Structural and U-Pb isotopic age constraints on the tectonothermal evolution at Glassvik, Halland
Abstract: The topic of this paper include an attempt to interpret the mutually timing of the tectonothermal events at Glassvik, considered to constitute a key locality in the southern part of the Southwestern Granulite Province (SGP). The province is characterized by numerous occurrences of mafic granulites and charnockites as well as high-grade gneisses of a polymetamorphic origin. Three different distinguishable types of granitic dykes and segregates have been studied. A thick pegmatitic dyke was sampled and dated by the U-Pb zircon method. The upper intercept age is 1510 Ma and is interpreted as the intrusion age, while the significance of the lower intercept age of c. 630 Ma is unclear. Three planar structures are distinguished by differences in mineralogy, deformational textures and structural relationships to intrusive pegmatitic dykes and segregates. The intrusion of the pegmatitic dykes postdates the formation of the compositional banding (S1) exposed in the amphibolites, and predates a high-grade event D2. During this event (D2), migmatitic gneisses were formed and pre-existing fabrics and intrusive dykes were reworked. Retrogression of the high-grade metamorphic assemblages in amphibolitic S3-lenses is interpreted to be associated to late or post-Sveconorwegian uplift.
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