Anders Wikström

A U–Pb dating of the Askersund granite and its marginal augen gneiss

Abstract U–Pb zircon ages of 1848±15 and 1842+23 -13 Ma have been obtained for the coarse porphyritic Askersund granite (‘Filipstad type’) and for its marginal augen gneiss, respectively. The results indicate that the onset of magmatism in the Transscandinavian Igneous Belt (TIB) took place in connection with the late Svecofennian orogenic development. They also support a structural model implying that regionally deformed and folded marginal parts of older granitoid intrusions in the TIB have similar ages as the isotropic and discordant central parts. The deformation is reflected in the morphology of the zircons but does not affect the isotopic ages.

Growth-related 1.85-1.55 Ga magmatism in the Baltic Shield; a review addressing the tectonic characteristics of Svecofennian, TIB 1-related, and Gothian events - A discussion

The concept of the Transscandinavian igneous belt (TIB; Patchett et al. 1987), assigned to rocks traditionally considered postorogenic in relation to the Svecofennian (or Svecokarelian) orogeny needs reevaluation, as high-lighted by the recent review by Åhäll & Larson (2000; hereafter Å&L). During the last decade new work has increased the knowledge such that the concept has become expanded and subdivided in various ways by different authors. As the petrographic/geochemical and postorogenic structural characteristics, on which the original definitions were based, have been shown to be less homogeneous and reliable than previously thought, new more tenable subdivisions have to be sought. The geochronological data which have accumulated during that time show an age spread among TIB-defined rocks that now span c. 200 m.y. (1.85–1.65 Ga). Å&L use the spatial distribution of ages to put forward interpretations regarding possible plate tectonic scenarios. We find this to be an interesting and thought-provoking approach, but feel that their conclusions are based on an overinterpretation of the available geochronological data. We would like to take this opportunity to discuss some details and current problems concerning the interpretation of TIB evolution prompted by their review. We will focus our discussion on some marginal problems of the Småland-Värmland belt (SVB) in the south.

Charnockites, pyroxene granulites, and garnet-cordierite gneisses at a boundary between Early Svecofennian rocks and Småland-Värmland granitoids, Karlskoga, southern Sweden

Abstract A lobe of Early Svecofennian high-grade, metamorphic rocks surrounded and intruded by rocks of the Smaland-Varmland batholith east of Karlskoga, central southern Sweden, has been studied. Application of geothermobarometry reveals that these rocks have suffered granulite facies metamorphism at conditions constrained to 670–770°C, 4.0–4.5 kbar and aH2O0.1–0.3. The metamorphism has transformed biotite granite into charnockite, intermediate volcanite into pyroxene granulite, and lower grade presumably semipelitic gneiss into garnet-cordierite gneiss. Extensive partial melting accompanied the metamorphism in the garnet-cordierite gneisses and granulites, but not in the charnockites. The metamorphism is attributed to a local contact metamorphic peak, associated with the emplacement of the Smaland-Varmland granitoids and related mafic plutonics, in the penecontemporaneous, amphibolite facies, regional “serorogenic Svecofennian” episode.

Radiometric dating of the postorogenic Graversfors granite, south central Sweden

Abstract A Rb—Sr whole rock isochron dating has yielded an age of 1692 ±7 Ma on the Graversfors granite, south central Sweden. The 280 Ma higher U—Pb discordia age of 1971+51 −43 Ma suggests a previous history for the zircons. The Rb-Sr mineral systems finally closed 1641 ±17 Ma ago.

The relationship between 1.88 Ga old magmatism and the Baltic-Bothnian shear zone in northern Sweden

Abstract New geochemical data and U-Pb ages for Proterozoic mafic and felsic dykes and their granitic host rocks in the Bläsberget Massif, proximal to the Baltic-Bothnian shear zone, indicate that these rocks are part of a terrane distinct from Svecofennian terranes in Central and Southern Sweden. The dykes comprise a NNE-trending swarm which can be traced over a strike length of about 20 km. Most of the mafic dykes are internally deformed and have systematic internal structures showing a sinistral horizontal component and an eastern-side-up sense of movement. This sinistral shearing is also observed within the nearby Baltic-Bothnian shear zone. The mafic dykes have a uniformly basaltic composition. High content of elements such as Ti and Zr illustrate the typical ‘within-plate’ character. The felsic dykes are chemically homogeneous and have a SiO2 content of about 77 wt%. High content of Y, Yb and Nb confirms the ‘within-plate’ character of the dyke swarm. Zircons extracted from one of the felsic dykes indicate a U-Pb age of c. 1.88 Ga. Approximately the same age has been obtained from the host rock granite and from a nearby occurrence of a similar late-orogenic granite. Since the dykes cross-cut most deformation and migmatite structures and most likely represent the waning stages of orogenic deformation in the area, it is evident that the timing of the orogenic events in the area is different and older than the Svecofennian development in central and southern Sweden. A genetic relationship is suggested between the magmatism and the development of the Baltic-Bothnian shear zone.

A precursory study of magnetic fabrics in the transition zone between magmatic and metamorphic textures in the Askersund granitoid, Sweden

Abstract The Askersund granitoid (age 1845–1850 Ma) is the oldest member of the Transscandinavian Igneous Belt which forms a major structure in the Scandinavian peninsula. The external, metamorphic and gneissic parts of this older generation have the same age as the more central and isotropic parts. A petrographical investigation of samples collected in two traverses from the macroscopically isotropic part to the gneissic part of the intrusion has been complemented with studies of the anisotropy of the magnetic susceptibility of the rocks. The mean degree of anisotropy (PJ ) is different from site to site and increases from the macroscopically isotropic part to the gneissic part. The shape of the susceptibility ellipsoid is slightly oblate in the former part and prolate in the latter. Under the microscope this change can be correlated with a change in textures indicating deformation in a partly crystallized magma to more solid state deformation structures. A major conclusion of this investigation with reg...

Geothermometry of garnet-cordierite rocks in Kilsbergen and northern Tiveden, southern Sweden

Abstract Samples of garnet-cordierite rocks have been investigated in order to check if the estimated high-amphibolite facies conditions of the sampling areas (absence of orthopyroxene), also are reflected in different geothermometers (garnet-cordierite and garnet-biotite) when compared with a similar investigation of similar rocks in a nearby granulite facies area. The sampled garnet-cordierite rocks differ somewhat in geological character. At Svenshyttan they are similar to other high-grade Svecofennian veined gneisses in the region, at Kinkhyttan they have a characteristic, massive appearance, while the rocks at Nygarden are mobilized and back-vein a norite. A thermal metamorphic peak is present in the area, displayed by the Kinkhyttan samples which, according to the garnet-cordierite thermometers, have reached temperatures of at least 650–700°C. This metamorphism is most likely associated with a near lobe of Smaland-Varmland granitoids. Along the western contact of this lobe, the granulite facies cond...

The Loftahammar augen gneiss; a discussion

Discussion sur les articles de Wikstrom, A. 1984 A possible relationship between augen gneiss and postorogenic granites in S.E. Sweden Journal of Structural Geology no 6, pp. 409-415. Wikstrom, A. 1987 Magmatic diapirism of granites in south-eastern Sweden Revista Brasileira de Geosciencias 17, in print. Wikstrom, A., Aaro, S. 1986 The Finspang augen gneiss massif. Geology, geophysics and relationship to postorogenic granites Sveriges geologiska undersokning C. 813, pp. 1-39, Kresten P., 1986 The granites of the Vastervik area south-eastern Sweden. Sveriges geologiska undersokning C. 814, pp. 1-35

Radiometric dating of the Finspång augen gneiss, south central Sweden

Abstract A Rb-Sr whole rock isochron for the Finspang augen gneiss, south central Sweden, yields an age of 1834 ± 21 Ma which is compared with intrusive ages for similar rocks in the neighbourhood. The augen gneiss is interpreted as an outer ‘skin’ surrounding younger granite intrusions.

The Breven dolerite dike in south central Sweden and an eastward extension of dolerite-granophyre breccia

Abstract The Breven dolerite dike in the eastern part of south central Sweden has been remapped. A large central part of the dike is occupied by a red granophyre, most probably a late differentiate of the dolerite. To the east of the major dike there is a hybrid dolerite-breccia with xenoliths of granophyre showing different stages of assimilation. The latter dike has a width of around 100 m and a length of at least 8 km. These observations suggest several pulses of dolerite in the dike system.