EARLY PALEOZOIC EVENTS AT THE GONDWANA MARGIN

The comparison of pre-Variscan basement relicts hidden in the Variscan mountain chain of Europe, the Alps included, brings evidence (von Raumer 1998, von Raumer et al. 2001), that most of the pre-Variscan basement units originated, before the Ordovician, at the Gondwana margin, in the former eastern prolongation of future Avalonia, and had an identical evolution until the break-off of Avalonia.

A Late Proterozoic active margin setting containing volcanic arc sequences in the entire length of the future micro-continents at the Gondwanan border, and the many Late Cadomian granites may indicate a plate tectonic evolution leading to a cordillera stage. Drift of Avalonia and opening of the Rheic ocean were enhanced after the subduction of the mid-oceanic ridge of the ProtoTethys, characterized by large-scale magmatic pulses around 500 Ma indicating a rising thermal activity.

In the eastern continuation of Avalonia, only early stages of the Rheic ocean may have existed. Drifting was hampered by the still existing oceanic ridge whose collision with the detaching terranes triggered the consumption of this embryonic eastern Rheic ocean and the amalgamation of volcanic arcs and continental ribbons with Gondwana in a rather short lived orogenic pulse. Chemical evolution of granitoids is the mirror of the general evolution from Cambrian rifting to Cambro-Ordovician active margin-setting and Ordovician amalgamation.

The cordillera, resulting from mid-ocean ridge subduction during the Ordovician, started to collapse already during the Late Ordovician leading to the opening of the PaleoTethys rift, followed by the Late Silurian drift of the composite Hun-superterrane. Well known from the Alpine basement areas, few relicts of this episode, either sedimentation in a back-arc-situation, or the Late Ordovician/Early Silurian active margin setting are known, but they have still to be identified in many basement areas.

The complex Variscan evolution since the Silurian (Stampfli et al. 2001) includes a Devonian collision between terranes derived from Laurussia and Gondwana and ended with the Late Carboniferous final collision of Gondwana with Laurussia, leading to the complex Late Variscan structures known from the European part of the orogen.

Summarizing, striking parallels of pre-Silurian evolution show that the pre-Variscan elements of Europe had a comparable lateral location at the Gondwana margin. They may contain Cadomian basement and related Late Proterozoic detrital sediments and volcanic arcs, relicts of a Rheic ocean, Cambro-Ordovician accretionary wedges, relicts of an Ordovician orogenic event and related granites, and volcanites and sediments indicating the opening of PaleoTethys, as well as active margin settings during the Silurian, involving the Rheic-ProtoTethys southward subduction.

Stampfli G M, von Raumer J F, Borel G, 2001, From peri-Gondwana to Variscan collision. The Palaeozoic evolution of pre-Variscan relicts in the Variscan domain of Europe – a discussion, Special Papers (GSA), submitted

von Raumer J F, 1998, The Palaeozoic evolution in the Alps - from Gondwana to Pangea. Geologische Rundschau, 87, p. 407-435

von Raumer J F, Stampfli G M, Borel G, Bussy F, 2001, The organization of pre-Variscan basement areas at the Gondwana margin, International Journal of Earth Sciences, 90, (online published)