DERIVATION OF 500-MA ECLOGITES FROM THE PASSIVE MARGIN OF BALTICA ANDA NOTE ON THE TECTONOMETAMORPHIC HETEROGENEITY OF ECLOGITE-BEARING CRUST

Several recent plate reconstructions of the Iapetus Ocean describe the margins of Baltica as passive until Silurian collision with Laurentia . Yet there is a variety of evidence to suggest that the accretion of the Scandinavian Caledonides began by latest Cambrian-early Ordovician subduction and imbrication of the passive continental margin. One suc h evidence is provided by eclogites occurring in the Seve Nappe Comple x. Previous work by others dated the high-pressure metamorphism at 503 +/-14 Ma (Sm-Nd garnet-omphacite age), and the uplift through the c. 5 00 degrees C isotherm at 491+/-8 Ma (Ar-40/Ar-39 hornblende plateau ag es). The protolith dolerites of the eclogites have been correlated wit h Iapetan rift-facies dolerites of the Baltoscandian margin. If valid, such a correlation implies early Caledonian destruction of the margin , and thus modification of those plate reconstructions which require p assive margins around Baltica in latest Cambrian-early Ordovician time . This paper provides a substantially improved basis for the concept t hat the protoliths of eclogites and their host rocks derived from Balt oscandian rift basins. The chemical similarity between coronitic doler ites and dolerites of the rift basins pertains not only to element con centrations and variations but also to the specific T-MORB signature s hared by the two groups. The variation of psammitic and pelitic schist s, graphitic schists, calc-silicate gneisses and marbles of the eclogi te host rocks equates with sequences of sandstones, siltstones, shales , black shale, quartzite, dolomite and limestones of Baltoscandian pal aeobasins. At the same time, the paper calls attention to the remarkab le preservation of structural and metamorphic contrasts within the ecl ogite-bearing thrust sheets of the Seve Nappe Complex. Such disequilib rium is generally ascribed to the kinetics of localized deformation an d fluid infiltration into dry crust. This paper presents evidence that disequilibrium is found also within inferred subducted sedimentary co mplexes, which are generally assumed to be pervasively flushed by flui ds. Preservation of sedimentary, volcanic and magmatic structures and fabrics,and of both underformed dolerite dykes and eclogitized dykes d emonstrates that neither deformation nor high-pressure metamorphism we re pervasive.