THE TECTONIC SIGNIFICANCE OF PRE-SCANDIAN AR-40 AR-39 PHENGITE COOLING AGES IN THE CALEDONIDES OF WESTERN NORWAY/

Pre-Silurian continental-margin deposits in western Norway, non-confor mably overlying allochthonous continental orthogneisses retain Ordovic ian Ar-40-Ar-39 cooling ages for phengites, implying either rapid cool ing immediately after a Late Ordovician orogenic event, or less likely , a slow cooling following an Early Ordovician or older orogeny. The D alsfjord Suite-Hoyvik Group basement-cover pair are probably a lateral equivalent to Late Proterozoic sandstones ('sparagmites') covering th e Jotun Nappe gneisses of the Middle Allochthon in central-south Norwa y. The Hoyvik Group underwent polyphase deformation, greenschist-facie s metamorphism (T-max <450 degrees C) and exhumation prior to depositi on of the unconformably overlying Wenlockian continental-margin deposi ts of the Herland Group. The Hoyvik Group was only weakly metamorphose d during obduction of the Solund-Stavfjord Ophiolite and the Scandian continental collision between Baltica and Laurentia. Phengitic white m icas from the Hoyvik Group yield cooling ages of 446.1 +/- 3.0, 449.1 +/- 2.2 and 447.5 +/- 4.0 Ma, respectively, identical within experimen tal error. One sample gives a plateau over 72% of the gas analysed, wh ereas the other samples were slightly disturbed after initial cooling, as indicated by systematically lower apparent ages at low experimenta l extraction temperatures. Minor Ar-40 loss probably occurred during s ubsequent Scandian deformation and late to post-orogenic extension. Th e Hoyvik Group rocks were unroofed before the Wenlock time (423-428 Ma ) and cooled through the temperature for argon retention in phengite a t c. 447 +/- 4 Ma, indicating a maximum cooling rate between 14 and 22 degrees C/Ma(-1) through Ashgill and Llandovery times before being su bjected to low-grade metamorphism during the Scandian orogeny. Rapid p re-Scandian cooling, combined with peak metamorphic conditions of 450 degrees C or less, may indicate that the Dalsfjord-Hoyvik basement ove r pair were affected by an orogenic event during the Late Ordovician ( Caradoc) time. The data also suggest that the Caledonian margin of Bal tica may have experienced a more protracted tectonism during the Caled onian cycle than previously models focusing on Early Caledonian and Tr emadoc (or older) ophiolite obduction and the Scandian continental col lision between Baltica and Laurentia.