The tectonic significance of pre-Scandian 40 Ar/ 39 Ar phengite cooling ages in the Caledonides of western Norway
| Autor: | Arild Andresen, Henry N. Berry, Torgeir B. Andersen, Daniel R. Lux |
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| Rok vydání: | 1998 |
| Předmět: | |
| Zdroj: | Journal of the Geological Society. 155:297-309 |
| ISSN: | 2041-479X 0016-7649 |
| DOI: | 10.1144/gsjgs.155.2.0297 |
| Popis: | Pre-Silurian continental-margin deposits in western Norway, non-conformably overlying allochthonous continental orthogneisses retain Ordovician 40 Ar/ 39 Ar cooling ages for phengites, implying either rapid cooling immediately after a Late Ordovician orogenic event, or less likely, a slow cooling following an Early Ordovician or older orogeny. The Dalsfjord Suite–Hoyvik Group basement–cover pair are probably a lateral equivalent to Late Proterozoic sandstones (‘sparagmites’) covering the Jotun Nappe gneisses of the Middle Allochthon in central-south Norway. The Hoyvik Group underwent polyphase deformation, greenschist-facies metamorphism ( T max 40 Ar loss probably occurred during subsequent Scandian deformation and late to post-orogenic extension.The Hoyvik Group rocks were unroofed before the Wenlock time (423–428 Ma) and cooled through the temperature for argon retention in phengite at c . 447±4 Ma, indicating a maximum cooling rate between 14 and 22C/Ma- 1 through Ashgill and Llandovery times before being subjected to low-grade metamorphism during the Scandian orogeny. Rapid pre-Scandian cooling, combined with peak metamorphic conditions of 450C or less, may indicate that the Dalsfjord–Hoyvik basement–cover pair were aVected by an orogenic event during the Late Ordovician (Caradoc) time. The data also suggest that the Caledonian margin of Baltica may have experienced a more protracted tectonism during the Caledonian cycle than previously models focusing on Early Caledonian and Tremadoc (or older) ophiolite obduction and the Scandian continental collision between Baltica and Laurentia. |
| Databáze: | OpenAIRE |
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