MID ORDOVICIAN TO EARLY SILURIAN GRANITOID PLUTONISM IN THE SCANDINAVIAN CALEDONIDES

The Scandinavian Caledonides, extending ca 2000 km along strike, are composed of a series of thrust complexes that were juxtaposed during multiple events culminating with the Laurentia-Baltica (Scandian) collision in the Early Devonian. The lowest allochthonous units originated along the margin of Baltica, whereas the highest (parts of the Upper and the Uppermost Allochthons) are of exotic origin and were probably derived from the Laurentian margin, arc complexes and unknown micro-continents. Granitoid plutons are common in many of these units, and their origin and setting are important for understanding the history of the exotic rock complexes.

An Early to Mid Ordovician (ca 465-480 Ma) magmatic and tectonometamorphic event is well documented in the Karmøy-Bergen area (southern Norway) and in the Helgeland Nappe Complex (Uppermost Allochthon, north-central Norway). Granitoids related to this event are peraluminous and essentially produced during high-grade metamorphism and anatexis of crustal protoliths during collision and amalgamation of continental fragments above an east-dipping (present coordinates) subducting slab near the Laurentian margin. The north-western parts of Scottish and Irish Caledonides, which were derived from the Laurentian margin, experienced time-equivalent deformation, metamorphism and magmatism, suggesting a possible link with exotic units of the Scandinavian Caledonides.

In the Late Ordovician to Early Silurian (ca 450-430 Ma) a gabbroic to granitic plutons were emplaced into the earlier assembled oceanic and continental rock units. The plutons show evidence of mixed crust and mantle sources and probably represent continued magmatism along the Laurentian margin.

The Ordovician to Early Silurian evolution of the exotic rock units in the Scandinavian Caledonides may be regarded as Taconian. Evidence of Mid to Late Ordovician tectonic events, including high-pressure metamorphism, have been recorded in different parts of the upper nappe units of the Scandinavian Caledonides and may relate to contrasting tectonic settings. These events pre-date eastward thrusting across subjacent nappes during Scandian contractional deformation.