IAPETUS CLOSURE – VIEW FROM BALTICA

For many years, the Western Gneiss Region (WGR) of southwestern Norway in the Scandinavian Caledonides has been known for the remarkable occurrence of HP and UHP metamorphism. This evidence of mostly Early Devonian subduction has been inferred to result from the underthrusting of the outer margin of Baltica beneath Laurentia, after a long period of convergence of the two continents. More recently, UHP metamorphism has been recognized farther east and at higher structural levels in the orogen in the Caledonian allochthons where eclogites have been reported previously. Existing age and PT data indicate that this subduction occurred in the Ordovician to earliest Silurian during the c. 60 million years of closure of the Iapetus Ocean and initial collision of the continents.

New evidence for the Ordovician UHP metamorphism has been documented in the Seve Nappe Complex (SNC) of the Middle Allochthon and the Tromsø Nappe (TN) of the Uppermost Allochthon. In both cases UHP parageneses were documented not only in the eclogites, but also in the metasediments of continental affinity. This UHP metamorphism in the SNC and TN was coeval and of similar PT conditions, thus it is possible that TN belongs rather to the Baltoscandian margin than the Laurentian. In any case, this evidence for UHP metamorphism in the allochthons suggests that the continental margin was subducted down to at least 100 km already back in the Ordovician.

Of particular importance are also the (U)HP rocks occurring within the "parautochthonous" basement and yielding pre-Devonian ages. Such an eclogite province is located in the Lofoten Islands in the northern part of the orogen. The exact age of these eclogites is not known; however available age data suggest an Ordovician timing of crystallization. Much farther south, in the north-easternmost WGR, the Vindøladalen eclogite, located within a basement-derived allochthon, has yielded a mid-late Silurian age, which is still significantly older than most of the WGR rocks. Both, the Lofoten and Vindøladalen eclogites show that the Baltican continental crust was subducted to the great depths earlier than previously thought. Attenuation of the SNC and underlying allochthons towards the hinterland and their presence within the WGR may well explain the occurrences of "anomalous" pre-Devonian ages in southwestern Norway.