RECONSTRUCTING THE SCANDINAVIAN CALEDONIDES: GEOCHEMICAL DISTINCTIONS BETWEEN THE BLÅHØ NAPPE OF CENTRAL NORWAY AND THE SEVE NAPPE OF WESTERN SWEDEN AND ADJACENT EASTERN NORWAY

The Scandinavian Caledonides can be thought of as a stack of four thrust-bounded packages, the Lower, Middle, Upper, and Uppermost Allochthons. The materials for these were derived from regions progressively farther outboard from Baltica, which developed and were assembled in different parts of Iapetus and on the Iapetan margins. They were thrust onto Baltica during the Silurian-early Devonian Scandian collision between Laurentia and Baltica. The intensely deformed Blåhø Nappe of central Norway has long been correlated with the Seve Nappe (Middle Allochthon), which is exposed over a length of 1000 km in the orogen. Both are in the same tectonostratigraphic position, have a similar range of lithologies, and have been metamorphosed to mostly amphibolite facies with local eclogites and garnet peridotites. The Seve Nappe contains slices of Baltican crust, Baltica-derived clastic sediments, and dike swarms associated with latest Proterozoic rifting of Rodinia. The Seve Nappe has two eclogite formation age populations: late Cambrian–early Ordovician, and middle to late Ordovician, but the Blåhø Nappe has only Scandian eclogites, as also do underlying rocks in the Middle and Lower Allochthons. In addition, metamorphosed igneous rocks of the Blåhø Nappe geochemically appear to be from an oceanic arc–back-arc system, different than a rift environment for late Proterozoic and younger Seve Nappe igneous rocks. Our model derives the Seve Nappe from the distal Baltican margin and detached outboard fragments. Late Cambrian–Ordovician west-directed subduction of these, near the Baltican margin, produced the older Seve eclogites. Most models of other workers are similar. However, we suggest a split (or scissors-like) subduction zone, collapsing earlier in the north and later, and developing a mature arc–back-arc system, in the central Norway region. The Blåhø Nappe material formed on the upper plate of this subduction zone, and merged with Baltica during the middle Ordovician. During convergence and merger, it received sediments with zircon populations of Baltican affinity. Having been on the upper plate, the Blåhø Nappe contains no Ordovician or older eclogites. During the Scandian collision, the Blåhø Nappe was part of the Baltican margin (the lower plate), and thus contains Scandian eclogites.