Igneous and metamorphic geochronologic evolution of granitoids in the central Eastern Segment, southern Sweden

摘要

The Eastern Segment abutting the Transscandinavian Igneous Belt (TIB) mostly consists of rocks with overlapping igneous ages. In the Eastern Segment west of Lake Vattern, granitoids of clear TIB affinity exhibit strong dcformational fabrics. This article presents U-Pb zircon ages from 21 samples spanning the border zone between these deformed TIB rocks in the east, and more thoroughly reworked rocks in the west. Magmatic ages fall in the range 1710-1660 million years, irrespective of the degree of deformation, confirming the overlapping crystallization ages between deformed TIB rocks and orthogneisses of the Eastern Segment. A common history is further supported by lcucocratic rocks of similar ages. Prolonged orogenic (magmatic) activity is suggested by continued growth of zircon at 1.66-1.60 Ga. Six of the weakly gncissic rocks show zircons with cathodoluminescence-dark patches and embayments, possibly partly replacing metamict parts of older magmatic crystals, with ~(207)Pb/~(206)Pb ages dominantly between 1460 and 1400 million years, whereas three of the gneisses have zircon rims with calculated ages of 1440-1430 million years. Leucosome formation took place at 1443 ± 9 and 1437 ± 6 Ma. The minimum age of SE-NW folds was determined by an undeformed 1383 ± 4 million years crosscutting aplitic dike. Sveconorwegian zircon growth was not found in any of the samples from the studied area. To our knowledge, 1.46-1.40 Ga metamorphism affecting the U-Pb zircon system has not previously been reported this far northeast in the Eastern Segment. We suggest that the E-W- to SE-NW-trending deformation fabrics in our field area were produced during the Hallandian-Danopolonian orogeny and escaped later, penetrative Sveconorwegian reworking.