Mesoproterozoic tectonothermal development of southwestern Sweden : new age constraints using field relationships and U-Pb geochronology
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The Atran terrane of southwestern Sweden is a Mesoproterozoic, polydeformed terrane and a product of crustal growth westward away from an Archean core. Both Gothian (Labradorian) and Sveconorwegian (Grenvillian) deformations are recorded, but it is difficult to identify the effects of each deformation because the resultant fabrics are essentially parallel. Recent debate regarding the timing of regional gneiss-formation revolves around two distinct hypotheses; one assigns the bulk of the deformation to the Gothian Orogeny and the other attributes it to the Sveconorwegian Orogeny. U-Pb ages from four lithotectonic units constrain the timing of tectonic and thermal events for the region. Primary, igneous zircons from a felsic orthogneiss yield a protolith crystallization age of 1664±7 Ma. The first dyke suite intruded the gneisses immediately after the earliest deformation and was then metamorphosed at 1654±9 Ma during the second major deformation and migmatization event. Migmatitic layers in the gneisses and mafic dykes are cross-cut by less deformed dykes of the second dyke suite that crystallized between 1426+9/-4 Ma and 1399+7/-6 Ma. The Gothian Orogeny was thus responsible for the majority of the deformation and migmatization recorded in the study area. In addition to ca. 1.4 Ga magmatism, metamorphism at 1438+12/-8 Ma resulted in new zircon growth in the mafic gneiss. This magmatism and metamorphism are interpreted to be a local manifestation of a regional interorogenic thermal event that may be related to ca. 1.4 Ga magmatism known elsewhere in southwestern Sweden. Sveconorwegian deformation, constrained by the undeformed third dyke suite, occurred before 946+6/-4 Ma. Metamorphic (re)crystallization of monazite (948±9 Ma), titanite (932±5 Ma and 935±7 Ma) and rutile (878±9 Ma) indicate late-Sveconorwegian static metamorphism and suggest slow cooling rates of 4.5-11°C/million years from 948 Ma to 932 Ma and 2.5°-5°C/million years from 932 Ma to 878 Ma during erosion and collapse of the Sveconorwegian Orogen.