Sveconorwegian (-Grenvillian) deformation, metamorphism and leucosome formation in SW Sweden, SW Baltic Shield: constraints from a Mesoproterozoic granite intrusion

The presence of partly well-preserved Mesoproterozoic (similar to 1.4 Ga) g ranites in the eastern part of the Sveconorwegian orogen in SW Scandinavia has previously been used as evidence for a Gothian age (1.70-1.55 Ga) for t he regional ductile structures and leucosome formation in the region, Accor dingly, deformation during the Sveconorwegian (Grenville-) orogeny (1.1-0.9 Ga) has been described as subordinate and mainly restricted to discrete du ctile shear zones. Structural, metamorphic and U-Pb zircon data from the Mesoproterozoic Tjarn esjo (meta-) granite demonstrate that the granite intruded prior to a major gneiss-forming event. The U-Pb age of 1368+/-4 Ma for igneous zircon from the granite defines the upper limit of this deformation. The Tjarnesjo mass if is composed of coarse-grained, K-feldspar megacrystic granite to quartz monzonite in various strain states. The heterogeneous deformation of the ma ssif, with tectonic lenses of better-preserved but rarely completely undefo rmed granite, is triggered by the high-competence of the coarse-grained int rusion. Ductile deformation structures include penetrative or near-penetrat ive upper amphibolite and high-pressure granulite facies gneissic foliation , as well as discrete amphibolite facies shear zones. Leucosome formation i s observed throughout the massif; continuous exposures show transformation of the granite by deformation and migmatization into a veined gneissic rock similar to the country rock gneisses. Zircon from strongly deformed veined Tjarnesjo granite, and from pegmatoid filling in boudinaged amphibolite wi thin the granite, date the partial melting at 992+/-24 Ma and 954+/-21 Ma r espectively. The regional foliation in the surrounding country rock gneisse s is concordant with the gneissic foliation in the Tjarnesjo meta-granite a nd the regional foliation locally cross-cuts the lithological boundary betw een these units. The: lower age limit for the deformation and metamorphism is set by a post-tectonic pegmatite dyke within the massil dated at 947+/-1 2 Ma. The data from the Tjarnesjo meta-granite show that the Sveconorwegian high- grade ductile deformation and metamorphism was pervasive, or near pervasive in competent lithologies, and commonly associated with partial melting. Th ese data call for re-evaluation of previous tectonic models inferring that the main ductile deformation and veining of the crust in SW Sweden is Gothi an in age (1.70-1.55 Ga). (C) 1999 Elsevier Science B.V. All rights reserve d.