Bulletin of the Geological Society of Finland (Dec 2004)
Timing of Svecofennian crustal growth and collisional tectonics in Åland, SW Finland
Abstract
In an effort to quantify the time parameter in the tectonomagmatic evolution of what has been called the Southern Svecofennian Arc Complex (SSAC) of SW Finland, advanced radiometric dating techniques have here been applied to rock groups of key importance in that area. In this paper we report the results of 131 high-resolution ion microprobe spot analyses (SIMS) of zircons, and 33 measurements using isotope dilution mass spectrometry (ID-TIMS) on zircon, monazite and titanite, and employing both large-sample multi-grain as well as single-grain techniques. The Torsholma area of the Åland archipelago, situated between southern Finland and central Sweden, is a key structural area significant to resolve the time dimension in Svecofennian tectonics. There a collage of imbricated rock slabs was formed by tectonic shortening representing the culmination of large-scale penetrative Svecofennian deformation. Another structurally significant feature investigated is the South Finland Shear Zone (SFSZ) that transects the southwest-Finnish archipelago and further east follows the southern coast of Finland. This shear zone forms the southern limit of the c. 1830 Ma Late Svecofennian Granite and Migmatite Zone (LSGM) and also features deformations of a later stage when the considered region of Svecofennian crust was consolidated. The obtained age results and their tectonic analysis can be summarized as follows. The Enklinge volcanic sequence (1885 ±6 Ma) is within error limits coeval with the intrusion of abundant early-kinematic gneissose granodiorites whose average age of 1884 ±5 Ma marks the formation of new crust in this region. Some of these geisses contain a significant amount of 2000-2080 Ma zircon. Although many Svecofennian granitoids are known to contain heterogeneous zircon populations, mainly formed c. 1890 Ma ago but also containing an inherited component, the Kökar gneiss is, to the best of our knowledge, the first case where inheritance from c. 2030 Ma sources has been unequivocally demonstrated in a syntectonic Svecofennian intrusive rock. At Torsholma, granodiorites (1879 ±6 Ma) have similarly intruded the supracrustal series, but there they were later metamorphosed into granulite orthogneisses. The mesoscopic recumbent folds and subhorizontal schistosity of these gneisses were transected by a set of steep amphibolitic dykes indicating an episode of extension. During a preceding stage, these gneisses had been sandwiched like an allochthonous slab unit between the supracrustal rocks. Younger sheet-like granodioritic intrusions (1861 ±19 Ma) with associated dykes (1865 ±7 Ma) reflect an even later stage of collisional thrusting. Altogether this tectonic evolution lasted for approximately 15 Ma (c. 1875-1860 Ma) and records a period of significant deformations during the Svecofennian orogeny. Monazites and zircon rims yield concordant U-Pb ages of c. 1830 Ma. This is in agreement with previously obtained ages of late Svecofennian granites and migmatites in the LSGM zone in S Finland. Weakly deformed pegmatites and even-grained granite dykes were emplaced 1790 ±6 Ma ago and intersect the other rocks of the area. They characterize the vaning stages of shearing along the northwestern part of the SFSZ, and the establishment of a consolidated crust where deformation was about to cease by this time.
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