Age constraints on the regional deformation within the Eastern Segment, S Sweden: Late Sveconorwegian granite dyke intrusion and metamorphic-deformational relations

A post-deformational granitic dyke and the metamorphic-deformational relations of the rocks in the Gallared area place important constraints on the timing of regional deformation and metamorphism within the southern, internal part of the Eastern Segment of the SW Swedish Gneiss Province. The petrologic data suggest a Sveconorwegian clockwise P-T evolution for the rocks in the Ullared-Gallared area, characteristic for tectonically thickened continental crust. Metamorphic-deformational relations show that the deformation of the rocks in the Ullared-Gallared area took place under Sveconorwegian high- to medium-grade metamorphic conditions. The intrusion of the Gallared granite dyke sets a clear lower limit for the deformation and the high-grade metamorphism. Five Pb-evaporation datings of single idiomorphic zircons from the dyke have provided a very well defined plateau age of 956 +/- 7 Ma. The granite dyke is unmetamorphosed except for very low-grade local alterations, and truncates a high-grade metamorphic, strongly planar mineral fabric in the host amphibolite. The Gallared granite dyke shows that granitic melts were present during late stages of the Sveconorwegian orogeny. The P-T evolution of the rocks in the area implies that high geothermal gradients prevailed during exhumation; the late granitic and pegmatitic dykes can therefore have been generated by partial melting of underlying crustal levels. The P-T-t history and the metamorphic-deformational relations evidence a pervasive Sveconorwegian tectonometamorphic evolution.