Geochronology and structural setting of the 1.38 Ga Torpa granite; implications for charnockite formation in SW Sweden

Abstract

In the Varberg region of the Southwest Scandinavian Domain in SW Sweden, the megacrystic Torpa granite forms a sheet‐like, partly charnockitic intrusion in intensely migmatised gneisses. The granite has been interpreted 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 portion of the Torpa granite is interpreted as the emplacement age and is also suggested to represent the age for the entire Varberg Charnockite‐Granite Association. Together, this age and the field relationships demonstrate that the regional gneiss‐forming event(s) in the Varberg region is older than 1.38 Ga, and that the pervasive deformation of the Mylonite Zone, a major N‐S trending shear zone and tectonic boundary in the region, is younger than 1.38 Ga and therefore probably Sveconorwegian 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 extensive anorogenic, bimodal magmatism in SW Sweden at 1.38–1.36 Ga. This magmatism is coeval with abundant anorogenic granite magmatism in SE Sweden and wide‐spread volcanic‐plutonic activity elsewhere in Laurentia‐Baltica. The charnockitic assemblages of the 1.38 Ga Torpa granite are interpreted to be primary in origin, and Sveconorwegian metamorphism is not evident in the U‐Pb systematics of the analysed zircon fractions. In contrast, granulite‐facies rocks from several localities in the Varberg region, including rocks belonging to the Varberg Charnockite‐Granite Association, have yielded Sveconorwegian isochron ages. Consequently, models based on two discrete events of charnockite‐granulite formation must be considered for the Proterozoic evolution of SW Sweden. Åhall, K.‐I., Samuelsson, L. & Persson, P.‐O., 1997: Geochronology and structural setting of the 1.38 Ga Torpa granite; implications for charnockite formation in SW Sweden. GFF, Vol. 119 (Pt. 1, March), pp. 37–43. Stockholm. ISSN 1103–5897.