Lower Palaeozoic volcanism of the Eastern Alps and its geodynamic implications

Abstract

A geodynamic model for the lower Palaeozoic evolution of the Eastern Alps (upper Ordovician to middle Devonian; Gurktal Nappe, Styrian Greywacke Zone, Palaeozoic of Graz) is discussed, based on new field work and numerous geochemical analyses of volcanic rocks. It is suggested that in the upper Ordovician there existed on the one hand a passive continental margin or continental rift flooded by the sea, where mildly-alkaline basalts erupted (Magdalensberg-Series/Carinthia; Kaser-Series/Gurktaler Alpen). On the other hand, a convergent plate boundary in the form of an active continental margin or island arc or in the form of an orogen formed by continent-continent collision which can be recognized by the eruption of calc-alkaline volcanic rocks, probably also existed in the upper Ordovician (Blasseneck »Porphyroid«/Styrian Greywacke Zone; NockSeries/Gurktaler Alpen). In the lower Silurian a reorganization of the plate boundaries took place, resulting in the disintegration of the upper Ordovician collision zone along deep-reaching faults. Further extensional movements during the Silurian and Devonian led to the disintegration of a broad shelf platform and to the eruption of alkaline within-plate basalts. The extensional movements were responsible for the thinning of the continental crust and a general subsidence, which probably lasted until Lower Carboniferous time.