40Ar/39Ar geochronology across the mylonite zone and the southwestern granulite province in the Sveconorwegian orogen of S Sweden

The crustal segment between the Mylonite Zone (MZ) and the Protogine Zone in southern Sweden (the Southwestern Granulite Province, SGP) is characterized by numerous occurrences of granulite and upper amphibolite-facies rocks including migmatitic gneisses, mafic granulites, garnet amphibolites and charnockites. The southwest part of the Mylonite Zone (north of Varberg) marks a tectonic upper boundary of the SGP. A 40Ar/39Ar study was initiated across the Mylonite Zone and within the SGP in order to constrain and compare the ages obtained from within the SGP with those obtained north of and within the Mylonite Zone. 23 hornblendes and one muscovite have been analyzed. The results of these analyses indicate the following. (1) Hornblendes from within and near the MZ yield 40Ar/39Ar hornblendes ages of similar to 915 Ma. These ages are interpreted to date the dynamic crystallization or cooling below 500 degrees C of the deformation associated with the MZ. (2) Away from the MZ there is a distribution of ages between those which yield younger Sveconorwegian ages of 931-934 Ma, and those which yield older Sveconorwegian ages generally between 960 and 1007 Ma. The younger age may be associated with orogenic uplift, while the older one may be associated with an earlier Sveconorwegian crustal thickening event. (3) The one muscovite age of 904 Ma would indicate a late cooling rate of at least 6-15 degrees C/Ma. Cooling rates of this magnitude may suggest that the final uplift of the Sveconorwegian orogen was related to large-scale crustal extension.