PRE-SCANDIAN AMALGAMATION OF ROCKS NOW EXPOSED IN THE UPPER AND UPPERMOST ALLOCHTHONS OF THE SCANDINAVIAN CALEDONIDES: EVIDENCE AND INTERPRETATIONS IN RELATION TO LAURENTIA AND GANDERIA

The Caledonides of Scandinavia and the British Isles have their counterparts in the Appalachians and elsewhere. They are a complex assemblage of continental margins, MORB ocean floor, and oceanic and continental arcs. Assembly took place via a series of collisions in different regions at times ranging from Middle Ordovician to the culminating Silurian–Early Devonian Scandian collision. The Scandian Upper and Uppermost Allochthons (UA and UmA) in mid Norway hold a diverse record of Iapetan arcs, fossils, sedimentation, and collisions that point to improved tectonic reconstructions. Major belts from southeast to northwest include: 1) The Meråker Nappe (UA) consisting of ~Late Cambrian to ~Middle Silurian sedimentary, volcanic, and intrusive rocks, and MORB-like ophiolites near its eastern margin. A serpentinite conglomerate unconformable on the ophiolite contains a Mid Ordovician cold-water Celtic fauna like that on the Penobscot Unconformity, eastern Maine, considered diagnostic of the Gander terrane in eastern Canada. 2) The Gula Nappe (UA) consists mostly of continental margin clastic rocks of uncertain age, possibly developed on a high-latitude peri-Gondwanan continental fragment. 3) The Støren Group (UA), composed mostly of 500-480 Ma basaltic rocks of oceanic arc ophiolites with arc tonalites, is overlain unconformably by the Lower Hovin group with a Middle Ordovician warm-water Laurentian fauna. This arc was evidently accreted onto the Laurentian margin before 470 Ma to accommodate the overlying sediments, and 467-438 Ma continental arc plutonic rocks (4) that include abundant adakites, a magma type probably produced within deep continental crust. In the UmA, these plutons also cut Laurentian margin carbonates as old as Late Proterozoic. Subduction associated with that arc juxtaposed the Meråker (1) and Gula (2) Nappes with the Støren Group (3) on the Laurentian margin by ~438 Ma. New subduction beneath Laurentia emplaced 439-424 Ma plutons into the Meråker, Gula, and Støren, close to onset of the Scandian Baltica-Laurentia collision. This transferred the Meråker (1), Gula (2), Støren (3), and continental arc plutons (4) onto Baltica as the Upper Allochthon, and a more inboard part of the Laurentian margin, with its carbonate rocks and identical arc plutons, onto Baltica as the Uppermost Allochthon.