0.7061. The early Variscan rocks of the Hintertal Plutonic Suite can be subdivided into (a) the meta- to peraluminous granodioritic suite of the Pletzen Pluton and (b) the peraluminous granitic suite of the Griessstein Pluton. The Pletzen Pluton shows typical magmatic fractionation trends for most of the major oxides and trace elements plotted against SiO2. On a chondrite-normalized diagram, metagranitoids are strongly enriched in LREE and show no significant negative Eu anomaly. Metagranitoids of the Griessstein Pluton have a more peraluminous character and similar major and trace element fractionation trends compared to the Pletzen Pluton. However, the contents in SiO2, major and trace elements clearly point towards a more evolved melt with generally lower TiO2, Al2O3, MgO and CaO values and higher K2O content. Metagranitoids of the Griessstein Pluton are additionally characterized by a slight negative Eu anomaly of about 0.81 on a chondrite-normalized REE plot. Initial 87Sr/86Sr values calculated back to the time of emplacement (~ 353 Ma) of the Pletzen Pluton and the Griessstein Pluton vary between 0.7051—0.7061 and 0.7054—0.7063, respectively, and suggest the same magmatic source for both units. Application of rhyolite-MELTS modelling to the Hintertal Plutonic Suite revealed that the Griessstein Pluton formed by fractional crystallization (~ 30%) from the more primitive Pletzen Pluton. Our geochemical data from the Hochreichart Suite granitoids suggest the existence of a Late Cambrian to Early Ordovician active margin with its remnants now exposed in the Seckau Complex. The early Variscan grainitoids of the Seckau Complex are inferred as part of a magmatic arc along the southern Bohemian active continental margin that was related to the subduction of differently termed oceanic domains (Galicia-Moldanubian Ocean or Paleotethys), prior to the final collision of Gondwana and Laurussia. The general paleogeographic position of the Seckau Complex during the Variscan orogeny is considered to be south to southeast of the Bohemian Massif, adjacent to the eastern Hohe Tauern, the Schladming Tauern, and the Western Carpathians.