AR-40 AR-39 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 Z one in southern Sweden (the Southwestern Granulite Province, SGP) is c haracterized by numerous occurrences of granulite and upper amphibolit e-facies rocks including migmatitic gneisses, mafic granulites, garnet amphibolites and charnockites. The southwest part of the Mylonite Zon e (north of Varberg) marks a tectonic upper boundary of the SGP. A Ar- 40/Ar-39 study was initiated across the Mylonite Zone and within the S GP in order to constrain and compare the ages obtained from within the SGP with those obtained north of and within the Mylonite Zone. 23 hor nblendes and one muscovite have been analyzed. The results of these an alyses indicate the following. (1) Hornblendes from within and near th e MZ yield Ar-40/Ar-39 hornblendes ages of similar to 915 Ma. These ag es are interpreted to date the dynamic crystallization or cooling belo w 500 degrees C of the deformation associated with the MZ. (2) Away fr om the MZ there is a distribution of ages between those which yield yo unger Sveconorwegian ages of 931-934 Ma, and those which yield older S veconorwegian ages generally between 960 and 1007 Ma. The younger age may be associated with orogenic uplift, while the older one may be ass ociated with an earlier Sveconorwegian crustal thickening event. (3) T he one muscovite age of 904 Ma would indicate a late cooling rate of a t least 6-15 degrees C/Ma. Cooling rates of this magnitude may suggest that the final uplift of the Sveconorwegian orogen was related to lar ge-scale crustal extension.