Abstract
The pre-Alpine basement of the Eastern Alps contains several ultramafic/mafic rock complexes, four of which have been investigated. Although polyphase deformation and metamorphism obscures much of the primary features, field, petrographical and geochemical evidence favours ophiolitic origins in mid-ocean ridge and back-arc tectonic settings. The Stubach Complex belongs to the Penninic basement of the Tauern Window and is interpreted as an obducted back-arc ophiolite with ultramafic cumulates, gabbro, and minor amounts of basalt, which shows the geochemistry of subduction-related magma generation. The Rating Complex belongs to a group of strongly dismembered basement slices carrying the Palaeozoic Austroalpine sequence of the Northern Greywacke Zone. Basaltic rocks are accompanied by former metamorphic peridotite containing chromite concentrations. Nearly all members of an ophiolite suite are present in the Speik Complex in the Austro-Alpine crystalline basement. The source rocks are metamorphic peridotite (harzburgite) with chromite occurrences, cumulate ultramafic and mafic rocks, isotropic gabbro, basalt and/or dolerite, and oceanic sediments. The Plankogel Complex, also part of the Austro-Alpine crystalline basement, represents a tectonic mélange. Blocks of serpentinite and amphibolite derived from metamorphic peridotites and mid-ocean ridge basalts, together with marble and quartzite, are contained in a micaschist matrix. Admixed is a rock assemblage with alkali basaltic amphibolites, interpreted as a seamount environment. The Ritting, Plankogel, and possibly parts of the Speik Complexes contain relict high-pressure minerals and therefore went through a stage of subduction. The inferred ages of all complexes is Late Precambrian to Early Palaeozoic. The ophiolite suites have their distinct characteristics and contrasting tectonic settings, and are therefore considered to be genetically unrelated to each other.
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