GEOCHRONOLOGY AND STRUCTURAL SETTING OF THE 1.38 GA TORPA GRANITE - IMPLICATIONS FOR CHARNOCKITE FORMATION IN SW SWEDEN

In the Varberg region of the Southwest Scandinavian Domain in SW Swede n, the megacrystic Torpa granite forms a sheet-like, partly charnockit ic intrusion in intensely migmatised gneisses. The granite has been in terpreted as part of an igneous suite, termed the Varberg Charnockite- Granite Association. The gneisses are made up of ca. 1.75-1.55 Ga old crust that has been subjected to high-grade Sveconorwegian-Grenvillian metamorphism, A U-Pb zircon age of 1380 +/- 6 Ma for a charnockitic p ortion of the Torpa granite is interpreted as the emplacement age and is also suggested to represent the age for the entire Varberg Charnock ite-Granite Association. Together, this age and the field relationship s demonstrate that the regional gneiss-forming event(s) in the Varberg region is older than 1.38 Ga, and that the pervasive deformation of t he Mylonite Zone, a major N-S trending shear zone and tectonic boundar y in the region, is younger than 1.38 Ga and therefore probably Svecon orwegian in age (1.1-0.9 Ga). The similarity of the Torpa granite age to other megacrystic granites in SW Sweden confirms the existence of e xtensive anorogenic, bimodal magmatism in SW Sweden at 1.38-1.36 Ga. T his magmatism is coeval with abundant anorogenic granite magmatism in SE Sweden and wide-spread volcanic-plutonic activity elsewhere in Laur entia-Baltica. The charnockitic assemblages of the 1.38 Ga Torpa grani te are interpreted to be primary in origin, and Sveconorwegian metamor phism is not evident in the U-Pb systematics of the analysed zircon fr actions. Tn contrast, granulite-facies rocks from several localities i n the Varberg region, including rocks belonging to the Varberg Charnoc kite-Granite Association, have yielded Sveconorwegian isochron ages. C onsequently, models based on two discrete events of charnockite-granul ite formation must be considered for the Proterozoic evolution of SW S weden.