DUNKS AND REDUNKS: REPEATED SUBDUCTION OF HP/UHP TERRANES IN THE WESTERN GNEISS REGION OF THE SCANDINAVIAN CALEDONIDES

W. Gary Ernst pioneered a model for the development of high pressure (HP) and ultrahigh pressure (UHP) crustal terranes involving subduction of slabs of continental crust into the mantle to depths that cause the formation of eclogite-facies mineral assemblages. Crustal subduction juxtaposes the crustal slab with the deep mantle so that peridotite can be transferred form the mantle into the subducted slab to form the "orogenic" or "alpine type peridotite" bodies that are characteristic of HP/UHP terranes. The crustal terranes, less dense than the surrounding mantle, ultimately return buoyantly towards the surface along essentially the same route they went down (Ernst's "two way street") to generate the high-grade core zones of mountain systems. Some mountains, such as the Caledonides of Scandinavia, contain several HP/UHP terranes of different ages, implying several episodes of crustal subduction during the evolution of the orogen. Surprisingly, some HP/UHP terranes may have been subducted more than once, potentially recording two generations of eclogite-facies assemblages and entraining two sets of mantle-derived peridotite bodies. For example, the Western Gneiss Region contains eclogite and peridotite at several different lithotectonic levels including the basement gneiss complex and units correlated with the middle(?) and upper allochthons of Sweden and eastern Norway. The eclogite is assumed to have formed during the subduction of the WGR into the mantle during the 400-420 Ma Scandian Orogeny. However, both 450 and 500 my old eclogite facies assemblages occur in the upper allochthon (i.e. the Seve Nappe) in Sweden; they could also have developed at these times in the correlative units of the WGR especially at 450 Ma since there is evidence for a Taconic event within the WGR. Some confusing radiometric dates and complex mineral textures from WGR eclogite would be explained by postulating "dunks" of the upper allocthon into the mantle at 450 Ma, followed by a "dunk" of the WGR and "redunk" of the associated allochthons into the mantle during the Scandian Orogeny. This scenario is consistent with evidence that eastern Iapetus underwent successive accretionary events that maintained a consistent geometry of lower plate subduction to the west followed by lower plate eduction to the east (present coordinates).