Formula omitted] mineral age record of a polyorogenic evolution within the Seve and Köli nappes, Trøndelag, Norway

Abstract

In the central 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. Hornblende within amphibolite at one locality within westernmost exposures of Seve Nappe units in Trandelag, Norway, displays an internally discordant 40Ar 39Ar age spectrum which suggests a polyphase thermal evolution including: 1. (1) initial post-metamorphic cooling through argon retention temperatures at c. 475–480 Ma; 2. (2) partial rejuvenation during a Silurian (c. 400–425 Ma) thermal overprint. Muscovite within the exposure was more extensively rejuvenated during the Silurian overprint. At twelve other Trandelag sample locations, hornblende, muscovite and biotite within both the Seve and Köli Nappe Complexes record post-metamorphic cooling ages between 420 and 435 Ma and display no direct evidence of a pre-Silurian thermal record. However, the widespread occurrence of intracrystalline extraneous argon components and textural complexities clearly point toward a pre-Silurian tectonothermal history. Comparison with 40Ar 39Ar mineral ages reported elsewhere in the central Scandinavian Caledonides suggests a polyorogenic evolution consisting of: 1. (1) variable high-pressure metamorphism of distal Baltoscandian miogeoclinal sequences during entrainment in an accretionary wedge which developed in the Late Cambrian-Early Ordovician over a west-dipping subduction zone; related arc volcanic activity occurred in more outboard, western sectors; 2. (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. (3) ultimate accretion of a variety of eugeoclinal terranes along the Baltica continental margin; 4. (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. Crystalline Baltica basement and overlying structural cover units were depressed to significant crustal depths during eastward nappe transport with resultant formation of high-temperature eclogues. Previously reported crystallization ages of c. 425 Ma suggest that the eclogites formed at approximately the same time as minerals within the superencumbent nappe units cooled through argon closure temperatures. Following eastward nappe transport, the basement experienced rapid uplift and resultant adiabatic decompression. This was followed by rapid cooling through argon retention temperatures at 390–400 Ma.