/sup 40/Ar//sup 39/Ar mineral age record of early Caledonian tectonothermal activity in the Baltoscandian miogeocline, central Scandinavia

Journal Article · Sat Jun 01 04:00:00 EDT 1985 · Am. J. Sci.; (United States)

DOI:https://doi.org/10.2475/ajs.285.6.532· OSTI ID:6441194

Dallmeyer, R D; Gee, D G; Beckholmen, M

In central portions of the Scandinavian Caledonides, greenschist facies volcanosedimentary successions within the Koeli Nappe Complex have been thrust several hundred kilometers eastward onto the Baltoscandian platform. These were derived from eugeoclinal terranes situated outboard (west) of the Baltica continent during the early Paleozoic. The Koeli Nappe Complex is tectonically underlain by higher grade units within the Seve Nappe Complex. These are composed of amphibolite and granulite facies rocks and locally contain eclogites. The Seve Nappes tectonically separate Koeli units from structurally lower allochthons derived from more inboard environments along the Baltoscandian miogeocline. Previous mineral isotopic age-determinations from Seve and Koeli units have been in the 430 to 390 Ma range and have been interpreted to presumably date cooling following Scandian (Middle Silurian to Early Devonian) metamorphism. However, incremental-release /sup 40/Ar//sup 39/Ar dates recorded by minerals within some of the Koeli and Seve Nappes exposed in Jaemtland, Sweden (Taennforsen and Are districts) provide evidence of earlier tectonothermal activity. Hornblendes from the Seve and Koeli Nappe Complexes display variably discordant age spectra as a result of low-temperature, experimental evolution of loosely bound extraneous argon components. However, in most analyses plateau ages of 510 to 475 Ma (Koeli) and 465 to 455 Ma (Seve) are defined. In contrast, muscovite and biotite from all tectonic units record Scandian cooling ages between 245 and 410 Ma. The older events recorded by hornblende within these Seve and Koeli units are evidence of early Caledonian tectonothermal activity and subsequent diachronous cooling during the Early-Middle Ordovician.

• Research Organization: Univ. of Georgia, Athens
• OSTI ID: 6441194
• Journal Information: Am. J. Sci.; (United States), Journal Name: Am. J. Sci.; (United States) Vol. 285:6; ISSN AJSCA
• Country of Publication: United States
• Language: English

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510101* -- Environment
Terrestrial-- Basic Studies-- Radiometric Techniques-- (-1989)
54 ENVIRONMENTAL SCIENCES
58 GEOSCIENCES
580100 -- Geology & Hydrology-- (-1989)
AGE ESTIMATION
AMPHIBOLITES
DATA
EUROPE
EXPERIMENTAL DATA
GEOLOGIC FAULTS
GEOLOGIC FRACTURES
GEOLOGIC STRUCTURES
GRANULITES
INFORMATION
ISOTOPE DATING
METAMORPHIC ROCKS
NORWAY
NUMERICAL DATA
OROGENESIS
ROCKS
SCANDINAVIA
SWEDEN
TECTONICS
WESTERN EUROPE