Håkan Sjöström

Multistage growth and reworking of the Palaeoproterozoic crust in the Bergslagen area, southern Sweden: evidence from U-Pb geochronology

The Svecofennian Domain of the Fennoscandian Shield constitutes a considerable volume of Palaeoproterozoic crustal growth, 2.1-1.86 Ga ago, in between the Archaean craton in the NE and the 1.85-1.65 Ga Transscandinavian Igneous Belt (TIB) in the south and west. The Bergslagen area is a classical ore province located in the southwestern part of the Svecofennian Domain of southcentral Sweden. Its northern part is dominated by volcanic and plutonic rocks of a magmatic are with continental affinity, while the SE part is made up by a sedimentary basin. The Bergslagen area shows a metamorphic zonation from lower to middle amphibolite facies in the north to upper amphibolite facies and locally granulite facies in the south; a small greenschist area exists in the west. Identifying the age spectra of inherited components, magmatic crystallization, as well as metamorphic episodes, provide important constraints on the geodynamic evolution of this centrally located piece of the Shield. U-Pb zircon SIMS data presented in this paper complement the previous, regionally scattered TIMS data from this area. Magmatic zircons from two felsic metavolcanic rocks and two amphibolites (metagabbros) yield 1888 +/- 12, 1892 +/- 7 and 1887 +/- 5, 1895 +/- 5 Ma, respectively; i.e. within the 1.91-1.86 Ga range previously obtained for Early Svecofennian magmatism in Bergslagen. An augen gneiss from southern Bergslagen, assigned to the earliest TIB generation, yield an intrusive age of 1855 6 Ma. Metamorphic monazites from the same rock indicate that deformation and elevated thermal activity prevailed 1.83-1.82 Ga ago (TIMS). Metamorphic zircons in high-grade metasedimentary rocks from the south and west yield ages of 1793 5 and 1804 10 Ma, in accordance with ages for regional peak metamorphism and migmatite formation found elsewhere in the southern Svecofennian province of Sweden. More importantly, a few zircon crystals and overgrowths in rocks from the north indicate an early metamorphic episode at c. 1.87 Ga, indicating that Bergslagen has experienced two major metamorphic events. Detrital and inherited zircons span the range 2.78-1.90 Ga, with an apparent gap at 2.45-2.1 Ga, which further emphasize previous observations of a major juvenile (< 2.1 Ga) and a minor Archaean provenance. This, and in particular the 1.94-1.91 Ga crystals present in the c. 1.89 Ga amphibolites, support the suggestion of a former Palaeoproterozoic pre-1.91 Ga crust in the Bergslagen area. (Less)

Evidence for 1.82 Ga transpressive shearing in a 1.85 Ga granitoid in central Sweden: implications for the regional evolution

Abstract Two crustal-scale shear zone systems, the Storsjon–Edsbyn deformation zone and the Hassela shear zone join in the western part of central Sweden and form an anastomosing pattern of ductile shear zones. Several of these, up to 1 km wide, zones truncate an intrusion previously defined as a component of the major, ca. 1.80–1.77 Ga magmatic suite (the Revsund granitoids) in north central Sweden. However, within the southern part of the intrusion, titanites that are components of the fabric in a pervasively deformed shear zone (Forsaan) have a U–Pb TIMS age of 1816±2 Ma. Zircon porphyroclasts in the same fabric yield a U–Pb SIMS age of 1849±14 Ma, overlapping with the U–Pb TIMS age of 1851±2 Ma for titanites from an undeformed granitoid. These results are interpreted to define the ages of ductile shearing and magma emplacement, respectively. This interpretation is supported by microstructures as well as the regional structural pattern characterised by shear zones enveloping virtually undeformed pods of the granitoid. Previously reported and locally existing magmatic flow structures indicate some syn-emplacement deformation. The kinematic results, characterised by a high proportion of pure shear within the granitoid and oblique dextral shearing along its margin, reflect strain partitioning under transpressive conditions. The kinematic and geochronological results fit into the regional framework of late-orogenic localised deformation along crustal scale shear zones. Recent, absolute datings of shear fabrics defines ca. 1.82–1.80 Ga as a period of shear zone activity in central Sweden. Temporally, this period overlaps with the emplacement of S-type granites and granitoids derived from deeper crustal levels. A similar situation exists in southern Finland. Structural, geochronological and geophysical data suggest that the crustal scale shear zones in central Sweden and southern Finland may be related temporally and spatially. The age and the post-tectonic nature (on a regional scale) of the dated 1.85 Ga granitoid have important regional implications. As both the age of the intrusion and the ductile shearing are older than the established age range (1.80–1.77 Ga) of the Revsund granitoids, either an extended intrusive history for the suite is indicated, or the granitoids in this part of the type areas should be excluded from the Revsund suite. Their post-tectonic nature implies that the pervasive Svecokarelian deformation in the area occurred earlier than generally assumed (1.85–1.80 Ga). On the other hand, approximately coeval, calc-alkaline granitoids to the south-east of the investigated area are folded and foliated pervasively, indicating a later Svecokarelian evolution in that area, i.e. that the granitoids are located in domains with separate orogenic evolutions.

Reflection seismic investigations in the Dannemora area, central Sweden: Insights into the geometry of polyphase deformation zones and magnetite‐skarn deposits

The Bergslagen region is one of the most ore prospective districts in Sweden. Presented here are results from two nearly 25 km long reflection seismic profiles crossing this region in the Dannemora ...

Ductile shear zones related to crustal shortening and domain boundary evolution in the central Fennoscandian Shield

The Paleoproterozoic part of the Fennoscandian Shield is composed of crustal components formed in different tectonic settings and generally separated by well-defined shear ...

Late-orogenic progressive shearing in eastern Bergslagen, central Sweden

Abstract Structural studies of deformation zones in northeastern Bergslagen with emphasis on the NNE-SSW Österbybruk-Skyttorp Zone (ÖSZ) and the N-S Gimo Zone (GZ) indicate that both the ÖSZ and the GZ are splays from the Singö shear zone (SSZ) system where they originally enveloped tectonic lenses. Strain in the SSZ was possibly enhanced and later pinned by the emplacement and solidification of a c. 1.8 Ga granite within the SSZ. This distributed strain southwards leading to rotation and folding of the lenses. Fold axes are steep and rotate both the regional foliation as well as the stretching lineation demonstrating modification of pre-existing structures. The ÖSZ shows east-side-up kinematics related to the east-vergent folding. An earlier (?) west-side-up pattern along the western margin of the ÖSZ is here interpreted to be related to a dextral northeast-side-down oblique shear along the SSZ, and east-side-up shear along the GZ, prior to the folding. The GZ, which is located closer to the SSZ, generally shows a higher metamorphic grade than the ÖSZ. Microstructures and chlorite-bearing shearbands indicate upper greenschist facies conditions during deformation of the ÖSZ, whereas striped gneisses and a C-S fabric with stable hornblende and biotite along both planes indicate amphibolite facies conditions along the GZ. Overlapping shear zones in the hinge of one east-vergent fold were probably formed during flexural slip related to the folding. The structural pattern and indicated timing fit into the previously suggested S-N or SSW-NNE convergence of the accretionary, Svecokarelian orogen.

Character and timing of Svecokarelian, late-orogenic, ductile deformation zones in Jämtland, west central Sweden

Abstract An anastomosing pattern of NW-SE to NNW-SSE trending, ductile shear zones have affected the Precambrian bedrock in central Jämtland. Spatially these structures are related to two major shear zone systems, the Storsjön-Edsbyn Deformation Zone (SEDZ) and the Hassela Shear Zone (HSZ). Several of the local zones occur between Lake Näkten and Lake Locknesjön and coincide with linear magnetic anomalies. The dextral kinematics recorded are emphasised by clockwise rotation of early Svecofennian rocks, that partly are arranged in a large-scale C/S pattern. Three different, ductile shear zones yield U-Pb titanite ages of 1801±1, 1799±7, and 1794±3 Ma, interpreted to date the deformation. Pale titanite overgrowths, colourless titanite, and apatite yield younger U-Pb ages (c. 1.75 Ga) which argue for a tectonic reactivation of the zones, or a later thermal and/or hydrothermal event, as indicated by intrusions of cross-cutting pegmatites and felsic dykes. A hydrothermal activity during the Caledonian orogeny is recorded in one of the deformation zones. This is shown by an altered fabric containing titanites with pale outer parts which are severely discordant in a U-Pb concordia diagram with a Palaeozoic lower intercept. Sm-Nd analyses on re-precipitated zircon grains also deviate considerably from a Palaeoproterozoic trend given by other minerals in the assemblage.

Deep drilling in a Palaeoproterozoic hot orogen – potential for deciphering the orogenic accretion and physical properties of a tectonically layered crust

The Svecofennian is a large hot orogen composed of different accreted crustal units. The boundaries between these units are often characterised by major, steeply dipping shear zones with post-accretionary signatures. However, some of these shear zones have recorded an earlier and long-lived activity related to the accretionary episode, so have moderately eastward dipping shear zones identified in the eastern part of the Ljusdal Domain in the central part of the orogen. These shear zones, repeated at multiple lithostratigraphical levels, are associated with west verging asymmetric F2-folds indicating thrusting with imbricate slices in thickness comparable to those in the Caledonides. In the Ljusdal Domain these structures have been recognised in rock of significantly different metamorphic grade indicating thick-skin thrusting on the crustal scale possibly accompanied by channel flow. Information from deep drilling through these stacked units would shed light on the tectonostratigraphy and consequently the accretionary to post-accretionary evolution of hot orogens. In addition, information about groundwater circulation, geothermal energy potential and reservoir quality of tectonically layered rocks for e.g. CO2 sequestration experiments in crystalline rocks would be gained.