Polyorogenic40Ar/39Ar Mineral Age Record in the Seve and Köli Nappes of the Gäddede Area, Northwestern Jämtland, Central Scandinavian Caledonides

Abstract

In the Scandinavian Caledonides, eugeoclinal terranes represented in the Köli Nappes were thrust >500 km onto the Baltoscandian platform during the early to middle Paleozoic. Immediately underlying thrust sheets of the Seve Nappe complex are suspected to have been derived from the outer margin of continent Baltica. Contrasting tectonothermal histories are recorded in Seve and Köli Nappes exposed in the Gäd-dede area, northern Jämtland County, Sweden. Eclogite-bearing Seve rocks underwent an initial amphibolite-grade regional metamorphism (SMI) with different structural units experiencing contrasting high-to intermediate-pressure conditions. Subsequent ductile imbrication (SM2) of high-and intermediate-pressure structural units occurred prior to 460-470 Ma cooling through temperatures required for intracrystalline retention of argon in hornblende. A later thermal overprint (SM3) everywhere rejuvenated Seve biotite and muscovite (and locally hornblende) argon systems at plateau ages of hornblende suggest attainment of peak metamorphic conditions (KM1) in the Lower and Middle Köli Nappes at ca. . Plateau dates of muscovite and biotite in both Seve and Köli units range down to 417 Ma, suggesting relatively rapid post-metamorphic cooling. Initial outboard assembly (KD1) of the Lower and Middle Köli Nappes preceded the regional metamorphic peak. Ductile translation (KD2) of the assembled Köli units occurred during maintenance of near peak KM1 metamorphic conditions. Although this thrusting was clearly later than SM2 ductile imbrication within the underlying Seve Nappe Complex, a widespread thermal overprint (SM3) appears to have accompanied structural emplacement of the Köli Nappes. Comparison with mineral ages reported elsewhere in the central Scandinavian Caledonides suggests a polyorogenic evolution consisting of: (1) variable high-pressure metamorphism of distal Baltoscandian miogeoclinal sequences during their entrainment in an accretionary wedge that developed in the Late Cambrian-Early Ordovician over a westdipping subduction zone; related arc volcanic activity occurred in more outboard, western terranes; (2) ductile imbrication of high-and intermediate-pressure sequences within the accretionary wedge (diachronous cooling and/or chronologically distinct tectonic episodes are indicated throughout the Early and Middle Ordovician); (3) ultimate accretion of a variety of eugeoclinal terranes along the Baltic continental margin; and (4) imbrication and metamorphism of the eugeoclinal terranes and previously deformed portions of the miogeocline during eastward translation onto the Baltoscandian platform in the Silurian and Devonian.